First-principles study of materials for solar thermochemical gas splitting

Metal oxide materials may be used in two-step solar thermochemical water-splitting cycles to renewably produce hydrogen. Here, we use first-principles density functional calculations to investigate promising oxide materials for use in these cycles. We present a general analysis of the equilibrium thermodynamics of a two-step metal oxide water splitting cycle, and survey a large number (more than 100) binary oxide redox couples. CeO₂ is promising, but the reoxidation at low temperature is kinetically limited due to slow surface processes, however, very little is known about the mechanism. We use DFT to identify stable surface structures containing surface or subsurface oxygen vacancies, H₂O or H. We calculate barriers for diffusion of oxygen vacancies, and barriers for dissociation of water and the desorption of hydrogen. Our results show that the surface coverage of adsorbed H may play a key role in the kinetics of water splitting on ceria surfaces.