Plasma–ionic liquid-assisted CO2 capture and conversion: A novel technology

The present study focused on CO2 capture, storage, and conversion through the innovative integration of plasma–ionic liquid (IL) technology. For the first time, we employed plasma-IL technology to confront climate change challenges. We utilized 1-Butyl-3-methylimidazolium chloride IL to capture and store CO2 under atmospheric pressure, and subsequently employed plasma to induce the transformation of IL-captured CO2 into CO. Furthermore, we performed computer simulations to enhance our understanding of the CO2 and CO capture processes of water and IL solutions. This comprehensive approach provides valuable insights into the potential of plasma–IL technology as a viable solution for climate change.


S
ince the onset of the industrial revolution, there has been a steady increase in CO 2 emissions into the environment.][5] The absorption of CO 2 by ILs results from physical interactions between the anions and cations of the ILs and CO 2 molecules. 6)n alternative approach to reducing the CO 2 concentration in the atmosphere involves the conversion of CO 2 into valuable products.In recent years, non-thermal plasma (NTP) has experienced substantial growth to enhance the breakdown and conversion of CO 2 . 7)[10] These studies showed that ILs are stable in the presence of NTPs and do not denature the proteins.Nevertheless, no study to date has demonstrated the capture and conversion of CO 2 using a plasma-IL combination.In a previous study, Li and Gallucci 11) reported the capture of CO 2 using hydrotalcite pellets and later the desorption and CO 2 conversion using an Ar feed gas coaxial DBD reactor.Moreover, Masaaki et al. reported that plasma-based desorption was higher than thermal desorption of CO 2 . 12)ence, in this pioneering effort, we employed an IL for CO 2 capture [Fig.1(a)] and storage [Fig.1(b)] followed by the conversion of the captured CO 2 into CO using NTP [Fig.1(c)].This study used 1-Butyl-3-methylimidazolium chloride ([Bmim]Cl) IL, obtained from Sigma-Aldrich.We implemented streamer plasma (as described in our previous articles 13,14 ) to convert captured CO 2 to CO, as shown in Fig. 1.In the prior investigation, Jang et al. showed the solubility of CO 2 in [Bmim]Cl IL at a temperature range of 353.15-373.15K under high pressure conditions at approximately 40 MPa. 15)[Bmim]Cl IL also showed versatility in applications such as supercritical carbon dioxide preparation, 16) cellulose dissolution, 17) etc.Therefore, we utilized [Bmim]Cl IL to effectively capture and store CO 2 at room temperature and converted the stored CO 2 into CO using NTP.
In this study, we introduced CO 2 with a purity of 99.99% into a 45 ml solution of deionized (DI) water and DI water + [Bmim]Cl (4, 12, and 20 wt%) along with 5 ml of ionic strength adjuster for CO 2 .CO 2 was introduced into the reactor at a flow rate of 0.372 l min −1 through a gas tube with a diameter of 4.35 mm for 2 min.Subsequently, we measured the absorbed CO 2 concentration using the sensor (details provided in the supporting files).We used pure CO 2 in the present study to avoid the influence of other gases (diluting gas) on holding the CO 2 in the IL solution and the role of dilute gas in CO 2 conversion.After 10 min of CO 2 supply, the CO 2 concentration in the solution was 630 ± 50 mg l −1 , which later decreased to 9 ± 4 mg l −1 after 1440 min, as displayed in Fig. 2(a).Hence, the loss of CO 2 concentration after 1440 min was 621 mg l −1 .On the other hand, for [Bmim]Cl (4 wt%), the CO 2 concentration was 860 ± 55 mg l −1 after 10 min of CO 2 supply, although the final CO 2 concentration after 1440 min was 310 ± 20 mg l −1 [see Fig. 2(a)].Furthermore, with an increase in the [Bmim] Cl concentration to 10 wt%, the CO 2 concentration after 10 min was 830 ± 60 mg l −1 , and after 1440 min, it was 330 ± 11 mg l −1 [Fig.2(a)].This observation indicates that CO 2 absorption decreases at higher CO 2 concentrations but sustains a substantial level over time.
Later, we increased the [Bmim]Cl concentration to 20 wt%, and after 10 min of CO 2 supply, the measured CO 2 concentration was 730 ± 40 mg l −1 , which was lower than the concentrations observed with 4 wt% and 10 wt% [Bmim]Cl [see Fig. 2(a)].However, after 1440 min, the captured CO 2 concentration was 370 ± 15 mg l −1 , which was higher than the concentrations observed with other wt% values of [Bmim]Cl.Note that the study did not explore concentrations of [Bmim]Cl higher than 20 wt% due to the elevated solution viscosity, making it challenging to measure CO 2 accurately with the sensor.The results suggest that the CO 2 absorption decreases as the [Bmim]Cl concentration increases.This trend occurs due to the rising viscosity of the solution, leading to a reduction in CO 2 solubility, and this correlation is also supported by previous studies. 3,18)eanwhile, the respective losses in CO 2 concentration after 1440 min were 550, 500, and 360 mg l −1 for 4, 10, and 20 wt% [Bmim]Cl solutions.This study demonstrates that at higher concentrations of [Bmim]Cl, the IL in the solution can effectively retain the captured CO 2 for a longer time at atmospheric pressure and room temperature than lower concentrations of [Bmim]Cl solutions.
