Cobalt-phosphate complexes catalyze the photoelectrochemical water oxidation of BiVO(4) electrodes

Abstract

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 BiVO4 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 BiVO4/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 O-2 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.