Electrochemical Reduction of Amine-Captured Carbon Dioxide Catalyzed by Transition-Metal Substituted Polyoxometalates

The low temperature electrocatalysis of CO₂ to CO typically requires purified CO₂, adding complexity, cost, and energy penalties due to the need for separate CO₂ capture and purification processes. Electrochemical reactive capture (e-RCC) is emerging as a simplification of the process. The method combines CO₂ capture with amines to form ammonium carbamates which are then reduced to produce CO and H₂. Heterometallic polyoxometalate catalysts were evaluated for e-RCC by cyclic voltammetry showing that [SiCu₂^IIGa^III(H₂O)₃W₉O₃₇]^9- had the lowest overpotential and highest turnover frequency. A combination of cyclic voltammetry and controlled potential electrolysis and transport measurements of carbamates through Nafion membranes revealed that commonly used small-molecule carbamates easily traversed membranes and were oxidized at the anode to CO and CO₂. To avoid anodic amine oxidation, polyammonium carbamates were prepared, and the method provides an efficient way to convert CO₂ captured into valuable carbon monoxide. Controlled potential electrolysis confirmed that [SiCu₂^IIGa^III(H₂O)₃W₉O₃₇]^9- reduced polyammonium carbamate at a negative potential of -1 V vs. Ag/AgCl, forming CO and H_2, while other polyoxometalate catalysts yield CO and H₂ at more negative potentials of -1.3 V. This research combines the use of polyoxometalates for electrocatalysts for e-RCC and provides a potential pathway to avoid detrimental amine/carbamate anodic oxidation.