Potassium carbonate-activated carbon materials for the direct air capture of carbon dioxide

Abstract

Fabricating a composite for direct air capture of carbon dioxide includes soaking an activated carbon material in an aqueous solution that includes potassium carbonate to yield an infused activated carbon material, removing the infused activated carbon material from the aqueous solution, drying the infused activated carbon material to yield a dried activated carbon material, and equilibrating the dried activated carbon material in air for a length of time, thereby increasing the porosity of the dried activated carbon material to yield the composite for direct air capture of carbon dioxide.

Description

POTASSIUM CARBONATE-ACTIVATED CARBON MATERIALS FOR THE DIRECT AIRCAPTURE OF CARBON DIOXIDECROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Patent Application No. 63 / 550,426 filed on February 6, 2024, which is incorporated herein by reference in its entirety.STATEMENT OF GOVERNMENT SUPPORT

[0001] This invention was made with government support under 80NSSC18K1508 awarded by National Aeronautical & Space Administration. The government has certain rights in the invention.TECHNICAL FIELD

[0002] This invention relates to materials and methods for capture of carbon dioxide from a gas mixture.BACKGROUND

[0003] Direct air capture is a technology that uses chemical or physical processes to selectively extract carbon dioxide (CO2) from ambient air. The capture of CO2 is typically achieved using liquid aqueous alkaline solutions such as potassium or sodium salts of carbonates or hydroxides or using alkanolamine solvents like monoethanolamine or diethanolamine. Slow adsorption kinetics, a high regeneration energy requirement, and evaporative losses due at least in part to sorbent volatility are some of the common concerns faced. Furthermore, alkanolamines suffer from chemical degradation, a low CO2 loading, and environmental safety concerns. Systems using liquid sorbents also suffer from high pressure drops leading to higher overall energy requirements, limitations in air contractor design, and in regeneration methodology.

[0004] Solid sorbents tend to have lower regeneration energy requirements, are generally easier and safer to handle, are less prone to evaporative losses, and have better CO2 adsorption capacity / selectivity, thus overcoming some of the limitations of liquid sorbents. Zeolites and amine-based solid sorbents, such as amine-supported polymers / resins and silica, have been explored as solid direct air capture sorbents. Amine-based sorbents in particular have shown highCCh capture capacity, CO2 selectivity, and lower regeneration energy requirement. However, these sorbents suffer from chemical or physical instability. Since zeolites utilize a physisorptive uptake mechanism, they have been found to perform poorly at low CO2 partial pressures and / or high relative humidity while also suffering from poor CO2 selectivity over N2. Other state of the art materials such as metal-organic frameworks, covalent-organic frameworks, and functionalized two-dimensional materials such as graphene and boron nitride have also gained significant interest. However, a high cost of synthesis, poor scalability, and lack of understanding of their environmental impact prevent large scale adoption of these materials for direct air capture.SUMMARY

[0005] This disclosure describes fabrication and use of high surface activated carbon K2CO3 composites. Suitable activated carbon materials include powders, granules, nonwoven sheets (e.g., felts), beads, extruded activated carbon, polymer-coated activated carbon, woven fabrics and sheets, and also backers or substrates that have been coated with activated carbon. The high surface area of the activated carbon materials allows for efficient contact of K2CO3 with air, considerably improving sorption performance, compared to bulk solid K2CO3.

[0006] In a first general aspect, making a composite for direct air capture of carbon dioxide includes soaking an activated carbon material in an aqueous solution that includes potassium carbonate to yield an infused activated carbon material, removing the infused activated carbon material from the aqueous solution, drying the infused activated carbon material to yield a dried activated carbon material, and equilibrating the dried activated carbon material in air for a length of time, thereby increasing the porosity of the dried activated carbon material to yield the composite for direct air capture of carbon dioxide.

[0007] Implementations of the first general aspect can include one or more of the following features. In some cases, the composite includes 25 wt% to 95 wt% potassium carbonate. In certain implementations, the first general aspect further includes concentrating the aqueous solution under vacuum while the activated carbon material is soaking. In some embodiments, drying the infused activated carbon material includes contacting the infused activated carbon material with an absorbent material. In some cases, contacting the infused activated carbon material with the absorbent material includes transferring some of the aqueous solution from theinfused activated carbon material to the absorbent material. Transferring some of the aqueous solution from the infused activated carbon material to the absorbent material can inhibit formation of dried potassium carbonate on a surface of the activated carbon material.

[0008] Drying the infused activated carbon material can include heating the infused activated carbon material for a length of time between 5 minutes and 10 hours. In some cases, drying the infused activated carbon material includes heating the infused activated carbon material at a temperature in a range between 60 °C and 200 °C.

