(296c) Active Phases, Reaction Centers and Catalytic Mechanism of Ni-Based and Co-Based Layered Double Hydroxides for the Oxygen Evolution Reaction
AIChE Annual Meeting
Tuesday, November 9, 2021 - 1:20pm to 1:40pm
In this talk, we provide the first direct atomic-scale evidence that, under applied anodic potentials, NiFe and CoFe LDHs oxidize from as-prepared Î±-phases to activated Î³-phases. The OER-active Î³-phases are characterized by about 8% contraction of the lattice spacing and switching of the intercalated ions from carbonate to potassium. The calculated surface phase diagrams indicate that surface O sites are saturated with H by forming bridge OH, and coordinatively unsaturated metal sites are poisoned by OH adsorption under OER conditions. These structures, and the associated reaction free energies, suggest that the OER proceeds via a Mars van Krevelen mechanism, starting with the oxidation of bridge OH at the reaction centers with dual metal sites, i.e. M1-OH-M2.
Our study suggests that the compound-dependent activity originates from the dual-metal site feature of the reaction centers. While this feature does not influence the OH-OOH scaling relationship, it leads to diverse OH-O scaling relationships, including those with near-zero slopes and negative slopes. In the literature, breaking OH-OOH scaling relationships were frequently discussed, as it determines the minimum overpotential. However, our study showed that, to approach the minimum overpotential dictated by a specific OH-OOH scaling relationship, the key is to break the OH-O scaling relationship. A possible route is to form binary metal oxyhydroxides with dual metal sites at the reaction centers or introduce a third element into NiFe LDH or CoFe LDH.
Nature Communications 2020, 11 (1), 2522.
Angewandte Chemie 2021, DOI: 10.1002/anie.202100631