(494b) Tailored Polymers for Lower-Energy Direct Air Capture

Post industrial revolution, the increase in greenhouse gas emissions has led to adverse weather conditions such as increased global surface temperatures, heavy rainfalls, droughts, and an increase in mean sea level [1,2]. The EPA has emphasized multiple times that carbon dioxide is the most prominent driver of climate change [3], and IPCC reports state that the concentration of CO₂ in the atmosphere has risen by more than 100 ppm over the last century [4]. To counter the inimical effects, nearly all countries have committed to reducing the global temperature rise below 2 °C by 2100 under the Paris Climate Agreement to confront the challenges of global warming [5,6]. These initiatives have led to the active development and deployment of several Negative Emissions Technologies (NETs) [7]. Direct air capture (DAC) emerges as a promising NET due to its ability to capture CO2 directly from the dilute atmosphere, unlike prevailing point source capture technologies targeting concentrated emissions [8].

The core idea of most DAC technologies is using a sorbent for CO₂ sorption. The type of sorption mechanism dictates the classification of sorbents, which in turn influences their effectiveness in various DAC applications. Solid sorbents are preferred because of their superior kinetics for sorption. They are less prone to losing volatiles into the atmosphere, and they avoid heat losses from evaporating liquid [9]. Sorbent development for sorption-based DAC necessitates balancing high sorption capacity with low regeneration energy requirements, as regeneration is a critical step in the DAC process. The commonly used regeneration method is thermal swing, utilizes heat for regeneration. Amines are commonly used sorbents in thermal swing, for which CO₂ adsorption is spontaneous in most cases and has considerably high sorption capacities. However, they typically require temperatures around 80−120 °C to desorb CO₂, with heats of sorption ranging from 60–80 kJ/mol [10]. To overcome the high energy costs involved with regeneration, Dr. Lackner proposed a novel technique called moisture swing, in which the sorption process is strictly controlled by moisture [11]. Many moisture-swing sorbents are strong basic ion-exchange resins with quaternary ammonium ions (NR₄⁺), with carbonate, bicarbonate, and hydroxides as counter-ions. These sorbents adsorb and desorb CO₂ in dry and wet states, respectively. Moisture-swing sorbents change their affinity to CO2 through interaction with H₂O, and the energy for the sorption process is also provided by this interaction. This way, moisture-swing technique exhibits lower energy demands compared to thermal swing [12].

Previous studies have reported sorption capacities for some moisture-swing sorbents ranging from 0.3 to 0.8 mmol CO₂/g at 20 °C. These capacities are significantly lower compared to those typically observed for thermal-swing sorbents [13-15]. These lower sorption capacities could pose a significant challenge for scaling up this energy-efficient technique. It is crucial to design novel sorbents that not only have considerable sorption capacities but also require relatively minimal energy for regeneration.

Motivated by the need for novel sorbents, this study investigates the development of polymers that exhibit thermomechanical stability, achieve CO₂ capture and release solely via water activity modulation, surpass the capacity limitations of conventional moisture-swing sorbents, and eliminate the need for excess energy inputs. This work explores a polymeric sorbent design incorporating both moisture-swing and thermal-swing functionalities within a single backbone, aiming to achieve high COâ‚‚ capture capacity and minimize regeneration energy requirements.

We hypothesize that adjacent moisture- and thermal-swing sites will have excess heats of water adsorption that can be utilized to drive desorption from amine-tethered CO₂. In moisture-swing sorbents, the electrostatic fields within charged sorbents result in high heats of water sorption, around 63−69 kJ/mol, exceeding the latent energy of condensation, approximately 42 kJ/mol. So, during the sorption process, the thermal sites bind to CO₂ spontaneously, and upon maintaining dry conditions, moisture-swing sites bind to the CO₂. When water vapor activity increases, the CO₂ desorbs from the moisture-swing sites, and the heat of the sorption of water aids in the desorption of thermal sites. Thermodynamically, the heat of water sorption and the energy required for the thermal site desorption are within the required range. The current research emphasizes designing different architectural polymers like random and block copolymers that will combine styrene (backbone), vinylbenzyl triethyl ammonium with bicarbonate counter-ion (moisture-swing site), and vinylbenzyl dimethyl amine (thermal site).

References

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