(315b) The Role of Charge Compensating Dopants in Decreasing the O-vacancy Formation Energy in ceria
Solar thermochemical water and carbon dioxide splitting provide a pathway to high efficiency solar fuels. To-date, ceria remains the benchmark material due to its stability and rapid kinetics. Of the many doping strategies investigated, only Hf- and Zr-doping has shown increased performance over undoped ceria. Here we use paired charge compensating dopants (PCCD) to improve the performance of ceria. Specifically, we use density functional theory calculations explore the reduction behavior of ceria co-doped with tri- and pentavalent dopants. We show that this strategy increases the range of available reduction energies available in ceria and provide a fundamental understanding for the found behavior. The identity of the trivalent determines the reduction energy of the system while pentavalent dopants only have a minor effect, due to their individual mechanisms of action. The pentavalent dopants cluster and cause large local restructuring which highly suppresses the O-removal energy in its vicinity to level below that needed to drive gas splitting. The trivalent dopants however weaken the bonds with their coordinating O anions through decreased ionic interaction; this interaction only slightly decrease the O-vacancy formation energy, making these sites more redox active. Overall we predict relative reduction energies of PCCD ceria of Hf doped ceria (Hf-CeO2) < ScNb-CeO2 < YNb-CeO2 < LaNb-CeO2 < CeO2. Experimental studies using thermos-gravimetric analysis confirm the predicted reduction energies. Although these materials are investigated in light of solar thermochemistry, their behavior is of potential interest to the metal oxide catalytic community as a whole.