Effect of a Cobalt-Based Oxygen Evolution Catalyst on the Stability and the Selectivity of Photo-Oxidation Reactions of a WO3 Photoanode

A bare WO3 electrode and a WO3 electrode coupled with a layer of Co−Pi oxygen evolution catalyst (OEC) were prepared to investigate the effect of Co−Pi OEC on the selectivity of photo-oxidation reactions and photostabilities of WO3 photoanodes. WO3 photoanodes have been reported to produce peroxo species as well as O2 during photo-oxidation reactions, and the accumulation of peroxo species on the surface is known to cause a gradual loss of photoactivity of WO3. The photocurrent to O2 conversion efficiencies of the WO3 and WO3/Co−Pi OEC electrodes were obtained by simultaneously measuring the photocurrent and O2 gas generated during illumination at 0.8 V vs Ag/AgCl. The result shows that the presence of OEC increases the photocurrent to O2 conversion efficiency from approximately 61% to approximately 100%. The complete suppression of peroxo formation provided the WO3/Co−Pi OEC photoelectrode with long-term photostability. The photocurrent−potential characteristics show that the presence of OEC effectively reduces the electron−hole recombination near the flat band potential region and shifts the onset potential of photocurrent by 0.17 V to the negative direction. However, when the applied potential became more positive than approximately 0.35 V vs Ag/AgCl, the WO3/Co−Pi OEC electrode produced less initial photocurrent than the bare WO3 electrode. Mott−Schottky plots reveal the presence of interface states at the WO3/OEC junction that induce more electron−hole recombination when the Fermi level moves below these states. Regardless of the adverse effect on recombination present at 0.8 V vs Ag/AgCl, the WO3/Co−Pi OEC achieved a more efficient and sustainable solar to O2 conversion owing to the ability of Co−Pi OEC to significantly increase the photocurrent to O2 conversion efficiency and prevent the photocurrent decay of the WO3 electrode.