© 2020 Yongrui Yang ALL RIGHTS RESERVED

Yongrui Yang

i FLEXIBLE SUPERCAPACITORS WITH NOVEL GEL ELECTROLYTES

A Thesis

Presented to

The Graduated Faculty of The University of Akron

In Partial Fulfillment

of the Requirement for the Degree

Master of Science

Yongrui Yang

May 2020

i FLEXIBLE SUPERCAPACITORS WITH NOVEL GEL ELECTROLYTES

Yongrui Yang

Thesis

Approved: Accepted:

______Advisor Department Chair Dr. Xiong Gong Dr. Mark D. Soucek

______Committee Member Dean of the College Dr. Kevin A. Cavicchi Dr. Ali Dhinojwala

______Acting Dean, Graduate School Committee Member Dr. Marnie Saunders Dr. Weinan Xu

______Date

ii ABSTRACT

In this thesis, we study flexible supercapacitors. Recent progress in flexible supercapacitors is reviewed in chapter I. Ionic liquid gel electrolytes used for approaching high-energy density supercapacitors are investigated in chapter II. In chapter III, we study the double-network hydrogel with redox additives as the solid- state electrolytes for flexible supercapacitors

In Chapter I, after a brief introduction to the current situation of flexible supercapacitors and the mechanisms of different supercapacitors, we summarize the recent progress in both electrode materials and electrolyte materials for flexible supercapacitors, which includes carbon materials, conducting polymers, transition metal oxides/chalcogenides/nitrides, MXenes, metal-organic frameworks (MOFs), and covalent-organic frameworks (COFs) for electrode materials and the polymer-based gel electrolytes with different supporting electrolytes. In the end, we give a brief discussion on current challenges and an outlook for the future directions.

In Chapter II, a novel 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) : poly(vinyl alcohol) (PVA) : tetrabutylammonium tetrafluoroborate (TBABF4)

(BMIMBF4:PVA:TBABF4) is developed for enhancing the energy density of supercapacitors. It is found that the BMIMBF4:PVA:TBABF4 gel electrolytes exhibit an ionic conductivity of 21.7 mS/cm, which is much higher than that of

BMIMBF4:PVA gel electrolyte. The quasi-solid-state asymmetric supercapacitors assembled by using the BMIMBF4:PVA:TBABF4 gel as electrolyte, and MnO2 coated carbon cloth as the positive electrode and reduced graphene oxide coated on carbon cloth as the negative electrode, respectively, exhibit a wide operational window of 3V, iii an energy density of 61.2 Wh/kg at the power density of 1049 W/kg, an operational temperature ranging from 20 oC to 90 oC, and more than 80% capacitance retention after 3000 charge/discharge cycles.

In Chapter III, we developed pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes for approaching high energy density and flexibility of supercapacitors. The pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes exhibit superior mechanical strength, good flexibility, and fast self-recovery performances. The symmetric supercapacitors (SSCs) constructed by using pHEAA-gelatin-chitosan

Na2MoO4 DN hydrogel as the quasi-solid-state electrolytes and carbon cloth as electrodes show an energy density of 0.07 mWh/cm3 (~34 Wh/kg) at the power density of 3.8 mW/cm3 (~1800 W/kg), and more than 95% capacitance retention after 3000 charge/discharge cycles.

iv ACKNOWLEDGEMENT

First of all, I am grateful to my advisor Prof. Xiong Gong for his kindness and continuous support. His unselfish and wise suggestions teach me not only how to do research and study but also to be a good person.

Then I would like to show my extreme gratitude to my beloved parents for their love, support, and enouncement all the time.

Then I would like to give my appreciation to my group mates: Mr. Zhihao Ma, Mr.

Luyao Zheng, Mr. Tao Zhu, Mr. Xiuyuan Zhang, Mr. Baosen Zhang, Mr. Suyuan Zhou

Mr. Cheng Chi, and Mr. Sian Xiao, especially for Mr. Zhihao Ma, Mr. Luyao Zheng, and Mr. Tao Zhu for their selfless help and encouragement.

Finally, I want to express my gratitude to all the faculty members and my classmates in the Department of Polymer Engineering for their sincere guidance and suggestions.

v TABLE OF CONTENTS

LIST OF TABLES…………………………………………………………………….ix

LIST OF FIGURES……………………………………………………………………x

LIST OF SCHEMES………………………………………………………………...xvi

CHAPTER

I. RECENT PROGRESS IN SOLID-STATE FLEXIBLE SUPERCAPACITORS

1.1 Introduction ...... 1

1.2 Mechanisms of supercapacitors ...... 3

1.2.1 Electric double-layer capacitors ...... 3

1.2.2 Pseudocapacitors ...... 4

1.2.3 Hybrid-capacitors ...... 5

1.2.4 Device performance parameters used for evaluation of supercapacitors ...... 6

1.3 Materials for high-performance flexible electrodes ...... 9

1.3.1 Carbon materials ...... 10

1.3.2 Conducting polymers...... 13

1.3.3 Metal oxides/chalcogenides/nitrides ...... 17

1.3.4 MXenes ...... 24

1.3.5 Metal-organic frameworks (MOFs) ...... 28

1.3.6 Covalent-organic frameworks (COFs) ...... 30

vi 1.4 Polymer based solid-state electrolyte materials for flexible supercapacitors ...... 32

1.4.1 Aqueous gel electrolytes ...... 32

1.4.2 Organic gel electrolytes ...... 36

1.4.3 Ionic liquid gel electrolytes...... 39

1.5 Summary and Outlook ...... 43

II. QUASI-SOLID-STATE ASYMMETRIC SUPERCAPACITORS WITH

NOVEL IONIC LIQUID GEL ELECTROLYTES

2.1 Introduction ...... 45

2.2 Experimental section ...... 46

2.2.1 Materials ...... 46

2.2.2 Synthesis of GO ...... 47

2.2.3 Synthesis of TBABF4:PVA:BMIMBF4 gels ...... 47

2.2.4 Preparation of the electrodes ...... 48

2.2.5 Fabrication of ASCs ...... 48

2.2.6 Characterization of ASCs ...... 48

2.3 Results and discussion ...... 49

2.4 Conclusions ...... 55

III. FLEXIBLE SUPERCAPACITORS WITH DOUBLE-NETWORK

HYDROGEL REDOX-ELECTROLYTES

vii 3.1 Introduction ...... 57

3.2 Experimental section ...... 59

3.2.1 Materials ...... 59

3.2.2 Preparation of the pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel

electrolytes ...... 59

3.2.3 Tensile measurement ...... 60

3.2.4 Hysteresis measurement ...... 60

3.2.5 SEM-EDX measurement ...... 60

3.2.6 FTIR-ATR Spectroscopy ...... 60

3.2.7 Preparation and electrochemical characterization of SSCs ...... 60

3.3 Results and Discussion ...... 62

3.4 Conclusion ...... 74

REFERENCES ...... 76

APPENDICES ...... 81

viii LIST OF TABLES

Table 1.1 Recent progress in CPs based supercapacitors…….……………………….17

Table 1.2 Recent progress in supercapacitors with metal oxides/chalcogenides/nitrides electrodes……………………………………………………………………………..23

Table 1.3 Recent progress in supercapacitors with MXene electrodes……………..…27

Table 1.4 Recent progress in supercapacitors with MOF/COF electrodes………..…..31

ix LIST OF FIGURES Figure 1.1 Figure 1.1: (a) Schematic illustration of ripple-like ruthenium oxide

(RuO2)/carbon nanotube (CNT) composites. Reprinted with permission from ref. 148,

(c) schematic illustration of FeOF/Ni(OH)2 core-shell composites. Reprinted with permission from ref. 168, Copyright 2017 Wiley-VCH Verlag GmbH & KGaA; (d) cyclic stability of the supercapacitors based on ultralong MnO2 nanowire composite electrodes. Reprinted with permission from ref. 163, Copyright 2018 Wiley-VCH

Verlag GmbH & KGaA; (e) cyclic stability of the supercapacitors based on porous

Verlag GmbH & KGaA; (h) schematic illustration of the intercalation behaviors between 1T phase MoS2 and the ions in the electrolytes. Reprinted with permission from ref. 187, Copyright 2015, Nature Publishing Group…………………………….21

Group; (b) schematic illustration of MXene/rGO hybrids. Reprinted with permission from ref. 64, Copyright 2017 Wiley-VCH Verlag GmbH & KGaA; (c) schematic illustration of the charge storage process of hydrated electrolyte ions in MXenes (left) x and N-doped MXenes (right). Reprinted with permission from ref. 224, Copyright 2017

Elsevier; (d) the left part is a schematic graph of Polypyrrole (PPy) intercalated into the l-Ti3C2 layers. the right part is the atomic-scale of the schematic illustration; (e)

VCH Verlag GmbH & KGaA; (f) CV graphs of the asymmetric sandwich device of

RuO2//Ti3C2Tx at different scan rates; (g) Ragone plot of the RuO2//Ti3C2Tx device in comparison to the other microsupercapacitors; (h) Coulombic efficiency and cycling stability of the asymmetric RuO2//Ti3C2Tx device. (f), (g), and (h) reprinted with permission from ref. 225, Copyright 2018 Wiley-VCH Verlag GmbH & KGaA……..24

Figure 1.3: (a) The left part is the idealized schematic illustration of relative size of pores in Ni3(HITP)2 MOF. White, brown, grey, blue, lime and green spheres represent

H, B, C, N, F and Ni atoms, separately. The right part is the molecular structure of

Ni3(HITP)2; (b) Comparison of the areal capacitance of Ni3(HITP)2 MOF with other

EDLC electrode materials (a), (b) reprinted with permission from ref. 235, Copyright

2017, Nature Publishing Group; (c) Synthetic route, SEM image and idealized schematic illustration of nMOF-867. (d) Idealized schematic illustration of nMOF supercapacitors (e) Relationship of stack capacitance of nMOF-867, activated carbon and graphene versus current density, inset is the schematic image of nMOF supercapacitors. (c), (d), and (e) reprinted with permission from ref. 236, Copyright

2014 American Chemical Society; (f) stability tests of the asymmetric supercapacitors.

(g) schematic routes of the synthesis of Dq-2,6-diaminoanthraquinone, Da-2,6- xi diaminoanthracene, and Tp-1,3,5-triformylpholoroglucinol; (h) CV curves of CT-

Figure 1.4: (a) Bending stability of supercapacitor with MnOx electrodes and PVA-

Elsevier; (b) schematic illustration of supercapacitor with bio-inspired, quasifractal metallic network electrodes and LiCl-PVA gel electrolytes. Reprinted with permission from ref. 294, Copyright 2019 Wiley-VCH Verlag GmbH & KGaA; (c) bending stability of supercapacitor with boron element nanowire electrodes and PVA-H2SO4 electrolytes. Reprinted with permission from ref. 295, Copyright 2018 Wiley-VCH

Verlag GmbH & KGaA; (d) the ionic conductivity of H5BW12O40 (BWA) electrolytes.

(e) schematic illustration of supercapacitor with NiCo2O4@carbon nanotube (CNT) electrodes and PVA-KOH gel electrolytes. Reprinted with permission from ref. 298,

Copyright 2017 Wiley-VCH Verlag GmbH & KGaA; (f) CV curves of different polymer matrices in solid-state electrolytes with porous carbon cathode and SnO2-TiO2 anode. Reprinted with permission from ref. 299, Copyright 2019 American Chemical

Society; (g) schematic illustration of grafted PVA-KOH gel electrolytes. Reprinted with permission from ref. 99, Copyright 2019 Wiley-VCH Verlag GmbH & KGaA; (h) stretchability of poly(AMPS-co-DMAAm)/Laponite/Graphene oxide (GO) hydrogel electrolytes. Reprinted with permission from ref. 300, Copyright 2019, Nature

Publishing Group……………………………………………………………………33

Society; (b) CV curves of CNT/DmFc-TBAP-THF/CNT devices. Reprinted with permission from ref. 309, Copyright 2014 American Chemical Society; (c) the gas evolution behavior of AN, PC, and GBL as the solvents in the electrolytes. Reprinted with permission from ref. 313, Copyright 2008 Elsevier; (d) Nyquist plot of NaTFSI-

PC, PEO-NaTFSI-PC, and PEO-PC-EC-DMC-NaTFSI as electrolytes. Reprinted with permission from ref. 314, Copyright 2014 American Chemical Society; (e) ionic conductivity of the modified PVDF/LiTFS polymer electrolytes. Reprinted with permission from ref. 315, Copyright 2019 Elsevier…………………………………..37

Figure 1.6: (a) CV curves of supercapacitor with graphene doped carbon materials and

GO-EMIMBF4-(P(VDF-HFP) ion gel as electrodes and electrolytes, respectively.

EMIMTFSI as electrodes and electrolyte, respectively. Reprinted with permission from ref. 343, Copyright 2017 American Chemical Society; (c) the ionic conductivity of 1- ethyl-3-methylimidazolium tetracyanoborate (EMIMTCB) entrapped in poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) gel electrolytes.

(d) the temperature range of the mixture of (PIP13FSI) and (PYR14FSI). Reprinted with permission from ref. 347, Copyright 2011 American Chemical Society; (e) schematic illustration of the intercalation pseudocapacitance provided by EMIM+ and FeOOH.

Chemistry…………………………………………………………………………….42

Figure 2.1 (a), the high and (b) low magnification SEM images of the BMIMBF4-PVA gel electrolytes, (c), the high and (d) low magnification SEM images of

BMIMBF4:PVA:TBABF4 gel electrolytes (a BMIMBF4:TBABF4 molar ratio of

4:1)………………………………………………………………………….………..49

Figure 2.2 (a) the CV curves of BMIMBF4:PVA:TBABF4 gel electrolytes, (b) the

Nyquist plots of BMIMBF4:PVA:TBABF4 and BMIMBF4:PVA gel electrolytes, (c) the CV curves and (d) the Nyquist plots of supercapacitors with

BMIMBF4:PVA:TBABF4 gel electrolytes at different working temperatures……....51

Figure 2.3 (a) the CV and (b) the GCD curves of the MnO2@carbon cloth electrode, (c) the CV and (d) the GCD curves of the rGO@carbon cloth electrode, (e) the cycling stability of the MnO2@carbon cloth and the rGO@carbon cloth electrodes……….…52

Figure 2.4 (a) Schematically illustration of MnO2@carbon cloth//BMIMBF4:PVA:TBABF4//rGO@carbon cloth ASCs, (b) the CV curves, (c) the

GCD curves and (d) the cycling stability of MnO2@carbon cloth//BMIMBF4:PVA:TBABF4//rGO@carbon cloth ASCs………………………………54

xiv recovery of pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes with six loading-unloading cycles……………………………………………………………..62

Figure 3.2. a) The SEM images and (b) The EDX mapping images of pHEAA-gelatin- chitosan Na2MoO4 DN hydrogel electrolytes, c) The ATR-FTIR spectra of DN electrolytes with and without Na2MoO4, respectively………………………………...65

Figure 3.3. a) The CV curves and b) The Nyquist plots of ACC//DN electrolytes//ACC

SSCs with different supporting electrolytes; c) The CV curves and d) The Nyquist plots of ACC//Na2MoO4-DN electrolytes//ACC SSCs with different acidic supporting electrolytes…………………………………………………………………………...67

Figure 3.4. a) The CV curves of ACC//Na2MoO4-DN electrolytes//ACC SSCs at different scan rates; b) The GCD curves of ACC//Na2MoO4-DN electrolytes//ACC

SSCs at different current densities; c) The specific capacitance of ACC//Na2MoO4-DN electrolytes//ACC SSCs at different charging-discharging current densities; d) The cycling stability of ACC//Na2MoO4-DN electrolytes//ACC SSCs…………………...69

o SSCs bended at 90 for 1000 times; d) The tensile strain-stress curves of Na2MoO4-DN electrolytes before and after 1000 times bending. e) The CV curves of ACC//Na2MoO4-

DN electrolytes//ACC SSCs after being bleached in pure H2SO4 solution without

2- MoO4 for 24 hours…………………………………………………………..………72

xv LIST OF SCHEMES

Scheme 1.2: (a) Helmholtz, (b) Gouy-Chapman, and (c) Stern model of the electrical double-layer formed at a positively charged electrode in an aqueous electrolyte.

