A leading approach for large-scale electrochemical energy production with minimal global-warming gas emission is to use a renewable source of electricity, such as solar energy, to oxidize water, providing the abundant source of electrons needed in fuel synthesis. We report corrosion-resistant, nanocomposite anodes for the oxidation of water required to produce renewable fuels. Silicon, an earth-abundant element and an efficient photovoltaic material, is protected by atomic layer deposition (ALD) of a highly uniform, 2 nm thick layer of titanium dioxide (TiO(2)) and then coated with an optically transmitting layer of a known catalyst (3 nm iridium). Photoelectrochemical water oxidation was observed to occur below the reversible potential whereas dark electrochemical water oxidation was found to have low-to-moderate overpotentials at all pH values, resulting in an inferred photovoltage of ~550 mV. Water oxidation is sustained at these anodes for many hours in harsh pH and oxidative environments whereas comparable silicon anodes without the TiO(2) coating quickly fail. The desirable electrochemical efficiency and corrosion resistance of these anodes is made possible by the low electron-tunnelling resistance (<0.006 Ω cm(2) for p(+)-Si) and uniform thickness of atomic-layer deposited TiO(2).

We report on the effects on water oxidation performance of varying (1) the nanoscale TiO 2 thickness and (2) the catalyst material in catalyst/TiO 2 /SiO 2 /Si anodes. Uniform films of atomic layer deposited TiO 2 are prepared in the thickness range $1-12 nm on degenerately-doped p + -Si, yielding water oxidation overpotentials at 1 mA cm À2 of 300 mV to 600 mV in aqueous solution (pH 0 to 14). Electron/hole transport through Schottky tunnel junction structures of varying TiO 2 thickness was studied using the reversible redox couple ferri/ferrocyanide. The dependence of the water oxidation overpotential on ALD-TiO 2 thickness, with all other anode design features unchanged, exhibits a linear trend corresponding to $21 mV of added overpotential at 1 mA cm À2 per nanometer of TiO 2 for TiO 2 thicknesses greater than $2 nm. For thinner TiO 2 layers, an approximately thickness-independent overpotential is observed.The linear behavior for anodes with thicker TiO 2 layers is consistent with the predicted effect of bulk TiO 2 -limited electronic conduction on the voltage required to sustain the current density across the TiO 2 /SiO 2 insulator stack. Eight different oxygen evolution catalysts of thickness 1-3 nm are studied. For the anodes investigated, 3 nm of Ir or Ru gave the best water oxidation performance, but both thinner layers and other catalysts can be quite effective, suggesting the potential for reduced materials cost. Lastly, a flat band voltage analysis of solid state thin film capacitors was done for five different gate metals on n-Si to probe junction energetics directly relevant to a photoanode. The results are consistent with a Schottky junction in which the Fermi level at the semiconductor surface is unpinned. Broader contextEnergy storage is likely to be a requirement for grid-scale replacement of fossil fuel sources by renewable energy. Its value lies not only in the ability to compensate for intermittency, but also in the economics of peak leveling and its national security implications. The possibility of synthesizing fuels in a renewable and clean fashion has long fascinated academic and industrial researchers alike, but the high cost and poor efficiencies of such processes have prevented practical implementation of solar fuel or electro-fuel synthesis. In an effort to address these problems, researchers have been pursuing the development of photoelectrochemical cells: all-in-one units that convert solar energy into energy stored in chemical bonds. However, materials that are stable under harsh reducing and, especially, oxidizing conditions are rarely optimized for solar absorption and transport of electronic carriers. The ability to combine the properties of wide bandgap and stable materials, like TiO 2 , and efficient small bandgap absorbers, like silicon, constitutes a major advance in making viable photoelectrochemical cells. This report looks further at a novel device structure for fuel synthesis in which high quality absorbers are coupled to high quality water oxidation catalysts via an ALD...

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