Solar-Driven Water Oxidation and Decoupled Hydrogen Production Mediated by an Electron-Coupled-Proton Buffer

Abstract

Solar-to-hydrogen photoelectrochemical cells (PECs) have been proposed as a means of converting sunlight into H2 fuel. However, in traditional PECs, the oxygen evolution reaction and the hydrogen evolution reaction are coupled, and so the rate of both of these is limited by the photocurrents that can be generated from the solar flux. This in turn leads to slow rates of gas evolution that favor crossover of H2 into the O2 stream and vice versa, even through ostensibly impermeable membranes such as Nafion. Herein, we show that the use of the electron-coupled-proton buffer (ECPB) H3PMo12O40 allows solar-driven O2 evolution from water to proceed at rates of over 1 mA cm–2 on WO3 photoanodes without the need for any additional electrochemical bias. No H2 is produced in the PEC, and instead H3PMo12O40 is reduced to H5PMo12O40. If the reduced ECPB is subjected to a separate electrochemical reoxidation, then H2 is produced with full overall Faradaic efficiency.

Electrochemistry of Polyoxometalates: From Fundamental Aspects to Applications

“…H 6 SiW V 2 W VI 10 O 40 released protons to effectively generate highly pure H 2 (only 0.08 % O 2 ) on a Pt/C surface . A WO 3 ‐FTO photoanode in 1 M CH 3 SO 3 H (pH 1) under visible light generated electricity as well as oxygen, whereas H 3 PMo 12 O 40 was reduced in conjunction with proton coupling to H 5 PMo 12 O 40 on carbon felt as a cathode in water at pH 0.1 . When H 5 PMo 12 O 40 was reoxidized to H 3 PMo 12 O 40 , H 2 was evolved in a 100 % Faradaic yield, implying that this system should be an excellent H 2 storage method.…”

Tuning Redox Active Polyoxometalates for Efficient Electron‐Coupled Proton‐Buffer‐Mediated Water Splitting

“…Previously, we introduced the electron‐coupled‐proton‐buffer (ECPB) to separate the process of water splitting in time and space . In this way, the hydrogen evolution reaction (HER) is no longer coupled with the rate‐limiting step of the oxygen evolution reaction (OER).…”

Recent Progress in Decoupled H2 and O2 Production from Electrolytic Water Splitting

“…Therefore, decoupled water splitting represents an excellent strategy in avoiding this H 2 /O 2 mixing issue for solar‐driven water electrolysis. Following their success in utilizing phosphomolybdic acid as the ECPB for electrochemical decoupled water splitting, the Cronin group further demonstrated that this strategy could be applied in solar‐driven water splitting as well . As shown in Figure a, WO 3 was adopted as an effective photoanode due to its suitable band gap which straddles both O 2 evolution onset (1.2 V vs NHE) and the reduction potential of [PMo 12 O 40 ] 3− (∼0.6 V vs NHE), as well as great stability under strongly acidic condition.…”