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Maximum and Comparative Efficiency Calculations for Integrated Capture and Electrochemical Conversion of CO2

Cite this: ACS Energy Lett. 2024, 9, 2, 768–770
Publication Date (Web):January 31, 2024
https://doi.org/10.1021/acsenergylett.3c02489

Copyright © 2024 American Chemical Society. This publication is available under these Terms of Use.

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CO2 as a C1 feedstock is receiving increasing attention, but it typically exists in dilute or impure streams. (1) The capture and conversion of CO2 to fuels or chemicals using carbon-neutral energy mitigates carbon emissions when CO2 is captured from point sources and results in net or negative carbon emissions when CO2 is obtained by direct air capture. CO2 capture and conversion steps are typically viewed as separate processes that optimize either capture and concentration or catalytic conversions of pure streams of CO2. (2)

However, there is an increasing interest in the reduction of CO2 from dilute streams without prior separation and concentration. Utilization of CO2 from dilute sources can occur either by direct reduction of the impure CO2 stream or by integrated capture and conversion, in which CO2 is captured by a sorbent and reduced directly in this captured (sorbed) form. Either of these approaches offers advantages in process intensification, both in energetic demand and in infrastructure, compared to sequential capture and concentration followed by reduction. (3) In CO2 capture and concentration cycles, capture is typically passive, but liberation of the CO2 and regeneration of the sorbent require the largest input of energy. (4) Combining capture and conversion avoids the energetic and capital requirement for liberating CO2 into a pure form, which has a minimum energy defined by eq 1 (in which Pi and Pf are, respectively, the initial and final partial pressures of CO2):

ΔG=RTln(Pf/Pi)
(1)

The overall energetic efficiency of CO2 conversion is defined by the consumed energy vs the thermodynamic minimum for conversion. In electrochemical reduction, this overall efficiency manifests as the overpotential. Starting from dilute streams of CO2 has an impact on the thermodynamic minimum free energy for conversion. Additionally, the use of sorbents to capture CO2 followed by reduction of the sorbed-CO2 results in additional considerations in evaluating and comparing electrochemical conversion or electrocatalytic efficiency. This Viewpoint discusses these considerations in evaluating efficiency for integrated capture and electrochemical conversion of CO2.

The thermodynamic potential for reduction of CO2 in dilute streams (blue reaction path in Scheme 1) requires an adjustment from the standard state potential (), which is defined at 1 atm of CO2. The potentials for electrochemical reactions are routinely adjusted from the standard state potential () to the thermodynamic potential (E) under nonstandard state conditions, as few chemical transformations are run at 1 M or 1 atm activities for all reactants and products. The electrochemical reduction of CO2 to CO is shown in eq 2 as an example reaction to illustrate the required corrections to the standard state:

CO2(g)+2H(solv)++2eCO(g)+H2O
(2)
The correction from standard state to actual reaction conditions is applied using the Nernst equation, eq 3:
E=E°RTnFlnQ
(3)
For eq 3, R is the universal gas constant, T is the temperature in Kelvin, n is the number of electrons in the transformation, F is Faraday’s constant, and Q is the reaction quotient. For the reaction in eq 2, the Nernst equation can be written as eq 4:
E=E°0.0296log(PCO[H2O]PCO2[H+]2)
(4)
Eq 4 can be separated to emphasize the impact of the different concentrations and partial pressures of the reactants. Eq 5 illustrates the impact of the partial pressure of CO2 or pH:
E=E°+0.0296log(PCO2)0.0592pH0.0296log(PCO[H2O])
(5)
Using eq 5, the impact of capture of CO2 from a point source or from the atmosphere can be readily calculated. This change in potential with pressure of CO2 is equivalent to the free energy determined using eq 1.

Scheme 1

Scheme 1. Impact of Partial Pressure of CO2 and Sorbent Binding Equilibria on Reaction Profile for an Electrocatalytic Conversion

The impact of the partial pressure of CO2 upon the reduction potential for CO2 to CO can be calculated from eq 5 and is illustrated in Table 1. The shift in potential for this 2e conversion is shown, as well as the equivalent value in kcal/mol, which can be analogously applied for hydrogenation reactions.

Table 1. Impact of Concentration (Partial Pressure) of CO2 on Combined Capture and Conversion of CO2 to COa
CO2 Concn (%)ΔE (mV)bΔΔGc (kcal/mol)
10000
10–29.61.36
1–59.22.73
0.1–88.84.09
0.04 (400 ppm)–100.64.63
a

The presented values are for the conversion of CO2 to CO, using calculations based on eq 4. Analogous values could be calculated to illustrate the impact of CO2 concentration upon the reduction potential for other products, such as CH4 or C2+ products.

b

The term ΔE represents the difference in potential vs the standard potential for the reduction of CO2 to CO, and thereby the impact of CO2 concentration upon the potential.

c

Similarly to ΔE, ΔΔG represents the difference in reaction free energy vs standard state for the reduction of CO2 to CO, and thereby the impact of CO2 concentration.

