Alkaline closed-loop operation of bipolar membrane electrodialysis for efficient CO2 capture from seawater

Bipolar membrane electrodialysis (BPMED) enables electrochemical pH swing for CO 2 capture from seawater, but conventional in-situ operation suffers from scaling and energy losses that limit stability. Here, a closed-loop alkaline BPMED configuration is introduced to continuously regenerate hydroxide while suppressing scaling via recirculation of a softened stream. System performance was evaluated as a function of current density (6–32 mA/cm 2 ), flow velocity (1–3 cm/s), and salinity (35–100 mS/cm) using model seawater and desalination brines. Optimal operation was achieved at current densities of 8–12 mA/cm 2 , extracting over 80% of the dissolved inorganic carbon with a specific energy consumption (SEC) of 3 kWh/kg CO 2 . Increasing flow velocity enhanced mass transport and reduced SEC from 4.5 to less than 3 kWh/kg CO 2 , while high salinity caused co-ion leakage and diminished hydroxide selectivity. Under identical conditions, the closed-loop system produced NaOH with SEC values of 1.8–2.2 kWh/kg and current efficiencies of 60–85%. Direct acid/base generation from NaCl brines provided current efficiency benchmarks of up to 90% and SEC as low as 1.8 kWh/kg NaOH, supporting electrochemical consistency. Overall, the closed-loop alkaline BPMED enabled stable, energy-efficient CO 2 capture from seawater while simultaneously generating a NaOH-rich stream. • Alkaline closed-loop BPMED enables continuous CO 2 capture from seawater with no scaling. • Optimal operation at 8–12 mA/cm 2 achieved >80% DIC removal at ~3 kWh/kg CO 2 . • Higher flow velocity (1–3 cm/s) reduced CO 2 -capture SEC from 4.5 to <3 kWh/kg CO 2 . • Closed-loop operation simultaneously generated NaOH at 1.8–2.2 kWh/kg with 60–85% current efficiency.