[0009] Equilibrating the dried activated carbon material in air for a length of time can include allowing the dried activated carbon material to sit under ambient conditions for a length of time between 2 hours and 36 hours. The first general aspect can further include, after equilibrating the dried activated carbon material in air for a length of time, removing residual liquid present in the dried activated carbon material. In some cases, removing the residual liquid includes heating the dried activated carbon material at a temperature between 60 °C and 200 °C for a length of time between 5 minutes and 10 hours. The first general aspect can further include shaping the infused activated carbon material (e.g., winding the infused activated carbon material with a spacer to yield a spiral wound configuration).

[0010] The activated carbon material can include activated carbon powders, granules, nonwoven sheets, beads, extruded activated carbon, polymer-coated activated carbon, woven fabrics and sheets, and activated-carbon-coated substrates. In one example, the nonwoven sheets include activated carbon felt. In some cases, regenerating the composite includes heating the composite at a temperature between 80 °C and 250 °C.

[0011] A second general aspect includes a composite formed by the first general aspect. In some cases, the composite is in a spiral wound configuration.

[0012] A third general aspect includes a direct air capture system that includes the composite of the second general aspect. The composite can be in a spiral wound configuration. The direct air capture system can further include a column into which the composite is packed.

[0013] The resulting composites are therm omechanically stable. These high surface area, porous, and robust materials have a low regeneration energy requirement and good cycle performance. The activated carbon-K2CO3 composites can be characterized using thermogravimetric analysis, Fourier transform infrared spectroscopy, Brunauer-Emmet-Teller analysis, and scanning electron microscopy. For activated carbon K2CO3 composite fiber felts,sorption testing reveals that a temperature of 125 °C for 30 minutes can be used to regenerate the composites. The composite felts also perform better under conditions of low humidity and are able to achieve good direct air capture sorption for 10 cycles with an average sorption capacity of 478 pmol / g in a span of 4 hours.

[0014] The details of one or more embodiments of the subject matter of this disclosure are set forth in the accompanying drawings and the description. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.BRIEF DESCRIPTION OF DRAWINGS

[0015] FIG. 1 shows the preparation of composite felts containing activated carbon fiber and K2CO3.

[0016] FIG. 2 shows the open circuit test system used in Example 2.

[0017] FIG. 3 shows scanning electron microscopy images of activated carbon felts before and after K2CO3 loading.

[0018] FIG. 4A shows fabricated activated carbon-IGCCh felt. FIG. 4B is an image showing a cross sectional view of the spiral wound activated carbon K2CO3 sorbent module.

[0019] FIG. 5 A shows Fourier transform infrared spectra of activated carbon felts. FIG. 5B shows thermogravimetric analysis data of activated carbon-I COi composite felts.

[0020] FIG. 6 shows Brunauer-Emmet-Teller N2 adsorption plot showing isotherms of the bare activated carbon felt, activated carbon-IGCOi prepared via the protocol described in Example 1, and activated carbon-K2CO3 prepared while omitting the air equilibration step.

[0021] FIG. 7A shows effect of regeneration temperature (shown as dew point temperature) on direct air capture performance. FIG. 7B shows effect of humidity (shown as dew point temperature) on direct air capture performance.

[0022] FIG. 8A shows 2h sorption capacity of the activated carbon-IGCCh composite felts over multiple sorption and regeneration cycles. FIG. 8B shows 4h sorption capacity of the activated carbon-K2CO3 composite felts over multiple sorption and regeneration cycles. Each 2- and 4-hour cycle was performed after 30 minutes at 125 °C in air.

[0023] FIG. 9A shows Avrami kinetic model adsorption fits of activated carbon-KzCCh samples under various conditions. FIG. 9B shows pseudo-first order kinetic model adsorption fits of activated carbon-FGCOs samples under various conditions. FIG. 9C shows pseudo-second order kinetic model adsorption fits of activated carbon-KiCCh samples under various conditions. All data points were measured at a dew point of 5 °C unless specified otherwise.

[0024] FIG. 10A shows total CO2 absorbed over time by 14 g of activated carbon-KzCCh felt when exposed to a gas mixture of 400 ppm CO2 and balance N2. FIG. 10B shows equilibrium sorption capacity of activated carbon-BGCCh felt when exposed to 4000 ppm CO2 / N2 mixture.DETAILED DESCRIPTION

