Molecular cobalt salophen catalyst-integrated BiVO4 as stable and robust photoanodes for photoelectrochemical water splitting

Abstract

Photoelectrochemical (PEC) water splitting is a promising method for the conversion and storage of solar energy. A combination of catalysts with photoelectrodes is generally required for the development of active photoanodes in PEC devices. In this work, we present two BiVO₄ photoanodes modified with cobalt salophen (Co(salophen)) complexes for PEC water oxidation. The resulting photoanodes show significantly enhanced PEC performance. Under simulated sunlight illumination (AM 1.5G, 100 mW cm⁻²), high photocurrents of 3.89 mA cm⁻² and 4.27 mA cm⁻² were obtained for Co1/BiVO₄ and Co2/BiVO₄, respectively at 1.23 V (vs. the reversible hydrogen electrode (RHE)) in a neutral solution, an almost three-fold enhancement over that of the unmodified BiVO₄. Intensity-modulated photocurrent spectroscopy (IMPS) analysis shows that the Co(salophen) complexes not only accelerate the water oxidation reaction but also reduce the surface recombination. The half-cell solar energy conversion efficiencies for Co1/BiVO₄ and Co2/BiVO₄ were 1.09% and 1.18% at 0.7 V, respectively. Due to their hydrophobic nature, the Co(salophen) complexes can bind strongly to the surface of BiVO₄. When the Co2 complex featuring four hydrophobic tert-butyl groups in a salophen ligand was anchored to BiVO₄, an extremely stable photocurrent of more than 3.5 mA cm⁻² at 1.23 V vs. RHE is sustained for at least 3 h without decay. Such a stable and robust photoanode based on a molecular WOC surpasses those attained by most of the state-of-the-art heterogeneous catalysts.