KGaA………………………………………………………………………………….4

Scheme 1.3: Different types of reversible redox mechanisms that give rise to pseudocapacitance: (a) underpotential deposition, (b) redox pseudocapacitance, and (c) intercalation pseudocapacitance. Reprinted with permission from ref. 35, Copyright

2013 Springer Science & Business Media……………………………………………..5

Wiley-VCH Verlag GmbH & KGaA; (b) schematic illustration of supercapacitor with cellulose nanofibril (CNF)/reduced graphene oxide (RGO)/carbon nanotube (CNT) hybrid aerogel electrodes. Reprinted with permission from ref. 86, Copyright 2015

American Chemical Society; (c) processing of large-area hierarchical graphene films.

Reprinted with permission from ref. 96, Copyright 2019 Wiley-VCH Verlag GmbH & xvi KGaA; (f) schematic illustration of Ni microtube/carbon nanosphere (CNSs) composites. Reprinted with permission from ref. 100, Copyright 2015 Wiley-VCH

Verlag GmbH & KGaA………………………………………………………………10

Scheme 1.6: (a) Schematic illustration of graphene/PANi fabric composites. Reprinted with permission from ref. 117, Copyright 2015 Royal Society of Chemistry; (b) schematic illustration of the tandem graphene/PANi supercapacitors. Reprinted with permission from ref. 118, Copyright 2015 Elsevier; (c)schematic illustration of the cellulose-mediated PEDOT: PSS/MWCNT composite films. Reprinted with permission from ref. 122, Copyright 2017 American Chemical Society; (d)schematic illustration of the processing of conducting PEDOT/cellulose paper flexible electrodes.

(e) the structure of P(DEBT/TETPA). Reprinted with permission from ref. 125,

Scheme 1.7: The structures of typical IL ions. Reprinted with permission from ref. 312,

xvii CHAPTER I

RECENT PROGRESS IN FLEXIBLE SOLID-STATE SUPERCAPACITORS

1.1 Introduction

With the decreasing availability of fossil fuel and the developing of the global economy, the eco-friendly, renewable, sustainable energy sources, and the technologies associated with energy storage play more and more important roles. In this case, rechargeable electrical energy storage devices which store energy by the transition between chemical energy and electrical energy have attracted more and more attention.1,

2 There are three main types of rechargeable energy storage devices, which are capacitors, electrochemical batteries and supercapacitors (SCs). Scheme 1.1 presents the ragone plot of various electrical energy storage devices. In the ragone plot, rechargeable batteries have been widely developed due to their high energy density.

However, there are some limitations of the rechargeable batteries, such as low power

Scheme 1.1: Ragone plot of various electrical energy storage devices. Reprinted with permission from ref. 3, Copyright 2008, Nature Publishing Group. density and short cyclic stability. In order to address these issues, supercapacitors which present higher power density than batteries, higher energy density than traditional

1 capacitors, and excellent cyclic stability3-5 become a bridge filling the gap between batteries and traditional capacitors with the preponderance in both energy density and power density. For supercapacitors, the main difference from batteries is the surface charge storage mechanism comparing to the bulk charge storage in batteries.6-9

The flexible energy storage devices play crucial roles in the wearable and portable electronics, such as biosensors10, 11, nanorobots12, 13, wearable energy conversion devices14, 15, self-powered systems16, 17, and so on, which cause an urgent demand for lightweight, flexible, long-life term, high-performance energy storage devices. In order to power these wearable and portable devices, the energy storage devices are required both high energy density and power density to provide long-time energy supply and high-rate pulse current, respectively.15, 18-20 As promising candidates for next- generation flexible energy storage devices, flexible solid-state supercapacitors have attracted abundant attention in both industry and academia. Flexible solid-state supercapacitors usually consist of flexible thin-film electrodes incorporated soft current collectors and gel polyelectrolytes serving as both electrolytes and separators.

Comparing to traditional commercial supercapacitors which need to be sealed in solid metallic crust, the solid-state supercapacitors can provide excellent flexibility and efficiently prevent the leakage of the toxic organic liquid electrolytes.21-24

Currently, there are still some challenges in flexible solid-state supercapacitors, such as finding high pseudocapacitive electrode materials and super-flexible electrolyte materials with excellent ionic conductivity.24-27 In this review, we summary the recent development in the flexible supercapacitors. In the second part, we give a brief introduction of the different energy storage mechanisms of supercapacitors. In the third part, we review the recent development in the flexible electrode materials, in particular pseudocapacitive nanomaterials including conducting polymers, transition metal

2 oxides/chalcogenides/nitrides, MXenes, metal-organic frameworks (MOFs), and covalent-organic frameworks (COFs). In the next part, we summary the recent progress on the flexible gel electrolyte in both aqueous and organic systems. Finally, we give a summary on the existing problems and an outlook for the future development in flexible supercapacitors.

1.2 Mechanisms of supercapacitors

Before the discussion of materials, it’s better to introduce the mechanism of supercapacitors. In this part, we will list three different mechanism of supercapacitors and give a brief introduction.

1.2.1 Electric double-layer capacitors

The mechanism of electric double-layer capacitors (EDLCs) can be described by three models shown in Scheme 1.2.28 The energy stored by EDLC is achieved by the electrostatic adsorption and desorption of ions on the interface between electrodes and electrolytes. The Helmholtz model is the simplest one to describe this behavior arose by H. Helmholtz in 1853.29 As shown in Scheme 1.2a, when the electrode surface is polarized, the ions carried opposite charge in the electrolyte will drift to the interface between electrodes and electrolytes then form a several-nanometer condensed layer.

The capacitance of this double-layer can be defined by Equation 1:

퐶 = 휀 휀 푆 (1) 퐻 0 푟 푑

-12 In Equation 1, 휀0 is the vacuum permittivity (8.854*10 F/m), 휀푟 stands for the relative permittivity of the dielectric electrolyte, d is the thickness of the electric double layer, S is the area of the interface between electrode and electrolyte. For the Helmholtz model, d usually is in nanometer magnitude, if choose the large-surface material as the electrodes, the total capacitance will be much larger than the traditional parallel plate capacitors. Based on the Helmholtz model, Gouy and Chapman investigated a new

3 model (shown in Scheme 1.2b) which considers the diffusion behaviors of ions and the interaction between the electrodes and the solvents.30 This model pointed that the electrical potential will decrease in an exponential way from the potential of electrode

(Φe) to the potential of the bulk solution (Φs). Stern combined the two former model and arose a new model (shown in Scheme 1.2c) which can fit the highly charged double-

Scheme 1.2: (a) Helmholtz, (b) Gouy-Chapman, and (c) Stern model of the electrical double-layer formed at a positively charged electrode in an aqueous electrolyte. Reprinted with permission from ref. 28, Copyright 2014 Wiley-VCH Verlag GmbH & KGaA. layer.31 The stern plane shows the minimum distance between the ions and the electrode surface.

1.2.2 Pseudocapacitors

Comparing to EDLCs, Pseudocapacitors or Faradaic supercapacitors store energy in the fast and reversible redox reactions at the surface of active materials, which is similar to the process in the batteries.32-34 As shown in Scheme 1.335, pseudocapacitors can be divided into three types by different types of redox reactions. (1) Underpotential

4 deposition is a kind of reversible adsorption of metal ions on the different metal surface36, such as the adsorption between gold and lead ions as shown in Scheme 1.3a36,

37; (2) Redox pseudocapacitance usually happens on the surface of transition metal chalcogenides/nitrides and conducting polymers with the faradaic charge-transfer, such

38-40 as the pseudocapacitive behavior of RuO2 (shown in Scheme 1.3b) . (3) Intercalation pseudocapacitance occurs when the ions (like Li+ shown in Scheme 1.3c) intercalate into the tunnels of the redox-active materials.33, 41 The materials storing energy by intercalation pseudocapacitance usually have structure that will not undergo crystallographic phase transformation. In this way, although some part of the charge is stored in the bulk materials, the electrode can still show the pseudocapacitance behaviors.42

1.2.3 Hybrid-capacitors

Hybrid capacitors usually contains an electrical double-layer (EDL) electrode and the other pseudocapacitive or battery-type electrode as shown in Scheme 1.4.25 So the

5 concept ‘‘hybrid capacitor’’ can only be used to describe the whole supercapacitors, not electrodes.43 These asymmetric supercapacitors can provide both high energy density and power density, such as MXene//Na+, active carbon//Na+, carbon//Li+.44-47

When these kind of devices under working condition, the surface adsorption/desorption

Scheme 1.4: Schematic diagram of a hybrid-capacitor. Reprinted with permission from ref. 25, Copyright 2015 Royal Society of Chemistry. of the EDLC electrode and the intercalation/deintercalation of the battery-type electrode occur simultaneously, which can deliver high energy density at high power density without the degradation in the lifetime.48-51

1.2.4 Device performance parameters used for evaluation of supercapacitors

Capacitance: For the whole devices of supercapacitors, the capacitance of two electrodes can be considered as two capacitors in series. If the capacitance of the positive electrode and negative electrode is Cp and Cn, respectively, the overall capacitance CT can be described by Equation 2:

6 1 1 1 = + (2) 퐶푇 퐶푝 퐶푛

If the two electrodes are same (Cp=Cn=Ce), the overall capacitance is half of the single electrodes. From Equation 2, it is clear that for the asymmetric structure (two electrodes are different), the overall capacitance mainly depends on the smaller one. The capacitance of a real supercapacitor will be influenced by the amount of electrode materials. So it is necessary to use the specific capacitance to describe the performance of the supercapacitors. The specific capacitance can be described by Equation 3,

퐶푇 퐶푠 = (3) 푚푒 in which Cs means the specific capacitance, CT is the overall capacitance of the single electrodes or the whole devices, and me is the mass of the active materials on the electrodes. For the active mass, some works use the total mass including the binders, current collectors, or other additives, to be the denominator of the specific capacitance, which should be pointed before discussing the device performance. The specific capacitance usually is calculated by Equation 4 from the galvanostatic charge/discharge test:

∫ 퐼×푑푡 퐶 = (4) 푠 ∆푉

In Equation 4 Cs stands for the specific capacitance, I is the current density in galvanostatic discharge test, dt is the differential of the time, ΔV means the discharge potential range.

Energy density: Energy density means how much electric energy can be stored in the supercapacitors, which can also be calculated from the galvanostatic charge/discharge test. For the devices with linear charge/discharge curves, the energy density can be evaluated form Equation 5: 7 1 퐸 = 퐶 푉2 (5) 2 푠

Where Cs is the specific capacitance, V is the operation potential window in charge/discharge curves.

For some asymmetric devices with nonlinear charge/discharge curves, the energy density should be calculated by Equation 6:

퐸 = 퐼 ∫ 푉(푡)푑(푡) (6)

Where I is the constant current density in galvanostatic discharge curves, V(t) is the function of potential versus time, and d(t) is the differential of the time.

Power density and equivalent series resistance (ESR): Compared with energy density, power density stans for how much power can be transmitted during specific time, which can be calculated by Equation 7:

푉2 푃 = (7) 4퐸푆푅

Where V is the charged potential of supercapacitors, and ESR means the equivalent series resistance. ESR can be influenced by (1) the resistance from the intrinsic electrodes, electrolytes, and current collectors; (2) the resistance form the interface between current collectors and electrodes, electrodes and electrolytes; (3) the ion transport resistance in electrolytes and separators.52, 53 ESR can be get from the sum of the series resistance (Rs) and the charge-transport resistance (Rct) in the electrochemical impedance spectrum (EIS).

Cyclic stability and self-discharging rate: Long-term operation stability is also one of the preponderances compared with other energy storage devices. Generally, the cyclic stability can be described by comparing the initial and final specific capacitance

8 of single electrodes or whole devices after continuous charging and discharging for certain cycles. The EDLC devices usually have an excellent cyclic stability due to the completely reversible physical charge-storage mechanism.54, 55 However, when introduce the redox pseudocapacitance into the systems, the cyclic stability will decrease due to some irreversible redox-reactions between electrode materials and electrolyte materials.56, 57 Besides electrode materials and electrolyte materials, the cyclic stability also depends on the charge/discharge current density, temperature, and cell type, which should be considered during the test.

Another factor related to the stability is the self-discharging rate, which describes the energy loss after being charged for a period of time.35 The current leakage will lead reduction on the potential of the devices, which will have negative influence on the long-term energy retention applications.

In the research of flexible supercapacitors, the main challenges are to develop the high- performance flexible electrodes and solid-state flexible electrolytes. In the following parts, we will give a review of materials for both flexible electrodes and solid-state electrolytes reported in recent years.

1.3 Materials for high-performance flexible electrodes

The capacitance and charge storage of supercapacitors are mainly related to the electrode materials used on account of the charge storage mechanism of supercapacitors.

To achieve the flexible supercapacitors, the key factor is to find the high-performance and flexible electrodes. In general, the electrode materials of supercapacitors can be fell into three categories: (1) carbon-based materials with high specific surface area;58, 59 (2) conducting polymers;60, 61 (3) metal oxides62, 63. Moreover, many novel materials such as Mxenes,64-67 metal-organic frameworks (MOFs)/covalent-organic frameworks

(COFs),68-71 transition-metal dichalcogenides (TMDs)72-74 are also introduced as the

9 electrode materials for the applications of supercapacitors.

1.3.1 Carbon materials

Carbon-based materials are considered as the most promising electrode materials for the industrialization of supercapacitors.5 Carbon-based materials such as graphene,75, 76 active carbon,4 and carbon nanotube77 have been used in supercapacitors due to their low cost, good electronic conductivity, high chemical stability, light weight, and wide operating temperature range.78-80 Carbon-based materials store the energy by adsorption and desorption of the charges on the interface between electrode and electrolyte. Therefore, specific surface area, pore size and structure, electrical conductivity are the important factors for carbon-based materials. Yao et al.81 investigated a flexible 2D supercapacitor based on hierarchical porous carbon (2D-HPC)

Scheme 1.5: (a) Schematic illustration of 2D supercapacitor based on hierarchical porous carbon (2D-HPC). Reprinted with permission from ref. 81, Copyright 2018 Wiley-VCH Verlag GmbH & KGaA; (b) schematic illustration of supercapacitor with cellulose nanofibril (CNF)/reduced graphene oxide (RGO)/carbon nanotube (CNT) hybrid aerogel electrodes. Reprinted with permission from ref. 86, Copyright 2015 American Chemical Society; (c) processing of large-area hierarchical graphene films. Reprinted with permission from ref. 92, Copyright 2015 Wiley- VCH Verlag GmbH & KGaA; (d) carbon nano-cages with various functional groups. Reprinted with permission from ref. 95, Copyright 2017 Elsevier; (e) hydrophilic carbon cloth (HCC). Reprinted with permission from ref. 96, Copyright 2019 Wiley-VCH Verlag GmbH & KGaA; (f) schematic illustration of Ni microtube/carbon nanosphere (CNSs) composites. Reprinted with permission from ref. 100, Copyright 2015 Wiley-VCH Verlag GmbH & KGaA. 10 nanosheets, as shown in Scheme 1.5a. The devices show a high surface area of 2406 m2/g, an energy density of 8.4 mWh/cm3 at a power density of 24.9 mW/cm3, as well as a good cyclic stability (96% retention after 10000 cycles). Moreover, the devices also achieve good machinal performance. After 10000 bending cycles, the devices can still keep 78% of the initial volumetric capacitance.

