Polyoxometalate-Based Electrocatalysts for Anodes in Electrolytic Water Splitting

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Background Polyoxometalates (POMs) are molecular metal oxides with the general formula [MxOy]n- where M = V, Nb, Ta, W or Mo and it is possible to substitute M for other metals and to remove or substitute oxygen for other groups. POMs in which one or more M atoms are replaced with other transition metals have been shown to catalyse water oxidation, which is a vital step in the production of hydrogen from water - a key process for renewable energy conversion and storage. Aims This project aims to provide a platform for detailed mechanistic studies of water activation at substituted POMs, leading to a theoretical understanding of the electron- and proton-transfer processes involved, the design of active catalysts and their use in a new type of electrolyser.Objectives 1. Preparation of reactive transition-metal-substituted POMs based on the Lindqvist-type {M6} structure.Transition-metal-substituted POMs have potential as highly active, stable water oxidation catalysts, and those studied to date have large structures containing upwards of 22 metal atoms. In order to gain insight into how these compounds react with water we will develop a series of simpler transition metal containing POMs based on the simpler Lindqvist structure, with only six metal atoms (Figure 1). This will allow us to probe more easily the important interactions between these POMs and water during oxidation catalysis.Novel procedures previously used by our group to access the cobalt-substituted POM [(CoW5O18H)2]6- (Figure 1) will be adapted in order access new {M'M5} POMs, using organic cations to impart solubility in non-aqueous solvents (Eur. J. Inorg. Chem., 2009, 5240-5246). Initial targets will be {MW5} analogues containing manganese, iron or ruthenium, and attempts will then be made to extend the series to {MMo5} molybdates.In addition to elemental microanalysis, the {M'M5} structure will be confirmed by infrared spectroscopy and single-crystal X-ray crystallography. In addition, X-ray absorption spectroscopy (XAS) will be used to probe structure and oxidation states in solution and in the solid state. While the use of multinuclear NMR atom is likely to be limited for paramagnetic compounds, EPR will provide information about spin states of the anions. Linear and cyclic voltammetry studies will establish the redox properties of the POMs and determine whether they have the appropriate potentials to drive water oxidation. 2. Theoretical understanding of the effects on reactivity of metal substitution in {M'M5} Lindqvist POMs.Density Functional Theory (DFT) modelling of our POMs will be carried out in parallel with experimental studies with the assistance of a newly appointed PDRA from the Poblet group at URV, Tarragona. The studies will provide insight into how the transition metal present in the POM structure influences the interaction with water by examining energies associated with proton and electron transfers, and the calculated redox potentials and electrostatic potential distributions can be tested experimentally. Computational modelling of POM water oxidation should be easier with the smaller Lindqvist POMs, but such systems have received little attention to date.3. Incorporation of active POMs as redox mediators into an electrolytic cell. The end goal of the project is to develop effective water oxidation catalysts that can be used in a working electrolytic cell for water splitting. Electrolysis experiments will be carried out with Dr Mohamed Mamlouk, a chemical engineer with extensive experience of fuel cell and electrolyser design and testing. The role of the POMs is to reduce the oxygen evolution over-potential and thereby increase the efficiency of the electrolysis process.