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The structure of active sites of enzymes involved in bioenergetic processes can inspire design of active, stable and cost-effective catalysts for renewable-energy technologies. For these materials to reach maturity, the benefits of bioinspired systems must be combined with practical technological requirements.

Hydrogen production through water splitting is one of the most promising solutions for the storage of renewable energy. [NiFe] hydrogenases are organometallic enzymes containing nickel and iron centres that catalyse hydrogen evolution with performances that rival those of platinum. These enzymes provide inspiration for the design of new molecular catalysts that do not require precious metals. However, all heterodinuclear NiFe models reported so far do not reproduce the Ni-centred reactivity found at the active site of [NiFe] hydrogenases. Here, we report a structural and functional NiFe mimic that displays reactivity at the Ni site. This is shown by the detection of two catalytic intermediates that reproduce structural and electronic features of the Ni-L and Ni-R states of the enzyme during catalytic turnover. Under electrocatalytic conditions, this mimic displays high rates for H 2 evolution (second-order rate constant of 2.5 × 10 4  M −1  s −1 ; turnover frequency of 250 s −1 at 10 mM H + concentration) from mildly acidic solutions. [NiFe] hydrogenases are enzymes containing nickel and iron centres that catalyse hydrogen evolution with performances that rival those of platinum catalysts. Now, a NiFe model complex has been reported that mimics the structure and the Ni-centred hydrogen evolution activity found at the active site of [NiFe] hydrogenases.

Molybdenum sulfides are very attractive noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER) from water. The atomic structure and identity of the catalytically active sites have been well established for crystalline molybdenum disulfide ( c -MoS 2 ) but not for amorphous molybdenum sulfide ( a -MoS x ), which exhibits significantly higher HER activity compared to its crystalline counterpart. Here we show that HER-active a -MoS x , prepared either as nanoparticles or as films, is a molecular-based coordination polymer consisting of discrete [Mo 3 S 13 ] 2− building blocks. Of the three terminal disulfide (S 2 2− ) ligands within these clusters, two are shared to form the polymer chain. The third one remains free and generates molybdenum hydride moieties as the active site under H 2 evolution conditions. Such a molecular structure therefore provides a basis for revisiting the mechanism of a -MoS x catalytic activity, as well as explaining some of its special properties such as reductive activation and corrosion. Our findings open up new avenues for the rational optimization of this HER electrocatalyst as an alternative to platinum. Molybdenum sulfides are attractive electrocatalysts for the hydrogen evolution reaction. The polymeric structure of amorphous molybdenum sulfide can now be formulated as a coordination polymer based on [Mo 3 S 13 2− ] clusters sharing disulfide ligands.

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. Innovative solutions for the design of sustainable and efficient systems for the conversion and storage of renewable energy sources are needed, and one promising option is the production of hydrogen through water splitting. A nanoparticulate electrocatalytic material consisting of metallic cobalt coated with a cobalt-oxo/hydroxo-phosphate layer is now found to exhibit active hydrogen evolution, and can also be converted into a cobalt oxide film catalysing oxygen evolution.

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.

The status, concepts and challenges toward catalysts free of platinum group metal (pgm) elements for proton-exchange membrane fuel cells (PEMFC) are reviewed. Due to the limited reserves of noble metals in the Earth's crust, a major challenge for the worldwide development of PEMFC technology is to replace Pt with pgm-free catalysts with sufficient activity and stability. The priority target is the substitution of cathode catalysts (oxygen reduction) that account for more than 80% of pgms in current PEMFCs. Regarding hydrogen oxidation at the anode, ultralow Pt content electrodes have demonstrated good performance, but alternative non-pgm anode catalysts are desirable to increase fuel cell robustness, decrease the H2 purity requirements and ease the transition from H2 derived from natural gas to H2 produced from water and renewable energy sources.

