The efficient interconversion of electrical energy and chemical fuels will be crucial for our future sustainable energy economy. For example, fuel cells utilize the electrochemical oxidation of fuels to directly generate electricity, offering higher theoretical efficiencies than standard combustion engines. However, current fuel cells utilize heterogenous catalysts that are based upon precious metals and require high overpotentials to achieve reasonable rates. We are developing molecular first-row transition metal catalysts for the electro-oxidation of fuels such as hydrogen and formic acid. The oxidation of both substrates by a transition metal catalyst likely involves hydride transfer from the substrate to the metal center, setting a thermodynamic requirement for the hydricity of the resulting metal-hydride intermediate. Using established thermochemical cycles, we identified a series of cobalt(III) complexes with bidentate phosphine ligands, [CpCo(P-P)(MeCN)]2+, that are predicted to favor the oxidation of hydrogen and formic acid under mildly basic conditions. We have synthesized the corresponding cobalt(III)-hydride complexes and experimentally determined their pKa and hydricity values as a function of ligand properties. By 1H NMR and cyclic voltammetry studies, we find that addition of formate to cobalt(III)-MeCN species leads to rapid formation of a cobalt(III)-formate species. With unfunctionalized alkylphosphine ligands, subsequent hydride transfer gradually occurs over the course of several hours to generate the Co(III)-hydride complex. Notably, this reaction is significantly faster for complexes containing phosphine ligands bearing pendent amine groups, occurring over several minutes under ambient conditions. While these complexes strongly favor formate oxidation from a thermodynamic standpoint, this result highlights the important role that the outer coordination sphere plays in lowering the kinetic barrier for hydride transfer. We have also found that the pendent amine systems are capable of heterolytically cleaving hydrogen in the absence of an exogenous base without pressurization, setting the stage of electrochemical hydrogen oxidation under mild conditions. We are currently working on understanding the role that exogenous bases may play in the kinetics of H2 cleavage in addition to elucidating potential relationships that may exist between the thermodynamics and kinetics H2 cleavage.

Developing molecular electrocatalysts for fuel oxidation

ACS Spring Meeting 2021

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