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 (NiO _x ) nanoparticles as a water oxidation catalyst in hybrid photoanodes based on polymeric carbon nitride (CN _x ) supported on electron-collecting mesoporous TiO₂ support. The performance of the resulting TiO₂–CN _x /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 <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 IrO _x or CoO(OH) _x , the TiO₂–CN _x /NiO _x photoanodes excel by the very negative photocurrent onset potential (0 V vs. RHE), which we ascribe to good hole-extracting properties of NiO _x . 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 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.