As a typical one-dimensional carbon material, carbon nanotube (CNT) is one of the most promising candidates for flexible electrodes due to the good electrical conductivity and mechanical performance.82-84 Based on these excellent properties,

CNT can be used as both EDLC materials and the support of pseudocapacitive materials in supercapacitors. For example, Li at el.85 anchored the activated carbon (AC) on a 3D framework built by carbon nanotube (CNT) a reduced graphene oxide (RGO). The supercapacitors fabricated by the self-supporting and flexible carbon electrodes show a specific capacitance of 101 F/g at a current density of 0.2 A/g, an energy density of 30.0

Wh/kg with a wide potential window from 0.001 V to 3.0 V in organic electrolytes.

Similarly, Zheng et al.86 developed a flexible all-solid-state supercapacitor with cellulose nanofibril (CNF)/reduced graphene oxide (RGO)/carbon nanotube (CNT) hybrid aerogel electrodes. The devices show a specific capacitance of 252 F/g at a current density of 0.5 A/g, and an energy density of 28.4 µWh/cm2 at the power density of 9.5 mW/cm2 with low cost and ease of large-scale manufacturing (as shown in

Scheme 1.5b).

Graphene is another promising material for the high-performance carbon electrodes in flexible supercapacitors. Since Geim and Novoselov exfoliated a single-layer graphene sheet in 2004,87 graphene have attracted much attention in both batteries and supercapacitors due to high electrical conductivity, large-surface 2D structures, wide operational window, and excellent mechanical flexibility.88-91 Xiong et al.92

11 investigated large-area hierarchical graphene films for the flexible supercapacitors by blade-casting of graphene hydrogel and reduction (as presented in Scheme 1.5c). The devices represent an energy density of 8.4 µWh/cm2 at the power density of 4.9 mW/cm2, a high capacitance retention of 96.7% after 5000 cycles. After being bent to

180º, the performance do not shows any degradation and the capacitance even shown an increase of 1.4%.

However, specific capacitance is not directly proportional to the specific surface area due to the access of the micropores in the electrode layers, which limit the transfer of the electrolyte ions. Therefore, the surface functional groups and heteroatoms can also assist the adsorption of the ions and improve the hydrophilicity/lipophilicity of the carbon materials, achieving a great wettability and rapid electrolyte ions transport within the micropores.5, 93, 94 Jiang et al.95 introduced various functional groups into the carbon nano-cages by nitric treatment (as shown in Scheme 1.5d). After introducing the oxygen-containing groups, the specific capacitance increased to 96 F/g compared to the value of 50 F/g before treatment. Chen et al.96 introduced a hydrophilic carbon cloth

(HCC) prepared by chemical oxidation and further hydrazine gas reduction process on commercial carbon cloth. HCC fibers exhibit a more rough surface compared with pristine carbon cloths, suggesting the modification of carbon fibers’ surface during the oxidation and reduction processes Moreover, HCC possesses completely hydrophilic surface with distinct liquid bead, indicating hydrophilic functional groups confirmed on HCC and a higher electrical conductivity than that of pristine carbon cloth as shown in Scheme 1.5e.

Carbon materials can also be used to combine with pseudocapacitive materials, such as conducting polymers and metal oxides to provide higher electrical conductivity and stability.97-99 For example, as shown in Scheme 1.5f, Xia et al.100 developed Ni

12 microtube/carbon nanosphere (CNSs) composites as the flexible and stable supercapacitor electrodes. The carbon materials provide supporting matrix for metallic microtubes, which enhances the stability of the metallic microtubes. The symmetric supercapacitors with metal/CNSs electrodes reach the specific capacitance of 227 F/g at a current density of 2.5 A/g, and the capacitance retention of 97% after 40000 cycles.

1.3.2 Conducting polymers

Conducting polymers (CPs) have many advantages as the candidates of pseudocapacitive electrode materials for supercapacitors such as, high voltage window, high storage capacity, and adjustable redox activity through chemical modification.101-

103 CPs offer the capacitance behavior by the redox process where ions are transferred to the polymer backbone when oxidation happens and released back to the electrolyte in the reduction process.26, 104, 105 Due to the entire bulk redox reactions without any structural alterations in charge and discharge process, CPs demonstrate a high capacitance and great reversibility.35, 106 CPs can be charged with ion insertion in the polymer matrix positively or negatively as p-type conducting polymers (p-CPs) and n- type conducting polymers (n-CPs).5 The common CPs for supercapacitor applications includ polyaniline (PANi)107, 108, polypyrrole (PPy)109, polythiophene (PTh)103, 110, poly(3,4-ethylenedioxythiophene) (PEDOT)111, 112 and their corresponding derivatives.

Unfortunately, the swelling and shrinking of CPs occur during the charging and discharging process, leading to the mechanical degradation of the electrodes and fading of the electrochemical performance. To mitigate the challenge of low stability, the composites of CPs with other materials is the most efficient approach.60, 80, 113 Table 1.1 summarizes recent advances in CPs based supercapacitors.

PANi is the one of the most promising materials for CPs-based supercapacitors, due to easy synthesis, good open-air stability, and reversable doping/de-doping process.114-

13 116 PANi can be easily polymerized in aqueous solution with different morphology. For example, Zang et al.117 synthesized a graphene/PANi fabric composites by electropolymerization. After adding PANi as pseudocapacitive materials, the specific capacitance increased from 2 to 23 mF/cm2 without significant decrease after 2000 cycles. The devices also show a stable mechanical performance. After 500 times bending, the specific capacitance remains stable (as shown in Scheme 1.6a). What’s more, Chen et al.118 reported the tandem graphene/PANi supercapacitors by directly growing graphene-PANi electrodes on the separators. The tandem supercapacitors demonstrate the energy density of 26.1 Wh/kg at the power density of 3000 W/kg, and excellent stability after 10000 cycles (as shown in Scheme 1.6b). Poly(3,4- ethylenedioxythiophene) (PEDOT) is another kind of promising conducting polymers for supercapacitors. The conjugated backbone of PEDOT allows the delocalized electron transport through the whole system, which leads to a good electrical conductivity.119-121 Scheme 6c reveals the cellulose-mediated PEDOT: PSS/MWCNT composite film as supercapacitor electrodes reported by Zhao et al.122 This composite

14 was assembled by incorporating multiwalled carbon nanotubes (MWCNTs) into the cellulose/PEDOT: PSS, which exhibited low resistance of 0.45 Ω, high specific capacitance of 485 F/g, and good cycling stability with a capacity retention of 95% after

2000 cycles. Similarly, Anothumakkool et al.123 developed a highly conducting

PEDOT/cellulose paper flexible electrode by interfacial polymerization at the interface of two immiscible liquids, which is shown in Scheme 1.6d. The all-solid-state supercapacitors fabricated by PEDOT/cellulose paper flexible electrodes and PVA-

H2SO4 electrolytes shows a specific capacitance of 115 F/g, an energy density of 1

15 mWh/cm3, and excellent stability after 3800 charge/discharge cycles.

Besides traditional conducting polymers, some new molecules with large conjugated system are also synthesized for the electrode materials of supercapacitors to get enhanced capacity and stability.113, 124 For example, Li et al.125 reported a conjugated polymer gel, poly(4,7-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5- yl)benzo[c][1,2,5] thiadiazole (DEBT)-co-tris(4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-

5-yl)phenyl) amine (TETPA), for the high performance electrode materials. The structure of P(DEBT/TETPA) is shown in Scheme 1.6e. The large conjugated skeleton improved both electrical conductivity and ionic conductivity. The electrodes show good specific capacitance of 149 F/g and a wide voltage window of 1.3 V, as well as the cyclic stability of 92% retention after 2000 cycles.

16 Table 1.1 Recent progress in CPs based supercapacitors

1.3.3 Metal oxides/chalcogenides/nitrides

Mental oxides, chalcogenides, or nitrides can provide a much higher energy density than conventional carbon-based materials and better electrochemical stability than conducting polymers.57, 136-141 These mental oxides/chalcogenides/nitrides store the energy not only like the electrostatic carbon materials but also show electrochemical redox reactions between electrode materials and electrolyte ions within the potential windows.44, 137, 142, 143

In general, there are several requirements for metal oxides/nitrides used as supercapacitors electrode materials:35 (1) the oxides/chalcogenides/nitrides should be electronically conductive; (2) the metal can exist in two or more oxidation states, which coexisting in a continuous range with no phase changes; (3) the protons can freely transport into the oxide/nitrides lattice on reduction. According to these requirements, the oxides/ chalcogenides/nitrides of ruthenium, manganese, cobalt, nickel, and 17 vanadium are commonly used as the electrode materials of supercapacitors.140, 144-146

Table 1.2 summarizes recent advances in supercapacitors with metal oxides/chalcogenides/nitrides electrodes.

Ruthenium oxide is considered as the earliest and most extensively studied metal oxide as electrode material in supercapacitors due to the wide potential window, highly reversible redox reactions, high proton conductivity and high specific capacitance.147-

149 148 Wang et al. introduced a ripple-like ruthenium oxide (RuO2)/carbon nanotube

(CNT) composite synthesized by using cathodic deposition technique, as shown in

Figure 1.1a. The composite exhibits a specific capacitance of 272 mF/cm2. Moreover, the symmetric cell, using the composite as the electrode materials, achieves a high specific capacitance as 37.23 mF/cm2 and specific power density of 19.04 mW/cm2.

Although RuO2 can provide an extremely high specific capacitance, the high cost and environmental harmfulness prevent it from the commercialization of supercapacitors.150 However, the toxicity and high cost of ruthenium restricts the further commercialization of the ruthenium based supercapacitors.62, 148, 151

Manganese oxide (MnO2) as an alternative material for ruthenium oxide has been rapidly developed, dur to the low cost, environmental friendly, low toxicity and great theoretical capacities ranging from 1100 to 1300 F/g depending on the redox reaction as expressed in Equation 8.152-155

+ - MnO2 + C + e ↔ MnOOC...... (8)

where C+ denotes the protons and alkali metal cations in the electrolyte, e- stands for the electrons. Moreover, Among the transition metal oxides, MnO2 has the easiest and lowest-cost approach of synthesizing, such as traditional hydrothermal method156-

158 and electrochemical deposition159-161. As shown in Figure 1.1b, the fibrous

162 PANI@MnO2 nanocomposite reported by Ansari et al. exhibits a high capacitance

18 of 525 F/g and good cycling stability of 76.9% after 1000 cycles. Besides forming composites with conducting polymers, Lv et al.163 directly waved the flexible electrodes with the ultralong MnO2 nanowire composites. The hive-like stretchable supercapacitors show a specific capacitance of 227.2 mF/cm2, 500% elongation without electrochemical performance decay, and 98% capacitance retention after 10000 stretch- and-release cycles under 400% tensile strain (as shown in Figure 1.1d).

Nickel oxides are considered as another promising alternative electrode materials for supercapacitors due to the ultra-high theoretical specific capacitance, nontoxicity, and low cost.164-166 The redox reaction of nickel oxides can be expressed as Equation 9:

NiO + OH- ↔ NiOOH- + e- (9)

The theoretical specific capacitance calculated from Equation 9 is 3750 F/g, which is much higher than the value of manganese oxides.167 Wang et al.168 developed

FeOF/Ni(OH)2 core-shell composites to fabricate flexible positive electrodes. FeOF has a low intrinsic resistance, which can enhance the charge transfer process. Large-surface

Ni(OH)2 provide enough capacitance for the asymmetric supercapacitors, which is shown in Figure 1.1c. The composite electrodes show a high specific capacitance of

169 1425 F/g at a current density of 1 A/g. Guan et al. also reported a porous NiCo2O4 nanostructure as flexible electrode material for supercapacitors. The full cells show an energy density of 31.9 Wh/kg at a power density of 2.9 kW/kg, and 86.7% capacitance retention after 20000 cycles (shown in Figure 1.1e).

Vanadium oxides are also promising candidates for the Faradic electrode materials due to the variety of the oxidation states of vanadium ions, low cost, and low toxicity.170-172 However the low electrical conductivity has become the bottle neck which restricts the further development of the vanadium based electrodes.173, 174 To enhance the electrical conductivity of the vanadium based electrodes, lots of Vanadium-

19 based composite have been developed.174, 175 For example, Foo et al.176 developed a flexible V2O5-rGO composite electrode. Due to the high electrical conductivity of rGO, the free-standing electrodes have a low sheet resistance without any current collectors.

The V2O5-rGO composite electrodes show specific capacitance of 178.5 F/g at a current density of 0.05 A/g, and a wide working

20 Figure 1.1: (a) Schematic illustration of ripple-like ruthenium oxide (RuO2)/carbon nanotube (CNT) composites. Reprinted with permission from ref. 148, Copyright 2015 Elsevier; (b) CV curves of fibrous PANI@MnO2 nanocomposites. Reprinted with permission from ref. 162, Copyright 2016 Royal Society of Chemistry; (c) schematic illustration of FeOF/Ni(OH)2 core-shell composites. Reprinted with permission from ref. 168, Copyright 2017 Wiley-VCH Verlag GmbH & KGaA; (d) cyclic stability of the supercapacitors based on ultralong MnO2 nanowire composite electrodes. Reprinted with permission from ref. 163, Copyright 2018 Wiley-VCH Verlag GmbH & KGaA; (e) cyclic stability of the supercapacitors based on porous NiCo2O4 electrodes. Reprinted with permission from ref. 169, Copyright 2017 Wiley-VCH Verlag GmbH & KGaA; (f) the Nyquist polt of supercapacitors based on V2O5-rGO composite electrodes. Reprinted with permission from ref. 176, Copyright 2014 Wiley-VCH Verlag GmbH & KGaA; (g) supercapacitors with corn- like TiN as the electrodes. Reprinted with permission from ref. 183, Copyright 2016 Wiley-VCH Verlag GmbH & KGaA; (h) schematic illustration of the intercalation behaviors between 1T phase MoS2 and the ions in the electrolytes. Reprinted with permission from ref. 187, Copyright 2015, Nature Publishing Group.

21 potential range of 1.6 V in nonaqueous electrolytes (1 M LiClO4 in propylene carbonate). The rGO/V2O5-rGO (VGO) asymmetric supercapacitors present a low equivalent series resistance of 3.36 Ω with around 20 mg active materials, maximum energy density of 13.3 Wh/kg, and good flexibility and cyclic stability (the Nyquist plot is shown in Figure 1.1f). Similarly, Boukhalfa et al.177 investigated the high- performance electrodes with MWCNT-vanadium oxide composite materials. By optimizing the thickness of vanadium oxide coated on the MWCNT, the selected electrodes show a remarkable capacitance of up to 1550 F/g, which is much exceeding the commercial activated carbons.

Compared with the transition metal oxides, transition metal nitrides can also provide pseudocapacitance by redox reactions and the electrical conductivity is usually higher than transition metal oxides.174, 178 For titanium nitride, the electrical conductivity can reach the range of 4000 to 55500 S/cm, which is close to the value of metals.179, 180 The high electrical conductivity makes the transition metal nitrides become an emerging kind of materials for the high-performance electrodes.178, 181, 182

Yang et al.183 developed a corn-like TiN as the electrode materials for supercapacitors.

The symmetric devices using TiN as electrodes show the specific capacitance of 20.7

F/cm3, the maximum energy density of 1.5 mWh/cm3, the maximum power density of

150 W/cm3. In addition, the devices show the extremely stable performance. After

20000 charge/discharge cycles, no significant change was observed in the electrochemical performance (shown in Figure 1.1g).

The 2D transition metal chalcogenides are considered as the ideal materials for the next-generation electronics due to the ultrathin thickness, high transparency, and excellent electronic performance.184-186 Recently, Acerce et al.187 studied the intercalation behaviors between 1T phase MoS2 and the ions in the electrolytes (such

22 + + + + + as H , Li , Na , K , and TEA ) (shown in Figure 1.1h). 1T MoS2 shows remarkable capacitive behavior in both aqueous and organic electrolytes. The devices reach the specific capacitance in the range of 400 to 700 F/cm3, the energy density of 0.11

Wh/cm3 at the power density of 1.1 W/cm3. And the devices can remain 90% initial capacitance after 5000 charge/discharge cycles.

