An electrochemical membrane processes for CO2 capture

Electrochemically driven CO₂ capture is a rapidly growing field of research that represents a wide yet largely unexplored parameter space. We have developed a first-of-its-kind methodology that uses an anion exchange membrane sandwiched between Pt/C catalyst-coated gas diffusion electrodes within a cell, as shown in Figure 1. OH- ions that are generated on the feed (cathode) side through the hydrogen evolution reaction (HER) react with CO₂ molecules present in the feed gas stream to form HCO₃⁻ and CO₃²⁻ ions (carbonation). These ions then electromigrate across the membrane to the permeate (anode) side, where they react with a stream of H2 to form a mixture of CO₂ and H₂O through the hydrogen oxidation reaction (HOR). The net result is the transport of CO₂ from the feed side to the permeate side of the cell, where the resulting mixture of CO₂ and residual H2 could be used for a downstream fuel synthesis process, such as methanol synthesis. We performed cell polarization experiments with current densities up to 50 mA·cm-2 using 0.1-100% CO2 in N2 as the feed gas, and permeate CO₂ concentrations were monitored using on-line gas analysis. The bar plot in Figure 1 shows the molar specific energy consumption of the CO₂ separation process (log scale) at the current densities of 5 and 10 mA·cm-2 for various feed gas concentrations, with the minimum theoretical separation work for the given gas mixture shown for reference. As shown in Figure 1, the energy consumption is heavily dependent on the feed gas concentration, with values of 16-26 kJ·mol^-1 for 15% CO₂ (similar to the concentration found in coal flue gas) and 189-390 kJ·mol^-1 for 0.1% CO₂ (similar to what is found in air). This is largely an effect of increased fraction of OH- pumping at low CO₂ concentrations, which limits the applicability of the approach for sub-percent concentrations of CO₂. Further experiments successfully demonstrated the feasibility of electrochemical CO₂ pumping against concentration and pressure gradients. Finally, we employed a simplified techno-economic model to examine the cost dynamics of the system, showing that current densities of >50 mA·cm^-2 and faradaic efficiencies of 50% are necessary for the process to be economically viable. The results represent an important step in mapping out the current capabilities and limitations of a membrane electrochemical process for CO₂ capture, showing where efforts should be directed in future experimental campaigns.