Using coupled Ni and Zn oxides based on ZIF8 as efficient electrocatalyst for OER

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In the present work, we report the synthesis and characterization of efficient and stable electrocatalysts for oxygen evolution reaction (OER) in alkaline medium constructed using Ni and/or Co oxides dispersed on Zn oxide based on the zeolitic imidazole framework (ZIF8) operated through a combined application of solvothermal and calcination methods. Once subjected to calcination, the calcined, solvothermally treated Ni/ZIF8, named Ni/ZIF8-900A composite, recorded the most impressive improvement in OER electrocatalytic activity among the electrocatalysts synthesized in this study. Ni/ZIF8-900A recorded an overpotential (η) of 0.34 V at 10 mA cm−2; this was the same value obtained for RuO2 and was relatively close to the value obtained for IrO2 (0.29 V) - both widely regarded as state-of-the-art electrocatalysts. Long-term electrochemical tests revealed that Ni/ZIF8-900A had a negligible loss in electrocatalytic activity, demonstrating its high electrocatalytic efficiency in OER in alkaline medium. The physicochemical characterization of the Ni/ZIF8-900A composite showed that the catalyst is characterized by overlapping and merged structures of ZnO and Ni0.8Zn0.2O, as well as a high Ni content on its surface (Ni(1):Zn(2) in %w/w) in comparison with the Ni content in the bulk composition (Ni(1):Zn(14) in %w/w). The large amount of Ni3+ on the Ni/ZIF8-900A surface (before and after the stability tests) and the presence of N atoms bonded to the metals were found to be responsible for the improved OER activity of the electrocatalyst. The electrochemical results showed that when exposed to oxidative potentials during OER experiments, the surface structure of the Ni oxide/oxyhydroxide-enriched Ni/ZIF8-900A catalyst is activated. The outstanding OER electroactivity of the Ni/ZIF8-900A composite can be attributed to surface activation fueled by oxide formation, which resulted in an increase in active surface area along the composite and caused a possible displacement of Ni3+ states to Ni4+ states on the oxides formed.