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

Nickel bisdiphosphine complexes bearing pendant amines form a unique series of catalysts (so-called DuBois catalysts) capable of bidirectional or even reversible electrocatalytic oxidation and production of dihydrogen. While this unique behaviour is directly linked to the presence of proton relays installed within the molecular structure, close to its metal center, quantitative activity descriptors are still lacking to guide the rational design of molecular catalysts with enhanced activity. We report here for the arginine derivative [Ni(P2CyN2Arg)2]6+ on a detailed kinetic treatment based on a mechanistic model that applies to the whole DuBois catalyst series and show that, with a unique set of parameters, it allows, for a good fit of the experimental data measured in a wide range of pH values, catalyst concentrations and partial hydrogen pressures. The bidirectionality of catalysis results from the balanced equilibrium constant of the kinetically critical hydrogen uptake and evolution chemical step and the corresponding rate constants being large enough in both directions, as well as a very fast intramolecular proton transfer, both being likely due to concentration effects resulting from the presence of proton relays at the immediate vicinity of the catalysts. In that specific case, we show that hydrogen oxidation, kinetically limited by H2 insertion, has a larger turnover frequency than hydrogen evolution that is kinetically limited by H2 release. The reversibility of catalysis appears also to result from a subtle balance between the characteristics of two sequential proton-coupled electron transfer square schemes and the equilibrium constants as well as the kinetic constants of both chemical steps. We illustrate experimentally that reversibility does not required that the energy landscape be flat, with in the present case redox transitions occurring at potentials ~250 mV away for the equilibrium potential. Still, large deviations from a flat energy landscape requires interfacial electron transfers to occur far from their equilibrium potential, which impacts their kinetics and the overall rate of catalysis. At that point, the rate of catalysis may be limited by the efficiency of deprotonation/reprotonation of the relays, a concern that also holds for the design of improved monodirectional electrocatalysts.