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

Efficient photoanodes for visible-light-driven water oxidation are eagerly desired to construct practical water splitting systems to produce O 2 and H 2 using solar energy, which is one of the most prospective approaches for sustainable H 2 production. To further improve photoelectrochemical (PEC) water oxidation by a unique nitrogen-doped CuWO 4 (N-CuWO 4 ) 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-CuWO 4 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 O 2 ) 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 O 2 value (6.7 s –1 ) of SnO x /N-CuWO 4 electrodes was lower than that (10.2 s –1 ) for the Fe-SnO x /N-CuWO 4 electrode but higher than that (5.5 s –1 ) for bare N-CuWO 4 . Both SnO x and Fe-SnO x layers on the N-CuWO 4 surface improved k O 2; however, the mechanism of improved k O 2 is distinct between these layers: passivation effect of the SnO x layer to prevent electron tunneling at the interface of the N-CuWO 4 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-CuWO 4 electrode was 1.3 times lower than that (8.9 s –1 ) of the bare N-CuWO 4 electrode, which clearly shows the effect of the passivating SnO x layer on the suppression of the surface recombination of photogenerated carriers.