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

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.