Direct ocean capture

Abstract

Direct ocean capture using bipolar membrane electrodialysis (BPMED) enables indirect CO2 removal from the atmosphere and, when combined with storage, can deliver negative emissions indispensable to achieving carbon neutrality. While previous research demonstrated functionality, a comprehensive comparison of membrane arrangements is missing. We model and compare two BPMED arrangements: BPM/CEM ( in-situ capture) and BPM/CEM/AEM ( ex-situ capture), both using concentrated seawater (brine) and pure water (permeate). BPM/CEM achieves lower electrochemical energy consumption ( e ele) at low current densities, while H+ crossover hinders BPM/CEM/AEM. As current increases, BPM/CEM becomes limited by pumping losses due to the increasing flow rate required for in-situ treatment, whereas BPM/CEM/AEM eliminates these losses. Higher current also mitigates H+ crossover, yielding substantially lower total energy consumption for BPM/CEM/AEM. Tuning the compartment thickness and recycle ratio further reduces energy losses, leaving the bipolar membrane as the dominant contributor. In contrast, BPM/CEM faces an intrinsic trade-off between high pumping losses and high e ele. BPM/CEM/AEM additionally achieves higher productivity and lower permeate consumption. Experimentally, we investigate BPM/CEM/AEM and show that using pure water in the alkaline compartment is insufficient to prevent fouling. Fouling originates from the cation-exchange membrane and prevention requires complete removal of Mg2+ and Ca2+ from brine. Experimentally, we measure e ele of 170 kJ mol−1 at 10 mA cm−2 and project 220 kJ mol−1 at 100 mA cm−2 with an additional 60 kJ mol−1 from pumping, excluding the energy penalty for removing multivalent ions. This establishes the BPM/CEM/AEM configuration competitive to other pH-swing methods for CO2 extraction from seawater brine.