Studies of Ion Exchange Membranes with Particular Focus on Synthesis of Cation

Exchange Membranes

Shanxue Jiang

A thesis submitted in fulfilment of the requirements for the degree of

Doctor of Philosophy

Department of Chemical Engineering

Imperial College London

August 2019

Declaration of Originality

I hereby declare that this thesis was composed by myself and that the work reported herein is my own unless explicitly stated otherwise in the text. This work has not previously been submitted for any degree.

Shanxue Jiang

August 2019

The copyright of this thesis rests with the author and is made available under a

Creative Commons Attribution Non-Commercial No Derivatives licence.

Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work.

Shanxue Jiang

August 2019

Abstract

A series of studies on ion exchange membranes (IEMs), with a particular focus on synthesis of cation exchange membranes (CEMs), were carried out in this thesis.

Firstly, a comprehensive statistical study was carried out on the research of IEMs via a scientometric approach. It was found that, from 2001 to 2016, over 18000 articles were published on IEMs. Also, these articles were spread across over 1000 different journals, nearly 100 countries/regions and over 5000 research institutes.

Secondly, in this thesis, different methods to prepare CEMs were discussed. It was found that membranes prepared without support was very fragile. Therefore, a support was necessary. Also, it was found that linear polystyrene sulfonate was not suitable for making CEMs. In order to prepare good CEMs, the functional polymers had to be cross-linked. Therefore, chemical reaction was necessary which could introduce new bonds to make crosslinking happen.

Thirdly, a new methodology was proposed for making CEMs with high ion exchange capacity (IEC) using porous membrane support and functional polymers.

The synthesized membranes demonstrated superior IEC. Besides, the semi- finished membrane demonstrated hydrophobic property while the final membranes showed super hydrophilic property. In addition, when sulfonation reaction time increased, the conductivity of membranes also showed a tendency to increase.

Fourthly, another effective methodology to prepare high performance CEMs was

Abstract proposed. Water as selected as the solvent and also no sulfonation was needed. A series of membranes were synthesized using this methodology and different preparation conditions were investigated. Water played an important role in membrane synthesis. IEC could be greatly increased by reducing the amount of water used for synthesis. The synthesized membranes demonstrated high electrodialysis performance. It is envisioned that this one-step synthesis methodology may open up new possibilities for synthesis of IEMs in an effective and environment-friendly way.

Acknowledgements

Time flies so fast. It has been three years since I started my PhD journey.

Undoubtedly, it has been a wonderful experience studying at Imperial College. I would like to thank my supervisor, Dr. Bradley Ladewig, for his support, trust and encouragement during my PhD study. I am extremely grateful for his rapid response to my questions/problems. Also, I would like to thank my academic mentor, Professor Geoffrey Kelsall for his generous support. I would also like to thank the past and present postgraduate members in our group whom I have met and worked with, namely Ben Slater, Chen Chen, Luqman Hakim Bin Mohd Azmi,

Marine Michel, Nicholaus Prasetya, Haimiao Jiao, Siyao Li, Tingwu Liu, Anthony

Houghton, Xin Dong, and Zeru Wang. I would also like to thank David Crawford for building up the electrodialysis system and providing detailed procedures for system operation and maintenance. I would also like to thank Kimberly Hagesteijn for her enthusiasm in learning scientometrics and her contribution to the scientometric study. In addition to the group members, I would also like to thank the other members in the office whom I have worked with, namely Dr. Ruiyi Liu,

Dr. Zhiwei Jiang, and Marc Plunkett. I would also like to thank Dr. Bo Wang, Dr. Tao

Li, Yunsi Chi, and Vatsal Shah from Professor Kang Li’s group for their help using their lab and lab equipment. I would also like to thank Patricia Carry and Kaho

Cheung for their help using the analytical lab. I would also like to thank Dr.

Mahmoud Ardakani for his help with SEM. I would also like to thank Susi

Underwood for her enthusiastic help. I would also like to thank the administrative and supporting staff at the department and the college for their support. Finally, I would like to thank my family and friends for their support during my PhD study.

Table of Contents

Declaration of Originality ...... 2

Abstract ...... 4

Acknowledgements ...... 6

List of Figures ...... 12

List of Tables ...... 20

Nomenclature...... 22

Chapter 1 Introduction ...... 25

1.1 Background ...... 25

1.2 Aims and Objectives ...... 26

1.3 Thesis Structure ...... 27

Chapter 2 Literature Review ...... 29

2.1 Types and Mechanism of Ion Exchange Membranes ...... 30

2.2 Applications of Ion Exchange Membranes ...... 34

2.2.1 Electrodialysis ...... 34

2.2.2 Reverse Electrodialysis ...... 41

2.2.3 Fuel Cells ...... 43

2.2.4 Electrolysis for Hydrogen Production ...... 44

2.2.5 Chlor-Alkali Industry ...... 46

2.2.6 Redox Flow Battery ...... 47

Table of Contents

2.2.7 Membrane Capacitive Deionization...... 49

2.2.8 Electro-Electrodialysis ...... 51

2.2.9 Electrodeionization...... 52

2.2.10 Diffusion Dialysis ...... 52

2.3 Synthesis of Ion Exchange Membranes ...... 53

2.4 Summary ...... 62

Chapter 3 Materials and Methods ...... 63

3.1 Materials ...... 64

3.2 Scientometric Method ...... 64

3.2.1 Basic Processing Procedures ...... 66

3.2.2 Advanced Processing Procedures ...... 69

3.3 Basic Experiments ...... 72

3.3.1 Solution Preparation ...... 72

3.3.2 Density and Thickness Measurements ...... 73

3.4 Membrane Preparation ...... 73

3.4.1 Early Stages of the Membrane Preparation Journey ...... 73

3.4.2 Membrane Preparation via Polymerization and Sulfonation ...... 78

3.4.3 Membrane Preparation via Polymerization Without Sulfonation ...... 80

3.5 Membrane Characterisation ...... 83

3.5.1 Scanning Electron Microscope ...... 83

3.5.2 Fourier Transform Infrared Spectroscopy ...... 83

3.5.3 Ion Exchange Capacity ...... 83

8 Table of Contents

3.5.4 Water Uptake...... 84

3.5.5 Water Contact Angle ...... 84

3.5.6 Loading Ratio ...... 85

3.5.7 Proton Conductivity ...... 85

3.5.8 Thermogravimetric Analysis and Differential Scanning Calorimetry ...... 86

3.6 Electrodialysis Test...... 87

3.6.1 Electrodialysis System Setup ...... 87

3.6.2 Test Procedures ...... 88

Chapter 4 Scientometric Study of Ion Exchange Membranes ...... 91

4.1 Introduction ...... 92

4.2 Results and Discussion ...... 92

4.2.1 Document Types ...... 92

4.2.2 Publishing Languages ...... 93

4.2.3 Publishing Trend ...... 94

4.2.4 Publishing Journals ...... 96

4.2.5 Publishing Countries/Regions ...... 98

4.2.6 Publishing Institutions ...... 101

4.2.7 Most-Cited Papers ...... 105

4.2.8 Title Analysis ...... 106

4.2.9 Abstract Analysis ...... 107

4.2.10 Keywords Analysis...... 111

4.2.11 Research Areas Analysis ...... 113

9 Table of Contents

4.3 Conclusion ...... 115

Chapter 5 Two-Step Synthesis of Cation Exchange Membranes via Polymerization and Sulfonation ...... 117

5.1 Introduction ...... 118

5.2 Results and Discussion ...... 120

5.2.1 Membrane Morphology and Sulfur Distribution ...... 120

5.2.2 Membrane Chemical Structure ...... 124

5.2.3 Theoretical Ion Exchange Capacity ...... 126

5.2.4 Experimental Ion Exchange Capacity ...... 130

5.2.5 Water Contact Angle, Water Uptake and Conductivity ...... 132

5.2.6 Membrane Thermal Properties ...... 136

5.2.7 Mass Production of Membranes ...... 138

5.2.8 Electrodialysis Test ...... 141

5.3 Conclusion ...... 145

Chapter 6 Solubility Study of LiSS in DMSO and Water ...... 147

6.1 Introduction ...... 148

6.2 Results and Discussion ...... 149

6.2.1 Solvent Evaporation Method ...... 149

6.2.2 Ultraviolet–Visible Spectroscopy Method ...... 154

6.2.3 Membrane Preparation Using DMSO/Water as the Solvent ...... 161

6.3 Conclusion ...... 163

Chapter 7 One-Step Synthesis of Cation Exchange Membranes via Polymerization Without Sulfonation...... 164

10 Table of Contents

7.1 Introduction ...... 165

7.2 Results and Discussion ...... 167

7.2.1 Membrane Synthesis and Thickness ...... 167

7.2.2 Membrane Chemical Structure ...... 171

7.2.3 Ion Exchange Capacity and Loading Ratio ...... 173

7.2.4 Water Uptake and Contact Angle ...... 178

7.2.5 Membrane Morphology and Sulfur Distribution ...... 179

7.2.6 Membrane Thermal Properties ...... 181

7.2.7 Relationship Between Conductivity and Concentration...... 183

7.2.8 Membrane Electrodialysis Performance ...... 186

7.3 Conclusion ...... 189

Chapter 8 Conclusions and Recommendations for Future Work ...... 191

List of Publications ...... 195

References ...... 197

Appendices ...... 224

Appendix 1 Copyright Permissions ...... 224

Appendix 2 Standard Solution Calibration for ICP Measurement...... 229

List of Figures

Figure 1-1. Flow chart of thesis structure...... 28

Figure 2-1. Schematic illustration of two basic types of IEMs...... 30

Figure 2-2. Schematic illustration of bipolar membranes and amphoteric IEMs. 33

Figure 2-3. Schematic illustration of monovalent selective IEMs...... 34

Figure 2-4. Schematic illustration of a typical ED process...... 35

Figure 2-5. Schematic illustration of a typical BMED process for acid and base recovery. The electrode solution is not shown...... 38

Figure 2-6. Schematic illustration of a typical RED process for energy production...... 41

Figure 2-7. Schematic illustration of a typical PEMFC...... 43

Figure 2-8. Schematic illustration of a PEM-based electrolysis cell for H2 production...... 45

Figure 2-9. Schematic illustration of the discharging process of a vanadium redox flow battery...... 48

Figure 2-10. Schematic illustration of membrane capacitive deionization...... 49

Figure 2-11. Structure of chloromethyl methyl ether (CMME) and trimethylamine (TMA)...... 56

Figure 2-12. Conventional routes and relatively green routes for synthesizing IEMs. (a) Conventional route for synthesizing AEMs. (b) An example of green synthesis of AEMs. (c) Conventional route for synthesizing CEMs. (d) An example of green synthesis of CEMs. CMME refers to chloromethyl methyl ether; PVA refers to polyvinyl alcohol; TEOS refers to tetraethyl orthosilicate; THOPS refers to 3- trihydroxysilyl-1-propanesulfonic acid; TMA refers to trimethylamine; VBTAC

List of Figures refers to (ar-vinylbenzyl)trimethylammonium chloride...... 57

Figure 3-1. Schematic illustration of membrane preparation via polymerization and sulfonation. O-M refers to original porous membrane support; S-M refers to semi-finished membrane synthesized by copolymerization of St and DVB in O-M...... 79

Figure 3-2. Schematic illustration of membrane preparation via polymerization without sulfonation...... 80

Figure 3-3. Schematic illustration of membrane cross-sections prepared via polymerization without sulfonation. M refers to membrane; LiSS refers to lithium styrene sulfonate; DVB refers to divinylbenzene; PSS refers to polystyrene sulfonate or sulfonated polystyrene; CER refers to cation exchange resin; PDVB refers to polydivinylbenzene...... 81

Figure 3-4. Polymerization reactions during membrane synthesis. Synthesis of

MDLX and MCLX mainly involves reaction (a); synthesis of MWL0.5 mainly involves reaction (c); synthesis of MCDX mainly involves reaction (a) and (b). The above reaction equations are for demonstration purposes only, as the ratios of different reactants are not considered in these equations...... 82

Figure 3-5. Schematic diagram of proton conductivity testing cell. L refers to the distance between the inner electrodes, and L = 0.5 cm. W refers to the width of hydrated membrane sample, and W = 0.5 cm. T refers to the thickness of the hydrated membrane sample, and T = 0.26 mm...... 86

Figure 3-6. Real pictures of the ED cell, membrane size and active membrane area for ED test. (a)-(c) Real picture of the ED cell and membrane stack. (d) Schematic illustration of membrane size and active membrane area...... 88

13 List of Figures control and flow rate display panel; 15. Potentiostat control panel. (a) Diluting compartment; (b) Concentrating compartment; (c) Electrode compartment. EI refers to electrode solution in; DI refers to solution in (to be diluted); CI refers to solution in (to be concentrated); EO refers to electrode solution in; DO refers to diluted solution out; CO refers to concentrated solution out...... 89

Figure 4-1. Percentage distribution of document types...... 92

Figure 4-2. Number of publications in different languages...... 93

Figure 4-3. Number of publications per year and cumulative number of publications on IEMs since 2001...... 94

Figure 4-4. Number of publications by different journals...... 95

Figure 4-5. Number of publications, average number of citations per paper, and h- index of the top 20 most publishing journals...... 97

Figure 4-6. Number of publications and corresponding percentage of the top 20 most publishing countries...... 98

Figure 4-7. Network graph showing collaborations between countries. Countries with collaborations of more than 20 times are connected with lines...... 100

Figure 4-8. Number of publications, average number of citations per paper, and h- index of the top 20 most publishing research institutes...... 103

Figure 4-9. Research institute collaboration network graph. Institutes whose collaborations exceeded 10 times are connected with lines...... 104

Figure 4-10. Word cloud generated from titles with frequency no less than 200...... 106

Figure 4-11. Word cloud generated from abstracts with frequency no less than 1000...... 108

Figure 4-12. Keywords network graph. Keywords whose cooccurrence exceeded 30 times were connected with lines...... 112

14 List of Figures

Figure 4-13. Number of publications among different research areas...... 114

Figure 4-14. Number of publications in the top six research areas every year from 2001 to 2016...... 114

Figure 5-1. Conceptual illustration of the methodology in this chapter. (a) Interconnected rebar. (b) Schematic diagram of reinforced concrete. (c) SEM image of membrane support used in this work. (d) SEM image of semi-finished membrane in this work...... 120

Figure 5-2. Surface morphology, sulfur mapping and energy spectrum diagram of F-M with different sulfonation reaction time. (a) Membrane F-M-22. (b) Membrane F-M-30. (c) Membrane F-M-38. (d) Membrane F-M-46. F-M refers to the final- membrane. The number following F-M means sulfonation hours...... 121

Figure 5-3. Energy spectrum diagram of F-M cross-sections...... 123

Figure 5-4. FTIR spectra of O-M, S-M and F-M. O-M refers to the original porous membrane support; S-M refers to the semi-finished membrane synthesized by copolymerization of St and DVB in O-M; F-M refers to the final-membrane synthesized by sulfonation of S-M. The number following F-M means sulfonation hours...... 124

Figure 5-5. Effect of sulfonation reaction time on IEC. Values are averages of at least three replicates. Error bars represent one standard deviation...... 131

Figure 5-6. Comparison of IEC between prepared membranes in this chapter and common commercial membranes...... 131

Figure 5-7. Measurement of the water contact angle of (a) original porous membrane support (O-M), (b) semi-finished membrane (S-M), and (c) final- membrane (F-M). Comparison of water contact angles of O-M, S-M, and F-M is shown in (d). Values are averages of at least three replicates. Error bars represent one standard deviation...... 133

Figure 5-8. Complex plane plots obtained by electrochemical impedance

15 List of Figures spectroscopy. (a) Background without membranes. (b) Membrane F-M-22. (c) Membrane F-M-30. (d) Membrane F-M-38. (e) Membrane F-M-46. Z′′ refers to imaginary impedance and Z′ refers to real impedance. At least four repeated measurements are carried out for each membrane sample...... 135

Figure 5-9. TGA diagrams of original porous membrane support (O-M), semi- finished membrane (S-M), and final-membrane (F-M) with different sulfonation reaction time. The number following F-M means sulfonation hours...... 136

Figure 5-10. DSC diagrams of original porous membrane support (O-M), semi- finished membrane (S-M), and final-membrane (F-M) with different sulfonation reaction time. The number following F-M means sulfonation hours...... 137

Figure 5-11. Methodology diagram for mass production of membranes. The left diagram is used in the synthesis of F-M series where aluminium sheet is the spacer. The right diagram shows mass production of membranes where original aluminium sheet is replaced by polyester sheet...... 139

Figure 5-12. Real pictures of the synthesized CEMs for ED test. The sulfonation reaction time was 6 h. (a) Without holes. (b) With holes. The holes are channels for electrolyte solutions. (c) Six pieces of membranes synthesized at one time.140

Figure 5-13. Water contact angle of the synthesized membranes for ED test. .... 141

Figure 5-14. Real-time monitoring of the ED system. (a) Flow rate in the electrode compartment. (b) Flow rate in the concentrating compartment. (c) Flow rate in the diluting compartment. (d) Voltage applied to the ED cell. (e) Current applied to the ED cell...... 142

Figure 5-15. ED performance. (a) Conductivity changes of the electrolyte solution in the concentrating and diluting compartments. (b) Changes of sodium concentration in the concentrating and diluting compartments...... 144

Figure 6-1. Structural formula of NaSS (left) and LiSS (right)...... 148

Figure 6-2. Real pictures of supernatant (i.e., saturated LiSS solution). (a) LiSS-

16 List of Figures water solution. (b) LiSS-DMSO solution...... 150

Figure 6-3. Real pictures of the LiSS-DMSO system under different LiSS/DMSO mass ratios...... 155

Figure 6-4. Relationship between density of LiSS solution (solvent: DMSO) and mass fraction of LiSS in the solution...... 156

Figure 6-5. UV-Vis spectra of LiSS-DMSO system under different LiSS/DMSO mass ratios. Values are averages of at least three replicates...... 158

Figure 6-6. R2 of quadratic curve fittings at different wavelengths...... 159

Figure 6-7. Relationship between absorbance and mass fraction at different wavelengths. (a) 348 nm. (b) 349 nm. (c) 350 nm. (d) 351 nm...... 160

Figure 7-1. Thicknesses of different membrane series. (a) Membrane thicknesses under different DVB/LiSS ratios. (b) Membrane thicknesses under different

CER/LiSS ratios. (c) Membrane thicknesses under different CER/DVB ratios. MDL0.6 from (a) is shown in (b) and (c) again for comparison. (d) Membrane thicknesses under different water/LiSS ratios. CER refers to cation exchange resin; DVB refers to divinylbenzene; LiSS refers to lithium styrenesulfonate...... 167

Figure 7-2. Schematic illustration of the dramatic increase in thickness observed in the MCDX series...... 169

Figure 7-3. FTIR spectra of different membrane series and materials. (a) MDLX series. (b) MCDX series and PDVB. (c) MCLX series and CER. (d) LiSS, MDL0.5, MWL0.5, and FMWL0.5. PDVB refers to polydivinylbenzene; FM refers to functional material in the membranes...... 171

Figure 7-4. IEC and LR of different membrane samples and materials. (a) Theoretical effect of DR and LR on IEC. (b) IEC and LR under different DVB/LiSS ratios. (c) Effect of LR on IEC. (d) IEC and LR under different CER/DVB ratios. (e) IEC and LR under different CER/LiSS ratios. (f) IEC and LR under different water/LiSS ratios. FM refers to functional material in the membranes. The black

17 List of Figures square points correspond to experimental IEC values and the red circle points correspond to theoretical IEC values...... 177

Figure 7-5. Water uptake and contact angle of different membrane series. (a) Water uptake and contact angle under different DVB/LiSS ratios. (b) Water uptake and contact angle under different CER/LiSS ratios. (c) Water uptake and contact angle under different CER/DVB ratios. (d) Water uptake and contact angle under different water/LiSS ratios...... 179

Figure 7-6. Membrane morphologies and sulfur distributions. (a)-(d) Surface morphology of different membrane series. (e)-(g) Surface sulfur distribution (red color) of different membrane series. (h)-(k) The corresponding energy spectrum diagrams of different membrane series. (i)-(o) The cross-section morphology of different membrane series. The red curves refer to sulfur distribution at a certain position across the membranes (yellow line)...... 180

Figure 7-7. Thermal properties of different membrane series and material. (a) DSC heating curves of different membrane series and material. Pm1 and Pm2 refer to the first and second melting peak temperature, respectively. (b) DSC cooling curves of different membrane series and material. Pc1 and Pc2 refer to the first and second crystallization peak temperature, respectively. (c) Melting and crystallization enthalpies of different membrane series. ΔHm and ΔHc refer to melting and crystallization enthalpy, respectively. (d) TGA curves of different membrane series and material (first heating). (e) TGA curves of different membrane series and material (second heating)...... 183

Figure 7-8. Relationship between conductivity and concentration...... 184

Figure 7-9. Voltage and current changes during the ED tests. (a) Record for synthesized membranes. (b) Record for commercial membranes...... 186

18 List of Figures

(b) Changes of electrolyte concentration in the concentrating and diluting compartments using synthesized membranes MWL0.5. (c) Conductivity changes of the electrolyte solution in the concentrating and diluting compartments using commercial membranes. (d) Changes of electrolyte concentration in the concentrating and diluting compartments using commercial membranes...... 188

Figure 7-11. Schematic illustration of large-scale production of membranes using the methodology proposed in the chapter...... 189

List of Tables

Table 2-1. Common functional groups for CEMs ...... 32

Table 2-2. Common functional groups for AEMs ...... 32

Table 2-3. Comparison of common membrane technologies for water treatment ...... 40

Table 2-4. Twelve principles of green chemistry...... 55

Table 2-5. Recent studies on synthesis of CEMs ...... 60

Table 3-1. Materials used in this thesis ...... 64

Table 3-2. Examples of search terms for topic ...... 65

Table 3-3. Summary of the acronyms used to describe different membrane series ...... 81

Table 4-1. The top 20 most publishing journals ...... 96

Table 4-2. Number of countries/regions in terms of number of articles published ...... 98

Table 4-3. Number of countries/regions in terms of percentage of articles published ...... 99

Table 4-4. The top 10 most collaborating country pairs ...... 100

Table 4-5. Number of research institutes in terms of number of articles published ...... 102

Table 4-6. The top 20 most publishing research institutes ...... 102

Table 4-7. The top 10 most collaborating research institute pairs ...... 104

Table 4-8. The top 20 most-cited articles ...... 105

Table 4-9. Common applications of IEMs ...... 108

List of Tables

Table 4-10. Common properties of IEMs...... 109

Table 4-11. Common studied polymers for IEMs ...... 110

Table 4-12. The top 20 most used keywords for IEMs ...... 112

Table 5-1. Water uptake and conductivity of synthesized CEMs ...... 133

Table 5-2. Melting temperatures of O-M, S-M and F-M ...... 138

Table 5-3. Volume changes of the solution in the diluting and concentrating compartments before and after test ...... 143

Table 6-1. Mass fraction of LiSS in DMSO ...... 153

Table 6-2. Solubility of LiSS in DMSO calculated via the UV-Vis method ...... 160

Table 7-1. Summary of synthesized membranes and commercial membranes for the ED test ...... 187

Nomenclature

The items below are listed in alphabetical order.

ABS Acrylonitrile butadiene styrene AC Alternating current ACS American Chemical Society AEMFC Anion exchange membrane fuel cell AEMs Anion exchange membranes AMW Average molecular weight ASME American Society of Mechanical Engineers BMED Bipolar membrane electrodialysis BPO Benzoyl peroxide CEMs Cation exchange membranes CER Cation exchange resin CMME Chloromethyl methyl ether CSM Cation selective membrane DFAFC Direct formic acid fuel cell DMAc Dimethylacetamide DMF Dimethylformamide DMSO Dimethyl sulfoxide DR DVB and styrene to LiSS mass ratio DSC Differential scanning calorimetry DVB Divinylbenzene ECS Electrochemical Society ED Electrodialysis EDI Electrodeionization EDS or EDX Energy-dispersive X-ray spectroscopy EIS Electrochemical impedance spectroscopy FM Functional material

Nomenclature

F-M Final-membrane FTIR Fourier-transform infrared spectroscopy HCl Hydrochloric acid HER Hydrogen evolution reaction HIPS High impact polystyrene IARC International Agency for Research on Cancer ICP Inductively coupled plasma IEC Ion exchange capacity

IECthr IEC threshold value IEMs Ion exchange membranes LiSS Lithium p-styrenesulfonate LR Loading ratio MCDI Membrane capacitive deionization MFC Microbial fuel cell NaCl Sodium chloride NaOH Sodium hydroxide NaSS Sodium 4-vinylbenzenesulfonate or sodium styrenesulfonate NMP N-methyl-2-pyrrolidone O-M Original membrane support PBI Polybenzimidazole PDVB Polydivinylbenzene PE Polyethylene PAES Poly(arylene ether sulfone) PEEK Polyether ether ketone PEI Polyethylenimine Proton exchange membrane fuel cell or PEMFC Polymer electrolyte membrane fuel cell PEMs Proton exchange membranes PES Polyether sulfone PP Polypropylene PPO Polydimethyl phenylene oxide

23 Nomenclature

PS Polystyrene PSS Polystyrene sulfonate or sulfonated polystyrene PTFE Polytetrafluoroethylene PVA Polyvinyl alcohol PVC Polyvinyl chloride PVDF Polyvinylidene fluoride PVP Polyvinyl pyrrolidone QA Quaternary ammonium RED Reverse electrodialysis RSC Royal Society of Chemistry SAFC Solid acid fuel cell SCIE Science Citation Index Expanded SEM Scanning electron microscope S-M Semi-finished membrane SPEEK Sulfonated poly(ether ether ketone) TEOS Tetraethyl orthosilicate TGA Thermogravimetric analysis THF Tetrahydrofuran UV-Vis Ultraviolet–visible spectroscopy

Note: some acronyms may have more specific and detailed meanings in this thesis.

For example, PSS is also short for poly(sodium 4-styrenesulfonate).

Chapter 1 Introduction

1.1 Background

Water and energy are two biggest challenges in world today. A lot of effort has been made to address these two issues. Among the various methods and techniques developed or being developed, ion exchange membranes (IEMs) have demonstrated their capacity in dealing with these issues. As an example, IEMs are used in electrodialysis to address the water issue and in fuel cells to address the energy issue. Actually, IEMs are widely used in many applications, including but not limited to conventional fuel cells [1–3], microbial fuel cells [4], conventional electrodialysis (ED) [5–7], bipolar membrane electrodialysis (BMED) [8], reverse electrodialysis (RED) [9, 10], electrolysis for hydrogen production [11], redox flow battery [12, 13], membrane capacitive deionization (MCDI) [14], diffusion dialysis

[15, 16], and electrodeionization (EDI) [17]. Correspondingly, there are many types of IEMs, including but not limited cation exchange membranes (CEMs) [18,

19], anion exchange membranes (AEMs) [20–22], bipolar membranes [23, 24], proton exchange membranes (PEMs, also known as polymer electrolyte

Note: The work presented in section 1.1 is adapted from the following publications:

[1] Shanxue Jiang, Kimberly F. L. Hagesteijn, Jin Ni, and Bradley P. Ladewig, A scientometric study of the research on ion exchange membranes, RSC Advances, vol. 8, no. 42, pp. 24036–24048, 2018. Copyright 2018 Royal Society of Chemistry. Adapted with permission.

