Polyoxometalate Multi‐Electron‐Transfer Catalytic Systems for Water Splitting

Abstract

The viable production of solar fuels requires a visible-light absorbing unit, a H2O (or CO2) reduction catalyst (WRC) and a water oxidation catalyst (WOC) that work in tandem to split water or reduce CO2 with H2O rapidly, selectively and for long periods of time. Most catalysts and photosensitizers developed to date for these triadic systems are oxidatively, thermally and/or hydrolytically unstable. Polyoxometalates (POMs) constitute a huge class of complexes with extensively tunable properties that are oxidatively, thermally and (over wide and adjustable pH ranges) hydrolytically stable. POMs are some of the fastest and most stable WOCs to date. This Microreview updates the very active POM WOC field, reports the first POM WRCs and initial selfassembling metal oxide semiconductor-photosensitizer-POM catalyst triad photoanodes. The complexities of investigating these POM systems, including but not limited to the study of POM-hydrated metal ion-metal oxide speciation processes, are outlined. The achievements and challenges in POM WOC, WRC and triad research are outlined. IntroductionMeasurements and models make it ever more certain that the planet will face a serious energy shortage as the availability of economically accessible fossil fuels fails to keep pace with global energy needs. [1] Data and analysis also indicate that the environmental change caused by fossil fuel combustion will become increasingly problematic. Although green and alternative energy sources are rapidly becoming more available and less expensive, the net consumption of environmentally worrisome fossil fuel is not dropping significantly. Increases in both global population and average global standard of living paint a less-thanrosy picture for our energy future. [1b, 1g, 2] Solar remains the most likely source of sustainable energy for the medium and longer-term future. The other renewable sources of energy, with the arguable exception of biofuels provided the energy production efficiency (photosynthesis and other efficiencies) can be significantly increased, will not likely be sufficient to power the planet. In addition, high density energy will be needed in enormous quantities moving forward; electricity and other sources of energy will not provide sufficient energy density for our major transportation needs (ships, aircraft). Unlike the production of solar electricity, which is a now a rapidly maturing technical area and a major and growing market sector, production of solar fuel is in its infancy.The principal reactions for the generation of solar fuel are H2O splitting to produce H2 and O2 (eq. 1) and H2O splitting coupled to CO2 reduction (eq. 2). Technology is needed so both these processes can be driven by terrestrial sunlight and proceed with high rates and selectivity to the desired products. A factor in the slow rates observed for H2O oxidation by many systems is that it is a four-electron, four-proton process, hence the need for a catalyst that can facilitate the multiple proton-coupled electron transfer (PCET) processes with lo...

A Keggin Polyoxometalate Shows Water Oxidation Activity at Neutral pH: POM@ZIF‐8, an Efficient and Robust Electrocatalyst

“…[4,5] In last few decades, various research groups have contributed towards understanding of the process of WO and designing of robust and efficient WOCs. [8][9][10][11][12][13][14][15][16][17][18] Development of polyoxometalates (POMs) as WOCs came as am ajor breakthrough in this regard. [8][9][10][11][12][13][14][15][16][17][18] Development of polyoxometalates (POMs) as WOCs came as am ajor breakthrough in this regard.…”

Self-assembly of rare-earth Anderson polyoxometalates on the surface of imide polymeric hollow fiber membranes potentially for organic pollutant degradation

“…Isopolyanions are composed of a metal oxide framework without the internal heteroatom/heteroanion, which are mostly represented by the Lindqvist structure [5] and decavanadates [6]. Heteropolyanions centring on a heteroatom that Because of the versatile nature in terms of structure, size, and tunable properties such as redox potential, acidity, charge distribution and solubility in various media, which can be manipulated by choosing proper constituent elements and counter cations, POMs are receiving considerable attention in extensive fields such as catalysis, materials, energy, medicine and biology [7][8][9][10]. Among their diversified properties, their function as catalysts is the most popular research field for either oxidation reactions or acid-dependent processes.…”

Multi‐Electron Oxidation of Anthracene Derivatives by Nonheme Manganese(IV)‐Oxo Complexes

“…Multi‐electron transfers are of major importance in a wide range of fields including renewable energy and bioinspired catalysis. For example, natural photosynthesis proceeds through multi‐electron transfer processes such as the four‐electron oxidation of water and the multi‐electron reduction of CO 2 . Unlike single electron transfers, mechanisms of multi‐electron transfer reactions have yet to be clearly elucidated .…”

“…For example, natural photosynthesis proceeds through multi‐electron transfer processes such as the four‐electron oxidation of water and the multi‐electron reduction of CO 2 . Unlike single electron transfers, mechanisms of multi‐electron transfer reactions have yet to be clearly elucidated . Among multi‐electron oxidations of various substrates, the oxidation of anthracene and its derivatives merits special attention because the number of electrons to be transferred is different depending on the substituents on the anthracene derivatives .…”