The exploitation of bio-inspired molecular and supramolecular photosystems or purified biocatalysts (e.g. hydrogenases) requires that they can be stably and durably integrated into electrode surfaces. This calls for original methods of attaching molecules (preferably covalently) to inexpensive electrodes (carbon, transparent ITO-type oxides, etc.). The various modern grafting methods, with examples, are discussed: (i) ligands modified by carboxylate, phosphonate or acetylacetonate functions, for attachment to oxides; (ii) electrochemical reduction of diazonium salts; (iii) electropolymerization; (iv) functionalization of carbon nanotubes. A fine example of electrode realization involving the development of an original efficient non-noble metal catalyst and its attachment to an electrode is presented in detail(Science, 2009). This result was obtained at the Laboratory of Chemistry and Biology of Metals (CEA-CNRS-Université J. Fourier, at CEA Grenoble), in collaboration with a team from the Laboratory of Chemistry of Surfaces and Interfaces (CEA Saclay) and a team from the Laboratory of Innovation for New Energy Technologies and Nanomaterials (CEA Grenoble).
For the first time, we have been able to develop a material capable, in electrochemical devices, of catalyzing, as platinum does, both the production of hydrogen from water (for use in electrolyzers) and its oxidation (for use in fuel cells). This original material consists of a small nickel complex, which reproduces certain characteristics of hydrogenases, grafted onto carbon nanotubes chosen for their large potential catalyst-binding surface area and high electrical conductivity. Deposited on an electrode, it proves extremely stable, resistant to CO and capable of operating, without overvoltage, in highly acidic environments, making it compatible with proton exchange membranes (such as Nafion), which are used almost universally in fuel cells. Even if the electrical current densities obtained are still low, the implementation of this new material could remove a major scientific barrier to the large-scale development of the hydrogen economy.
