The charge efficiency (CE) signifies the ratio of adsorbed CO2 over the charges stored in the electrodes. Biomass-derived activated carbons (GP-AC, GR-AC, and GS-AC) exhibit higher CE than commercial activated carbons (S-AC, BPL-AC, and Y-AC) and follow the sequence GP-AC (0.24) > GS-AC (0.17) > GR-AC (0.16) > S-AC (0.13) > BPL-AC (0.12) > Y-AC (0.10) (Figure ). High charge efficiency of biomass-derived electrodes is still far less than the theoretical maximum value of 1.0 (assuming the ionic liquid-solid mechanism as the only mechanism). Co-ion expulsion, competition of H + /HCO3 -with electrolyte ions in the double layer, and unwanted faradaic reactions (e.g., carbon oxidation, oxygen reduction) may hinder the CE to reach its maximum value. In addition, the conversion of HCO3 -ions to form CO3 2-that have a greater affinity to the anode due to their higher negative charge may reduce the charge efficiency. CE ranging from 0.5 to 0.7 has been achieved by using ion-selective membrane, thinner electrodes (0.25 mm vs 0.70 mm used in this study), and pure DI water instead of salt solutions in a related membrane-capacitive deionization process (MCDI). However, the added cost of membranes along with their mass-transfer limitations, mechanically weak electrodes with less absolute capacitance, and low ionic conductivity of the CO2 sparged DI water are disadvantageous for capturing CO2, especially at low concentrations. Separate studies requiring in-situ characterization are needed to gain insights about the adsorbed species at the double-layer, and possible faradaic surface reactions. These insights may then be used to improve the selectivity of the electrodes for H + /HCO3 -over the electrolyte ions.