Table 1.2 Recent progress in supercapacitors with metal

oxides/chalcogenides/nitrides electrodes

23 1.3.4 MXenes

Since their discovery in 2011204, MXenes, containing nitrides, transition metal carbides and carbonitrides, have become an emerging and attracting branch of 2D materials, leading a rapid expansion of its family.205-212 MXenes share a general formula

Figure 1.2: (a) Schematic illustration of the intercalation of cation between Ti3C2Tx layers. Reprinted with permission from ref. 205, Copyright 2017, Nature Publishing Group; (b) schematic illustration of MXene/rGO hybrids. Reprinted with permission from ref. 64, Copyright 2017 Wiley-VCH Verlag GmbH & KGaA; (c) schematic illustration of the charge storage process of hydrated electrolyte ions in MXenes (left) and N-doped MXenes (right). Reprinted with permission from ref. 224, Copyright 2017 Elsevier; (d) the left part is a schematic graph of Polypyrrole (PPy) intercalated into the l-Ti3C2 layers. the right part is the atomic-scale of the schematic illustration; (e) Bending performance of symmetric flexible supercapacitor based on PPy/l-Ti3C2 hybrid films. (d), (e) reprinted with permission from ref. 217, Copyright 2016 Wiley-VCH Verlag GmbH & KGaA; (f) CV graphs of the asymmetric sandwich device of RuO2//Ti3C2Tx at different scan rates; (g) Ragone plot of the RuO2//Ti3C2Tx device in comparison to the other microsupercapacitors; (h) Coulombic efficiency and cycling stability of the asymmetric RuO2//Ti3C2Tx device. (f), (g), and (h) reprinted with permission from ref. 225, Copyright 2018 Wiley-VCH Verlag GmbH & KGaA. as Mn+1XnTx(n=1-3), where M stands for an early transition metal (e.g. Sc, Ti, Zr, Hf,

24 Ta, Cr, Mo), X is carbon and/or nitrogen while Tx represents the surface terminations

(such as oxygen, hydroxyl or fluorine)213. Due to the unique properties, such as good mechanical properties, hydrophilicity and high electrical conductivity, MXenes have drawn huge attention as a promising candidate of the energy storage applications, including supercapacitors214, 215 Some examples have been reported, including

207 212 211 216, Nb4C3Tx , Ti2CTx and Ti3C2Tx . For instance, as is widely studied and reported

217 , titanium carbide (Ti3C2Tx) shows a broad electronic conductivity range from <1000

S/cm to 6500 S/cm, due to the different synthetic route218, 219. Table 1.3 summarizes recent advances in supercapacitors with MXene electrodes.

Compared to the dense stacking feature in other two-dimensional materials, such as graphene and conducting polymers, the unique layer-by-layer structure of MXenes can provide a relatively porous structure, which is helpful for ion transport205. When spontaneously intercalated with metal ions215, 220 and polar organic molecules220-222, the electrochemical active cites on the surfaces of MXenes can be occupied by these polar particles, which can subsequently participate in energy storage215, 223(Figure 1.2a). Thus, increasing interlayer spacing of MXenes is considered to be an effective way to improve the capacitance of the electrodes. In this context, Yan et al.64 demonstrated a method of electrostatic self-assembly to effectively prevent the self-restacking of MXene nanosheets, by inserting poly(diallyldimethylammoniumchloride) (PDDA)-modified reduced graphene oxide (rGO) nanosheets in-between Ti3C2Tx layers, in order to considerably increase the interlayer space(Figure 1.2b). Therefore, the accelerated diffusion of ions in electrolyte can enable more accessible electroactive sites in the material. As a result, a high volumetric energy density of 32.6 Wh/L is shown by the as-fabricated binder-free symmetric supercapacitor. Also, the as-reported MXene/rGO electrode demonstrates a volumetric capacitance of 1040 F/cm3 at a scan rate of 2 mV/s,

25 and no significant performance decay after 20000 charge/discharge cycles. Similarly,

224 Wen et al. synthesized a new type of nitrogen-doped MXene (N-Ti3C2Tx) by post- etch annealing Ti3C2Tx in ammonia at different annealing temperatures. After the ammonia treatment at 200°C, the distance between two layers of MXene sheets increases from 1.92 nm in Ti3C2Tx to 2.46 nm in N-Ti3C2Tx. The devices using as- synthesized doped MXenes have drastic improvement on capacitance (Figure 1.2c), which is 192 F/g in 1 M H2SO4 and 82 F/g in 1 M MgSO4, comparing to the values of un-doped ones, which is 34 F/g in 1 M H2SO4 and 52 F/g in 1 M MgSO4, respectively.

Moreover, Zhu et al.217 reported a method of enhancing the performance of supercapacitor electrodes by hybridizing polypyrrole (PPy) chains with MXene. As illustrated in Figure 1.2d, PPy chains are intercalated into layered Ti3C2 (l-Ti3C2). The

MXene material effectively prevents dense PPy stacking and benefits the electrolyte infiltration. Consequently, the freestanding conductive hybrid films show the capacitance of 203 mF/cm2(406 F/cm3) and the cycling stability of almost 100% capacitance retention after 20000 charge-discharge cycles. The authors announced that the strong bonds formed between surfaces of l-Ti3C2 and PPy backbones not only provides good conductivity and benefits charge-carrier transport, but improve the stability of PPy backbone structures as well. Moreover, the ultra-thin all-solid-state supercapacitor based on the freestanding PPy/l-Ti3C2 film exhibits a significant capacitance of 35 mF/cm2, stable performance at bending angles from 0º to 120º (Figure

1.2e), and almost 100% capacitance retention over 10000 charge/discharge cycles.

However, because of the possible oxidation at high anodic potentials, a very limited voltage window, which is around 0.6 V, is a common feature of the symmetric MXene supercapacitors225. Forming asymmetric structure is a considerable method to widen the voltage window. In the work reported by Jiang et al.225, an all-pseudocapacitive

26 MXene(Ti3C2Tx)-RuO2 asymmetric supercapacitor with an impressive electrochemical property to operate at a voltage window of 1.5 V (Figure 1.2f), is developed.

Subsequently, an energy density of 37 μW h/cm at a power density of 40 mW/cm can be delivered by the device (Figure 1.2g), with 86% capacitance retention after 20000 charge/discharge cycles (Figure 1.2h).

Table 1.3 Recent progress in supercapacitors with MXene electrodes

27 1.3.5 Metal-organic frameworks (MOFs)

Due to the high porosity and chemical tunability by their open channels and nanosized cavities, Metal-organic frameworks (MOFs) have become new candidates for precursors or sacrificial templates of nanostructured materials.229-231 MOFs are the combination of metal ions/clusters and organic ligands with extremely light feature, well-defined controlled pore sizes, huge pore volumes, and consequently very high

Figure 1.3: (a) The left part is the idealized schematic illustration of relative size of pores in Ni3(HITP)2 MOF. White, brown, grey, blue, lime and green spheres represent H, B, C, N, F and Ni atoms, separately. The right part is the molecular structure of Ni3(HITP)2; (b) Comparison of the areal capacitance of Ni3(HITP)2 MOF with other EDLC electrode materials (a), (b) reprinted with permission from ref. 235, Copyright 2017, Nature Publishing Group; (c) Synthetic route, SEM image and idealized schematic illustration of nMOF-867. (d) Idealized schematic illustration of nMOF supercapacitors (e) Relationship of stack capacitance of nMOF-867, activated carbon and graphene versus current density, inset is the schematic image of nMOF supercapacitors. (c), (d), and (e) reprinted with permission from ref. 236, Copyright 2014 American Chemical Society; (f) stability tests of the asymmetric supercapacitors. Reprinted with permission from ref. 158, Copyright 2015 American Chemical Society; (g) schematic routes of the synthesis of Dq-2,6-diaminoanthraquinone, Da-2,6-diaminoanthracene, and Tp-1,3,5- triformylpholoroglucinol; (h) CV curves of CT-DqTp and CT-Dq1Da1Tp COF supercapacitors. (g) and (h) reprinted with permission from ref. 239, Copyright 2018 American Chemical Society.

28 specific surface area (SSA), such as 46000 m2/g in the first reported MOFs by Yaghi in

1995232. The ability of incorporating redox-metal and controllable pore size in MOFs have made it a popular nominee as electrode materials of supercapacitors.68, 70, 233, 234

235 Sheberla et al. reported Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2 (Ni3(HITP)2), a

MOF with high porosity and electrical conductivity (Figure 1.3a), as the active electrode materials in EDLCs. It shows a high gravimetric capacitance of 111 F/g at a scan rate of 50 mA/g and a voltage window of 1 V. The device represents a significant areal capacitance which exceeds most carbon-based materials (Figure 1.3b) and the capacity retention more than 90% over 10000 cycles. Hybridizing MOFs with other 2- dimentional materials also shows great promise as electrode materials. Choi et al.236 synthesized nanocrystalline MOFs (nMOFs) (Figure 1.3c) doped with graphene, and successfully incorporated it into supercapacitor devices. (Figure 1.3d) The authors find that a zirconium MOF shows notably high stack capacitance of 0.64 mF/cm2, the areal capacitance of 5.09 mF/cm2, which is 6 times of commercial activated carbon based supercapacitors (Figure 1.3e), and good stability over 10000 charge/discharge cycles.

However, most synthesized MOFs are nonconductive, which limits their direct application as the electrode materials of supercapacitors.231, 233, 237, 238 Nevertheless, as porous sacrificial templates and precursors, MOFs still show excellent potential in synthesizing nanostructured materials with high porosity. Salunkhe et al.239 synthesized nanoporous carbon and nanoporous cobalt oxide (Co3O4) as negative and positive electrode materials, respectively, from a single MOF named zeolitic imidazolate frameworks-67 (ZIFs-67). The nanoporous carbon materials possess a high specific

2 surface area (SSA) of 350 m /g, and the ZIF-derived nanoporous Co3O4 has a SSA of

2 148 m /g. The nanoporous carbon and Co3O4 tested for electrode materials show significant specific capacitance of 272 F/g and 504 F/g, separately. The as-fabricated

29 asymmetric supercapacitors show a high specific energy of 36 W h/kg, a wide voltage window from 0.0 to 1.6V, and considerable 2000 cycles stability (Figure 1.3f).

1.3.6 Covalent-organic frameworks (COFs)

Like MOFs, covalent organic frameworks (COFs) are crystalline materials with high porosity, controllable pore sizes, high surface areas, and highly designed molecular structures240-242. However, the building blocks of COFs are made of light elements, such as H, C, N and O, and connected by covalent bonds. Through various synthetic routes and options, the crystalline structures in COFs can be predictable and well-designed240, 242, 243, comparable to MOFs. Table 1.4 summarizes recent advances in supercapacitors with MOF/COF electrodes. Since the first successful example of

COFs reported in 2005244, this rapidly growing field has increasingly attracted researchers for its vast potential in various energy storage devices.245, 246 DeBlase et al.247 describe a β-ketoenamine-linked 2-dimentional COF, which possesses reversible electrochemical processes of anthraquinone subunit. The material exhibits valuable nanostructured properties as it possesses the highest surface areas of the enamine- or imine- linked 2-dimentional COFs. Comparing to non-redox-active one, the modified redox-active COF represents higher capacitance (311 F/g) and energy density (31

Wh/kg) during long-term charge-discharge cycles. Khayum M et al.243 synthesized the convergent COF flexible freestanding thin sheets via solid-state molecular baking method (Figure 1.3g). Redoxactive 2,6-diaminoanthraquinone (Dq) and π-electron-rich

2,6-diaminoanthracene (Da) are chosen as linkers in a β-ketoenamine-linked 2- dimentional COF. The π-electron-rich anthracene linker improves the mechanical properties of the freestanding thin sheets by enhancing noncovalent interaction between crystallites. A symmetrical flexible all-solid-state COF supercapacitor (Figure 1.3h) was fabricated using carbon tape (CT) as the current collector. The CT-COF

30 supercapacitor shows a considerable specific capacitance of 3 F/g and long-term cyclic stability of 90% capacitance retention after 2500 charge-discharge cycles.

Table 1.4 Recent progress in supercapacitors with MOF/COF electrodes

31 1.4 Polymer based solid-state electrolyte materials for flexible supercapacitors

As mentioned before, the polymer based solid-state electrolytes in supercapacitors serve as both electrolytes and separators. Comparing with conventional liquid electrolytes, solid-state electrolytes efficiently prevent the leakage of the toxic liquid electrolytes without the clumsy metal crusts. What’s more, the solid-state electrolytes also provide excellent mechanical stability and durability.

Generally, polymer based solid-state electrolytes are consisted of three parts which contains the polymer matrices, supporting electrolytic salts, and plasticizers. The supporting electrolytic salts and plasticizers are absorbed in the polymer matrices, which prevents the leakage of the liquid electrolytes. These polymer matrices include polyvinyl alcohol (PVA),255-258 polyoxyethylene (PEO),259-261 polyacrylamide

(PAM)262-264, poly(methylmethacrylate) (PMMA),265, 266 poly(amineester) (PAE)267, 268 poly(vinylidene fluoride) (PVDF)269-271. These single or multi component polymer hosts provide good flexibility and stretchability for the application of supercapacitors in wearable and portable devices. Besides polymer hosts, conducting electrolytic salts, and plasticizers also play crucial roles in the solid-state electrolytes. Different conducting electrolytic salts, and plasticizers can be divided into three main types, which are aqueous electrolytes, organic electrolytes, and ionic liquid electrolytes. The following part will be divided into three sections: aqueous gel electrolytes, organic gel electrolytes, and ionic liquid gel electrolytes.272, 273

1.4.1 Aqueous gel electrolytes

Aqueous gel electrolytes have attracted extensive attention in the past few years due to their high ionic conductivity, low toxicity, easy processing, and low cost. In aqueous gel electrolytes, the plasticizer is water.99, 274, 275 The electrolytic salts in aqueous gel

276-278 279 117, 280, 281 electrolytes contain strong acids (like H2SO4 , HCl , H3PO4 ), strong

32 282-284 285, 286 287-289 bases (KOH ) or some neutral salts (like Na2SO4 and LiCl ). Compared with commercial organic electrolyte devices, these aqueous gel electrolytes are much cheaper and less toxic. What’s more the higher ionic conductivity of aqueous gel

Figure 1.4: (a) Bending stability of supercapacitor with MnOx electrodes and PVA- Na2SO4 gel electrolytes. Reprinted with permission from ref. 291, Copyright 2016 Elsevier; (b) schematic illustration of supercapacitor with bio-inspired, quasifractal metallic network electrodes and LiCl-PVA gel electrolytes. Reprinted with permission from ref. 294, Copyright 2019 Wiley-VCH Verlag GmbH & KGaA; (c) bending stability of supercapacitor with boron element nanowire electrodes and PVA-H2SO4 electrolytes. Reprinted with permission from ref. 295, Copyright 2018 Wiley-VCH Verlag GmbH & KGaA; (d) the ionic conductivity of H5BW12O40 (BWA) electrolytes. Reprinted with permission from ref. 296, Copyright 2015 Royal Society of Chemistry; (e) schematic illustration of supercapacitor with NiCo2O4@carbon nanotube (CNT) electrodes and PVA-KOH gel electrolytes. Reprinted with permission from ref. 298, Copyright 2017 Wiley-VCH Verlag GmbH & KGaA; (f) CV curves of different polymer matrices in solid-state electrolytes with porous carbon cathode and SnO2-TiO2 anode. Reprinted with permission from ref. 299, Copyright 2019 American Chemical Society; (g) schematic illustration of grafted PVA-KOH gel electrolytes. Reprinted with permission from ref. 99, Copyright 2019 Wiley-VCH Verlag GmbH & KGaA; (h) stretchability of poly(AMPS-co-DMAAm)/Laponite/Graphene oxide (GO) hydrogel electrolytes. Reprinted with permission from ref. 300, Copyright 2019, Nature Publishing Group.

