Atomically controllable in-situ electrochemical treatment of metal-organic-framework-derived cobalt-embedded carbon composites for highly efficient electrocatalytic oxygen evolution

Co-based metal-organic frameworks (Co-MOFs) have attracted significant interest as intermediate templates to obtain Co-embedded nitrogen-doped carbon (NC) composites for highly catalytic oxygen evolution reaction. However, the electrocatalytic performance of Co-MOF-derived composite (Co@NC) for oxygen evolution remains unsatisfactory owing to its insufficient active sites and the detachment of Co nanoparticles. As catalytic reactions are mostly dominated by surface atoms, it is important to artificially control the surface catalytic sites of Co@NC composites to obtain highly efficient electrocatalysts. Herein, we proposed an in-situ electrochemical treatment to change the surface of Co nanoparticles to catalytic Co³⁺/Co⁴⁺ couple exchange states, thus obtaining a core/shell structure consisting of conductive Co metal and a highly active CoOOH layer. To achieve this, pyrolyzed Co@NC composites were subjected to a two-step in-situ electrochemical treatment; in the first step, the naturally formed inactive oxidation states were removed, and in the second step, the core/shell-structured nanoparticles were formed. The electrochemically designed Co@NC composite exhibited excellent electrocatalytic reaction with desirable overpotential (η₁₀ = ~320 mV) and Tafel slope (78–80 mV·dec⁻¹), in addition to long-term durability.