Moisture Swing Direct Air Capture Utilizing Different Ion Exchange Resins – Exploring Possible Solutions for Carbon Capture –

Abstract

Rising atmospheric CO₂ concentrations in the recent century has highlighted the urgent need for effective carbon capture technologies. Among these, direct air capture (DAC) offers a unique carbon-negative solution by extracting CO₂ directly from ambient air. Traditional DAC approaches using thermal or pressure swings are energy-intensive, prompting interest in moisture swing capture, where CO₂ is absorbed at low humidity and released at high humidity. This study explores the use of cation exchange resins (CERs) as a potential alternative to the commonly studied anion exchange resins (AERs) for moisture swing DAC. By testing CERs with aminophosphonic and iminodiacetic functional groups and comparing them to AERs with phosphate and carbonate counterions, the study investigates CO₂ capture capacities, capture/regeneration rates, and performance across multiple cycles. Results suggest that CERs go through a similar but marginally different mechanism as AERs, where functional groups are hydrolyzed to produce hydroxyl ions, which then react to capture CO2 in the form of bicarbonate. The captured CO2 is released at high humidity as bicarbonate, and the acid form of functional groups neutralizes. However, their CO₂ capture capacity and rates are significantly lower than those of AERs, largely due to differences in hydrolysis mechanisms and functional group pKa values. Nevertheless, CERs demonstrated stable performance over repeated cycles, suggesting potential for improvement and application in scenarios where water purity is a limiting factor. This research offers insights into the feasibility of CERs for low-energy, water-tolerant DAC systems and highlights pathways for further material optimization.