Bipolar membrane electrolyzers for co-upgrading of CO2 capture solutions and sulfide contaminants to syngas and sulfur

Abstract

Direct electrolysis of CO₂ capture solutions (e.g. (bi)carbonate) streamlines upstream carbon supply, yet faces challenges including high cell voltage, low-value anode byproduct, and gaseous product impurity owing to incomplete CO₂ utilization. Herein, we demonstrate a bipolar membrane (BPM) electrolyzer coupling CO₂ capture solution reduction with sulfion oxidation reaction (SOR) for cogeneration of syngas and sulfur. Tailoring BPMs with rapid water dissociation kinetics and mass transfer facilitates paired reactions through pH gradients, with cathode acidification triggering in situ CO₂ production for electroreduction while sustaining the alkaline environment necessary for anodic SOR. Leveraging gas-liquid extraction between the cathodic product stream and anolyte enables simultaneous syngas purification and sulfur precipitation, establishing a self-sustained system. With these material and process innovations, the paired electrolyzer achieves low energy consumptions (cell voltage <2.5 V), high carbon utilization (>97%), and long-term stable operation (>300 h) at 100 mA cm^-2, continuously producing syngas (CO/H₂ ratios = 2/1-1/1, with CO₂ content <3%) and pure elemental sulfur.

Keywords: CO2 capture solutions electrolysis; bipolar membranes; paired electrolyzers; self-sustained system; sulfion oxidation.

Figure 1.

Design concept of the CO₂RR|BPM|SOR paired electrolyzer. (a) Schematic illustration of the CO₂RR|BPM|SOR paired electrolyzer. The electrolyzer employs a BPM to separate the cathode and anode. Under reverse bias, water dissociation at the BPM interface generates H⁺ that migrate to the cathode, acidifying bicarbonate to produce CO₂ for subsequent reduction. Simultaneously, water dissociation creates an alkaline cathodic environment favorable for SOR. (b) Comparison of electrode potentials and product value-added between SOR and OER. SOR exhibits lower potential and is more economically favorable than OER.

Figure 2.

Paired electrolyzer performed with PEM. (a) Polarization curves of the CO₂RR||SOR and CO₂RR||OER electrolyzers equipped a Nafion 117 PEM. The CO₂RR||SOR electrolyzer requires a lower voltage than CO₂RR|| OER below 200 mA cm⁻², but the voltage convergence at higher current densities indicate a transition from SOR to OER as the dominant anodic reaction. (b) Current density-dependent FE_CO and CO₂ utilization rate of the electrolyzers. (c) Schematic illustration of the main reactions at the cathode, where HER prevails as the main pathway with H₂ production under CO₂-starved conditions. (d) Modeled pH distribution within the cathode for different applied current densities. (e) Evolution of current density, FE_CO, and electrode pH values of the CO₂RR||SOR electrolyzer during stability testing at a cell voltage of 1.5 V.

Figure 3.

Paired electrolyzer performed with commercial BPM. (a) Schematic illustration of the main reactions at the cathode. H⁺ from the BPM interface migrate to the cathode (step 1), reacting with bicarbonate to generate CO₂ (step 2), which is subsequently reduced to CO (step 3) alongside competitive HER (step 4). (b) Modeled pH distribution within the cathode for different water dissociation voltages. (c) Modeled CO₂ concentration within the cathode for different CO₂RR current densities. (d) Polarization curves of the CO₂RR||SOR and CO₂RR||OER electrolyzers equipped with a commercial FBM-PK BPM. Commercial membranes exhibit low water dissociation efficiency and slow mass transport, resulting in high cell voltages that limit the effective current density window to below 50 mA cm⁻². (e) Current density-dependent FE_CO and CO₂ utilization rate of the electrolyzers. (f) Evolution of current density, FE_CO, and electrode pH values of the CO₂RR||SOR electrolyzer during stability testing at a cell voltage of 1.5 V.

Figure 4.

Paired electrolyzer performed with tailored BPM. (a) Structural authentication of BPSn-BM. Molecular formula and scale photograph (30 × 15 cm). (b) Ohmic and water dissociation resistances of the commercial FBM-PK and home-made BPSn-BM membranes for different current densities. (c) Polarization curves of the CO₂RR||SOR and CO₂RR||OER electrolyzers equipped a BPSn-BM BPM. The electrolyzer with advanced BPMs demonstrates significantly reduced cell voltage compared to commercial counterparts, thereby extending the current density window to 200 mA cm⁻². (d) Current density-dependent FE_CO and CO₂ utilization rate of the electrolyzers. Evolution of (e) cell voltage, electrode pH values and (f) FE_CO, CO₂ utilization rate, and anodic S²⁻ concentration of the CO₂RR||SOR electrolyzer during stability testing at a current density of 100 mA cm⁻². (g) CO₂-mediated anolyte acidification process. (h) XRD pattern of the collected sulfur product with S₈ reference standard (PDF#78–1888).

Figure 5.

Self-sustained system design for the CO₂RR||SOR paired electrolyzer. (a) Schematic illustration of the self-sustained CO₂RR||SOR paired electrolyzer. Gas–liquid extraction between the cathode out stream and anolyte enables simultaneous cathodic syngas purification and anolyte acidic precipitation. (b) Evolution of cell voltage, FE_CO, CO₂ utilization rate, and CO/H₂ ratio of the self-sustained electrolyzer during stability testing at a current density of 100 mA cm⁻². (c) Energy consumption comparison of our paired electrolyzer versus conventional (bi)carbonate- and gaseous CO₂-fed systems. (d) Carbon emission comparison of the optimized bicarbonate electrolyzer (CO₂RR|BPSn-BM|SOR) versus commercial BPM-integrated (CO₂RR|FBM-PK|SOR and CO₂RR|FBM-PK|OER) and SOR-free configurations (CO₂RR|BPSn-BM|OER).