From Hydrogenases to Noble Metal–Free Catalytic Nanomaterials for H 2 Production and Uptake

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.