Direct deposition of amorphous cobalt-vanadium mixed oxide films for electrocatalytic water oxidation

Efficiency of water oxidation catalysts in terms of overpotential, current density, and voltage stability over time with facile methods of their fabrication remains a key challenge in developing competent mechanisms of storing energy in the form of green hydrogen fuels. In this work, a rapid one-step aerosol-assisted chemical vapor deposition (AACVD) method is employed to synthesize amorphous and highly active cobalt-vanadium mixed oxide catalysts (CoVO_x) directly over fluorine-doped tin oxide (FTO) substrates. Morphological and structural characterizations made by field emission scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy techniques revealed the formation of pure-phase amorphous films with a gradual variation of topography as a function of deposition time. Of these films, the most active film (CoVO_x-20) was obtained in 20 min deposition, showing a spongy networking of interwoven nanofibers with a homogeneous distribution of 3-4 nm pores, achieving an overpotential of 308 mV at 10 mA/cm² current density. A much higher current density of 175 mA/cm² could be achieved just at 380 mV of overpotential with Tafel slope as low as 62 mV/dec for this whole range while exhibiting long-term stability. Mass activity, electrochemical impedance spectroscopy data, and the estimation of electrochemically active surface area all endorsed this high catalytic performance of CoVO_x-20, which is unprecedented for a low-cost, upscalable, and relatively less conductive substrate such as FTO used here. Our findings, thus, not only highlight the benefits of using AACVD in preparing two-dimensional amorphous catalysts but also prove the high efficiency of CoVO_x materials thus obtained, as outlined in a plausible reaction mechanism.