A bio-inspired molecular catalyst that selectively catalyzes water oxidation in seawater, without significant chlorine formation

Most transport fuels are derived from fossil fuels, generate greenhouse gases, and consume significant amounts of water in the extraction, purification, and/or burning processes. The generation of hydrogen using solar energy to split water, ideally from sea water or other non-potable sources, could potentially provide an unlimited, clean fuel for the future. Solar, electrochemical water splitting typically combines a photoanode at which water oxidation occurs, with a cathode for proton reduction to hydrogen. In recent work we have found that a bioinspired tetra-manganese cluster catalyzes water oxidation at relatively low overpotentials (0.38 V) when doped into a Nafion proton conduction membrane deposited on a suitable electrode surface, and illuminated with visible light. We report here that this assembly is active in electrolyte solutions containing a range of different salts in varying concentrations. Similar photocurrents were obtained using sodium sulfate, sodium perchlorate, or sodium chloride over various electrolyte concentrations (0.0 - 0.5 M). The photocurrent declined only at and above 5.0 M sodium perchlorate. Remarkably, the photocurrent did not increase in the presence of chloride ions, either in saturated aqueous NaCl or in non-aqueous acetonitrile. Moreover, testing of the system in sea water generated the same photocurrent as aqueous 0.1 M Na2SO4. In a normal water electrolysis cell, chlorine gas would be preferentially formed from chloride (before water oxidation) because, while the standard potential of water oxidation is lower than that of chloride, its overpotential is substantially higher. The catalyst-induced decline in overpotential and, possibly, the impermeability of Nafion to chloride ions, reverses this situation in the present system. This finding raises the possibility of developing a novel, solar-powered desalination process.