Murielle Chavarot-Kerlidou

Splitting Water with Cobalt

The future of energy supply depends on innovative breakthroughs regarding the design of cheap, sustainable, and efficient systems for the conversion and storage of renewable energy sources, such as solar energy. The production of hydrogen, a fuel with remarkable properties, through sunlight-driven water splitting appears to be a promising and appealing solution. While the active sites of enzymes involved in the overall water-splitting process in natural systems, namely hydrogenases and photosystem II, use iron, nickel, and manganese ions, cobalt has emerged in the past five years as the most versatile non-noble metal for the development of synthetic H(2)- and O(2)-evolving catalysts. Such catalysts can be further coupled with photosensitizers to generate photocatalytic systems for light-induced hydrogen evolution from water.

Molecular engineering of a cobalt-based electrocatalytic nanomaterial for H 2 evolution under fully aqueous conditions

The viability of a hydrogen economy depends on the design of efficient catalytic systems based on earth-abundant elements. Innovative breakthroughs for hydrogen evolution based on molecular tetraimine cobalt compounds have appeared in the past decade. Here we show that such a diimine-dioxime cobalt catalyst can be grafted to the surface of a carbon nanotube electrode. The resulting electrocatalytic cathode material mediates H(2) generation (55,000 turnovers in seven hours) from fully aqueous solutions at low-to-medium overpotentials. This material is remarkably stable, which allows extensive cycling with preservation of the grafted molecular complex, as shown by electrochemical studies, X-ray photoelectron spectroscopy and scanning electron microscopy. This clearly indicates that grafting provides an increased stability to these cobalt catalysts, and suggests the possible application of these materials in the development of technological devices.

Artificial Photosynthesis: From Molecular Catalysts for Light-driven Water Splitting to Photoelectrochemical Cells

Photosynthesis has been for many years a fascinating source of inspiration for the development of model systems able to achieve efficient light‐to‐chemical energetic transduction. This field of research, called “artificial photosynthesis,” is currently the subject of intense interest, driven by the aim of converting solar energy into the carbon‐free fuel hydrogen through the light‐driven water splitting. In this review, we highlight the recent achievements on light‐driven water oxidation and hydrogen production by molecular catalysts and we shed light on the perspectives in terms of implementation into water splitting technological devices.

Hydrogen Evolution Catalyzed by Cobalt Diimine–Dioxime Complexes

Mimicking 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 10 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, H2 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 H2 to be evolved at modest overpotentials, that is, close to the thermodynamic equilibrium over a wide range of acid-base conditions in nonaqueous 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 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, H2-CoCat, mediates H2 evolution from neutral aqueous buffer at low overpotentials. Finally, the potential of diimine-dioxime cobalt complexes for light-driven H2 generation has been attested both in water/acetonitrile mixtures and in fully aqueous solutions. All together, these studies hold promise for the construction of molecular-based photoelectrodes for H2 evolution and further integration in dye-sensitized photoelectrochemical cells (DS-PECs) able to achieve overall water splitting.

Phosphine Coordination to a Cobalt Diimine–Dioxime Catalyst Increases Stability during Light-Driven H2 Production

The combination of cobalt diimine-dioxime complexes with a cyclometalated iridium photosensitizer gives efficient systems for hydrogen generation under visible-light irradiation using triethylamine as a sacrificial electron donor. Interestingly, the addition of triphenylphosphine (PPh(3)) to the medium results in a significant improvement of the stability of the system, with up to ∼700 turnovers achieved within 10 h. UV-visible spectroscopic monitoring of the reaction allows identification of a PPh(3)-coordinated Co(I) intermediate as the active species. Mechanistic issues regarding (i) the photogeneration of the Co(I) species, (ii) the nature of the active species, and (iii) the influence of PPh(3) on the H(2)-evolution mechanism are discussed.

