Spin evolution and flip in the oxygen reduction reaction: a theoretical study of Cu(Ni)XP2S6 (X = In, Bi and Cr)

Abstract

The local spin state has attracted increasing attention because of its critical role in improving the catalytic performance in catalysis. However, the mechanism behind it has not been clearly understood so far. Here, we carry out a theoretical study systematically to reveal the spin effect on the catalytic performance of metal compounds in the oxygen reduction reaction (ORR) based on density-functional theory (DFT). We find that Ni-incorporated two-dimensional (2D) CuXP₂S₆ (CNXPS, X = In, Bi, and Cr) show excellent activity for the ORR, which is highly related to the spin transport and flip. Our results demonstrate that: (1) the high spin (HS) state of Ni ions is beneficial to the ORR because of the strengthened Ni–O bond and reduced total Gibbs free energy of the system, which dominate the potential determining step; (2) spin flip occurs during the formation of the Ni–O bond and breaking of the O–O bond; (3) the charge transfer during the ORR is spin-sensitive due to the formation of the O–H bond. Our findings specifically illustrate the effects of magnetic moment and spin on the system energy and bond strength, and the evolution of the spin during the ORR, which may provide insightful understanding of the mechanism of the spin-related ORR and guidance for the design of novel ORR catalysts with high performance.