In the subsequent section, we measured the CO formation after 1440 min from the stored CO 2 in DI water and [Bmim] Cl solutions, which were exposed to streamer plasma for 1 min at atmospheric pressure without any feed gas (discharge occurs without gas flow, maintaining a relative humidity of approximately 45%.Following the plasma discharge, N 2 gas flowed to remove the CO or other gases produced during the discharge from the reactor).Five milliliters of the solution was transferred to the plasma reactor and then treated with plasma for 1 min (see Fig. 1).A 5 ml solution was used due to current reactor design limitations; however, we plan to treat a larger volume of the solution in future studies.The plasma discharge power was 1.3 W for DI water and [Bmim]Cl (2 wt%), while for [Bmim] Cl (10 wt% and 20 wt%), the discharge power was slightly decreased to 1.2 W due to an increase in the ionic state of the solution (see the supporting file for details).The solution's plasma treatment resulted in CO production in the gas phase, which was detected by a CO sensor (described in the supporting file).The plasma treatment on the DI water without CO 2 flow solution resulted in a concentration of 8.28 × 10 −7 ± 0.85 × 10 −7 moles of CO.Meanwhile, the CO concentrations for solutions of 4, 10, and 20 wt% [Bmim] Cl without CO 2 flow were 1.49 × 10 −6 ± 0.71 × 10 −7 , 1.58 × 10 −6 ± 0.47 × 10 −7 , and 1.62 × 10 −6 ± 0.81 × 10 −7 moles, respectively [see Fig.
( ) ( ) ( ) OH H e 5 2 ( ) ( ) ( ) ( ) ( ) The detection of CO in the gas phase from those solutions that were not treated with CO 2 could be due to the decomposition of the To address any possibility of [Bmim]Cl degradation with longer treatment durations, we measured the absorbance spectra after 3, 5, and 7 min, as shown in Fig. S1. Figure S1(a) represents the absorbance spectra of DI water after NTP treatment for different treatment times.Figures S1(b), S1(c), and S1(d) show a slight increase in intensity in the absorbance spectra for the 7 min NTP treatment to IL solution.However, this might be attributed to the accumulation of reactive species in the solution or the possibility of hypochlorite formation, as illustrated in Eqs. ( 10)- (12).
( ) Figure 4 shows no visible change in the [Bmim]Cl solution before and after NTP treatment (1 min and 7 min).The abstraction of hydrogen or adding OH in 1-Butyl-3-methylimidazolium is possible, but it requires high energy to break the carbon chain from 1-Butyl-3-methylimidazolium to produce CO.However, we will conduct further detailed studies on the degradation of 1-Butyl-3-methylimidazolium in the future.
Later, we calculated the percentage of energy efficiency for CO production (η %) using Eq. ( 13 For water, the η% was 0.60%, while for [Bmim]Cl IL 4 wt%, 10 wt%, and 20 wt%, the η% was 0.67%, 0.75%, and >0.94%, respectively.This indicates that a 60 s NTP treatment can generate CO, but simultaneously, plasma energy was also utilized in the CO 2 desorption, and production of reactive oxygen and nitrogen species (RONS) due to water dissociation [Eqs.( 1)-( 12)], heat, etc.There is no direct comparison of our work with any reported studies, as our CO 2 absorbent IL is mixed with water, and we have not used any other gas flow that can aid in desorption.Nevertheless, in a previous study, researchers reported a maximum energy efficiency of 0.98% for the first 400 s, which later decreased to 0.68% for 1000 s, using Ar plasma with hydrotalcite pellets as a CO 2 sorbent. 11)Compared to the reported work by Li and Gallucci, 11) our energy efficiency for CO production was slightly less for [Bmim]Cl IL 20 wt%.However, there are significant differences in the treatment conditions and the target of the work (use of water-soluble sorbent).Furthermore, we conducted non-reactive molecular dynamics (MD) simulations to calculate the free energy profiles (FEPs) of CO 2 and CO in both water and a mixture of water + [Bmim]Cl.MD simulations were carried out using umbrella sampling (US) simulations. 20,21)The US simulation technique provides comprehensive FEPs along the reaction coordinates for CO 2 and CO.Details of the simulation procedures can be found in the supporting file.The ΔG gs values were calculated to represent the free energy change for the vacuum-water interface, whereas ΔG sl was associated with the free energy barriers transitioning from the vacuum-water interface into the liquid phase.Hydration-free energy (ΔG hydr ) values were used to elucidate the transport of CO 2 and CO molecules from the gaseous phase into the solution (water or water + [Bmim]Cl).Comprehensive explanations of the ΔG gs , ΔG sl , and ΔG hydr measurements are available in previous studies. 22,23)The ΔG gs , ΔG sl , and ΔG hydr values for CO 2 in water were determined to be −5.52,8.97, and 3.45 kJ mol −1 , respectively [see Fig. 5 I].These US simulations align with our experimental results, as the ΔG gs and ΔG hydr values were lower for CO permeation in water than in water + [Bmim]Cl.This implies that after CO production by plasma, there is a reduced chance of absorption in IL compared to water.In summary, we have introduced a novel technology that integrates plasma and [Bmim]Cl IL for the simultaneous capture, storage, and conversion of CO 2 for the first time.Our experimental results indicate that the water + [Bmim]Cl solution can store CO 2 under atmospheric pressure and room temperature.Moreover, the release of CO 2 during plasma treatment produces CO.Our MD simulation supports our experimental findings, suggesting that CO 2 molecules easily transition from the gaseous phase into the water + [Bmim]Cl solution.In contrast, the penetration of CO molecules into the water + [Bmim]Cl solution is more challenging than water alone.This observation implies that once plasma produces CO, its solubility in the IL solution may be limited, showcasing the potential of this technology for efficient CO 2 capture and conversion.

