Evaluation of the charge transfer kinetics of spin-coated BiVO4 thin films for sun-driven water photoelectrolysis

• Dense and porous BiVO4 thin film were prepared through the spin-coating technique.
• Charge transfer kinetics in dense BiVO4 was 3-fold faster than in a porous film.
• Increased surface recombination in the porous film prevailed with respect to its high surface area.
• Optimized photo-electrodeposition of CoPi on BiVO4 enhanced by 70% the charge transfer kinetics.
• Non-uniformity and defects on the spin-coated films is an issue for big electrodes preparation.

The present research work focuses on bismuth vanadate (BiVO4) thin films deposited on FTO-coated glass electrodes through the spin-coating technique, and discusses the influence of different film morphologies (dense and porous) on the physicochemical properties and photoelectrochemical (PEC) performance of the as-prepared photoanodes, for the water splitting reaction. The surface recombination phenomenon, which is one of the main issues of BiVO4, has been quantified by means of two distinct approaches: transient photocurrent measurements and electrochemical impedance spectroscopy (EIS). This phenomenon has resulted to be higher in the porous material, thus a poorer performance has been observed than in the dense material. In order to increase the BiVO4 efficiency, a cobalt phosphate (CoPi) catalyst has been photo-electrodeposited onto the best BiVO4 electrode, employing an optimized technique and a photocurrent of up to 3 mA/cm2 at 1.23 V vs. RHE under neutral pH and 1 sun irradiation (100 mW/cm2) has been achieved. The charge transfer kinetics of the BiVO4 photoanodes, with and without CoPi, has also been quantified. The beneficial effect of this water oxidation catalyst, as well as the influence of the preparation method on the uniformity of the film and on its actual performance, is discussed in view of its prospective application in a real PEC device.

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