NiMnO₃/NiMn₂O₄ Oxides Synthesized via the Aid of Pollen: Ilmenite/Spinel Hybrid Nanoparticles for Highly Efficient Bifunctional Oxygen Electrocatalysis

Abstract

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.