Role of ion exchange membranes and capacitive electrodes in membrane capacitive deionization (MCDI) for CO₂ capture

Recently we showed that membrane capacitive deionization (MCDI) can be used to capture CO₂, but we found that the performance decreases with decreasing current density. In the present study, we investigate the effect of electrodes and ion exchange membranes by performing experiments with two membranes (CO₂-MCDI), with one membrane (cation or anion exchange membrane), and without membranes (CO₂-CDI). We find that the anion exchange membrane is essential to keep high CO₂ absorption efficiencies ( [Formula: see text] /n_charge), while the absorption efficiency of the CO₂-CDI cell was lower than expected (Λ_a≈0.5 for CO₂-MCDI against Λ_a≈0.18 for CO₂-CDI). Moreover, we theoretically investigate ion adsorption mechanisms in the electrodes by comparing experimental data of a CO₂-CDI cell with theoretical results of the classic amphoteric-Donnan model developed for conventional CDI. By comparing the experimental results with the amph-D model, we find that the model overestimates the absorption efficiency in CO₂-CDI experiments. To understand this discrepancy, we investigate the effects of other phenomena, i.e., (i) low ion concentration, (ii) passive CO₂ absorption, and (iii) the effect of acid-base reactions on the chemical surface charge.