(575a) Simultaneous CO2 Capture and Conversion Using Bipolar Membrane Electrodialysis

The rising concentration of atmospheric carbon dioxide (CO₂), largely driven by fossil fuel use in industrial processes, pushes our community towards a negative emission world where we must sequester existing CO2. This necessitates a streamlined approach that captures, stores, and utilizes CO₂ efficiently. While various strategies target CO₂ from the atmosphere, seawater, and industrial effluents, traditional thermochemical separation and conversion routes are often decoupled and energy-intensive, limiting their sustainability and economic viability. Electrochemical separation technologies powered by renewable energy offer a promising path toward process intensification and decarbonization. Among these, bipolar membrane electrodialysis (BMED) has emerged as a viable strategy for separating aqueous CO₂ from liquid feedstocks via pH-swing–induced gas evolution. However, the integration of CO₂ capture and electrochemical conversion within a single system remains underexplored. This disconnection between separation and conversion introduces added capital and energy costs due to the need for multiple unit operations. For instance, a direct air capture (DAC) unit must be carefully paired with an electrolyzer to avoid storing captured CO₂ as unutilized inventory.

To address this challenge, we present an integrated bipolar membrane electrolysis electrodialysis (BMEED) system capable of capturing CO₂ from aqueous bicarbonate streams and directly converting it to carbon monoxide (CO) in a single module. We first investigate the system’s performance for CO2 capture across varying applied current densities (50–100 mA/cm²), electrolyte concentrations (0.0025–0.5 M KHCO₃), flow rates, and number of stacked unit cells (1–5). CO₂ released through water dissociation at the BPM interface is directed to a cathode chamber, where it is reduced to CO using a nickel single-atom (NiSA) catalyst (2.5 cm × 2.5 cm). Iridium-coated titanium on stainless steel serves as the anode for the oxygen evolution reaction (OER). Analytical techniques include gas chromatography for product quantification, pH and conductivity measurements, and potentiometric analysis to assess energy consumption.

The BMEED architecture employs alternating bipolar membranes (BPM) and anion exchange membranes (AEM), with KHCO₃ as the feedstock. Bicarbonate ions migrate across the AEM into the circulation channel, where protons generated at the BPM interface acidify the stream, converting bicarbonate into gaseous CO₂. This in-situ generation and delivery of CO₂ to the cathode enables seamless electrochemical reduction. A current density sweep and flow rate variation (5–50 sccm CO₂) identify optimal operating regions for selective CO₂-to-CO conversion.

Experimental results demonstrate effective CO₂ separation across a wide range of bicarbonate concentrations—from seawater-level dilute solutions (0.0025 M) to concentrated conditions (0.5 M)—with complete bicarbonate conversion achievable under optimized current and flow parameters. Stacking multiple unit cells reduces the required current while maintaining high CO₂ throughput, enhancing energy efficiency. The system also mitigates carbonate crossover across the cathode by recirculating carbonate species into the circulation channel. CO₂ reduction at 100 mA/cm² with >10 sccm feed flow achieves CO selectivity above 95%, and increased flow rates improve product selectivity and stability. Preliminary techno-economic analysis indicates that integrating capture and conversion into a unified system reduces equipment redundancy and improves capital utilization compared to conventional multi-unit setups.

This work highlights the potential of BMEED as a modular, electrified platform for simultaneous CO₂ separation and conversion. Future research will focus on deeper integration of capture and catalysis within a single cell, optimization of stack design, and expanded techno-economic modeling at the system level.