Fuel cells are attractive alternatives to combustion engines for electrical power generation because of their very high efficiencies and low pollution levels. While many types of fuel cells, differentiated primarily by the type of electrolyte, are under development, few arecommercially available. This is, in part, due to fundamental limitations in materials properties. For example, while solid oxide fuel cells offer excellent fuel efficiency, their high temperature operation, necessitated by the poor ionic conductivity of the zirconia electrolyte, results in high costs. Similarly, the heat and moisture sensitivity of polymeric proton conductors results in complex and costly fuel cell systems. To address these challenges, there is a clear need for fuel cells that operate at intermediate temperatures, in the 200 – 600 ºC range, which ensures high catalytic activity while avoiding the inherent difficulties of high-temperature operation. We describe here recent advances in this direction, where the fuel cell electrolyte materials span both proton and oxide ion conductors (i.e. solid acids and ceria based oxides), and the electrode materials are mixed ion and electron conductors (ceria-based oxides as anodes and cobaltite oxides as cathodes).

While fuel cells offer exceptionally high conversion efficiencies from chemical to electrical energy, they continue to rely on chemical energy inputs for operation. In a truly sustainable energy scenario, the chemical energy serves as a storage medium for a carbon-free energy input (i.e. renewable or nuclear energy). Of viable carbon-free energy sources, solar energy is available in excess, by several orders of magnitude, of current global energy consumption rates. We show here that the mixed conductors useful for fuel cell electrode applications have the necessary characteristics for thermochemical conversion of heat energy, ideally obtained from solar sources, to chemical energy. Using water, carbon dioxide and thermal energy as inputs, we demonstrate production of fuels ranging from hydrogen, carbon monoxide, syngas, and methane with moderate efficiency and extremely rapid kinetics.