While the discussion above dictates the minimum energy required to turn a dilute CO2 stream to a product, another approach is to use a sorbent (sorb) to capture CO2 (sorb-CO2) from dilute streams (eq 6), which has distinct considerations. Sorbents can be useful for separating CO2 from contaminants that may otherwise interfere with subsequent reduction and/or for effectively concentrating the CO2 in its sorbed form, offering a kinetic advantage based on concentration of sorbed CO2 available to the catalyst. Sorbents can also be designed to activate CO2 to give different products than unactivated CO2 reduction. However, the use of sorbents can result in an energetic penalty for the overall conversion of dilute CO2 to products, which is described for electrochemical processes by the orange traces in Scheme 1. Sorbents capture CO2 with an equilibrium constant Ksorb:

sorb+CO2(g)sorb‐CO2Ksorb=[sorb‐CO2][sorb]PCO2
(6)

The reduction potential for this sorb-CO2 complex can be determined using Hess’s law through the summation of the net reaction at standard conditions (without sorbent) and Ksorb, as illustrated through eqs 7 and 8, which sum to eq 9, with the value of the resulting free energy expressed with eq 10:

CO2(g)+2H++2eCO(g)+H2OEsorb‐CO2/CO°
(7)
sorb‐CO2CO2(g)+sorb1/Ksorb
(8)
sorb‐CO2+2H++2e+CO(g)+sorb+H2OΔGsorb‐CO2/CO°
(9)
ΔGsorb‐CO2/CO°=nFE°RTln(1Ksorb)
(10)

This energy incorporates the equilibrium of capture from various streams as described above. Thus, appreciable capture from more dilute streams requires a greater equilibrium constant for binding CO2, corresponding to a larger sorption free energy. As subsequent reduction requires cleavage of the sorb-C bond, the negative sorption free energy will increase in the overall free energy required for carbon functionalization. Thus, sorbents that bind CO2 more tightly than required for capture from a specific dilute stream will not operate as efficiently as a sorbent that is just strong enough for capture from the targeted CO2 concentration. In other words, the most efficient sorbent for reactive capture will depend on the partial pressure of CO2 in the feed stream. For example, although a strongly binding sorbent (sorb2 in Scheme 1) that can capture CO2 from air (direct air capture) can also be used for flue gas capture (5–15% CO2), it will operate will lower overall efficiency compared to a sorbent better matched to the concentration (sorb1 in Scheme 1). The binding energy in excess of the minimum will contribute to overall inefficiency.

The standard potential for this reaction (sorb-CO2/CO) can be calculated from Δsorb-CO2/CO and can be adjusted to nonstandard state conditions, as illustrated by eq 11:

Esorb‐CO2/CO=Esorb‐CO2/CO°0.0296log(PCO[sorb][H2O][sorb‐CO2][H+]2)
(11)

For reactions that involve the reduction of sorb-CO2, the difference between Esorb-CO2/CO in eq 11 and the applied potential can be used to calculate the overpotential (η). Using the corrected thermodynamic potential from eq 11 provides a more accurate depiction of catalyst efficiency for specific sorbed-CO2 substrates to more easily compare the effectiveness of varying catalysts for sorbed-CO2 conversion.

Another metric for evaluating the efficiency for reactive capture is focused on changes introduced by the use of a sorbent, specifically the change in minimum energy required for conversion. The impact of sorbent strength on capture efficiency has been previously described; (5) however, our focus here is on the impact of sorbent strength on capture and conversion. For electrocatalysis, we describe this contribution to the as ΔERCC, stemming from its origin in reactive capture and conversion with a sorbent. The value of this parameter can be calculated from Ksorb and PCO2 by rearranging the free energy expression shown in eq 6 to yield eq 12:

Ksorb×PCO2=[sorb‐CO2][sorb]
(12)
Based on the aim of having equilibrium binding of CO2, and thereby [sorb-CO2] equal to [sorb], the free energy associated with sorption based on Ksorb and PCO2 can be calculated using eq 13:
ΔG=RTln(Ksorb×PCO2)
(13)

For values of Δ < 0, the sorbent adds to the minimum thermodynamic potential. This free energy can be converted to ΔERCC but is dependent upon the number of electrons in the transformation of interest. For a 2e conversion, such as CO2 reduction to CO, the value of ΔERCC can be calculated using eq 14:

ΔERCC=0.0296log(Ksorb×PCO2)
(14)

Values of ΔERCC > 0 contribute to the overall energetic requirement because of the stronger than necessary binding of the sorbent to CO2. This effect is illustrated in Scheme 1 and exemplified in Table 2. As ΔERCC adds to the minimum energetic requirement for integrated capture and conversion, it should be minimized by matching sorbent binding energies with the concentration of CO2 in the feedstream.