[2] Shanxue Jiang and Bradley P. Ladewig, High Ion-Exchange Capacity Semihomogeneous Cation Exchange Membranes Prepared via a Novel Polymerization and Sulfonation Approach in Porous Polypropylene, ACS Applied Materials & Interfaces, vol. 9, no. 44, pp. 38612–38620, 2017. Copyright

2017 American Chemical Society. Adapted with permission.

Chapter 1 membranes) [25, 26], and monovalent selective membranes [27]. The research on

IEMs is also very prosperous.

Given researchers’ great interest in IEMs, it is necessary and important to give a timely update on this field so readers will not get lost in front of thousands of new papers published every year in this field. Although quite a few review papers have been published on the topic of IEMs, they are mainly based on the authors’ subjective experience. There are very limited, if not zero, number of publications on the topic of IEMs using comprehensive statistical approach, or quantitative data analysis.

Furthermore, the widespread applications of IEMs also necessitate different methods for synthesizing IEMs with different properties to meet different requirements [28–33]. However, many synthesis methods or procedures have one or more of the following issues: use of organic solvents [34, 35], use of additional chemicals to improve solubility of effective reactants or functional components

[36], heterogeneous casting solution [37], complicated procedures [38], reduced membrane long-term stability [39], etc. Therefore, it is important to develop new synthesis methods and procedures to tackle the above issues as much as possible.

1.2 Aims and Objectives

The main aims of this thesis are:

(a) to investigate the development of ion exchange membranes in a comprehensive quantitative way;

26 Chapter 1

(b) to explore novel method to synthesize cation exchange membranes with specific properties;

The main objectives of this thesis are:

(a) to gain a quantitative understanding of the research on ion exchange membranes using scientometric approach;

(b) to synthesize cation exchange membranes with high ion exchange capacity using polymerization and sulfonation approach without toxic organic solvents;

(c) to synthesize cation exchange membranes via polymerization of monomers containing sulfonate groups using water as the solvent and without using corrosive acids;

(d) to characterise the synthesized membranes to gain a better understanding of the properties of the membranes.

1.3 Thesis Structure

As shown in Figure 1-1, there are eight chapters in this thesis. Chapter 1 briefly introduces the background of the project (i.e., ion exchange membranes), and also contains the aims and objectives of this project. Chapter 2 gives a literature review on ion exchange membranes in terms of their types and mechanism, applications, and synthesis methods. Chapter 3 contains the materials and methods for preparing and characterising cation exchange membranes, and also introduces the procedures for the scientometric study of ion exchange membranes. Chapter 4

27 Chapter 1 presents the scientometric study of ion exchange membranes. Chapter 5 proposes a two-step method to synthesize cation exchange membranes with high ion exchange capacity. No toxic organic solvents are used in this method. Chapter 6 presents the solubility study of lithium styrenesulfonate in dimethyl sulfoxide and water, which provides an important foundation for the following synthesis in

Chapter 7. Chapter 7 proposes a methodology to synthesize cation exchange membranes using water as the media to replace conventional organic solvents. No sulfonation is needed in this method. The method in Chapter 7 is originated from the method in Chapter 5. Chapter 8 gives the conclusions of this thesis, and also puts forwards a few recommendations for future research work in this field.

Figure 1-1. Flow chart of thesis structure.

Chapter 2 Literature Review

Abstract

This chapter gives a review on ion exchange membranes (IEMs). Firstly, the types of IEMs are introduced, including cation exchange membranes (CEMs), anion exchange membranes (AEMs), proton exchange membranes (PEMs), bipolar membranes, amphoteric IEMs, and ion selective membranes. Also, mechanism of

IEMs is discussed. Secondly, common applications of IEMs are introduced, including electrodialysis, reverse electrodialysis, fuel cells, electrolysis for hydrogen production, chlor-alkali industry, redox flow battery, membrane capacitive deionization, electro-electrodialysis, electrodeionization, and diffusion dialysis. Thirdly, synthesis of IEMs are reviewed, including the chemical-reaction route, which usually involves polymerization and/or functionalization, and the non-chemical-reaction route, which usually uses organic solvents to dissolve the functional polymers. Especially, green synthesis is discussed.

Note: Part of the work presented in section 2.3 is adapted from the following publication:

[1] Shanxue Jiang and Bradley P. Ladewig, Green synthesis of polymeric membranes: recent advances and future prospects, Current Opinion in Green and Sustainable Chemistry, vol. 21, pp. 1- 8, 2020. Copyright 2019 Elsevier. Adapted with permission.

Chapter 2

2.1 Types and Mechanism of Ion Exchange Membranes

Figure 2-1. Schematic illustration of two basic types of IEMs.

As shown in Figure 2-1, based on the type of ions that is desired to pass through a membrane, there are two basic types of ion exchange membranes (IEMs), i.e., cation exchange membranes (CEMs) and anion exchange membranes (AEMs).

CEMs contain fixed anion groups and exchangeable cations in the polymers. In contrast, AEMs contain fixed cation groups and exchangeable anions in the polymers.

It should be noted that the mechanism of IEMs is different to that of ion exchange resins (IER). As indicated by its name, IER works by exchanging ions. For example, a sodium-based cation exchange resin (CER) can be used to remove calcium ions and magnesium ions from hard water and therefore the hardness is reduced. The underlying mechanism is that sodium ions and calcium/magnesium ions are exchanged. In other words, calcium ions “enter” the resin and sodium ions are released from resin to bulk solution. Therefore, although calcium/magnesium ions are decreased in the product water, more sodium ions appear in the product water.

After all the sodium ions are exchanged, the resin has to be regenerated. However,

Chapter 2 this is not the case for IEMs. In fact, the name itself is quite misleading because it seems that the IEM process is also an ion exchange process. However, the mechanism of IEM is actually not an ion exchange process. The basic mechanism of IEM is permselectivity. For example, a CEM allows cations to pass through it but blocks anions and neutral molecules, while an AEM allows anions to pass through it but blocks cations and neutral molecules. The reason is that, as mentioned earlier, CEMs contain fixed functional groups which are negatively charged. Due to charge repulsion, anions are less likely to pass through a CEM. In contrast, there are forces of attraction between negatively charged ions (i.e., functional groups in

CEMs) and positively charged ions (i.e., cations in the bulk solution), therefore cations are more likely to pass through a CEM. The mechanism of an AEM allowing anions to pass through it is similar.

It should also be noted that, in order for IEMs to work properly, the related ions should have the ability to move freely, and therefore directional movement of ions can be achieved. What does that mean and how to achieve it? Let us take sodium chloride as an example. The sodium ions and chloride ions in solid sodium chloride cannot move freely because they are fixed in their positions and can only vibrate.

Therefore, solid sodium chloride cannot conduct electricity. In order to conduct electricity, the ions have to move freely. There are two common ways to achieve this. The first one is to dissolve solid sodium chloride in water to form a sodium chloride solution, and the second one is to heat solid sodium chloride to change it into a molten state. Water is the most common environment for IEMs applications.

For example, in electrodialysis applications, when power is applied to the electrodes, cations in the aqueous solution/salt solution can move towards the

31 Chapter 2 cathode and anions can move towards the anode. In a correctly installed system, cations then pass through a nearby CEM but are prevented from moving further to the cathode by an AEM, while at the same time anions pass through a nearby AEM but are prevented from moving further to the anode by a CEM. In common fuel cell applications, humidification is necessary to promote ion movement and transport

[40].

As shown in Table 2-1, common functional groups for CEMs include sulfonic acid group [38, 41, 42], carboxylic acid group [43], and phosphonic acid group [44]. As shown in Table 2-2, common functional groups for AEMs include quaternary ammonium (QA) group [13, 45, 46], quaternary phosphonium group [47], and tertiary sulfonium group [48, 49]. In addition, metal-cation-based group is an emerging type of functional groups for AEMs in recent years [22, 50].

Table 2-1. Common functional groups for CEMs Functional group Formula Exchangeable ions Sodium salt form + Sulfonic acid R-SO3H H R-SO3Na Carboxylic acid R-COOH H+ R-COONa + Phosphonic acid R-PO3H2 H R-PO3Na2

Table 2-2. Common functional groups for AEMs Functional group Formula Exchangeable ions Chloride salt form - Quaternary ammonium R4NOH OH R4NCl - Quaternary phosphonium R4POH OH R4PCl - Tertiary sulfonium R3SOH OH R3SCl

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Figure 2-2. Schematic illustration of bipolar membranes and amphoteric IEMs.

In addition to common CEMs and AEMs, there are also a few special types of IEMs.

The first one is proton exchange membranes (PEMs), which is mainly used to conduct protons in fuel cells [51, 52]. The second one is bipolar membranes, which are mainly used for electrodialysis applications [8, 53, 54]. Bipolar membranes contain both cation exchange groups and anion exchange groups [55–57]. Another similar type of IEMs which contain both cation exchange groups and anion exchange groups is amphoteric IEMs, which are promising for vanadium redox flow battery applications [58–60]. The advantage of amphoteric IEMs over conventional CEMs lies in that CEMs usually have high permeability of vanadium ions which is not desired [61]. Meanwhile, the advantage of amphoteric IEMs over conventional AEMs lies in that amphoteric IEMs have higher conductivity [62]. The structural difference between bipolar membranes and amphoteric IEMs is illustrated in Figure 2-2.

As shown in Figure 2-3, another special type of IEMs is ion selective membranes

[27, 63], which can be further classified into monovalent selective CEMs [53–55]

33 Chapter 2 and monovalent selective AEMs [67, 68]. As indicated by its name, monovalent selective IEMs have the ability to separate monovalent ions from solutions and retain multivalent in solutions. Monovalent selective IEMs can be used to remove arsenic and nitrate ion from groundwater [69], to concentrate reverse osmosis brines via electrodialysis [70], to generate energy through reverse electrodialysis

[68], and so on.

Figure 2-3. Schematic illustration of monovalent selective IEMs.

2.2 Applications of Ion Exchange Membranes

IEMs are widely used in a variety of applications, such as electrodialysis [71–73], reverse electrodialysis [74–76], fuel cells [77–80], and so on. These applications are discussed in detail below.

2.2.1 Electrodialysis

Electrodialysis (ED) is one the most important applications of IEMs [81]. ED is a separation process driven by electricity [28]. Schematic illustration of the ED process is shown in Figure 2-4.

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Figure 2-4. Schematic illustration of a typical ED process.

An ED cell usually have two half parts where each part has an electrode (either anode or cathode). Before the two parts are fastened together, a set of cation exchange membranes (CEMs) and anion exchange membranes (AEMs), which are separated by spacers, are placed between the two parts. The anode and cathode of the ED cell are then connected to a power source. Besides, the cell also has inlets and outlets which are connected to three external compartments, including the electrode compartment, concentrating compartment, and diluting compartment.

Usually the solutions are pumped into the cell where solution in the diluting compartment will be desalted while salt concentration in the concentrating compartment will be increased. To be more specific, when power is supplied to the cell, an electric current will pass through the cell. As a result, cations have the tendency to move to the cathode while anions have the tendency to move towards the anode. However, since the cathode and anode is separated by CEMs and AEMs, and also since the solutions are flowing through the cell at a certain rate and the flow direction is perpendicular to the current direction, only a small amount of the cations and anions will reach at the cathode and anode, respectively.

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To elaborate, when cations from the diluting compartment reach a CEM, they will pass through the CEM and arrive at the concentrating compartment. However, when cations from the concentrating compartment reach an AEM, normally they cannot pass through the AEM and thus stays where they are. Similarly, when anions from the diluting compartment reach a AEM, they will pass through the

AEM and arrive at the concentrating compartment. However, when anions from the concentrating compartment reach an CEM, normally they cannot pass through the AEM and thus stays where they are. In other words, AEMs act as barriers for cations while CEMs act as barriers for anions. It should be noted that AEMs and

CEMs are carefully arranged in an ED cell to ensure that when power is supplied, cations in the concentrating compartment will not move towards CEMs and anions in the concentrating compartment will not move towards AEMs. In a word, ions move from the diluting compartment to the concentrating compartment. As a result, the salt concentration in the diluting compartment will be reduced.

A lot of ED studies on a variety of applications (e.g., seawater desalination, brackish water desalination, wastewater treatment, chlor-alkali industry, and food industry) have been carried out [82–86]. For example, Zhang et al [27] designed an ED system to treat seawater concentrate, which was produced by reverse osmosis plant, to produce coarse salt using monovalent selective IEMs under continuous operation. Alternatively, the concentrated brine after ED treatment can also be used as raw material for chlor-alkali industry [87]. Doornbusch et al [88] investigated the practicability of ED for seawater desalination from the perspective of energy consumption. They found that it was not possible to achieve full desalination in a single stage so multiple stages were necessary. The energy

36 Chapter 2 consumption was close to that of state-of-the-art reverse osmosis desalination technology. Xu et al [89] conducted bench and pilot-scale studies of brackish groundwater desalination using ED technology and they investigated the effects of operating conditions such as current density, linear velocity, hydraulic retention time, and staging on desalination performance. Shah et al [90] designed a feed- forward voltage-controller which could be used in batch ED system to improve desalination performance by changing voltage. ED technology is also widely used in food industry. However, food industry solutions usually have high concentrations of organic and inorganic matters, colloidal particles, microorganisms and macromolecules [91]. These complex components in food industry solutions make membrane fouling easier to occur during ED process.

Bdiri et al [91] proposed a membrane fouling mechanism during ED treatment of food industry solutions containing polyphenols and they also found that saline solutions were only effective for membrane surface cleaning while acidified water- ethanol solution was effective for both internal and surface cleaning of IEMs.

Bipolar membrane electrodialysis (BMED) is a special type of ED which involves using bipolar membranes [8]. One of the most obvious advantages of BMED technology compared to conventional hydrolysis process is that hydrogen ions and hydroxide ions can be efficiently produced through water splitting in the BPMs

[21]. Therefore, BMED is widely used in acid and base production from salt solutions [24] and a typical BMED process for acid and base recovery is illustrated in Figure 2-5. In addition to the electrode compartment, there are three main compartments/channels, namely the acid channel, base channel, and the wastewater channel. When power is applied to the cell, in the wastewater channel,

37 Chapter 2 anions such as sulfate ions and chloride ions pass through an AEM and arrive at the acid channel while cations such as sodium ions pass through a CEM and arrive at the base channel. Meanwhile, the hydrogen ions produced in a BPM are driven to the acid channel and the hydroxide ions produced in a BPM are driven to the base channel. Therefore, acid is formed in the acid channel and base is formed in the base channel.

Figure 2-5. Schematic illustration of a typical BMED process for acid and base recovery. The electrode solution is not shown.

A lot of studies are carried out on BMED. Bunani et al [8] found that BMED was effective in simultaneously separation and recovery of lithium (recovered as LiOH) and boron (recovered as H3BO3) from aqueous solution. Zhang et al [54] adopted ion exchange technology to remove calcium ions and magnesium ions from desulfurization wastewater, and then the desulfurization wastewater was further treated via BMED process, during which high-purity acid and base were produced.

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Szczygiełda et al [92] developed a BMED process to produce alpha-ketoglutaric acid and they found that high concentration of alpha-ketoglutaric acid, high current efficiency, and low energy consumption could be achieved simultaneously under optimized conditions. Lv et al [93] recovered hydrochloric acid and ammonium hydroxide from ammonium chloride wastewater using BMED technology and the process cost could be decreased by optimizing operation conditions such as current density and initial concentration of ammonium chloride. Sun et al [94] found that BMED was promising in recovering citric acid from fermented liquid due to its low energy consumption and high recovery rate of citric acid under optimized conditions. Shi et al [95] found that BMED could be employed to effectively recover ammonium ions, phosphate ions, and volatile fatty acids from pig manure hydrolysate.

In addition to IEMs-based water treatment technologies (e.g., electrodialysis), there are also many other technologies for water treatment, including but not limited to sand filtration [96], coagulation-flocculation-sedimentation [97], activated carbon adsorption [98], ion exchange technology [99], thermally-driven membrane distillation technology [100], low-pressure-driven microfiltration and ultrafiltration membrane technologies [101, 102], high-pressure-driven nanofiltration and reverse osmosis membrane technologies [103, 104], osmotically-driven forward osmosis membrane technology [105], disinfection technologies such as UV irradiation and chlorination [106, 107], advanced oxidation technologies such as ozonation [108], and biological treatment technologies [109, 110].

The above technologies can be divided into membrane technologies and non-

39 Chapter 2 membrane technologies. Generally, membrane technologies are more advanced water treatment technologies and can produce water with high quality and purity

[111]. As the cost of membrane technologies has decreased significantly in recent years, they are being widely used in water treatment. In fact, membrane technologies are employed in most of the desalination plants today [112].

Table 2-3. Comparison of common membrane technologies for water treatment Membrane Driven force Main Disadvantages Main Advantages technology (1) Low membrane fouling; (1) Low water recovery (2) capable of treating high Membrane Thermally- rate; (2) high energy salinity water; (3) high distillation driven consumption. product water purity.

(1) Easy membrane fouling (1) Space saving and easy and scaling; (2) low water control; (2) no chemical recovery rate; (3) sensitive MF, UF, NF, Pressure- additives needed; (3) no to pH and temperature; (4) RO driven phase change. high hydraulic pressure needed for RO.

(1) High reverse solute (1) Low membrane fouling; flux; (2) high consumption (2) no external pressure of energy due to Forward Osmotically- needed; (3) high water regeneration of draw osmosis driven recovery rate compared to solution (DS) and water RO. recovery from DS.

(1) High water recovery (1) High cost; (2) cannot rate compared to RO; (2) remove other impurities Electrically- low membrane fouling and ED such as electrically neutral driven long membrane lifetime; (3) organic pollutants and selective separation of suspended particles. monovalent ions. Note: MF refers to microfiltration; UF refers to ultrafiltration; NF refers to nanofiltration; RO refers to reverse osmosis; ED refers to electrodialysis.

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The main advantages and disadvantages of different membrane technologies are summarized in Table 2-3 [112–114]. Compared to other water treatment technologies, electrodialysis (ED) is especially effective and efficient in removing ions from water. Therefore, ED can be used as a pretreatment technology for reverse osmosis (RO) process to reduce RO membrane scaling. Also, ED can be used to treat RO concentrate. However, from the other hand, ED cannot remove other water pollutants such as suspended particles, colloids, bacteria, and large organic molecules. Therefore, in order to get high purity water, other water treatment technologies, such as activated carbon adsorption, filtration and disinfection technologies, are needed to remove these pollutants.

2.2.2 Reverse Electrodialysis

Figure 2-6. Schematic illustration of a typical RED process for energy production.

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Though reverse electrodialysis (RED) contains the word “electrodialysis”, it is actually quite different from ED. For example, ED is mainly used for water treatment, e.g., to remove salts from water, while RED is used to produce energy

[115]. In other words, energy is consumed in an ED process while energy is produced in a RED process. In terms of driving force, ED is driven by external power source, e.g., electricity, while RED is driven by concentration difference or salinity difference. Therefore, RED usually involves mixing two aqueous solutions with different salinities [115]. The energy arisen from this salinity difference is known as salinity gradient power [116]. To be more specific, due to salinity difference, seawater and river water/freshwater possess different electrochemical potentials. As ion transport happens between the two aqueous solutions via IEMs, the electrochemical potentials are reduced [76]. In the meantime, energy is produced (Figure 2-6).

RED is a promising renewable energy technology for large-scale power generation

[115]. High power densities obtained from RED could be achieved by changing cell design with particular focus on membrane resistance and the cell length [117]. It should be noted that reverse electrodialysis (RED) and electrodialysis reversal

(EDR) are two different terms. EDR refers to reversing the polarity of the electrodes at regular time intervals and EDR is developed to reducing membrane fouling [118].

A lot of studies on RED have been carried out. For example, as seawater and river water contain multivalent ions which would limit RED performance, researchers developed a method to improve membrane selectivity towards monovalent ions and multivalent ions by coating a layer on commercial membranes [68].

42 Chapter 2

Fontananova et al [119] investigated the effect of solution concentration and composition on ionic resistance and permselectivity of membranes which would affect power density obtained from RED. Fan et al [120] developed a membrane transport model to elucidate the tradeoff relationship between conductivity and permselectivity in ED and RED. They concluded that using hypersaline streams of saltworks brine and seawater concentrate for power generation via RED would not be feasible due to the significantly diminished permselectivity which could significantly reduce energy extraction efficiency.

2.2.3 Fuel Cells

Figure 2-7. Schematic illustration of a typical PEMFC.

One of the most widely studied applications of IEMs is fuel cells. Fuel cell is defined to be an electrochemical device which can convert chemical energy of fuels into

43 Chapter 2 electrical energy without combustion [51]. Proton exchange membrane fuel cell

(PEMFC) is the most popular one [3, 25, 52]. As shown in Figure 2-7, fuel (e.g., hydrogen) is diffused to the anode side of the fuel cell, and oxygen or air is diffused to the cathode side of the fuel cell. At the anode, in the presence of a catalyst, hydrogen splits into protons (i.e., H+) and electrons. The protons can pass through the PEM, while the electrons have to arrive at the cathode via an external circuit.

As a result, electricity is generated. At the same time, water is produced at the cathode [51].

Currently, PEMFC is an expensive technology because the high production cost of high purity hydrogen. Impurities in hydrogen can deactivate the catalytic electrode materials such as platinum in PEMFC[121]. In addition, research is needed on developing low-cost membrane materials and membranes that can function well at high temperatures [121, 122]. In addition to PEMFC, anion exchange membrane fuel cell (AEMFC) is another type of fuel cell which is being actively studied in recent years due to its advantages over PEMFC, such as more choices of catalysts and high oxygen reduction reaction rate [123–125]. Another kind of fuel cell involving IEMs is called microbial fuel cell (MFC), which is an emerging technology for energy production [4, 126, 127]. The main advantages of MFC over PEMFC include that microorganism is used as the catalyst to replace expensive catalysts like platinum and wastewater is used as the fuel to replace other fuels such as hydrogen [127].

2.2.4 Electrolysis for Hydrogen Production

There are three main types of water electrolysis for hydrogen production,

44 Chapter 2 including alkaline electrolysis, proton exchange membrane (PEM) electrolysis, and solid oxide electrolysis [128–130]. For PEM-based electrolysis for hydrogen production (Figure 2-8), water is used as the source for hydrogen production. At the anode, protons (i.e., H+) and electrons are produced. Protons pass through a

PEM and arrive at the cathode, while electrons arrive the cathode via an external wire. Therefore, at the cathode, protons and electrons combine together to form hydrogen.

Figure 2-8. Schematic illustration of a PEM-based electrolysis cell for H2 production.

Electrolysis for hydrogen production is regarded as a clean energy because no pollutants are produced. However, currently it is not an economically competitive technology due to its high operational cost. Furthermore, compared to conventional alkaline electrolysis, PEM-based electrolysis typically uses more expensive catalytic electrode materials (e.g., platinum) [131]. So a lot of studies are

45 Chapter 2 focused on developing cost-effective alternatives to platinum. For example,

Rozenfeld et al [132] synthesized an exfoliated molybdenum di-sulfide catalyst which contributed to the production of highly purified hydrogen. Kim et al [133] prepared a blended material as cathode using Nickel powder and activated carbon to reduce cost for hydrogen production.

2.2.5 Chlor-Alkali Industry

IEMs are also used in chlor-alkali industry to produce chlorine, hydrogen and sodium hydroxide simultaneously. The chlor-alkali process one of the most important industrial processes which is realized via electrolysis of sodium chloride solution using membranes such as Nafion membranes which are used to separate the two electrodes and thus prevent chlorine from reacting with hydroxide ions [134, 135]. At the anode, chloride ions lose electrons and are oxidized into chlorine gas. At the cathode, water molecules gain electrons and are reduced to hydrogen gas. At the same time, hydroxide ions are produced. The cation exchange membrane allows sodium ions to pass through from the anode side to the cathode side. Therefore, sodium ions combine with hydroxide ions to produce sodium hydroxide solution at the cathode.

The chlor-alkali process is very energy-consuming and costly, so quite a few studies have been reported to address this issue. For example, Kiros et al [135] found that a lot of energy (over 30%) could be saved during electrolysis by using highly active electrocatalysts as cathode materials compared to conventional electrolysis using cathode which involves high voltage-consuming hydrogen evolution reaction. Furuya et al [136] compared the performance of oxygen

46 Chapter 2 cathodes with silver catalyst and platinum catalyst in chlor-alkali electrolysis process. They found that although platinum demonstrated better catalytic activity compared to silver, it had a much shorter lifetime. Therefore, operation cost could be reduced by using oxygen cathodes loaded with silver catalyst rather than those with platinum catalyst. Otashu et al [137] developed a dynamic model to optimize operations of chlor-alkali membrane electrolysis plant so as to reduce energy cost.

This model could be used to study demand response strategies and it was possible to modulate the cell power demand rapidly without affecting cell concentration and temperature. Moussallem et al [138] developed a novel method to prepare high performance silver-based gas diffusion electrodes for chlor-alkali electrolysis with oxygen depolarized cathodes. Electrode thickness could be easily controlled by this method and also this method could be used for large scale production of electrodes. Jalali et al [139] investigated the effects of different operating conditions on cell voltage and current efficiency of chlor-alkali membrane cell.

They found that current density and cell temperature were two deciding factors for cell voltage while brine concentration was the deciding factor for current efficiency.

2.2.6 Redox Flow Battery

Basically, redox flow battery (RFB) works by converting chemical energy into electricity. In a RFB system, two electrolyte solutions are pumped into the regions which contain the electrodes and are separated by an ion exchange membrane

(IEM). Redox reactions take place in these regions. Ions pass through the IEM (e.g., cations pass through a cation exchange membrane) while electrons pass through the external wires. As a result, electricity is generated.

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Figure 2-9. Schematic illustration of the discharging process of a vanadium redox flow battery.

Vanadium redox flow battery (VRFB) is most common type of redox flow battery and the vanadium ions involved in the system include VO2+, VO2+, V3+, and V2+ [140,

141]. As shown in Figure 2-9, at the cathode, VO2+ ions gain electrons and are reduced to VO2+ ions. At the anode, V2+ ions lose electrons and are oxidized to V3+ ions. VRFB technology is promising for large-scale energy storage due to its low cost, long cycle life, high efficiency, easy operation, good safety, and high design flexibility compared to other technologies [58, 60, 142]. IEMs play a key role in

VRFB applications and a lot of studies are focused on IEMs for VRFB. For example, in order to have high voltage efficiency, IEMs used in VRFB should have low membrane resistance [140]. Kim et al [141] developed a series of IEMs for VRFB applications and one of the IEMs demonstrated better cell capacity and efficiency, as well as lower capacity decay compared to that with Nafion membranes. Liu et al [142] developed novel acid-base IEMs incorporating modified graphene oxides

48 Chapter 2 which showed great potential for VRFB applications. Especially, amphoteric ion exchange membranes (AIEMs) are being widely studied in VRFB applications due to their advantages compared to conventional membranes, such as having low vanadium ion permeability while maintaining high conductivity [62]. Liao et al [60] developed AIEMs which demonstrated higher battery efficiencies and lower capacity loss compared to that with Nafion membranes. Sharma et al [58] prepared amphoteric ion-exchange membranes based on styrene sulfonate and vinyl benzyl chloride and the membranes demonstrated lower vanadium ion permeability than that with Nafion membranes. Wang et al [61] synthesized novel

AIEMs with significantly low permeability of vanadium ions in VRFB due to the presence of quaternary ammonium groups in the membranes.

