Dual Salt Cation-Swing Process for Electrochemical CO2 Separation

Electrochemical CO2 separations, which use electricity rather than thermal energy to reverse sorption of CO2 from concentrated point sources or air, are emerging as compelling alternatives to conventional approaches given their isothermal, ambient operating conditions, and ability to integrate with renewable energy inputs. Despite several electrochemical approaches proposed in previous studies, further explorations of new electrochemical CO2 separation methods are crucial to widen choices for different emissions sources. Herein, we report an electrochemical cation-swing process that is able to reversibly modulate the CO2 loading on liquid amine sorbents in dimethyl sulfoxide (DMSO) solvent. The process exploits a reversible carbamic acid-to-carbamate conversion reaction that is induced by changing the identity of Lewis acid cations (e.g. K+, Li+, Ca2+, Mg2+, and Zn2+) coordinated to the amine-CO2 adduct in the electrolyte. Using ethoxyethylamine (EEA) as a model amine, we present NMR-based speciation studies of carbamic acid-to-carbamate conversion as a function of amine/salt concentrations and cation identity. The reaction is further probed using gas-flow reaction microcalorimetry, revealing the energetic driving forces between cations and the amine-CO2 adduct that play a key role in the described re-speciation. A prototype electrochemical cell was further constructed comprising a Prussian white (PW) potassium (K+) intercalation cathode, zinc (Zn) foil anode, and EEA/DMSO electrolyte containing a dual KTFSI/Zn(TFSI)2 salt. A low CO2 separation energy of ∼22–39 kJ/mol CO2 (0.1–0.5 mA cm–2) was achieved with a practical CO2 loading delta of ∼0.15 mol CO2/mol amine. Further optimizations in electrolyte design and cell architectures toward continuous CO2 capture-release are expected to enhance rate performance while retaining favorable separation energies.

