In the urgent quest to combat the mounting crisis of climate change, the development of efficient technologies for carbon dioxide removal from the atmosphere has taken center stage. While numerous strategies exist, direct air capture (DAC) technologies hold significant promise by enabling the removal of CO₂ directly from ambient air, thereby addressing emissions from legacy sources and hard-to-abate sectors. However, traditional DAC systems have been hampered by high complexity and cost, often reliant on intricate sorbent and solid chemical loops paired with large-scale air handling infrastructures. A groundbreaking study now suggests a radical departure from these conventional methods by harnessing the power of ultraconcentrated potassium hydroxide (KOH) solutions to accelerate the direct capture of CO₂ through a fundamentally new mechanism—evaporative carbonate crystallization.
For decades, the prevailing liquid sorbent-based DAC systems have focused on enhancing the kinetics of CO₂ absorption and the sorbent regeneration processes. This generally involves multiple chemical steps and significant energy inputs to sustain cyclic sorbent use. The new research overturns this approach by demonstrating that KOH solutions at concentrations exceeding 9 molar exhibit a remarkable ability to precipitate carbonate crystals rapidly at the air-liquid interface. This discovery means CO₂ can be sequestered passively and continuously as a stable solid without the need for complex air circulation or large solvent volumes, marking a radical simplification of the capture process.
The scientific innovation pivots on establishing a crystalline carbonate phase directly at the air interface, which forms as atmospheric CO₂ diffuses through the ultraconcentrated KOH solution. This crystallization process effectively locks CO₂ into a solid matrix without requiring the cyclical transformations that previously constrained system scalability and cost efficiency. By leveraging the natural evaporation to increase KOH concentration on a wicking substrate, the researchers created a novel “carbonate crystallizer” that maintains a steady, efficient capture flux. This passive approach sidesteps the energy-intensive mechanical blowers and compressors characteristic of existing DAC technologies, representing a paradigm shift toward easier-to-deploy solutions.
A striking advantage of this approach lies in its dissolution and regeneration properties. Following CO₂ capture and carbonate crystallization, the captured solid can be regenerated through an electrochemical process, reinstating the KOH sorbent and enabling a single-chemical-loop capture cycle. This mechanism sidesteps traditional thermochemical regeneration’s high energy demands, reducing operational complexity and cost. Early demonstrations showed a module containing 100 such crystallizers performing with unprecedented reliability and scalability, sustaining operation seamlessly over multiple capture-regeneration cycles spanning 25 days.
Quantitative performance analysis reveals the passive evaporative crystallization method achieves a CO₂ capture flux exceeding conventional sorbent-based DAC systems by more than sixfold. When scaled up in modular arrays, the capture flux averaged over three times higher than traditional contactor setups, validating both laboratory-scale viability and promising potential for industrial application. These remarkable flux enhancements could directly translate into smaller, more cost-effective installations, lowering barriers to large-scale atmospheric carbon removal and deployment in diverse environments.
Beyond the improved kinetics and scalability, this method brings considerable economic benefits. The researchers estimate that adopting this technology could slash capital expenditure by approximately 42% and reduce levelized costs by nearly a third compared with current liquid-based DAC systems. Such cost savings may prove critical in fostering commercial viability, supporting carbon management policies, and accelerating global decarbonization efforts. The simplified passive system design offers a compelling route toward democratizing DAC technology, enabling adoption across regions lacking robust infrastructure.
The scientific foundation of direct air capture via evaporative carbonate crystallization challenges conventional wisdom about CO₂ sorption chemistry in dilute atmospheric conditions. The ultraconcentrated KOH solutions behave differently than dilute alkali absorbers, favoring carbonate solid precipitation over aqueous bicarbonate ion formation. This insight paves the way for revisiting fundamental capture chemistries and engineering new materials and devices tailored for ambient CO₂ removal with minimal energy penalties and maximal stability.
