A schematic of silicotungstic acid–mediated H2 evolution from water. At the anode, H2O is split into O2, protons, and electrons, while the mediator is reversibly reduced and protonated at the cathode in preference to direct production of H2. The reduced H6[SiW12O40] (dark blue) is then transferred to a separate chamber for H2 evolution over a suitable catalyst and without additional energy input after two-electron reduction of the mediator to H6[SiW12O40].
