Study on nanoporous CuBi₂O₄ photocathode coated with TiO₂ overlayer for photoelectrochemical CO₂ reduction

Investigation on the electrode/electrolyte interface of nanotextured electrodes is very challenging but critical to understand the CO₂ reduction reactions. Here we present that the nanoporous CuBi₂O₄ photocathodes coated by a TiO₂ overlayer with gradient thickness show improved CO₂ reduction performance compared to pristine CuBi₂O₄. Different activity and selectivity for photoelectrochemical CO₂ reduction are observed when the thickness of TiO₂ coating layer is set at two extreme values, i.e., less and longer than its Debye length. The CuBi₂O₄ with a thin TiO₂ layer shows higher CO/H₂ yield ratio, and the one with a thick layer exhibits lower CO/H₂ ratio but higher yield for both CO and H₂. All these are elucidated based on the results of Mott-Schottky plots, photocurrent response and SEM/TEM images. Results indicate that the TiO₂ overlayer on CuBi₂O₄ is in favor of the generation and separation of electron/hole pairs, and thus facilitate CO and H₂ production, while the nanoporous structure and the nonuniform TiO₂ overlayer on CuBi₂O₄ have a great impact on mass transport, local pH, and exposed active sites and, thereby, on the CO production selectivity.