Water Oxidation by Copper–Amino Acid Catalysts at Low Overpotentials

Graphical Cu-based water oxidation catalysts : A self-assembling Cu II –4 Gly complex is reported as a catalyst precursor for water oxidation. Electrolysis with this complex precursor results in in situ formation of a surface-accumulated copper(II) deposit, dominated by an amorphous, phosphate-incorporated CuO/Cu(OH) 2 mixture. An onset overpotential of 380 mV and a current density of 1 mA cm −2 could be achieved at an overpotential of 450 mV with a Tafel slope of 64 mV dec −1 . Although notable progress has been made in developing first-row transition-metals-based water oxidation catalysts, continuous efforts are required to identify more inexpensive, efficient, and robust catalysts. Here, we demonstrate the utility of a series of readily available complexes formed by earth-abundant copper and amino acids as catalyst precursors. In phosphate solution at pH 12, electrolysis with 1 m m Cu II and 4 m m Gly results in in situ formation of an amorphous surface precipitate of a CuO/Cu(OH) 2 mixture, which incorporates a substantial amount of phosphate anions. This surface-bound solid can catalyze water oxidation with an impressive onset overpotential of 380 mV, an overpotential of 450 mV for a current density of 1 mA cm −2 , and a low Tafel slope of 64 mV dec −1 , which make it among the most active Cu-based heterogeneous electrocatalysts. The copper-based reactivity toward water oxidation is shared by Cu II complexation to other amino acids as catalyst precursors.