[0025] This disclosure describes the incorporation of K2CO3 into activated carbon materials (e g., used as supports) as well as the resulting composites for direct air capture of carbon dioxide. Suitable activated carbon materials include powders, granules, nonwoven sheets (e.g., felts), beads, extruded activated carbon, polymer-coated activated carbon, woven fabrics and sheets, and also backers or substrates that have been coated with activated carbon. The morphology of the activated carbon combined with the high surface area and porosity of activated carbon allows for efficient contact of air with the active K2CO3 sorbent. K2CO3 is hygroscopic and can form potassium carbonate sesquihydrate (K2CO3 1.5 H2O) in the presence of moisture. CO2 can react with either the anhydrous or hydrated carbonate form to produce KHCO3. The reaction be reversed to regenerate the carbonate by the addition of heat to the system. The reactions with CO2 are as follows:K2CO3+ 1.5 H2O (g) K2CO31.5 H2O AH27°C = - 101.0 kJ mol’1(1)K2CO3+ C AH27°C - -141.7 kJ mol’1(2)K2C031.5AH27°C = -40.7 kJ mol-1 (3)

[0026] Upon exposure to temperatures of 100-200 °C, KHCO3 decomposes to regenerate K2CO3, releasing CO2 and H2O. The theoretical CO2 sorption capacity of K2CO3 assuming 1 :1 mokmol sorption of CCUKzCCh is 7,221 pmol / g. However, the actual observed capacity is lower, suggesting other reaction pathways for CO2 sorption or the need to optimize the presentation of the sorbent to the C Ch-laden air. Depending on reaction conditions, other possible reaction pathways for capture are:2 K2CO3+ 2.5 (4)K4H2(CO3)3- 1.(5)

[0027] The composites containing activated carbon and K2CO3 are thermally stable, environmentally benign, and have good passive direct air capture sorption performance over multiple cycles. Some composite felts collected an average of 478 pmol of CO2 per gram of composite during 4 hours of exposure to ambient air with 24% relative humidity. The performance of the composites felts confirms that the composite felts can be used in stand-alone direct air capture systems as well as integrated to HVAC systems in buildings to facilitate negative emissions technologies.

[0028] FIG. 1 depicts a method 100 of making a composite for direct air capture of carbon dioxide. The method 100 includes soaking an activated carbon material 102 in an aqueous solution 104. The aqueous solution 104 includes potassium carbonate. Soaking the activated carbon material 102 in the aqueous solution 104 yields an infused activated carbon material 106. The infused activated carbon material 106 is removed from the aqueous solution 104. The infused activated carbon material 106 is dried to yield a dried activated carbon material 108. The dried activated carbon material 108 is equilibrated in air for a length of time. Equilibrating the dried activated carbon material 108 increases porosity of the dried activated carbon material 108 and yields the composite for direct air capture of carbon dioxide. The composite for direct air capture of carbon dioxide includes 25 wt% to 95 wt% of the potassium carbonate. The composite can be in a spiral wound configuration.

[0029] The activated carbon material 102 can include activated carbon powders, granules, nonwoven sheets, beads, extruded activated carbon, polymer-coated activated carbon, woven fabrics and sheets, activated-carbon-coated substrates, or a combination thereof. The nonwoven sheets can include activated carbon felt. Soaking the activated carbon material 102 in aqueous solution 104 can further include concentrating the aqueous solution 104 under vacuum while the activated carbon material 102 is soaking.

[0030] Drying the infused activated carbon material 106 includes contacting the infused activated carbon material 106 with an absorbent material. Contacting the infused activated carbon material 106 with the absorbent material includes transferring some of the aqueous solution 104 from the infused activated carbon material 106 to the absorbent material. Transferring some of the aqueous solution 104 from the infused activated carbon material 106 tothe absorbent material inhibits formation of dried potassium carbonate on a surface of the activate carbon material 102. Drying the infused activated carbon material 106 includes heating the infused activated carbon material 106 for a length of time between 5 minutes and 10 hours (e.g., between 1 hour and 10 hours) and at a temperature in a range between 60 °C and 200 °C (e.g., between 90 °C and 140 °C).

[0031] Equilibrating the dried activated carbon material 108 in air for a length of time includes allowing the dried activated carbon material 108 to sit under ambient conditions for a length of time between 2 hours and 36 hours (e.g., between 12 hours and 36 hours). The method 100 can further include, after equilibrating the dried activated carbon material 108 in air for a length of time, removing residual liquid present in the dried activated carbon material 108. Removing the residual liquid includes heating the dried activated carbon material 108 at a temperature between 60 °C and 200 °C (e.g., between 125 °C and 175 °C) for a length of time between 5 minutes and 10 hours (e.g., between 1 hour and 10 hours).

[0032] The method 100 can further include shaping the infused activated carbon material 106. The method 100 can further include winding the infused activated carbon material 106 with a spacer to yield a spiral wound configuration. Regenerating the composite includes heating the composite at a temperature between 80 °C and 250 °C (e.g., between 100 °C and 150 °C).