1.It is common practice in the literature, but somewhat misleading, to cite an energetic cost for CO2 separation for a process that captures/releases CO2 from/to a constant-pCO2 environment (100% CO2 in this case).In practice, one would capture and concentrate CO2 from a dilute stream and release it at a higher partial pressure/purity, as shown here (DOI: 10.1038/s41467-022-29791-7). The authors should at least comment on the feasibility of running such a process with their cation-swing cell, and how much of an increase in energetic cost might be expected.
2. The initial discharge capacity for Zn|PW cell in Figure 5a appears to be 45 mAh/g, not 65 mAh/g as written in the text.This apparent discrepancy should be clarified.
Author's Response to Peer Review Comments: Dear Dr. Editor, Thank you for forwarding the reviewer comments on our manuscript entitled, "Dual Salt Cation-Swing Process for Electrochemical CO2 Separation".We were pleased to see that both Reviewers are supportive of publication in ACS Central Science subject to appropriate revisions.We found the Reviewers' comments very helpful and have been able to address them in the revised manuscript, submitted here for your consideration.
As requested, we are returning to you a separate response letter that includes point-to-point response to the Reviewers comments, a manuscript file (with changes highlighted), and an SI file (with changes highlighted).
Thank you in advance for your handling of our revised manuscript.
On behalf of all authors, Sincerely, Betar M. Gallant 1.The Coulombic efficiencies of initial cycles of Zn plating-stripping were lower in the presence of EEA (Figure S12).Was Zn lost to solution in the form of Zn 2+ or afforded side products on the electrode surface?
Author response: We thank the reviewer for the good question.To further investigate the Coulombic inefficiencies (CIE) of initial cycles, we have conducted additional experiments using Zn/Cu cells with 0.5 M EEA-CO 2 in the electrolyte.The following experimental procedure was added on p. S12 of SI: For analysis of Coulombic inefficiencies of Zn|Cu cells with 0.5 M EEA-CO 2 in the initial cycles, the cells were opened after 9 cycles (conditions: current 0.25 mA/cm 2 , capacity 0.25 mAh/cm 2 , and 9 plating + stripping cycles).The separators were soaked in 1 ml DMSO for 3 h, and the solution was then filtered with a syringe filter to remove any solids.The filtrate was mixed with 3 wt% HNO 3 solution to a final composition of 10 vol% of DMSO and 90 vol% of 3 wt% HNO 3 solution as the ICP sample for quantification of final Zn 2+ concentration in the electrolyte.
We have added the following figure on p. S18 of SI: We have added the following discussion regarding the result of Figure S14 on p. 17 of manuscript.We also took the opportunity to further clarify the entire discussion regarding capacity fading in addition to CE, so we copy the entire paragraph below for the convenience of the reviewer: Finally, the long-term cycling performance of the PW/Zn cell was evaluated (Figure 5c).With amines and CO 2 in the cell, 94.2% of capacity was retained after 30 cycles (Figure 5d), which is only slightly lower than that of cells without amine (96.2%, Figure S10; three-electrode cell measurements in Figure S11-S12).Given that these cells utilize a Zn metal anode which acts as a quasi-infinite reservoir, the observed losses arise at the PW cathode.To further understand the intrinsic Zn cyclability in the two electrolytes, two-electrode Zn/Cu cells were also examined (Figure S13).The Zn plating/stripping CEs of the amine-containing cell were initially lower than without amine, but eventually approached and even exceeded that with amine, yielding 96.4% from the 20th to the 50th cycle compared to 95.1% without.A higher degree of Zn plating/stripping polarization was also observed with amine present.The initial CE discrepancy of Zn/Cu cells with and without amine was not due to Zn loss (corrosion) to the solution during cycling, given that the Zn 2+ concentration in the electrolyte remained constant at 0.10 M after cycling with amines present (Figure S14).We could also confirm that no H 2 and CO evolution, such as from possible parasitic decomposition of amines, occurred during Zn cycling by extracting the headspace gas from the cell for GC gas quantification (Figure S15).Therefore, the different initial CE values can be attributed to initial electrode conditioning differences to form the SEI on the Cu current collector, which is common for metal plating/stripping reactions.Given the above observations, the small capacity and cycling differences without and with amine in full cells is hypothesized to arise from cell polarization differences arising primarily at the Zn anode, which can lead to capacity slippage of PW in the full cell configuration and will be the subject of focused future work, including development of optimized cycling protocols.Regardless, the good Zn CE and overall PW/Zn cell cycling performance indicates reasonable stability suitable for further development.
New Figure S9.Integrated CO 2 peak area of GC TCD signals from acid titration of electrolytes after cycling cells to different states as indicated (nomenclature: 'XDYC' indicates X discharge and Y charge half-cycles).
3. It is understandable that as proof-of-concept work, the CO 2 modulation capacity is limited by the capacity of the electrode.It can be a good idea to discuss the electrode capacity required for the practical deployment of this process, considering the CO 2 solubility in DMSO under different headspace CO 2 concentrations.This can be particularly important if we want to release CO 2 at pressure.
Author response: Thank you for the excellent question.We have added the following analysis on p. S21 of SI to discuss the target CO 2 loading modulation in practical applications for capture of CO 2 from a dilute stream and release at a higher partial pressure:

