Antenna molecule drives solar hydrogen generation

It would be convenient if gigantic reservoirs of molecular hydrogen H2(g) existed in the earth’s crust that could be safely tapped as an energy source. Instead, most hydrogen found near the earth’s surface is bound up in water as protons, H+(aq) that must be reduced before utilization as a fuel (1). When the electrons for proton reduction come from the oxidation of water to O2(g), this redox chemistry is commonly referred to as “water splitting”:2H2O(aq)→2H2(g)+O2(g) (ΔG°=4.92eV).2H2O(aq)→2H2(g)+O2(g) (ΔG°=4.92eV).[1] As shown in Eq. 1, water splitting is thermodynamically uphill and hence provides a means for storing energy in chemical bonds. The reverse reaction produces water and energy. The identification of an inexpensive scalable process for water splitting has been a “holy grail” of science for decades that could one day enable a “hydrogen economy” (2). Electrolysis accomplishes water splitting, but practical utility requires an inexpensive source of electrical power that ideally would not involve greenhouse gas formation from fossil fuel combustion. Electricity generated from wind or photovoltaic panels could be used for this purpose, but this does not yet appear to be cost-effective. Approaches that integrate solar energy harvesting and catalysis are particularly attractive as they afford the real possibility for inexpensive production of H2(g). In 1972, Fujishima and Honda (3) reported sustained water splitting from a relatively simple photoelectrochemical cell based on TiO2 and Pt electrodes separated by a membrane (Fig. 1). The drawback was that sunlight absorption was limited to the UV region, comprising less than 3% of the solar spectrum. Since that time, alternative semiconductor materials have been identified that effectively harvest sunlight …