Mechanistically, the interfacial crystallization is driven by a delicate interplay between solution concentration gradients and water evaporation. Water loss near the surface escalates KOH supersaturation, inducing rapid nucleation and growth of carbonate crystals as supplied CO₂ reacts chemically. The wicking substrate sustains this evaporation and concentration cycle naturally, creating a self-regulating platform that maintains consistent capture activity over extended periods without external energy inputs for pumping or blowing air. This elegant coupling of evaporation with crystallization exemplifies nature-inspired engineering optimized for climate remediation technologies.
Integration of the carbonate crystallizer modules into arrays highlights the modularity and practical deployment potential of this technology. The size and configuration can be tailored for urban rooftop installations, remote industrial complexes, or distributed capture infrastructures. The passive, atmospheric-only interface minimizes spatial footprint and infrastructure complexity, opening avenues for decentralized atmospheric CO₂ mitigation efforts aligned with circular economy principles.
Looking forward, key technical challenges include optimizing electrochemical regeneration efficiency, crystal harvesting techniques, and long-term material durability under varying environmental conditions. Further research into catalyst-supported electrochemical cells and sorbent reinvigoration pathways could enhance overall system sustainability and energy efficiency, consolidating this approach as a pioneering player among the emerging generation of direct air capture technologies.
The environmental impact of this evaporative crystallization DAC is poised to be transformative. By passively converting atmospheric CO₂ into stable solid carbonate with minimal inputs, this method offers a scalable, low-energy sink for persistent atmospheric carbon. It aligns with ambitious net-zero frameworks and supports negative emission goals without competing significantly for land or water resources, addressing some of the key sustainability metrics that currently limit bioenergy with carbon capture and sequestration (BECCS) or afforestation-based approaches.
This discovery also signals new directions for material science and chemical engineering, where concentration gradients and interfacial phenomena are harnessed for selective gas capture and separation technologies. It bridges CO₂ chemistry with crystallization science, potentially inspiring innovations in pollutant removal, hydration cycles, or industrial gas scrubbing with improved energy profiles and material recyclability.
The socioeconomic benefits of affordable, passive DAC technologies could extend far beyond environmental remediation. Creating scalable pathways for atmospheric carbon removal may legitimize carbon-neutral or carbon-negative certifications, foster emerging green markets, and catalyze investment flows into clean energy transitions. Furthermore, modular passive design accommodates ease of maintenance and operational resilience, particularly valuable in lower-income regions where technical complexity and cost often pose insurmountable obstacles for climate technologies.
Taken together, the findings by Kim, Liu, Devasagayam and colleagues redefine the landscape of atmospheric CO₂ capture by demonstrating that simplicity—embodied in ultraconcentrated KOH and evaporative crystallization—can yield performance breakthroughs with profound implications. This minimalist chemical loop approach not only challenges the established paradigm of energy-intensive air capture but also charts a viable path forward to scalable, passive, and economically viable atmospheric carbon removal solutions urgently needed to address the climate emergency.
Subject of Research:
Direct air capture of atmospheric CO₂ using ultraconcentrated potassium hydroxide solutions and evaporative carbonate crystallization.
Article Title:
Passive direct air capture via evaporative carbonate crystallization.
Article References:
Kim, D., Liu, S., Devasagayam, T. et al. Passive direct air capture via evaporative carbonate crystallization. Nat Chem Eng (2025). https://doi.org/10.1038/s44286-025-00308-5
Image Credits:
AI Generated
DOI:
https://doi.org/10.1038/s44286-025-00308-5
Tags: climate change mitigation strategiesCO2 absorption kineticsdirect air capture advancementsefficient carbon dioxide removal methodsevaporative carbonate crystallizationhigh-concentration KOH applicationsinnovative carbon capture mechanismsliquid sorbent DAC systemspassive carbon capture technologypotassium hydroxide CO2 removalrenewable energy in carbon capturesustainable carbon sequestration solutions
