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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.

Fuel cell reactions invariably involve an oxygen reduction reaction (ORR) at the cathode, which is one of the main rate-decreasing steps on platinum (Pt)-catalysts in the water formation reaction and energy conversion efficiency in polymer electrolyte membrane fuel cells (PEMFCs). The Pt scarcity and cost have led to the development of alternative catalyst materials for fuel cell applications. This paper reviews ORR catalysts with regard to their classification, mechanism, activity and performances. From conventional Pt-based catalysts to non-noble metal or bio-inspired catalysts, we show how significant progresses were made in ORR catalysis.

Molecules of bisthiolterthiophene have been adsorbed on the two facing gold electrodes of a mechanically controllable break junction in order to form metal-molecule(s)-metal junctions. Current-voltage (I-V) characteristics have been recorded at room temperature. Zero bias conductances were measured in the 10-100 nS range and different kinds of non-linear I-V curves with step-like features were reproducibly obtained. Switching between different kinds of I-V curves could be induced by varying the distance between the two metallic electrodes. The experimental results are discussed within the framework of tunneling transport models explicitly taking into account the discrete nature of the electronic spectrum of the molecule.Comment: 12 pages, 12 figures to appear in Phys. Rev. B 59(19) 199

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.

International audienc

International audienceCovalent surface modification of conductive, semiconductive, and insulating substrates with thin organic polymers films induced by redox activation of aryl diazonium salts in the presence of vinyl monomers has been investigated in acidic aqueous media. This new process, called diazonium-induced anchoring process (DIAP), is an efficient technique to impart covalent adhesion of polyvinyl coatings onto raw inorganic or organic surfaces without any conductivity requirement. Typically, aryl diazonium salts are reduced with iron powder to give surface-active aryl radicals leading (i) to the formation of a grafted polyphenylene-like film on the substrate surface and (ii) to the initiation of the radical polymerization of the vinylic monomer in solution. The resulting radical-terminated macromolecular chains formed in solution are then able to react with the polyphenylene primer layer to form a very homogeneous thin organic film on the surface. The final organic thin coating is strongly grafted on materials surfaces, as evidenced by its persistence after a long ultrasonic treatment in a good solvent of the polymer. We speculate this process is supported by the large concentration of aryl and hydrogen radicals formed when iron powder is added in the acidic aqueous solution. The thickness of the polymer film can be controlled as a function of time, typically a few minutes, and was measured between 10 and several hundred nanometers. Infrared reflection–absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and contact angle measurements were used to characterize the surface modification of metals, glass, carbon nanotubes, or polytetrafluoroethylene (PTFE). This very simple and efficient grafting method provides a powerful tool for the covalent coating of organic or inorganic surfaces possessing complex geometrical shapes

The spontaneous reaction of diazonium salts on various substrates has been widely employed since it consists of a simple immersion of the substrate in the diazonium salt solution. As electrochemical processes involving the same diazonium salts, the spontaneous grafting is assumed to give covalently poly(phenylene)-like bonded films. Resistance to solvents and to ultrasonication is commonly accepted as indirect proof of the existence of a covalent bond. However, the most relevant attempts to demonstrate a metal-C interface bond have been obtained by an XPS investigation of spontaneously grafted films on copper. Similarly, our experiments give evidence of such a bond in spontaneously grafted films on nickel substrates in acetonitrile. In the case of gold substrates, the formation of a spontaneous film was unexpected but reported in the literature in parallel to our observations. Even if no interfacial bond was observed, formation of the films was explained by grafting of aryl cations or radicals on the surface arising from dediazoniation, the film growing later by azo coupling, radical addition, or cationic addition on the grafted phenyl layer. Nevertheless, none of these mechanisms fits our experimental results showing the presence of an Au-N bond. In this work, we present a fine spectroscopic analysis of the coatings obtained on gold and nickel substrates that allow us to propose a chemical structure of such films, in particular, their interface with the substrates. After testing the most probable mechanisms, we have concluded in favor of the involvement of two complementary mechanisms which are the direct reaction of diazonium salts with the gold surface that accounts for the observed Au-N interfacial bonds as well as the formation of aryl cations able to graft on the substrate through Au-C linkages.