(548d) Stimuli-Responsive Molecules for Carbon Capture

The development of molecular systems that respond to external stimuli such as electrochemical potential or light presents promising strategies for carbon capture. These approaches offer alternatives to conventional thermal regeneration by enabling CO2 binding and release under mild conditions, thereby reducing energy consumption and improving compatibility with intermittent renewable energy sources. Their modularity, tunability, and integration potential make them attractive for next-generation carbon separation technologies. This abstract highlights our recent research on electrochemical and photochemical CO2 capture, with particular attention to redox-active systems and light-triggered desorption enabled by photoactive molecules.

One strategy involves redox-active organic molecules that reversibly bind CO2 via electrochemical cycling. In recent work by Seo and Hatton (2023), neutral red, a commercially available phenazine-based dye, was demonstrated as an efficient carrier for electrochemical direct air capture (DAC). Operating in aqueous media, the system captures CO2 from ambient air (~420 ppm) upon electrochemical reduction of neutral red to its base form, which is subsequently protonated, enabling CO2 uptake. Re-oxidation releases CO2 and regenerates the carrier. The system avoids the need for organic solvents or specialized electrolytes, and its narrow redox potential window allows the use of simple, stable electrodes. These features suggest strong potential for scalable, low-cost electrochemical CO2 separation using tunable organic redox species.

Beyond organic molecules, redox-active coordination complexes are also being explored for CO2 capture. A promising example is Fe(III)-EDDHA (ethylenediamine-N,N′-bis(2-hydroxy-5-sulfonylbenzyl)acetic acid), a water-soluble iron chelate widely used as an agricultural micronutrient. EDDHA provides a robust coordination environment that preserves iron’s redox activity and solubility in aqueous systems. Our studies show that electrochemical reduction of Fe(III)-EDDHA leads to CO2 capture with high electron utilization exceeding 1.43—surpassing the theoretical limit of 1 often associated with organic redox-active molecules. Oxidation reverses the binding, triggering CO2 release. This system is attractive due to its low cost, commercial availability, aqueous compatibility, and redox tunability. Further optimization through ligand design may enhance reactivity, selectivity, and catalytic performance.

In parallel, we have developed a photochemical strategy for CO2 release using pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid), a visible-light-responsive photoacid generator. Upon irradiation, pyranine undergoes excited-state proton transfer, increasing the local proton concentration and driving CO2 desorption from bicarbonate-rich aqueous media by shifting the carbonate equilibrium. This fully aqueous, non-toxic system operates under low-intensity visible light and requires no external heating. The photogenerated pH swing is reversible and tunable, enabling spatial and temporal control over CO2 release. This platform is particularly well suited for low-footprint, solar-integrated photoreactors. Current efforts focus on optimizing release kinetics, quantum efficiency, and coupling with upstream capture phases to realize closed-loop photochemical CO2 separation modules.

Together, these three systems—neutral red, Fe(III)-EDDHA, and pyranine—illustrate how reversible CO2 capture can be enabled through molecular architectures that respond predictably to external stimuli. Whether modulated by redox state or photochemical excitation, the central challenge is to achieve efficient, selective, and stable CO2 binding and release. Translating these concepts from laboratory demonstrations to real-world applications will require advances in transport engineering, kinetic optimization, and system integration. Ongoing work aims to expand the scope of active molecules, integrate capture and conversion processes, and validate long-term stability under operational conditions.

These developments mark a shift toward non-thermal, energy-efficient strategies for CO2 management. Stimuli-responsive molecules provide the adaptability needed to support distributed, compact carbon capture units—including point-of-use separation devices, ambient air collectors, and closed-loop systems for aerospace and defense applications. Their operation under ambient conditions simplifies infrastructure and facilitates integration with renewable energy sources.

In summary, the design of molecular systems responsive to electrochemical or optical stimuli offers a compelling foundation for next-generation CO2 capture. Their modularity, responsiveness, and energy efficiency position them as promising tools in the broader effort to address global carbon emissions through innovative materials chemistry and systems design.