(296c) Active Phases, Reaction Centers and Catalytic Mechanism of Ni-Based and Co-Based Layered Double Hydroxides for the Oxygen Evolution Reaction

Ni-based and Co-based layered double hydroxides (LDHs) are among the most active and the most studied catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. Because it happens under extremely oxidative aqueous conditions, however, in-situ crystal structures of the OER active phase are still largely unknown, which has significantly hindered the establishment of structure-property relationships.

In this talk, we provide the first direct atomic-scale evidence that, under applied anodic potentials, NiFe and CoFe LDHs oxidize from as-prepared α-phases to activated γ-phases. The OER-active γ-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.

References:

Nature Communications 2020, 11 (1), 2522.

Angewandte Chemie 2021, DOI: 10.1002/anie.202100631