One-pot synthesis of a mesoporous NiCo2O4 nanoplatelet and graphene hybrid and its oxygen reduction and evolution activities as an efficient bi-functional electrocatalyst

Abstract

Mesoporous NiCo₂O₄ nanoplatelets and graphene sheets (NiCo₂O₄–G) are combined as a hybrid material via a one-pot synthesis process to demonstrate excellent bi-functional catalytic activity towards both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Physical characterizations have confirmed the formation of NiCo₂O₄ nanoplatelets created by selective adsorption of PVP onto specific crystal orientations, which provides spatial confinement for an anisotropic growth into 2-dimensional nanostructures. In addition, the decomposition of surface adsorbed PVP during the calcination process creates uniformly distributed meso-sized pores in the NiCo₂O₄ nanoplatelets. The beneficial hybrid and PVP effects are investigated via half-cell testing with NiCo₂O₄–G in comparison to graphene-free NiCo₂O₄ and PVP-free NiCo₂O₄–G, respectively, where much lower activation energy and higher current densities are observed with the mesoporous NiCo₂O₄–G hybrid for both ORR and OER. Furthermore, the positive impact of Ni incorporation was exclusively demonstrated, whereby NiCo₂O₄–G outperformed Co₃O₄–G in terms of onset potential and current densities for both ORR and OER. This is attributed to the increased electrical conductivity and the creation of new active sites with much lower activation energy due to the incorporation of Ni cations into the octahedral sites of the spinel crystal structure. This cost effective and highly efficient bi-functional catalyst is highly suitable for rechargeable metal–air battery technologies.