Spectroelectrochemical Identification of CuO Impurities Catalyzing Water Reduction on CuBi₂O₄ Photocathodes

We identify the function of CuO impurities in CuBi 2 O 4 photocathodes significantly improving the photoelectrochemical water reduction efficiency. Spectroelectrochemical photo/voltage-induced absorption spectroscopies are employed to investigate the charge carrier dynamics of pure, CuO-impure, and CuO-covered CuBi 2 O 4 photocathodes during photoelectrochemical water reduction, in comparison with the CuO/FTO for electrochemical water reduction. Electrons after photoexciation are found to accumulate at the surface CuO in both CuO-covered and impure CuBi 2 O 4 photocathodes, rather than directly reducing water from the CuBi 2 O 4 conduction band. A rate law analysis is employed to compare the kinetics of water reduction in CuO/FTO electrochemically and CuBi 2 O 4 photoelectrochemically, showing a faster water reduction rate constant in CuO than CuBi 2 O 4 . The kinetic comparison demonstrates that the CuO impurity functions as a cocatalyst on the surface of CuBi 2 O 4 during water reduction due to the faster kinetics. Our results demonstrate the importance of considering the catalytic function of surface impurities responsible for the enhancement of water reduction efficiency.