Low Iridium Content Confined inside a Co3O4 Hollow Sphere for Superior Acidic Water Oxidation

Noble-metal-oxide support catalysts have been demonstrated to be unique for electrocatalytic water oxidation in acidic media. Highly porous three-dimensional oxide supported can serve as an ideal platform to confine ultrasmall metal catalysts on specific sites and modulate their reactivity, resulting in the reduction of noble metal content in the catalyst by boosting the mass activity. However, due to poor control over the support morphology, geometric-driven shifts in mass activity of metal-oxide support catalysts for the oxygen evolution reaction in acidic media have not been realized. Herein, a nanoscale Kirkendall effect is exploited to produce and control a structural evolution yielding an oxygen-evolving catalyst that is highly efficient and robust in acidic medium. By selective reaction-diffusion under oxidizing conditions, the starting solid CoIr NC is directly transformed into an unprecedented Ir-Co3O4@Co3O4 porous-core@shell hollow nanospheres (ICO PCSHS), in which an ultrasmall Ir catalyst is spatially isolated within a porous Co3O4-backbone core, encapsulated by a hollow Co3O4 outer shell. With a low Ir content of 14 wt %, the iridium mass activity exhibited by ICO PCSHS-400 catalyst is 24 times higher than that of benchmark RuO2, substantially exceeding the known oxide-supported metal catalysts. More importantly, the electrocatalyst shows high stability during 8 h of continuous testing in acidic medium.