Scalable H2-redox-mediated direct air capture powered by renewable electricity

The increasing concentration of atmospheric CO2 drives global climate change and motivates the development of electrochemical direct air capture (eDAC) technologies to surpass conventional energy-intensive methods based on temperature or pressure swing1, 2. Among them, the anion exchange membrane (AEM) -based systems offer the advantage of direct gas-phase CO2 capture. Fundamentally driven by pH swing, an AEM-based eDAC system requires a reduction reaction to generate hydroxides for CO2 capture and an oxidation reaction to consume hydroxides for CO2 release. Coupling the hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) is preferable due to zero thermodynamic equilibrium voltage, facile kinetics, and non-oxidative electrochemical environment, leading to high energy efficiency and long lifetime. However, this pairing remained unrealized because HER is incompatible with oxygen in air. Here, we demonstrate an electrochemical H2-redox-mediated CO2 separator (eHCS) that incorporates an anion-conducting isolation membrane between the cathode and airflow, which blocks O2 access to the HER side while supporting back-diffusion of hydroxides for CO2 capture, and simultaneously enables internal hydrogen recycling to the HOR side for CO2 release. Once optimized, the eHCS shows a CO2 flux of 330 kg/m2/yr with an electricity usage of 1. 08 MWh/tCO2 and minimal H2 loss. Durability and scalability were validated through 720-hour continuous operation and a 3-cell stack demonstration. Rapid load-following response under dynamic conditions reveals its potential for integration with renewable electricity sources. For a 1 MtCO2/yr eHCS plant, technoeconomic analysis projects an eDAC cost of 97/tCO2. The strategies herein establish a diffusion-driven rather than typical migration-driven electrochemical process, opening new avenues for electrochemical device design when selective exclusion of unwanted species is critical.