Figure 2-10. Schematic illustration of membrane capacitive deionization.

2.2.7 Membrane Capacitive Deionization

As shown in Figure 2-10, for membrane capacitive deionization (MCDI), CEMs and

AEMs are used simultaneously. The cations from the salted water (e.g., brackish

49 Chapter 2 water) arrive at the CEM side, pass through the CEM and are absorbed by the porous electrode which is usually made of carbon materials such as carbon nanotubes and carbon aerogels [41, 143]. In contrast, the anions from the salted water arrive at the AEM side, pass through the AEM and are absorbed by the porous electrode [143]. MCDI technology combines the advantages of capacitive deionization (CDI) technology and IEM technology. Compared to CDI, MCDI can remove ions from water more efficiently and is regarded as a promising technology for brackish water desalination [143]. Salt removal efficiency during MCDI process is strongly affected by membrane resistance/conductivity and a lot of studies are focused on developing IEMs with low resistance [144]. Zhang et al [145] prepared a series of novel anion exchange membranes and they found that by incorporating reduced graphene oxide/polyaniline into the membranes, the electrical conductivity of the membranes was improved greatly, and the salt removal efficiency and adsorption capacity during MCDI process increased greatly. Kim et al [41] developed novel pore-filled cation exchange membranes (CEMs) which demonstrated high selectivity coefficients for multivalent cations and during MCDI process using these membranes, high removal efficiency for multivalent cations and high energy recovery efficiency were achieved. Qiu et al [14] prepared very thin CEMs that had very low electrical resistance, and the current efficiency during

MCDI process using these membranes was comparable to that with commercial membranes. Kim et al [146] developed CEMs via radiation grafting technique and during MCDI process using these membranes, the salt removal rate was about three times as fast as that of CDI process.

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2.2.8 Electro-Electrodialysis

As indicated by its name, electro-electrodialysis (EED) technology combines electrolysis and electrodialysis. In a three-chamber EED cell, the cathode and anode are separated by a cation exchange membrane (CEM) and an anion exchange membrane (AEM) [147]. Salt solution is pumped into the middle chamber, and the cations pass through the CEM and reach at the cathode chamber while the anions pass through the AEM and reach at the anode chamber.

Meanwhile, water splitting occurs at the two electrodes. To be specific, at the cathode, hydrogen ions gain electrons and are reduced to hydrogen gas, leaving hydroxide ions in the solution, which combine with the cations and therefore alkaline solution is produced. At the anode, hydroxide ions loss electrons and are oxidized to oxygen gas, leaving hydrogen ions in the solution, which combine with the anions and therefore acid is produced. Miao et al [148] developed a three- chamber EED cell to produce tetrabutyl ammonium hydroxide with high purity.

The EED cell can also have only two chambers. In a two-chamber EED cell, the cathode chamber and anode chamber is separated by either an AEM or a CEM. Das et al [149] developed a CEM-based two chamber EED cell to increase hydrogen iodide concentration in the mixture of hydrogen iodide and iodine solution. Wu et al [150] developed an AEM-based two-chamber EED cell to remove phenol from wastewater, which cannot be easily removed by conventional ED. Wei et al [151] compared the performance of EED and bipolar membrane electrodialysis (BMED) in regenerating sodium hydroxide from spent caustic and they concluded that EED is less economically feasible for large scale application because each EED unit needs one pair of electrodes.

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2.2.9 Electrodeionization

Electrodeionization (EDI) is technology which combines ion exchange and electrodialysis (ED) together. The main difference between an EDI cell and ED cell lies in that ion exchange resins are integrated into the diluting compartment in an

EDI cell. EDI technology can reduce the undesired concentration polarization occurred in an ED process [17, 152]. Meanwhile, for EDI process, no additional chemicals are needed to regenerate ion exchange resins, which is a main advantage compared to standalone ion exchange technology [17]. Furthermore, the ion exchange resin in the diluting compartment can reduce the resistance of the EDI cell, especially when the salt concentration in the diluting compartment becomes very low [17]. EDI can produce very high purity water and is often used to polish reverse osmosis permeate [17, 153].

2.2.10 Diffusion Dialysis

Basically, in a diffusion dialysis (DD) cell, there are two parts which are separated by an ion exchange membrane (IEM). The two parts contain solutions with different concentrations. Driven by concentration difference, ions can diffuse through the IEM from the high concentration solution part to the low concentration solution part. Small counter-ions will also pass through the IEM to balance the charges. DD is regarded as an effective method for acid recovery [16].

DD is a spontaneous process which virtually does not need external energy input

[154]. Therefore, it is a process with low cost and low energy consumption [16].

However, the DD process is very slow because the driving force is very weak. In order to speed up the process, large membrane area is needed, which will increase

52 Chapter 2 the capital cost, rendering it uneconomical [155].

2.3 Synthesis of Ion Exchange Membranes

There are different kinds of membranes, including but not limited to reverse osmosis, nanofiltration, ultrafiltration, microfiltration, forward osmosis, ion exchange membranes, etc. The most common technique for making membranes is phase inversion [156–160], which can be further divided into three main types, including thermally induced phase separation (TIPS) [161–163], vapor-induced phase separation (VIPS) [164], and non-solvent induced phase separation (NIPS)

[165–167]. For the TIPS process, solution is prepared by dissolving polymer in a solvent at an elevated temperature, and then the solution is cooled down to induce phase separation and polymer solidification, and then the solvent is removed via solvent exchange to form microporous membrane [162, 163]. TIPS is usually used to prepared membranes when a suitable solvent is not available at room temperature [162]. For the VIPS process, the wet film is exposed to a gaseous non- solvent atmosphere for a period of time which can reduce mass transfer rate between the solvent and non-solvent, and then immersed in a non-solvent bath to get the membranes [164, 168]. VIPS is used to tailor membranes with desired pore structure and morphologies. For the NIPS process, a polymer solution is prepared and then is cast into a thin film, and then the wet film is immersed in a non-solvent bath to induce phase separation [169]. Membrane pore size can be effectively controlled via NIPS but it is not easy to precisely control the phase separation process [170]. Other techniques include sintering [170, 171], electrospinning [172,

173], melt extrusion [174, 175], and so on. Sintering is a widely adopted technique to fabricate commercial membranes [170]. Electrospinning is a widely adopted

53 Chapter 2 technique to fabricate fiber membranes with diameters from micron to nanoscale

[166]. Melt extrusion can produce membranes without using solvent [174].

A good ion exchange membrane (IEM) is expected to possess high conductivity (i.e., low resistance), high ion exchange capacity, high permselectivity (e.g., a perfect

CEM only allows cations to pass through), high dimensional stability (i.e., low membrane swelling and water uptake), and high chemical, mechanical and thermal properties [176–179]. However, it is very challenging to prepare perfect

IEMs which have all of the above merits.

A lot of synthesis methods have been reported to synthesize IEMs with desired properties. Generally, there are two routes to synthesize IEMs, where one involves chemical reactions while the other one does not involve chemical reactions. The chemical-reaction route usually involves polymerization and/or functionalization.

For the non-chemical-reaction route, polymers which contain functional groups are usually dissolved in organic solvents. Then, solution casting technique is adopted [63, 180–182]. Using this technique, solvent needs to be removed after casting membrane onto a plate (e.g., glass plate). Usually, the solvent can be removed via phase inversion. One typical method is to immerse the plate into water, and gradually the polymer will change from the liquid phase to solid phase.

Another common method is solvent evaporation, which can be done by heating the plate in an oven.

In 1991, Paul Anastas and John Warner developed twelve principles of green chemistry (Table 2-4) [183, 184]. Green synthesis is becoming more and more important in membrane synthesis.

54 Chapter 2

Table 2-4. Twelve principles of green chemistry No. Principle Explanation It is better to prevent waste than to treat or clean up waste 1 Prevention after it has been created. Synthetic methods should be designed to maximize 2 Atom economy incorporation of all materials used in the process into the final product. Less hazardous Wherever practicable, synthetic methods should be 3 chemical designed to use and generate substances that possess little syntheses or no toxicity to human health and the environment. Design safer Chemical products should be designed to preserve efficacy 4 chemicals of function while reducing toxicity. The use of auxiliary substances (e.g., solvents, separation Safer solvents 5 agents, etc.) should be made unnecessary wherever and auxiliaries possible and, innocuous when used. Energy requirements should be recognized for their Design for environmental and economic impacts and should be 6 energy efficiency minimized. Synthetic methods should be conducted at ambient temperature and pressure. A raw material or feedstock should be renewable rather Use of renewable 7 than depleting whenever technically and economically feedstocks practicable. Unnecessary derivatization (use of blocking groups, protection/deprotection, temporary modification of Reduce 8 physical/chemical processes) should be minimized or derivatives avoided if possible, because such steps require additional reagents and can generate waste. Catalytic reagents (as selective as possible) are superior to 9 Catalysis stoichiometric reagents. Chemical products should be designed so that at the end of Design for 10 their function they break down into innocuous degradation degradation products and do not persist in the environment. Real-time Analytical methodologies need to be further developed to analysis for 11 allow for real-time, in-process monitoring and control prior pollution to the formation of hazardous substances. prevention Inherently safer Substances and the form of a substance used in a chemical chemistry for 12 process should be chosen to minimize the potential for accident chemical accidents, including releases, explosions, and fires. prevention

55 Chapter 2

Generally, green synthesis can be partly achieved by replacing conventional toxic organic solvents with water or non-toxic organic solvents. A completely green synthesis should avoid using any chemicals, reactions, and processes that are dangerous to humans and/or the environment. Meanwhile, the raw materials used for synthesis should also be fabricated greenly. In other words, life cycle assessment technique should be used when designing green synthesis routes.

Nevertheless, although there are various synthesis methods, such as radiation induced grafting [185–187] and pore filling [188–190], only a small portion of these methods can be classified as green synthesis methods. Usually, organic solvents are inevitable in membrane synthesis. Common organic solvents used for synthesis include acetone [191], dimethylformamide (DMF) [192], dimethylacetamide (DMAc) [45], toluene [176], N-methyl-2-pyrrolidone (NMP)

[193], dimethyl sulfoxide (DMSO) [71], and tetrahydrofuran (THF) [191]. Among these solvents, many are toxic to human health and hazardous to the environment.

Besides, many membrane synthesis processes involve dangerous reactants or produce a lot of hazardous waste.

Figure 2-11. Structure of chloromethyl methyl ether (CMME) and trimethylamine (TMA).

56 Chapter 2

Figure 2-12. Conventional routes and relatively green routes for synthesizing IEMs. (a) Conventional route for synthesizing AEMs. (b) An example of green synthesis of AEMs. (c) Conventional route for synthesizing CEMs. (d) An example of green synthesis of CEMs. CMME refers to chloromethyl methyl ether; PVA refers to polyvinyl alcohol; TEOS refers to tetraethyl orthosilicate; THOPS refers to 3-trihydroxysilyl-1-propanesulfonic acid; TMA refers to trimethylamine; VBTAC refers to (ar-vinylbenzyl)trimethylammonium chloride.

57 Chapter 2

Typically, there are two successive steps for synthesizing anion exchange membranes (AEMs), including chloromethylation followed by quaternization to introduce quaternary ammonium groups (Figure 2-11 and 2-12a) [194, 195].

However, toxic and/or carcinogenic chemicals are usually unavoidable in this method [80, 188]. For example, chloromethyl methyl ether (CMME) is often used for chloromethylation [177, 191, 194]. According to the International Agency for

Research on Cancer (IARC), CMME is carcinogenic to human beings. Also, organic solvents are often used in this method [77]. In some studies, CMME is replaced by other chemicals (e.g., paraformaldehyde and chlorotrimethylsilane [125]).

However, even if these chemicals are not as dangerous as CMME, they are only relatively safer than CMME. Therefore, researchers have developed various methods to skip the chloromethylation process and to directly quaternize polymers that contain nitrogen groups. For example, Hou et al. [196] reported a route to prepare AEMs by quaternizing PBI using bromoethane. This route is very simple and no dangerous chloromethylation reagents are used. However, on the other hand, a lot of DMAc is used to dissolve PBI. According to the IARC, DMAc is possibly carcinogenic to humans. Qaisrani et al. [197] synthesized a new kind of

AEMs through thermal treatment of benzoxazine monomer on PTFE support.

Similarly, no chloromethylation is needed in this route. However, formaldehyde is used for benzoxazine synthesis. According to the IARC, formaldehyde is a human carcinogen. Besides, as a solvent, 1,4-dioxane is used for both benzoxazine synthesis and the after membrane preparation. According to the IARC, 1,4-dioxane is a probable human carcinogen. Also, other hazardous chemicals, although not classified as carcinogenic because of lack of evidence, are used in the aforementioned routes. Hu et al. [192] developed a more environment-friendly

58 Chapter 2 route to prepare AEMs via plasma grafting, polymerization of (ar- vinylbenzyl)trimethylammonium chloride (VBTAC) which contains functional quaternary ammonium groups, and solution casting technique. Neither chloromethylation nor quaternization is needed for this route, which is its main advantage over other synthesis routes. The main drawback is that organic solvent

DMF is still needed due to the use of solution casting. However, this study opens up new doors for preparing AEMs, which is to directly use monomers containing functional groups (Figure 2-12b). It should be pointed out that when using this methodology, other strategies (e.g., plasma grafting, crosslinking reagents) have to be applied at the same time to make the polymers (in salt form) insoluble in water and more resistant to membrane swelling.

Recent studies on synthesis cation exchange membranes (CEMs) are summarized in Table 2-5. As revealed by Table 2-5 and Figure 2-12c, the synthesis of CEMs generally involves sulfonation to introduce functional sulfonic groups. Dangerous acids such as concentrated sulfuric acid or chlorosulfonic acid are typically used in sulfonation. The acid waste is also hazardous to the environment and additional efforts are needed to handle the waste properly. Another common method to synthesize CEMs is to disperse cation exchange resins into polymer solution, followed by solution casting and solvent removal via phase inversion (e.g., solvent evaporation by heating or solvent exchange with water) to get the membrane [63].

However, organic solvents are inevitable in this route. In order to solve these problems, Hao et al. [198] developed a green route to synthesize PVA-based CEMs

(Figure 2-12d). Since PVA is soluble in water, all the preparation is carried out in aqueous media so no organic solvent is used. Also, no sulfonation is needed.

59 Chapter 2

Table 2-5. Recent studies on synthesis of CEMs Main No. Main Synthesis Procedures Solvents

(1) PVDF is dehydrofluorinated; (2) Polymerization of NaSS 1 DMAc takes place on PVDF; (3) PVDF-g-PSS is dissolved in DMAc, and [199] DMSO the solution is cast on a film; (4) The membrane is annealed at various temperatures.

(1) PVDF is sulfonated using HSO3Cl; (2) S-PVDF is dissolved in

NMP and PVDF is dissolved in NMP; (3) the S-PVDF/PVDF 2 NMP blended solution is cast on a glass plate and dried; (4) membrane [200] surface is modified (e.g., via polymerization of aniline) to improve monovalent cation selectivity.

(1) Copolymers of PEEK and PES containing cardo (lactone) 3 DMAc group are synthesized in DMAc and toluene; (2) copolymers are

[201] Toluene sulfonated using HSO3Cl; (3) S- and non-S-copolymers are dissolved in DMAc and cast on a glass plate and dried.

(1) Polyketone solution is cast on a glass plate and dried; (2) 4 DMSO polyketone membrane is immersed into NaSS-DMSO solution [146] and exposed to irradiation to initiate the grafting reaction.

(1) Fullerene is phosphorylated; (2) PVA is sulfonated using Toluene 5 HSO3Cl and then the S-PVA solution is cast on a glass plate and CHCl3 [202] dried; (3) phosphorylated fullerene powder is added to S-PVA H2O solution, then the mixture is cast on a glass plate and dried.

(1) PVC, HIPS, and ABS with different ratios are dissolved in

6 THF; (2) CER and nanoparticles are added into the polymer THF [203] solution, and then the mixture is cast on a glass plate, and then dried and immersed in water to get the membrane.

(1) PPO in dissolved in CHCl3, and is sulfonated using HSO3Cl; (2)

7 CHCl3 S-PPO is dissolved in DMF, and then cast and dried; (3) TEOS is [204] DMF added to S-PPO solution, and the mixture is cast on a glass plate and dried to get the composite membrane.

(1) Zn(II) nanoparticles are prepared; (2) PVC is dissolved in 8 Methanol THF; (3) CER and Zn(II) nanoparticles are added to the PVC [205] THF solution, and then the mixture is cast on a glass plate, and then dried and immersed in water to get the membrane.

60 Chapter 2

Table 2-5. Recent studies on synthesis of CEMs (continued)

Main No. Main Synthesis Procedures Solvents

(1) PVDF is dissolved in NMP and NaSS is dissolved in DMSO; (2)

PVDF-NMP solution undergoes ozone treatment via O3/O2; (3)

NMP the PVDF solution and NaSS solution are mixed and 9 DMSO polymerization takes place, producing the PVDF-g-PSS [206] Methanol copolymers; (4) PVDF-g-PSS is dissolved in NMP, and then cast onto a glass plate, and then dried and immersed in water to get the membrane.

(1) PES is sulfonated using concentrated H2SO4; (2) the

10 sulfonated PES is dissolved in DMAc; (3) PVP is added into the DMAc [207] solution; (4) the solution is cast on a glass plate, and then dried and immersed in water to get the membrane.

(1) Graphene oxide is prepared and sulfonated using

concentrated H2SO4; (2) PVC is dissolved in cyclohexanone; (3) styrene and DVB are added to the PVC solution and then 11 Cyclohexa- undergo polymerization; (4) after reaction, the product is cast [208] none on a glass plate and dried; (5) the membrane is sulfonated using

HSO3Cl; (6) sulfonated graphene oxide is dissolved in cyclohexanone, and then added into the copolymer solution, and then the resulting solution is cast on a glass plate, dried and sulfonated to get the composite membrane.

(1) Copolymer of sulfonated PBI, PBI containing pyridine, and

polyimide are synthesized; (2) the copolymer is dissolved in 12 NMP NMP, and then the solution is cast on a glass plate, dried and [209] immersed in water to get the membrane; (3) the membrane is

immersed in phosphoric acid to get the PEM.

Note: DMAc refers to dimethylacetamide; DMSO refers to dimethyl sulfoxide; PVDF refers to polyvinylidene fluoride; NaSS refers to sodium 4-vinylbenzenesulfonate; PSS refers to polystyrene sulfonate; NMP refers to N-methyl-2-pyrrolidone; S- refers to sulfonated; PEEK refers to polyether ether ketone; PES refers to polyether sulfone; PVA refers to polyvinyl alcohol; THF refers to tetrahydrofuran; PVC refers to polyvinyl chloride; HIPS refers to high impact polystyrene; ABS refers to acrylonitrile butadiene styrene; CER refers to cation exchange resin; DMF refers to dimethylformamide; PPO refers to polydimethyl phenylene oxide; TEOS refers to tetraethyl orthosilicate; PVP refers to polyvinyl pyrrolidone; PBI refers to polybenzimidazole; PEM refers to proton exchange membrane. 61

Chapter 2

With the continuous improvement of our society, in combination with the growing concern of global environmental pollution, green and sustainable synthesis is the only way forward. Unfortunately, as discussed earlier, green synthesis studies on

IEMs only account for a small portion of the total studies. Furthermore, if life cycle assessment is adopted, these green synthesis studies have to be reevaluated in a more comprehensive manner. For example, VBTAC in Figure 2-12b is usually synthesized through reaction of vinylbenzyl chloride and trimethylamine [210]. In other words, quaternization is still needed. To conclude, there is still a long way to go in green synthesis.

2.4 Summary

To sum up, there are a variety of ion exchange membranes (IEMs) which are widely used in different applications. The research on IEMs are very prosperous and diversified. Thousands of IEM-related papers are published every year so it is necessary to conduct a quantitative statistic study on IEMs. This will be addressed in Chapter 4. In addition, as discussed above, many reported synthesis methods have drawbacks. For example, organic solvents are used in most of the current synthesis methods, which increases synthesis cost and also post-disposal of these organic solvents are necessary. In addition, many synthesis methods are too complicated. So it is important to explore new synthesis methods to address these issues, which will be discussed in Chapter 5, 6 and 7.

Chapter 3 Materials and Methods

Abstract

This chapter summarizes the materials and methods used in this thesis. To be specific, the materials used are summarized in section 3.1. The scientometric methods including basic processing procedures and advanced processing procedures are introduced in section 3.2. Different membrane preparation methods are discussed in section 3.4. Also, the membrane characterisation techniques used in this thesis are introduced in section 3.5. Finally, electrodialysis system setup and test procedures are discussed in section 3.6. In a nutshell, this chapter gives the materials and methods used in this thesis, while the following chapters (4, 5, 6, and 7) give the results and discussion.

Note: The work presented in Chapter 3 (excluding 3.1 and 3.3) is adapted from the following publications:

2017 American Chemical Society. Adapted with permission.

[3] Shanxue Jiang and Bradley P. Ladewig, High performance cation exchange membranes synthesized via in situ emulsion polymerization without organic solvents and corrosive acids, Journal of Materials Chemistry A, vol. 7, no. 29, pp. 17400–17411, 2019. Copyright 2019 Royal Society of Chemistry. Adapted with permission.

Chapter 3

3.1 Materials

The materials used in this thesis were summarized in Table 3-1.

Table 3-1. Materials used in this thesis Name Description Supplier Benzoyl peroxide BPO, Luperox® A75 Sigma Aldrich Cation exchange resin Amberlyst® 15 H form Sigma Aldrich Clear and flat polyester films PMX727, T = 125 microns HiFi Industrial Film Dimethyl sulphoxide DMSO Sigma Aldrich Divinylbenzene DVB Sigma Aldrich Emulsifier Kolliphor® EL Sigma Aldrich Hydrochloric acid HCl VWR Lithium p-styrenesulfonate LiSS Tosoh Corporation Phenolphthalein solution Indicator Sigma Aldrich Poly(sodium 4- PSS, AMW ~70000 Sigma Aldrich styrenesulfonate)

Polyethylenimine solution PEI, 50 % w/v in H2O Sigma Aldrich Polystyrene PS, AMW ~192000 Sigma Aldrich Porous non-woven fabric Freudenberg Filtration Novatexx 2471 support Technologies ~3 mol/L, for Ag/AgCl Potassium chloride solution Sigma Aldrich electrodes Sodium 4- NaSS Sigma Aldrich vinylbenzenesulfonate Sodium chloride NaCl Sigma Aldrich Sodium hydroxide NaOH VWR TraceCERT® , 1000 mg/L Sodium standard Sigma Aldrich Na in nitric acid, for ICP Styrene St Sigma Aldrich Note: H is short for hydrogen; T is short for thickness; AMW is short for average molecular weight; ICP is short for inductively coupled plasma.

3.2 Scientometric Method

The data used in this study was obtained from the database of Web of Science Core

Collection with time span of 2001-2016, with citation index of Science Citation

64 Chapter 3

Index Expanded (SCI-EXPANDED) and with the following search terms for topic:

("*ion* exchange* membrane*" or "*bipolar membrane*" or "proton* exchange* membrane*" or "*polymer* electrolyte* membrane*" or "*ion* *selective membrane*" or "*ion* conduct* membrane*" or "proton* conduct* membrane*" or

"*ion* exchange* film*")

Table 3-2. Examples of search terms for topic

Separate Search Term Examples of Separate Term Term *ion* ion, cation, anion, ionic, cationic, anionic "*ion* exchange* exchange, exchanger, exchangeable, exchange* membrane*" exchanged membrane* membrane, membranes *bipolar bipolar, ambipolar "*bipolar membrane*" membrane* membrane, membranes proton* proton, protonic "proton* exchange* exchange, exchanger, exchangeable, exchange* membrane*" exchanged membrane* membrane, membranes *polymer* polymer, copolymer, polymeric "*polymer* electrolyte* electrolyte* electrolyte, electrolytes membrane*" membrane* membrane, membranes *ion* ion, cation, anion, ionic, cationic, anionic "*ion* *selective *selective selective, permselective membrane*" membrane* membrane, membranes "*ion* conduct* *ion* ion, cation, anion, ionic, cationic, anionic membrane*" conduct* conducting, conductive proton* proton, protonic "proton* conduct* conduct* conducting, conductive membrane*" membrane* membrane, membranes *ion* ion, cation, anion, ionic, cationic, anionic exchange, exchanger, exchangeable, "*ion* exchange* film*" exchange* exchanged film* film, films

Examples of search terms were listed in Table 3-2. For example, "*ion* exchange*

65 Chapter 3 membrane*" included terms like "ion exchange membrane", "ion exchange membranes", "cation exchange membrane", "anionic exchange membrane", and so on. A total of 21123 publications met this search criteria. Full record of these publications was downloaded as .txt files. After importing all the data from .txt files into Excel, the data was further processed. Before further processing, it was found that one publication record had some format problems, and thus was excluded from the data set. Firstly, the parts of full record data studied included title, address, publishing journal, language, document type, keywords, abstract, citations, yeas published, and research areas. All the other columns were removed.

Then, the Find & Replace tool in Excel was used to remove some punctuations as described below. The first modification was to remove the space after comma. The second modification was to remove the space after semicolon. The third modification was to remove the double quotes. Further data processing was discussed in the respective sections below.

3.2.1 Basic Processing Procedures

Document Type (DT): For DT data processing, the Pivot Table tool in Excel was used to summarize different document types. The processing procedures for

Language (LA), Publishing Year (PY), and Publishing Journal (SO) were similar to

DT processing. It should be clarified that SO was used as the abbreviation for publishing journal because it was the default name for the publishing journal column when the data was imported into Excel. Some other abbreviations also followed this “rule”.

Total Citations (TC): As is known, citations for a paper tend to increase as time goes

66 Chapter 3 on. Since the data was downloaded on December 06, 2017, the total citations were updated to this date as well. For a specific journal, its average number of citations per paper was calculated by dividing the total citations by number of articles published in this journal. The h-index of a journal used in this study referred to the highest number of articles published in this journal which had h or more citations each while the other articles had less than h citations each. Besides, the h-index was calculated based on articles published since 2001. Also, the h-index was calculated based on the data downloaded on December 06, 2017. Therefore, the h- index used in this thesis has its limitations. The above method was also adopted in calculating the h-index and average number of citations per paper of the research institutes.

Addresses (C1): The C1 data column contained addresses which were used to acquire research institutes and countries/regions. Out of the 18166 articles,

18151 articles contained address information in the C1 column and therefore constituted the samples for acquiring research institutes and countries/regions.

