Vincent Artero

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.

From Hydrogenases to Noble Metal–Free Catalytic Nanomaterials for H2 Production and Uptake

Electrolysis at Nickel One drawback of solar and wind power is the need for an efficient storage system to release accumulated energy when neither source is readily available (during still nights, for example). Hydrogen derived from electrolysis of water is potentially a useful medium for this purpose, but catalyzing the interconversion efficiently at large scale would currently require a substantial amount of the scarce precious metal platinum. An alternative approach would be to mimic natural enzymatic reactions, which accomplish the interconversion using hydrogenases that incorporate the more abundant metals iron and nickel. In this vein, Le Goff et al. (p. 1384; see the Perspective by Hambourger and Moore) have lightly modified a hydrogenase-inspired nickel complex in order to append it to a conductive carbon nanotube support. The resulting hybrid material shows promising catalytic efficiency for reversible aqueous electrolysis in a standard apparatus. A nickel electrocatalyst supported on carbon nanotubes shows promising activity for proton-hydrogen interconversion in water. Interconversion of water and hydrogen in unitized regenerative fuel cells is a promising energy storage framework for smoothing out the temporal fluctuations of solar and wind power. However, replacement of presently available platinum catalysts by lower-cost and more abundant materials is a requisite for this technology to become economically viable. Here, we show that the covalent attachment of a nickel bisdiphosphine–based mimic of the active site of hydrogenase enzymes onto multiwalled carbon nanotubes results in a high–surface area cathode material with high catalytic activity under the strongly acidic conditions required in proton exchange membrane technology. Hydrogen evolves from aqueous sulfuric acid solution with very low overvoltages (20 millivolts), and the catalyst exhibits exceptional stability (more than 100,000 turnovers). The same catalyst is also very efficient for hydrogen oxidation in this environment, exhibiting current densities similar to those observed for hydrogenase-based materials.

A Janus cobalt-based catalytic material for electro-splitting of water

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. The production of hydrogen through water splitting seems a promising and appealing solution. We found that a robust nanoparticulate electrocatalytic material, H(2)-CoCat, can be electrochemically prepared from cobalt salts in a phosphate buffer. This material consists of metallic cobalt coated with a cobalt-oxo/hydroxo-phosphate layer in contact with the electrolyte and mediates H(2) evolution from neutral aqueous buffer at modest overpotentials. Remarkably, it can be converted on anodic equilibration into the previously described amorphous cobalt oxide film (O(2)-CoCat or CoPi) catalysing O(2) evolution. The switch between the two catalytic forms is fully reversible and corresponds to a local interconversion between two morphologies and compositions at the surface of the electrode. After deposition, the noble-metal-free coating thus functions as a robust, bifunctional and switchable catalyst.

Biomimetic assembly and activation of [FeFe]-hydrogenases

Hydrogenases are the most active molecular catalysts for hydrogen production and uptake, and could therefore facilitate the development of new types of fuel cell. In [FeFe]-hydrogenases, catalysis takes place at a unique di-iron centre (the [2Fe] subsite), which contains a bridging dithiolate ligand, three CO ligands and two CN– ligands. Through a complex multienzymatic biosynthetic process, this [2Fe] subsite is first assembled on a maturation enzyme, HydF, and then delivered to the apo-hydrogenase for activation. Synthetic chemistry has been used to prepare remarkably similar mimics of that subsite, but it has failed to reproduce the natural enzymatic activities thus far. Here we show that three synthetic mimics (containing different bridging dithiolate ligands) can be loaded onto bacterial Thermotoga maritima HydF and then transferred to apo-HydA1, one of the hydrogenases of Chlamydomonas reinhardtii algae. Full activation of HydA1 was achieved only when using the HydF hybrid protein containing the mimic with an azadithiolate bridge, confirming the presence of this ligand in the active site of native [FeFe]-hydrogenases. This is an example of controlled metalloenzyme activation using the combination of a specific protein scaffold and active-site synthetic analogues. This simple methodology provides both new mechanistic and structural insight into hydrogenase maturation and a unique tool for producing recombinant wild-type and variant [FeFe]-hydrogenases, with no requirement for the complete maturation machinery.

Cobalt and nickel diimine-dioxime complexes as molecular electrocatalysts for hydrogen evolution with low overvoltages

Hydrogen production through the reduction of water appears to be a convenient solution for the long-run storage of renewable energies. However, economically viable hydrogen production requests platinum-free catalysts, because this expensive and scarce (only 37 ppb in the Earths crust) metal is not a sustainable resource [Gordon RB, Bertram M, Graedel TE (2006) Proc Natl Acad Sci USA 103:1209–1214]. Here, we report on a new family of cobalt and nickel diimine-dioxime complexes as efficient and stable electrocatalysts for hydrogen evolution from acidic nonaqueous solutions with slightly lower overvoltages and much larger stabilities towards hydrolysis as compared to previously reported cobaloxime catalysts. A mechanistic study allowed us to determine that hydrogen evolution likely proceeds through a bimetallic homolytic pathway. The presence of a proton-exchanging site in the ligand, furthermore, provides an exquisite mechanism for tuning the electrocatalytic potential for hydrogen evolution of these compounds in response to variations of the acidity of the solution, a feature only reported for native hydrogenase enzymes so far.

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.

H2 Evolution and Molecular Electrocatalysts: Determination of Overpotentials and Effect of Homoconjugation

In an effort to standardize the determination of overpotential values for H(2)-evolving catalysts in non-aqueous solvents and allow a reliable comparison of catalysts prepared and assayed by different groups, we propose to adopt the half-wave potential as reference potential. We provide a simple method for measuring it from usual stationary cyclic voltammograms, and we derive the formulas to which the measured potential should be compared, taking into account the effect of homoconjugation. We also revisit tabulated values of the standard reduction potential of protons in nonaqueous solvents, E(H+/H(2))°.

Noncovalent Modification of Carbon Nanotubes with Pyrene‐Functionalized Nickel Complexes: Carbon Monoxide Tolerant Catalysts for Hydrogen Evolution and Uptake

Hydrogen production through the reduction of water appears to be a very attractive solution for the long-term storage of renewable energy. However, economically viable processes require platinum-free catalysts, since this expensive and scarce metal is not a sustainable resource. We recently showed that the combination of a bioinspired molecular approach with nanochemical tools, through the covalent attachment of mimics 3] of the active site of hydrogenase enzymes onto carbon nanotubes (CNTs), results in a noblemetal-free electrocatalytic nanomaterial with low overpotential and exceptional stability for H2 evolution or uptake. [4,5] In this initial study, we used the electroreduction of a diazonium salt to decorate multiwalled carbon nanotubes (MWCNTs) deposited on the electrode support with a polyphenylene layer bearing amino groups. These amino groups were then used to attach an activated ester derivative [Ni(P2N Ar 2)2] 2+

Copper molybdenum sulfide: a new efficient electrocatalyst for hydrogen production from water

A new inorganic solid state electrocatalyst for the hydrogen evolution reaction (HER) is reported. Highly crystalline layered ternary sulfide copper-molybdenum-sulfide (Cu2MoS4) was prepared by a simple precipitation method from CuI and [MoS4]2− precursors. In aqueous solution and over a wide pH range (pH 0 to 7), this Cu2MoS4 showed very good catalytic activity for HER with an overvoltage requirement of only ca. 135 mV and an apparent exchange current density of 0.040 mA cm−2 (Tafel slope of ca. 95 mV per decade was found irrespective of the pH value). This Cu2MoS4 catalyst was found to be stable during electrocatalytic hydrogen generation. Therefore, it represents an attractive alternative to platinum.