Nitrocellulose-Based Membranes for Electrocatalytic Bicarbonate-to-Formate Conversion via In Situ Generated CO ₂

CO2 conversion using aqueous carbonate and bicarbonate solutions is a promising route for reactive CO2 capture, but sustaining high Faradaic efficiency (FE) over a broad pH range remains challenging. Introducing a mixed cellulose ester (MCE) porous membrane between the proton exchange membrane (PEM) and the bismuth cathode electrocatalyst enhances the FE for formate production during bicarbonate-fed electrolysis. In this configuration, bicarbonate ions (HCO3-) are protonated within the electrolyzer, enabling direct utilization of in situ-generated CO2 (i-CO2) for electroreduction. Here, we investigate the role of the intermediate porous membrane and the effects of different cellulose materials on electrolysis performance at current densities above 100 mA cm-2. Although direct contact between the cathode and PEM significantly decreases formate FE with increasing pH, incorporating the MCE membrane enhanced formate FE across all pH values, maintaining 78% even at pH 9.2 (2.0 mol L-1 KHCO3 + 0.5 mol L-1 K2CO3), demonstrating its effectiveness in sustaining i-CO2 generation. In contrast, α-cellulose and acetyl cellulose exhibit lower formate FE and higher full-cell voltage owing to increased cell resistance. Operando electrochemical impedance spectroscopy and real-time visualization of the bubble formation reveal that these celluloses produced large CO2 bubbles, whereas i-CO2 remained predominantly dissolved when using the MCE membrane, which is mainly composed of nitrocellulose. These results indicate that nitrate ester groups facilitate i-CO2 generation and its transport to the cathode catalyst. Furthermore, a 3.0 mol L-1 KOH solution that captures CO2 gas achieves 91% formate FE at 100 mA cm-2, with a full-cell voltage of 1.5 V and 80% i-CO2 utilization, highlighting the potential of this MCE-based electrolyzer design for reactive CO2 capture and utilization.