Recent developments in hydrogen evolving molecular cobalt(II)–polypyridyl catalysts

The search for efficient noble metal-free hydrogen-evolving catalysts is the subject of intense research activity. A new family of molecular cobalt(II)-polypyridyl catalysts has recently emerged. These catalysts prove more robust under reductive conditions than other cobalt-based systems and display high activities under fully aqueous conditions. This review discusses the design, characterization, and evaluation of these catalysts for electrocatalytic and light-driven hydrogen production. Mechanistic considerations are addressed and structure-catalytic activity relationships identified in order to guide the future design of more efficient catalytic systems.

Covalent Design for Dye-Sensitized H2-Evolving Photocathodes Based on a Cobalt Diimine–Dioxime Catalyst

Dye-sensitized photoelectrochemical cells (DS-PECs) for water splitting hold promise 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 dye-catalyst assembly able to achieve photoelectrochemical visible light-driven H2 evolution in mildly acidic aqueous conditions when grafted onto p-type NiO electrode substrate.

Combined experimental-theoretical characterization of the hydrido-cobaloxime [HCo(dmgH)2(PnBu3)].

A combined theoretical and experimental approach has been employed to characterize the hydrido-cobaloxime [HCo(dmgH)(2)(PnBu(3))] compound. This complex was originally investigated by Schrauzer et al. [Schrauzer et al., J. Am. Chem. Soc. 1971, 93,1505] and has since been referred to as a key, stable analogue of the hydride intermediate involved in hydrogen evolution catalyzed by cobaloxime compounds [Artero, V. et al. Angew. Chem., Int. Ed. 2011, 50, 7238-7266]. We employed quantum chemical calculations, using density functional theory and correlated RI-SCS-MP2 methods, to characterize the structural and electronic properties of the compound and observed important differences between the calculated (1)H NMR spectrum and that reported in the original study by Schrauzer and Holland. To calibrate the theoretical model, the stable hydrido tetraamine cobalt(III) complex [HCo(tmen)(2)(OH(2))](2+) (tmen = 2,3-dimethyl-butane-2,3-diamine) [Rahman, A. F. M. M. et al. Chem. Commun. 2003, 2748-2749] was subjected to a similar analysis, and, in this case, the calculated results agreed well with those obtained experimentally. As a follow-up to the computational work, the title hydrido-cobaloxime compound was synthesized and recharacterized experimentally, together with the Co(I) derivative, giving results that were in agreement with the theoretical predictions.

A Computational Study of the Mechanism of Hydrogen Evolution by Cobalt(Diimine‐Dioxime) Catalysts

Cobalt(diimine-dioxime) complexes catalyze hydrogen evolution with low overpotentials and remarkable stability. In this study, DFT calculations were used to investigate their catalytic mechanism, to demonstrate that the initial active state was a Co(I) complex and that H2 was evolved in a heterolytic manner through the protonation of a Co(II)-hydride intermediate. In addition, these catalysts were shown to adjust their electrocatalytic potential for hydrogen evolution to the pH value of the solution and such a property was assigned to the presence of a H(+)-exchange site on the oxime bridge. It was possible to establish that protonation of the bridge was directly involved in the H2-evolution mechanism through proton-coupled electron-transfer steps. A consistent mechanistic scheme is proposed that fits the experimentally determined electrocatalytic and electrochemical potentials of cobalt(diimine-dioxime) complexes and reproduces the observed positive shift of the electrocatalytic potential with increasing acidity of the proton source.

Dye-sensitized PS-b-P2VP-templated nickel oxide films for photoelectrochemical applications

Moving from homogeneous water-splitting photocatalytic systems to photoelectrochemical devices requires the preparation and evaluation of novel p-type transparent conductive photoelectrode substrates. We report here on the sensitization of polystyrene-block-poly-(2-vinylpyridine) (PS-b-P2VP) diblock copolymer-templated NiO films with an organic push–pull dye. The potential of these new templated NiO film preparations for photoelectrochemical applications is compared with NiO material templated by F108 triblock copolymers. We conclude that NiO films are promising materials for the construction of dye-sensitized photocathodes to be inserted into photoelectrochemical (PEC) cells. However, a combined effort at the interface between materials science and molecular chemistry, ideally funded within a Global Artificial Photosynthesis Project, is still needed to improve the overall performance of the photoelectrodes and progress towards economically viable PEC devices.