Column C1 was then refined to remove square brackets, the content in the square brackets as well as the blank space following the square brackets. This was realized through the Replace tool in Excel. Also, an article could have one or more institution addresses. These addresses were originally contained in one cell for one article. Therefore, the C1 column was separated into several columns so that one address was contained in one cell. This was realized through the Text To

Columns tool in Excel, using semicolon as the delimiter. After processing, the cell content of each article was separated into multiple cells if there is more than one address. Further, the addresses were refined twice independently, one was to keep

67 Chapter 3 the institution name only while the other was to keep country/region only. This was realized through the Replace tool in Excel. As is known, an article could have two or more institution addresses with the same institution name. Therefore, the next step was to remove the repetitive institution names in each article. This was realized through the MATCH function together with the Filter tool in Excel. Then, the Pivot Table tool in Excel was used to determine the count of each institution and each country. The RANK function in Excel was used to calculate the rank of each research institute and country in terms of articles number. It should be pointed out that occasionally two research institutes, which were actually the same institute, were stored in the database under different names. For example,

Tsinghua Univ. and Tsing Hua Univ. refer to the same university. It was really time- consuming and difficult to find all these “bugs”. On the other hand, considering the large sample size in this study, most of these “bugs” would not have a noticeable effect on the results. Therefore, only the top ranking research institutes were carefully scrutinized to prevent or minimize the possible effect. To be specific,

Tsinghua Univ. and Tsing Hua Univ. were actually the same institution, and therefore were put together as Tsinghua Univ. in this study. Besides, in this study,

Chinese Acad. Sci. YICCAS and Chinese Acad. Sci. were regarded as two institutes.

Keywords (DE): Out of the 18166 articles, 14054 articles contained keywords in the relevant column (i.e., DE column in Excel). As is known, an article could have several keywords. These keywords were originally contained in one cell for one article. Therefore, similarly to the C1 column, the DE column was separated into several columns so that one keyword was contained in one cell. Again, this was realized through the Text To Columns tool in Excel, using semicolon as the

68 Chapter 3 delimiter. After processing, the cell content of each article was separated into multiple cells. The next step was, all the keywords in different columns were put into one column. Then, the Pivot Table tool in Excel was used to determine the count of each keyword. There were 22674 different keywords which appeared

68992 times in total in the 14054 articles. The RANK function in Excel was used to calculate the rank of each keyword in terms of articles number. Alternatively,

BibExcel can also be used to get the above results (frequency distribution in the .cit file). The processing procedures for Research Areas (SC) were similar to the procedures for DE.

3.2.2 Advanced Processing Procedures

Network Graph Generation: The software program named Gephi was used to generate network graphs showing research institute collaboration, collaborations between countries, and keywords cooccurrence. For detailed instructions on using it, please visit its official website at gephi.org. However, before using Gephi, another software program named BibExcel was needed to general the pairs data information which was then used for Gephi.

Word Cloud Generation: Title (TI) column and Abstract (AB) column were used to create word clouds. For word cloud generation using TI column, the TI column was firstly saved as .txt file. Then the double quotes in the .txt file was removed via the

Replace tool. Then, this file was imported into the software ATLAS.ti. Then word cloud could be generated using the Word Cloud function. It should be pointed out that the common words like a, an, and the were automatically screened out by this software program. For AB column analysis, among the 18166 articles, 18060

69 Chapter 3 contained abstracts. Therefore, the following AB analysis was based on these abstracts. To begin with, as AB column contained copyright information, it needed to be removed as it was not related to this study. This was realized through the

Replace tool in Excel. Then the double quotes in the .txt file was removed via the

Replace tool. Then, this file was imported into the software ATLAS.ti. Then similarly, word cloud could be generated using the Word Cloud function.

Alternatively, another software named Python can be used to generate word cloud.

Applications of IEMs: Common applications of IEMs were obtained based on the abstracts analysis using the Filter tool in Excel. The following “?” was used to represent any single character. The numbers in Table 4-9 in Chapter 4 came from the following results. 10261 abstracts contained “fuel cell” or “fuel-cell”. 843 abstracts contained “electrodialysis” or “electro?dialysis”, where “electro?dialysis” included “electro dialysis” and “electro-dialysis”. 580 abstracts out of the 843 abstracts did not contain “reverse?electrodialysis” and “bipolar membrane”. It revealed that 580 abstracts mentioned electrodialysis but did not mention bipolar membrane and reverse electrodialysis. 447 abstracts contained “electrolysis”. 256 abstracts contained “desalination”. 225 abstracts contained “redox flow batter” or

“vanadium * batter”, where “batter” included battery and batteries; “vanadium * batter” included but was not limited to “vanadium redox batter” and “vanadium flow batter”. 182 abstracts contained “bipolar membrane” and “electrodialysis”.

157 abstracts contained “donnan dialysis” or “diffusion dialysis”. 104 abstracts contained “water treatment”. 82 abstracts contained “reverse?electrodialysis”, which included “reverse electrodialysis” and “reverse-electrodialysis”. 52 abstracts contained “capacitive deionization”. 43 abstracts contained

70 Chapter 3

“electrodeionization” or “electro-deionization”. 31 abstracts contained “water purification”. 25 abstracts contained “electro-electrodialysis”. It should be pointed that this method had its limitations. In other words, it might omit some applications. Or the current applications listed in Table 4-9 in Chapter 4 might have less or more number of publications than listed. That is why their meanings were introduced here.

Properties of IEMs: Similarly, the numbers in Table 4-10 in Chapter 4 came from the following results. 4008 abstracts contained “properties”. 4952 abstracts contained “conductivity”. 3301 abstracts contained “stability”. 2175 abstracts contained “water uptake” or “water content”. 2158 abstracts contained

“resistance”. 1600 abstracts contained “ion?exchange?capacity” or “IEC”. 1465 abstracts contained “permeability”. 1150 abstracts contained “morphology”. 1145 abstracts contained “thermal stability” or “thermally stable”. 999 abstracts contained “thermal stability”. 1033 abstracts contained “swelling”. 979 abstracts contained “hydrophilic”. 874 abstracts contained “hydrophobic”. 833 abstracts contained “mechanical properties”. 519 abstracts contained “chemical properties”.

466 abstracts contained “chemical stability”. 337 abstracts contained “oxidative stability” or “oxidation stability”. 310 abstracts contained “oxidative stability”. 306 abstracts contained “dimensional stability”. 298 abstracts contained “contact angle”. Again, it should be pointed that this method had its limitations. In other words, it might omit some properties. Or the current properties listed in Table 4-

10 in Chapter 4 might have less or more number of publications than listed. That is why their meanings were introduced here.

Polymers for IEMs: The number listed in Table 4-11 in Chapter 4 was strictly

71 Chapter 3 limited to the corresponding polymer name itself. That is why two actually same polymers were listed simultaneously. In other words, the polymers listed in Table

4-11 were not necessarily mutually exclusive. Limitations: poly(ether ether ketone) could have other forms such as poly (ether ether ketone), polyether ether ketone.

Similarly, polybenzimidazole could have other forms such as poly(benzimidazole), poly(2,5-benzimidazole). That is why they were not listed as a whole. In other words, it was very difficult to list the various forms of the polymers completely.

3.3 Basic Experiments

3.3.1 Solution Preparation

To prepare 1 mol/L NaCl solution, 29.22 g NaCl is weighed using a balance, and then dissolved in water in a beaker, which is then transferred to a 500 mL volumetric flask. The beaker is washed with water for several times, and the resulting solution is also transferred to the volumetric flask. Then water is carefully added into the volumetric flask to achieve the precise volume. In practice, the accurate readings from the balance are taken down so the accurate concentration can be calculated.

The procedures to prepare 2 mol/L NaCl solution, 0.01 mol/L NaOH solution, as well as other solutions are similar to the above preparation procedure.

To prepare 2% dilute poly(sodium 4-styrenesulfonate) (PSS) solution (w/w), 4 g

PSS is dissolved in 196 g water. It should be clarified that 4 g PSS refers to the original PSS solution bought from Sigma Aldrich so in fact the real PSS concentration is less than 2%. However, this is not important as long as it is well

72 Chapter 3 defined here.

3.3.2 Density and Thickness Measurements

The bulk density of the porous non-woven fabric membrane support is measured using the following relationship: density = weight/(width*length). The calculated density is around 8.2 mg/cm2. The thickness of the support and synthesized CEMs are measured using a micrometer. The measured thickness of the support is around 0.164 mm.

3.4 Membrane Preparation

3.4.1 Early Stages of the Membrane Preparation Journey

Initially, the idea was to make cation exchange membranes (CEMs) using the conventional solution casting technique because it was simple and also usually no chemical reactions were involved using this technique. In addition, CEMs with permselectivity among cations are playing an important role in applications where a special cation (e.g., sodium) is of particular interest which needs to be separated from other cations (e.g., magnesium and calcium) such as sodium chloride production and reverse electrodialysis [64, 66, 70, 211]. It is reported that amino groups have influence on permselectivity [44]. Polyethylenimine (PEI), polystyrene sulfonate (PSS), and polystyrene (PS) were selected as the polymers for membrane preparation because PEI contains amino groups, PSS contains sulfonate groups, and PS serves as the supporting polymer matrix.

Solution viscosity has a positive relationship with polymer concentration [212,

73 Chapter 3

213]. When polymer concentration is increased, the solution viscosity will also increase. Neither too dilute nor too viscous solutions are suitable for casting membranes. In order to get a suitable solution, a series of tests were carried out to get a general idea of the solubility of polystyrene (PS) in dimethylformamide

(DMF). To be specific, 10 g DMF was firstly transferred into a 50 mL beaker, and then 2 g PS beads was added into the beaker. The resulting mixture was stirred overnight at 300 revolutions per minute. The result showed that the PS beads were completely dissolved. Furthermore, the solution was too dilute to be suitable for casting membrane. In other words, it was feasible to get a uniform solution when the mass ratio of PS to DMF was 1 to 5. However, DMF should be reduced in order to make the solution more viscous. Therefore, the weight of PS beads was increased from 2 g to 2.5 g, and the other procedures were kept the same. The results showed that the PS beads were still completely dissolved. That means it was feasible to get a uniform solution when the mass ratio of PS to DMF was 1 to

4. Also, the resulting solution was suitable for casting membranes. More experiments were carried out and it was concluded that when the mass ratio of PS to DMF was about 1 to 2.8~4.0, a uniform solution could be achieved, and the resulting solution demonstrated suitable viscosity for casting membranes. When the mass ratio of PS to DMF was less than 1 to 2.8, the resulting solution is too viscous to be suitable for casting membranes. In contrast, when the ratio is greater than 1 to 4.0, the fluidity of the solution is too high and therefore not suitable for casting membranes. For polyethylenimine (PEI), the original PEI solution was too viscous to be suitable for casting membranes. Therefore, dilution was necessary.

The dilution procedures to determine the suitable ratio of PEI to water was similar to that of PS to DMF.

74 Chapter 3

In the beginning, the main procedures to make CEMs with permselectivity among cations were designed as follows. Firstly, PS was dissolved into DMF in a glass vial.

Meanwhile, PSS was dissolved into water in another glass vial. Then, the two solutions were blended slowly. Next, an appropriate amount of PEI solution was added into the mixed solution. Then, the mixture was cast onto a clean and dry glass plate, followed by evaporating the solvents in an oven. Finally, the membrane was formed and detached from the glass plate. However, there was an obvious flaw in this procedure, which was discovered immediately when carrying out the experiments. That was, water and DMF was incompatible with each other. When

PSS solution was added into PS solution, PS was separated out from DMF immediately. A “simple” strategy was proposed to solve this issue. The strategy was to avoid using water. To be specific, instead of dissolving PSS into water, PSS powder was added into the DMF solution directly to form a dispersion system. The new issue aroused which was that the particle size was too big and obvious in the resulting membrane. As a result, the membrane thickness was very high and meanwhile the membrane was very fragile.

In order to make the membrane more “robust”, a membrane support was used. At the very beginning, the membrane was prepared by the immersing method together with phase inversion method. To elaborate, porous membrane support was immersed in PSS solution, and then the support was taken out and placed in an oven to evaporate water. However, only a small amount of PSS was loaded on the membrane support. In order to make more PSS load on the support, a revised procedure was designed. To be more specific, a piece of porous fabric membrane support was weighed and placed in a glass dish, then a certain amount of 2% PSS

75 Chapter 3 solution was added into the glass dish to fully cover the membrane support. Next, the glass dish was dried in an oven overnight to remove water. The resulting membrane was peeled off and weighed. The good aspect was that more PSS was loaded on the support. The mass ratio of loaded PSS to membrane support was about 0.73 to 1. However, the resulting membrane had “wrinkles”, possibly due to the high temperature in the oven (60-80 ℃) during phase inversion process. In addition, the membrane was not suitable for a second heating treatment as the membrane shape will be twisted more seriously. So this immersion method was abandoned for a time and the conventional casting solution method was reconsidered.

In order to increase the mass ratio of PSS to porous support, PSS solution with high concentrations (e.g., mass fraction at around 32% and 40%) was prepared and then cast onto the support which was fixed to a glass plate. To prepare one piece of membrane, a series of casting thickness were adopted (e.g., begun with 0.1 mm, and then reduced to 0.04 mm, 0.02 mm, and 0.01 mm successively). There were two reasons for changing thicknesses. The first one was to make sure there was enough PSS on the porous support and PSS solution can permeate into the pores of the support due to its own gravity as well as the pressure applied onto it by the casting machine. The second one was to ensure that no much PSS was left on the surface. Next, the glass plate was dried in an oven overnight to get the PSS membrane. Meanwhile, PS solution (in DMF) with mass fraction of 20% was prepared. Then the PS solution was cast onto the PSS membrane, and then dried in an oven to get the PSS-PS membrane. The reason to cast a PS layer on top of the

PSS membrane was that PSS was soluble in water while PS was not. Therefore, it

76 Chapter 3 was thought that a top layer of PS could prevent the inner PSS to be dissolved into water when the membrane was immersed in water. The density of the prepared

PSS-PS membranes increased by around 100% compared to that of the pure membrane support.

The ion exchange capacity (IEC) of the PSS-PS membranes was tested using the titration method. The IEC was calculated to be at around 2 mmol/g, which was very high. However, unfortunately, it was found later that the IEC values were incorrect because the residues of HCl on the membrane were not washed off completely. As a result, more NaOH was consumed during the titration process. In addition, the weight of the membranes decreased dramatically after IEC test, indicating that PSS was dissolved during the exchange process.

In order to make PSS stay in the membrane, a couple of strategies were developed and tested. For example, the PSS membrane was immersed into 20% PS solution and very dilute PSS solution successively, followed by drying it in an oven. However, using this method, PSS could not stay in the membrane when immersing it in water and thus the IEC was near zero. Another strategy tested was the heat-treatment method. To elaborate, the PSS-PS membrane was heat-treated at around 150 ℃ in an oven to “trigger” the fusion of the membrane so PSS could have better interactions and connections with PS and the support. However, the resulting membrane was more crispy. In addition, the IEC was still unsatisfactory. The possible reason was that part of the PSS molecules were fully surrounded by PS or the support during the fusion so these PSS molecules lost the ability to exchange cations with HCl, while the other PSS molecules were not firmly connected to the bulk support so they could still dissolve in water.

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Finally, it was concluded that it was not possible to make good CEMs using linear water-soluble PSS to provide functional sulfonate groups. More generally, it was further concluded that, without chemical reactions, it was very difficult to prepare good CEMs for water-related applications using water-soluble polymers which contain the necessary functional groups. In order to make PSS insoluble in water, it had to be cross-linked. Therefore, methods involving chemical reactions (e.g., polymerization) were considered in the following membrane synthesis.

For the chemical-reaction route, styrene and DVB were selected as the monomers.

At the beginning, porous support containing reaction liquid was placed between two glass plates, and then the plates were clamped and placed in an oven for synthesis. However, the reactants were not well sealed so only a small amount of polystyrene was formed in the support. In order to make sure the polymerization took place in the support, a reactor was designed. At first, a 3D printer, which used plastic materials for the printing, was used to print the reactor. However, it was found later that the reaction liquid could deform this reactor. Therefore, stainless steel plate was selected as the material for the reactor, and worked out quite well.

3.4.2 Membrane Preparation via Polymerization and Sulfonation

The membrane preparation scheme is illustrated in Figure 3-1. To begin with, styrene (St), divinylbenzene (DVB), and benzoyl peroxide (BPO) with mass ratio of 7:1:0.07 are mixed together at room temperature, and the mixture is then stirred at 300 revolutions per minute at room temperature to get a homogeneous solution. Then porous membrane support is immersed into the solution for at least

10 min to ensure maximum uptake of the solution. Then the porous support is

78 Chapter 3 taken out and placed between two aluminium sheets, and then the aluminium sheets are sandwiched between two stainless steel plates. The stainless-steel plates are then compressed tightly to prevent monomers from escaping the membrane support and to ensure that polymerization takes place in the membrane support. The polymerization takes place at 80 ℃ for 8 hours. After the reaction, the membrane is peeled off and heated at 60 ℃ for several hours to remove unreacted monomers. Alternatively, the membrane can be immersed into acetone to dissolve the unreacted monomers and then be dried at room temperature to remove acetone residues.

Then the membrane is immersed into concentrated sulfuric acid and the sulfonation reaction takes place at 80 ℃ for different hours (e.g., 22, 30, 38, and 46 hours). After reaction, the membrane is taken out and immersed into ice-water mixture to remove excess acid. Then the membrane is washed with DI water. After wash, the water is collected into a beaker. Next, two drops of phenolphthalein (as the indicator) and one drop of 0.01 mol/L NaOH solution are added into the beaker, successively. If the solution changes to red, then it indicates that no acid residues exist in and on the membranes.

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3.4.3 Membrane Preparation via Polymerization Without Sulfonation

Membrane preparation procedures are demonstrated in Figure 3-2. The general procedures are described as follows. Firstly, lithium p-styrenesulfonate (LiSS) is dissolved in water. Next, a certain amount of DVB or styrene is added into the solution. Then, a small amount of emulsifier is added into the mixture to make the mixture uniform. Then, a certain amount of CER could be added into the mixture.

Next, a small amount of BPO is added into the mixture. Then, the porous support is immersed into the mixture to adequately absorb the mixture. Then, the support is taken out of the mixture and placed between two clear polyester films, and then placed between two stainless-steel plates. Finally, the stainless-steel plates are fastened and placed in an oven to activate the polymerization process. The reaction temperature and time are set to 80 ℃ and 8 hours, respectively. After polymerization, the membranes are peeled off from the polyester films.

Figure 3-2. Schematic illustration of membrane preparation via polymerization without sulfonation.

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Table 3-3. Summary of the acronyms used to describe different membrane series

M X meaning X values Other parameters

MDLX DVB/LiSS 0.2, 0.4, 0.5, 0.6, 0.8, 1.0 Water/LiSS ratio was 1.2

MCDX CER/DVB 0.2, 0.4, 0.6 DVB: water: LiSS was 0.6: 1.2: 1.0

MCLX CER/LiSS 0.2, 0.4, 0.6 DVB: water: LiSS was 0.6: 1.2: 1.0

MWLX Water/LiSS 0.5, 1.2 (DVB+styrene)/LiSS ratio was 0.5

Note: M is short for membrane; DVB is short for divinylbenzene; LiSS is short for lithium p-styrenesulfonate; CER is short for cation exchange resin.

As shown in Table 3-3 and Figure 3-3, four series of membranes are prepared. For the MDLX, the subscript DLX means that the mass ratio of DVB to LiSS is X where X is a number. Different DVB/LiSS ratios (i.e., X) are studied, including 0.2, 0.4, 0.5,

0.6, 0.8, 1.0. Meanwhile, the mass ratio of water to LiSS to water is kept the same, namely 1.2. For the MCDX, the subscript CDX means that the mass ratio of CER to

DVB is X. Different CER/DVB ratios are studied, including 0.2, 0.4 and 0.6.

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Meanwhile, the mass ratio of DVB: water: LiSS is kept the same, namely 0.6: 1.2:

1.0. For the MCLX, the subscript CLX means that the mass ratio of CER to LiSS is X.

Different CER/LiSS ratios (i.e., X) are studied, including 0.2, 0.4 and 0.6. Meanwhile, the mass ratio of DVB: water: LiSS is kept the same, namely 0.6: 1.2: 1.0. For the

MWLX, the subscript WLX means that the mass ratio of water to LiSS is X. Different water/LiSS ratios (i.e., X) are studied, including 0.5 and 1.2. Meanwhile, the mass ratio of DVB and styrene to LiSS to water is kept the same, namely 0.5. It should be noted that MWL1.2 is replaced by MDL0.5 because they are actually identical. Also, the procudures to prepare MCDX series are slightly different compared with the other three series. To elaborate, for MCDX series, two different mixtures are used together for synthesis. The first mixture is mainly composed of DVB, water and LiSS. The second mixture is mainly composed of CER and DVB. The main polymerization reactions occur during membrane synthesis are shown in Figure 3-4.

Figure 3-4. Polymerization reactions during membrane synthesis. Synthesis of MDLX and

MCLX mainly involves reaction (a); synthesis of MWL0.5 mainly involves reaction (c); synthesis of MCDX mainly involves reaction (a) and (b). The above reaction equations are for demonstration purposes only, as the ratios of different reactants are not considered in these equations.

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3.5 Membrane Characterisation

3.5.1 Scanning Electron Microscope

Scanning electron microscope (SEM) analysis and energy-dispersive X-ray spectroscopy (EDS or EDX) analysis were performed (JSM 6400, JEOL) to study membrane morphology as well as sulfur distribution in the synthesized membranes.

3.5.2 Fourier Transform Infrared Spectroscopy

The molecular structure of synthesized membranes was characterized using

Fourier Transform Infrared Spectroscopy (FTIR) technique (Spectrum-100,

Perkin-Elmer). The FTIR spectra of the membrane samples were taken from 500 cm-1 to 4000 cm-1.

3.5.3 Ion Exchange Capacity

Ion exchange capacity (IEC) of membranes is often defined as the amount of exchangeable ions per dry weight of the membrane, with the unit of meq/g or mmol/g [214]. Titration method was adopted to determine IEC of the synthesized membranes [215]. For membranes synthesized via polymerization and sulfonation, the membrane (H+ form) was immersed in 2 M NaCl solution to convert H+ form into Na+ form. During the conversion process, H+ was released into the solution. The membrane sample was then taken out from the solution and completely washed with DI water, which was then mixed with the solution.

Following this, the solution was titrated with 0.01 M NaOH using phenolphthalein

83 Chapter 3 as the indicator. For membranes synthesized via polymerization without sulfonation, the membrane samples were firstly immersed in 1 M HCl solution to convert Li+ form into H+ form. The following procedures were the same. Finally, the

IEC of membranes could be calculated using the following equation: IEC=C*V/W where C was the concentration of NaOH solution, V was the volume of NaOH solution consumed during titration, and W referred to the dry weight of samples.

3.5.4 Water Uptake

To measure water uptake (or salt solution uptake) of the synthesized membranes, the samples were firstly immersed in water or 2M NaCl solution for 24 h, and then excess water on sample surface was removed with tissue paper, and then wet weight of the samples was measured using a balance. After that, the samples were dried in an oven at 60 ℃. Water uptake was equal to the difference of wet weight and dry weight, divided by dry weight of the sample.

3.5.5 Water Contact Angle

Water contact angles of the synthesized membranes were determined by sessile- drop technique, using standard goniometer (Ramehart Instrument, USA, model

250-U1) equipped with a high-speed digital camera which operates at 100 frames per second. Briefly, the sample was placed onto a piece of glass, which was then placed on the platform of the goniometer. The platform was adjusted to be horizontal, and a droplet of water was carefully placed on the membrane sample, and the image of the droplet was recorded by the camera, and the contact angle could be measured by the software.

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3.5.6 Loading Ratio

For membranes synthesized via polymerization without sulfonation, loading ratio

(LR) is one of the most important parameters and is studied in detail. As its name implies, LR is used to evaluate the amount of functional materials loaded onto and into the porous support after polymerization. As porous support contained no functional groups (i.e., sulfonate groups), the loaded functional materials are the only source of sulfonate groups. Obviously, LR has a direct effect on IEC. Generally, the higher the LR, the higher the IEC. To calculate LR, the weight of the synthesized membrane samples (denoted as W1) and weight of corresponding porous support

(denoted as W2) were recorded. Then, the following equation could be used to calculate LR: LR = (W1 - W2)/W2.

3.5.7 Proton Conductivity

The proton conductivity for the membranes synthesized via polymerization and sulfonation was determined using electrochemical impedance spectroscopy (EIS) technique and four-electrode in-plane proton conductivity testing method [216–

218]. Complex plane plots can be obtained via EIS. Figure 3-5 showed the schematic diagram of conductivity testing method. For the testing procedure, the membrane sample was placed into the testing device that was connected with

BioLogic VSP potentiostat. The testing device was immersed into ultra-pure water

(~18 MΩ·cm) at room temperature. Alternating current (AC) impedance data was collected in frequency range of 0.1 Hz to 1 MHz with a sinus amplitude of 100 mV across the electrodes. After the test, the membrane sample was taken out and the device was immersed into water again to measure background resistance.

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Membrane resistance could be calculated using the following equation:

R=RtRb/(Rb-Rt) where R is the membrane resistance, Rt is the total resistance (with membrane) and Rb is the background resistance (without membrane) [216, 219].

Membrane conductivity was then calculated using the following equation:

σ=L/RTW where σ is membrane conductivity, L is the distance between the inner electrodes, T is membrane thickness and W is membrane width [219].

3.5.8 Thermogravimetric Analysis and Differential Scanning Calorimetry

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to study thermal stability of synthesized membranes. The test was

86 Chapter 3 conducted via a Netzsch instrument (STA 449 F5 Jupiter®) under nitrogen flow of

20 cm3/min. For membranes prepared via polymerization and sulfonation, the test was conducted with a heating rate of 10 °C/min in the range of 25–800 °C. For membranes prepared via polymerization without sulfonation, the samples were firstly heated to 170 °C followed by an isothermal process for 10 min, and then cooled down to 30 °C followed by another isothermal process for 10 min, and then heated to 650 °C. The second DSC heating curve was used. Both the heating rate and cooling rate were kept at 10 °C/min.

3.6 Electrodialysis Test

3.6.1 Electrodialysis System Setup

Figure 3-6 showed real pictures of the electrodialysis (ED) cell. Also, as shown in

Figure 3-6, each membrane had a square shape, and the length/width was 11 cm.

The active membrane surface area was 64 cm2, where the membrane came in contact with the electrolyte solution. Figure 3-7 showed real pictures of the ED system used in this study to test membrane ED performance. To be specific, the water tanks with a volume capacity of 12 L were supplied by Tanks Direct. The aluminium framework was supplied by KJN Aluminium Profiles. The pump with a maximum flow rate of 23 L/min (brand Xylem Totton), flow meter, and connecting fittings were supplied by RS Components. Valve 1, valve 2, and valve 3 were purchased from RS Components, Hoses Direct, and Cole-Palmer, respectively. The tubing, hose connectors, and tape were supplied by Department of Chemical

Engineering at Imperial College London. The torque wrench and wrench head, which were used for assembly and disassembly of the ED cell, were purchased

87 Chapter 3 from Amazon and RS Components, respectively. The control power interface unit was supplied by National Instruments. The potentiostat (VMP3) and current booster (VMP3B-10) were supplied by BioLogic Science Instruments.

