Photoelectrochemical CO reduction activity of a hybrid photocathode, based on a Ru(II)-Re(I) supramolecular metal complex photocatalyst immobilized on a NiO electrode (NiO-RuRe) was confirmed in an aqueous electrolyte solution. Under half-reaction conditions, the NiO-RuRe photocathode generated CO with high selectivity, and its turnover number for CO formation reached 32 based on the amount of immobilized RuRe. A photoelectrochemical cell comprising a NiO-RuRe photocathode and a CoO/TaON photoanode showed activity for visible-light-driven CO reduction using water as a reductant to generate CO and O, with the assistance of an external electrical (0.3 V) and chemical bias (0.10 V) produced by a pH difference. This is the first example of a molecular and semiconductor photocatalyst hybrid-constructed photoelectrochemical cell for visibl-light-driven CO reduction using water as a reductant.

Dye-sensitized photo-electrochemical cells (DS-PECs) for water splitting hold promises for the large-scale storage of solar energy in the form of (solar) fuels, owing to the low cost and ease to process of their constitutive photoelectrode materials. The efficiency of such systems ultimately depends on our capacity to promote unidirectional light-driven electron transfer from the electrode substrate to a catalytic moiety. We report here on the first noble-metal free and covalent dyecatalyst assembly able to achieve photo-electrochemical visible light-driven H 2 evolution in mildly acidic aqueous conditions when grafted onto p-type NiO electrode substrate.Photosynthesis has inspired for many years the development of water splitting dye-sensitized photo-electrochemical cells (DS-PECs) for hydrogen production.1-3 A key step has been achieved very recently with the report of the first fully operative tandem DS-PEC.4 In such devices, limitation currently arises from the photocathode performances. Therefore different architectures based on the co-grafting4,5 of catalyst and dye onto nickel oxide (NiO), layerby-layer6 or supramolecular linkage of the catalyst to a grafted dye7 have been investigated. NiO is a p-type transparent conducting oxide specifically suitable for fast hole injection from the highest occupied molecular orbital (HOMO) of the excited dye.8 Then H 2 evolution requires that the photogenerated electron is efficiently and rapidly transferred to a catalyst. In that perspect, push-pull organic dyes appear as particularly attractive since they combine large absorptivity in the visible spectrum and spatial charge separation in the Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts excited state that limits the undesired recombination reaction from the reduced dye to the NiO electrode.9,10 The push-pull architecture is also instrumental to foster unilateral electron transfer through direct connection of the acceptor moiety, where the lowest unoccupied molecular orbital (LUMO) is centered and the photogenerated electron located, to the catalyst. We report here the first example of a covalent dye-catalyst molecular assembly integrated in an operative photoelectrode for H 2 evolution (Figure 1).We previously reported that, upon grafting on NiO, an easily affordable push-pull dye based on a triarylamine electron-donor part and an ethyl cyanoacetate electron-acceptor part separated by a thiophene unit generates large photocurrents in the presence of an irreversible electron acceptor in mildly acidic aqueous solution (pH 4-5).11 Cobalt diimine-dioxime complexes are proven catalysts for H 2 evolution at low overvoltage.12,13 When grafted onto electrode surfaces, they display sustained activity in pH = 4.5 aqueous solution14 and tolerance to oxygen.15 These features make them particularly attractive for incorporation into dye-sensitized H 2 -evolving photoelectrodes. To prepare a covalent dye-catalyst assembly, we first synthesized a terminal alkyne derivative (1, Figure 2) of the a...

ConspectusMimicking photosynthesis and producing solar fuels is an appealing way to store the huge amount of renewable energy from the sun in a durable and sustainable way. Hydrogen production through water splitting has been set as a first-ranking target for artificial photosynthesis. Pursuing that goal requires the development of efficient and stable catalytic systems, only based on earth abundant elements, for the reduction of protons from water to molecular hydrogen. Cobalt complexes based on glyoxime ligands, called cobaloximes, emerged ten years ago as a first generation of such catalysts. They are now widely utilized for the construction of photocatalytic systems for hydrogen evolution.In this Account, we describe our contribution to the development of a second generation of catalysts, cobalt diimine-dioxime complexes. While displaying similar catalytic activities as cobaloximes, these catalysts prove more stable against hydrolysis under strongly acidic conditions thanks to the tetradentate nature of the diimine-dioxime ligand. Importantly, H 2 evolution proceeds via proton-coupled electron transfer steps involving the oxime bridge as a protonation site, reproducing the mechanism at play in the active sites of hydrogenase enzymes. This feature allows H 2 to be evolved at modest overpotentials, i.e. close to the thermodynamic equilibrium over a wide range of acid-base conditions in non-aqueous solutions.Derivatization of the diimine-dioxime ligand at the hydrocarbon chain linking the two imine functions enables the covalent grafting of the complex onto electrode surfaces in a more convenient manner than for the parent bis-bidentate cobaloximes. Accordingly we attached * to whom correspondence should be addressed. vincent.artero@cea.fr. Europe PMC Funders Group Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts diimine-dioxime cobalt catalysts onto carbon nanotubes and demonstrated the catalytic activity of the resulting molecular-based electrode for hydrogen evolution from aqueous acetate buffer. The stability of immobilized catalysts was found to be orders of magnitude higher than that of catalysts in the bulk. It led us to evidence that these cobalt complexes, as cobaloximes and other cobalt salts do, decompose under turnover conditions where they are free in solution. Of note this process generates in aqueous phosphate buffer a nanoparticulate film consisting of metallic cobalt coated with a cobalt-oxo/hydroxo-phosphate layer in contact with the electrolyte. This novel material, H 2 -CoCat, mediates H 2 evolution from neutral aqueous buffer at low overpotentials.Finally, the potential of diimine-dioxime cobalt complexes for light-driven H 2 generation has been attested both in water/acetonitrile mixtures and in fully aqueous solutions. All together, these studies hold promises for the construction of molecular-based photoelectrodes for H 2 evolution and further integration in dye-sensitized photo-electrochemical cells (DS-PECs) able to achieve overall water splitting. Introducti...

