Cobalt-phosphate complexes catalyze the photoelectrochemical water oxidation of BiVO₄ electrodes

BiVO₄ semiconductor electrodes were coupled with cobalt-phosphate complexes (CoPi) to enhance the photoelectrochemical (PEC) performance for water oxidation reaction. CoPi was deposited on a 550 nm-thick BiVO₄ film via electrodeposition (ED) and photodeposition (PD) methods for comparison of their effects. The CoPi on BiVO₄ exhibited Co:P atomic ratios of approximately 1:7 for the electrodeposited sample and approximately 1:18 for the photodeposited sample, and Co²⁺ and Co³⁺ co-existed in both samples. Optimized CoPi ED resulted in a CoPi overlayer of approximately 850 nm thick, which showed an electrochromic-like behavior that was likely due to limited access of phosphate into BiVO ₄ across the CoPi layer. Optimized CoPi PD, however, had very thin and rather uniform CoPi dispersion and did not show electrochromic-like behavior. Despite the lesser amount of CoPi, the PEC performance of BiVO ₄/CoPi (PD) was comparable to that of BiVO₄/CoPi (ED). Real-time measurements of the headspace molecular oxygen that evolved from water oxidation indicated that CoPi enhances O₂ production and photocurrent generation at BiVO₄ by a factor of around 15 and a maximum of 20, respectively, at 0.576 V_SCE (equivalent to 1.23 V _RHE) under air mass 1.5 irradiation (400 mW cm^-2). Prolonged irradiation of BiVO₄/CoPi (ED) resulted in a reduced Co:P ratio to 1:1.77 without changing the mixed valency of Co(ii/iii). This finding indicates that incorporation of phosphate into the CoPi was kinetically slower than water oxidation. The primary role of CoPi has been suggested as a hole-conducting electrocatalyst making the photogenerated electrons more mobile and, consequently, increasing conductivity and boosting the PEC water oxidation performance of BiVO₄.