3.6.2 Test Procedures

Electrodialysis (ED) tests were conducted using an electrodialysis cell unit (PCCell

ED 64004) provided by PCCell GmbH. The salt used for ED demonstration was sodium chloride. Commercial IEMs were provided by PCA GmbH. In order to

88 Chapter 3 compare the performance of synthesized CEMs in this study to commercial CEMs, the same AEMs from PCA GmbH were used for all the tests. The membrane stack contained 5 cell pairs, which were composed of 6 cation exchange membranes, 5 anion exchange membranes, and 10 spacers.

Figure 3-7. Real pictures of ED system. 1. Tank; 2. Framework; 3. Valve 1; 4. Flow meter; 5. Tubing; 6. Pump; 7. ED cell; 8. Valve 2; 9. Valve 3; 10. Hose connector; 11. Potentiostat; 12. Current booster; 13. Control power interface unit; 14. Pump control and flow rate display panel; 15. Potentiostat control panel. (a) Diluting compartment; (b) Concentrating compartment; (c) Electrode compartment. EI refers to electrode solution in; DI refers to solution in (to be diluted); CI refers to solution in (to be concentrated); EO refers to electrode solution in; DO refers to diluted solution out; CO refers to concentrated solution out.

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As shown in Figure 2-4 and 3-7, the system contained three compartments, namely diluting compartment, concentrating compartment and electrode compartment.

Salts were removed in the diluting compartment and were collected in the concentrating compartment. The flow rate of the solution in the diluting compartment was adjusted to be the same with that in the concentrating compartment. An initial current was applied to the ED system using a potentiostat.

Current, voltage and flow rates were recorded continuously in real time. The duration for the electrodialysis test was 60 minutes. Samples from the diluting compartment and concentrating compartment were collected every ten or twelve minutes. The initial volume of the electrolyte solution (i.e., sodium chloride solution) in the diluting compartment was the same with that in the concentrating compartment, namely 2.5 L. The initial volume of the sodium sulfate solution in the electrode compartment was 4.0 L. The conductivity of these samples was measured using a TDS meter (model C100, Cole-Parmer, UK). Furthermore, for membranes prepared via polymerization and sulfonation, the sodium concentration of the samples was measured using an inductively coupled plasma

(ICP) instrument (Perkin-Elmer, USA, model Optima 2000 DV). The concentration was calculated based on the relationship between conductivity and concentration

(chapter 7.2.7).

Chapter 4 Scientometric Study of Ion Exchange Membranes

Abstract

In this chapter, a comprehensive scientometric approach was adopted to study the research on ion exchange membranes (IEMs). The statistical analysis was conducted based on over 21000 publications which were related to the topic of

IEMs. Specifically, from 2001 to 2016, over 18000 articles were published on IEMs, indicating researchers' great interest in this topic. Especially, the number of articles published in 2016 was about six times that of articles published in 2001.

This trend continued in 2017 since over 2000 articles were published in the year of 2017. Also, these articles were distributed in more than 1000 different journals, nearly 100 countries/regions and more than 5000 research institutes, revealing the importance of IEMs as well as the broad research interest in this field. Besides, the properties and applications of IEMs were also discussed statistically.

Furthermore, keywords analysis indicated that fuel cell and proton exchange membrane had the highest cooccurrence frequency. Finally, research areas analysis revealed that IEMs had a close relation with chemistry, energy and materials.

Note: The work presented in this chapter (Chapter 4) has been published:

Shanxue Jiang, Kimberly F. L. Hagesteijn, Jin Ni, and Bradley P. Ladewig, A scientometric study of the research on ion exchange membranes, RSC Advances, vol. 8, no. 42, pp. 24036–24048, 2018. Copyright 2018 Royal Society of Chemistry. Adapted with permission.

Chapter 4

4.1 Introduction

As discussed in Chapter 1, the field of ion exchange membranes (IEMs) is prosperous and plenty of studies on IEMs have been published. In the age of big data, a lot of useful information can be obtained through data analysis. Therefore, we can conduct quantitative analysis as an insightful supplement to conventional qualitative analysis or review. Scientometrics is a quantitative study of the progress of science and scientific research via data analysis of published studies

[220], and is being widely used by researchers in various fields, such as sustainability [221], building information modelling [222], antibiotics [223], and so on. This chapter aims to provide a comprehensive statistical study on the topic of IEMs through scientometric approach.

4.2 Results and Discussion

4.2.1 Document Types

Figure 4-1. Percentage distribution of document types.

The publications were divided into 10 document types, where article was the dominating type accounting for 86.0% of the total. As shown in Figure 4-1, the next two types were proceedings paper and review, which had a percentage of 8.0%

92 Chapter 4 and 3.7%, respectively. The fourth document type was meeting abstract, which accounted for 1.7% of the total. The remaining six document types with a total percentage of 0.6% were correction, book chapter, editorial material, letter, news item, and retracted publication, respectively. The following analysis was based on articles only since it was the main document type with a total number of 18166.

Figure 4-2. Number of publications in different languages.

4.2.2 Publishing Languages

As shown in Figure 4-2, English was the language used in most articles (97.5%).

Chinese was the second most popular language (accounting for 1.5%) but its popularity was far lower than that of English. Articles published in other languages

(e.g., Japanese, Korean) made up only 1.0% of the total articles. It was not surprising, though, that English was the most popular language since it was the

93 Chapter 4 default language for the vast majority of journals that publish IEMs-related work.

On the other hand, the results revealed a fact that Science Citation Index Expanded

(SCIE)-indexed journals are not necessarily published exclusively in English. For example, a Chinese journal can also be indexed by SCIE. Compared to a English journal, the main drawback of a Chinese journal is that its readers are restricted to those who can read the Chinese language. As a result, the impact of this journal and the articles published in it can also be limited. That being said, as more and more people are learning Chinese, the gap is expected to be gradually reduced in the future.

Figure 4-3. Number of publications per year and cumulative number of publications on IEMs since 2001.

4.2.3 Publishing Trend

As shown in Figure 4-3, from 2001 to 2016, over 18000 articles were published on

94 Chapter 4 the topic of IEMs, indicating researchers' great interest in this topic. To be more specific, the number of articles published in 2016 was about six times that of articles published in 2001. In comparison, the total number of articles published in all research fields/topics in 2016 (1342248 articles) was only about twice that of the total articles published in 2001 (657542 articles). Generally, there was an increasing trend on the publications from 2001 to 2016. More precisely, the trend was very obvious from 2001 to 2006, and from 2007 to 2011. From 2011 to 2013, and from 2014 to 2016, it seemed that the number of annual publications was stabilized at around 1600 and 1800, respectively. That being said, it does not necessarily mean that researchers are less interested in IEMs in recent years. It is more likely that the annual number of publications is already significant enough.

In fact, this increasing trend continued in 2017 since over 2000 articles were published in the year of 2017.

Figure 4-4. Number of publications by different journals.

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4.2.4 Publishing Journals

The journal ID No. in Figure 4-4 was also the ranking of these journals in terms of publications, from the highest being No. 1 to the lowest being No. 1092. If two journals had the same number of publications, they were ranked in alphabetical order. As shown in Figure 4-4, the articles on IEMs were distributed in nearly 1100 different journals, which indicated the popularity as well as diversity of this research field. On the other hand, the number of publications decreased dramatically with increasing journal ID No., which revealed that the top ranked journals contributed greatly to the total publications.

Table 4-1. The top 20 most publishing journals ID No. Journal Name Publisher 1 Journal of Power Sources Elsevier 2 International Journal of Hydrogen Energy Elsevier 3 Journal of Membrane Science Elsevier 4 Journal of the Electrochemical Society ECS 5 Electrochimica Acta Elsevier 6 Journal of Applied Polymer Science John Wiley & Sons 7 RSC Advances RSC 8 Fuel Cells John Wiley & Sons 9 Desalination Elsevier 10 Journal of Fuel Cell Science and Technology ASME 11 Macromolecules ACS 12 Polymer Elsevier 13 Journal of Materials Chemistry A RSC 14 Journal of Physical Chemistry C ACS 15 Journal of Physical Chemistry B ACS 16 Electrochemistry Communications Elsevier 17 Solid State Ionics Elsevier 18 Separation and Purification Technology Elsevier 19 ACS Applied Materials & Interfaces ACS 20 Physical Chemistry Chemical Physics RSC Note: ECS refers to Electrochemical Society; RSC refers to Royal Society of Chemistry; ASME refers to American Society of Mechanical Engineers; ACS refers to American Chemical Society.

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To be more specific, more than half of the total articles were published in the top twenty most publishing journals summarized in Table 4-1. Besides, as revealed by

Table 4-1, Elsevier, American Chemical Society (ACS), and Royal Society of

Chemistry (RSC) were the top three most popular publishers for IEMs-related studies.

As shown in Figure 4-5, in general, h-index was higher than average number of citations per paper. But the really interesting thing was that Figure 4-5 revealed a positive correlation between h-index and citations, which was comprehensible as h-index was a composite of the number of papers and citations (namely impact).

To sum up, as revealed by the journals analysis, the field of IEMs was quite popular and diversified, but it did have its own focused-areas. In fact, as discussed below,

IEMs had many important applications, with water and energy being the two most concerned ones.

Figure 4-5. Number of publications, average number of citations per paper, and h-index of the top 20 most publishing journals.

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4.2.5 Publishing Countries/Regions

Table 4-2. Number of countries/regions in terms of number of articles published Number of Articles Number of Countries/Regions No less than 20 52 No less than 50 37 No less than 100 30 No less than 150 21 No less than 200 20 No less than 250 15 No less than 300 14 No less than 400 13 No less than 500 10 No less than 1000 5 No less than 2000 2

Figure 4-6. Number of publications and corresponding percentage of the top 20 most publishing countries.

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Table 4-3. Number of countries/regions in terms of percentage of articles published Percentage of Articles Number of Countries/Regions No less than 0.10% 52 No less than 0.50% 31 No less than 1.00% 20 No less than 1.50% 14 No less than 2.00% 13 No less than 2.50% 13 No less than 3.00% 8 No less than 4.00% 8 No less than 5.00% 6 No less than 10.00% 2 No less than 20.00% 1

In general, nearly 100 countries/regions in total had contributions to these articles, which revealed that the field of IEMs was of global interest. Among these countries/regions, more than 50 countries/regions published at least 20 articles while only 5 countries/regions published at least 1000 articles during the 16-year period (see Table 4-2).

As revealed by Figure 4-6, China and USA were the top two most publishing countries. The following three countries whose article number exceeded 1000 were South Korea, Japan and Canada. Table 4-3 revealed the number of countries/regions in terms of percentage of articles published. For example, as the top three most publishing countries, China, USA, and South Korea contributed to

25.94%, 19.82%, and 9.81% of the total articles published, respectively. This seemed that China, USA, and South Korea contributed more than half of the total publications.

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Figure 4-7. Network graph showing collaborations between countries. Countries with collaborations of more than 20 times are connected with lines.

Table 4-4. The top 10 most collaborating country pairs Country 1 Country 2 No. of Collaborations China USA 291 South Korea USA 145 Canada China 135 Canada USA 113 France Russia 80 China Japan 77 China UK 77 Japan USA 72 China Singapore 67 China South Korea 66

However, It should be pointed out that the sum of the publishing percentage of

100 Chapter 4 each country was expected to be greater than 100% due to country/region collaborations. To be more specific, 17.28% of the articles were published as a result of collaborations between two or more countries/regions. As shown in

Figure 4-7, around 40 pairs of countries published more than 20 articles together, where the USA/China pair had the most fruitful collaborations (see Table 4-4).

4.2.6 Publishing Institutions

In general, over 5000 research institutes had contributions to these articles, which again uncovered the broad research interest in the field of IEMs. Specifically, among these institutes, 34 institutes published 100 articles or more during the 16- year period (see Table 4-5). As shown in Table 4-6 and Figure 4-8, Chinese

Academy of Sciences was the No.1 most publishing research institute (over 650 articles), followed by University of Science and Technology of China. From another perspective, it should be pointed out that institutes like Chinese Academy of

Sciences, Indian Institutes of Technology, and so on are centrally funded clusters of research institutes, which enabled their high production. In other words, the institutes listed in Table 4-6 can be roughly divided into two groups, namely independent universities such as Jilin University and “institute clusters” such as

Chinese Academy of Sciences. Actually, there are quite a few research centres/groups in Chinese Academy of Sciences that focus on the field of IEMs as well as membrane applications, including but not limited to fuel cells, electrodialysis and electrolysis.

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Table 4-5. Number of research institutes in terms of number of articles published Number of Articles Number of Research Institutes No less than 20 324 No less than 50 115 No less than 100 34 No less than 150 15 No less than 200 5 No less than 250 2 No less than 300 1

Table 4-6. The top 20 most publishing research institutes ID No. Research Institute 1 Chinese Academy of Sciences 2 University of Science and Technology of China 3 Jilin University 4 Shanghai Jiao Tong University 5 Seoul National University 6 Tsinghua University 7 Korea Institute of Science and Technology 8 National Research Council Canada 9 Pennsylvania State University 10 Wuhan University of Technology 11 Russian Academy of Sciences 12 Indian Institutes of Technology 13 Hanyang University 14 Tianjin University 15 Yuan Ze University 16 Tokyo Institute of Technology 17 Dalian University of Technology 18 University of South Carolina 19 Kuban State University 20 University of Connecticut

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Figure 4-8. Number of publications, average number of citations per paper, and h-index of the top 20 most publishing research institutes.

Besides, 48.13% of the articles were published as a result of collaborations between two or more research institutes. Furthermore, there were around 1000 pairs of research institutes that published two or more articles together. Chinese

Academy of Sciences and University of Chinese Academy of Sciences had the most fruitful collaborations with each other (see Table 4-7). As revealed by Figure 4-9,

Chinese Academy of Sciences, Korea Institute of Science and Technology, and

National Taiwan University ranked top three in terms of number of collaborating institutes. To be specific, take Chinese Academy of Sciences for example, it had collaborations with 6 different research institutes and published more than 10 articles together with each of these six institutes.

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Table 4-7. The top 10 most collaborating research institute pairs Institute 1 Institute 2 No. of Collaborations Chinese Acad. Sci. Univ. Chinese Acad. Sci. 65 Oak Ridge Natl. Lab. Univ. Tennessee 37 CICCSE Tianjin Univ. 37 Wageningen Univ. Wetsus 36 CNRS Univ. Grenoble Alpes 33 Korea Inst. Sci. & Technol. Korea Univ. 32 Korea Inst. Sci. & Technol. Seoul Natl Univ. 32 Arak Univ. Razi Univ. 31 Chinese Acad. Sci. Jilin Univ. 31 Natl. Res. Council Canada Simon Fraser Univ. 29 Note: CICCSE is short for Collaborative Innovation Center of Chemical Science and Engineering (Tianjin).

Figure 4-9. Research institute collaboration network graph. Institutes whose collaborations exceeded 10 times are connected with lines.

Note: 1-Chinese Academy of Sciences; 2-Korea Institute of Science and Technology; 3- National Taiwan University; 4-National Research Council Canada; 5-Hanyang University; 6-Seoul National University.

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4.2.7 Most-Cited Papers

Table 4-8. The top 20 most-cited articles No. Articles TC PY Research category 1 V.R. Stamenkovic [224] 2096 2007 PEMFC 2 B. Lim et al. [225] 1811 2009 PEMFC 3 M. Lefèvre et al. [226] 1512 2009 PEMFC 4 J. Greeley et al. [227] 1118 2009 PEMFC 5 G. Nagel et al. [228] 1083 2003 CSM 6 H. Liu et al. [229] 1017 2004 MFC 7 F. Wang et al. [230] 868 2002 PEMFC 8 E.J. Popczun et al. [231] 859 2013 HER 9 D.B. Levin et al. [232] 816 2004 PEMFC 10 P.J. Ferreira et al. [233] 781 2005 PEMFC 11 K. Schmidt-Rohr et al. [234] 706 2008 PEMFC 12 C.G. Van de Walle et al. [235] 666 2003 PEMFC* 13 J.H. Nam et al. [236] 627 2003 PEMFC 14 C. Wang et al. [237] 625 2004 PEMFC 15 P. Xing et al. [238] 622 2004 PEMFC 16 Z.H. Wang et al. [239] 609 2001 PEMFC 17 B. Logan et al. [240] 603 2007 MFC 18 S. Cheng et al. [241] 594 2006 MFC 19 S.M. Haile et al. [242] 591 2001 SAFC 20 C. Rice et al. [243] 537 2002 DFAFC Note: TC refers to total citations; PY refers to publishing year; PEMFC refers to proton exchange membrane fuel cell or polymer electrolyte membrane fuel cell; CSM refers to cation selective membrane; MFC refers to microbial fuel cell; SAFC refers to solid acid fuel cell; HER refers to hydrogen evolution reaction; DFAFC refers to direct formic acid fuel cell. *The more direct research topic of this paper was hydrogen, understanding of which is important for PEMFC development.

The top 20 most-cited articles on the topic of IEMs since 2010 were summarized in Table 4-8, including total citations, year published as well as research direction.

It should be pointed out that the research directions were not necessarily mutually exclusive. For example, direct formic acid fuel cell (DFAFC) was a subcategory of proton exchange membrane fuel cell or polymer electrolyte membrane fuel cell

(PEMFC). Among the articles listed in Table 4-8, it seemed that although IEMs have

105 Chapter 4 many applications, fuel cells were the most studied ones, which corresponded well with the following title and abstract analysis.

4.2.8 Title Analysis

Figure 4-10. Word cloud generated from titles with frequency no less than 200.

As shown in Figure 4-10, “fuel” was the most common word in titles, which had a frequency of 7515, meaning that over 40% of the articles contained the word “fuel” in their titles. Following “fuel” were “membrane”, “cell” and “membranes”, which ranked 2nd, 3rd, and 4th, respectively. There was no surprise that the two words

“membrane” and “membranes” had top rankings in terms of frequency. A further analysis revealed that 7350 articles out of the 18166 articles contained “fuel cell” or “fuel cells”, which indicated that fuel cell(s) was the most frequently studied subfield in the field of IEMs. The following two were “exchange” and “proton”.

Similarly, a further analysis showed that 2901 articles contained “proton exchange

106 Chapter 4 membrane” or “proton exchange membranes”, which indicated that proton exchange membrane(s) was the most frequently studied membrane type of IEMs.

Another two common words appearing in the titles were “polymer” and

“electrolyte”. A further analysis revealed that 1404 articles contained “polymer electrolyte membrane” or “polymer electrolyte membranes” in their titles. In fact, the term polymer electrolyte membrane fuel cells (PEMFCs) was often used as a whole. Furthermore, 141 titles contained “cation exchange membrane” or “cation exchange membranes” while 517 titles contained “anion exchange membrane” or

“anion exchange membranes”. However, this did not necessarily mean that cation exchange membranes were less popular compared to anion exchange membranes.

Another noteworthy high-frequency word was “sulfonated”, which was contained in the titles of 1624 articles out of the 18166 articles. Given the high frequency of proton exchange membrane(s) which was a kind of cation exchange membranes, it was understandable that “sulfonated” also had a high frequency as well, since sulfonate group was the most common functional group for cation exchange membranes. On the other hand, it indicated that a lot of studies on sulfonated polymers had been done, which was further verified by the fact that 1252 articles contained “sulfonated poly”, among which sulfonated poly(ether ether ketone)

(SPEEK) was the most studied one (Nafion was excluded from this comparison and was analyzed specifically below).

4.2.9 Abstract Analysis

Figure 4-11 showed similar frequency distributions as Figure 4-10. The word

“water” appeared more frequently in the abstracts than in the titles. To be specific,

“water” was the fifth most common word in abstracts, which appeared in 6689

107 Chapter 4 abstracts, revealing the important relationship between water and IEMs.

Figure 4-11. Word cloud generated from abstracts with frequency no less than 1000.

Table 4-9. Common applications of IEMs Application Number of Abstracts Fuel Cell 10261 Electrodialysis 843 Electrolysis 447 Desalination 256 Redox Flow Battery 225 Bipolar Membrane Electrodialysis 182 Diffusion Dialysis/Donnan Dialysis 157 Water Treatment 104 Reverse Electrodialysis 82 Capacitive Deionization 52 Electrodeionization 43 Water Purification 31 Electro-electrodialysis 25 Note: The applications listed in Table 4-9 are not necessarily mutually exclusive.

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As shown in Table 4-9, fuel cell was the most common application of IEMs, followed by electrodialysis. As revealed by Figure 4-11 and Table 4-10, conductivity was the most studied membrane property, indicating its important position in IEMs. Besides, stability was another most studied membrane property.

In order to make membranes commercially feasible and more competitive, it is crucial that excellent membrane stability is guaranteed. Furthermore, the word

“Nafion” appeared 3143 times in these abstracts, meaning that 17.40% of the abstracts contained “Nafion”, which indicated its important role in the field of IEMs.

As discussed earlier, Nafion is a sulfonated tetrafluoroethylene based perfluorinated polymer [244] which was widely used in many applications such as fuel cells [245]. Other common studied polymers included poly(arylene ether sulfone) (PAES), polybenzimidazole (PBI), and so on (see Table 4-11).

Table 4-10. Common properties of IEMs Property Number of Abstracts Conductivity 4952 Stability 3301 Water Uptake 2175 Resistance 2158 Ion Exchange Capacity 1600 Permeability 1465 Morphology 1150 Thermal Stability 1145 Swelling 1033 Hydrophilic 979 Hydrophobic 874 Mechanical Properties 833 Chemical Properties 519 Chemical Stability 466 Oxidative Stability 337 Dimensional Stability 306 Note: The properties listed in Table 4-10 are not necessarily mutually exclusive.

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Table 4-11. Common studied polymers for IEMs Polymer Number of abstracts Polybenzimidazole 408 Poly(ether ether ketone) 364 Polystyrene 341 Polyimide 304 Poly(arylene ether sulfone) 280 Polysulfone 256 Poly(vinyl alcohol) 224 Polyethylene 218 Polytetrafluoroethylene 174 Polyaniline 153 Poly(vinylidene fluoride) 128 Poly(ether sulfone) 111 Polyvinyl alcohol 103 Poly(arylene ether) 99 Poly(2,6-dimethyl-1,4-phenylene oxide) 86 Polypyrrole 85 Poly(arylene ether ketone) 83 PEEK 603 SPEEK 506 PBI 502 PVA 367 PTFE 331 PVDF 314 Note: The polymers listed in Table 4-11 are not necessarily mutually exclusive.

From another perspective, the above analysis is mainly focused on the statistical aspect of IEMs. In other words, it is powerful in revealing the “major trend” or

“broad interest” of this field but is less powerful to reveal the latest research breakthroughs or issues. Therefore, it is important to give a “human-knowledge- based” analysis on this topic, as a supplement. For example, bipolar membrane is gaining more research interest in recent years due to its high energy-efficiency as well as environmental benefits. One special research interest is bipolar membrane water splitting [246]. In addition to conventional preparation methods (e.g., casting technique), researchers are also exploring other novel technologies, such

110 Chapter 4 as electrospinning to control the layer thickness precisely [247]. Another very promising topic is water electrolysis membranes. This technology is gaining growing interest due to its capacity to produce hydrogen from cheap and readily available resources, i.e., water. However, it does not mean the hydrogen production process is cheap. So one research interest is to reduce high capital cost as well as uncertainty related to this process [248]. Besides, another interesting aspect of water electrolysis membranes is product gas crossover [249]. Finally, researchers are also developing new membranes, including mixed matrix membranes [250], short side chain perfluorosulfonic acid membranes [251], and so on. However, more work needs to be done to make these membranes more robust. For example, the mechanical stabilities of mixed matrix membranes could be a problem due to the addition of particles.

4.2.10 Keywords Analysis

As revealed by Table 4-12, “Fuel cell” was obviously the keyword with highest frequency. This result agreed well with the above discussions. In brief, the development of IEMs was mainly driven by two worldwide issues/concerns, i.e. energy and water. Fuel cell was on the energy side. Besides, as shown in Figure 4-

12, “Fuel cell” and “Proton exchange membrane” had the highest cooccurrence frequency, followed by “Fuel cell” and “Proton conductivity”. Interestingly, as shown in Figure 4-12, there were three independent keywords networks. The Fuel cell-centered network and the Electrodialysis-centered network revealed the two most studied applications of IEMs. This result was consistent with the analysis from the previous sections.

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Table 4-12. The top 20 most used keywords for IEMs Keyword Number of Articles Percentage (%) Rank Fuel cell 1386 9.86 1 PEMFC 824 5.86 2 Proton exchange membrane 789 5.61 3 Proton exchange membrane fuel cell 743 5.29 4 Fuel cells 709 5.04 5 Proton conductivity 699 4.97 6 PEM fuel cell 575 4.09 7 Electrodialysis 504 3.59 8 Membranes 369 2.63 9 Proton exchange membrane fuel cells 346 2.46 10 Polymer electrolyte membrane 326 2.32 11 Nafion 325 2.31 12 Membrane 323 2.30 13 Direct methanol fuel cell 315 2.24 14 Polymer electrolyte membrane fuel cell 310 2.21 15 Anion exchange membrane 307 2.18 16 Oxygen reduction reaction 297 2.11 17 Gas diffusion layer 270 1.92 18 PEM fuel cells 232 1.65 19 Durability 222 1.58 20

Figure 4-12. Keywords network graph. Keywords whose cooccurrence exceeded 30 times were connected with lines.

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4.2.11 Research Areas Analysis

The research areas discussed in this section referred to the categories in the SC attribute in the database where SC was short for research areas as defined by Web of Science. The 18166 articles was categorized into 68 research areas, and these research areas appeared 37873 times in total, meaning that one article were categorized into two research areas on average. Specifically, 18 of the 68 research areas had 100 articles or more, and only 8 out of the 68 research areas had 1000 articles or more (see Figure 4-13). Similar to Figure 4-4, the research area ID No. in Figure 4-13 was also the ranking of these research areas in terms of publications, from the highest being No. 1 to the lowest being No. 68. If two research areas had the same number of publications, then they were ranked in alphabetical order.

Interestingly, there seemed to be a big gap between the 8th research area and the

9th research area, where the former had more than 1000 articles while the latter had less than 500 articles. Moreover, from 1st to 8th, the number of publications decreased very quickly compared to that from 8th to the last one. The most common research area was “Chemistry”, and 8227 articles were categorized into this area. In other words, 45.3% of the total articles were categorized into

“Chemistry”. The next top five research areas were “Electrochemistry” (36.6%,

2nd), followed by “Energy & Fuels” (29.2%, 3rd), “Materials Science” (27.7%, 4th),

“Engineering” (22.3%, 5th), and “Polymer Science” (17.6%, 6th). As revealed by

Figure 4-14, almost all of these research areas (except “Engineering”) seemed to reach a plateau, some of which even showed a tendency to decrease. These trends were generally in line with the overall publication trend discussed in section 4.2.3.

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Figure 4-13. Number of publications among different research areas.

Figure 4-14. Number of publications in the top six research areas every year from 2001 to 2016.

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4.3 Conclusion

A comprehensive statistical study on the research of IEMs was conducted via a scientometric approach. Based on statistical analysis of over 21000 publications which were related to the topic of IEMs, it was found that article was the dominating type for these publications and accounted for 86.0% of the total.

