Cobalt-Salen Complexes as Catalyst Precursors for Electrocatalytic Water Oxidation at Low Overpotential

Water oxidation is an important half-reaction to achieve overall water splitting. In this present study, we show that a series of molecular cobalt-salen complexes can serve as catalyst precursors, to form nanostructured and amorphous cobalt-based thin films during electrodeposition, which can catalyze the water oxidation, reaction at low overpotentials. Cyclic voltammetry and bulk electrolysis using the cobalt-based film electrodes demonstrated obvious catalytic currents in 0.1 M KBi solution at pH 9.2. The onset catalytic potentials of the catalyst films are at similar to 0.84 V (vs Ag/AgCl) with a film made by electrodeposition of cobalt-salen complex 2 on FTO and at similar to 0.85 V for complex 4. Oxygen gas bubbles were clearly seen on the FTO electrode when the applied potential was above the onset potential. The Tafel plots using a catalyst film made of complex 4 showed that appreciable catalytic current was Observed starting at eta = 0.26 V for the film (a current density of 0.01 mA/cm(2) required eta = 290 mV), accompanied by a Faradaic efficiency >93% at 1.2 V. The catalyst film was further characterized by scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS).