Spatiotemporally and Chemically Resolved Imaging of Electrocatalytic Oxygen Evolution on Single Nanoplates of Cobalt-Layered Hydroxide.

Transition metal-layered hydroxides have been extensively studied in order to address the key challenge of slow kinetics of the oxygen evolution reaction (OER). However, how the catalytically active sites are evolved and the corresponding heterogeneous structure-property relationship remain unclear. Herein, using cobalt-layered hydroxide as a representative catalyst, we report a strategy for the comprehensive in situ investigation of the electrocatalytic OER process at the single electrocatalyst level using combined electrochemiluminescence (ECL) and vis-absorption microscopy. The stepwise heterogeneous electrocatalytic responses of single-cobalt hydroxide nanoplates are unveiled with ECL imaging, and the corresponding valence state changes are revealed by vis-absorption imaging. The correlated in situ and ex situ multimode analyses indicate that, during the oxidation process, the Co 2+ cations in the tetrahedral sites (Co Td 2+ ) turned into Co Td 3+ and even the highly unstable Co Td 4+ , assisted by the interlayer water in a metastable CoOOH· x H 2 O phase. Crucially, the Co Td 4+ sites are mainly distributed in the inner part of the nanoplates and show superior electrocatalytic properties. The correlative single-particle imaging approach for electrocatalytic process analysis with high spatiotemporal and chemical resolution enables in-depth mechanistic insights to be generated and, in turn, will benefit the rational design of electrocatalysts with enhanced performance.