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

Abstract

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 TiO2 support. The performance of the resulting TiO2-CNx/NiOx photoanodes is evaluated with respect to our previous results on hybrid TiO2-CNx photoanodes modified with IrOx and CoO(OH)x cocatalysts. The deposition of NiOx into TiO2-CNx photoanodes enhances significantly the photocurrent (from <8 µA to >250 µA cm-2 at 1.23 V vs. RHE) under visible light irradiation (λ > 420 nm, ~200 mW cm−2) and triggers the photoelectrocatalytic oxygen evolution. No oxygen evolution was observed without a cocatalyst. As compared to photoanodes modified with IrOx or CoO(OH)x, the TiO2-CNx/NiOx photoanodes excel by the very negative photocurrent onset potential (0 V vs. RHE), which we ascribe to good hole-extracting properties of NiOx. However, the comparatively low Faradaic efficiencies for oxygen evolution (~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 NiOx particles and increase thus the oxidative photodegradation of the photoanodes. Our study highlights the fact that employing outstanding electrocatalysts like NiOx 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.