Activated Chemical Bonds In Nanoporous And Amorphous Iridium Oxides Favor Low Overpotential For Oxygen Evolution Reaction

Oxygen evolution reaction (OER) is the key anodic catalytic reaction for many important clean energy processes. To date, the search for an active, selective, and stable electrocatalysts has not ceased and a detailed atomic-level design of the OER catalyst remains an outstanding (if not, compelling) problem. Only recently, a computational high-throughput study of iridium oxides (for both \ce{IrO2} and \ce{IrO3}) has highlighted the role of polymorphism and stoichiometry to precisely engineer iridium oxides for efficient and stable OER catalysis. However, it seems surprising that nanoporous (i.e. crystal structures containing nanopores and nanochannels) and amorphous iridium oxides -- which have been proposed in various experiments -- were not examined (and excluded) in that study. In this work, we have further extended the search to include many metastable nanoporous iridium oxide polymorphs -- inspired both from experiments and also analogous nanoporous crystal structures of known manganese oxides, as well as amorphous iridium oxides of different stoichiometries. Using van der Waals corrected density-functional theory calculations, we investigate the thermodynamic and electrochemical stability, intercalation properties, and electronic structure of these nanoporous and amorphous iridium oxides. Using the correlation between the Ir-O bond strength/length and the associated charge states of Ir/O, we demonstrate how the weaker Ir-O bonds and the availability of more electrophilic oxygens in these nanoporous and amorphous iridium oxides can explain their superior OER performance via the known Mars van Krevelen surface mechanism