Abstract
This review describes major advances in the use of functionalized molecular metal oxides (polyoxometalates, POMs) as water oxidation catalysts under electrochemical conditions. The fundamentals of POM-based water oxidation are described, together with a brief overview of general approaches to designing POM water oxidation catalysts. Next, the use of POMs for homogeneous, solution-phase water oxidation is described together with a summary of theoretical studies shedding light on the POM-WOC mechanism. This is followed by a discussion of heterogenization of POMs on electrically conductive substrates for technologically more relevant application studies. The stability of POM water oxidation catalysts is discussed, using select examples where detailed data is already available. The review finishes with an outlook on future perspectives and emerging themes in electrocatalytic polyoxometalate-based water oxidation research.
Highlights
The splitting of water into hydrogen and oxygen is a key technology for sustainable energy conversion and storage [1,2,3]
Electrocatalytic water oxidation catalyst (WOC) research can be broadly subdivided in homogenous catalysis using molecular catalysts and heterogeneous catalysis, mainly focusing on solid-state, often nanostructured materials
Note that while homogeneous WOC catalysis occurs in one phase, under electrocatalytic conditions, the role of interfacial electrochemical processes between the molecular catalyst and the electrode surface have to be considered when rationalizing reactivities and the mechanism
Summary
The splitting of water into hydrogen and oxygen is a key technology for sustainable energy conversion and storage [1,2,3]. Functionalization of the cluster shell with suitable reaction sites, mainly to materials which harsh conditions [13].POM-WOCs. In PEM water electrolysis, typically transition metals, is can usedwithstand as central these design principle for active [23,24,25,26]. (1) One or several redox-active transition metals need to be incorporated in the POM framework to enable the proton-coupled, four-electron water oxidation process. (3) The proton-coupled electron transfer reactions required for water oxidation can be facilitated by nearby proton transfer sites; i.e., Bronsted bases capable of supporting the release and transfer of protons from the water substrate Due to their general structures, POM-WOCs typically feature terminal and bridging oxo-ligands close to the reaction sites, so that oxo-ligand-assisted proton transfer is possible. We return to discussing the question of POM stability under different experimental conditions, (Section 5), and provide a brief outlook at future developments from the authors’ point of view. (Section 6)
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