Atomic-scale asymmetric local electric fields in BiOCl: An efficient “electron pump” for photoelectrochemical seawater splitting

Despite the promise of BiOCl for photoelectrochemical seawater splitting due to excellent corrosion resistance, its practical application is hindered by wide bandgap and carrier recombination. Here, a strategy of Cu/In co-doping in BiOCl microplates is demonstrated to create atomically asymmetric local electric fields (LEFs), leading to highly enhanced PEC efficiency. The co-doping introduces a unique combination of multi-valent ions (Bi^{5 +}, Cu⁺, Cu²⁺, Cu⁰) and lattice strain, breaking the periodic field of pristine BiOCl. Cu atoms, especially Cu⁰, are revealed to act as highly active HER sites. The asymmetric LEFs, most pronounced around Cu⁰/Bi⁵⁺ configurations, are demonstrated to serve as a powerful “electron pump”, synergizing with photo-carriers to directionally drive electrons towards Cu sites, thereby significantly suppressing recombination and optimizing the free energy of OH^- desorption. Furthermore, the co-doping finely tunes the valence band maximum to ∼2.12 eV, effectively suppressing the chlorine evolution reaction (CER) while maintaining oxygen evolution reaction (OER) efficiency. Consequently, the optimized Cu_0.2In_0.2-BiOCl cathode achieves a 4-fold increase in PEC current density (-0.038 mA/cm² at −0.2 V vs. RHE) compared to pristine BiOCl and demonstrates exceptional stability for over 250 h in seawater. This work provides a profound insight into the role of multi-element doping in engineering local electronic fields for high-performance PEC water splitting.