Efficient BiVO₄/CoFeO_xH_y photoanodes using controlled annealing and conformal linear-sweep electrocatalyst photodeposition

Although monoclinic bismuth vanadate (BiVO₄) is a promising photoanode for solar water splitting, its practical use is hindered by imperfect photocurrent generation/collection, low photovoltage compared to the bandgap, and corrosion side reactions that limit durability. Here, we introduce a controlled-annealing sol–gel process for BiVO₄ thin-film photoanodes along with an optimized linear-sweep-voltammetry photodeposition of CoFeO_xH_y cocatalysts. The resulting BiVO₄ films annealed at 550 °C exhibited a photocurrent density of 4.1 mA/cm² at 1.23 V_RHE under 1 sun AM 1.5G solar simulation and a low onset potential of 0.26 V_RHE due to high majority carrier conductivity, a crystalline bulk with reduced defects as evidenced by x-ray photoelectron spectroscopy and photoluminescence lifetime analysis, and thus enhanced photocarrier collection. However, significant degradation in performance was found due to interfacial photocorrosion. To protect the surface and speed the oxygen-evolution reaction CoFeO_xH_y cocatalyst layers were deposited. By varying the number of consecutive sweeps and adjusting the applied bias range, an ultra-thin (∼15 nm) CoFeO_xH_y cocatalyst layer was uniformly grown deposited over 30 cycles on the BiVO₄ surface. The resulting BiVO₄/CoFeO_xH_y yielded 4.03 mA/cm² at 1.23 V_RHE and onset potential of 0.24 V_RHE, with stable operation (∼15 % loss in photocurrent at 1.23 V_RHE relative to ∼60 % loss in the uncatalyzed control sample). These conformal CoFeO_xH_y catalytic layers function simultaneously to selectively collect photoexcited holes from the BiVO₄, catalyze the water-oxidation reaction, and protect the BiVO₄ from photodegradation.