Basic concepts of photoelectrochemical solar energy conversion systems

Abstract

The development of novel oxides, nanocomposites and architectures is in demand for the direct conversion of solar energy into other forms of energy, such as chemical and electrical energy. Especially, those oxides and devices, which can be used for photolysis of water (hydrogen and oxygen evolution) and for water purification, are of interest. So far, semiconducting oxides such as TiO₂, Fe₂O₃, WO₃, SrTiO₃, tantalates and niobates are the only class of materials which have shown high stability as photoelectrodes towards corrosion in the rate limiting step in the oxygen evolution reaction (OER) under illumination at the electrode/electrolyte interface during photolysis of water. Oxides have to be developed to be highly conductive and have a bandgap, which can be achieved by tailoring the defect chemistry of the oxides or by formation of a suited mixed oxide phase. Of special interest is the preparation of highly conductive p- and n-type TiO₂ and WO₃ as well as their alloys as corrosion stable and photoelectrocatalytically active electrodes. Bandgap reduction of pure TiO₂ involved the formations of solid solutions of TiO₂–FeO and TiO₂–Fe₂O₃, which were reported to have a bandgap of ∼2·2 eV.¹ Besides TiO₂, WO₃ also has a superior stability as a photoelectrode material in the OER. Alloying with FeO also leads to lowering of the bandgap. Alternatively, ternary oxides of the systems Ni–Co–O and Ni–Fe–O are known for their high catalytic activity in the OER. They are considered as potential cocatalysts in the process of water oxidation. The materials can also be used in hybrid photoelectrodes consisting of a photovoltaic structure to absorb the sunlight with a corrosion stable and catalytically active window layer, which is in contact with the electrolyte.