Enhancement Effects on Visible-Light-Driven Water Oxidation by a Bifunctional Fe-Co-deposited SnOx Catalyst Layer Deposited on an N-Doped CuWO4 Photoanode

Abstract

Efficient photoanodes for visible-light-driven water oxidation are eagerly desired to construct practical water splitting systems to produce O₂ and H₂ using solar energy, which is one of the most prospective approaches for sustainable H₂ production. To further improve photoelectrochemical (PEC) water oxidation by a unique nitrogen-doped CuWO₄ (N-CuWO₄) photoanode, a Fe-co-deposited SnO_x (Fe-SnO_x) layer was formed on the photoanode surface by photoassisted electrodeposition. The incident photon-to-current conversion efficiency (IPCE) of the N-CuWO₄ photoanode was improved by 1.9 times at 1.23 V vs a reverse hydrogen electrode (RHE) by the Fe-SnO_x layer, which is contributed by the increased catalytic efficiency (η_cat) from 41.7 to 67.0%. Photoelectrochemical impedance spectroscopy (PEIS) measurement showed that the rate constant (k_O2) of water oxidation at the surface increased from 5.5 to 10.2 s^–1 and the rate constant (k_rec) of surface recombination of photogenerated carriers decreased a little from 8.9 to 8.6 s^–1 by the Fe-SnO_x layer. This indicates a bifunctional role of the Fe-SnO_x catalyst layer in promoting the water oxidation reaction at the surface and suppressing the surface recombination of photogenerated carriers. For comparison with the case of a SnO_x layer (no Fe-co-deposition), the k_O2 value (6.7 s^–1) of SnO_x/N-CuWO₄ electrodes was lower than that (10.2 s^–1) for the Fe-SnO_x/N-CuWO₄ electrode but higher than that (5.5 s^–1) for bare N-CuWO₄. Both SnO_x and Fe-SnO_x layers on the N-CuWO₄ surface improved k_O2; however, the mechanism of improved k_O2 is distinct between these layers: passivation effect of the SnO_x layer to prevent electron tunneling at the interface of the N-CuWO₄ surface/electrolyte and promotion effect of the Fe-SnO_x layer on water oxidation at the surface. The k_rec value (6.9 s^–1) for the SnO_x/N-CuWO₄ electrode was 1.3 times lower than that (8.9 s^–1) of the bare N-CuWO₄ electrode, which clearly shows the effect of the passivating SnO_x layer on the suppression of the surface recombination of photogenerated carriers.