[0033] FIG. 2 depicts a direct air capture system 200. System 200 includes the composite for direct air capture of carbon dioxide. The composite in the system 200 can be in a spiral wound configuration. The system 200 further includes a column into which the composite is packed. A compressor 202 is used to fill air into a tank 204. Air flows from tank 204 through a mass flow controller 206, from mass flow controller 206 into a dew point generator 208, from dew point generator 208 into a sample chamber 210, and from sample chamber 210 into an infrared gas analyzer 212. The air was then vented to the ambient air.EXAMPLESMaterials and Methods

[0034] Activated carbon felts (CF1600) were purchased from CeraMaterials, USA and used as received. K2CO3 was purchased from Sigma Aldrich, USA and used as received. Deionized water with a conductivity of 0.5 pS / cm was used to make saturated K2CO3 solutions. Scanning electron microscopy (SEM) images were obtained at a gun voltage of 5 kV using a Zeiss AurigaSEM to characterize the composite morphology. Image Analysis was done using the measure feature in Image J software. Thermogravimetric analysis (TGA) of the composite samples was performed using a TA Instruments TGA 5500 at a heating rate of 10 °C / min within a temperature range of 50-900 °C. The samples were equilibrated in air for 24 hours prior to testing. Fourier transform infrared spectra (400-4000 cm ' ) were obtained with a Nicolet™ iS™ spectrometer at 4 cm1resolution and averaged over 64 scans. The surface area characterization was determined by N2 adsorption at 77 K using a Micromeritics TriStar II 3020 Brunauer- Emmet-Teller instrument. The samples were degassed at 100 °C under N2 for 24 hours prior to testing.Example 1. Preparation of composite felts containing activated carbon and K2CO3.

[0035] A 7 M solution of K2CO3 in deionized water was made by dissolving 483.71 g of K2CO3 in 500 mb deionized water. The solution was stirred overnight to ensure complete dissolution. The K2CO3 was kept in an oven set to 120 °C prior to dissolution to ensure dryness. Then, 400 m of the solution was transferred to a 500 mb beaker. CeraMaterial activated carbon felt samples were then cut into roughly two squares weighing approximately 0.5 g each and the felts were immersed in the K2CO3 solution and kept submerged using a weighted wire gauze.

[0036] The beaker was then moved to a vacuum chamber to concentrate the solution under vacuum (gauge pressure of -90 kPa) for 1 hour at room temperature. Bubbling of the K2CO3 solution and the felt was observed during evacuation. The vacuum also releases gases present in the pores of the activated carbon felt and aids in solution penetration into the activated carbon fiber. The felt was taken out of the remaining solution and patted dry using a clean paper towel. The felt was later wrapped in a paper towel and left overnight to absorb any excess K2CO3 solution. This drying process prevents the formation of a thick K2CO3 crust on the surface of the felt, which drastically decreases CO2 sorption performance. The felt was then dried in an oven kept at 110 °C for 4 hours and later allowed to equilibrate in ambient air for 24 hours. The air equilibration process improves the porosity of the composite material. Finally, the felt was heated in an oven at 150 °C for 4 hours for simultaneous regeneration and removal of any excess residual liquid present in the sample. An overview of the preparation process is shown in FIG. 1. Example 2. Sorption testing.

[0037] The open-circuit system was designed to assess sorbents under conditions relevant for direct air capture, as shown in FIG. 2. To minimize fluctuations of CO2 air in an indoorenvironment, ambient lab air was filled into a tank using a compressor. This ensured that the inlet air concentration remained around 400 450 ppm depending on ambient laboratory conditions. During the experiment, air flowed from the tank through a mass flow controller, a Li- Cor 610 dew point generator, the sample chamber, and a Li-Cor 850 Infra-Red Gas Analyzer (IRGA) before it was vented to the ambient air. The gas flow rate was controlled at a fixed rate of 1 L / min. This air flow rate was chosen to mimic slow ambient air wind speeds and the sample chamber was not packed to minimize pressure drop across the sample chamber to simulate passive direct air capture conditions. CCh and water vapor content of the inflow stream were constant throughout the experiment, whereas CO2 and water vapor concentration downstream of the sample chamber were measured continuously. The inlet CO2 concentration (Gw) is measured before the sample is placed in the sample chamber and is averaged over a time period of at least 60 seconds. The IRGA records the outlet CO2 concentration (Cout (t)) of air coming from the sample chamber every second.

