Utilizing Direct Membrane Deposition to Improve the Performance of Forward-Bias Bipolar Membrane CO2 Electrolysers

Abstract

The electrochemical conversion of CO2 to multi-carbon products (C2+), such as ethylene and ethanol, is an attractive technology towards achieving net zero carbon emission goals. Among the available configurations of CO2 electrolysers, those employing a bipolar membrane (BPM) in forward-bias mode (f-BPM) reduce CO2 loss to the anode – a problem commonly faced in traditional anion exchange membrane (AEM) electrolysers. Therefore, carbon efficiency (the percent of input CO2 that is converted to C2+ products) and system operational costs can be improved.In f-BPM electrolysers, (bi)carbonate ions move through the AEM then combine with protons moving through the cation exchange membrane (CEM) to regenerate CO2 and H2O at the AEM|CEM interface. Recent reports have implemented a porous AEM structure or an added AEM|CEM interface channel to aid the movement of CO2 from the AEM|CEM interface to the cathode and, thus, avoid blistering. Despite the advantage of f-BPM CO2 electrolysers, their performance is limited by the parasitic H2 evolution reaction (>20% Faradaic efficiency, or FE) and the low C2+ FE (<40%) at industrially relevant reaction rates (≥200 mA cm-2). We attribute this performance limitation to the unintended gaps between the used AEM and CEM. Inadequate contact between the AEM and CEM is anticipated to reduce the extent of local CO2 regeneration and promote unwanted proton crossover to the cathode; both phenomena could be contributing to the commonly observed FEs.In this report, we enhanced the contact between the AEM and CEM through the use of direct membrane deposition (DMD) over a Cu-based cathode. The DMD approach enabled modular control over the AEM and CEM thicknesses and structures, which improved ion and gas transport. Through electrochemical impedance spectroscopy, the DMD system was observed to improve the mass transport by >58% compared to a system that is similar to current f-BPM electrolysers (i.e., control system). The facilitated mass transport resulted in a reduction of the operating cell potential (by 0.84 V) and an enhancement of the C2+ FE to 65% (from 29% in the control system) at 300 mA cm-2. The flexibility of the DMD approach also enabled the fabrication of asymmetric BPMs, resulting in a record low H2 FE of 12% at 300 mA cm-2. We also demonstrated a 79% single-pass CO2 conversion (SPC) with 67% C2+ FE, resembling the highest simultaneous SPC and C2+ FE achievement at high current density (300 mA cm-2)among current f-BPM electrolysers. The DMD benefit was also applicable to other types of CO2 electrolysers, such as CO2-to-CO silver catalyst electrolysers, in which the CO FE was boosted to 90% (from 65% in the control system) at 90 mA cm-2. Figure 1