Hybrid photoanodes for visible light-driven water oxidation: the beneficial and detrimental effects of nickel oxide cocatalyst

Hybrid photoanodes comprising polymer-based light absorbers coupled to oxygen-evolving cocatalysts represent a promising, yes still underdeveloped, approach to photoelectrochemical splitting of water into hydrogen and oxygen. In this study, we investigate nickel oxide (NiOx) nanoparticles as a water oxidation catalyst in hybrid photoanodes based on polymeric carbon nitride (CNx) supported on electron-collecting mesoporous TiO(2)support. The performance of the resulting TiO2-CNx/NiO(x)photoanodes is evaluated with respect to our previous results on hybrid TiO2-CN(x)photoanodes modified with IrO(x)and CoO(OH)(x)cocatalysts. The deposition of NiO(x)into TiO2-CN(x)photoanodes enhances significantly the photocurrent (from mu A to >250 mu A cm(-2)at 1.23 Vvs.RHE) under visible light irradiation (lambda> 420 nm, similar to 200 mW cm(-2)) and triggers the photoelectrocatalytic oxygen evolution. No oxygen evolution was observed without a cocatalyst. As compared to photoanodes modified with IrO(x)or CoO(OH)(x), the TiO2-CNx/NiO(x)photoanodes excel by the very negative photocurrent onset potential (0 Vvs. RHE), which we ascribe to good hole-extracting properties of NiOx. However, the comparatively low Faradaic efficiencies for oxygen evolution (similar to 18%) and dramatically decreased operational stability of the photoanodes indicate that the extracted holes do not efficiently oxidize water to dioxygen, but instead accumulate in the NiO(x)particles and increase thus the oxidative photodegradation of the photoanodes. Our study highlights the fact that employing outstanding electrocatalysts like NiO(x)in photoelectrochemical water-splitting systems does not necessarily lead to satisfactory results, especially when the photoelectrode cannot be operated at optimal pH due to light absorber stability issues.