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

Abstract

Solar-driven direct seawater electrocatalysis is a promising technology for sustainable large-scale green-H 2 fuel generation. In this work, we systematically investigate the influence of altervalent cation doping into the Bi 3+ and V 5+ sites of scheelite BiVO 4 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 + , Ba 2+ , Co 2+ , and In 3+ ) doping favors monoclinic-phase formation; however, higher-valent cations (Hf 4+ , Nb 5+ , and Mo 6+ ) facilitated the thermodynamically unfavorable tetragonal-zircon type BiVO 4 . Interestingly, mixed phases of monoclinic-tetragonal BiVO 4 have been obtained upon codoping of Co and Mo, exhibiting enhanced photocurrent density (J p = 5.8 mA cm −2 ) among other doped BiVO 4 samples. To increase the overall charge-transfer kinetics, we construct a nanoporous carbon with a Co and Mo codoped BiVO 4 hybrid photoanode showing a remarkable (∼5-fold) enhancement in photoelectrochemical (PEC) freshwater splitting with the highest recorded photocurrent density of J p = 6.9 mA cm −2 at 1.23 V vs RHE, AM1.5 G in 0.5 M Na 2 SO 4 electrolyte solution, in comparison to pristine BiVO 4 (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 H 2 fuel. Significantly, we observed an enhanced J p 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.