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University of Southampton Research Repository Eprints Soton University of Southampton Research Repository ePrints Soton Copyright © and Moral Rights for this thesis are retained by the author and/or other copyright owners. A copy can be downloaded for personal non-commercial research or study, without prior permission or charge. This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the copyright holder/s. The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the copyright holders. When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given e.g. AUTHOR (year of submission) "Full thesis title", University of Southampton, name of the University School or Department, PhD Thesis, pagination http://eprints.soton.ac.uk UNIVERSITY OF SOUTHAMPTON Faculty of Engineering and the Environment Energy Technology Research group Heteropolyacids and non-carbon electrode materials for fuel cell and battery applications by Maria Kourasi Thesis for the degree of Doctor of Philosophy February 2015 UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF ENGINEERING AND THE ENVIRONMENT Doctor of Philosophy HETEROPOLYACIDS AND NON-CARBON ELECTRODE MATERIALS FOR FUEL CELL AND BATTERY APPLICATIONS by Maria Kourasi Heteropolyacids (HPAs) are a group of chemicals that have shown promising results as catalysts during the last decades. Since HPAs have displayed encouraging performance as electrocatalysts in acidic environment, in this project their redox activity in acid and alkaline aqueous electrolytes and their electrocatalytic performance as additives on a bifunctional gas diffusion electrode in alkaline aqueous electrolyte are tested. The results from the electrochemical characterisation of two different HPAs, the phos- phomolybdic acid (PMA) and the phosphotungstic acid (PWA) dissolved in acidic and alkaline environment showed that both heteropolyacids demonstrate a redox activity but they also suffer from low stability issues. A series of gas diffusion electrodes were manufactured having PMA and PWA incorpo- rated in their catalyst layer. The electrode support was carbon Toray paper and each heteropolyacid was mixed with Ni to create the catalyst layer of the electrode. From the electrochemical characterisation oF these electrodes in alkaline electrolyte, it was shown that the addition of HPAs enhances the activity of the nickel towards OER and ORR. During the constant current measurements on the manufactured gas diffusion electrodes it was noticed that the electrodes fail after a period of time which could be attributed to the corrosion of the carbon support. In order to find alternative, non-carbon materials to be used as the electrode support, electrochemical characterisation on Magn´eliphase bulk materials, Magn´elispray coated electrodes and PVD coated electrodes was performed. The results from this investigation showed that Magn´eliphase materials can support electron transfer reactions but their electron conductivity is rather low and it needs to be enhanced. Additionally, it was presented that the Magn´elicoating protects the substrate over the potential region where OER and ORR take place. Hence, Magn´eli materials could be used as a support for the bifunctional HPA gas diffusion electrodes. Contents Abstract i List of Figures ix List of Tables xix Declaration of Authorship xxiii Acknowledgements xxv Nomenclature xxx 1 Introduction 1 1.1 Background and motivation . 1 1.2 Aims and Objectives . 7 1.3 Thesis outline . 8 2 Literature review 11 2.1 Novel electrocatalysts: Heteropolyacids . 11 2.1.1 Applications of Heteropolyacids . 16 2.1.1.1 HPAs in fuel cell electrolytes . 18 2.1.1.1.1 Heteropolyacids in PEMFC electrolytes . 19 iii Contents 2.1.1.1.2 Heteropolyacids in DMFC electrolytes . 21 2.1.1.2 HPAs immobilised on fuel cell electrodes . 22 2.1.1.2.1 Heteropolyacids in PEMFC electrodes . 24 2.1.1.2.2 Heteropolyacids in DMFC electrodes . 28 2.1.1.2.3 Heteropolyacids in DAFC electrodes . 32 2.2 Bifunctional oxygen/air electrodes . 32 2.2.1 Catalyst layer . 34 2.2.2 Catalyst support . 36 2.2.2.1 Magn´eliphase materials . 37 2.3 Summary . 39 3 Experimental details 41 3.1 Materials and chemicals . 41 3.2 Electrolyte preparation . 42 3.3 Electrochemical techniques . 42 3.4 Microscopy . 44 3.5 HPA redox activity experiments . 45 3.5.1 Cyclic voltammetry . 45 3.6 HPA gas diffusion electrodes . 45 3.6.1 Catalyst ink preparation . 47 3.6.2 Printing the electrodes . 48 3.6.3 Cyclic voltammetry . 48 3.6.4 Constant current potentiometry . 50 3.7 Magn´eliphase electrodes . 50 3.7.1 Cyclic voltammetry . 52 iv Contents 3.8 APS & PVD coated substrates . 