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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 TiO 2  support. The performance of the resulting TiO 2 –CN x /NiO x photoanodes is evaluated with respect to our previous results on hybrid TiO 2 –CN x photoanodes modified with IrO x and CoO(OH) x cocatalysts. The deposition of NiO x into TiO 2 –CN x photoanodes enhances significantly the photocurrent (from  250  μ A cm –2  at 1.23 V  vs.  RHE) under visible light irradiation ( λ  &gt; 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 TiO 2 –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.

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