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

This work develops the NiMnO 3 /NiMn 2 O 4 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 NiMnO 3 and NiMn 2 O 4 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 Ni 3+ facilitates OER activity, while the surface Mn 3+ with a small amount of Mn 4+ favors ORR processes. Through optimization, 0.61NiMnO 3 /NiMn 2 O 4 shows the highest OER activity, while 1.57NiMnO 3 /NiMn 2 O 4 can outperform the others in promoting ORR processes. Between them, 0.61NiMnO 3 /NiMn 2 O 4 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.