The intermittency of solar and wind power has prompted investigations into efficient methods for long-term energy storage in chemical fuels and for on-demand release. One promising avenue for the latter is through the use of fuel cells; however, current technologies typically utilize hydrogen as the fuel, which is challenging to store and transport and has a low volumetric energy density, even at high pressures. Carbon-based liquid fuels, such as formate and methanol, have shown significant promise as fuels as they are much easier to handle, are relatively energy dense, and can be produced from the direct hydrogenation or electrochemical reduction of CO<sub>2</sub> using renewable energy sources. However, there is a dearth of molecular electrocatalysts for the oxidation of methanol and formate, with most examples based on precious metals and/or high energy metal-oxo species. Catalysts that facilitate hydride transfer from these substrates to a metal center are likely to be more energy efficient and operate at more mild potentials. To that end, we have targeted first-row transition metal complexes with favorable thermodynamics for hydride transfer oxidation. In our efforts, we have synthesized and characterized a series of Co compounds featuring pendent amine ligands for the oxidation of formate and alcohols via direct hydride transfer from the substrate. Density functional theory (DFT) calculations predict these reactions to be highly exergonic under ambient conditions, which we have confirmed experimentally by 1H NMR studies. The reactivity of the Co complexes toward formate and isopropanol oxidation under stoichiometric and electrocatalytic conditions will be discussed. The efficacy and stability of the complexes for facilitating oxidative catalysis are assessed as a function of ligand properties, notably electron-donating strength, steric bulk, and basicity of the pendent amine.

Exploration of cobalt catalysts for the electro-oxidation of liquid fuels

ACS Spring Meeting 2021

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