A Versatile Photoelectrochemical System with Feooh/MoOx/BiVO4 for Reactive Halogen Species Generation Coupled with Hydrogen Evolution
Authors: Park Doyeon , Cho Kangwoo 
Published: December 2024

Solar-driven seawater splitting is a promising approach for sustainable green hydrogen generation. Chlorine evolution reaction (ClER) can provide much faster kinetics than oxygen evolution reaction (OER), due to lower number of electrons involved in the interfacial charge transfer. Designing stable and efficient photoelectrodes for a photoelectrochemical (PEC) seawater splitting presents challenges due to the corrosive nature of seawater. BiVO4 has been a promising photoanode for solar energy conversion to molecular hydrogen (H2), due to desirable bandgap (2.4 eV), favorable band position and chemical stability. Nevertheless, BiVO4 has been mostly employed for OER, and shows limitation of poor charge transport and insufficient active sites for interfacial charge transfer. This study proposed a composite photoanode comprising MoOx hole transfer layer and iron oxyhydroxide (FeOOH) electrocatalysts on BiVO4, to enhance photocurrent and stability in the PEC seawater splitting. The MoOx layer, containing Mo6+ as electron donors, was deposited on BiVO4 using a simple drop-casting method. A simple reductive electrodeposition introduced the FeOOH electrocatalysts, while simultaneously forming MoOx with surface defect states (FeOOH/MoOx/BiVO4) to facilitate the facilitating hole transfer through the segregated MoOx layer. The surface structure of FeOOH/MoOx/BiVO4 photoanode was investigated through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) confirmed successful synthesis of MoOx and FeOOH layers on BiVO4. The charge separation efficiency was four times higher on the FeOOH/MoOx/BiVO4 photoanode (78%) compared to the intact BiVO4 photoanode (18%). Additionally, the FeOOH/MoOx/BiVO4 photoanode exhibited improved photocurrent density for OER and ClER under simulated AM 1.5 G sunlight, reaching 3.3 mA cm-2 at 1.23 VRHE in circum-neutral pH, which is approximately three times higher than that of the intact BiVO4. The applied bias photon-to-current efficiency (ABPE) of the FeOOH/MoOx/BiVO4 photoanode reached 0.9% at 1.23 VRHE, five times higher than that of intact BiVO4. Such excellent PEC activity can be ascribed to introduced surface defects (such as oxygen vacancy) on MoOx/BiVO4 and alleviated kinetic barrier by FeOOH. Moreover, the photocurrent density of the FeOOH/MoOx/BiVO4 photoanode in a 0.5 M NaCl electrolyte (for ClER) was 0.5 mA cm-2 higher than that in 0.5 M Na2SO4 electrolyte (for OER). Transient absorption spectroscopy (TAS) confirmed the improved charge transport behavior of the modified BiVO4 for OER or ClER. The generation of reactive chlorine species could compromise the stability of FeOOH, which could be mitigated by an added aqueous pollutants. For instance, in the PEC chlorine evolution reaction system, ammonia or 4-chlorophenol as model pollutant enhanced the stability. Notably, the FeOOH/MoOx/BiVO4 exhibited a 10-fold higher rate for ammonia oxidation compared to the intact BiVO4. A moderated concentration of bromide ion could further elevate the photocurrent density. The increased photocurrent activity of the modified FeOOH/MoOx/BiVO4 photoanode in PEC splitting of blended seawater and wastewater can serve multifaceted purposes, enhancing the efficiency of pollutants degradation (or chemical upgrading) and H2 evolution.