[0038] The rate of CO2 sorption (Cads(t)) by the sample is calculated from the difference between the inlet and outlet CO2 concentration (equation 6). The combination of the compressor tank along with the use of a single IRGA to measure the baseline CO2 concentration and the outlet concentration after absorption minimizes errors from calibration and measurement errors from the use of multiple IRGA’s. The cumulative CO2 adsorbed Q(t)) is the total CO2 adsorbed over a time t (equation 7).The IRGA measures CO2 concentrations in ppm. The cumulative CO2 adsorbed (equation 8) by the sample in pmol / g is obtained as follows:where F is the flow rate (mL / s) of air in the open-circuit device, VM is the molar volume (mL / mol) of air (assumed to be at normal temperature and pressure), and m is the mass (g) of the sample.

[0039] The average sorption rate is calculated over a time period t, Rt (pmol / g / s) by equation 9 as:The experimental data obtained from the open circuit was fitted using three kinetic models. The pseudo-first order model (equation 10) and pseudo-second order model (equation 11) have been widely used to model the sorption of gases into solid sorbents. The pseudo-first order kinetic model describes the surface diffusion-controlled adsorption process. On the other hand, the pseudo-second order kinetic model describes chemisorption at the gas-solid interface. The Avrami model (equation 12) was developed to simulate phase transition and crystal growth. This fractional order kinetic model has been recently used to describe sorption phenomena that involve more than one mechanism. The Avrami model can fit both physical and chemical sorption processes. The equations for the models are shown below.Pseudo-First Order:Pseudo-Second Order:Avrami Model:where Qt is the amount of CO2 adsorbed at a given time t, Qeis the CO2 sorption capacity at equilibrium, kf, ks, and kaare the rate constants for pseudo-first order, pseudo-second order and Avrami models respectively. The parameter n is the order of the reaction as described by the Avrami model. The fit quality of the models in predicting experimental data was assessed by plugging in the observed equilibrium capacity and using the coefficient of determination R2to evaluate fitting correlation. The fitting parameters were obtained via nonlinear regression using ORIGIN software.

[0040] The amount of K2CO3 loaded into the composite felts was measured by recording the mass of the activated carbon felt before and after loading. The activated carbon felt used was cut to a size of approximately 4 cm x 4 cm (length x width) and the thickness of the pre-loaded felt was 0.2 cm. The felt was then soaked in saturated K2CO3 solution under vacuum and dried using the protocol described in the experimental section. The vacuum and heat treatment protocol allows for good distribution of the K2CO3 into the felt, without the formation of a thick K2CO3 crust on the felt surface. The percentage loading remains fairly consistent with an average loading of 344 wt% with respect to the pre-loaded activated carbon felt.

[0041] The scanning electron microscopy images show that the activated carbon fibers have a diameter of around 15-20 pm, and K2CO3 crystals are clearly visible in the scanning electron microscopy images after loading, lining the fiber surface, as shown in FIG. 3. The micro-sizedcrystals tend to form aggregates on the fiber surface with a majority of the crystals sharing boundaries. The activated carbon fiber structure and network itself remains intact after K2CO3 loading and heat treatment. The K2CO3 coverage appears to be non-homogenous but covers the entire bulk activated carbon fiber network. The fiber network and distribution of K2CO3 enable high surface area of contact with air, which is one factor that can lead to good direct air capture performance.

[0042] The activated carbon fiber material remains intact and flexible after K2CO3 loading and heat treatment, as shown in FIG. 4A. Prior to drying, the felt can shaped to the required form factor after the soaking process in K2CO.3 solution. Even without a pre-shaping step, the fiber felt is flexible enough to be rolled, as shown in FIG. 4B. Accordingly, the felt can be formed into spiral wound CO2 sorbents (e.g., with one or more spacers) and packed into a column to increase sorption performance.

[0043] FIG. 5 A shows Fourier transform infrared spectra of two activated carbon-KsCCh composed felts that were partially and fully equilibrated with CO2 from air. Both samples were heated in an oven at 125 °C for 1 hour in air prior to equilibration. Partial equilibration was achieved by exposing the sample to air for 2 h, whereas full equilibration was achieved by exposing the sample to air for 24 hours. To assess the effect of thermal desorption, pieces of the partially and fully equilibrated samples were kept aside and examined using Fourier transform infrared spectroscopy after heating the samples for 15 minutes at 125 °C in air (labelled after heat treatment (AHT) in FIG. 5A). After equilibration or desorption, the samples were stored in a vial filled with nitrogen. The samples were tested in the Fourier transform infrared spectroscopy instrument within 15 minutes of equilibration to ensure minimal degassing of CO2 into the headspace of the vial. After heating, the infrared spectra for both the partially and fully equilibrated samples exhibit a complete reduction in the water peak (3157 cm'1), which can be attributed to water loss from the sample in the oven. The peak at 1628 cm'1increases in intensity from partially equilibrated to fully equilibrated for the samples examined before heat treatment (BHT). This peak can be attributed to KHCO3 formation from CO2 sorption. The peak starts disappearing after thermal regeneration implying that the KHCO3 decomposes back to K2CO3. When fully equilibrated, the strongest peak at 1391 cm'1, that corresponds to asymmetric C-0 vibration in the anhydrous carbonate, starts developing a shoulder with new peaks appearing at 1356 cm'1and 1447 cm'1due at least in part to hydration of the carbonate by water molecules inair. Furthermore, only the air equilibrated samples that have hydrated carbonates showed a peak at around 1058 cm'1. The bare activated carbon felt shows no peaks indicating it has minimal surface functionalization.

