Polyoxometalate multi-electron-transfer catalytic systems for water splitting

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 CO ₂ 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 under optimal conditions. This Microreview updates the very active POM WOC field; it reports the application of POMs as WRCs and initial self-assembling 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. Polyoxometalates (POMs) have enjoyed recent success as catalysts in the multi-electron water oxidation and reduction reactions. They have also been incorporated into photoanodes for heterogeneous water oxidation. We address the challenges that remain and highlight upcoming advances in the field.