Ultrastable, High Photoelectrocatalytic Performance of Altervalent Cation-Doped BiVO<sub>4</sub> Photoanode and Effect of Interfacial Contact with Nanoporous Carbon for Seawater Splitting Using a 3D-Printed Flow Device
Tulsi Satyavir Dabodiya, Twinkle George, Feba Ann Mathew, A. Vadivel Murugan
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.