33 electrolytes leads to lower ESR and higher power density. Besides power density, another prominent advantage of the aqueous electrolyte is the significant higher specific capacitance than organic systems. The same faradic-active electrode shows the highest specific capacitance in aqueous systems rather than in organic electrolytes nor ionic liquids electrolytes, due to the higher ionic conductivity and smaller ion radius.290

For the neutral system, Pan et al.291 investigated an endurable flexible all-solid-state supercapacitor with MnOx electrodes and PVA-Na2SO4 gel electrolytes. The devices achieve the specific capacitance of 906.6 mF/cm2 at a current density of 1 A/cm2, the energy density of 1.16 mWh/cm3 at a current density of 1 mA/cm2. The specific capacitance doesn’t have significant change under different binding angle from 0 to

180º and maintain over 85% initial capacitance after 200 bending cycles (show in

Figure 1.4a). Compared to Na+, Li+ have a smaller ion radius which will lead to a higher ionic conductivity.292, 293 LiCl is another promising electrolytic salt in neutral aqueous systems. Chen et al.294 recently reported a flexible supercapacitor with bio-inspired, quasifractal metallic network electrodes and LiCl-PVA gel electrolytes (shown in

Figure 1.4b). The transparent devices show good long-term mechanical stability. After

500 bending cycles, the devices can keep 93.9% initial specific capacitance.

Due to the fast transportation of protons in the aqueous system, the polymer gel electrolytes with proton-donor acids were widely investigated. These acid gel electrolytes show the ionic conductivity from 10 to 100 mS/cm at room temperature.4,

257, 274 Xue et al.295 reported a flexible sold-state supercapacitors with boron element nanowire electrodes and PVA-H2SO4 electrolytes. The devices based on PVA-H2SO4 electrolytes show low internal resistance and charge-transfer resistance which are 1.15 and 0.64 Ω, respectively. In addition, the devices also present excellent flexibility. After

1000 times bending, the supercapacitor can still keep more 80% initial capacitance, as

34 shown in Figure 1.4c. Gao et al.296 investigated a novel proton-conducting heteropoly acid gel electrolyte for solid supercapacitors. The H5BW12O40 (BWA) electrolytes show higher ionic conductivity than H2SO4 at same concentrations (as shown in Figure 1.4d) and achieve a maximum cell voltage of 1.6 V.

Among different alkaline electrolytic salts in gel electrolytes, KOH is the most widely used in alkaline gels due to its low cost, high ionic conductivity, and environmental friendliness.5, 25, 297 For example, Wu et al.298 reported an all-solid-state asymmetric supercapacitor with NiCo2O4@carbon nanotube (CNT) electrodes and

PVA-KOH gel electrolytes (as shown in Figure 1.4e). The devices show high energy density around 27.6 Wh/kg at the power density of 0.55 kW/kg, 95% capacitance retention after 5000 cycles. In addition, the supercapacitors keep most electrochemical performance under different bending angle from 0 to 180º. To investigate the influence of the polymer matrices on the electrochemical performance, Pal et al.299 evaluated the performance of different polymer matrices in solid-state electrolytes with porous carbon cathode and SnO2-TiO2 anode. The cyclic voltammetry curves of supercapacitors with different polymer gel electrolytes are shown in Figure 1.4f.

Among the solid-state electrolytes based on poly(vinyl alcohol) (PVA), poly(ethylene oxide) (PEO), and poly(ethylene glycol) (PEG)-KOH, the asymmetric supercapacitors based on PEG-KOH electrolytes show the highest specific capacitance due to the lower bulk and charge transfer resistance, quicker ion diffusion rate, and less ion pairing.

Besides the single-component polymer based electrolytes, more and more multi- component polymer gel based solid-state electrolytes have been developed to further enhance both the electrochemical and mechanical performance. Chen et al.99 developed a multi-component PVA-KOH gel electrolyte by grafting graphene oxide and amino groups on the PVA back bone, as shown in Figure 1.4g. The grafted PVA-KOH gel

35 electrolytes show much higher ionic conductivity of 108.7 mS/cm than the value (30.5 mS/cm) in the pure PVA-KOH gel electrolytes. While enhancing the electrochemical performance, multi-component polymer gel based solid-state electrolytes also have impressive improvement on the mechanical performance. Recently, Li et al.300 have investigated a multifunctional hydrogel polyelectrolyte with co-polymerization of 2- acrylamido-2-methylpropane sulfonic acid (AMPS) and N, N-dimethylacrylamide

(DMAAm). The supercapacitors based on poly(AMPS-co-

DMAAm)/Laponite/Graphene oxide (GO) hydrogel electrolytes show superior mechanical stretchability (up to 1000%) and excellent self-healing behavior under either heating or light treatment (shown in Figure 1.4h).

1.4.2 Organic gel electrolytes

Suffering from the narrow voltage windows of the aqueous electrolytes,43, 301 organic electrolytes become promising candidates for the high energy density applications. Generally, the organic gel electrolytes contain high molecular weight polymer matrices with some organic conducting salts dissolved in aprotic solvents. For example, Nguyen et al.302 synthesized a solid polymer electrolyte with four interbonding layers (shown in Figure 1.5a). Linear poly(ethylene imine) (LPEI) and poly(ethylene oxide) (PEO) are used to enhance the dissolution of lithium salt and the ionic transport. Poly(acrylic acid) (PAA) serve as the bridge to combine the LPEI and

PEO. The dry solid electrolytes show ionic conductivity of 10-5 S/cm at room temperature described by Vogel-Fulcher-Tammann equation. In organic electrolytes, the organic ammonium salts, such as tetraethylammonium tetrafluoroborate

(TEABF4)303-305, Tetrabutylammonium hexafluorophosphate (TBAPF6)306, 307, and

Bis(trifluoromethylsulfonyl)amine (TFSI-)308 play the most common roles as the conducting salts. To further increase the performance, Park et al.309 added

36 decamethylferrocenium (DmFc) into tetrabutylammonium perchlorate (TBAP)- tetrahydrofuran (THF) electrolyte as the redox-active materials. The redox reaction of

DmFc can provide extra pseudocapacitance for the devices. After the addition of DmFc redox couples, the voltage widow of the CNT/DmFc-TBAP-THF/CNT devices increased from 1.1 to 2.1 V (shown in Figure 1.5b), the energy density increased from

1.3 Wh/kg to 36.8 Wh/kg, and the specific capacitance increased from 8.3 F/g to 61.3

F/g compared with the devices without DmFc. For the solvents, acetonitrile (AN) can dissolve most types of conducting salts used in the organic electrolytes and has a higher ionic conductivity compared with other solvents under the same concentration of

Figure 1.5: (a) Schematic illustration of the polymer electrolyte with four interbonding layers. Reprinted with permission from ref. 302, Copyright 2011 American Chemical Society; (b) CV curves of CNT/DmFc-TBAP-THF/CNT devices. Reprinted with permission from ref. 309, Copyright 2014 American Chemical Society; (c) the gas evolution behavior of AN, PC, and GBL as the solvents in the electrolytes. Reprinted with permission from ref. 313, Copyright 2008 Elsevier; (d) Nyquist plot of NaTFSI-PC, PEO-NaTFSI-PC, and PEO-PC-EC- DMC-NaTFSI as electrolytes. Reprinted with permission from ref. 314, Copyright 2014 American Chemical Society; (e) ionic conductivity of the modified PVDF/LiTFS polymer electrolytes. Reprinted with permission from ref. 315, Copyright 2019 Elsevier. conducting salts.310, 311 For example, dissolving 1M TEABF4 in acetonitrile (AN) and propylene carbonate (PC), the solution prepared by AN has an ionic conductivity of 60 37 mS/cm which is more than five times larger than the value of PC solution (11 mS/cm).312 Kötz et al.313 studied the gas evolution behavior of AN, PC, and γ- butyrolactone (GBL) as the solvents in the electrolytes. As shown in Figure 1.5c, after testing under the voltage of 2.5 V, 2.75 V, 3.0 V, and 3.25 V for 24 hours, respectively,

GBL has a significant gas evolution at 2.5 V while PC starts to release gas at 3.0 V and

AN keeps low gas evolution rate even at 3.25 V. These results suggest AN is the preferred choice for the high-potential window supercapacitors. However, considering about the flambility and toxicity of AN, the electrolytes based on PC are more environmental-friendly and safe.5 Recently, some mixed solvents have been used in the organic electrodes to further enlarge the voltage windows. For example, Ramasamy et al.314 reported a propylene carbonate (PC)- ethylene carbonate (EC)- dimethyl carbonate (DMC)- sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) mixed system which shows a higher ionic conductivity than the gels with single solvent. As shown in

Figure 1.5d, NaTFSI-PC, PEO-NaTFSI-PC, and PEO-PC-EC-DMC-NaTFSI are corresponding to system 1, 2, and 3, respectively. The multi-solvent gel electrolytes shown better impedance performance than the single-solvent gel electrolytes. The utilization of the organic gel electrolytes also broadens the working temperature range of supercapacitors due to the low freezing point of the organic solvents. For instance,

Cai et al.315 investigated a modified polyvinylidene fluoride (PVDF)/lithium trifluoromethanesulfonate (LiTFS) polymer electrolyte for the low-temperature solid- state supercapacitors. After using K4Fe(CN)6/K3Fe(CN)6 as the additive below the concentration of 4 vol% in the electrolytes. The modified PVDF/LiTFS polymer electrolytes show the ionic conductivity around 10-3 S/cm at -20 ºC and 10-2 S/cm at 20

ºC (as shown in Figure 1.5e), which guarantees the applications under some extreme conditions. By using nonaqueous solvents, the voltage windows of organic electrolyte

38 can easily get 2.5-3V187, 303-305, 316 , which are markedly higher than the maximum operation voltages in aqueous systems. Although organic electrolytes can provide much larger voltage windows than aqueous systems, the biggest issue for organic electrolytes is the toxic, volatile, and flammable organic solvents and the moisture sensitive conducting salts, the supercapacitors with organic electrolytes have to be sealed in the shells to prevent the volatilization of solvents, which restricts the application of organic electrolytes in the portable or wearable devices.

1.4.3 Ionic liquid gel electrolytes

The solvent-free ionic liquids are regarded as another promising candidates of solid state-electrolytes in high-performance supercapacitors which have large potential windows (up to 6.0V)317 . Ionic liquids (ILs) are a kind of organic salts which consist of large cations and charge delocalized anions with low melting points, low vapor

39 pressure and noninflammability318-320. The performances of the ILs based electrolytes mainly rely on the chemical structures of ions. To choose the suitable IL molecules for the supercapacitors, three main parameters should be considered, which are ionic conductivity, working voltage window, and the melting point. As electrolytes in supercapacitors, several kinds of ionic liquids have be developed, such as quaternary ammonium321, 322, sulfonium323-325, imidazolium326-328, pyrrolidinium329, 330,

331, 332 - 321, 333, 334 piperidinium cations. For anions, tetrafluoroborate (BF4 ) ,

- 335 - 323, 326, 332, 336, hexafluorophosphate (PF6 ) , bis(trifluoromethanesulfonyl)imide (TFSI )

337, bis(fluorosulfonyl)imide (FSI-)338, 339, dicyanamide (DCA-)338, 340 are most common.

Scheme 1.7 shows the structures of typical IL ions. Compared with organic electrolytes, the ILs based electrolytes further widen the voltage windows of supercapacitors to 3.5-

6 V, commonly around 4 V, and remain the high ionic conductivity in the order of 10-3 to 10-2 S/cm.28, 341 For example, Yang et al.342 developed a graphene oxide (GO) doped poly(vinylidene fluoride-hexafluoro propylene (P(VDF-HFP))- 1-ethyl-3- methylimidazolium tetrafluoroborate (EMIMBF4) ion gel electrolytes with a high ionic conductivity around 25 mS/cm at the room temperature. The all-solid-state supercapacitors with graphene doped carbon materials and GO-EMIMBF4-(P(VDF-

HFP) ion gel as electrodes and electrolytes, respectively, show a stable working potential window from 0 to 3.5 V (CV curves are shown in Figure 1.6a), the specific capacitance of 190 F/g, the energy density of 76 Wh/kg at the current density of 1 A/g.

Similarly, She et al.343 investigated a high-voltage supercapacitor with ionic liquid decorated graphene oxide and polypropylene (PP)-EMIMTFSI as electrodes and electrolyte, respectively. The surfactant microemulsion system of ionic liquid can accelerate the diffusion of ionic liquid ions into graphene oxide. The supercapacitors show a working window of 3 V, the specific capacitance of 302 F/g, the maximum

40 energy density of 45 Wh/L at the power density of 571.4 W/L at room temperature.

(The ragone plots are shown in Figure 1.6b). In addition, supercapacitors based on ILs electrolytes show the excellent stability in a wide temperature range from -95 ºC to more than 300 ºC 344, 345, which is inaccessible in the aqueous electrolytes and organic electrolytes. For example, Pandey et al.346 reported an 1-ethyl-3-methylimidazolium tetracyanoborate (EMIMTCB) entrapped in poly(vinylidene fluoride-co- hexafluoropropylene) (PVdF-HFP) gel electrolyte. The gel electrolytes can provide a wide working window about 3.8 V and a high ionic conductivity of 9×10-3 S/cm at room temperature (shown in Figure 1.6c). What’s more the gel electrolytes can keep stable up to 310 ºC, which promise the application in some extreme situations. For the low temperature condition, Lin et al.347 developed an ionic liquid electrolyte which show an operating range from -50 to 100 ºC by mixing N-methyl-N-propylpiperidinium bis(fluorosulfonyl)imide (PIP13FSI) and N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR14FSI) in a molar ratio of 1:1. The eutectic mixture formed by two ionic liquids has a much lower melting point (-80 °C) than the melting points of (PIP13FSI) (6 °C) and (PYR14FSI) (-18 °C). The devices with the mixed ionic liquid electrolytes and nanostructured carbon electrodes show a wide potential of 3.5 V and a wide operating range from -50 to 100 ºC (as shown in Figure 1.6d). The devices

41 with IL gel-based electrolytes also show the outstanding mechanical performance while keeping the high electrochemical performance. Shen et al.344 reported a flexible asymmetric supercapacitor based on FeOOH electrodes and ionic liquid electrolytes.

The γ-FeOOH@carbon cloth, N-doped active carbon, and 1-ethyl-3-

Figure 1.6: (a) CV curves of supercapacitor with graphene doped carbon materials and GO-EMIMBF4-(P(VDF-HFP) ion gel as electrodes and electrolytes, respectively. Reprinted with permission from ref. 342, Copyright 2014 Elsevier; (b) ragone plot of supercapacitor with ionic liquid decorated graphene oxide and polypropylene (PP)-EMIMTFSI as electrodes and electrolyte, respectively. Reprinted with permission from ref. 343, Copyright 2017 American Chemical Society; (c) the ionic conductivity of 1-ethyl-3-methylimidazolium tetracyanoborate (EMIMTCB) entrapped in poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) gel electrolytes. Reprinted with permission from ref. 346, Copyright 2013 Royal Society of Chemistry; (d) the temperature range of the mixture of (PIP13FSI) and (PYR14FSI). Reprinted with permission from ref. 347, Copyright 2011 American Chemical Society; (e) schematic illustration of the intercalation pseudocapacitance provided by EMIM+ and FeOOH. Reprinted with permission from ref. 344, Copyright 2011 Royal Society of Chemistry. methylimidazolium bis(trifluoromethylsulfonyl) imide (EMIM-NTF2)-fumed SiO2 are used as negative electrode, positive electrode, and electrolyte, respectively. The intercalation pseudocapacitance provided by EMIM+ and FeOOH (shown in Figure

1.6e) promise the electrochemical performance and the (EMIM-NTF2)-fumed SiO2 gel electrolytes provide the great stability. The devices reach the maximum energy density 42 of 1.44 mWh/cm3 at 200 ℃ and stable energy-storage ability at a 180º bending angle.

