Abstract: In this work, the effect of ion-selective membranes on the detailed carbon balance was systematically analyzed for high-rate CO2 reduction in GDE-type flow electrolyzers. By using different ion-selective membranes, we show nearly identical catalytic selectivity for CO2 reduction, which is primarily due to a similar local reaction environment created at the cathode/electrolyte interface via the introduction of a catholyte layer. In addition, based on a systematic exploration of gases released from electrolytes and the dynamic change of electrolyte speciation, we demonstrate the explicit discrepancy in carbon
balance paths for the captured CO2 at the cathode/catholyte interface via reaction with OH when using different ion-selective membranes: (i) the captured CO2 could be transported through an anion exchange membrane in the form of CO32-, subsequently releasing CO2 along with O2 in the anolyte, and (ii) with a cation exchange membrane, the captured CO2 would be accumulated in the catholyte in the form of CO32-, while (iii) with the use of a bipolar membrane, the captured CO2 could be released at the catholyte/membrane interface in the form of gaseous CO2. The unique carbon balance path for each type of membrane is linked to ion species transported through the membranes.