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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 phosphomolybdic 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 incorporated 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 Magneli phase bulk materials, Magneli spray coated electrodes and PVD coated electrodes was performed. The results from this investigation showed that Magneli phase 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 Magneli coating protects the substrate over the potential region where OER and ORR take place. Hence, Magneli materials could be used as a support for the bifunctional HPA gas diffusion electrodes. |
