Model-Driven Analysis of Ion Transport for High Electron Efficiency Membrane-Based Electrochemical CO 2 Capture

Reducing energy consumption is essential for electrochemical direct air capture (DAC) of CO 2 to reach the Department of Energy (DOE) cost target of $100/tonCO 2 . However, membrane-based electrochemical DAC systems typically exhibit electron efficiencies below 40% (mol CO 2 /mol e − ), in part due to an incomplete understanding of ion transport. To address this limitation, we developed a one-dimensional electrochemical model for hydrogen-powered hydroxide exchange membrane carbon capture (H 2 -HEMCC) systems. Validated against experiments, the model revealed two dominant inefficiencies: carbonate diffusion losses and hydroxide diffusion losses at low and high CO 2 flux, respectively. Guided by these insights, we experimentally show improved electron efficiency at low flux through higher membrane resistance, achieved via thermal degradation of quaternary ammonium sites. To overcome high-flux losses, we introduced a thick interlayer between the membrane and cathode, which suppresses hydroxide diffusion losses that dominate at high flux, where capital costs are lower. This strategy raised experimental peak electron efficiency from 38.7% to 63.8%, the highest reported for any membrane-based DAC system. It translates to $23/tonCO 2 in energy savings—nearly one quarter of the DOE target. Model projections further indicate that employing a carbonate-rejecting ionomer could elevate electron efficiencies beyond 75%, providing a new pathway for high-performance, low-energy electrochemical DAC systems.