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 (NiMnO and MnNiO) is investigated as a platform to elucidate the correlation with catalytic performance in methanol electro-oxidation. The occupation of Ni 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 NiO octahedral structure in inverse spinel MnNiO exhibits superior catalytic performance and stability compared to the NiO tetrahedral structure in normal NiMnO spinel. Specifically, at 1.50 V RHE, the MnNiO inverse spinel delivers mass activity and specific activity for methanol oxidation that are 1.9 and 3.5 times those of the normal NiMnO spinel, respectively. Furthermore, it also maintains a stable current density of 33.5 mA cm at 1.56 V RHE for 25 hours. Theoretical calculations reveal that Ni sites in MnNiO exhibit a significantly lower activation energy barrier and enhanced CO anti-poisoning capability compared to those in NiMnO. The Ni site-dependent coordination environment in spinel structures provides useful insights into catalyst development and the methanol oxidation mechanism.