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 [SiCu2IIGaIII(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 [SiCu2IIGaIII(H2O)3W9O37]9- reduced polyammonium carbamate at a negative potential of -1 V vs. Ag/AgCl, forming CO and H2, 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.