1.5 Summary and Outlook

Flexible solid-state supercapacitors are playing more and more important roles diverse energy storage systems, such as wearable electronics, self-powered devices, smart medical devices. This review systemically introduces the basic mechanisms of different types of supercapacitors and recent progress in both electrode and electrolyte materials for solid-state flexible supercapacitors. For the electrode materials, besides the traditional carbon materials, conducting polymers, and metal oxides, several new electrode materials are also reviewed, such as metal nitrides, metal chalcogenides,

MXenes, COFs, and MOFs. These emerging new materials are becoming promising candidates for the high-performance electrodes due to their unique electrochemical performance. For the electrolyte materials, polymer gel based solid-state electrolytes have attracted more attention in both academia and industry instead of the traditional liquid electrolytes. The multi-component polymer matrices provide both higher mechanical and electrochemical performance for the flexible supercapacitors. And the solvent-free ionic liquids with large voltage and temperature windows boost the developments of the high-energy density devices and the applications in extreme situations.

Although many impressive and encouraging developments have been achieved, there are still some challenges for the future research:

a. The energy density of most flexible supercapacitors is still lower than the values in secondary batteries, which restrict the long-time endurance of the wearable and portable devices. Investigating the new pseudocapacitive materials with both high capacitance and large working windows is always the primary objective;

b. The emerging 2D layered materials, such as MXenes, transition metal nitrides,

43 and chalcogenides show promising high-performance as electrode materials. However, suffering from the collapse between layer and layer the stability of these 2D materials still have some issues (as mentioned in part 3);

c. More research are needed to further investigate the energy storage mechanisms in solid-state electrolytes. There are few works studied on the interaction between electrolyte ions and the polymer matrices. The mechanisms of the influence from different polymer chains on the ionic conductivity are still unclear. Finding the influence from the polymer chains in the matrices on the ionic conductivity will leading a better development of multi-component polymer matrices;

d. A standardized evaluation system should be established to batter evaluate the performance of flexible supercapacitors. In the past decades, most researchers were used to describe the performance of the supercapacitors by the mass specific capacitance. The mass to calculate the specific capacitance can be both the mass of active materials on electrodes and the whole devices But for the wearable and portable electronics, it is more reasonable to use the volumetric specific capacitance since these electronic usually are light. So the standard parameters should be set up to better describe the device performance.

e. High processing cost still restrict the widely commercialization of flexible supercapacitors. For the applications of supercapacitors, the expensive electrodes materials and low processability restrict the further developments of large-scale devices.

Investigating the low cost and easily processable materials will benefit the developments of the large-scale processing such as blade cast and roll-to-roll (R2R).

44 CHAPTER II

Quasi-Solid-State Asymmetric Supercapacitors with Novel Ionic Liquid Gel

Electrolytes

2.1 Introduction

In the past decades, supercapacitors as one of energy storage devices, have drawn great attentions in both academic and industrial sectors due to its high power density, low-cost and environmentally friendly.28, 348, 349 In supercapacitors, the charge carriers are stored in the interfaces between the electrodes and electrolytes,5, 350 which results in high power densities. However, poor energy densities restrict the practical applications of supercapacitors in the wearable and smart electronics.351-354

Many approaches have been attempted to enhance the energy densities of supercapacitors.42, 352, 355 It was reported that enlarging the operational voltage window

(V) was a simple way to enhance the energy density since the energy density is proportional to the square of V.5, 350 Towards the end, ionic liquids were widely used as electrolytes since supercapacitors with ionic liquids as electrolytes could generate a large operational window (3-5 V).58, 356-358 In addition, ionic liquids possess low melting point, low vapor pressure and noninflammability,358 which allowed supercapacitors to be operated at broad working temperature range.310, 342, 346, 359 However, poor ionic conductivities of ionic liquids originated from large ionic sizes and high viscosities restricted supercapacitors to have high power densities.359, 360

To boost power densities of ionic liquid based supercapacitors, ionic liquids were mixed with polar or redox additives as electrolytes.361, 362 Yang et al.342 reported an ionic conductivity of ~ 25 mS/cm from graphene oxide (GO) mixed with poly(vinylidene fluoride-hexafluoro propylene (P(VDF-HFP))-1-ethyl-3- methylimidazolium tetrafluoroborate (EMIMBF4) ionic gels and further found out that

45 quasi-solid-state supercapacitors incorporated with GO-(P(VDF-HFP)-EMIMBF4 gel electrolytes exhibited a stable working potential window from 0 to 3.5 V, the specific capacitance of 190 F/g, the energy density of 76 Wh/kg at the current density of 1 A/g.

However, fluoric copolymers were typically used as matrix for solid-state ionic liquid electrolyte,328, 342 and fluoric copolymers are neither biodegradable nor low-cost.44 Thus, cost-effective and biodegradable polymer matrices are required to be developed for approaching high ionic conductivity solid-state electrolytes for environmental friendly supercapacitors.342, 346

In this work, we report novel ionic liquid BMIMBF4:PVA:TBABF4 gel electrolytes. The BMIMBF4:PVA:TBABF4 gel electrolytes exhibit an ionic conductivity of 21.7 mS/cm. We further assemble asymmetric supercapacitors (ASCs) by using MnO2@carbon cloth as the positive electrode, rGO@carbon cloth as the negative electrode, and the BMIMBF4:PVA:TBABF4 gels as solid-state electrolytes.

ASCs exhibit an operational window of 3 V, the energy density of 61.2 Wh/kg at the power density of 1049 W/kg, an operational temperature ranging from 20 oC to 90 oC, and more than 80% capacitance retention after 3000 charge/discharge cycles.

2.2 Experimental section

2.2.1 Materials:

1-Butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4), tetrabutylammonium tetrafluoroborate (TBABF4), poly(vinyl alcohol) (PVA) (Mw 89,000-98,000, 99+% hydrolyzed), potassium permanganate (KMnO4), graphite (powder, <20 μm, synthetic), manganese acetate (Mn(CH3COO)2), sodium sulphate (Na2SO4), and sodium dodecyl sulfonate (SDS) were purchased from Sigma-Aldrich. Carbon cloth (0.5 mm thickness) was purchased from Fuel Cells Earth. Acetonitrile (ACN), dimethyl sulfoxide (DMSO) were purchased from Fisher Chemical Co. Ltd. Phosphorus (V) oxide (P2O5) and

46 potassium peroxydisulfate (K2S2O8) were purchased from Alfa Aesar. All materials are used as received without further purification.

2.2.2 Synthesis of GO:

GO were synthesized followed with an improved Hummers’ method.363 Firstly, 3.0 g graphite powder was mixed with 12 mL 98% H2SO4. After that, 2.5 g K2S2O8 and 2.5 g P2O5 powders were added into above mixture solution and the mixture solution was heated to 80 ℃ under stirring for 6 hours (hrs). Afterward, above solution was cooled down to room temperature naturally. Under continuously stirring, 120 mL cold H2SO4 solution was poured into above mixture solution, followed with adding 15 g KMnO4.

This mixture solution was kept in the temperature not over 35 ℃ for 6 hrs. After that,

700 mL deionized (DI) water was added into above solution. 20 mL H2O2 (30%) was added into above solution to form clear and yellowish color solution. Above yellowish color solution was further centrifuged and washed by 1M HCl and then DI water for several times. The product is GO solution. Afterwards, 20 µL hydrazine hydrates was added into 10 mL GO solution (~ 2 mg/mL). The mixture solution was heated in an autoclave at 60 °C for 6 hrs. Finally, reduce GO (rGO) was dried in an oven at 60 ℃ for 12 hrs.

2.2.3 Synthesis of TBABF4:PVA:BMIMBF4 gels:

The TBABF4:PVA:BMIMBF4 gels were synthesized by a solution-cast method.99

Firstly, PVA solution was prepared by dissolving 1 g PVA powder into 10 mL DMSO under stirring. After 1 h stirring, 2.26 g BMIMBF4 was mixed with above solution. The mixture solution was stirred for 2 hrs at 60 ºC. Then 0 g or 0.822 g or 1.645 g of

TBABF4 powder was separately dissolved into 1 mL DMSO to make three different

TBABF4 DMSO solutions. TBABF4 DMSO solution was dropped into the

BMIMBF4:PVA solution under vigorous stirring at 60℃ for 4 hrs to make

47 homogeneous solutions, which are with different molar ratios of BMIM:TBA (1:0, 2:1 and 4:1). Finally, above solution was put into a petri dish and then dried in a vacuum oven at 80℃ for 8 hrs to produce transparent gels.

2.2.4 Preparation of the electrodes:

135 According to our previous reports, MnO2 was electrochemical deposited on the top of pretreated carbon cloth (1cm x 1cm) to form MnO2@carbobn cloth, which is used as the positive electrode. The rGO was blended with carbon black and polyvinylidene difluoride in a weight ratio of 8:1:1 in 1-methyl-2-pyrrolidone. Then the slurry containing rGO, carbon black and polyvinylidene difluoride was coated on the hydrophilic carbon cloth (1cm x 1cm) and dried at 60 ºC for 12 hrs.364

2.2.5 Fabrication of ASCs:

The MnO2@carbon cloth//BMIMBF4:PVA:TBABF4//rGO@carbon cloth ASCs were fabricated by assembling of the MnO2@carbon cloth as the positive electrode, rGO@carbon cloth as the negative electrode, and the BMIMBF4:PVA:TBABF4 gels as the solid-sate electrolytes with a sandwich-type device configuration. Finally, ASCs were packaged by PET film.

2.2.6 Characterization of ASCs:

48 Scanning electron microscopy (SEM) images of samples were performed by JEOL-

7401. The cycling voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements were performed by

Gamry reference 3000 electrochemical workstations (Gamry Instruments, U.S.A). The single electrode was measured in a three-electrode configuration using the platinum wire and the Ag/Ag+ electrode as the counter electrode and reference electrode,

Figure 2.1 (a), the high and (b) low magnification SEM images of the BMIMBF4- PVA gel electrolytes, (c), the high and (d) low magnification SEM images of BMIMBF4:PVA:TBABF4 gel electrolytes (a BMIMBF4:TBABF4 molar ratio of 4:1). respectively. The EIS was measured at the frequency ranging from 10 mHz to 100 kHz and an impedance amplitude of ±5 mV at the open circuit potential condition. The cyclic stability was measured by LAND CT2001A (Landt Instruments, Wuhan China).

2.3 Results and discussion

Figure 2.1a, b presents the high and low magnification SEM images of the

BMIMBF4:PVA gel electrolytes. The BMIMBF4:PVA gel electrolytes possess very smooth surface, which indicates that bulk compact structure is formed. Such compact structure is ascribed to good solubility of BMIMBF4 and PVA in DMSO solutions.

49 Such compact structure, but on the other hand, could slow down ionic transporting from electrolytes to the electrodes. To circumvent above deficiency, TBABF4 is mixed with

BMIMBF4:PVA gels to form BMIMBF4:PVA:TBABF4 gel electrolytes with tuned film morphology since TBABF4 possess different solubility in DMSO and BMIMBF4.

Figure 2.1c, d presents the high and low magnification SEM images of the

BMIMBF4:PVA:TBABF4 gel electrolytes (where the mole ratio of

BMIMBF4:TBABF4 is 4:1). The BMIMBF4:PVA:TBABF4 gel electrolytes possess porous structure as compared with the BMIMBF4:PVA gel electrolytes. Such porous structure is probably ascribed to TBABF4 has different solubility in DMSO compared to that of BMIMBF4.365 Most importantly, such porous structure could provide channels for counter ions to be transported from electrolytes to the electrode, resulting in boosted ionic conductivity.99 However, as shown in Figure S1, high concentration of

TBABF4 in the BMIMBF4:PVA:TBABF4 gel electrolytes would induce phase separation and generate TBABF4 precipitation, which would result in decreased ionic conductivity.

50 Figure 2.2a shows the CV curves of supercapacitors with a device configuration of carbon cloth/BMIMBF4:PVA:TBABF4/carbon cloth. The areas of the CV curves of supercapacitors with the BMIMBF4:PVA:TBABF4 gel electrolytes are larger than those with the BMIMBF4:PVA gel electrolytes ( Figure S2) at the same scan rates.

Such enlarged areas certainly indicates that more counter ions are transported.

Furthermore, it is found that the areas of the CV curves of supercapacitors with the

BMIMBF4:PVA:TBABF4 gel electrolytes are dramatically increased as along with the increased scan rates, whereas, the areas of the CV curves of supercapacitors with the

BMIMBF4:PVA gel electrolytes exhibit a negligible change as along with the increased scan rates. These results demonstrate that the BMIMBF4:PVA:TBABF4 gel

Figure 2.2 (a) the CV curves of BMIMBF4:PVA:TBABF4 gel electrolytes, (b) the Nyquist plots of BMIMBF4:PVA:TBABF4 and BMIMBF4:PVA gel electrolytes, (c) the CV curves and (d) the Nyquist plots of supercapacitors with BMIMBF4:PVA:TBABF4 gel electrolytes at different working temperatures. electrolytes possess efficient electrochemical double layers at the high scan rates as compared to that of the BMIMBF4:PVA gel electrolytes.366 In addition,

51 supercapacitors with the BMIMBF4:PVA:TBABF4 gel electrolytes maintain unchanged shapes of the CV curves under the scan rates from 10 mv/s to 75 mv/s, which indicates that the BMIMBF4:PVA:TBABF4 gel electrolytes have a good electrochemical stability.

An operational window observed from supercapacitors with the

BMIMBF4:PVA:TBABF4 gel electrolytes is 3V, which is smaller than that (4 V) with the BMIMBF4:PVA gel electrolytes (Figure S3). Such reduced operational window is probably originated from quaternary ammonium (TBA), which typically exhibited a smaller operational window.367

Figure 2.2b presents the Nyquist plots of supercapacitors with either

BMIMBF4:PVA:TBABF4 gel electrolytes or the BMIMBF4:PVA gel electrolytes. It is found that the series resistance (Rs) is ~ 9.2 Ω for supercapacitors with the

BMIMBF4:PVA:TBABF4 gel electrolytes, and Rs is ~ 100 Ω for supercapacitors with

52 the BMIMBF4:PVA gel electrolytes.

As a result, the ionic conductivity of the BMIMBF4:PVA:TBABF4 gel electrolytes is calculated to be ~ 21.7 mS/cm, which is much higher than that (~ 1.9 mS/cm) for the

BMIMBF4:PVA gel electrolytes. Such boosted ionic conductivity indicates that the

BMIMBF4:PVA:TBABF4 gel electrolytes could enhance the electrochemical performance of supercapacitors.99

Figure 2.2c displays the CV curves of supercapacitors with the

BMIMBF4:PVA:TBABF4 gel electrolytes operated in different temperatures. It is found that the shapes of the CV curves at different temperatures are similar, which is different to those observed from supercapacitors with the BMIMBF4:PVA gel electrolytes (Figure S5). These results demonstrate that supercapacitors with the

BMIMBF4:PVA:TBABF4 gel electrolytes are workable in a broad temperature window.

Figure 2.2d presents the Nyquist plots of supercapacitors with the

BMIMBF4:PVA:TBABF4 gel electrolytes tested at different temperatures. It is found

o that the Rs is decreased to 4.3 Ω at 90 C from 9.2 Ω at room temperature. These results further indicate such supercapacitors are able to work at high temperature.

Figure 2.3a,b presents the CV and GCD curves of the MnO2@carbon cloth positive electrode in BMIMBF4:TBABF4 (4:1 mole ratio) mixed acetonitrile solution electrolyte. The MnO2@carbon cloth electrode shows a stable operational window from

0 to 1.5 V. No significant change is observed from the shapes of the CV curves at the scan rates from 10 mV/s to 100 mV/s, indicating that the MnO2@carbon cloth positive electrode could provide a rapid and reversible pseudocapacitance. Moreover, based on the GCD curves (Figure 2.3b), the largest specific capacitance is calculated to be 125.2

F/g at the current density of 0.5 A/g.

