Optimization of BiVO₄ coatings for high yield photoelectrochemical production of reactive chlorine species

Photoelectrochemical (PEC) synthesis of valuable chemicals, such as hydrogen and oxidants, is widely studied as a potential alternative to replace conventional synthesis processes with high carbon footprint. In the present study two different approaches were used to improve photoelectrochemical activity of BiVO₄ coatings, i.e. doping with molybdenum and heterostructuring with BiOCl. Crystalline structure, chemical composition and morphology of the coatings were characterized using X-ray diffraction, energy-dispersive spectroscopy and scanning electron microscopy techniques. Performance of the photoelectrodes was tested in the solutions of Na₂SO₄ and NaCl. PEC activity was found to increase in the sequence BiVO₄ < Mo_BiVO₄ < Mo_BiVO₄/BiOCl and the enhancement was much more pronounced in chloride medium. Mo doping was found to double incident photon to current conversion efficiency (from 5% to 10%) as well as Faradaic efficiency of photoinduced generation of reactive chlorine species (RCS) (from an average of 36% to 78%). Formation of hypochlorite and chlorite (ClO⁻ and ClO₂⁻) was determined experimentally. Construction of heterojunction between Mo_BiVO₄ and BiOCl promoted separation of photogenerated electrons and holes and led to significant enhancement of photocurrent. Moreover, presence of BiOCl layer was found to catalyze photooxidation of ClO⁻ to ClO₂⁻. Findings of the study demonstrate the potential of BiVO₄-based coatings for photoelectrochemical production of RCS via saline water splitting.