Fig. 1 .Fig. 2 . 2 ©
Fig. 1.Schematic description of (a) CO 2 capture, (b) CO 2 storage, and (c) conversion of CO 2 using plasma-[Bmim]Cl ionic liquid combination.(a)(b) Figure4shows no visible change in the [Bmim]Cl solution before and after NTP treatment (1 min and 7 min).The abstraction of hydrogen or adding OH in 1-Butyl-3-methylimidazolium is possible, but it requires high energy to break the carbon chain from 1-Butyl-3-methylimidazolium to produce CO.However, we will conduct further detailed studies on the degradation of 1-Butyl-3-methylimidazolium in the future.Later, we calculated the percentage of energy efficiency for CO production (η %) using Eq.(13) from Ref. 11 ⎛

Figure 5
illustrates that the FEPs of CO 2 and CO decrease at the vacuum-water interface, indicating minimum values of ΔG that facilitate the accumulation of CO 2 and CO.This suggests an enhancement of the solute molecule concentration at the vacuum-water or vacuum-water + [Bmim]Cl interface.
(a) and Table I].In contrast, the corresponding ΔG gs , ΔG sl , and ΔG hydr values for CO 2 in water + [Bmim]Cl were −6.83, 8.47, and 1.64 kJ mol −1 , respectively [see Fig. 5(a) and Table I].These simulation results closely align with our experimental findings, indicating that the ΔG gs and ΔG hydr values are lower in water + [Bmim]Cl compared to water, clearly demonstrating that CO 2 encounters less resistance in the IL solution, potentially increasing the amount of CO 2 molecules entering into the solution (water + [Bmim]Cl) compared to water.For CO in water, the ΔG gs , ΔG sl , and ΔG hydr values were −8.93, 13.36, and 4.43 kJ mol −1 , respectively (see TableI ).Conversely, for CO in water + [Bmim]Cl, the ΔG gs , ΔG sl , and ΔG hydr values were −1.11, 14.45, and 13.34 kJ mol −1 , respectively [see Fig. 5(b) and Table
13e electron-based reaction, and we assume that NTP treatment can break the physical binding of CO 2 with IL and convert it to CO [Eq.(1)].The O generated from the reaction is later converted to O 2 [see Eq. (2)] and released into the air or bound with the IL solution.The fate of the O 2 bound with the IL solution is unknown.There are many possible CO 2 and H 2 O dissociation products, as explained in our previous study.13)Some are mentioned below; refer to ). Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd due to Bmim]Cl IL (10 wt% and 20 wt%) control (without NTP treatment) and 1 min NTP-treated [Bmim]Cl IL (10 wt% and 20 wt%), as shown in Figs.3(b) and 3(c).Although we are not neglecting the degradation of [Bmim]Cl IL caused by the plasma, the current study concludes that if the CO production were due to [Bmim]Cl IL degradation, then it would be a very small or insignificant amount.
19)-treated water,[Bmim]Cl IL (4 wt%) control (without NTP treatment), and 1 min NTP-treated [Bmim]Cl IL (4 wt%).No peak shift was observed before and after NTP treatment.The absorbance spectra of the [Bmim]Cl IL control (without NTP treatment) were similar to the literature.19)Thisshows that a 1 min plasma treatment did not degrade the [Bmim]Cl.Similar results were also observed for the [

Table I .
Calculated ΔG gs , ΔG sl , and ΔG hydr values using US simulation.