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

Abstract

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.

Fig. 1. (Left) Molecular structure of 1. (Right) Thermal ellipsoid plot of the X-ray structure of 1 (50% probability). Hydrogen atoms and counter anions are omitted for clarity.

Fig. 2. (a) CVs of 1, CuSO₄, and buffer-only solution at a 100 mV s^–1 scan rate. (b) Normalized CVs of 0.50 mM 1 at different scan rates in mA cm^–2 V^–1/2 s^1/2. (c) CVs of 1 at different concentrations. (d) Plot of catalytic peak current versus [1]. Conditions: FTO working electrode (S = 0.25 cm²), 0.10 M pH 7.0 phosphate buffer, and 20 °C.

Fig. 3. (a) 20 successive CV cycles of 0.75 mM 1 with the FTO working electrode (S = 0.25 cm²) at a 50 mV s^–1 scan rate. (b) Catalytic currents in CPE with or without 1 (0.75 mM) using the FTO electrode (S = 1.0 cm²) at an applied potential of 1.30 V. (c) CVs of the FTO electrode after 10 h CPE with 1, the FTO electrode after 20 successive CV cycles with 1, and a freshly cleaned FTO electrode in the buffer-only solution. Conditions: 0.10 M pH 7.0 phosphate buffer, 100 mV s^–1 scan rate, and 20 °C. (d) UV-vis spectra of 1 before and after 10 h CPE.

Fig. 4. (a) LSV curves of 0.50 mM 1 in 0.10 M pH 2.0–7.0 phosphate buffers. (b) Plot of potential measured at j = 0.45 mA cm^–2versus pH. (c) DPV curves of 0.50 mM 1 in 0.10 M pH 2.52–4.45 phosphate buffers. (d) Plots of the first and second oxidation peak potentials versus pH from 2.52 to 4.45. Conditions: GC working electrode, scan rate 100 mV s^–1, and 20 °C.

Fig. 5. CVs of 1.0 mM 1 in 0.10 M pH 2.52 phosphate buffer by reversing the scan at 1.35 V with different scan rates (Left) and the ratio of the reduction peak current versus the oxidation peak current i_red/i_oxi (Right). Conditions: GC working electrode and 20 °C.

Fig. 6. (a) CVs of 0.75 mM 1 with the addition of H₂O₂ using the GC electrode at a 100 mV s^–1 scan rate. (b) DPV curves of 0.50 mM 1 with or without 1.0 mM H₂O₂. Conditions: 0.10 M phosphate buffer and 20 °C. (c) RRDE analysis of 0.5 mM 1 in 0.10 M pH 7.0 phosphate buffer. (d) RRDE analysis of 0.50 mM 1 in 0.10 M pH 3.0 phosphate buffer. The potential of the ring electrode is maintained at 0.70 V. Rotation rate is 1000 rpm, and scan rate is 10 mV s^–1.