We investigated a range of different mesoporous NiO electrodes prepared by different research groups and private firms in Europe to determine the parameters which influence good quality photoelectrochemical devices. This benchmarking study aims to solve some of the discrepancies in the literature regarding the performance of p-DSCs due to differences in the quality of the device fabrication. The information obtained will lay the foundation for future photocatalytic systems based on sensitized NiO so that new dyes and catalysts can be tested with a standardized material. The textural and electrochemical properties of the semiconducting material are key to the performance of photocathodes. We found that both commercial and non-commercial NiO gave promising solar cell and water-splitting devices. The NiO samples which had the two highest solar cell efficiency (0.145% and 0.089%) also gave the best overall theoretical H2 conversion.

The perspective of integrating molecular catalysts for hydrogen evolution into operating devices requires the benchmarking of their activity preferentially in aqueous media. Within a series of cobalt complexes assessed in that way, cobalt diimine–dioxime derivatives were shown to be the most active catalysts with onset overpotential for proton reduction as low as 260 mV in phosphate buffer (pH = 2.2) (McCrory et al. J. Am. Chem. Soc. 2012, 134, 3164–3170). Combining a set of analytical techniques (electrochemistry, gas chromatography, SEM, and XPS), we demonstrate here that the electrochemical wave previously assigned to H2 evolution catalyzed by the molecular complex actually corresponds to low levels of catalytic hydrogen production (≤27% faradaic yield). Instead, we assign this wave to the reductive degradation of the molecular complex and to the formation of a nanoparticulate deposit at the electrode. Actually, this coating is responsible for the high faradaic yields for hydrogen evolution observed at more cathodic potentials. The catalytic nanoparticulate material is metastable and readily redissolves, so that rinse-test experiments were insufficient here to rule out the formation of solid-state materials. This point accounts for the previous misidentification of the active species in H2 evolution mediated by a cobalt diimine–dioxime complex in aqueous phosphate buffer (pH = 2.2). Our finding, exemplified on a cobalt complex, may be extended to other molecular systems and suggests that the routine use of rinse-test experiments may not be sufficient to ascertain the molecular nature of active water-splitting catalytic species.

The active sites of hydrogenases have inspired the design of molecular catalysts for hydrogen evolution and oxidation. In this feature article, we showcase key elements of bio-inspiration before embarking on a tour of a representative series of molecular hydrogen evolving catalysts (HECs) and describing the toolbox available for benchmarking their performances. We then show how such catalysts can be immobilized on conducting substrates to prepare electrode materials active for hydrogen evolution and oxidation with a special emphasis on cobalt diimine-dioxime complexes and DuBois' nickel diphosphine compounds. We finally discuss the optimization required for implementing molecular-engineered materials into operational devices and illustrate how such molecular approaches can be expanded to other fuel-forming processes such as the electrochemical valorisation of carbon dioxide and the oxygen reduction or water oxidation reactions.

Improvement of the oxygen evolution reaction (OER) is a challenging step toward the development of sustainable energy technologies. Enhancing the OER rate and efficiency relies on understanding the water oxidation mechanism, which entails the characterization of the reaction intermediates. Very active Ru-bda type (bda is 2,2'-bipyridine-6,6'-dicarboxylate) molecular OER catalysts are proposed to operate via a transient 7-coordinate Ru═O intermediate, which so far has never been detected due to its high reactivity. Here we prepare and characterize a well-defined supported Ru(bda) catalyst on porous indium tin oxide (ITO) electrode. Site isolation of the catalyst molecules on the electrode surface allows trapping of the key 7-coordinate Ru═O intermediate at potentials above 1.34 V vs NHE at pH 1, which is characterized by electron paramagnetic resonance and in situ X-ray absorption spectroscopies. The in situ extended X-ray absorption fine structure analysis shows a Ru═O bond distance of 1.75 ± 0.02 Å, consistent with computational results. Electrochemical studies and density functional theory calculations suggest that the water nucleophilic attack on the surface-bound Ru═O intermediate (O-O bond formation) is the rate limiting step for OER catalysis at low pH.

Co-grafting of a cobalt diimine–dioxime catalyst and push–pull organic dye on NiO yields a photocathode evolving hydrogen from aqueous solution under sunlight, with equivalent performances compared to a dyad-based architecture using similar components.