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_ycatalyst 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⁻²and 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.

Strategies to generate high-valence metal species capable of oxidizing water often employ composition and coordination tuning of oxide-based catalysts, where strong covalent interactions with metal sites are crucial. However, it remains unexplored whether a relatively weak “non-bonding” interaction between ligands and oxides can mediate the electronic states of metal sites in oxides. Here we present an unusual non-covalent phenanthroline-CoO₂ interaction that substantially elevates the population of Co⁴⁺ sites for improved water oxidation. We find that phenanthroline only coordinates with Co²⁺ forming soluble Co(phenanthroline)₂(OH)₂ complex in alkaline electrolytes, which can be deposited as amorphous CoO_xH_y film containing non-bonding phenanthroline upon oxidation of Co²⁺ to Co^3+/4+. This in situ deposited catalyst demonstrates a low overpotential of 216 mV at 10 mA cm⁻² and sustainable activity over 1600 h with Faradaic efficiency above 97%. Density functional theory calculations reveal that the presence of phenanthroline can stabilize CoO₂ through the non-covalent interaction and generate polaron-like electronic states at the Co-Co center.

A substantial effort is devoted to the development of efficient electrolyzers made of earth-abundant elements for low-temperature industrial-scale water electrolysis. However, a large current density leads to the decline of the reaction kinetics that result from the decrease of local pH, the irreversible redox states of active metal sites, and the structure and composition collapse. Currently, the transition metal layered double hydroxides (LDHs) are proven as efficient alkaline oxygen evolution catalysts and demonstrate promising current density, generally at the scale of 10 mA cm^–2 for the potential solar-driven catalysis concerning 10% solar-to-fuels efficiency. However, there is very limited progress in understanding the activity and stability degradation mechanism of LDHs at high current density, for instance above 100 mA cm^–2. Here we introduce the current advances in achieving activity enhancement by tuning the composition, structure, and morphology of LDHs and present the degradation mechanism during the electrolysis under oxidative alkaline environments, long-term operation, and voltage fluctuations. Finally, we present the state-of-the-art approaches to stabilize the overall performance of LDHs for water oxidation and provide an outlook in this field.

Stable seawater oxidation at a large current density of 500 mA cm⁻² for 1000 h is achieved with a self-healing borate-intercalated nickel–cobalt–iron oxyhydroxide (NiCoFe-B_i) catalyst in highly alkaline artificial seawater at pH 14.95.