In cells, di-iron hydrogenases require three maturases to facilitate proper assembly of metal clusters. Reconstitution experiments with synthetic cofactor mimics coupled with functional and spectroscopic characterization now show these helper proteins are not needed in vitro to form highly active H 2 -producing catalysts. Hydrogenases catalyze the formation of hydrogen. The cofactor ('H-cluster') of [FeFe]-hydrogenases consists of a [4Fe-4S] cluster bridged to a unique [2Fe] subcluster whose biosynthesis in vivo requires hydrogenase-specific maturases. Here we show that a chemical mimic of the [2Fe] subcluster can reconstitute apo-hydrogenase to full activity, independent of helper proteins. The assembled H-cluster is virtually indistinguishable from the native cofactor. This procedure will be a powerful tool for developing new artificial H 2 -producing catalysts.

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 Earth's crust) metal is not a sustainable resource [Gordon RB, Bertram M, Graedel TE (2006) Proc Natl Acad Sei 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.

The structure of the hydrogenase-maturation protein HydF in the holo form with its [4Fe-4S] cluster reveals a labile glutamate ligand that allows binding of artificial 2Fe subcluster mimics, thus endowing HydF with its own hydrogenase activity. [FeFe] hydrogenase (HydA) catalyzes interconversion between 2H + and H 2 at an active site composed of a [4Fe-4S] cluster linked to a 2Fe subcluster that harbors CO, CN − and azapropanedithiolate (adt 2− ) ligands. HydE, HydG and HydF are the maturases specifically involved in the biosynthesis of the 2Fe subcluster. Using ligands synthesized by HydE and HydG, HydF assembles a di-iron precursor of the 2Fe subcluster and transfers it to HydA for maturation. Here we report the first X-ray structure of HydF with its [4Fe-4S] cluster. The cluster is chelated by three cysteines and an exchangeable glutamate, which allows the binding of synthetic mimics of the 2Fe subcluster. [Fe 2 (adt)(CO) 4 (CN) 2 ] 2− is proposed to be the true di-iron precursor because, when bound to HydF, it matures HydA and displays features in Fourier transform infrared (FTIR) spectra that are similar to those of the native HydF active intermediate. A new route toward the generation of artificial hydrogenases, as combinations of HydF and such biomimetic complexes, is proposed on the basis of the observed hydrogenase activity of chemically modified HydF.

Herein, the synthesis and optoelectronic properties of a novel push–pull organic dye, pRK1, specifically designed for use in dye‐sensitized photocathodes for hydrogen generation, are reported. The chemical structure of this dye, which incorporates a benzothiadiazole moiety, is inspired by RK1, a dye previously reported as a photosensitizer in n‐type dye‐sensitized solar cells (DSSCs) with power conversion efficiencies above 10% and high stability. The photoelectrochemical activity for hydrogen evolution of pRK1 after grafting onto NiO photocathodes in combination with a cobalt tetrapyridyl catalyst [Co(bapbpy)(OH2)2](BF4)2 in aqueous solution is evaluated and compared with two reference dyes from the literature, RuP2‐bpy and P1. It is shown that among the three photocathodes studied in this work, NiO|pRK1 is the most efficient, producing up to 1.9 μmol cm−2 of hydrogen with a faradaic efficiency of 66%, under visible light irradiation in aqueous electrolyte at pH 4.5. pRK1 shows a turnover number (TONdye) of up to 145 during the 6 h chronoamperometric test, almost twice that of P1. This study demonstrates that the chemical structure of high performance dyes commonly used in DSSCs can be successfully modified to meet the requirements for light‐driven water splitting in dye‐sensitized photoelectrochemical cells. A novel push–pull dye, pRK1, is reported and used in dye‐sensitized photoelectrochemical cells. The photoelectrochemical activity for hydrogen evolution of this photosensitizer is evaluated after grafting onto NiO photocathodes using a cobalt tetrapyridyl catalyst, [Co(bapbpy)(OH2)2](BF4)2, in aqueous solution. The sensitized photocathodes produce up to 1.9 μmol cm−2 of hydrogen with a faradaic efficiency of 66%.