As water is the most abundant molecule on the planet, and given the huge amounts of solar energy that strike the earth every day, the production of hydrogen by using sunlight to split water has the potential to provide large amounts of clean, renewable fuel. This can be achieved by coupling a water electrolyzer to photovoltaic cells, which has been previously demonstrated to yield solar-to-hydrogen conversion efficiencies of up to 7%. However, due to the large overpotentials required to oxidize water, typical electrolyzers operate at voltages of around 2 V. Thus, when using conventional silicon photovoltaic devices, four cells need to be assembled in series, making the process prohibitively expensive. A wide range of catalysts that lower the required overpotential have been developed in response to this challenge. An ideal water oxidation catalyst would remove the overpotential, so that only the thermodynamic energy would be required to drive the water-splitting reaction; equivalent to a voltage of 1.23 V (pH 0). A diverse range of metal oxides, including multimetal oxides containing various combinations of Ti, Nb, Ta, W, Ga, In, Ge, Sn, and Sb; narrow-band-gap semiconductors, such as CdS and CdSe; and other materials have been developed in an effort to achieve this goal. 5–13] This is a very active area of research because achieving the right balance between energy absorption, catalytic activity, and materials stability has proven difficult for a single material. Some of these limitations have been overcome by coupling appropriate combinations of materials. For example, the unsuitable band edge positions of WO3 and Fe2O3 can be surmounted by coupling these materials to a photovoltaic device in tandem. In this case, the extra potential required to oxidize water and reduce protons is provided by the photovoltaic device. Solar-to-hydrogen conversion efficiencies of 6% have been reported for these tandem systems. In other examples, the challenge has been addressed by integrating multiple materials into a single electrode, creating multi-junction devices in which the photoanode contains a layer of a water oxidation photocatalyst, such as GaInP2, and a layer of photovoltaic material, such as a GaAs p/n junction, which provides the extra potential required to complete the circuit. Other approaches include depositing doped thin-film oxides (NiFeO2 and Fe2O3) on multi-junction photocells. 7] These previous examples have focused on the use of solid films as the catalytic material. In addition to these systems, a wide range of molecular water oxidation catalysts have been developed. The majority of these catalysts are based on inorganic Ru, Ir, or Mn complexes. Of these catalysts only a few have been successfully attached to electrode surfaces, which is a prerequisite for their incorporation into photoelectrochemical devices. To the best of our knowledge there are no reports of the successful integration of these types of water oxidation catalysts with a solar cell into a tandem water-splitting device. We recently reported that a tetranuclear Mn-oxo cluster, [Mn4O4L6] + (1 ; L= (p-Me-C6H4)2PO2; Scheme 1A), [18,19] is able to catalyze the oxidation of water for extended periods when doped within the proton-conducting channels of a Nafion membrane, polarized at 1 V (vs Ag/AgCl) and illuminated with visible light. The development of this catalyst was inspired by the presence of a tetranuclear Mn cluster in the water oxida-