English was the dominating publishing language for these articles. Specifically, from 2001 to 2016, over 18000 articles were published on IEMs, indicating researchers' great interest in this topic. Especially, the number of articles published in 2016 was around six times that of articles published in 2001. This trend continued in 2017 since over 2000 articles were published in the year of

2017. Also, these articles were spread across over 1000 different journals, nearly

100 countries/regions and over 5000 research institutes, revealing the importance of IEMs as well as the broad research interest in this field. Besides, the properties, polymers and applications of IEMs were also discussed statistically.

Furthermore, keywords analysis indicated that “fuel cell” and “proton exchange membrane” had the highest cooccurrence frequency. Finally, research areas analysis revealed that IEMs had a close relation with chemistry, energy and materials. To conclude, this scientometric study provides a statistical analysis on

IEMs and may provide an avenue for future research work in this field.

On the other hand, it should be pointed out that the scientometric approach has its limitations. To elaborate, similar to literature review, the scientometric study is based on past publications. As a result, the results and findings are based on past achievements. Though the results and findings can be used for future predictions, these kind of predictions are usually general predictions, such as prediction of

115 Chapter 4 future publishing trend. Furthermore, it is difficult to reveal new emerging methods and techniques on IEMs development. This is because scientometrics is a statistical method so only common studies can be disclosed correctly, such as common polymers used for synthesis of IEMs. Also, the data used for analysis cannot be updated in real-time. For example, the data used in this chapter was obtained in December 2017 so all the results are up to that time. There are possible solutions for this problem, though. One feasible solution is to apply artificial intelligence or machine learning techniques to realize real-time update of the results. But that is beyond the scope of this thesis and will not be discussed in detail. Despite its limitations, the scientometric method provides valuable information on the research of IEMs in a quantitative and objective way.

As revealed by scientometric study, electrodialysis is one of the most studied applications (Table 4-9) and polystyrene is one of the most studied polymers

(Table 4-11) for ion exchange membranes. In the next chapters, sulfonated polystyrene-based membranes are synthesized and their application in electrodialysis is studied.

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Chapter 5 Two-Step Synthesis of Cation Exchange Membranes via Polymerization and Sulfonation

Abstract

Cation exchange membranes (CEMs) with superior ion exchange capacity (IEC) were synthesized via a two-step approach (i.e., polymerization and sulfonation) in porous non-woven fabric support. The IEC of membranes could reach up to 3 mmol/g, due to high mass ratio of functional polymer to membrane support.

Especially, theoretical IEC threshold value agreed well with experimental threshold value, revealing the possibility to specifically design IEC without carrying out extensive experiments. Also, sulfonate groups were distributed both on membrane surface and across the membranes, which corresponded well with high IEC of the synthesized membranes. Besides, the semi-finished membrane showed hydrophobic property due to formation of polystyrene. In contrast, the final membranes demonstrated super hydrophilic property, indicating the adequate sulfonation of polystyrene. Furthermore, when sulfonation reaction time increased, the conductivity of membranes also showed a tendency to increase, revealing the positive relationship between conductivity and IEC. In addition, the synthesized membranes showed sufficient thermal stability for electrodialysis

Note: The work presented in this chapter (Chapter 5) has been published:

Shanxue Jiang and Bradley P. Ladewig, High Ion-Exchange Capacity Semihomogeneous Cation Exchange Membranes Prepared via a Novel Polymerization and Sulfonation Approach in Porous Polypropylene, ACS Applied Materials & Interfaces, vol. 9, no. 44, pp. 38612–38620, 2017. Copyright

2017 American Chemical Society. Adapted with permission.

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Chapter 5 applications such as water desalination. Finally, mass production method of membranes was introduced and electrodialysis performance was tested and discussed.

5.1 Introduction

Ideally, ion exchange membranes (IEMs) are supposed to have high permselectivity, high conductivity, high ion exchange capacity (IEC) as well as reliable chemical, mechanical, and thermal stabilities [119, 252–254]. Among these properties, IEC is undoubtedly an important parameter for IEMs. In industrial electrodialysis (ED) applications, electricity consumption is a main source of operation cost. Increasing IEC could reduce electricity consumption because when IEC is increased, membrane resistance is usually reduced.

Meanwhile, when IEC is increased, more functional groups (e.g., sulfonate groups) are present in the membranes. Therefore, the ED efficiency could be increased since the processing time is reduced [255]. However, it was not easy to prepare membranes with high IEC. Dutta et al. [256] prepared high IEC Nafion membranes through casing blend solutions of Nafion and sulfonated polyaniline. The synthesized membranes demonstrated improved IEC (1.43 mmol/g) compared to pristine Nafion (0.8 mmol/g). Li et al. [257] synthesized novel high IEC proton exchange membranes using ring-opening metathesis polymerization and the IEC could reach up to 2.3 mmol/g. However, in addition to the complicated procedures, solution casting method was adopted in these procedures, where organic solvents were used, which may pose a threat to the environment.

In this chapter, a novel methodology, without using the conventional solution

118 Chapter 5 casting technique, was proposed for making cation exchange membranes (CEMs) with high IEC by controlling the mass ratio of functional polymers to porous membrane support. The inspiration for this methodology was originated from formation of reinforced concrete, one of the most widely used modern building materials. As illustrated in Figure 5-1, the interconnected rebar provides solid support for concrete, and the former is filled with the latter, resulting reinforced concrete after solidification of concrete slurry. Similarly, membranes fabricated using this methodology is composed of two parts. The first part is membrane support and the second part is functional polymers. The relationship between membrane support and the functional polymers is analogous to that of rebar and concrete in reinforced concrete. Specifically, membrane support should have strong chemical, mechanical and thermal stability. More importantly, it should also have porous structure (voids) and low density, thus making it possible to have high liquid uptake capacity and to provide enough space for functional polymers as well.

In other words, the secret of high IEC produced using this methodology lies in high amount of functional polymers introduced to the synthesized membranes. Briefly, the functional polymers are formed by functionalization of polymers and the polymers are formed by polymerization of monomers. Especially, all the reactions take place in the membrane support. Therefore, the finally formed functional polymers are closely interconnected with membrane support. In this chapter, porous non-woven fabric Novatexx 2471 was used as membrane support, which had a porous structure as revealed by the SEM image (Figure 5-1c), and high liquid uptake capacity. Sulfonated polystyrene (PSS) was used as functional polymers, which was formed by sulfonation of cross-linked polystyrene (PS) that “grew up” in the membrane support through polymerization of styrene and divinylbenzene

119 Chapter 5

(Figure 5-1d). Not limited to synthesis of CEMs, this methodology may also be applied to synthesis of AEMs. Therefore, this methodology may provide an avenue for future research work on IEMs with high IEC.

5.2 Results and Discussion

5.2.1 Membrane Morphology and Sulfur Distribution

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Figure 5-2. Surface morphology, sulfur mapping and energy spectrum diagram of F-M with different sulfonation reaction time. (a) Membrane F-M-22. (b) Membrane F-M-30. (c) Membrane F-M-38. (d) Membrane F-M-46. F-M refers to the final-membrane. The number following F-M means sulfonation hours.

121 Chapter 5

In this work, membranes with different sulfonation reaction time were synthesized using the proposed methodology. It should be noted that S-M referred to semi-finished membranes obtained by polymerization of styrene and divinylbenzene in porous support while F-M referred to final membranes obtained by sulfonation of S-M. Generally, surface morphology of the synthesized membranes was strongly influenced by preparation conditions. As revealed by

Figure 5-1c, the original membrane support (i.e., O-M) had an intertwined porous structure (voids), which contributed to its liquid uptake capacity (preliminary experiments revealed that it could take in equal amount of water compared to its own weight). Similar to the role of rebar in reinforced concrete, the intertwined porous structure enabled polymerization reaction to take place in O-M, producing compacted semi-finished membrane (S-M). As shown in Figure 5-1d, the voids in

O-M were filled with cross-linked polystyrene (confirmed by FTIR), the role of which was similar to that of concrete in reinforced concrete. Also, it is interesting to mention that the surface of S-M was not smooth, which might be attributed to a synthetic effect of multi-factors, such as the porous structure of O-M, surface smoothness of the aluminium sheets between which O-M was sandwiched, as well as compression/sealing force. Therefore, one interesting future research topic is to systematically investigate the key parameters for fabricating membranes with smooth surface. Besides, it is obvious that the surface of F-M was also not smooth

(shown in the first column of Figure 5-2) due to rough surface of S-M.

Sulfur mapping technique was applied to investigate the distribution of functional groups (i.e., sulfonate groups) in the synthesized membranes. As revealed by the second column of Figure 5-2, the presence of sulfur on membrane surface was

122 Chapter 5 confirmed. The presence of sulfonate group was further confirmed by FTIR technique. Also, it is interesting to point out that the appearance of sulfur mapping was consistent with the uneven membrane surface. This is actually one limitation of elemental mapping technique. In other words, membrane surface topography could have an important effect on the signals of energy-dispersive X-ray spectroscopy (EDX or EDS). Therefore, it does not necessarily mean that sulfur was less abundant in relatively darker area of the mapping.

A further analysis method was applied to gain a better understanding of the presence of sulfonate groups on membrane surface. Two typical points were selected on each membrane and their energy spectrum diagrams were obtained.

As shown in the third column of Figure 5-2, both energy spectrum diagrams in all membranes shown high sulfur peaks. In brief, combing sulfur mapping and energy spectrum diagram, sulfur was successfully introduced to membrane surface.

Figure 5-3. Energy spectrum diagram of F-M cross-sections.

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Besides, in order to investigate whether sulfonation reaction also took place across the membrane, two typical points across the membrane samples were selected and their energy spectrum diagrams were analyzed. As shown in Figure 5-3, sulfur peaks were observed in energy spectrum diagrams of all the membranes.

Therefore, it was safe to say that sulfonation reaction took place both on the surface and across the membrane.

5.2.2 Membrane Chemical Structure

The chemical structures of O-M, S-M and F-M were revealed by FTIR spectra

124 Chapter 5

(Figure 5-4). Polypropylene/polyethylene as O-M composition was verified by

FTIR spectrum [258–262]. The most prominent peaks were observed at around

2915 cm-1, 2849 cm-1, 1462 cm-1, 1473 cm-1, 1376 cm-1, 718 cm-1, and 732 cm-1, which were attributed to the various vibrations of alkyl C-H groups (i.e., CH2 and

CH3).

For S-M spectrum, the first prominent peak at 3025 cm-1 was due to aromatic C-H stretching vibration, the peaks at 2920 cm-1 and 2849 cm-1 were due to alkyl C-H stretching vibrations, the next peaks at 1601 cm-1, 1493 cm-1, and 1451 cm-1 were due to carbon-carbon bond stretching vibration in the aromatic ring, and the peaks at 755 cm-1 and 695 cm-1 indicated aromatic C-H bending vibration [263]. In short,

S-M spectrum revealed that polystyrene was successfully formed in membrane support.

For F-M spectrum, the most remarkable peaks for confirming sulfonation of polystyrene were observed at around 1174 cm-1, 1127 cm-1, 1038 cm-1 and 1009 cm-1, where the first two peaks were attributed to the asymmetric stretching vibration of sulfonate group while the latter two peaks were attributed to the symmetric stretching vibration of sulfonate group [263, 264]. These characteristic peaks were also present in the FTIR spectra in Figure 7-3 (chapter 7) where no sulfuric acid was not used, which indicated that these peaks came from sulfonated polystyrene rather than sulfuric acid. Besides, the range 3000 – 3700 cm-1 was due to O-H stretching vibration of H2O, the peaks at 2920 cm-1 and 2849 cm-1 were due to alkyl C-H stretching vibrations, the peak at 1634 cm-1 was due to O-H bending vibration of H2O, the weak peaks at 1496 cm-1, 1451 cm-1 and 1412 cm-1 corresponded to carbon-carbon bond stretching vibration in the aromatic ring, the

125 Chapter 5 peaks at 832 cm-1, 775 cm-1 and 673 cm-1 were due to aromatic C-H out of plane bending vibration, and the peak at 618 cm-1 was due to ring in-plane bending vibration [265–268]. In a nutshell, F-M spectra demonstrated that functional sulfonate groups were successfully introduced to F-M through functionalization of polymers. Furthermore, FTIR spectrum of S-M indicated that the porous fabric support was fully covered by polystyrene. These results corresponded well with membrane surface morphology and sulfur distribution discussed above.

5.2.3 Theoretical Ion Exchange Capacity

As discussed in Chapter 3, IEC is defined as the amount of exchangeable ions per dry weight of the membrane (unit: mmol/g). Assuming a membrane is made up of polymer A, and the molecular weight of polymer A is MWA (unit: g/mol), then the amount of polymer A molecules contained in 1 g polymer A could be expressed as

1/MWA (unit: mol). Therefore, the theoretical maximum IEC (unit: mmol/g) can be calculated using the following equation:

1000 IECmax = (5-1) MWSO3 + MWStyrene

Where:

• IECmax is the theoretical maximum IEC of sulfonated polystyrene (in hydrogen

form).

• MWSO3 is the molecular weight of the SO3 group (i.e., 80 g/mol).

• MWStyrene is the molecular weight of styrene (i.e., 104 g/mol).

Therefore, the calculated IECmax is around 5.43 mmol/g. In other words, for polystyrene sulfonate (PSS)-based cation exchange membrane/resin, the

126 Chapter 5 maximum IEC that the membrane/resin could theoretically achieve is around 5.43 mmol/g. But in practice it is very difficult to achieve this value due to different factors such as sulfonation degree, the effect of divinylbenzene (DVB), etc. Besides, the resin usually has higher IEC compared to membranes (by dry weight). This is mainly because that for membranes, membrane support is usually required in order to increase the thermal and mechanic reliability of membranes, which has a negative effect on IEC. Therefore, for theoretical calculation, a practice coefficient p is applied. p is defined as the ratio of highest IEC of common commercial PSS- based cation exchange resins (mmol/g) to IECmax. Since the highest IEC of common commercial PSS-based cation exchange resins is around 4.8 mmol/g, the value of p can then be calculated. The calculation process is: p = 4.8/5.43 ≈ 0.88.

Besides, the semi-finished membranes were composed of cross-linked polystyrene

(PS) and original porous non-woven membrane support (O-M). Their relationship could be expressed as follows:

WS−M = WO−M + WPS (5-2)

Where:

• WS-M was the weight of semi-membrane synthesized by copolymerization of

styrene and DVB in porous fabric support.

• WO-M was the corresponding weight of the original porous membrane support.

• WPS was the corresponding weight of the cross-linked PS.

The synthesized CEMs were composed of porous fabric support and sulfonated PS.

Their relationship could be expressed as follows:

127 Chapter 5

WF−M = WO−M + WPSS (5-3)

Where:

• WF-M was the weight of CEM synthesized by sulfonation of semi-membrane via

concentrated sulfuric acid.

• WPSS was the corresponding weight of the sulfonated PS.

The difference between PSS and PS was that PSS contained sulfonate groups.

Therefore, their relationship could be expressed as follows:

WPSS = WPS + WS (5-4)

Where:

• WS was the weight of sulfonate groups formed in the porous support.

MWSO3 × WPS WS = (5-5) MWStyrene

Equation 5-5 was based on the following assumptions/conditions: (1) The sulfonation of PS was complete. In other words, every benzene ring was sulfonated.

(2) The effect of DVB was not considered. Instead, its effect was reflected by practice coefficient.

Furthermore, the mass ratio of PS to O-M in semi-membrane could be expressed using the following equation:

W r = PS (5-6) WO−M

Furthermore, the mass fraction of PSS in final-membrane could be expressed using

128 Chapter 5 the following equation:

W m = PSS (5-7) WF−M

Combining equation 5-3 and 5-7, the following equation could be obtained:

W m = PSS WO−M + WPSS

1 = W (5-8) 1 + O−M WPSS

Combining equation 5-4 and 5-8, the following equation could be obtained:

1 m = W 1 + O−M WPS + WS

1 = 1 1 + W W (5-9) PS + S WO−M WO−M

Combining equation 5-5, 5-6, and 5-9, the following equation could be obtained:

1 m = 1 1 + MW × W r + SO3 PS MWStyrene × WO−M

1 = 1 1 + MW (5-10) r × (1 + SO3 ) MWStyrene

The value of r was around 1 based on experimental results. Therefore, based on equation 5-10, the value of m was around 0.64.

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Finally, the theoretical IEC threshold value (IECthr) could be calculated using the following equation:

IECthr = p × m × IECmax (5-11)

Therefore, the value of IECthr could be calculated. The calculation process is: IECthr

= 0.88 × 0.64 × 5.43 ≈ 3.06 mmol/g.

5.2.4 Experimental Ion Exchange Capacity

As shown in Figure 5-5, all the synthesized membranes showed high IEC. IEC of the synthesized membranes (F-M-30, F-M-38 and F-M-46) were compared with common commercial CEMs (Figure 5-6) [21, 269]. As revealed by Figure 5-6, the synthesized membranes demonstrated excellent IEC performance compared with other membranes. The high IEC was mainly due to high mas ratio of polystyrene to support in S-M, which could reach around 1:1 based on experiment results. The high IEC was also indicated by sulfur distribution as well as FTIR analysis discussed above. Furthermore, IEC increased with increasing sulfonation reaction time, but reached a threshold value (~3mmol/g), which was consistent with theoretical IEC threshold value discussed above. One significant point of this agreement lied in that it provided a straightforward way to design membranes with different IEC using this novel methodology. In other words, the IEC of membranes could be designed to meet different requirements simply by controlling the mass ratio of functional polymers to membrane support as well as the sulfonation reaction time. And the IEC (threshold value) could be predicted by theoretical calculation without carrying out extensive experiments.

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Figure 5-5. Effect of sulfonation reaction time on IEC. Values are averages of at least three replicates. Error bars represent one standard deviation.

Figure 5-6. Comparison of IEC between prepared membranes in this chapter and common commercial membranes.

131 Chapter 5

Although the IEC of membranes can be designed by controlling the ratio of functional polymers to membrane support, there are some issues that need further study. To be specific, the design is based on the assumption that the sulfonation of polystyrene is complete. However, as will be discussed below, when the semi- finished membranes are fully sulfonated, the obtained membranes demonstrate high water uptake and swelling. In other words, the dimension stability of the membranes will be affected. As a result, the ED performance will be affected.

Therefore, there is a trade-off relationship between sulfonation degree and membrane dimension stability.

One solution to the above issue is to reduce the sulfonation degree. However, when all the other conditions are same, reducing sulfonation degree means reducing IEC.

Therefore, in order to maintain the desired IEC, the ratio of polystyrene to membrane support has to be increased. However, the quantitative relationships among the ratio (i.e., r in equation 5-6), sulfonation time, sulfonation degree, and

IEC are not clear. Therefore, one future research direction is to quantify these relationships through experimental and theoretical studies.

5.2.5 Water Contact Angle, Water Uptake and Conductivity

Water contact angle of O-M decreased quickly and changed from a1 to a4 in less than 10 seconds, indicating the porous structure of O-M (Figure 5-7a). S-M was hydrophobic due to the formation of cross-linked polystyrene (Figure 5-7b). When a water droplet was placed down on F-M, it spread very quickly and completely, and was much faster than O-M, and thus making it unable to observe the changes of contact angle shown in O-M (Figure 5-7c). In a nutshell, the water contact angles

132 Chapter 5 of membranes experienced fluctuating changes at different fabricating stages

(Figure 5-7d), indicating the successful reactions of polymerization and sulfonation. It is important to point out that F-M demonstrated super hydrophilic property.

Figure 5-7. Measurement of the water contact angle of (a) original porous membrane support (O-M), (b) semi-finished membrane (S-M), and (c) final-membrane (F-M). Comparison of water contact angles of O-M, S-M, and F-M is shown in (d). Values are averages of at least three replicates. Error bars represent one standard deviation.

Table 5-1. Water uptake and conductivity of synthesized CEMs

Membrane Water uptake (%) Conductivity (mS/cm) F-M-22 72.5 ± 1.4 22.37 ± 1.43 F-M-30 101.7 ± 10.9 24.93 ± 0.66 F-M-38 94.1 ± 8.2 29.04 ± 0.36 F-M-46 92.9 ± 5.1 30.39 ± 0.56 Note: the conductivity values given in Table 5-1 correspond to proton conductivity.

Also, as revealed by Table 5-1, these membranes showed high water uptake.

However, it should be pointed out that although these two properties could play a

133 Chapter 5 positive role in reducing membrane resistance, super hydrophilic property and high water uptake could have a negative impact on membrane dimensional stability since they had a positive relationship with membrane swelling. In other words, they could have passive effects on desalination as water diffusion across the membranes could more easily took place.

As mentioned earlier, reducing sulfonation reaction time is a solution for the issue of membrane swelling/water diffusion. However, a disadvantage of this solution is that reducing sulfonation reaction time could also reduce the efficiency of electrodialysis due to high membrane resistance. Therefore, future research should aim to reduce membrane water uptake and hydrophilicity while maintaining their high IEC performance. One strategy adopted by researchers to reduce water uptake of ion exchange membranes was through incorporation of appropriate amount of phase separated hydrophobic polymers in the membranes

[270].

Complex plane plots (Figure 5-8) were obtained from measurements using the electrochemical impedance spectroscopy (EIS) function of the potentiostat [183,

218, 271]. As discussed in Chapter 3, complex plane plots are used to calculate the impedance of the membranes. Impedance is then used to calculate conductivity. As indicated by Table 5-1, the conductivity of membranes showed a tendency to increase with increasing sulfonation reaction time, indicating the positive relationship between conductivity and IEC.

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Figure 5-8. Complex plane plots obtained by electrochemical impedance spectroscopy. (a) Background without membranes. (b) Membrane F-M-22. (c) Membrane F-M-30. (d) Membrane F-M-38. (e) Membrane F-M-46. Z′′ refers to imaginary impedance and Z′ refers to real impedance. At least four repeated measurements are carried out for each membrane sample.

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5.2.6 Membrane Thermal Properties

Figure 5-9. TGA diagrams of original porous membrane support (O-M), semi-finished membrane (S-M), and final-membrane (F-M) with different sulfonation reaction time. The number following F-M means sulfonation hours.

As shown in Figure 5-9, O-M and S-M demonstrated excellent thermo-stable property which did not decompose until at high temperature (> 300 °C). For F-M, there were three stages of weight loss as temperature was increased, including the loss of absorbed water (i.e., dehydration stage), followed by thermal decomposition of sulfonate group (i.e., desulfonation stage), and finally degradation of polymer chains (i.e., carbonization stage). The presence of hydrate water was also indicated by FTIR spectra as discussed above. Besides, TGA results revealed that a significant mass fraction of F-M remained at about 800 °C, which was different from O-M and S-M whose weight percent already reached zero at

136 Chapter 5 about 500 °C indicating no residues or negligible residues left. This was most likely a result of the formation of a highly heat-resistant sulfur-bridged (e.g., -SO2-) polymer during pyrolysis [272, 273]. Furthermore, the mass fraction of F-M residues showed a trend to increase with increasing sulfonation reaction time, with F-M-22 having the lowest ratio of residues while F-M-46 having the highest ratio of residues. TGA results also revealed that F-M-22 had the lowest ratio of water content while F-M-38 and F-M-46 showed the highest ratio of water content.

These results indicated that the amount of functional sulfonate groups in F-M tended to increase with increasing sulfonation reaction time. This agreed well with the results revealed by other characterisation techniques discussed above.

Figure 5-10. DSC diagrams of original porous membrane support (O-M), semi-finished membrane (S-M), and final-membrane (F-M) with different sulfonation reaction time. The number following F-M means sulfonation hours.

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Table 5-2. Melting temperatures of O-M, S-M and F-M

Membrane Melting temperature 1 (°C) Melting temperature 2 (°C) O-M 132.3 166.1 S-M 132.3 170.4 F-M-22 133.1 / F-M-30 135.9 159.4 F-M-38 137.3 161.1 F-M-46 141.6 162.3

Additionally, in the derivative weight loss curves of F-M in the range of 400 to

500 °C, the maximum degradation rate tended to shift to higher temperatures with increasing sulfonation reaction time. Also, as revealed by DSC curves (Figure 5-10),

O-M showed multiple melting peaks (two obvious peaks and one less obvious peak) and the melting peaks for F-M tended to shift to higher temperatures with increasing sulfonation reaction time (see Table 5-2). These interesting results might indicate that thermal stability of F-M was slightly improved with increasing sulfonation reaction time. Given the modest operating temperature for desalination using electrodialysis, F-M demonstrated sufficient thermal stability to meet the requirements, which was comparable to many other kinds of cation exchange membranes (CEMs) [35, 37, 274].

5.2.7 Mass Production of Membranes

The methodology introduced in this work could be used to produce many pieces of membranes at one time. In other words, it could be used for mass production of membranes, thus making it possible for industrial applications. The key thing is to find an effective spacer which should have very smooth surface and should not be dissolved by monomer solution. The spacer used in this study for demonstration

138 Chapter 5 of mass production of membranes was transparent polyester films which were kindly provided by HiFi Industrial Film.

Figure 5-11 showed the methodology diagram for mass production of membranes.

To be specific, the aluminium sheet which was originally used as the spacer was replaced by polyester sheet, and O-M containing monomer solution was separated by the polyester film, one by one. Using this upgraded method, many pieces of membranes could be prepared at one time. Figure 5-12 showed the real pictures of the synthesized CEMs using the upgraded method. The six pieces of membranes were synthesized at one time rather than six times. These synthesized CEMs were used for further electrodialysis tests. Also, the membrane size was 11 cm × 11 cm, because the electrodialysis cell required this size of membranes. For industrial- scale production, much larger size membranes could be manufactured simply by enlarging the length of sides of stainless-steel plates.

139 Chapter 5

The following equation could be used to estimate the volume of membranes produced at one time.

A = L × W × N (5-12)

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Where:

• A is the area of membranes.

• L and W are the length and width of O-M, respectively.

• N is the number of O-M separated by polyester films.

For example, if L = 1 m, W = 1 m, N =10, then A = 10 m2, which means 10 m2 membranes can be synthesized at one time.

Also, the synthesized membranes for electrodialysis test demonstrated reduced water uptake (0.281±0.035) and increased and stable water contact angle (see

Figure 5-13), indicating that these membranes were less hydrophilic compared to

F-M-22, F-M-30, F-M-38, and F-M-46.

Figure 5-13. Water contact angle of the synthesized membranes for ED test.

5.2.8 Electrodialysis Test

As discussed above, due to the high water uptake of highly sulfonated F-M series

(F-M-22, F-M-30, F-M-38, F-M-46), they were not used for electrodialysis (ED) tests. Instead, membranes with reduced sulfonation time (6h) were used for demonstration. Figure 2-4 demonstrated the scheme of the ED process adopted in this study. To be specific, five pairs of ion exchange membranes including six cation exchange membranes and five anion exchange membranes were assembled

141 Chapter 5 together and separated by spacers.

Real-time monitoring of flow rates of diluting, concentrating and electrode compartments was shown in Figure 5-14 (a-c). As revealed by these figures, the

142 Chapter 5 flow rates of the entire system were very stable. To be specific, the average flow rates of the solutions in the diluting, concentrating and electrode compartments were 0.75 L/min, 0.73 L/min and 2.50 L/min, respectively. The flow rates in the diluting compartment and concentrating compartment were nearly the same, which was an important condition since great difference of flow rates in these two compartments would generate pressure difference across membranes. Pressure difference was bad for membranes and should be avoided or reduced to the minimum.

