Insight into Ni active site coordination in nickel-manganese spinels for methanol electrooxidation catalysis

As non-precious catalysts, Ni-based catalysts play a significant role in methanol oxidation for energy conversion technologies. At the same time, the effect of the complicated chemical environment on catalytic efficiency remains unclear. Here, the coordination environment of Ni active sites in spinel nickel-manganese (NiMn₂O₄ and MnNi₂O₄) is investigated as a platform to elucidate the correlation with catalytic performance in methanol electro-oxidation. The occupation of Ni²⁺ ions in these structures modulates the intrinsic activity of Ni active sites in NiMn spinels, resulting in different catalytic mechanisms and intrinsic active site efficiency, although they have similar morphology and structure. The high-symmetry NiO₆ octahedral structure in inverse spinel MnNi₂O₄ exhibits superior catalytic performance and stability compared to the NiO₄ tetrahedral structure in normal NiMn₂O₄ spinel. Specifically, at 1.50 V vs. RHE, the MnNi₂O₄ inverse spinel delivers mass activity and specific activity for methanol oxidation that are 1.9 and 3.5 times those of the normal NiMn₂O₄ spinel, respectively. Furthermore, it also maintains a stable current density of 33.5 mA cm^-2 at 1.56 V vs. RHE for 25 hours. Theoretical calculations reveal that Ni sites in MnNi₂O₄ exhibit a significantly lower activation energy barrier and enhanced CO anti-poisoning capability compared to those in NiMn₂O₄. The Ni site-dependent coordination environment in spinel structures provides useful insights into catalyst development and the methanol oxidation mechanism.