The Reactivity and Stability of Polyoxometalate Water Oxidation Electrocatalysts

molecules Review The Reactivity and Stability of Polyoxometalate Water Oxidation Electrocatalysts Dandan Gao 1, Ivan Trentin 1, Ludwig Schwiedrzik 2 , Leticia González 2,* and Carsten Streb 1,3,* 1 Institute of Inorganic Chemistry I, Ulm University, Albert-Einstein-Allee 11, 89081 Ulm, Germany; [email protected] (D.G.); [email protected] (I.T.) 2 Institute of Theoretical Chemistry, Faculty of Chemistry, University of Vienna, Währinger Str. 17, 1090 Vienna, Austria; [email protected] 3 Helmholtz-Institute Ulm (HIU), Helmholtzstr. 11, 89081 Ulm, Germany * Correspondence: [email protected] (L.G.); [email protected] (C.S.) Academic Editor: Mark Symes Received: 4 December 2019; Accepted: 27 December 2019; Published: 31 December 2019 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. Keywords: polyoxometalate; water oxidation catalysis; oxygen evolution reaction; self-assembly; electrocatalysis 1. Introduction The splitting of water into hydrogen and oxygen is a key technology for sustainable energy conversion and storage [1–3]. Particularly, electrocatalytic water splitting is relevant to generate hydrogen as secondary energy carrier from intermittent energy sources, such as solar or wind power [3–6]. In general, 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 (i.e., in solution), 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. Thus, in homogeneous electrocatalysis, the electron transfer to the catalyst is an interfacial process between electrode and solution, while the catalytic turnover (via a proton-coupled electron transfer mechanism, PCET) is expected to occur in solution (Figure1)[ 7]. In contrast, in heterogeneous catalysis, electron transfer and substrate turnover occur at the catalyst-solvent interface [3]; since water oxidation involves formation of a gaseous product, O2, triple-phase boundaries between solid catalyst, liquid electrolyte and gas have to be considered also [8]. In addition, eﬀective electron transfer between the heterogeneous catalyst and the actual electrode (often metal or glassy carbon) needs to be considered to fully understand the macroscopically observed WOC activity [3,9]. Molecules 2020, 25, 157; doi:10.3390/molecules25010157 www.mdpi.com/journal/molecules Molecules 2019, 24, x 2 of 20 electrode (often metal or glassy carbon) needs to be considered to fully understand the macroscopically observed WOC activity [3,9]. Further, when technological usage is targeted, the operating conditions of modern water electrolyzers have to be considered. Currently, three main types of water electrolyzers are employed: Alkaline water electrolyzers (AWE) operate in highly alkaline aqueous solution (pH > 13), which restricts catalyst choice due to stability considerations [10]. In AWEs, NiFe layered double hydroxides are often used as WOCs with high corrosion stability [10,11]. Additionally, current research is exploring other transition metal (oxy)hydroxides as alternative WOCs [12]. In contrast, proton exchange membrane (PEM) water electrolyzers operate under acidic conditions (pH < 2) using a solid membrane for proton transport instead of liquid electrolytes [13]. This also limits catalyst selection to materials which can withstand these harsh conditions [13]. In PEM water electrolysis, typically noble metal oxides, such as IrO2 or RuO2, have been used as catalysts [14]. More recent studies have reported noble-metal free metal oxides as potential alternative WOCs in PEMs [15]. Anion exchange membrane water electrolyzers (AEMs) can operate at near pH-neutral conditions [16], so that catalyst selection is less critical, resulting in the use of various metal oxides/hydroxides as WOCs (e.g., Co3O4, CuCoOx) [16,17]. Note that while the technology-readiness levels of these established systems and WOCs are often in the prototype stages, for the polyoxometalate-catalysts discussed here, many fundamentalMolecules 2020, 25 questions, 157 need to be addressed before advanced upscaling and deployment can2 of be 20 envisaged; see the following sections. Figure 1. ((a)) Simplified Simpliﬁed schematic illustration of the chemicalchemical