Ultrastable, High Photoelectrocatalytic Performance of Altervalent Cation-Doped BiVO4 Photoanode and Effect of Interfacial Contact with Nanoporous Carbon for Seawater Splitting Using a 3D-Printed Flow Device

Solar-driven direct seawater electrocatalysis is a promising technology for sustainable large-scale green-H2 fuel generation. In this work, we systematically investigate the influence of altervalent cation doping into the Bi3+ and V5+ sites of scheelite BiVO4 via a sustainable microwave-assisted hydrothermal (MW-HT) technique within a few minutes (12 min) at as low a temperature of 190 °C. We observed that lower-valent cation (Cs+, Ba2+, Co2+, and In3+) doping favors monoclinic-phase formation; however, higher-valent cations (Hf4+, Nb5+, and Mo6+) facilitated the thermodynamically unfavorable tetragonal-zircon type BiVO4. Interestingly, mixed phases of monoclinic-tetragonal BiVO4 have been obtained upon codoping of Co and Mo, exhibiting enhanced photocurrent density (Jp = 5.8 mA cm–2) among other doped BiVO4 samples. To increase the overall charge-transfer kinetics, we construct a nanoporous carbon with a Co and Mo codoped BiVO4 hybrid photoanode showing a remarkable (∼5-fold) enhancement in photoelectrochemical (PEC) freshwater splitting with the highest recorded photocurrent density of Jp = 6.9 mA cm–2 at 1.23 V vs RHE, AM1.5 G in 0.5 M Na2SO4 electrolyte solution, in comparison to pristine BiVO4 (1.45 mA cm–2) under simulated visible light. The superior performance is due to oxygen vacancy (OV)-related defect levels functioning as electron-trap sites promoting fast charge separation and surface adsorption for generating excess holes at the nanohybrid photoanode. In order to investigate how the nanohybrid photoanode affects the charge-carrier recombination rate and oxygen evolution reaction (OER) in seawater, we designed a compartmentalized three-dimensional (3D)-printed membrane-less, continuous-flow PEC device to produce massive H2 fuel. Significantly, we observed an enhanced Jp of 3.8 mA cm–2 accompanied by an outstanding long-term photostability of 4 h, achieved due to the rapid transfer of photostimulated holes from the nanohybrid photoanode to the electrolyte, promoted by the internal electric field over the constructed Mott–Schottky heterostructure. Thus, our work explicates the innovative design of a nanohybrid photoanode playing a crucial role in the efficient electronic interactions in the space-charge layer between the photoanode and seawater-electrolyte interface for effective seawater splitting.