Table 5-3. Volume changes of the solution in the diluting and concentrating compartments before and after test

Compartment Before test After test

Diluting 2.5 L 2.5 L

Concentrating 2.5 L 2.5 L

The volume changes of the solutions in the diluting and concentrating compartments before and after electrodialysis (ED) test were summarized in Table

5-3. No volume changes occurred after the test, indicating that no or negligible water diffusion across the membrane took place.

Also, as revealed by Figure 5-15, both conductivity and sodium concentration in the diluted solution decreased as time went on. In contrast, both conductivity and sodium concentration in the concentrated solution increased as time went on.

Admittedly, it was theoretically possible for the membranes to have defects which could let sodium and chloride ions to permeate through, and more study are needed on membrane defects and permselectivity between cations (e.g., sodium

143 Chapter 5 ions) and anions (e.g., chloride ions). However, in this study, after 60 minutes of ED test, there was a big difference of sodium ion concentration in the concentrated solution which was greater than 4.7 g/L and that in the diluted solution which was less than 3.0 g/L. The initial difference before ED test was less than 0.3 g/L. If the membranes had noticeable defects, then the final sodium ion concentrations in the concentrated solution and diluted solution would be the same. Therefore, these membranes demonstrated ED performance.

That being said, it should be pointed out that the ED performance was low due to high membrane resistance, as revealed by Figure 5-14 (d-e), where the current was not high enough even though the initial current was set to be 5 A. The high resistance was mainly due to the reduced sulfonation time. Therefore, one future research work is to quantitatively study the relationship between ED performance and sulfonation time so as to find the optimal sulfonation time.

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5.3 Conclusion

In this chapter, a novel methodology was proposed for making CEMs with high IEC using porous non-woven fabric as membrane support and sulfonated polystyrene as functional polymers. The polymerization and sulfonation reactions successfully took place in membrane support. The synthesized membranes demonstrated superior IEC (up to 3 mmol/g), attributing to high amount of functional polymers introduced in the synthesized membranes. Especially, theoretical IEC threshold value corresponded well with experimental threshold value. Further, sulfonate groups were distributed both on membrane surface and across the membranes, which agreed well with high IEC of synthesized membranes. Besides, the semi- finished membrane demonstrated hydrophobic property due to formation of polystyrene. By comparison, the final membranes showed super hydrophilic property as well as high water uptake. In addition, when sulfonation reaction time increased, the conductivity of membranes also showed a tendency to increase, revealing the positive relationship between conductivity and IEC. Finally, the final membranes showed sufficient thermal stability for electrodialysis applications such as water desalination. This novel methodology may open up new possibilities for fabricating CEMs with superior IEC.

However, there are some problems to be solved in future research work. Firstly, although the synthesized CEMs demonstrated high IEC, the membranes also had high water uptake and swelling. High IEC is preferable, but high water uptake is not. High water uptake usually indicates high membrane swelling and low dimension stability. That is why for ED demonstration, lower sulfonation time is adopted to reduce water uptake and swelling. However, reduced sulfonation also

145 Chapter 5 corresponds to reduced IEC. Therefore, one future research direction is to explore ways to maintain high IEC of membranes while lowering the water uptake.

Besides, concentrated sulfuric acid is used for synthesis. Concentrated sulfuric acid is a very dangerous chemical due to its corrosive, oxidizing, and dehydrating properties. A lot of efforts are needed to dispose this waste acid after sulfonation.

Therefore, it is important to develop new synthesis methods which can avoid using these kinds of acids. Chapter 6 and 7 will address this issue.

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Chapter 6 Solubility Study of LiSS in DMSO and Water

Abstract

In this chapter, solubility studies of LiSS in DMSO and water were carried out. The results revealed that DMSO and water demonstrated comparable ability in dissolving LiSS. However, because DMSO had a much higher boiling point than that of water and also LiSS-DMSO solution was more viscous than LiSS-water solution, membranes prepared using DMSO had some critical issues, such as low binding strength between functional polymers and porous support. Therefore, water was finally selected as the solvent for membrane preparation, which was further discussed in Chapter 7.

Note: The work presented in this chapter (Chapter 6) has been published as the supporting information for the following article:

Shanxue Jiang and Bradley P. Ladewig, High performance cation exchange membranes synthesized via in-situ emulsion polymerization without organic solvents and corrosive acids, Journal of Materials Chemistry A, vol. 7, no. 29, pp. 17400–17411, 2019. Copyright 2019 Royal Society of Chemistry. Adapted with permission.

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Chapter 6

6.1 Introduction

As will be discussed in Chapter 7, the two-step membrane synthesis involves sulfonation, where dangerous acids such as concentrated sulfuric acid, or chlorosulfonic acid are usually unavoidable. One solution to solve this problem is to replace styrene by styrene sulfonate salts, such as sodium styrenesulfonate

(NaSS) and lithium styrenesulfonate (LiSS) shown in Figure 6-1. However, one challenge is that unlike styrene, styrene sulfonate salts cannot be dissolved in divinylbenzene directly. At room temperature, styrene and divinylbenzene are liquids while styrene sulfonate salts (e.g., NaSS, LiSS) are powders. Therefore, a solvent is needed.

Figure 6-1. Structural formula of NaSS (left) and LiSS (right).

In this chapter, LiSS is used as the solute while DMSO and water are considered as the solvent candidates. One reason for considering DMSO as the solvent for LiSS is that, based on the information provided by the manufacturer of LiSS, DMSO is among the best organic solvents for LiSS.

It is important to get a good understanding of LiSS solubility in DMSO and water.

148 Chapter 6

Ideally, the higher the solubility, the better. There are several reasons. Firstly, the solvent is an “unrelated” chemical and the only role of the solvent is to provide a media for the reactants. Secondly and more importantly, for the synthesis method used in this thesis, the solvent will compete with the reactants for pores in the membrane support. If the solubility is low, more pores in the support will be occupied by the solvent. As a result, less amount of functional polymers can grow in and onto the support. Moreover, high amount of solvent could also have an adverse effect on the bonding strength between functional polymers and membrane support. Although the reaction is carried out in 80 °C for a couple of hours, it does not necessarily mean that all the solvent will be evaporated during the reaction. This is because the reaction is carried out in a “semi-sealed” reactor

(i.e., the stainless steel plate), due to the use of large-size spacers in combination with the pressure applied on the reactor. Moreover, DMSO has a high boiling point compared to the reaction temperature. As a result, a considerable proportion of solvent, especially DMSO, will remain in the membrane after polymerization.

Therefore, the key point here is to increase the ratio of LiSS to solvent as high as possible. That means it is important to carry out a solubility study of LiSS in DMSO and water.

6.2 Results and Discussion

6.2.1 Solvent Evaporation Method

In practice, it was found that the experiment to determine solubility of LiSS in water was much easier compared to that of LiSS in DMSO. The main reason was that it was much easier and faster to dissolve LiSS in water than in DMSO. The

149 Chapter 6 procedures were briefly discussed as follows. LiSS was added into the solvent, and the mixture was stirred. When the mixture became clear, added some more LiSS.

This process was repeated until undissolved LiSS remained in the mixture. Next, some supernatant (i.e., saturated solution) was taken out and put into another glass vial (shown in Figure 6-2). Then, the remaining mixture was dried in an oven until the weight did not change any more.

Figure 6-2. Real pictures of supernatant (i.e., saturated LiSS solution). (a) LiSS-water solution. (b) LiSS-DMSO solution.

The following relationships were established.

Ws−LiSS MFs−LiSS = (6-1) Wsupernatant

Where:

• MFs-LiSS was mass fraction of LiSS in the supernatant (i.e., saturated solution).

• Ws-LiSS was the weight of LiSS in the supernatant.

• Wsupernatant was the weight of the supernatant.

150 Chapter 6

Wsupernatant = Ws−LiSS + Ws−solvent (6-2)

Where:

• Ws-solvent was the weight of solvent in the supernatant.

Ws−LiSS = WLiSS − Wr−LiSS (6-3)

Where:

• WLiSS was the weight of LiSS powder added into the glass vial.

• Wr-LiSS was the weight of LiSS remained in the glass vial after supernatant was

taken out from the vial.

Ws−solvent = Wsolvent − Wr−solvent (6-4)

Where:

• Wsolvent was the weight of the solvent added into the glass vial.

• Wr-solvent was the weight of solvent remained in the glass vial after supernatant

was taken out from the vial.

Wtotal = Wvial + Wsolvent + WLiSS (6-5)

Where:

• Wtotal was the total weight of the mixture and the container.

• Wvial was the weight of the glass vial, including the glass body, the lid, and the

magnetic stir bar.

Wsupernatant = Wtotal − Wr−total (6-6)

Where:

• Wr-total was the total weight of the remaining mixture and the container after

151 Chapter 6

supernatant was taken out from the vial.

Wr−LiSS = Wr−total−dry − Wvial (6-7)

Where:

• Wr-total-dry was the total weight of the remaining mixture and the container

after supernatant was taken out from the vial, and then after the container was

dried in an oven to remove solvent.

Wr−solvent = Wr−total − Wr−total−dry (6-8)

In this chapter, the term “solubility” is defined as the mass fraction of LiSS in the saturated solution (solvent: DMSO or water). Based on the above relationships, the solubility of LiSS in DMSO or water can be calculated in a couple of ways (each way will return a very similar result), such as using the following equation:

WLiSS − Wr−total−dry + Wvial MFLiSS = (6-9) Wsolvent − Wr−total + WLiSS + Wvial

All the weight values on the right-hand side of equation 6-9 were experimentally obtained using the balance. Using equation 6-9, the solubility of LiSS in water was calculated to be 43.15%±0.22%, and the solubility of LiSS in DMSO was calculated to be 35.97%±0.86%. The above calculation was based on the assumption that the weight of LiSS did not change during the heating process. However, this assumption was not accurate. LiSS heating experiments revealed that the mass loss of LiSS during heating was about 7.63%±0.10%, possibly due to the loss of absorbed water in the LiSS sample. Therefore, the amended solubility of LiSS in water was 41.13%±0.20% while the amended solubility of LiSS in DMSO was

152 Chapter 6

34.26%±0.97%. The solubility result for LiSS in water was accurate because it agreed well with experimental observations and analysis. However, the solubility result for LiSS in DMSO was not accurate. The calculated value was based on the assumption that all DMSO was removed during the drying process. Unfortunately, this was not true for DMSO. It was found that it was very difficult to remove DMSO completely even though the glass vial containing the LiSS-DMSO mixture was dried for 3 days at 60 °C in a general oven, 60 °C in a vacuum oven, and 80 °C in a vacuum oven, successively. As a result, the remaining chemicals in the glass vial contained both LiSS and DMSO, rather than LiSS only. In other words, the experimental value of Wr-LiSS was higher than its real value while the experimental Wr-solvent value was lower than its real value (the weight of the remaining mixture was accurate).

Therefore, the experimental Ws-LiSS value was lower than its real value while the experimental Ws-solvent value was higher than its real value (the weight of the supernatant was accurate). As a result, the MFs-LiSS (i.e., solubility) value was lower than its real value.

Table 6-1. Mass fraction of LiSS in DMSO

More Total Mass Mass DMSO (g) LiSS (g) AD? AD? LiSS (g) LiSS (g) Fraction 1 Fraction 2 9.8597 6.8761 Yes 0.3013 7.1774 No 41.09% 42.13% 10.6092 6.5813 Yes 1.1436 7.7249 No 38.28% 42.13% 9.2831 6.6204 Yes 0.2124 6.8328 No 41.63% 42.40% Note: AD is short for all dissolved; g is short for gram; Mass Fraction 1 = LiSS (g)/[DMSO (g)+LiSS (g)]; Mass Fraction 2 = Total LiSS (g)/[DMSO (g)+Total LiSS (g)].

A revised procedure was designed aiming to solve this issue. Briefly speaking, an appropriate amount of LiSS-DMSO supernatant was dropwise added into a glass dish so the supernatant had a larger “surface evaporation area”. The dish was dried

153 Chapter 6 to remove DMSO. Using equation 6-1, the mass fraction of LiSS in the solution was calculated to be 48.50%±3.82%. The amended solubility of LiSS in DMSO was

52.41%±10.60%. However, this time, the calculated value was higher than its real value. The reason was the same, namely incomplete removal of DMSO. As a result, the experimental weight of LiSS in the solution was higher than its real value while the weight of DMSO in the solution was lower than its real value (the weight of the solution was accurate). In short, the real solubility of LiSS in DMSO should be a value between 34.26%±0.97% and 52.41%±10.60% (amended ranges based on mass loss of LiSS during the drying process). This range was obviously too large.

The good news was, during the experiments, LiSS was not added at one time, but was added into the solution after all the LiSS added previously was fully dissolved.

The results were summarized in Table 6-1. As shown in Table 6-1,the maximum value of mass fraction 1 (i.e., 41.63%) and the minimum value of mass fraction 2

(i.e., 42.13%) determined the range of the real solubility of LiSS in DMSO. Based on this, it was safe to conclude that the real solubility of LiSS in DMSO should be a value in the range of 41-42% (in terms of mass fraction).

6.2.2 Ultraviolet–Visible Spectroscopy Method

In order to get a better understanding of the solubility of LiSS in DMSO, a further technique was adopted and the procedures were designed. The principle of this methodology was that the relationship between absorbance and concentration of

LiSS solution could be quantified using ultraviolet–visible spectroscopy (UV-Vis) technique. When there were excess LiSS in the LiSS-DMSO mixture, the absorbance of the supernatant solution was expected to be the same. Therefore, a curve to describe unsaturated solution and a horizontal line to describe the saturated

154 Chapter 6 solution could be obtained. The solubility of LiSS in DMSO could be obtained by calculating the value at the intersection point of the curve and the line.

Figure 6-3. Real pictures of the LiSS-DMSO system under different LiSS/DMSO mass ratios.

As shown in Figure 6-3, as the mLiSS/mDMSO ratio (i.e., LiSS/DMSO mass ratio) increased, the color of the solution deepened, changing from colorless to light yellow, and finally yellow. When the ratio was below 0.7 (including 0.7), there were no precipitates at the bottom. In other words, the solution was unsaturated and some more LiSS could still be dissolved into the solution. When the ratio was higher than 0.8 (including 0.8), there were precipitates (i.e., undissolved LiSS) at the bottom. In other words, the solution was saturated. Therefore, the solubility of

LiSS in DMSO was expected to be a number between 0.7 and 0.8 (in terms of mLiss/mDMSO ratio). Though it was feasible to set more mLiss/mDMSO ratios between

0.7 and 0.8 using bisection method, a well-known method in mathematics, to find the boundary value between unsaturated and saturated solution, the work to find this value could be laborious. More importantly, theoretically it was not possible to find the exact value using bisection method. Therefore, instead of using the bisection method, UV-Vis spectrophotometer was used to measure solubility.

155 Chapter 6

Figure 6-4. Relationship between density of LiSS solution (solvent: DMSO) and mass fraction of LiSS in the solution.

First of all, the relationship between density of LiSS solution (solvent: DMSO) and mass fraction of LiSS in the solution was established via experiments. As shown in

Figure 6-4, density had a linear relationship with mass fraction. Their relationship was expressed in equation 6-10.

D = a × MF + b (6-10)

Where:

• MF was mass fraction of LiSS in the solution (unit: g/g or dimensionless).

• D was short for density of LiSS solution (unit: g/mL).

• a and b were constants, and based on Figure 6-3, a = 0.290, b = 1.099.

156 Chapter 6

Besides, the relationship between concentration, density, and mass fraction could be expressed using the following equation:

1000 × D × MF C = (6-11) MWLiSS

Where:

• C was short for concentration of LiSS in the solution (unit: mol/L).

• MWLiSS referred to molar mass of LiSS (190.15 g/mol).

Combining equation 6-10 and 6-11, the following equation could be established:

1000a × MF2 + 1000b × MF C = (6-12) MWLiSS

Based on Beer–Lambert law [275, 276], absorbance has a linear relationship with concentration, which is expressed in equation 6-13:

A = ε × l × C (6-13)

Where:

• A was short for absorbance.

• ε referred to molar absorption coefficient.

• l was the length of the absorbing medium.

Combining equation 6-12 and 6-13, the following equation could be established:

1000aεl × MF2 + 1000bεl × MF A = (6-14) MWLiSS

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As revealed by equation 6-14, absorbance had a quadratic relationship with mass fraction, instead of a linear relationship. That was why quadratic curve fitting was applied in the following discussion.

Figure 6-5. UV-Vis spectra of LiSS-DMSO system under different LiSS/DMSO mass ratios. Values are averages of at least three replicates.

Figure 6-5 showed the UV-Vis spectra LiSS/DMSO system at different LiSS/DMSO mass ratios. Quadratic curve fittings were conducted under different wavelengths and coefficient of determination was used to evaluate the fittings (Figure 6-6). The coefficient of determination, which is commonly denoted as R2, is a very useful tool to evaluate how well the regression model agrees with the experimental data [277,

278]. Generally, the R2 value ranges from 0 to 1. The closer the R2 value to 0, the

158 Chapter 6 worse the fitting. The closer the R2 value to 1, the better the fitting. If the R2 value equals to 1, it means the models agrees perfectly with the data.

Interestingly, as shown in Figure 6-6, the distribution of R2 seemed to be

“organized”. In the wavelength range of 348-351 nm, R2 values were the highest and three nines (i.e., 0.999) were achieved (Figure 6-7). Therefore, the relationships between absorbance and mass fraction in the wavelength range of

348-351 nm were used to calculate solubility. As shown in Table 6-2, the solubility of LiSS in DMSO was around 42%, which agreed well with the result calculated by the solvent evaporation method.

Figure 6-6. R2 of quadratic curve fittings at different wavelengths.

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Figure 6-7. Relationship between absorbance and mass fraction at different wavelengths. (a) 348 nm. (b) 349 nm. (c) 350 nm. (d) 351 nm.

Table 6-2. Solubility of LiSS in DMSO calculated via the UV-Vis method

W (nm) Relationship Between A and MF Threshold A Value Solubility 348 A = 23.473MF2 - 4.300MF + 0.480 2.808 41.96% 349 A = 23.540MF2 - 4.418MF + 0.486 2.800 42.11% 350 A = 23.962MF2 - 4.694MF + 0.508 2.782 42.12% 351 A = 23.976MF2 - 4.790MF + 0.513 2.764 42.22% Note: W is short for wavelength; A is short for absorbance; MF is short for mass fraction.

However, it should be pointed out that Beer–Lambert law, which was used in the theoretical calculations above, had its limitations. Normally, Beer-Lambert law is accurate only when the solution is very dilute and the absorbance is less than 2

[275, 276]. When the concentration is high, the interactions among the molecules

160 Chapter 6 cannot be ignored any more. However, the concentration of LiSS solution in this study was very high and the absorbance also exceeded 2 (Figure 6-7). Interestingly, despite this violation, the experimental results corresponded well with theoretical calculations. The underlying reason was not clear, but here was one possible explanation. Although the concentration was high in this study, the linear relationship between absorbance and concentration was still valid in certain concentration ranges (instead of the whole ranges). For example, the revised relationship could be: A = a’ × C + b’ where C was greater than 0 and a’ and b’ could be different values in different C ranges. Therefore, although equation 6-13 and 6-

14 were no longer accurate, the relationship between absorbance and mass fraction was still quadratic. This explanation was partly supported by a minor difference between the above theoretical calculations and experimental results. To be specific, as revealed by equation 6-14, when MF was 0, A also equaled to 0.

However, from Table 6-2, when MF was 0, A was not equal to 0 (around 0.5). In other words, the equations in Table 6-2 were not applicable in low mass fractions.

Based on the above discussions, it was concluded that water and DMSO demonstrated comparable performance in dissolving LiSS. In terms of solubility, both water and DMSO could be regarded as good solvent candidates for membrane synthesis. In the next section, the practicability of using DMSO as the solvent to prepare CEMs was further discussed.

6.2.3 Membrane Preparation Using DMSO/Water as the Solvent

At first, DMSO was used as the solvent to prepare CEMs. However, the prepared

CEMs had two critical issues. The first issue was that the amount of functional

161 Chapter 6 polymers formed in/onto the porous support was low. To be specific, the mass ratio of functional materials to porous support was only about 20%. The main reason was that the LiSS-DMSO solution was very viscous and thus its fluidity was very poor. As a result, it was difficult for the liquid mixture to permeate into the pores of the support. In other words, a majority of functional materials were formed on the surface instead of the pores inside. The second issue was that the surface of the membrane was viscous after reaction. In other words, DMSO did not evaporate completely during the heating process. As discussed before, the main reason was that DMSO had a very high boiling point. As a result, the synthesized membranes were not stable because the formed materials were not firmly connected with the porous support.

Although DMSO and water showed similar capacity for dissolving LiSS (based on solubility), some differences were observed. The first one was that the saturated

LiSS-DMSO solution was more viscous than the saturated LiSS-water solution

(Figure 6-2). The second one was that when there was an excessive amount of LiSS in the LiSS-DMSO mixture, the undissolved LiSS agglomerated together and precipitated at the bottom (Figure 6-3). The agglomerated and precipitated LiSS was very hard and thus it was not feasible to get it uniformly dispersed in the mixture even under stirring. However, the undissolved LiSS in the LiSS-water mixture did not agglomerate and thus it was very easy to get it uniformly dispersed under stirring. Therefore, it was practical to prepare a uniform mixture of LiSS- water under stirring where the amount of LiSS in the mixture exceeded the maximum amount of LiSS that could be dissolved. But this was not applicable for

DMSO.

162 Chapter 6

6.3 Conclusion

To conclude, compared to DMSO, water demonstrated more favorable properties as the solvent for LiSS. Therefore, water was finally chosen as the solvent for membrane synthesis without sulfonation, which was further discussed in Chapter

163

Chapter 7 One-Step Synthesis of Cation Exchange Membranes via Polymerization Without Sulfonation

Abstract

In this chapter, synthesis of high performance CEMs is achieved without organic solvents and sulfonation. The synthesis is carried out via in-situ polymerization of lithium styrene sulfonate in porous support. Different preparation procedures are developed and optimized. Functional sulfonate groups are successfully loaded onto and into the membrane support, as verified by FTIR. Besides, water plays an important role during membrane synthesis. By reducing the amount of water used, the ratio of functional polymers to membrane support in the synthesized CEMs is increased. Therefore, the synthesized CEMs show increased ion exchange capacity

(IEC). This is significant because it means that high IEC can be achieved without introducing cation exchange resins to the membranes. Finally, the synthesized membranes demonstrate high ED performance. This new methodology may shed new light on preparing CEMs in an efficient and eco-friendly way.

Note: The work presented in this chapter (Chapter 7) has been published:

164

Chapter 7

7.1 Introduction

Ion exchange membranes (IEMs) are undergoing prosperous development in recent years [52, 279–282]. A recent statistical study reveals that over 20,000 research papers relating to IEMs have been published since 2001 [283]. In order to meet the growing requirements for IEMs, many synthesis methods have been developed [284–288]. Among these methods, solution casting is a very popular one. However, there are several disadvantages using this method. Firstly, harmful organic solvents such as dimethylformamide (DMF) and dimethylacetamide

(DMAc) are often used in this method, and therefore post-disposal of these solvents are inevitable. Secondly, when ion exchange resins are added to the solution, additional procedures such as sonication are usually necessary so as to disperse the resins uniformly and to avoid aggregation of resins. Also, the amount of resins that can be added is usually limited. As a result, ion exchange capacity

(IEC) of the prepared membranes will be affected. Thirdly, mechanical properties of the membranes are usually not robust and the membranes can be easily broken if bent. Another common method for preparing IEMs with different properties is radiation induced grafting [185–187]. However, with this method, high-energy radiation is usually indispensable. In addition, organic solvents are usually required. Furthermore, in some procedures, strong acids are used to introduce functional groups. The sol-gel method is an emerging method to introduce inorganic particles to organic polymers [289, 290]. However, it is difficult to control the dispersion of inorganic particles, and therefore the membrane performance will be compromised. This method also inherits the issues and challenges of the solution casting method, since casting is usually needed for sol- gel method.

165 Chapter 7

In Chapter 5, CEMs with high IEC were synthesized using porous support [19].

Similar to other common polymerization methods for membrane preparation, concentrated sulfuric acid is used to introduce sulfonate groups to the membranes.

Since the acid waste is harmful to the environment, sodium hydroxide is needed to neutralize the waste. A similar method which also uses porous support is pore filling [41, 188]. However, with this method, organic solvents and/or sulfonation are usually needed, and therefore post-treatment of solvents and acid wastes are necessary [141, 189]. In addition, in some cases, in order to induce crosslinking, additional procedures such as annealing treatment are necessary [190, 291].

In this Chapter, a new methodology is developed to synthesize high performance

CEMs in an effective and environment-friendly way (Figure 3-2). This methodology includes avoiding the use of organic solvents and strong acids, reducing the use of unnecessary chemicals, and realizing rapid synthesis via optimizing preparation procedures. To achieve the above goals simultaneously, in-situ polymerization, emulsification, functional monomers, and porous support are combined together for the first time. To be specific, monomer containing functional sulfonate group (e.g., lithium p-styrenesulfonate) is used so sulfonation is not needed. Also, water is used to dissolve the monomer so no organic solvents are needed. The detailed process of choosing water as the solvent is discussed in

Chapter 6. In order to homogenize the mixture which mainly contains water, functional monomer, and crosslinking agent (e.g., divinylbenzene), a small amount of eco-friendly emulsifier was used. Porous support is used to absorb the liquid mixture and also to ensure mechanical strength of the membranes. The polymerization reaction takes place within and onto the porous support. This

166 Chapter 7 effective methodology may open up a new world of possibilities for scalable synthesis of ion exchange membranes in an eco-friendly way.

Figure 7-1. Thicknesses of different membrane series. (a) Membrane thicknesses under different DVB/LiSS ratios. (b) Membrane thicknesses under different CER/LiSS ratios. (c)

Membrane thicknesses under different CER/DVB ratios. MDL0.6 from (a) is shown in (b) and (c) again for comparison. (d) Membrane thicknesses under different water/LiSS ratios. CER refers to cation exchange resin; DVB refers to divinylbenzene; LiSS refers to lithium styrenesulfonate.

7.2 Results and Discussion

7.2.1 Membrane Synthesis and Thickness

167 Chapter 7

The effect of membrane thickness on membrane properties is complicated. An increase in membrane thickness indicates that more functional materials are loaded into and onto the membrane support. When a membrane contains more functional materials (e.g., sulfonate groups for CEM), the ED efficiency could be increased because more salt ions can pass the membrane (e.g., sodium ions for

CEM) per unit time. Nevertheless, when the membrane thickness is increased, the membrane resistance will also increase. As a result, the ED efficiency will be reduced. This contradictory result reveals that membrane thickness has a trade- off relationship with membrane properties.