53 3.8.1 Cyclic voltammetry . 54 4 Investigation of the redox activity of HPAs 57 4.1 Introduction . 57 4.2 Results and discussion . 58 4.2.1 Cyclic voltammetries . 58 4.2.1.1 Heteropolyacids dissolved in acidic solution . 58 4.2.1.2 Heteropolyacids dissolved in alkaline solution . 66 4.2.1.3 Glassy carbon electrode modified with heteropoly- acids in acidic solution . 69 4.3 Conclusions and Further Work . 71 5 HPA gas diffusion electrodes 73 5.1 Introduction . 73 5.2 Results and discussion . 74 5.2.1 Cyclic voltammetries . 74 5.2.2 Constant current potentiometry . 83 5.2.3 Microscopy . 87 5.2.4 Charge / discharge in a battery . 89 5.3 Conclusions and Further Work . 93 6 Further investigation on HPA gas diffusion electrodes 97 6.1 Introduction . 97 6.2 Results and discussion . 98 6.2.1 Cyclic voltammetries . 98 6.2.1.1 Electrodes with varying HPA content . 98 v Contents 6.2.1.1.1 Electrodes with varying PMA:Ni ratio . 100 6.2.1.1.2 Electrodes with varying PWA:Ni ratio . 103 6.2.1.1.3 Comparison between the best performing Ni PMA vs. the best performing Ni PWA electrode . 106 6.2.1.2 Nickel carbonate instead of nickel micro-powder . 107 6.2.1.3 Ink solid loading effect . 110 6.2.2 Constant current potentiometry . 114 6.3 Conclusions and Further work . 116 7 Alternative materials as electrode substrates 119 7.1 Introduction . 119 7.2 Results and discussion . 120 7.2.1 Studies on Magn´eliphase materials . 120 7.2.1.1 Microelectrodes with Magn´elidense fibres . 121 7.2.1.2 Electrodes with Magn´eliporous tablets . 125 7.2.2 Electrodes with APS Magn´elicoating . 128 7.2.2.1 APS Magn´eliphase coated electrodes with stain- less steel substrates . 129 7.2.2.2 APS Magn´eliphase coated electrodes with alu- minium substrates . 133 7.2.2.3 APS Magn´eliphase coated electrodes with carbon polymer substrates . 137 7.2.3 Electrodes with PVD coating . 140 7.3 Conclusions and Further Work . 148 vi Contents 8 Conclusions and Further work 151 8.1 Conclusions . 151 8.2 Further Work . 155 8.2.1 HPA solution-based redox activity . 155 8.2.2 Bifunctional HPA gas diffusion electrodes . 156 8.2.3 Alternative materials as electrode substrates . 156 Appendix A- HPAs as additives in electrolyte membranes in fuel cells 159 References 169 vii viii List of Figures 1.1 Schematic of a URFC in (a) water electrolyser mode and in (b) fuel cell mode (Design 1). 3 1.2 Schematic of a URFC in (a) water electrolyser mode and in (b) fuel cell mode (Design 2). 3 1.3 Schematic of a bifunctional oxygen/air electrode . 5 1.4 Project's tree diagram . 9 2.1 Ball and stick diagrams showing (A) Keggin and (B) Wells-Dawson HPA variant where, heteroatoms are black, addenda atoms are red and oxygen atoms are light grey. The diagrams were designed with Vestra (Ver.3.1.5) software. 13 2.2 Structural diagrams showing (A) Keggin and (B) Wells-Dawson HPA variant where, heteroatoms tetrahedra are black and addenda octahedra are red. The diagrams were designed with Vestra (Ver.3.1.5) software. 14 2.3 The model of Damjanovic et al. for oxygen reduction on metals in both alkaline and acidic media. Water can either be formed directly via the four-electron pathway, 1, or via the two-electron pathway, 2 and 3 in series. The subscript ad refers to an adsorbed species. 25 3.1 Cyclic voltammogram used as a qualitative example. It corresponds to the voltammogram presented in figure 4.2(b) and page 62. 44 3.2 The electrochemical cell used during the measurements of the HPA redox activity . 46 ix List of figures 3.3 The electrochemical cell used during the measurements with the HPA gas diffusion electrodes . 49 3.4 An image of the microelectrode with the Magn´elifibre . 51 3.5 The design of the electrode with the Magn´elitablet . 52 3.6 The glass electrochemical cell used for the characterisation of the different coated electrodes . 55 4.1 Cyclic voltammograms of 5 10-2 M PMA (black line) and 10 10-2 × × M PMA (dotted line) dissolved in 1.0 M H2SO4. The electrolyte was saturated with nitrogen gas. The fourth cycle of each voltam- mogram is presented. The scan rate of the voltammogram was 100 mV sec-1 and the temperature was 20oC. The working electrode area was 0.07 cm2. ........................... 59 -2 4.2 Cyclic voltammograms of 5 10 M PWA dissolved in 1.0 M H2SO4; × (a) is the overall voltammogram, (b) is the voltammogram of the first redox peak, (c) is the voltammogram of the second redox peak and (d) is the voltammogram of the third redox peak. The elec- trolyte was saturated with nitrogen gas. The second cycle of each voltammogram is presented. The scan rate of the voltammogram was 100 mV sec-1 and the temperature was 20oC.
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