Materials aspects of photo-electrochemical systems

Photo-electrochemical systems are midway between solid state photovoltaic devices and electrochemical cells. On the one hand this creates new material problems, such as corrosion of the photoactive material, while on the other hand the demand for lattice match is replaced by that of rapid kinetics of the electrochemical reactions occuring in the cell. Photocorrosion is dependent on the rate of reaction between light-generated charge carriers and solution species, as well as the rate of removal of reacted solution species from the electrode surface. The occurrence of more than one stable valence state in the semiconductor of the photoelectrode can prevent photocorrosion. Exchange of surface ions with the solution may create a dynamic equilibrium which can lead to effective stabilization of certain photoelectrodes. The substrate material of the photoelectrode should act as a poor electrocatalytic material for the reverse of the photogenerated reaction and its contact with the semiconductor material should be ohmic and physically coherent. The counter-electrode must be kinetically rapid for the reverse of the photogenerated reaction, which may cause complications if even small quantities of electrocatalyst reach the photoelectrode from the counter-electrode. If a storage system is added to the photo-electrochemical system its electrochemical as well as chemical properties have to be carefully matched to that of the photoconverting system. When chemicals are produced instead of electrical energy, the energy difference between the bandgap of the semiconductor and the photopotential obtained can be utilized to overcome overpotentials. This is of special importance for photodiodes, where appreciable overpotentials for dark reactions can prevent side-reactions and recombination, while the photoreaction has sufficient energy available for overcoming these overpotentials. Photodiodes and photocatalysts are conceptually close to each other, their difference being defined by the energetic requirements of the chemical reaction. While the combination of energy production, energy storage and the production of chemicals into one device creates many technical problems, it may turn out to be more economical in the end as has been shown to be true for parallel cases in chemical technology.