As shown in Figure 7-1, the synthesized membranes exhibited different membrane thicknesses under different preparation conditions. When other conditions were kept the same, DVB/LiSS ratios (i.e., MDLX series) did not have an obvious effect on membrane thicknesses (Figure 7-1a). This was due to the low loading ratios of functional materials in the membranes. When porous support was immersed in the liquid mixtures, water competed with functional monomers for limited places in the support. As the total liquid uptake capacity of the support did not change much, the loading of functional monomers was affected. During polymerization, water was evaporated gradually, which made more spaces available. At the same time, the monomers underwent polymerization. However, due to the pressure applied onto the support, most of the liquid mixture on the support was removed before reaction. As a result, very little monomer was available to fill in the spaces left by water. Therefore, the loading ratio of functional materials was reduced. Furthermore, the pressure applied on the reactor also showed a negative effect on membrane thickness. Although the loaded functional

168 Chapter 7 materials had a positive effect on membrane thickness, the negative effect of the pressure was greater than the positive effect of functional materials when the loading ratio was low. As a result, some synthesized membranes showed decreased thicknesses compared to the thickness of pure membrane support

(Figure 7-1).

Figure 7-2. Schematic illustration of the dramatic increase in thickness observed in the

MCDX series.

169 Chapter 7

When cation exchange resin (CER) was introduced, it had a positive effect on membrane thickness. However, its effect was different in different situations. For example, when the ratios of LiSS, water, and DVB were kept the same, CER/LiSS ratios (i.e., MCLX series) had a moderate impact on membrane thicknesses, especially when CER/LiSS ratio was low (Figure 7-1b). Nevertheless, when CER was mixed with DVB, the resulting mixture became very viscous. Moreover, the viscosity increased with increasing the CER/DVB ratio. As a result, the mixture had very poor mobility. When the mixture was applied onto the support surface, it could not spread out. Even when pressure was applied, the thickness of the mixture did not reduce much. As a result, the synthesized membranes (i.e., MCDX series) showed dramatic increase in thicknesses compared to pure support

(Figure 7-2). Moreover, the higher the CER/DVB ratio, the thicker the membranes

(Figure 7-1c). Besides, it was interesting to point out that when CER was introduced into DVB, its color changed to black, so possibly this was not a simple physical mixture. In other words, it was possible that unwanted chemical reactions happened after the two chemicals were mixed together. Therefore, one interesting future research topic is to investigate the interactions between CER and DVB.

The water/LiSS ratio also showed an effect on membrane thickness. As shown in

Figure 7-1d, reducing water/LiSS ratio could increase membrane thicknesses. The reason was that, as water usage was reduced, more spaces were occupied by functional monomers, and therefore the loading ratio was increased. At the same time, lowering water ratio also made the mixture more viscous, and thus the pressure applied on the support was less effective to remove the mixture on the

170 Chapter 7 surface. As a result, after polymerization, more functional materials were loaded into and onto the support, and therefore the membrane thickness was increased.

7.2.2 Membrane Chemical Structure

Figure 7-3. FTIR spectra of different membrane series and materials. (a) MDLX series. (b)

MCDX series and PDVB. (c) MCLX series and CER. (d) LiSS, MDL0.5, MWL0.5, and FMWL0.5. PDVB refers to polydivinylbenzene; FM refers to functional material in the membranes.

Chemical structures of the membranes were investigated by FTIR. The broad concave shape with shape center at around 3410±16 cm-1, in combination with another peak at 1632±3 cm-1 (denoted as c in Figure 7-3a, 3c and 3d), indicated the presence of water in these samples. In contrast, in the spectra of the MCDX series, these two peaks were not prominent, indicating the low amount of water. This

171 Chapter 7

corresponded well with the following water uptake analysis, as MCDX series exhibited much lower water uptake compared to other membrane series.

The peaks at 1173±2 cm-1, 1127±1 cm-1, 1039±2 cm-1, and 1009±1 cm-1 (denoted as b in Figure 7-3) were characteristic peaks of sulfonate group (salt form). As discussed above, the thicknesses of MCDX series were much higher compared to the thickness of pure membrane support. As a result, the mass fraction of polydivinylbenzene (PDVB) and cation exchange resin (CER) in the membranes was much higher than that of the support. That was why these membranes did not show characteristic peaks of the support. Instead, the spectra of these membranes exhibited spectra characteristics of PDVB (denoted as d in Figure 7-3b) and CER.

Besides, the two peaks around 2916 cm-1 and 2849 cm-1 (denoted as a in Figure 7-

3a, 7-3c and 7-3d) were characteristic peaks of the membrane support.

Furthermore, MWL0.5 and FMWL0.5 exhibited very similar FTIR spectra with each other, indicating the high ratio of functional materials in the membranes. This further revealed high IEC of the membranes, as discussed below. In addition, these spectra agreed well with those of cross-linked sulfonated polystyrene salts, indicating the successful polymerization of LiSS. Besides, the successful polymerization of LiSS could also be verified via the spectrum of LiSS. To be specific, the sharp peak of LiSS spectrum at 1647 cm-1 (denoted as e in Figure 7-

3d) revealed the presence of vinyl group. This sharp peak disappeared after reaction, which indicated that the vinyl group was involved in the reaction.

172 Chapter 7

7.2.3 Ion Exchange Capacity and Loading Ratio

Ion exchange capacity (IEC) is one of the most important properties of IEMs. In this work, preparation conditions demonstrated great influence on IEC of the synthesized membranes, mainly by influencing the loading ratio of functional materials (denoted as LR below) and also the IEC of loaded functional materials.

To be more specific, IEC of the synthesized membranes could be increased by increasing LR and/or IEC of loaded functional materials. Generally, LR could be controlled by controlling the immersion time of the porous support in the liquid mixture and/or by controlling the compositions of the liquid mixture. For example,

MCDX series demonstrated very high loading ratios compared to other membrane series. The reason for this was that the mixture of CER and DVB was very viscous.

As a result, more CER/DVB liquid mixture was loaded onto the membrane support.

Moreover, IEC of loaded functional materials had a positive relationship with the amount of sulfonate groups in the materials. Therefore, IEC of loaded functional materials could be increased by increasing the amount of sulfonate groups in it. If assuming all water was removed during membrane preparation and all water- soluble substances were removed during conversion, then IEC of the synthesized membranes were affected by two parameters, including LR and mass ratio of DVB and styrene to LiSS in the liquid mixture (denoted as DR below). The influence of

LR and DR on IEC can be quantitatively described. And the derivation procedures were provided below.

Because:

N = IECload × (Wm – Wp) and also N = IECm × Wm

173 Chapter 7

Where: N is the amount of exchangeable functional sulfonate groups in membrane samples (unit: millimole, mmol); IECload is the IEC of loaded functional materials;

IECm is the IEC of synthesized membrane samples; Wm is the weight of membrane samples (sodium form); Wp is the weight of membrane support.

Therefore:

IECload × (Wm – Wp) = IECm × Wm

Therefore:

Wm − Wp IECm = IECload × Wm

Wm − Wp = IECload × Wm − Wp + Wp W − W m p Wp = IECload × Wm − Wp + Wp Wp W − W m p Wp = IEC × load W − W 1 + m p Wp LR = IEC × load 1 + LR 1 + LR − 1 = IEC × load 1 + LR 1 = IEC × (1 − ) load 1 + LR

W −W Where: LR is short for loading ratio and LR = m p Wp

Furthermore:

174 Chapter 7

MWS RL × MWL IECload = IECmax × MWS 1 + RL × ( – 1) MWL

MWS 1000 RL × MW = × L MWS MWS 1 + RL × ( – 1) MWL 1000 = 1 MWL × ( – 1) + MWS RL 1000 = 1 190.15 × ( – 1) + 206.19 RL

Where: MWS is molar mass of sodium p-styrene sulfonate (206.19 g/mol); MWL is molar mass of lithium p-styrene sulfonate (190.15 g/mol); IECmax is theoretical maximum IEC of sodium p-styrene sulfonate and IECmax = 1000/206.19 ≈ 4.85 mmol/g; RL is the ratio of sulfonate groups (in terms of LiSS) in the loaded functional materials.

One purpose of the above derivation was to convert IEC (lithium form) to IEC

(sodium form) as the experimental IEC was measured in sodium form.

If assuming all water was removed during membrane preparation and all water soluble substances were removed during conversion, then:

WL RL ≈ WL + WD + WS

Where: WL is the weight of real LiSS in the liquid mixture used for synthesis; WD is the weight of DVB in the mixture; WS is weight of styrene in the mixture.

175 Chapter 7

Therefore: 1000 IEC ≈ load W + W 190.15 × D S + 206.19 WL Therefore:

1000 1 IEC ≈ × (1 – ) m W + W 190.15 × D S + 206.19 1 + LR WL

1000 × LR = (190.15 × DR + 206.19) × (1 + LR)

W + W Where: DR is the mass ratio of DVB and styrene to LiSS and DR = D S WL

The above quantitative relationship was also demonstrated in Figure 7-4a. It should be noted that the IEC in Figure 7-4 was identical to the IECm in the above derivations. As shown in Figure 7-4b, under the same preparation conditions, LR did not seem to be affected by DVB/LiSS ratio in the mixture. Furthermore, when

LR was about the same, DVB/LiSS ratio revealed a negative relationship with IEC, though the relationship was not prominent. At high DVB/LiSS ratios, experimental

IEC values corresponded well with theoretical IEC values. At low DVB/LiSS ratios, some deviations were observed. This was possibly due to that the above equation was obtained based on the assumption that all water was evaporated during reaction. However, when DVB/LiSS ratio was low, the amount of water used for synthesis was high, and therefore the above assumption was no longer valid. In other words, the effect of water could not be ignored at low DVB/LiSS ratios. Also, the IEC values of the membrane samples in Figure 7-4b were not high, due to the low LRs. One possible way to increase LR was to increase the amount of liquid mixture into and onto the membrane support before reaction. In addition, as

176 Chapter 7 revealed by Figure 7-4c, when other conditions were the same, LR had a positive relationship with IEC, which agreed well with theoretical predictions.

177 Chapter 7

As revealed by Figure 7-4d and Figure 7-4e, cation exchange resin (CER) had a positive effect on IEC. Moreover, compared to other membrane series, increasing

CER/DVB ratio could also dramatically increase LR (Figure 7-4d). As a result, IEC was also increased. In addition, the effect of CER/LiSS ratio on LR and on IEC showed a similar trend, due to the positive relationship between LR and IEC

(Figure 7-4e). Though CER could increase IEC, it was possible to prepare membranes with comparable or even higher IEC without using CER. One strategy to prepare high IEC membranes without CER was to reduce the amount of water used for dissolving LiSS in the mixtures (Figure 7-4f). When water amount was reduced, more functional polystyrene sulfonates could be formed into and onto the support, and therefore LR and IEC were also increased. This also agreed well with theoretical calculations.

7.2.4 Water Uptake and Contact Angle

Water uptake and contact angle were another two parameters revealing electrochemical properties of CEMs. As shown in Figure 7-5, all the membrane samples exhibited hydrophilicity but varied in degree. The MDLX series demonstrated highest water uptake and lowest contact angle compared to other series. In contrast, due to the dense PDVB layer formed on the support, the MCDX series exhibited lowest water uptake among all the series.

178 Chapter 7

7.2.5 Membrane Morphology and Sulfur Distribution

As revealed by Figure 7-6a to 7-6d, different series of membrane samples exhibited different surface morphologies. As sulfonate group played an important role in determining ion exchange capacity and membrane performance, its distribution within the membranes were investigated using sulfur mapping technique [19].

179 Chapter 7

As shown in Figure 7-6e, 7-6f, 7-6i, and 7-6j, both MWL0.5 and MCL0.6 showed rich and uniform sulfur distribution on membrane surface. As shown in Figure 7-6g, the surface of MCD0.4 was rough. To some extent, this uneven surface increased the difficulty to detect sulfur. As a result, the sulfur distribution of MCD0.4 was not as

180 Chapter 7

uniform as those of MWL0.5 and MCL0.6. In contrast, as shown in Figure 7-6h, MDL0.6 did not show sulfur distribution on the surface, due to the low loading ratio of functional materials in the membranes compared to those of other membrane series. To summarize, the surface sulfur distribution of different membranes shown in Figure 7-6 agreed well with the above IEC analysis.

In addition, as shown in Figure 7-6l to 7-6o, sulfur distributions of the membrane cross-sections were not uniform, which was due to the porous structure of membrane support. For example, the cross-section of MCD0.4 clearly showed three layers, due to the high loading ratio. The outer two layers did not contain membrane support while the inner layer contained both membrane support and functional materials. Besides, it seemed that there were some “pores” in the membranes, as shown in Figure 7-6l, 7-6m, 7-6n and 7-6o. However, it should be clarified that they were not pores. In fact, they were cross-sections of the membrane support, which had a fiber shape. To conclude, the synthesized membranes were semi-homogenous, where the fibers contained no sulfonate groups while the functional materials had uniform distribution of sulfonate groups.

7.2.6 Membrane Thermal Properties

As revealed by Figure 7-7a, the membranes showed two melting peaks, which were characteristic peaks of the membrane support. Also, all the peaks were there, only shifting slightly. Furthermore, as revealed by the TGA curve (Figure 7-7e), during the second heating process, there was no mass loss below 200 ℃. These

181 Chapter 7 phenomena indicated that no chemical reactions happened to the porous support during membrane preparation. In addition, the membranes also showed two crystallization peaks (Figure 7-7b). In contrast, the functional materials did not show any melting or crystallization peaks at relevant temperatures, indicating its amorphous structure [292]. Besides, many factors could cause shift of the melting peaks and crystallization peaks, such as the heating rates and cooling rates [293,

294]. As revealed by Figure 7-7c, MDL0.6 had highest melting enthalpies and crystallization enthalpies (absolute value, the same below) while MCD0.4 had lowest melting enthalpies and crystallization enthalpies. The reason lied in that MDL0.6 had the lowest loading ratio compared to those of other series, while MCD0.4 had the highest loading ratio (Figure 7-4). In other words, the mass ratio of porous support was highest in MDL0.6 and lowest in MCD0.4. As a result, MDL0.6 had the highest enthalpies while MCD0.4 had the lowest enthalpies.

As shown in Figure 7-7d, the weight of the samples decreased gradually, which was due to the loss of absorbed water in the membranes. The sulfonate groups in the membranes had a positive role in retaining the water molecules in the membranes, so the release of water molecules was a continuing process [295]. The following weight losses in Figure 7-7e were due to the decomposition of sulfonate group and degradation polymer matrix. In addition, the functional material which had highest sulfur amount showed highest residual weight. In contrast, MDL0.6 which had lowest sulfur amount showed lowest residual weight. One possible explanation was that during degradation, a highly thermostable sulfur-bridged polymer was formed [273].

182 Chapter 7

7.2.7 Relationship Between Conductivity and Concentration

As revealed by Figure 7-8, the relationship between conductivity (mS/cm) and concentration (g/1000g solution) was quadratic. Theoretical derivations also proved this quadratic relationship.

183 Chapter 7

Figure 7-8. Relationship between conductivity and concentration.

The relationship between concentration (mol/L) and mass fraction or concentration (g/1000g solution) could be quantitatively expressed using the follow equation:

D × Cg Cm = MWNaCl

Where: Cm is short for concentration (mol/L); D is short for density (g/mL); Cg is short for concentration (g/1000g solution); MWNaCl is the molar mass of NaCl,

58.44 g/mol.

Also, D had a liner relationship with Cg. Therefore, D could be expressed using the following equation:

184 Chapter 7

D = a × Cg + b

Where: a and b are constants.

Therefore:

(a × Cg + b) × Cg Cm = MWNaCl

2 a × Cg + b × Cg = MWNaCl

Also, conductivity had a direct linear relationship with concentration (mol/L):

′ C = a × Cm + b′

Where: C is short for conductivity (mS/cm); a' and b' are constants.

Therefore:

2 a × Cg + b × Cg C = a′ × + b′ MWNaCl

′ 2 ′ a × a × Cg + a × b × Cg = + b′ MWNaCl

′′ 2 ′′ = a × Cg + b × Cg + b′

Where: a'' and b'' are constants and their relationship with a, a', b and MWNaCl are expressed as follows:

a′ × a a′′ = MWNaCl a′ × b b′′ = MWNaCl

185 Chapter 7

According to Figure 7-8:

a′′ = 0.01221 b′′ = −1.15857 b′ = 150.69516

2 C = 0.01221 × Cg − 1.15857 × Cg + 150.69516

7.2.8 Membrane Electrodialysis Performance

Membrane electrodialysis (ED) performance was demonstrated using MWL0.5 due to its advantages over other membrane series. Firstly, MCDX had high thicknesses compared to other membrane series, which indicated that the membrane resistance could be very high. As a result, its ED efficiency was very low. Secondly,

MDLX had low IEC compared to other membrane series, which would also affect its

ED efficiency. Thirdly, although MCLX did not show obvious disadvantages over other membrane series, CER was used to improve its performance. In addition, it was much easier to get uniform sulfur distribution for MWL0.5 compared to MCLX.

Therefore, among all of the membrane series, MWL0.5 stood out.

Figure 7-9. Voltage and current changes during the ED tests. (a) Record for synthesized membranes. (b) Record for commercial membranes.

186 Chapter 7

Table 7-1. Summary of synthesized membranes and commercial membranes for the ED test

Synthesized membrane Commercial membrane

Supplier - PCA GmbH Company

ID MWL0.5 PC SK

Type CEM CEM

Charged group Sulfonate group Sulfonate group

Width (cm) 11 11

Length (cm) 11 11

Thickness (mm) 0.21±0.02 0.18±0.02

The scheme of the ED process adopted in this study was demonstrated in Figure

2-4. Again, five pairs of ion exchange membranes including six cation exchange membranes and five commercial anion exchange membranes were assembled together and separated by spacers. The basic properties of synthesized membranes and commercial membranes used for ED test were summarized in

Table 7-1. As mentioned earlier, the membranes have the same square shape with an active area of 64 cm2. During the ED tests, voltage and current changes were recorded (Figure 7-9). As revealed by Figure 7-10, in the ED process, the conductivity of the electrolyte solution (i.e., sodium chloride solution) increased in the concentrating compartment while decreased in the diluting compartment.

Also, the electrolyte concentration increased in the concentrating compartment while decreased in the diluting compartment. More importantly, the synthesized membranes showed high ED performance, which was comparable to commercial

187 Chapter 7 membranes, even given that the synthesized membranes had a higher thickness compared to commercial membranes. Therefore, it is envisioned that when the membrane thickness is reduced while its high IEC is maintained, the ED performance could be further increased. This is one interesting future research topic.

Figure 7-10. ED performance of synthesized membranes and commercial membranes. (a) Conductivity changes of the electrolyte solution in the concentrating and diluting compartments using synthesized membranes MWL0.5. (b) Changes of electrolyte concentration in the concentrating and diluting compartments using synthesized membranes MWL0.5. (c) Conductivity changes of the electrolyte solution in the concentrating and diluting compartments using commercial membranes. (d) Changes of electrolyte concentration in the concentrating and diluting compartments using commercial membranes.

188 Chapter 7

Furthermore, similar to that in Chapter 5, it was possible to achieve large-scale production of membranes using this method (Figure 7-11). Admittedly, as this method is new, there are still many aspects which need further study, such as membrane stability study. Especially, future research should focus on improving the stability of MWLX in dilute solutions.

Figure 7-11. Schematic illustration of large-scale production of membranes using the methodology proposed in the chapter.

7.3 Conclusion

In summary, a new and effective methodology to prepare high performance cation exchange membranes was proposed and studied. Instead of using conventional organic solvents and concentrated sulfuric acid for sulfonation, water as selected

189 Chapter 7 as the solvent and also no sulfonation was needed. A series of membranes were synthesized using this methodology and different preparation conditions were investigated in order to optimize the synthesis parameters. Polymerization successfully took place onto and within the membranes, and the support did not take part in the reactions. Also, when cation exchange resin was not used, ion exchange capacity (IEC) was determined by two factors, including the mass ratio of divinylbenzene and styrene to lithium p-styrenesulfonate, and loading ratio of functional materials. In addition, water played an important role in membrane synthesis. IEC could be greatly increased by reducing the amount of water used for synthesis. More importantly, high IEC could be achieved without cation exchange resin, and therefore the synthesis procedure could be more efficient and the synthesized membranes could be more homogeneous. Finally, the synthesized membranes demonstrated high ED performance. It is envisioned that this methodology may open up new possibilities for synthesis of ion exchange membranes in an effective and environment-friendly way. In order to further improve this methodology, more studies on membrane stability should be carried out in future research.

190

Chapter 8 Conclusions and Recommendations for Future Work

Ion exchange membranes (IEMs) with a particular focus on synthesis of cation exchange membranes (CEMs) were studied in this thesis.

Firstly, a comprehensive statistical study was carried out on the research of IEMs via a scientometric approach. It was found that, from 2001 to 2016, over 18000 articles were published on IEMs, indicating researchers' great interest in this topic.

Especially, the number of articles published in 2016 was around six times that of articles published in 2001. In comparison, the total number of articles published in all research fields/topics in 2016 was only about twice that of the total articles published in 2001. Also, these articles were spread across over 1000 different journals, nearly 100 countries/regions and over 5000 research institutes, revealing the importance of IEMs as well as the broad research interest in this field.

Furthermore, keywords analysis revealed that “fuel cell” and “proton exchange membrane” had the highest cooccurrence frequency. Finally, research areas analysis revealed that IEMs had a close relation with chemistry, energy and materials.

The scientometric approach adopted in this thesis has its limitations. For example, the data used for analysis cannot be updated in real-time. To elaborate, the data used in this thesis for scientometric study was obtained in December 2017 so all the results are up to that time. Therefore, it is recommended that future research should focus on solving this problem. One feasible solution is to apply artificial intelligence or machine learning techniques to realize real-time update of the results. 191

Chapter 8

Secondly, in this thesis, different methods to prepare CEMs were discussed. It was found that membranes prepared without support was very fragile. Therefore, a support was necessary. Also, it was found that linear functional polymers (e.g., sulfonated polystyrene) were not suitable for making CEMs because the polymers would be dissolved in water. In order to prepare good CEMs, the functional polymers had to be cross-linked. It was further concluded that methods involving chemical reactions (e.g., polymerization or/and sulfonation) were necessary which could introduce new bonds to make crosslinking happen.

Thirdly, a new methodology was proposed for making CEMs with high IEC by controlling the mass ratio of sulfonated polystyrene to porous membrane support.

The polymerization and sulfonation reactions successfully took place in membrane support. The synthesized membranes demonstrated superior IEC (up to 3 mmol/g), attributing to high amount of functional polymers introduced in the synthesized membranes. Especially, theoretical IEC threshold value corresponded well with experimental threshold value. Further, sulfonate groups were distributed both on membrane surface and across the membranes, which agreed well with high IEC of synthesized membranes. Besides, the semi-finished membrane demonstrated hydrophobic property due to formation of polystyrene.

By comparison, the final membranes showed super hydrophilic property as well as high water uptake. In addition, when sulfonation reaction time increased, the conductivity of membranes also showed a tendency to increase, revealing the positive relationship between conductivity and IEC.

Although the synthesized CEMs demonstrated high IEC, the membranes also had high water uptake and swelling. High IEC is preferable, but high water uptake is

192 Chapter 8 not. High water uptake usually indicates high membrane swelling and low dimension stability. Therefore, for ED demonstration, lower sulfonation time is adopted to reduce water uptake and swelling. However, reduced sulfonation also corresponds to reduced IEC. So one interesting future research direction is to explore ways to maintain high IEC of membranes while lowering the water uptake.

Fourthly, another new and effective methodology to prepare high performance

CEMs was proposed and studied. Instead of using conventional organic solvents and concentrated sulfuric acid for sulfonation, water as selected as the solvent and also no sulfonation was needed. In order to optimize the synthesis parameters, a series of membranes were synthesized using this methodology and different preparation conditions were investigated. Polymerization successfully took place onto and within the membranes, and the support did not take part in the reactions.

Also, when cation exchange resin (CER) was not used, IEC was determined by two factors, including the mass ratio of divinylbenzene and styrene to lithium p- styrenesulfonate, and loading ratio of functional materials. In addition, water played an important role in membrane synthesis. IEC could be greatly increased by reducing the amount of water used for synthesis. More importantly, high IEC could be achieved without CER, and therefore the synthesis procedure could be more efficient and the synthesized membranes could be more homogeneous.

Finally, the synthesized membranes demonstrated high ED performance.

It is envisioned that the one-step synthesis methodology proposed above may open up new possibilities for synthesis of IEMs in an effective and environment- friendly way. In order to further improve this methodology, more studies are needed. For example, future research should systematically study membrane

193 Chapter 8 stability. Especially, future research should focus on improving the stability of membranes in dilute solutions. Besides, one interesting future research topic is to investigate the interactions between CER and DVB. This is because when CER is introduced into DVB, its colour changes to black, so possibly this is not a simple physical mixture. In other words, it is possible that unwanted chemical reactions happen after the two chemicals are mixed together.

194

List of Publications

[1] Shanxue Jiang and Bradley P. Ladewig, High performance cation exchange

membranes synthesized via in-situ emulsion polymerization without

organic solvents and corrosive acids, Journal of Materials Chemistry A, vol. 7,

no. 29, pp. 17400–17411, 2019. (IF: 10.733)

[2] Shanxue Jiang and Bradley P. Ladewig, Green synthesis of polymeric

membranes: recent advances and future prospects, Current Opinion in Green

and Sustainable Chemistry, vol. 21, pp. 1-8, 2020.

[3] Shanxue Jiang and Bradley P. Ladewig, High Ion-Exchange Capacity

Semihomogeneous Cation Exchange Membranes Prepared via a Novel

Polymerization and Sulfonation Approach in Porous Polypropylene, ACS

Applied Materials & Interfaces, vol. 9, no. 44, pp. 38612–38620, 2017.

(IF: 8.456)

[4] Shanxue Jiang, Kimberly F. L. Hagesteijn, Jin Ni, and Bradley P. Ladewig, A

scientometric study of the research on ion exchange membranes, RSC

Advances, vol. 8, no. 42, pp. 24036–24048, 2018. (IF: 3.049)

[5] Shanxue Jiang, Yuening Li, and Bradley P. Ladewig, A review of reverse

osmosis membrane fouling and control strategies, Science of the Total

Environment, vol. 595, pp. 567–583, 2017. (IF: 5.589)

[6] Kimberly F. L. Hagesteijn, Shanxue Jiang, and Bradley P. Ladewig, A review

of the synthesis and characterization of anion exchange membranes, Journal

195

List of Publications

of Materials Science, vol. 53, no. 16, pp. 11131–11150, 2018. (IF: 3.442)

[7] Benjamin Slater, Zeru Wang, Shanxue Jiang, Matthew R. Hill, and Bradley P.

Ladewig, Missing Linker Defects in a Homochiral Metal-Organic Framework:

Tuning the Chiral Separation Capacity, Journal of the American Chemical

Society, vol. 139, no. 50, pp. 18322–18327, 2017. (IF: 14.695)

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Appendices

Appendix 1 Copyright Permissions

Below are permissions to reuse materials from the published articles where

Shanxue Jiang is the first author.

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Appendices

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Appendix 2 Standard Solution Calibration for ICP Measurement

Calibration of sodium standard solution for ICP measurement. The coefficient reached 0.99991, which is an excellent result.

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