Imaging and Tailoring Chemical Evolution Kinetics of (0001) Facet on Single β-Co(OH)2 Nanoplates for the Electrocatalytic Oxygen Evolution Reaction.

The (0001) crystal facet of β-Co(OH)₂ has been widely accepted as an inert facet for the oxygen evolution reaction (OER), while the (101̅0) facet is considered more active. However, mechanistic details regarding the origin of the differences at the coordination environment and the electronic state level remain unexplored to date. Herein, we used a multimode optical imaging method to track the evolving heterogeneous dynamics of cobalt species during the OER process and correlate it with the OER performance. Cobalt underwent the oxidation to Co_Oh³⁺, distorted to Co_Td³⁺, and subsequently oxidized to Co_Td⁴⁺, in which the distribution was mapped by vis-absorption imaging. It indicates that cobalt was negligibly oxidized to Co_Td⁴⁺ on the (0001) facet, because of the coordination-saturated environment, while it was relatively facile on the (101̅0) facet. The adsorbate evolution mechanism (AEM) process and lattice-oxygen-mediated mechanism (LOM) process during OER were spatiotemporally decoupled by electrochemiluminescence (ECL) imaging. Furthermore, the cobalt oxidation kinetics was tailored by the atom topping (Fe/Ni) strategy, which was accelerated by iron doping and retarded by nickel doping. On this basis, we propose a lattice O_v-involved mechanism for transforming Co_Oh³⁺ to Co_Td³⁺, which is a crucial step to Co_Td⁴⁺. The accelerated oxidation kinetics is from the enrichment of O_v, which induces abundant coordination-unsaturated cobalt, facilitating the transformation to Co_Td^3+/4+. This study examined the oxidation kinetics of cobalt with high spatiotemporal resolution and further tailors the distribution of Co_Td⁴⁺, which is expected to promote future research on the kinetic tuning of crystal facets.