This work develops the NiMnO3/NiMn2O4 ilmenite/spinel hybrid oxides as highly efficient bifunctional catalysts toward both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). They are prepared with the aid of pollen, using a two-step annealing method. The interaction between NiMnO3 and NiMn2O4 nanoparticles results in an enhanced charge transfer between nanoparticles and active species in electrolytes, which is favorable for their electrocatalytic activities. The surface oxidation states of Ni and Mn for the hybrid oxides can be tuned by pollen, which greatly influences the OER and ORR activities and the overall bifunctional activity. The surface Ni3+ facilitates OER activity, while the surface Mn3+ with a small amount of Mn4+ favors ORR processes. Through optimization, 0.61NiMnO3/NiMn2O4 shows the highest OER activity, while 1.57NiMnO3/NiMn2O4 can outperform the others in promoting ORR processes. Between them, 0.61NiMnO3/NiMn2O4 exhibits the higher overall bifunctional activity. Furthermore, both of the optimized hybrid oxides show excellent durability during both OER and ORR processes. They can be considered as promising bifunctional catalysts for OER/ORR.
Keywords: electrocatalysis; hybrid oxides; oxygen evolution reaction; oxygen reduction reaction; pollen.