A Bidirectional Bioinspired [FeFe]-Hydrogenase Model.

With the price-competitiveness of solar and wind power, hydrogen technologies may be game changers for a cleaner, defossilized, and sustainable energy future. H2 can indeed be produced in electrolyzers from water, stored for long periods, and converted back into power, on demand, in fuel cells. The feasibility of the latter process critically depends on the discovery of cheap and efficient catalysts able to replace platinum group metals at the anode and cathode of fuel cells. Bioinspiration can be key for designing such alternative catalysts. Here we show that a novel class of iron-based catalysts inspired from the active site of [FeFe]-hydrogenase behave as unprecedented bidirectional electrocatalysts for interconverting H2 and protons efficiently under near-neutral aqueous conditions. Such bioinspired catalysts have been implemented at the anode of a functional membrane-less H2/O2 fuel cell device.

Aerobic Partial Oxidation of Alkanes Using Photodriven Iron Catalysis.

Photodriven oxidations of alkanes in trifluoroacetic acid using commercial and synthesized Fe(III) sources as catalyst precursors and dioxygen (O2) as the terminal oxidant are reported. The reactions produce alkyl esters and occur at ambient temperature in the presence of air, and catalytic turnover is observed for the oxidation of methane in a pure O2 atmosphere. Under optimized conditions, approximately 17% conversion of methane to methyl trifluoroacetate at more than 50% selectivity is observed. It is demonstrated that methyl trifluoroacetate is stable under catalytic conditions, and thus overoxidized products are not formed through secondary oxidation of methyl trifluoroacetate.

How do H2 oxidation molecular catalysts assemble onto carbon nanotube electrodes? A crosstalk between electrochemical and multi-physical characterization techniques.

Molecular catalysts show powerful catalytic efficiency and unsurpassed selectivity in many reactions of interest. As their implementation in electrocatalytic devices requires their immobilization onto a conductive support, controlling the grafting chemistry and its impact on their distribution at the surface of this support within the catalytic layer is key to enhancing and stabilizing the current they produce. This study focuses on molecular bioinspired nickel catalysts for hydrogen oxidation, bound to carbon nanotubes, a conductive support with high specific area. We couple advanced analysis by transmission electron microscopy (TEM), for direct imaging of the catalyst layer on individual nanotubes, and small angle neutron scattering (SANS), for indirect observation of structural features in a relevant aqueous medium. Low-dose TEM imaging shows a homogeneous, mobile coverage of catalysts, likely as a monolayer coating the nanotubes, while SANS unveils a regular nanostructure in the catalyst distribution on the surface with agglomerates that could be imaged by TEM upon aging. Together, electrochemistry, TEM and SANS analyses allowed drawing an unprecedented and intriguing picture with molecular catalysts evenly distributed at the nanoscale in two different populations required for optimal catalytic performance.

Noncovalent Integration of a Bioinspired Ni Catalyst to Graphene Acid for Reversible Electrocatalytic Hydrogen Oxidation.

Efficient heterogeneous catalysis of hydrogen oxidation reaction (HOR) by platinum group metal (PGM)-free catalysts in proton-exchange membrane (PEM) fuel cells represents a significant challenge toward the development of a sustainable hydrogen economy. Here, we show that graphene acid (GA) can be used as an electrode scaffold for the noncovalent immobilization of a bioinspired nickel bis-diphosphine HOR catalyst. The highly functionalized structure of this material and optimization of the electrode-catalyst assembly sets new benchmark electrocatalytic performances for heterogeneous molecular HOR, with current densities above 30 mA cm-2 at 0.4 V versus reversible hydrogen electrode in acidic aqueous conditions and at room temperature. This study also shows the great potential of GA for catalyst loading improvement and porosity management within nanostructured electrodes toward achieving high current densities with a noble-metal free molecular catalyst.

Molecular engineered nanomaterials for catalytic hydrogen evolution and oxidation.

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.

Electrochemical and Infrared Spectroscopic Studies Provide Insight into Reactions of the NiFe Regulatory Hydrogenase from Ralstonia eutropha with O2 and CO.

The regulatory hydrogenase (RH) from Ralstonia eutropha acts as the H2-sensing unit of a two-component system that regulates biosynthesis of the energy conserving hydrogenases of the organism according to the availability of H2. The H2 oxidation activity, which was so far determined in vitro with artificial electron acceptors, has been considered to be insensitive to O2 and CO. It is assumed that bulky isoleucine and phenylalanine amino acid residues close to the NiFe active site "gate" gas access, preventing molecules larger than H2 interacting with the active site. We have carried out sensitive electrochemical measurements to demonstrate that O2 is in fact an inhibitor of H2 oxidation by the RH, and that both H(+) reduction and H2 oxidation are inhibited by CO. Furthermore, we have demonstrated that the inhibitory effect of O2 arises due to interaction of O2 with the active site. Using protein film infrared electrochemistry (PFIRE) under H2 oxidation conditions, in conjunction with solution infrared measurements, we have identified previously unreported oxidized inactive and catalytically active reduced states of the RH active site. These findings suggest that the RH has a rich active site chemistry similar to that of other NiFe hydrogenases.