[0044] The therm ogravimetric analysis plot of the activated carbon-K2COs composite felt is shown in FIG. 5B. The thermogravimetric analysis traces of the pristine activated carbon felt and K2CO3 are also shown for reference. Both the activated carbon felt and K2CO3 show high thermal stability until 850 °C, where the activated carbon felt slowly starts to show some degradation. The mass loss for activated carbon felt and K2CO3 over the temperature range (50- 900 °C) of the thermogravimetric analysis test is 6.3% and 0.9%, respectively. The overall mass loss for the activated carbon-IGCOi composite from a temperature of 50-900 °C is 64.5%. The initial mass loss of 3.2% (50-100 °C) in the composite can be attributed to water. The mass loss of 16% from 100-170 °C is due at least in part to a combination of the release of CO2 from the conversion of K4H2(CO.3)3 1.5 H2O, K2CO3 1.5 H2O and KHCO3 to K2CO3 as well as additional water loss. Evolved gas analysis using Fourier transform infrared spectroscopy connected to the thermogravimetric analysis exhaust shows that H2O is released from the sample until around 170 °C, which coincides with the mass loss observed in the thermogravimetric analysis. XRD analysis data confirms the presence of hydrated K2CO3 and KHCO3 in the composite, confirming that the hydrated forms of K2CO3 are present in the system. During the standard fabrication protocol, the activated carbon-K2CO3 composite felt is allowed to “rest” and equilibrate in air for 24 hours after heat treatment at 110 °C, prior to sorption testing. A “non-rested / no rest” sample was also prepared, to compare the effect of air equilibration soon after composite fabrication. The XRD data show that there is no significant difference in crystallinity between the “rested” and “non-rested” samples, suggesting that the improvement in surface area of the rested sample is related to an increase in porosity of the composite.

[0045] The Brunauer-Emmet-Teller surface area plot shows that the surface area of the activated carbon felt decreases significantly upon K2CO3 loading, as shown in FIG. 6. The Brunauer-Emmet-Teller data obtained, which is summarized in Table 1, shows that the resting period after fabrication can be important to ensure good composite sorption performance. The drop in surface area can lead to the poor performance of the “no rest” sample, and without being bound by theory, it is hypothesized that equilibration in air opens up pores of the composites(pore size from Brunauer-Emmet-Teller increases). This can be also reflected in a qualitatively better mechanical flexibility or ductility of equilibrated samples.Table 1. Brunauer-Emmet-Teller data of activated carbon-K CO3 composites.

[0046] The direct air capture performance of the activated carbon-K2COa composite mats was tested in the open circuit device, as shown in FIG. 2. The effect of various parameters such as regeneration temperature and humidity on sorbent performance is also assessed. Before sorption testing, the activated carbon-K2COa composite mats were regenerated in an air furnace for 30 minutes. The dependance of sorption performance on the regeneration temperature was first explored by varying the regeneration temperature from 100-150 °C. Higher temperatures drive up the energy requirement of regeneration, whereas lower temperatures result in a lower active site utilization and possibly a longer regeneration time. Referring to FIG. 7A, it can be concluded that an increase in regeneration temperature increases sorption performance. The increase in sorption capacity when the regeneration temperature increases from 100 °C to 125 °C is 135%; however, the increase is only 35% when the regeneration temperature increases from 125 °C to 150 °C. Thus, 125 °C is an acceptable regeneration temperature, as a qualitative optimization of energy requirement and sorption performance. All the tests to assess the effect of temperature were performed at a dew point of 5 °C (or 27.5% relative humidity (RH)), as shown in FIG. 7A.