and electrochemical steps occurring during homogeneous and heterogeneousheterogeneous electrocatalyticelectrocatalytic waterwater oxidation.oxidation. ( (bb)) Illustration Illustration of the functional componentscomponents required required within within a moleculara molecular POM-WOC. POM-WOC. ET—electron ET—electron transfer; transfer; PCET—proton PCET— protoncoupled coupled electron electron transfer; transf polyoxometalateer; polyoxometalate (POM); (POM); water oxidation water oxidation catalyst catalyst (WOC). (WOC). OneFurther, of the when most technological intriguing classes usage of ismolecular targeted, WOCs the operating are molecular conditions metal oxide of modern clusters, water so- calledelectrolyzers polyoxometalates have to be considered. (POMs). POMs Currently, are anioni threec main metal types oxo of clusters water electrolyzerstypically formed are employed: by pH-, Alkalinetemperature water and electrolyzers concentration-dependent (AWE) operate complex in highly self-assembly alkaline aqueous routes solutionin aqueous (pH solutions> 13), which [18– 22].restricts POMs catalyst are typically choice due based to stabilityon early, considerations high-valent transition [10]. In AWEs, metals NiFe (mainly layered Mo, double W and hydroxides V), where theare outer often surface used as of WOCsthe POM with anions high is corrosionstabilized stabilityby unreactive, [10,11 strong]. Additionally, metal-oxo double current bonds. research Non- is functionalizedexploring other POMs transition are, therefore, metal (oxy)hydroxides not suitable for as water alternative oxidation, WOCs as the [12 ].redox In contrast,potential protonof the high-valentexchange membrane central metals (PEM) (Mo(VI), water electrolyzers W(VI), V(V)) operate is not undersufficient acidic for conditions electron abstraction (pH < 2) using from awater solid ormembrane oxo ligands. for proton Thus, transport functionaliz insteadation of of liquid the cluster electrolytes shell [13with]. This suitable also limitsreaction catalyst sites, selection mainly transitionto materials metals, which is canused withstand as central thesedesign harsh principle conditions for active [13 ].POM-WOCs In PEM water [23–26]. electrolysis, There are typically several fundamentalnoble metal oxides,requirements such as for IrO the2 structuralor RuO2, haveand chemical been used design as catalysts of viable [ 14POM-WOCs:]. More recent studies have(1) reported One or noble-metalseveral redox-active free metal transition oxides metals as potential need to alternative be incorporated WOCs in in the PEMs POM [ 15framework]. Anion toexchange enable the membrane proton-coupled, water electrolyzers four-electron (AEMs) water can oxidation operate process. at near pH-neutralThe use of several conditions metal [16 sites], so facilitatesthat catalyst this selection approach, is less as the critical, four-electron resulting transf in theer use can of be various split up metal between oxides the/hydroxides redox-active as WOCs metal (e.g.,centers Co present.3O4, CuCoO Manyx)[ POM-WOCs16,17]. Note reported that while feature the technology-readiness four or more metal sites, levels some of thesespecies established featuring upsystems to nine and redox-acti WOCs areve oftencenters in the[27,28]. prototype stages, for the polyoxometalate-catalysts discussed here, many(2) fundamental For water oxidation questions to need proceed, to be addressed the catalysts before require advanced substrate upscaling binding and sites; deployment i.e., accessible can be coordinationenvisaged; see sites the where following aquo sections. ligands can bind [26]. While some reports recently discussed the use One of the most intriguing classes of molecular WOCs are molecular metal oxide clusters, so-called polyoxometalatesMolecules 2019, 24, x; doi: (POMs). POMs are anionic metal oxo clusters typicallywww.mdpi.com formed by pH-,/journal/molecules temperature and concentration-dependent complex self-assembly routes in aqueous solutions [18–22]. POMs are typically based on early, high-valent transition metals (mainly Mo, W and V), where the outer

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