Trade-Off between Redox Potential and the Strength of Electrochemical CO₂ Capture in Quinones

Abstract

Electrochemical carbon dioxide capture recently emerged as a promising alternative approach to conventional energy-intensive carbon-capture methods. A common electrochemical capture approach is to employ redox-active molecules such as quinones. Upon electrochemical reduction, quinones become activated for the capture of CO 2 through a chemical reaction. A key disadvantage of this method is the possibility of side-reactions with oxygen, which is present in almost all gas mixtures of interest for carbon capture. This issue can potentially be mitigated by fine-tuning redox potentials through the introduction of electron-withdrawing groups on the quinone ring. In this article, we investigate the thermodynamics of the electron transfer and chemical steps of CO 2 capture in different quinone derivatives with a range of substituents. By combining density functional theory calculations and cyclic voltammetry experiments, we support a previously described trade-off between the redox potential and the strength of CO 2 capture. We show that redox potentials can readily be tuned to more positive values to impart stability to oxygen, but significant decreases in CO 2 binding free energies are observed as a consequence. Our calculations support this effect for a large series of anthraquinones and benzoquinones. Different trade-off relationships were observed for the two classes of molecules. These trade-offs must be taken into consideration in the design of improved redox-active molecules for electrochemical CO 2 capture.

Direct Electrochemical CO2 Capture Using Substituted Anthraquinones in Homogeneous Solutions: A Joint Experimental and Theoretical Study

“…When excluding 1-NH 2 -4-OH-AQ and AQ, the comparison in Figure 11a reveals a clear correlation (red line) between the stability of the radical species in CO 2 saturated conditions and the shift of the second-reduction peak under CO 2 toward the first-reduction peak under N 2 . One possible reason for the observed deviation in the stability of AQ is the lack of any steric hindrance upon CO 2 binding, which was also recently suggested by Bui et al 33 Furthermore, it has to be stated that additional research is required to provide a comprehensive understanding of the stabilities of CO 2 :X-AQ •− species, since the experimentally determined value of ΔE p,2−1 still consists of equilibrium contributions of both the radical as well as the final X-AQ 2− species. Another possibility for the observed deviating behavior of certain derivatives is that the type of electrochemical and/or chemical mechanism might also be dependent on the actual substitutions of the AQ lead structure.…”

“…However, 1-NH 2 -4-OH-AQ displays a behavior that is notably deviating from the observed trend, although the thermodynamically more stable syn conformer is the one closer to the linear regression. A possible explanation for the peculiar behavior of AQ and 1-NH 2 -4-OH-AQ is that their reduction potential of around −0.66 V is at an optimum energetic value for CO 2 capture, as pointed by Bui et al 33 Despite the abovementioned differences, this work represents, to the best of our knowledge, the first instance in which the binding energies of CO 2 and X-AQ •− radical species are reported. They are found to be qualitatively in good agreement with the experimental observations of radical species measured by SEC, as given in section 3.2.…”

“…29,32 Very recently, Bui et al investigated the impact of multiple substitutions involving several AQ derivatives with CV and related their findings regarding electrochemical potentials with results obtained via density functional theory (DFT) calculations. 33 Besides the use of quinones for electrochemical CO 2 capture, also numerous applications of quinones for metalion batteries 34−40 as well as redox-flow batteries 41−43 are reported. As quinones in electrochemical applications are often limited due to reductive dissolution from the electrode surface, 44 metal-ion battery research is focused on the prevention of this dissolving process via polymerization or tuning of the substrate-quinone binding strength.…”

High-Capacity and High-Stability Electrochemical CO2 Capture Cell with Coupled Electricity Storage

“…A high priority for future research is the development of molecules with the advantages of 1,8-ESP but which also have sufficient stability against O2 to be deployed in DAC. 46 Another complementary approach is to develop additives that anodically shift the reduction potential of the redox active molecules.…”

Electrochemical carbon capture processes for mitigation of CO2 emissions

“…The absorbent can then be electrochemically oxidized to regenerate the organic absorbent and release pure CO 2 . 40,[52][53][54][55][56] Quinone-based nucleophiles have been widely studied as the redox-active organic component that can effectively capturerelease CO 2 at different reduction potentials. Early works on employing quinone chemistry for CO 2 capture mostly focused on understanding the mechanism involved during the electrochemical capture-release stages, investigating effective redox candidates, and optimizing the electrolyte media.…”