UC Berkeley

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 CoFeOxHy cocatalysts. The resulting BiVO4 films annealed at 550 °C exhibited a photocurrent density of 4.1 mA/cm² at 1.23 VRHE under 1 sun AM 1.5G solar simulation and a low onset potential of 0.26 VRHE 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 CoFeOxHy cocatalyst layers were deposited. By varying the number of consecutive sweeps and adjusting the applied bias range, an ultra-thin (∼15 nm) CoFeOxHy cocatalyst layer was uniformly grown deposited over 30 cycles on the BiVO4 surface. The resulting BiVO4/CoFeOxHy yielded 4.03 mA/cm² at 1.23 VRHE and onset potential of 0.24 VRHE, with stable operation (∼15 % loss in photocurrent at 1.23 VRHE relative to ∼60 % loss in the uncatalyzed control sample). These conformal CoFeOxHy catalytic layers function simultaneously to selectively collect photoexcited holes from the BiVO4, catalyze the water-oxidation reaction, and protect the BiVO4 from photodegradation.