Solar-hydrogen: A solid-state chemistry perspective

The present work considers the solid-state chemistry-related issues relevant to the photo-electrochemical generation of hydrogen from water using solar energy. The focus is on the properties of semiconducting photo-electrodes, which comprise the critical component for increasing the efficiency of the conversion of solar energy into chemical energy. The most important parameters of the photo-electrodes, which are essential for the conversion and to be modified to this end, are the band gap and the Fermi energy. The relationships between these properties and the materials properties that can be experimentally determined conveniently (electrical conductivity, thermoelectric power and work function) are outlined. The present paper brings together the concepts of photo-electrochemistry with the concepts of defect chemistry and solid-state electrochemistry. It is considered that the Fermi energy (the chemical potential of electrons) at the surface of the photo-electrode is the key quantity in the assessment of the reactivity of photo-electrode materials with water. Consequently, the desired reactivity between the photo-electrode and water may be achieved through appropriate modification of this quantity during processing or subsequent treatment of the photo-electrode material. Further, the optimal photoelectrode for water photolysis with high solar energy conversion efficiency should exhibit a band gap of ~2eV and be resistant to corrosion in water. Finally, the reactivity between the photo-electrode and water must be considered in terms of both collective properties, such as electronic structure, and local properties related to photocatalytic active surface sites at which the photocatalytic reaction between water and the photoelectrode takes place. The leading candidate for such photo-electrodes is titania (TiO₂-x), which exhibits outstanding resistance to corrosion and photo-corrosion in aqueous environments. The optimal reactivity of titania with water, leading to maximal solar energy conversion efficiencies, can be achieved through maximization of solar energy absorption and minimization of energy losses due to recombination and charge transport. This may be achieved through the modification of defect disorder and related properties including charge transport, charge separation and electronic structure. Progress in research on the determination of the relevant properties of titania essential to theperformance of photo-electrodes, including defect chemistry and related electroactivity, is discussed. Significance, benefits and advantages of the solar-hydrogen technology and cost-related estimates are briefly considered.