Ultrafine Co-doped ZnO nanoparticles on reduced graphene oxide as an efficient electrocatalyst for oxygen reduction reaction

Abstract

In this work, we developed a novel hierarchical hybrid nanostructure of reduced graphene oxide (rGO) supported cobalt-doped ZnO nanoparticles (Co-doped ZnO NPs@rGO) via a simple solution-refluxing strategy. The ultrafine CoZn-glycolate nanoparticles precursor was obtained from the alkoxide reaction among ethylene glycol and Co2+/Zn2+ ions absorbed onto GO support. The post-annealing process makes the precursor easily transferred into crystalline Co-doped ZnO NPs@rGO with well-retained morphology and the oxidation state of Co element to be Co2+ in the final hybrid. When evaluated as an oxygen reduction reaction (ORR) electrocatalyst in an O2-saturated 0.1M KOH solution, the Co-doped ZnO NPs@rGO hybrid with the optimal Co-doping concentration of 0.38 (i.e., Co0.38Zn0.62O NPs@rGO) exhibits highly enhanced ORR electrocatalytic activity compared to the ZnO@rGO composite with an ORR peak potential of 0.74V (vs. RHE), an onset potential of 0.90V, a current density of 3.94mAcm−2 (at 0.4V (vs. RHE)) and a nearly four-electron catalytic pathway. The good electrochemical stability and excellent methanol-tolerance capability of the resultant Co0.38Zn0.62O NPs@rGO hybrid, superior to those of the commercial Pt/C catalyst, further demonstrate its great potential as an efficient ORR electrocatalyst. This work proposes the feasibility of the semiconducting ZnO nanomaterials as novel ORR electrocatalysts via doping active oxygen catalytic element, such as Co2+, for the first time.