An MgO passivation layer and hydrotalcite derived spinel Co2AlO4 synergically promote photoelectrochemical water oxidation conducted using BiVO4-based photoanodes

Abstract

M_xCo_3−xO₄ co-catalysed photoanodes with high potential for improvement in PEC water-oxidizing properties are reported. However, it is difficult to control the recombination of photogenerated carriers at the interface between the catalyst and cocatalyst. Here, an ultra-thin MgO passivation layer was introduced into the M_xCo_3−xO₄/BiVO₄ coupling system to construct a ternary composite photoanode Co₂AlO₄/MgO/BiVO₄. The photocurrent density of the electrode is 3.52 mA cm⁻², which is 3.2 times that of BiVO₄ (at 1.23 V vs. RHE). The photocurrent is practically increased by 0.86 mA cm⁻² and 1.56 mA cm⁻² in comparison with that of Co₂AlO₄/BiVO₄ and MgO/BiVO₄ electrodes, respectively. Meanwhile, the Co₂AlO₄/MgO/BiVO₄ electrode has the highest charge separation efficiency, the lowest charge transfer resistance (R_ct) and best stability. The excellent PEC performance could be attributed to the inhibitive effect provided by the MgO passivation layer that efficaciously suppresses the electron–hole recombination at the interface and drives the hole transfer outward, which is induced by Co₂AlO₄ to capture the electrode/electrolyte interface for efficient water oxidation reaction. In order to understand the origin of this improvement, first-principles calculations with density functional theory (DFT) were performed. The theoretical investigation converges to our experimental results. This work proposes a novel idea for restraining the recombination of photogenerated carriers between interfaces and the rational design of efficient photoanodes.