Photoelectrocatalysis

One of the major roadblocks to large-scale usage of solar power is the storage of energy during periods of little to no sunlight. One possible solution is the direct conversion of sunlight into chemical fuels, including hydrogen and simple hydrocarbons such as methanol. We can show a simplified process where photogenerated charge carriers are used to promote a redox reaction (here water oxidation), thus storing solar energy in the form of chemical bonds. In order to promote this conversion, the electron-hole pair must be separated – typically by an electric field within the device. This places certain requirements on photoelectrode materials; namely, that they exhibit high charge carrier mobility and good light absorption in the solar spectrum.

To date, there are two predominant strategies for generating solar fuel via water splitting: (1) “direct” photoelectrocatalysis at the semiconductor-electrolyte interface, i.e. at a solid-liquid junction, and (2) coupling the electrochemical reaction directly to a buried p-n junction solar cell. In the first case, the band edge positions relative to the H+/H2 and O2/H2O redox couples and the band bending at the solid-liquid interface are essential energetic parameters for water splitting. This method should also lead to simpler device architectures. In the second case, existing photovoltaic expertise can be utilized to design a solar cell with the appropriate current-voltage characteristics for solar water splitting. In addition to the photoelectrode requirements of surface stability, good electronic properties and suitable light absorption characteristics, both approaches require the generation of a photovoltage sufficient to split water (> 1.23 V). This is a tall order for any one material, and therefore there are many research fronts for improving solar water splitting devices. In addition to the energetic considerations discussed above, the reactions taking place during water splitting can require large overpotentials to overcome kinetic limitations and proceed at appreciable rates. The use of a co-catalyst on the photoelectrode surface can reduce the overpotential required for a given redox reaction to take place, and in some cases promotes separation and diffusion of carrier species.