[0047] The effect of humidity on sorption performance of the activated carbon-l CCh composite mats was assessed by setting the inlet air dew point temperature to 0 °C (24% RH), 5 °C (35% RH), or 12 °C (44% RH). Above a dew point of 12 °C, the composite samples begin to lose their integrity due at least in part to deliquescence of K2CO3. From FIG. 7B it can be concluded that the activated carbon-lGCCb composite felts perform better under lower humidityconditions. The 4 hour sorption capacity of the felts significantly decreases from 470 pmol / g to 282 pmol / g when the dew point of the inlet air is increased from 0 °C to 12 °C. Though the carbonation reaction of K2CO3 into KHCO3 requires H2O to proceed, the H2O requirement for CO2 sorption is lower than the actual H2O available in the inlet air. In other words, since the concentration of CO2 is much lower than the concentration of H2O in air, the reaction proceeds even under low humidity conditions. An increase in relative humidity likely hinders CO2 transport into the composite reducing sorption performance. Furthermore, water from high humidity could hypothetically favor the formation of sesquihydrate, without the sesquihydrate further reacting with CO2.

[0048] To assess the effect of multiple cycles on sorption performance, the activated carbon- K2CO3 composite felt was subjected to 10 consecutive sorption-regeneration cycles, as shown in FIG. 8. Before the first cycle, the composite felt was regenerated for 30 minutes at 150 °C in an air furnace. However, for subsequent runs the regeneration temperature was kept at 125 °C. This was done to test whether an initial higher regeneration temperature allows for higher sorption performance in subsequent runs at lower regeneration temperature. Higher regeneration temperatures allow for higher sorption capacities because a larger quantity of the sorption products (KHCO3 and K2CO3 1.5 H2O) is converted back to K2CO3. However, since the sorption cycle time is fixed at 4 hours, the composite sample is not fully equilibrated before the desorption cycle. Thus, after the first cycle, this excess capacity in the form of some unused K2CO3 transfers to the second cycle, increasing the second cycle sorption capacity. However, we see that the sorption capacity reaches a lower equilibrium value after the first cycle. After three cycles, the system reaches equilibrium, losing the initial boost in sorption capacity attributed to increased regeneration temperature. The sorption capacity after three cycles is similar to the earlier sorption capacity observed for the 125 °C regeneration case discussed above. As shown in FIGS. 8A and 8B, the average sorption capacity for the 2 hour and 4 hour cycles remain stable at 332 ± 5 pmol / g and 478 ± 13 pmol / g, respectively, from cycles 3-10. These consecutive cycles were performed on the same sample after the completion of the previous assessments described above, bringing the total number of cycles performed on the same sample to 20. Thus, it was successfully demonstrated the continuous stable cycling performance of the activated carbon- K2CO3 composite felts over 10 cycles after heat regeneration at 125 °C for 30 minutes.

[0049] The simulation of the kinetics of sorption for the activated carbon-KiCCh composite felts was done using the pseudo-first order, pseudo-second order, and Avrami kinetic models. Since most of the cycling performance was assessed after subjecting the sample to regeneration at 125 °C and then sorption at a dew point of 0 °C, this sorption data was used to test the different models. The fitting was also performed on samples that had been subjected to regeneration temperatures of 150 °C, 130 °C, and 110 °C followed by exposure to inlet air with a dew point of 5 °C and -400 ppm CO2 to assess model efficacy and robustness. The kinetic model fits are plotted in FIG. 9A-FIG. 9C and the fitting parameters obtained are listed in Table 3. The equilibrium capacity for the samples was assumed to be achieved at 25,000 seconds and this value was used as an input to assess model fitting performance. From the correlation coefficients R2, it can be concluded that the Avrami model fits the sorption performance data of the composite samples most accurately. The correlation coefficients R2for pseudo-first order and pseudo-second order varies from 0.9769-0.9999 and 0.9589-0.9878, respectively. The corresponding correlation coefficients R2for the Avrami fit remains between 0.9984-0.9999. The pseudo-second order model provided the worst fit. Higher regeneration temperatures also seemed to drive the sorption kinetics towards a pseudo-first order fit. The reaction order n for the Avrami fit remains begins to approach 1 with increasing temperature. A value of 1 indicates that the probability of sorption in the composite felt sample remains homogenous at every location of the sample. Since higher temperatures drive out more CO2, the availability of sorption sites in the sample increases, thereby driving the reaction order closer to 1. The high surface area of the felts from the microfiber morphology and porosity of the composite also enables uniform CO2 sorption. The model also shows that the sorption mechanism more closely follows a pseudo-first order process.Table 3. Parameters of sorption kinetic simulation for activated carbon-l CCh composites subjected to various regeneration conditions.

[0050] As further demonstration of the direct air capture sorption capacity of the activated carbon-K2CO3 composite fiber disclosed herein, the capability of the felt to be used to remove CO2 from space cabin air environments was examined. FIG. 10A shows total CO2 absorbed over time by 14 g of activated carbon-K2CO3 felt when exposed to a gas mixture of 400 ppm CO2 and balance N2. When exposed to an inlet mixture containing 4000 ppm CO2 in N2, the felt was able to uptake 3 mmol / g of CO2 from the inlet gas stream, as shown in FIG. 10B. The performance remained consistent over multiple cycles after regeneration of the felt in an air oven at 150 °C. Thus, the felts show potential as a low cost, light, nonhazardous, efficient material for CO2 regulation in space.

