Researchers at MIT, the Skoltech Institute of Technology, and the University of Texas at Austin show that mobilizing oxygen atoms from the crystal surface of perovskite-oxide electrodes to participate in the formation of oxygen gas is key to speeding up water-splitting reactions. This work may carry great significance for the widespread adoption of water splitting to produce hydrogen fuel, a desirable way to move from traditional energy sources like fossil fuels towards clean, renewable energy sources.
In their study, perovskite oxide catalysts consisting of varying proportions of lanthanum and strontium with cobalt and oxygen were investigated. The scientists identified two key parameters controlling the catalytic performance of these materials: the covalency of the cobalt-oxygen bond and the number of oxygen vacancies. They then tuned their synthesis parameters to demonstrate that one particular material, strontium cobaltite (SrCoO2.7), exhibits highly active water electrolysis, much faster than the state-of-art catalyst iridium oxide, which contains precious metals. The team then managed to unravel how the reaction occurs on the atomic scale, illustrating an unprecedented new electrolysis pathway occurring on SrCoO2.7, and showed that it occurs as a result of the nature of the electronic structure in cobalt and oxygen. The researchers stated that this shortcut-like pathway with an exceptionally low energy barrier provides essential understanding not only for the development of highly-efficient catalysts, but also for future catalyst optimization.
Metal oxide perovskite catalysts in these electrochemical reactions are often viewed as unaffected by the chemical interactions they stimulate, but the team showed through theoretically based models and simulations that in the most-efficient process — essentially a fast-kinetic shortcut — dehydrogenated hydroxide ions adsorb to the catalyst surface and combine directly with oxygen from the oxide lattice. The consumed lattice oxygen atoms are replaced by hydroxides from the water, thereby returning the catalyst to its initial state to restart the cycle. This mechanism is in contrast to the conventionally assumed reaction pathway, in which only the transition metal lattice sites are involved.