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.
Highlights
O2 1 evolution from mA cm−2 on water WO3 photoanodes without the need for any additional electrochemical bias
In the less extreme case, where product gas crossover is slower, hydrogen permeating to the anode side of the cell is preferentially oxidized to protons and electrons, while oxygen on the cathode side is preferentially reduced to water
Assuming a current density of 10 mA cm−2 for hydrogen production[5] and a target of producing 500 g of H2 gas over the course of 8 h irradiation at 1 Sun,[7] an array of area ∼17 m2 is required (see Section SI-5 in the Supporting Information (SI) for calculation). This is possible in principle, but collecting H2 from such a high surface area array may be inefficient
Summary
Quantification of the amount of oxygen evolved was possible using an O2 fluorescence-quench probe, and comparison with the total charge passed during irradiation indicated that the Faradaic efficiency for oxygen production by photolytic water splitting was 84% (±6%), see Figure 4 These results suggest that solar-driven water oxidation can be coupled to reduction of [PMo12O40]3− to give a reduced ECPB without simultaneous hydrogen evolution, such that gas mixing in a working device could be kept to a minimum (even at current densities under irradiation on the order of only 1 mA cm−2). The charge passed during reoxidation was identical to the charge passed during initial reduction of the ECPB during the photodriven water oxidation step This ECPB makes a good mediator for potential solar-to-hydrogen applications, greatly reducing gas crossover and allowing O2 and H2 to be generated at different rates and at different times on account of these two processes being electrochemically decoupled.
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