Table 2. Impact of Sorbent Binding Constant on Energy Efficiency for Sorbent-Mediated Capture and Conversion of CO2 to CO
PCO2KsorbΔERCC (mV)
0.1100
0.110029.6
0.11,00059.2
0.12,50071.0
0.110,00088.8
0.011000
0.0011,0000
0.00042,5000

We describe two considerations for discussing the efficiency for utilization of dilute CO2 streams. The first provides the overall efficiency for conversion of a dilute stream to CO2, which is accurate whether the dilute stream is used directly or captured and concentrated followed by utilization, or if sorb-CO2 is used as an intermediate. The second calculation considers systems that use an intermediate sorbent where sorb-CO2 is reduced and the sorbent is regenerated. The sorption energy is accounted for in the conversion. While this method does not provide the overall efficiency for conversion, it accurately reflects the efficiency of the catalyst for the specific reaction of sorb-CO2 to product. Using sorbents that are not well matched for the dilute CO2 stream (i.e., a binding constant that is too high) will manifest in greater inefficiencies between the latter and former systems. Ultimately, a system that uses sorbents will require sorbents well-matched to the level of the dilution of the stream to approach the overall thermodynamic minimum.

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    • Notes
      Views expressed in this Viewpoint are those of the authors and not necessarily the views of the ACS.
      The authors declare no competing financial interest.

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    A.M.A. and J.Y.Y. are supported by the Center for Closing the Carbon Cycle, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award Number DE-SC0023427.

    References

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    This article references 5 other publications.

    1. 1
      Badgett, A.; Feise, A.; Star, A. Optimizing utilization of point source and atmospheric carbon dioxide as a feedstock in electrochemical CO2 reduction. iScience 2022, 25 (5), 104270  DOI: 10.1016/j.isci.2022.104270
    2. 2
      Fu, L.; Ren, Z.; Si, W.; Ma, Q.; Huang, W.; Liao, K.; Huang, Z.; Wang, Y.; Li, J.; Xu, P. Research progress on CO2 capture and utilization technology. J. CO2 Utilization 2022, 66, 102260  DOI: 10.1016/j.jcou.2022.102260
    3. 3
      Freyman, M. C.; Huang, Z.; Ravikumar, D.; Duoss, E. B.; Li, Y.; Baker, S. E.; Pang, S. H.; Schaidle, J. A. Reactive CO2 capture: A path forward for process integration in carbon management. Joule 2023, 7 (4), 631651,  DOI: 10.1016/j.joule.2023.03.013
    4. 4
      Heldebrant, D. J.; Kothandaraman, J.; Dowell, N. M.; Brickett, L. Next steps for solvent-based CO2 capture; integration of capture, conversion, and mineralisation. Chem. Sci. 2022, 13 (22), 64456456,  DOI: 10.1039/D2SC00220E
    5. 5
      Lively, R. P.; Realff, M. J. On thermodynamic separation efficiency: Adsorption processes. AIChE J. 2016, 62 (10), 36993705,  DOI: 10.1002/aic.15269

    Cited By

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    This article is cited by 1 publications.

    1. Rachel E. Siegel, Marcos Aceves, Louise A. Berben. Direct Electrochemical Conversion of CO2 Sorbent Solution to Formate by a Molecular Iron Catalyst. ACS Energy Letters 2024, 9 (6) , 2896-2901. https://doi.org/10.1021/acsenergylett.4c00901
    • Scheme 1

      Scheme 1. Impact of Partial Pressure of CO2 and Sorbent Binding Equilibria on Reaction Profile for an Electrocatalytic Conversion
    • References

      ARTICLE SECTIONS
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      This article references 5 other publications.

      1. 1
        Badgett, A.; Feise, A.; Star, A. Optimizing utilization of point source and atmospheric carbon dioxide as a feedstock in electrochemical CO2 reduction. iScience 2022, 25 (5), 104270  DOI: 10.1016/j.isci.2022.104270
      2. 2
        Fu, L.; Ren, Z.; Si, W.; Ma, Q.; Huang, W.; Liao, K.; Huang, Z.; Wang, Y.; Li, J.; Xu, P. Research progress on CO2 capture and utilization technology. J. CO2 Utilization 2022, 66, 102260  DOI: 10.1016/j.jcou.2022.102260
      3. 3
        Freyman, M. C.; Huang, Z.; Ravikumar, D.; Duoss, E. B.; Li, Y.; Baker, S. E.; Pang, S. H.; Schaidle, J. A. Reactive CO2 capture: A path forward for process integration in carbon management. Joule 2023, 7 (4), 631651,  DOI: 10.1016/j.joule.2023.03.013
      4. 4
        Heldebrant, D. J.; Kothandaraman, J.; Dowell, N. M.; Brickett, L. Next steps for solvent-based CO2 capture; integration of capture, conversion, and mineralisation. Chem. Sci. 2022, 13 (22), 64456456,  DOI: 10.1039/D2SC00220E
      5. 5
        Lively, R. P.; Realff, M. J. On thermodynamic separation efficiency: Adsorption processes. AIChE J. 2016, 62 (10), 36993705,  DOI: 10.1002/aic.15269