[0051] The activated carbon-K^CCh composite fiber felts show excellent direct air capture sorption capacity and kinetics at fairly short sorption times and low temperature regeneration conditions. It was demonstrated that the composites work better under lower humidity conditions (0 °C dew point), suggesting that such composite felts could possibly be used as an effective sorbent in dry climates or after air dehumidification. The stable, cyclic direct air capture performance of the composite felts combined with the relatively low regeneration temperature demonstrate that the material has the capability to be used as a reliable sorbent with good sorption capacity and kinetics. With further optimization of the sorption-desorption cycles, the energy requirement of the system could also be potentially reduced. Hence, herein isdemonstrated the potential of high surface area activated carbon fiber felts to be used as supports for K2CO3 in direct air capture applications.

[0052] Although this disclosure contains many specific embodiment details, these should not be construed as limitations on the scope of the subject matter or on the scope of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in this disclosure in the context of separate embodiments can also be implemented, in combination, in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments, separately, or in any suitable sub-combination. Moreover, although previously described features may be described as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can, in some cases, be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination.

[0053] Particular embodiments of the subject matter have been described. Other embodiments, alterations, and permutations of the described embodiments are within the scope of the following claims as will be apparent to those skilled in the art. While operations are depicted in the drawings or claims in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed (some operations may be considered optional), to achieve desirable results.

[0054] Accordingly, the previously described example embodiments do not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure.

Claims

WHAT IS CLAIMED IS:

1. A method of making a composite for direct air capture of carbon dioxide, the method comprising: soaking an activated carbon material in an aqueous solution comprising potassium carbonate to yield an infused activated carbon material; removing the infused activated carbon material from the aqueous solution; drying the infused activated carbon material to yield a dried activated carbon material; and equilibrating the dried activated carbon material in air for a length of time, thereby increasing a porosity of the dried activated carbon material to yield the composite for direct air capture of carbon dioxide.

2. The method of claim 1, further comprising concentrating the aqueous solution under vacuum while the activated carbon material is soaking.

3. The method of claim 1, wherein drying the infused activated carbon material comprises contacting the infused activated carbon material with an absorbent material.

4. The method of claim 3, wherein contacting the infused activated carbon material with the absorbent material comprises transferring some of the aqueous solution from the infused activated carbon material to the absorbent material.

5. The method of claim 4, wherein transferring some of the aqueous solution from the infused activated carbon material to the absorbent material inhibits formation of dried potassium carbonate on a surface of the activated carbon material.

6. The method of claim 1, wherein drying the infused activated carbon material comprises heating the infused activated carbon material for a length of time between 5 minutes and 10 hours.

7. The method of claim 1, wherein drying the infused activated carbon material comprises heating the infused activated carbon material at a temperature in a range between 60 °C and 200 »r8. The method of claim 1, wherein equilibrating the dried activated carbon material in air for a length of time comprises allowing the dried activated carbon material to sit under ambient conditions for a length of time between 2 hours and 36 hours.

9. The method of claim 1, further comprising, after equilibrating the dried activated carbon material in air for a length of time, removing residual liquid present in the dried activated carbon material.

10. The method of claim 9, wherein removing the residual liquid comprises heating the dried activated carbon material at a temperature between 60 °C and 200 °C for a length of time between 5 minutes and 10 hours.

11. The method of claim 1, further comprising shaping the infused activated carbon material.

12. The method of claim 1, further comprising winding the infused activated carbon material with a spacer to yield a spiral wound configuration.

13. The method of claim 1, wherein the activated carbon material comprises activated carbon powders, granules, nonwoven sheets, beads, extruded activated carbon, polymer-coated activated carbon, woven fabrics and sheets, and activated-carbon-coated substrates.

14. The method of claim 13, wherein the nonwoven sheets comprise activated carbon felt.

15. A method of regenerating the composite of claim 1, the method comprising heating the composite at a temperature between 80 °C and 250 °C.

16. The method of claim 1, wherein the composite comprises 25 wt% to 95 wt% potassiumcarbonate.

17. The composite formed by the method of claim 1.

18. The composite of claim 17, wherein the composite is in a spiral wound configuration.

19. A direct air capture system comprising the composite of claim 1.

20. The direct air capture system of claim 19, wherein the composite is in a spiral wound configuration.

21. The direct air capture system of claim 19, further comprising a column into which the composite is packed.

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