Low overpotential water oxidation at neutral pH catalyzed by a copper(ii) porphyrin

Low overpotential water oxidation under mild conditions is required for new energy conversion technologies with potential application prospects. Extensive studies on molecular catalysis have been performed to gain fundamental knowledge for the rational designing of cheap, efficient and robust catalysts. We herein report a water-soluble Cu^II complex of tetrakis(4-N-methylpyridyl)porphyrin (1), which catalyzes the oxygen evolution reaction (OER) in neutral aqueous solutions with small overpotentials: the onset potential of the catalytic water oxidation wave measured at current density j = 0.10 mA cm^-2 is 1.13 V versus a normal hydrogen electrode (NHE), which corresponds to an onset overpotential of 310 mV. Constant potential electrolysis of 1 at neutral pH and at 1.30 V versus NHE displayed a substantial and stable current for O₂ evolution with a faradaic efficiency of >93%. More importantly, in addition to the 4e water oxidation to O₂ at neutral pH, 1 can catalyze the 2e water oxidation to H₂O₂ in acidic solutions. The produced H₂O₂ is detected by rotating ring-disk electrode measurements and by the sodium iodide method after bulk electrolysis at pH 3.0. This work presents an efficient and robust Cu-based catalyst for water oxidation in both neutral and acidic solutions. The observation of H₂O₂ during water oxidation catalysis is rare and will provide new insights into the water oxidation mechanism.