53 Figure 2.3c,d presents the CV and GCD curves of rGO@carbon cloth negative electrode in BMIMBF4:TBABF4 (4:1 mole ratio) mixed acetonitrile solution electrolyte. The rGO@carbon cloth negative electrode exhibits an operational window from -1.7 V to 0 V. Furthermore, no significant change is observed from the shapes of the CV curves scanned from 10 mV/s to 100 mV/s, indicating that the rGO@carbon cloth negative electrode is stable at the high current density. Based on the GCD curves

(Figure 2.3d), the highest specific capacitance is calculated to be 98.5 F/g at the current density of 0.5 A/g.

Figure 2.3e presents the cycling stability of the MnO2@carbon cloth and rGO@carbon cloth electrodes, respectively. The increased capacitance in cycles from

300 to 500 from both MnO2@carbon cloth and rGO@carbon cloth electrodes is

Figure 2.4 (a) Schematically illustration of MnO2@carbon cloth//BMIMBF4:PVA:TBABF4//rGO@carbon cloth ASCs, (b) the CV curves, (c) the

GCD curves and (d) the cycling stability of MnO2@carbon cloth//BMIMBF4:PVA:TBABF4//rGO@carbon cloth ASCs.

54 ascribed to the diffusion process of counter ions from electrolytes to the electrodes.368

No significant degradation is observed from the rGO@carbon cloth electrode after 4000 charge/discharge cycles at the current density of 3 A/g. The MnO2@carbon cloth electrode maintains 80% capacitance retention after 4000 charge/discharge cycles at the current density of 3 A/g. These results reveal that the MnO2@carbon cloth and rGO@carbon cloth electrodes have good long-term cycling stabilities.

Figure 2.4a schematically displays ASCs device configuration, MnO2@carbon cloth //BMIMBF4:PVA:TBABF4//rGO@carbon cloth. The CV curves of ASCs are presented in Figure 2.4b. The ASCs exhibit a wide operational window from 0 V to 3.0

V. No significant change is observed from the shapes of the CV curves at the scan rates from 10 mV/s to 100 mV/s, indicating that ASCs have a good electrochemical stability.

Figure 2.4c presents the GCD curves of ASCs at the current densities from 0.5 A/g to 10 A/g. The ASCs show the highest specific capacitance of 49 F/g, the energy density of 61.2 Wh/kg, the power density of 1049 W/kg at the current density of 0.5 A/g. All these device performance parameters are compatible to those reported values (Figure

S6).369-371

Figure 2.4d presents the cycling stability of ASCs. The increased capacitance in the first 800 cycles is due to the diffusion of counter ions from BMIMBF4:PVA:TBABF4 gel electrolytes into the electrodes. The ASCs maintains greater than 80% capacitance retention after the 3000 charge/discharge cycles at the current density of 3 A/g. These results demonstrate that ASCs exhibit good cycling stability.

2.4 Conclusions

In this study, we reported quasi-solid-state asymmetric supercapacitors (ASCs) by using MnO2@carbon cloth as the positive electrode, rGO@carbon cloth as the negative electrode and the BMIMBF4:PVA:TBABF4 gels as solid-state electrolytes. It was

55 found that the BMIMBF4:PVA:TBABF4 gel electrolytes processed a wide working temperature ranging from 20 ºC to 90 ºC. Moreover, the BMIMBF4:PVA:TBABF4 gel electrolytes exhibited enhanced ionic conductivity of 21.7 mS/cm compared to that (1.9 mS/cm) from the BMIMBF4:PVA gel electrolytes. MnO2@carbon cloth as the positive electrode, rGO@carbon cloth ASCs exhibited an operational window of 3.0 V, the energy density of 61.2 Wh/kg at the power density of 1049 W/kg, and more than 80% capacitance retention after 3000 charge/discharge cycles. All these results demonstrated that ACS we developed have great potential applications in the high-performance wearable and portable devices.

56 CHAPTER III

FLEXIBLE SUPERCAPACITORS WITH DOUBLE-NETWORK HYDROGEL

REDOX-ELECTROLYTES

3.1 Introduction

The energy storage devices with advanced features such as environmental friendliness, adaptable shape, and low cost, have great applications in implantable biosensors, nano-robots, and remote sensors.5, 14, 372-377 Supercapacitors, one of energy storage devices, have attracted great attentions in the past decades due to their high power density, fast charging and discharging times, and low cost.14, 376 Currently, supercapacitors incorporated with carbon materials as the electrodes were widely developed for approaching commercial products since carbon materials process advanced features, such as large surface area, high electrical conductivity, adjustable pore size, and low cost5, 14, 28 However, low energy density and flexibility of carbon materials restrict the practical applications of carbon-based supercapacitors.5

Various methods have been attempted to improve the energy density and flexibility of supercapacitors99, 276, 378-384 It was reported that hydrogel electrolytes were widely used as the quasi-solid-state electrolytes for approaching flexible supercapacitors due to its better flexibility and less leakage comparing to traditional liquid electrolytes.24,

385 High-performance supercapacitors with single-component hydrogel electrolytes, such as poly(vinyl alcohol) (PVA) with LiCl,386 poly(ethylene oxide) (PEO) with

KOH,297 and poly(vinylidene difluoride) (PVDF) with lithium trifluoromethanesulfonate (LiTFS),315 have been widely reported in recent years.

However, due to the intrinsic rigid nature, these single-component hydrogel electrolytes exhibit poor mechanical strength and flexibility.386 To explore the applications of flexible supercapacitors in wearable electronics, quasi-solid-state hydrogel electrolytes

57 with high mechanical strength and flexibility are needed to be developed.387

To address the poor mechanical strength and flexibility, double network (DN) hydrogels, possessing two independent and interpenetrating polymer networks, are used as the hydrogel electrolytes for supercapacitors. The DN hydrogels can disassemble the partial stress and dissipate the energy by the interpenetrations between different polymer networks, which can provide high mechanical strength, flexibility, and extensibility to supercapacitors.262, 300, 388

In this study, we propose a new design strategy to develop new flexible symmetric supercapacitors (SSCs) equipped with double-network (DN) hydrogels as the quasi- solid-state electrolytes by mixing poly(N-hydroxyethyl acrylamide) (pHEAA) with gelatin, chitosan, and Na2MoO4 redox additives (denoted as the pHEAA-gelatin- chitosan Na2MoO4 DN hydrogels electrolytes), active carbon cloth (ACC) as electrodes

(denoted as the ACC//Na2MoO4-DN electrolytes//ACC). Flexible carbon cloth and pHEAA-gelatin-chitosan DN hydrogel structure offer high mechanical strength and excellent flexibility, while the oxygen functional groups on ACC electrodes and

2- anchored MoO4 ions in pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes provide reversible energy storage sites by redox reactions leading a higher mechanical strength and energy density comparing to normal carbon-based supercapacitors. What’s

2- more, due to the coordination between MoO4 and chitosan, the redox additives are anchored in gel electrolytes, which prevents the stability degradation and environmental pollution caused by the leakage of molybdenum.389 The resultant

ACC//Na2MoO4-DN electrolytes//ACC SSCs achieved a high areal specific capacitance of 84 mF/cm2 and an energy density of 0.07 mWh/cm3 (~ 34 Wh/kg) at a power density of 3.8 mW/cm3 (~ 1800 W/kg) while retaining excellent cyclic stability after charge/discharge 3000 times. Also, 90º bending deformation on SSCs device for

58 1000 times did not result in any observable degradation in both electrochemical performance and mechanical strength. This work demonstrates a new design strategy to fabricate new hydrogel-based supercapacitors with superior flexibility, mechanical strength, and energy density for next-generation flexible and wearable energy storage devices.

3.2 Experimental section

3.2.1 Materials:

Chitosan powder (deacetylated chitin), Gelatin (gel strength ~300, type A), 2- hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959), potassium hydroxide, and sodium molybdate dehydrate (>99.5%) were purchased from Sigma-

Aldrich. N-Hydroxyethyl acrylamide (HEAA, >98%), sulfuric acid, and lithium chloride were purchased from TCI. Phosphoric acid and hydrochloric acid were purchased from Fisher Scientific. All materials are used as received without further purification. Ultrapure water was purified by a Millipore water purification system with a resistivity of 18.2 MΩ/cm.

3.2.2 Preparation of the pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes:

Chitosan powders were firstly dissolved in acetic acid acidulated Milli-Q water to prepare 0.02 g/mL aqueous solution at 50 oC. Afterward, a mixture solution containing

3.5 g HEAA, 0.7 g gelatin, 68 mg I-2959, 0~1000 (0, 250, 500, 750, 1000) μL 0.02 g/mL chitosan aqueous solution and extra Milli-Q water were added into beakers to a volume of 7 mL, respectively. After ultrasonic treatments, the solution was heated to

65 oC for ~10 minutes. Then the transparent hydrogel precursor was immediately injected into a sealed glass mold with a 1.0 mm volume. Then the precursor was exposed to UV light (365 nm, 8 W) for 1.5 hours. The DN hydrogel electrolytes then immersed into Na2MoO4 aqueous solutions (0.01 mol/L) for 10 min to fabricate

59 corresponding hydrogel electrolytes.

3.2.3 Tensile measurement:

The pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes were cut into bone-shape with a width of 4.00 mm, a gauge length of 17 mm, and a thickness of 1.0 mm. Tensile measurement was all performed on a universal tensile machine (Instron

3345, USA) with a 500 N transducer at the stretching rate of 100 mm/mm. Tensile strain

(ε) was defined as the extension distance (△L) divided by the initial length (L0).

3.2.4 Hysteresis measurement:

The pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes were first stretched to a suitable extension ratio (λ=4) and then unloaded. After returning to the original length, the specimens were reloaded and stretched to the same extension ratio

(λ=4) at 100 mm/min. This loading-unloading cycle was repeated four times and the interval time was controlled to ~5 min. Here, dissipated energy was estimated by area below the stress-strain curves or surrounded by the loading-unloading curves.

3.2.5 SEM-EDX measurement.

The morphology and elemental analysis of the pHEAA-gelatin-chitosan Na2MoO4

DN hydrogel electrolytes were recorded by field emission scanning electron microscopy (SEM) (Hitachi S-4700, Japan) equipped with energy dispersive spectrometer detector. The pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes were sputter-coated with a thin layer of platinum to improve the surface conductivity for better observation.

3.2.6 FTIR-ATR Spectroscopy.

Fourier-transform infrared spectroscopic (FT-IR) analysis was recorded by using

Thermo Scientific Nicolet 380 FTIR with a resolution at 4 cm-1, which evaluated chemical structures of lyophilized pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel

60 electrolytes.

3.2.7 Preparation and electrochemical characterization of SSCs.

The electrochemical performance of SSCs is measured by a two-electrode testing cell. Two pieces of active carbon cloth and the Na2MoO4-DN electrolyte were pressed together into sandwich-type cell construction (electrode/hydrogel electrolyte/electrode).

Finally, the cells were packaged by PET film. The preparation of active carbon cloth is followed by our previous work.135

The cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectrum (EIS) performance were tested by a Gamry reference 3000 electrochemical workstation (Gamry Instruments, U.S.A). EIS measurements were performed with the frequency ranging from 10 mHz to 100 kHz and an impedance amplitude of ±5 mV at open circuit potential. The cyclic stability was performed using computer-controlled cycling equipment (LAND CT2001A,

Wuhan China).

61 3.3 Results and Discussion

The preparation procedures of the pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes are described in Experimental Section. Figure 3.1a presents the tensile

Figure 3.1. a) The tensile strain-stress curves of pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes with different chitosan concentrations; b) Young’s modulus of pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes ; c) The hysteresis loading-unloading measurements of pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes undergo six cycles; d) The statistics of stiffness recovery and toughness recovery of pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes with six loading-unloading cycles. strain-stress curves of the pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes with different chitosan concentrations. It is found that the pHEAA-gelatin DN hydrogel electrolytes without chitosan exhibit a tensile stress of 1.56 MPa at the tensile strain of

9.48 mm/mm. The maximum tensile stress of the pHEAA-gelatin Na2MoO4 DN hydrogel electrolytes is higher than those reported values,390, 391 which demonstrates 62 that the pHEAA-gelatin DN hydrogel electrolytes possess superior mechanical strength and extensibility. Such high mechanical strength and extensibility are attributed to the rapid energy dissipation through the combination of two networks.392 It is also found that as the concentrations of chitosan are increased, the tensile strains are enlarged, but the tensile stresses are reduced. For example, at the chitosan concentration of 0.7 g/L, the pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes exhibit a tensile strain of 10.20 mm/mm and a tensile stress of 1.48 MPa. Whereas, at the chitosan concentration of 2.8 g/L, the pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes exhibit a tensile strain of 12.10 mm/mm and a tensile stress of 1.08 MPa.

The increased tensile strain and decreased tensile stress along with the chitosan concentrations are originated from the increased flexible cationic chitosan chains, which induce unbalanced charge interactions in the networks within the pHEAA-

393-396 gelatin-chitosan Na2MoO4 DN hydrogel electrolytes.

Figure 3.1b presents Young’s modulus of the pHEAA-gelatin-chitosan Na2MoO4

DN hydrogel electrolytes as the function of chitosan concentrations. Young’s modulus is the slope of the linear part of the tensile strain-stress curves as shown in Figure 3.1a.

It is found that chitosan concentration has negligible influences on Young’s modulus of the hydrogel electrolytes indicating that the introduction of chitosan does not alter the chemical bonds of the first network within the pHEAA-gelatin-chitosan Na2MoO4

DN hydrogel electrolytes. Since the pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes with a chitosan concentration of 1.4 g/L exhibit both moderated tensile strain and tensile stress among all the electrolytes, this electrolyte is used as an example to conduct further investigations.

To explore the influence of the intercalation effect between gelatin, pHEAA, and chitosan networks on the mechanical-induced network deformation, the self-recovery

63 and energy dissipation of the pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes are investigated. Figure 3.1c presents the hysteresis loops of the pHEAA- gelatin-chitosan Na2MoO4 DN hydrogel electrolytes through six consecutive loading- unloading tests under a tensile strain of 4.0 mm/mm at room temperature. It is found that the pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes exhibit a large hysteresis loop with notable hysteresis energy (i.e. loop area) of ~ 0.36 MJ/m3 during the first loading-unloading test. The hysteresis energy is reduced to ~ 0.20 MJ/m3 in the second loading-unloading test and remains approximatively the same values in the further third to sixth loading-unloading test. These results demonstrate that the fracture in the first network cannot be recovered immediately during the first loading-unloading test, which results in very limited energy dissipation in the following cycles.392

However, It is also found that the second network, which is ascribed to the reversible interpenetration and electrostatic interaction could rapidly recover independently on the load-unloading test.392

Figure 3.1d displays quantitative stiffness/toughness recoveries for each cycle. The toughness/stiffness recovery ratios (%/%) of the pHEAA-gelatin-chitosan Na2MoO4

DN hydrogel electrolytes are 57/60, 57/57, 59/62, 57/58, and 58/59 for the second, third, fourth, fifth, and sixth cycles, respectively. This fast recovery indicates that the pHEAA-gelatin-chitosan network can efficient facilitate dissipating energy, and assist the second network to be rapidly recovered during the deformation.300

Figure 3.2a presents the scanning electron microscope (SEM) images of the pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes. It is found that the pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes present typical porous structures with average pore sizes of 50-100 μm, which could offer the diverse channels and pathways for electrolytic ions to be transported from the pHEAA-gelatin-chitosan

Na2MoO4 DN hydrogel electrolytes to ACC electrodes, resulting in efficient ionic transport in supercapacitors.28 Figure 3.2b shows the energy dispersive X-ray (EDX) mapping images of the marked area in Figure 3.2a. In the EDX mapping images, molybdenum and nitrogen element are marked as purple dots and green dots, respectively. It is found that most molybdenum is accumulated around nitrogen instead

Figure 3.2. a) The SEM images and (b) The EDX mapping images of pHEAA- gelatin-chitosan Na2MoO4 DN hydrogel electrolytes, c) The ATR-FTIR spectra of DN electrolytes with and without Na2MoO4, respectively. of a homogeneous distribution. Such overlaps between molybdenum and nitrogen

65 indicate that the coordination effect between protonated amine groups of chitosan

2- 394 chains and MoO4 ions in the electrolytes occurs. Moreover, the EDX results (Figure

S7) reveal that the pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes mainly consist of C, N, O, Na, and Mo elements.

