An Internally-Recycling Electrochemical Hydrogen Pump for Direct Air Capture

Carbon capture from ambient air driven by renewable electricity enables atmospheric CO 2 removal while supplying CO 2 as a feedstock for chemical manufacturing such as methanol production. As a result, electrochemical direct air capture (eDAC) has emerged as a promising alternative to conventional energy-intensive processes based on temperature or pressure swing. Among them, the anion exchange membrane (AEM)-based systems offer the advantage of direct gas-phase CO 2 capture. Fundamentally driven by pH swing, an AEM-based eDAC system requires a reduction reaction to generate hydroxides for CO 2 capture and an oxidation reaction to consume hydroxides for CO 2 release. Coupling the hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) for the direct capture-release process would be preferred due to its potential low energy consumption, high CO 2 flux, and long lifetime. The low energy consumption originates from the zero thermodynamic equilibrium cell voltage and the well-known fast hydrogen kinetics; the high CO 2 flux stems from the use of porous electrodes; and the long lifetime arises from the low absolute electrochemical potential environment, mitigating concerns of oxidative degradation. Despite the obvious advantages listed above, an HER/HOR-based eDAC configuration has never been realized because the cathodic HER, which is necessary for CO 2 capture, cannot tolerate the presence of oxygen—an unavoidable component of air. Here, we demonstrate an electrochemical H 2 -redox-mediated CO 2 separator (eHCS) as shown in Fig. 1 by incorporating an anion-conducting isolation membrane between the cathode and airflow, blocking O 2 access to the cathode. We also implemented internal hydrogen recycling so there is no net hydrogen consumption in an ideal scenario. The strategies herein establish a diffusion-driven rather than typical migration-driven electrochemical process, opening new avenues for electrochemical device design, particularly in applications where the selective exclusion of unwanted species is critical, such as electrochemical separation or electrochemical synthesis. Once optimized, the eHCS shows a CO 2 flux of 330 kg/m 2 /yr with an electricity usage of 1.08 MWh/tCO 2 and minimal H 2 loss, exhibiting performance comparable to a standard electrochemical H 2 pump capturing 400 ppm CO 2 in N 2 . The advantage of eHCS over other DAC systems is highlighted in Fig. 2, where it stands out as the only technology that simultaneously achieves low energy consumption and high CO 2 flux compared to state-of-the-art eDAC technologies. In addition to its high performance, a 720-hour stable operation and a 3-cell stack demonstration confirm the stability and scalability of the device. Rapid load-following response under dynamic conditions reveals its potential for integration with renewable electricity sources. For a 1 Mt CO2 /yr eHCS DAC plant, technoeconomic analysis projects a capture cost below US Department of Energy (DOE)’s target of $100/tCO 2 . Moreover, we show that eHCS can employ PGM-free catalysts, and is suitable for applications with high CO 2 concentrations, such as post-combustion capture. The high-purity CO 2 (~99%) captured demonstrates that the eHCS can function as a modular electrochemical CO 2 separation unit for electrified chemical manufacturing, providing conditioned CO 2 feedstocks under energy-efficient and low-degradation operating conditions. References 1 Matz, S. et al. Hydrogen-powered Electrochemically-driven CO2 Removal from Air Containing 400 to 5000 ppm CO2. Journal of The Electrochemical Society 169, doi:10.1149/1945-7111/ac7adf (2022). 2 Buchen, J. et al. Hydroxide exchange membrane carbon capture using a nickel hydroxide symmetric battery cell. PREPRINT (Version 1) available at Research Square, doi:10.21203/rs.3.rs-5627423/v1 (2025). 3 Zhu, P. et al. Continuous carbon capture in an electrochemical solid-electrolyte reactor. Nature 618, 959-966, doi:10.1038/s41586-023-06060-1 (2023). 4 McQueen, N. et al. A review of direct air capture (DAC): scaling up commercial technologies and innovating for the future. Progress in Energy 3, doi:10.1088/2516-1083/abf1ce (2021). This effort was sponsored by the U.S. Government under OTA W912CH-24-9-0006 with the University of Delaware. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation herein. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the U.S. Government. Figure 1