Deciphering Reversible Homogeneous Catalysis of the Electrochemical H₂ Evolution and Oxidation: Role of Proton Relays and Local Concentration Effects**

Abstract

Nickel bisdiphosphine complexes bearing pendant amines form a unique series of catalysts (so‐called DuBois’ catalysts) capable of bidirectional/reversible electrocatalytic oxidation and production of dihydrogen. This unique behaviour is directly linked to the presence of proton relays installed close to the metal center. We report here for the arginine derivative [Ni(P2CyN2Arg)2]6+ on a mechanistic model and its kinetic treatment that may apply to all DuBois’ catalysts and show that it allows for a good fit of experimental data measured at different pH values, catalyst concentrations and partial hydrogen pressures. The bidirectionality of catalysis results from balanced equilibria related to hydrogen uptake/evolution on one side and (metal)‐hydride installation/capture on the other side, both controlled by concentration effects resulting from the presence of proton relays and connected by two square schemes corresponding to proton‐coupled electron transfer processes. We show that the catalytic bias is controlled by the kinetic of the H2 uptake/evolution step. Reversibility does not require that the energy landscape be flat, with redox transitions occurring at potentials up to 250 mV away for the equilibrium potential, although such large deviations from a flat energy landscape can negatively impacts the rate of catalysis when coupled with slow interfacial electron transfer kinetics.

Proton transfer kinetics of transition metal hydride complexes and implications for fuel-forming reactions

“…system, where all of the kinetic constants were extracted for the proposed reversible reaction scheme. 26 Notably, this study shows the importance of the P R 2 N R 0 2 ligand scaffold not only for its conformational flexibility and proper positioning of the pendant amine for metal hydride formation, but also for the effective deprotonation of the nickel-hydride species.…”