Development of composite structures combining iridium oxide with cobalt oxide for highly efficient oxygen evolution reaction catalysts

Abstract

The development of highly-efficient noble metal oxides and cost-effective transition metal oxides composite catalysts with superior stability could promote the practical applications of hydrogen energy from water electrolysis by improving the sluggish kinetics for oxygen evolution reaction (OER). Here, we report a unique three-dimensional (3D) hierarchical structures consisting of well-dispersed $IrO_x$ nanoparticles (NPs) with a sub-2 nm size encapsulated in numerous $Co_3O_4$ nanosheets (NSs) with a sub-15 nm thickness denoted as ICO NSs. The formation of ICO NSs was readily achieved by utilizing the metal encapsulation and morphology evolution capabilities of zeolitic imidazolate framework-67 (ZIF-67), which is a kind of metal-organic frameworks (MOFs), via ion-assistant solvothermal treatment with subsequent low-temperature calcination at 300 °C. The ICO NSs possesses high OER activity and excellent durability as compared with the commercial Ir/C even with the lower weight percentage of Ir content. The experimental results are contributed to the (i) formation of numerous exposed active sites and mesopores derived from morphology evolution and decomposition of organic linkers enabling faster charge transfers and intimate contact with electrolytes, (ii) phase transformation to $Co_3O_4$ including numerous active oxidation phase of $Co^{2+}$ induced by low-temperature calcination, and (iii) prevention of dissolution and detachment of $IrO_x$ NPs due to the surrounding passive $Co_3O_4$ NSs matrix.