A novel strategy toward energy-efficient direct air capture: Harnessing HCO3−/CO32− perm-selectivity in electrodialysis

A novel strategy is proposed to lower CO 2 regeneration energy in DAC. • Monovalent-selective AEMs capable of selectively separating HCO 3 − and CO 3 2− were prepared. • The optimized membrane exhibited a superior HCO 3 − /CO 3 2− perm-selectivity of 24.2. • Specific energy consumption for CO 2 regeneration was reduced by 59.3% compared to commercial AEMs. • The fabricated AEMs exhibited both high HCO 3 − /CO 3 2− perm-selectivity and total carbon permeability. Direct air capture (DAC) is an emerging technology for reducing atmospheric CO 2 content directly, but its current deployment is limited by the high energy demand in CO 2 regeneration. As an energy-conserving alternative, bipolar membrane electrodialysis (BPMED) exploits pH differentials to drive CO 2 regeneration without external thermal inputs or pressurized environments. In this work, we propose a new strategy to construct anion exchange membranes (AEMs) with high carbon loading capacity, enabling preferential permeation of bicarbonate (HCO 3 − ) over carbonate (CO 3 2− ) to significantly reduce specific electrical energy consumption for CO 2 regeneration in BPMED. AEMs were synthesized in a single-casting-process fabrication that integrates the dual advantages of electrostatic regulation and self-crosslinking, achieved through the incorporation of trimethylamine and sulfamerazine (SF) into 2,6-dimethyl-1,4-phenylene oxide (BPPO). The resulting membranes exhibited low water uptake (<12.6%), minimal linear expansion rate (<3.5%), and an area resistance as low as 2.7 Ω cm 2 . The perm-selectivity ( P CO 3 2 ‐ HCO 3 ‐ ) declined with increasing applied current density. At 5 mA/cm 2 current density, the optimized AEM (BPPO-SF-0.12) exhibited a HCO 3 − permeability of 0.21 mmol/(cm 2 ·h) and a CO 3 2− permeability of 0.04 mmol/(cm 2 ·h), yielding a perm-selectivity value of 24.2, substantially higher than that of the commercial membrane (1.0). In BPMED-based CO 2 regeneration, BPPO-SF-0.12 achieved the highest CO 2 regeneration ratio of 45.3%, and its energy consumption was only 240 kJ/mol, corresponding to a 59.3% reduction relative to the commercial membrane (590 kJ/mol). These findings substantiate the efficacy of the proposed strategy, revealing that AEMs with elevated carbon loading concurrently augment CO 2 regeneration ratio and energy performance. Such results establish a practical framework for energy-minimal CO 2 liberation via BPMED technology. Download: Download high-res image (318KB) Download: Download full-size image