Discussion of Implementing Cation-Swing Process under Post-Combustion Capture Conditions:
For this proof-of-concept work, the cation-swing process operates under fixed (100%) CO 2 partial pressure conditions, which is how the energy requirement for separation was obtained.However, in practice, a CO 2 separation process would be required to capture from a dilute CO 2 stream and release at a higher partial pressure.Therefore, this section provides an estimation of the electrode capacity-to-electrolyte ratios and the increased energy cost to implement such a process compared to those under constant CO 2 partial pressure conditions.The below analysis uses 0.18 bar CO 2 partial pressure as the dilute stream, which is close to post-combustion capture conditions, and 1 bar CO 2 released pressure.Note that the CO 2 solubilities in DMSO are 0.023 and 0.138 M under these pressures, respectively. 5,6  1. Electrode capacity-to-electrolyte ratios For CO 2 to release at pressure, the amount of CO 2 must be sufficiently high to exceed the physically dissolved CO 2 in the electrolyte and drive CO 2 into the headspace.Under the operating conditions described above, the solution already has an initial 0.023 M of CO 2 physically dissolved from the previous equilibration at lower partial pressure during the capture step.In this case, the amount of CO 2 modulation in solution, driven electrochemically, must exceed 0.138 M -0.023 M = 0.115 M.This value can be further converted to electrode charge and yields 3.1 mAh/mL of electrolyte assuming 100% cation conversion efficiency.Note that the above calculation assumes that amines do not substantially alter the physical solubility of DMSO.
In our specific case, we used 250 μL of electrolyte and 0.5 M amine concentration and measured a ~0.15 mol CO 2 /mol amine loading delta, which corresponds to 0.075 M change of CO 2 (with respect to the solution volume) and 2.0 mAh/mL.Therefore, to advance the cell design to be suitable for real applications, it is necessary to increase the electrode-to-electrolyte mass ratios and/or identify cathodes with higher capacities for the weak Lewis acid cation, which may include higher-capacity conversion-type electrodes or other intercalation materials in future work.Based on the estimation above, a ~53% increase in electrode capacity would be necessary for practical application, which is not out of reach given that PW materials are far from optimized in terms of capacity compared to other possible available materials or electrode reactions.
We have added the following reference on p. S23 of SI: 6.
and the following on p. 17 of manuscript (please note that these edits also incorporate changes in response to Question 1 from Reviewer 2): Based on the above data obtained under pure (100%) CO 2 partial pressure, the electrical energy of the cation-swing capture-release process reported herein was calculated to be ~22-39 kJ/mol CO 2 at an equivalent areal current of 0.1-0.5 mA cm -2 (calculations in the experimental section).This range represents a minimum energy estimate, which will be larger if capture is conducted at lower partial pressures as relevant for practical applications.It was estimated that an additional minimum ~50% increase in electrode capacity and ~85% increase in energy cost per mol CO 2 separated would be required to capture CO 2 from a dilute stream (0.18 bar) and release at 100% purity (atmospheric pressure, 1 bar) for the same amine concentration and voltages used herein.These increases are attributed to the need to overcome the difference in CO 2 solubilities in DMSO under different partial pressures.On the other hand, increasing the amine concentration, and/or decreasing the CO 2 solubility of the solvent by moving beyond DMSO are effective strategies to limit this additional energy requirement to within ~20-30% (see discussion in SI).Author response: We thank the reviewer for pointing this out and we have added the suggested literature in the introduction.Also, the reviewer is of course correct that neutral red (ref 23) is a pH swing mechanism, and we have corrected the placement of this reference (ref 24) in the introduction section.

Peer-Reviewer #2 (Comments to the Author):
Kuo et al. demonstrate that an electrochemically driven cation-swing process can modulate CO 2 capture and release via reversible carbamic acid-to-carbamate conversion.The study is well written and the energetic analysis using microcalorimetry is an especially useful contribution.I recommend publication after two points are addressed.
Author response: We thank the reviewer for the constructive suggestions about our manuscript and the recommended modifications, which we have addressed in our revision as detailed below.

1.
It is common practice in the literature, but somewhat misleading, to cite an energetic cost for CO 2 separation for a process that captures/releases CO 2 from/to a constant-pCO 2 environment (100% CO 2 in this case).In practice, one would capture and concentrate CO 2 from a dilute stream and release it at a higher partial pressure/purity, as shown here (DOI: 10.1038/s41467-022-29791-7). The authors should at least comment on the feasibility of running such a process with their cation-swing cell, and how much of an increase in energetic cost might be expected.
Author response: Thank you for the good question.First, we wish to note that we provided an estimate of the operating conditions (capacity-to-electrolyte ratios) required to capture/release CO 2 at different partial pressures in response to Reviewer 1, Comment 3, assuming DMSO as solvent.For the estimation of the increase in energetic cost, we have added the following discussion on p. S21 of the SI:

Increase in energy cost
In considering the energy cost when operating between two different partial pressures, we first summarize the contributions to the energy requirement: where is the energy consumption for cell charge (kJ), is the energy recovered from cell discharge (kJ), E charge E discharge V charge and V discharge are the charge and discharge voltages, respectively, and Q is the electrode capacity (assumed for simplicity to be equal for charge and discharge, i.e., 100% Coulombic efficiency).Meanwhile, is the N CO2, amine amount of CO 2 released from amine (mol), and is the difference of the amounts of CO 2 dissolved in the ∆N CO2, dissolved solvent between the capture and released partial pressures (mol).Additionally, is the electrolyte volume (L), v electrolyte is the concentration of CO 2 released from amine (M), and is the difference of the CO 2 C CO2, amine ∆C CO2, dissolved solubilities in the solvent between the capture and released partial pressures (M).The denominator reflects the fact that, when releasing at higher partial pressure than that of the inlet stream, the CO 2 physical solubility is higher due to the higher headspace partial pressure, and must be overcome by releasing excess CO 2 .Therefore, higher electrical work would be required for the same amount of actually released CO 2 .Alternatively but equivalently, for the same electrical work, the amount of separated CO 2 must be discounted, identically increasing the per-CO 2 separation cost.
As an example, we assume that the electrolyte volume, electrode capacity Q, and V charge and V discharge are the same as those determined under the constant (100%) CO 2 partial pressure used in this work for simplicity of the estimation.Under these assumptions, (1) the total amount of CO 2 released from the amine ( ), directly determined from N CO2, amine Q, and (2) the total electrical energy cost per cycle under constant CO 2 pressure ( ) would be the E charge -E discharge same.However, the amount of CO 2 released to the atmosphere (the denominator of the above equation) would be less for reasons noted above.Effectively, then, greater total charge Q is required to yield CO 2 that actually leaves solution and can be flushed out of the headspace, which indeed increases the energy requirement.Also, this increase in energy cost would depend on the CO 2 partial pressures of the dilute stream and amine concentrations.Therefore, for various conditions, we can calculate the increase in energetic cost as shown in Table 1 below assuming the maximum CO 2 released with the cation-swing mechanism is half of the amine concentration, representing the full loading window accessible in DMSO.
The energy cost can be significant for relatively low amine concentration, but decreases with higher values because there is higher CO 2 released compared to what can be dissolved.It is also important to note that the above estimation does not account for the differences in pumping requirements in the process, overpotentials in the cell operation, and differences in amine speciation under lower CO 2 partial pressure, which could further affect the energy cost but requires more research to elucidate in full, and is beyond the scope of this work.However, this first-order analysis is sufficient to illustrate how future optimizations might be directed towards increasing amine concentration and using solvents with lower CO 2 solubility to reduce this energy penalty.
The following sentence is added on p. 17 of manuscript as the pointer for the above discussion in SI: Based on the above data obtained under pure (100%) CO 2 partial pressure, the electrical energy of the cation-swing capture-release process reported herein was calculated to be ~22-39 kJ/mol CO 2 at an equivalent areal current of 0.1-0.5 mA cm -2 (calculations in the experimental section).This range represents a minimum energy estimate, which will be larger if capture is conducted at lower partial pressures as relevant for practical applications.It was estimated that an additional minimum ~50% increase in electrode capacity and ~85% increase in energy cost per mol CO 2 separated would be required to capture CO 2 from a dilute stream (0.18 bar) and release at 100% purity (atmospheric pressure, 1 bar) for the same amine concentration and voltages used herein.These increases are attributed to the need to overcome the difference in CO 2 solubilities in DMSO under different partial pressures.On the other hand, increasing the amine concentration, and/or decreasing the CO 2 solubility of the solvent by moving beyond DMSO are effective strategies to limit this additional energy requirement to within ~20-30% (see discussion in SI).

Figure S14 .
Figure S14.The final electrolyte Zn 2+ concentration for Zn|Cu cells after 9 cycles (conditions: current 0.25 mA/cm 2 , capacity 0.25 mAh/cm 2 , and 9 plating + stripping cycles).The data are from the average of 5 independent cells and the error bars indicate standard deviation.

4.
Some recent publications on electrochemical carbon capture via direct redox of organic sorbent molecules can be included, such as Nature Energy, 2022, 7, 1065-1075.The citation on neutral red (ref 23) is actually a pH swing mechanism.

Table S5 .
Percentage of increased energy cost with CO 2 partial pressures in dilute stream and different amine concentrations assuming CO 2 released partial pressure of 1 bar (solubility of CO 2 in DMSO under 1 bar CO 2 is 0.138 M).