Molecular Electrocatalysts for the Oxidation of Hydrogen and the Production of Hydrogen – The Role of Pendant Amines as Proton Relays

Abstract

Electrocatalysts for efficient conversion between electricity and chemical bonds will play a vital role in future systems for storage and delivery of energy. Our research on functional models of hydrogenase enzymes uses nickel and cobalt, abundant and inexpensive metals, in contrast to platinum, a precious metal used in fuel cells. A key feature of our research is a focus on the use of pendant amines incorporated into diphosphane ligands. These pendant amines function as proton relays, lowering the barrier to proton transfers to and from the catalytically active metal site. The hydride acceptor ability of metal cations, along with the basicity of pendant amines, are key thermochemical values that determine the thermodynamics of addition of H₂ to a metal complex with a pendant amine incorporated into its ligand. Nickel catalysts for oxidation of H₂ have turnover frequencies up to 50 s^–1 (at 1 atm H₂ and room temperature). Nickel and cobalt catalysts for production of H₂ by reduction of protons were studied; one of them has a turnover frequency over 1000 s^–1.