Green synthesis of water splitting electrocatalysts: IrO₂ nanocages via Pearson's chemistry

Highly porous iridium oxide structures are particularly well-suited for the preparation of porous catalyst layers needed in proton exchange membrane water electrolyzers. Herein, we report the formation of iridium oxide nanostructured cages, via a water-based process performed at room temperature, using cheap Cu₂O cubes as the template. In this synthetic approach, based on Pearson's hard and soft acid-base theory, the replacement of the Cu₂O core by an iridium shell is permitted by the difference in hardness/softness of cations and anions of the two reactants Cu₂O and IrCl₃. Calcination followed by acid leaching allow the removal of residual copper oxide cores and leave IrO₂ hierarchical porous structures with outstanding activity toward the oxygen evolution reaction. Fundamental understanding of the reaction steps and identification of the intermediates are permitted by coupling a set of ex situ and in situ techniques including operando time-resolved X-ray absorption spectroscopy during the synthesis.