Covalent Attachment of the Water-insoluble Ni(PCy 2NPhe 2)2 Electrocatalyst to Electrodes Showing Reversible Catalysis in Aqueous Solution

Hydrogenases are a diverse group of metalloenzymes which catalyze the reversible conversion between molecular hydrogen and protons at high rates. The catalytic activity of these enzymes does not require overpotential because their active site has been evolutionarily optimized to operate fast and efficiently. These enzymes have inspired the development of molecular catalysts, which have dramatically improved in efficiency in recent years, to the point that some synthetic catalysts even outperform hydrogenases under certain conditions. In this work, we use a reversible noble-metal-free homogeneous catalyst, the [Ni(P^Cy <inf>2</inf>N^Phe <inf>2</inf>)<inf>2</inf>]²⁺ complex, and we covalently immobilize it on a functionalized highly oriented pyrolytic graphite “edge” (HOPG<inf>e</inf>) electrode surface. This catalyst is not water soluble, but once it is surface-confined on the electrode, it maintains its catalytic properties in aqueous solutions, showing reversibility for H<inf>2</inf> oxidation/reduction. Immobilization of the [Ni(P^Cy <inf>2</inf>N^Phe <inf>2</inf>)<inf>2</inf>]²⁺ complex onto a multi-walled carbon nanotubes coated electrode leads to even higher catalytic current densities and enhanced stability.