Alkaline closed-loop operation of bipolar membrane electrodialysis for efficient CO$_2$ 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. ... mehrOverall, the closed-loop alkaline BPMED enabled stable, energy-efficient CO$_2$ capture
from seawater while simultaneously generating a NaOH-rich stream.