Strontium Promoted Surface Passivation and Water Dissociation in CuBi₂O₄ Water‐Reduction Photocathodes

Abstract Through utilizing solar energy and semiconducting photoelectrodes, photoelectrochemical (PEC) water splitting to produce hydrogen provides a sustainable energy future. Among various photoelectrode candidates, tetragonal copper bismuthate (CuBi 2 O 4 ) stands out as a highly promising photocathode material with a bandgap of 1.6–1.8 eV. However, the PEC performance of CuBi 2 O 4 is now limited by its poor bulk charge transport, serious surface recombination, and sluggish water reduction kinetics. In this study, to mitigate these shortcomings, strontium (Sr) is intentionally added to CuBi 2 O 4 films. Roles of the introduced Sr are revealed by multiple characterization techniques and cataloged as follows. First, the spontaneously segregated surface Sr species in the form of SrCO 3 passivates surface defects of CuBi 2 O 4 , resulting in suppressed surface recombination and facilitated bulk charge separation (enhanced band bending; function in thermodynamics). Second, surface Sr species could also promote water dissociation, leading to accelerated water reduction over the photocathode (function in reaction kinetics). In addition, Sr doping at Bi lattice sites also increases the carrier density of CuBi 2 O 4 , as such, Sr‐incorporated CuBi 2 O 4 photocathode achieves an anodic shift of onset potential by 80 mV and an 81.3% increase in photocurrent at 0.3 V RHE (reversible hydrogen electrode) when compared with the pristine one.