Self-adaptive amorphous CoO_xCl_y electrocatalyst for sustainable chlorine evolution in acidic brine

Electrochemical chlorine evolution reaction is of central importance in the chlor-alkali industry, but the chlorine evolution anode is largely limited by water oxidation side reaction and corrosion-induced performance decay in strong acids. Here we present an amorphous CoO_xCl_y catalyst that has been deposited in situ in an acidic saline electrolyte containing Co²⁺ and Cl^- ions to adapt to the given electrochemical condition and exhibits ~100% chlorine evolution selectivity with an overpotential of ~0.1 V at 10 mA cm^-2 and high stability over 500 h. In situ spectroscopic studies and theoretical calculations reveal that the electrochemical introduction of Cl^- prevents the Co sites from charging to a higher oxidation state thus suppressing the O-O bond formation for oxygen evolution. Consequently, the chlorine evolution selectivity has been enhanced on the Cl-constrained Co-O^* sites via the Volmer-Heyrovsky pathway. This study provides fundamental insights into how the reactant Cl^- itself can work as a promoter toward enhancing chlorine evolution in acidic brine.