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 highlighted.