Heteropolyacids and non-carbon electrode materials for fuel cell and battery applications

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

Molybdophosphoric acid based nickel catalysts as bifunctional oxygen electrodes in alkaline media

An air-breathing electrode was fabricated using a catalyst combination of nickel nano-powder and molybdophosphoric acid (H3PMo12O40) immobilised in a Nafion® based ink. The first investigation of the use of H3PMo12O40, a heteropolyacid (HPA), as an oxygen evolution catalyst and its application in an alkaline media is presented. Incorporating molybdophosphoric acid with nickel nano-powder resulted in more than a 650 mV reduction in potential difference between the oxygen evolution and reduction reactions at a current density of 44 mA cm? 2 when compared with electrodes made with the nickel nanopowder alone. The electrodes showed good stability and are potentially suitable for applications such as water electrolysis, unitised fuel cells and secondary metal–air batteries

Heteropolyacids for fuel cell applications

Polyoxometalates, and in particular heteropolyacids, are being studied extensively for potential applications, including as surfactants, chemical cleaners, catalysts and additives in fuel cell component materials. Heteropolyacids comprise MOx polyhedra (usually octahedral), in which M is typically W, Mo or V, surrounding one or more heteroatoms. They are commonly characterised under the Wells-Dawson, Keggin or lacunary structural configurations. It is possible to tailor their properties by modifying the central metal ion and the addenda atoms. In the context of fuel cells they are used as additives to enhance catalyst performance, as membrane additives to improve ionic conductance, and as stand-alone catalysts (electrode immobilised or electrolyte solvated). More recently they have been employed as catholyte redox mediators in novel fuel cells. This review summarises the electrochemical properties of heteropolyacids and provides a critical assessment of their applications, with a focus on fuel cells. Progress with regards to heteropolyacid-based electrochemical science and technology is summarised and key challenges are highlighte

Improved, high conductivity titanium sub-oxide coated electrodes obtained by Atmospheric Plasma Spray

Coatings based on reduced stoichiometries of titanium dioxide obtained by Atmospheric Plasma Spray are presented for their application as electrodes in devices such as lead acid batteries, redox flow batteries and electrochemical reactors. A study of their microstructure, composition and mechanical properties is first presented in this paper to show that the layers are compact, free of cracking and well adhered to the substrate. The presence of Magneli phase oxides, particularly Ti8O15 was identified in the coatings, which enhances their electrical conductivity. In order to understand the behaviour of this material as an electrode, an electrochemical analysis of the coatings was also done. The coated samples showed higher oxygen and hydrogen evolution overpotentials and lower electrochemical corrosion compared to typically used commercial stainless steel and carbon polymer composite electrodes

Enhancing the performance of common electrode materials by means of atmospheric plasma spray coatings

Atmospheric plasma spraying is used here to obtain titanium sub-oxide coatings on the following electrode substrates: a 316 stainless steel sheet, an aluminium alloy 2024 sheet, a carbon–polymer composite and nickel foam. It is necessary to increase the roughness of the substrate to aid the adhesion of the coating to the smooth electrodes; this is especially critical for the fragile carbon–polymer composite. High-energy conditions increase the strain caused by the mismatch between coefficients of thermal expansion when coating the thin steel and aluminium sheets, and degraded the carbon–polymer composite and nickel foam. Therefore, it is necessary to adjust the spraying parameters to lower-energy conditions in order to achieve well-bonded and homogeneous coatings on all four substrates and avoid the presence of cracks in their cross-section area. The suitability of the coated materials as battery electrodes was studied

Enhanced performance of common electrode materials obtained by means of atmospheric plasma coatings

Atmospheric plasma spraying is used here to obtain titanium sub-oxide coatings on the following electrode substrates: a 316 stainless steel sheet, an aluminium alloy 2024 sheet, a carbon-polymer composite and nickel foam. It is necessary to increase the roughness of the substrate to aid the adhesion of the coating to the smooth electrodes; this is especially critical for the fragile carbon-polymer composite. High-energy conditions increase the strain caused by the mismatch between coefficients of thermal expansion when coating the thin steel and aluminium sheets, and degraded the carbon-polymer composite and nickel foam. Therefore, it is necessary to adjust the spraying parameters to lower-energy conditions in order to achieve well-bonded and homogeneous coatings on all four substrates and avoid the presence of cracks in their cross-section area. The suitability of the coated materials as battery electrodes was studied

Enhancing the performance of common electrode materials by means of atmospheric plasma spray coatings

Atmospheric plasma spraying is used here to obtain titanium sub-oxide coatings on the following electrode substrates: a 316 stainless steel sheet, an aluminium alloy 2024 sheet, a carbon-polymer composite and nickel foam. It is necessary to increase the roughness of the substrate to aid the adhesion of the coating to the smooth electrodes; this is especially critical for the fragile carbon-polymer composite. High-energy conditions increase the strain caused by the mismatch between coefficients of thermal expansion when coating the thin steel and aluminium sheets, and degraded the carbon-polymer composite and nickel foam. Therefore, it is necessary to adjust the spraying parameters to lower-energy conditions in order to achieve well-bonded and homogeneous coatings on all four substrates and avoid the presence of cracks in their cross-section area. The suitability of the coated materials as battery electrodes was studied