Photoelectrochemical hydrogen production in water using a layer-by-layer assembly of a Ru dye and Ni catalyst on NiO

Capture and conversion of sunlight into the storable energy carrier H-2 can be achieved through photoelectrochemical water splitting using light-absorbing cathodes and anodes bearing H-2 and O-2 evolving catalysts. Here, we report on the development of a dye-sensitised p-type nickel oxide (NiO) photocathode with a hexaphosphonated Ru(2,2'-bipyridine)(3) based dye (RuP3) and a tetraphosphonated molecular [Ni(P2N2)(2)](2+) type proton reduction catalyst (NiP) for the photoreduction of aqueous protons to H-2. A layer-by-layer deposition approach was employed, using Zr4+ ions to link the phosphonate units in RuP3 and NiP in a supramolecular assembly on the NiO photocathode. This approach keeps the dye in close proximity to the catalyst and semiconductor surface, but spatially separates NiP from NiO for advantageous electron transfer dynamics. The NiO vertical bar RuP3-Zr4+-NiP electrodes generate higher photocurrents and are more stable than photocathodes with RuP3 and NiP co-immobilised on the NiO surface in the absence of Zr4+ cations linking dye and catalyst. The generation of H-2 with the NiO vertical bar RuP3-Zr4+-NiP hybrid electrode in pH 3 aqueous electrolyte solution during irradiation with a UV-filtered solar light simulator (lambda > 400 nm, 100 mW cm(-2), AM1.5G) has been confirmed by gas chromatography at an underpotential of 300 mV (E-appl = +0.3 V vs. RHE), demonstrating the potential of these electrodes to store solar energy in the chemical bond of H-2.