Electrocatalytic oxygen evolution reaction at IrOx supported by Ni/Co-ZIF-67: Controlled ratio of metallic Ir and Ir3+ states

Abstract

Dispersion of catalytic metal oxides over metal-organic framework (MOF) constructed a fascinated assembly for oxygen evolution electrocatalysis. A devised synthetic method for uniform dispersion and controlled particle size of catalyst materials could improve the catalytic parameters. We demonstrated a facile, surfactant-free method to immobilize ultrafine iridium oxide (IrOx) nanoparticles over nanosheet-shaped zeolitic imidazolate framework (ZIF) surface of bimetallic Ni/Co-ZIF-67. The optimum Ni/Co atomic ratio (1:6.5) improved the electronic conductivity and the combination with the well-dispersed IrOx nanoparticles (average 2.3 nm) provided IrOx@Ni/Co-ZIF-67. The optimized electrode assembly with 2.8 at.% (compared to 1.6 and 6.5 at.%) loading amount of IrOx led to superior OER performance. It appeared that catalytic activity correlated to the balanced ratio of metallic Ir0 and Ir3+ states. The active Ir3+ site coordinated by a hydroxyl group would likely promote the first dehydration step in alkaline media to initiate the catalytic OER, while the metallic Ir stabilizes the Ir3+ state. The desirable large surface area of ultrafine IrOx nanoparticles and the evenly controlled Ir0/Ir3+ species resulted in the high catalytic parameters. Additionally, the close electronic contact at the interface of IrOx and conductive Ni/Co-ZIF-67 should provide high catalytic OER performance.