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

Abstract

Capture and conversion of sunlight into the storable energy carrier H₂ can be achieved through photoelectrochemical water splitting using light-absorbing cathodes and anodes bearing H₂ and O₂ 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 (<b>RuP3</b>) and a tetraphosphonated molecular [Ni(P₂N₂)₂]²⁺ type proton reduction catalyst (<b>NiP</b>) for the photoreduction of aqueous protons to H₂. A layer-by-layer deposition approach was employed, using Zr⁴⁺ ions to link the phosphonate units in <b>RuP3</b> and <b>NiP</b> 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 <b>NiP</b> from NiO for advantageous electron transfer dynamics. The NiO|<b>RuP3</b>-Zr⁴⁺-<b>NiP</b> electrodes generate higher photocurrents and are more stable than photocathodes with <b>RuP3</b> and <b>NiP</b> co-immobilised on the NiO surface in the absence of Zr⁴⁺ cations linking dye and catalyst. The generation of H₂ with the NiO|<b>RuP3</b>-Zr⁴⁺-<b>NiP</b> hybrid electrode in pH 3 aqueous electrolyte solution during irradiation with a UV-filtered solar light simulator (<i>λ</i> > 400 nm, 100 mW cm^-2, AM1.5G) has been confirmed by gas chromatography at an underpotential of 300 mV (<i>E</i>_appl = +0.3 V <i>vs.</i> RHE), demonstrating the potential of these electrodes to store solar energy in the chemical bond of H₂.