Solar-to-hydrogen photoelectrochemical cells (PECs) have been proposed as a means of converting sunlight into H-2 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 H-2 into the O-2 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 O-2 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 H-2 is produced in the PEC, and instead H3PMo12O40 is reduced to H5PMo12O40. If the reduced ECPB is subjected to a separate electrochemical reoxidation, then H-2 is produced with full overall Faradaic efficiency.