Improving WO3/SnO2 photoanode stability by inhibiting hydroxyl radicals with cobalt ions in strong acid

Photoelectrochemical (PEC) water splitting in acid is promising, but its development has been hindered by the lack of stable photoanodes and effective nonprecious cocatalysts. WO 3 is one of the few acid-stable semiconductors, but its fast performance decay under illumination remains elusive and unsolved. Herein, we present that the fast photocurrent decreases of both WO3 and WO3/SnO2 photoanodes were caused by the hydroxyl radicals (OH center dot) generated at the electrode/electrolyte interfaces, and we solved this issue by introducing cobalt (Co2+) ions into the electrolyte at pH 0.3, allowing for the efficient oxidation of H2O to O-2 rather than to detrimental OH. radicals, with the Faradaic efficiency toward oxygen evolution increasing from 40% to 95% and the photocurrent density increasing from 0.6 to 0.8 mA cm(-2) and being stable for 25 h at 1.2 V (reversible hydrogen electrode). Importantly, the scavenging of OH. radicals by vitamin C demonstrated the same photocurrent stability as the introduction of Co2+ ions, further implying the crucial inhibiting role of Co2+ ions. In-situ ultraviolet-visible and Raman spectroscopy indicated the trapping of surface holes by the oxidation of Co2+ to Co2+, and electron paramagnetic resonance revealed the role of Co2+ ions in the inhibition of OH. radicals. This study provides an ideal model for combining a homogeneous redox-active Co2+/Co3+ couple with a photoanode for water oxidation in strong acid.

Automated summary

Photoelectrochemical (PEC) water splitting in acid has potential, but stable photoanodes and effective nonprecious cocatalysts are lacking. In this study, the fast photocurrent decay of WO3 and WO3/SnO2 photoanodes under illumination is attributed to hydroxyl radicals generated at electrode/electrolyte interfaces. Introduction of cobalt (Co2+) ions into the electrolyte solves this issue, improving oxygen evolution efficiency and stabilizing the photocurrent density. The scavenging of hydroxyl radicals by vitamin C demonstrates the importance of Co2+ ions. This study provides an ideal model for water oxidation in strong acid.