The attenuated total reflectance Fourier transform infrared (ATR-FTIR) was further conducted to confirm the interactions between Na2MoO4 additives and DN hydrogel.

Figure 3.2c presents the ATR-FTIR spectroscopies of the pHEAA-gelatin-chitosan DN hydrogel electrolytes with and without Na2MoO4 redox additives. It is clear that the pHEAA-gelatin-chitosan DN hydrogel electrolytes with and without Na2MoO4 redox additives exhibit similar ATR-FTIR peaks from the pHEAA-gelatin-chitosan networks.

The peak located at ~3270 cm-1 is attributed to the N-H stretching. The peaks located at 1639, 1553, and 1425 cm-1 are attributed to the asymmetric stretching, bending, and symmetric stretching of C=O bond, respectively. The peak located at 1239 cm-1 is attributed to the C-C stretching.397 However, there is a new peak located at ~829 cm-1 observed from the pHEAA-gelatin-chitosan DN hydrogel electrolytes with Na2MoO4 redox additives. This new peak is attributed to molybdate ions.398 Moreover, the weakening of the peak located at ~3270 cm-1 further confirms the coordination effect

2- 399 between protonated amine groups of chitosan chains and MoO4 ions. These anchored molybdate ions could generate the redox valence variation and store energy, resulting in extra and stable pseudocapacitance.400 As a result, supercapacitors incorporated with the pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes and

ACC electrodes are expected to possess both high capacitance and energy density.

The electrochemical performance of SSCs with a device configuration of ACC//DN electrolytes//ACC or ACC//Na2MoO4-DN electrolytes//ACC was characterized, where

ACCs were used as the electrodes, while DN hydrogels adsorbed supporting

66 electrolytes act as both electrolytes and separators. The preparation and characterization of ACC electrodes are described in detail in our previous work.135 Figure 3.3a shows the cyclic voltammetry (CV) curves of ACC//DN electrolytes//ACC SSCs with DN electrolytes adsorbed KOH, LiCl, and H2SO4 as supporting electrolytes, respectively, without redox additives at a scan rate of 50 mV/s in order to explore the pH adaptation of DN electrolytes. It is found that the SSCs exhibit an operational window from 0 V to 0.8 V, independent of the supporting electrolytes. Moreover, the shapes of these CV

Figure 3.3. a) The CV curves and b) The Nyquist plots of ACC//DN electrolytes//ACC SSCs with different supporting electrolytes; c) The CV curves and d) The Nyquist plots of ACC//Na2MoO4-DN electrolytes//ACC SSCs with different acidic supporting electrolytes. curves presents similar quasi-rectangular shapes indicating that DN electrolytes are

67 workable in a wide range of pH values.5, 35 In addition, the largest CV area observed from SSCs with DN electrolytes adsorbed H2SO4 indicates the largest capacitance of the SSCs.

Figure 3.3b shows the Nyquist plots of ACC//DN electrolytes//ACC SSCs with different supporting electrolytes. It is found that all the three curves present small semicircles in the high-frequency range indicating the compatibility at the interface between electrode and electrolyte. This compatibility is probably due to that both surfaces of electrode and electrolyte are porous and the excellent wettability of ACC electrodes, which forms an efficient interface for ionic transport.35 What’s more, The smallest semicircle observed from the SSCs with the DN electrolytes adsorbed H2SO4 indicates that the SSCs with the DN electrolytes adsorbed H2SO4 have the smallest charge transfer resistance (RCT). The smallest RCT indicates that protons are transported the most efficiently in DN electrolytes adsorbed H2SO4. In addition, considering

+ Na2MoO4 will be added into the electrolyte as redox-additives, the H can facilitate the

2- 382, 401 redox reaction of MoO4 . Thus, the electrolytes in an acidic condition with pH<7

2- are expected to be suitable for the MoO4 redox system. After ascertaining the suitable pH condition for DN electrolytes, the Na2MoO4 was added into the system as the redox additive to enhance the performance of SSCs. Figure 3.3c presents three CV curves of

SSCs with Na2MoO4-DN electrolytes absorbed different acid (H2SO4, or H3PO4 or HCl) as supporting electrolytes, respectively. The areas surrounded by all the three CV curves observed from ACC//Na2MoO4-DN electrolytes//ACC SSCs are much larger than that observed from ACC//DN electrolytes//ACC SSCs, which confirms the

Na2MoO4 can provide pseudocapacitance in electrolytes. During the three CV curves observed from ACC//Na2MoO4-DN electrolytes//ACC SSCs, the largest CV area is observed from SSCs with Na2MoO4-DN electrolytes absorbed HCl indicating the

68 largest capacitance of SSCs. The CV area observed from SSCs with Na2MoO4-DN electrolytes absorbed H2SO4 is slightly smaller than that absorbed HCl, but larger than that absorbed H3PO4. Thus, from the CV curves, the SSCs with Na2MoO4-DN electrolytes absorbed HCl possess the largest capacitance than that of SSCs absorbed

H2SO4 or H3PO4.

Figure 3.3d presents the Nyquist plots of SSCs with Na2MoO4-DN electrolytes

Figure 3.4. a) The CV curves of ACC//Na2MoO4-DN electrolytes//ACC SSCs at different scan rates; b) The GCD curves of ACC//Na2MoO4-DN electrolytes//ACC SSCs at different current densities; c) The specific capacitance of ACC//Na2MoO4- DN electrolytes//ACC SSCs at different charging-discharging current densities; d) The cycling stability of ACC//Na2MoO4-DN electrolytes//ACC SSCs.

69 absorbed H2SO4, or H3PO4 or HCl, respectively. It is found that similar series resistances (RS) of ~ 3 Ω are observed from SSCs with Na2MoO4-DN electrolytes absorbed H2SO4 and HCl. However, SSCs with the Na2MoO4-DN electrolytes absorbed

H3PO4 exhibit a large RS of ~ 9 Ω. Such larger RS (~ 9 Ω) is attributed to the smaller

402 dissociation constant of H3PO4 compared with HCl and H2SO4 Furthermore, among all the SSCs, the RCT of SSCs with Na2MoO4-DN electrolytes absorbed HCl is the smallest. The RCT of SSCs with Na2MoO4-DN electrolytes absorbed H2SO4 is slightly larger than that absorbed HCl, but smaller than that absorbed H3PO4. The smallest RCT indicates that SSCs with the Na2MoO4-DN electrolytes absorbed HCl show the most efficient ion transport process which is probably due to the almost complete

402 dissociation of HCl comparing to the partial dissociation of H2SO4 and H3PO4.

However, since it is well known for HCl to be evaporated easily, it likely induces instability of SSCs. Thus, the Na2MoO4-DN electrolytes absorbed H2SO4 are selected as the quasi-solid-state redox-electrolytes in SSCs for the following tests.

More electrochemical performance of the ACC//Na2MoO4-DN electrolytes//ACC

SSCs with H2SO4 as supporting electrolyte is shown in Figure 3.4. Figure 3.4a presents the CV curves of SSCs at different scan rates. It is found that the CV curves do not show obvious redox peaks in the operational window from 0 to 0.8 V. Only two shoulder peaks can be observed located at 0.2-0.3 V and 0-0.1 V, which can be ascribed to the oxidation and reduction of the reduced molybdates and molybdates, respectively.403 The possible reactions occurred in the redox-electrolytes are proposed as follows382, 401, 404:

2- + Mo(VI)O4 +2H ⇌H2Mo(VI)O4 (1)

- + H2Mo(VI)O4+2e +2H ⇌H2Mo(IV)O3+H2O (2)

- + H2Mo(VI)O4+e +H ⇌HMo(V)O3+H2O (3)

70 The overall quasi-rectangular symmetric shapes of the CV curves at different scan rates are similar to each other even at a high scan rate of 100 mv/s, indicating that the

ACC//Na2MoO4-DN electrolytes//ACC SSCs have good reversible capacitive behaviors. And no gas evolution peak can be observed indicating that the SSCs can keep a stable operational potential window from 0V to 0.8V. Figure 3.4b presents the galvanostatic charge/discharge (GCD) curves of SSCs at different current densities. As calculated by the equations listed in 1.2.4, at the current density of 1 mA/cm2, SSCs

2 show a specific capacitance of 84 mF/cm (~420 F/g based on the mass of Na2MoO4 in the electrolyte), which is 3 times larger than that (28 mF/cm2) without redox electrolytes

(Figure S8). The enlarged specific capacitance further confirms that the extra energy can be stored in the Na2MoO4 redox additive due to the various redox reactions of molybdenum. Figure 3.4c shows the specific capacitance of SSCs at different charge/discharge current densities. It is found that at the current density of 1 mA/cm2,

SSCs show a specific capacitance of 84 mF/cm2. After further increasing of the discharge current densities to 10 mA/cm2, the SSCs still retained 62% of capacitance retention. The capacitance retention of SSCs is much higher than that (32%) of SSCs without redox-electrolytes (Figure S9), indicating that the molybdenum-induced pseudocapacitance is more stable than the electrochemical-double-layer capacitance provided by the interface between electrodes and electrolytes at higher current density.

Moreover, the SSCs exhibit the energy density of 0.07 mWh/cm3 (~34 Wh/kg based on

3 the mass of Na2MoO4) and 0.005 mWh/cm (~2.4 Wh/kg) at the power density of 3.8 mW/cm3 (~1800 W/kg) and 7.2 mW/cm3 (~3400 W/kg), respectively. These values are comparable to the values reported by others recently,4, 405-408 which promises the application in wearable electronics. The above values are calculated by the equations listed in 1.2.4. Figure 3.4d presents the cyclic stability of SSCs. It is found that the SSCs

71 present good cyclic stability after 3000 charge/discharge cycles. During the first 500 cycles, the specific capacitance of the SSCs shows a slight increase, which is due to the diffusion process of ions in the electrolytes to the porous structures of electrodes.409 As

Figure 3.5. a) The photos of ACC//Na2MoO4-DN electrolytes//ACC SSCs at different bending angles; b) The CV curves of ACC//Na2MoO4-DN electrolytes//ACC SSCs at different bending angles; c) The CV curves of o ACC//Na2MoO4-DN electrolytes//ACC SSCs bended at 90 for 1000 times; d) The tensile strain-stress curves of Na2MoO4-DN electrolytes before and after 1000 times bending. e) The CV curves of ACC//Na2MoO4-DN electrolytes//ACC SSCs after 2- being bleached in pure H2SO4 solution without MoO4 for 24 hours.

the ions fully diffusing into the porous of electrodes, SSCs were activated and keep stable. After 3000 charge/discharge cycles at the current density of 5 mA/cm2, SSCs 72 can still retain more than 95% of capacitance retention. The decrease probably can be attributed to the evaporation of water from the quasi-solid-state electrolytes, which can generate detachment between electrolyte and electrodes leading a lower ionic conductivity.410 This stable long-term device performance indicates the potential applications of SSCs as reliable energy-storage devices. For the applications on wearable devices, the supercapacitors are anticipated to be eminently flexible. Owning to the excellent mechanical strength and flexibility of both Na2MoO4-DN electrolytes and ACC electrodes, the ACC//Na2MoO4-DN electrolytes//ACC SSCs are prospected to keep stable electrochemical after deforming repeatedly. Figure 3.5a presents the photo of SSCs bent at different angles. It is found that SSCs can be bent at 180º without any breakage or irreversible deformation, due to the excellent mechanical flexibility and toughness of the Na2MoO4-DN electrolytes and the ACC electrodes. The carbon fibers in the carbon cloth are strong enough to support the supercapacitors in large bending angles, and the Na2MoO4-DN electrolytes disperse the partial stress by the combination of two polymer networks preventing the fracture of the electrolytes. To investigate the relationship between flexibility and electrochemical performance, the electrochemical performance of supercapacitors was tested under bending states. Figure

3.5b presents the CV curves of SSCs at different bending angles of 0º, 45º, 90º, 135º, and 180º, to show the electrochemical performance at different bending angles. It is found that after SSCs were bent from 45º to 180º, there is no obvious change in both shapes and areas of CV curves, indicating that SSCs exhibit strong stable electrochemical performance almost independent of mechanical bending.

Figure 3.5c shows the CV curves of SSCs after 1000 times bending. No significant change can be observed in shapes or areas of the CV curves after 1000 times bending indicating that SSCs have excellent flexibility and stability comparing to the devices

73 reported by others.369, 411, 412 Such outstanding mechanical stability is ascribed to the rapid energy dissipation in Na2MoO4-DN electrolytes during bending. The

2- electrochemical stability is due to the coordination between MoO4 and chitosan. The pseudocapacitive component anchored in the electrolytes is more stable than that attached to the current collectors with binder after repetitive bending. The tensile strain- stress curves in Figure 3.5d also show that the Na2MoO4-DN electrolytes remain most of the machinal strength with a small reduction after 1000 times bending. The small reduction is probably due to the inevitable loss of moisture in the DN hydrogel during the bending process.

To further confirm whether Na2MoO4 is effectively anchored in the electrolytes,

1M H2SO4 solution was used to bleach the Na2MoO4 in SSCs. Figure 3.5e presents the

CV curves of SSCs after bleached in 1M H2SO4 solution for 24 hours. Comparing to the SSCs without bleaching, the SSCs immersed in 1.0 M H2SO4 solution for 24 hours remain more than 90% CV area of its initial value. The small shrinkage of CV area is

2- due to the diffusion of free MoO4 ions from the electrolyte into H2SO4 solution.

However, due to the coordination between protonated amide groups and molybdate ions,

2- most MoO4 ions are stilled anchored in the quasi-solid-state electrolytes after being rinsed by H2SO4 solution, which can prevent the leakage of the toxic molybdenum.

3.4 Conclusion

In this study, we reported novel symmetric supercapacitors with the pHEAA- gelatin-chitosan Na2MoO4 DN hydrogel electrolytes and ACC electrodes. The

Na2MoO4-DN electrolytes exhibit excellent mechanical properties with a high tensile stress of ~1.4 MPa, a tensile strain of ~11 mm/mm, and fast self-recovery features. The

ACC//Na2MoO4-DN electrolytes//ACC SSCs exhibit a specific capacitance of 84 mF/cm2 at the current density of 1mA/cm2, an energy density of 0.07 mWh/cm3 at the

74 3 power density of 3.8 mW/cm , and excellent cycle stability. The ACC//Na2MoO4-DN electrolytes//ACC SSCs remain more than 95% capacitance after 3000 charge/discharge cycles. All these results demonstrate that the ACC//Na2MoO4-DN electrolytes//ACC SSCs possess great potential applications in wearable energy-storage systems.

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APPENDICES

Figure S1 The SEM images of BMIMBF4:PVA:TBABF4 gel electrolytes with a BMIMBF4:TBABF4 molar ratio of 2:1.

Figure S2 The CV curves of supercapacitors with BMIMBF4:PVA gel electrolytes.

76 Figure S3 CV curves of supercapacitors with BMIMBF4:PVA gel electrolytes and BMIMBF4:PVA:TBABF4 gel electrolytes at the scan rate of 75 mv/s.

Figure S4 The GCD curves of supercapacitors with (a) BMIMBF4:PVA and (b) BMIMBF4:PVA:TBABF4 gel electrolytes.

77 Figure S5 The CV curves of the supercapacitors with BMIMBF4:PVA gel electrolytes at different working temperatures.

Figure S6 The ragone plot of MnO2@carbon cloth //BMIMBF4:PVA:TBABF4//rGO@carbon cloth ASCs.

Figure S7. The EDX spectrum of the pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes.

Figure S8. The GCD curves of ACC//DN electrolytes//ACC SSCs with and without 2- MoO4 , respectively.

Figure S9. Specific capacitance of ACC//DN electrolytes//ACC SSCs at different charging-discharging current densities.