Trade-off between redox potential and strength of electrochemical CO2 capture in quinones

Abstract

Electrochemical carbon dioxide capture has recently emerged as a promising alternative approach to conventional energy-intensive carbon capture methods. The most common electrochemical capture approach is to employ redox-active molecules such as quinones. Upon electrochemical reduction, quinones become activated for the chemical capture of CO2. The main 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 CO2 capture in different anthraquinone derivatives with a range of substituents. By combining density functional theory calculations and cyclic voltammetry experiments, we discover a trade-off between redox potentials and the strength of CO2 capture. We show that redox potentials can readily be tuned to more positive values to impart stability to oxygen, but as a consequence, significant decreases in CO2 binding free energies are observed. This trade-off must be taken into consideration for the design of improved redox active molecules for electrochemical CO2 capture.

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

“…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 above-mentioned 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 .…”

“…When excluding 1-NH 2 -4-OH-AQ and AQ, the comparison in Figure a 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 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.…”

“… 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 …”

“…Recently, Hatton and co-workers published an in-depth study on the nature of the CO 2 -quinone binding in relation to the CO 2 binding strength mentioned above by comparing various different quinones, but without any hydrogen-bonding substituents . The same group also developed several prototype cells for direct air capture of CO 2 based on AQ polymers. − It has been reported that oxygen (O 2 ) binding and electrochemical O 2 reduction by these AQ-polymers are crucial side-reactions to be considered. , 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 …”

Anthraquinone and its derivatives as sustainable materials for electrochemical applications – a joint experimental and theoretical investigation of the redox potential in solution

“…The redox potential of AQ, and other organic carbonyl compounds, can be tailored by the introduction of electron-donating or -withdrawing groups, resulting in an increase or decrease of the respective electrochemical potentials. [31][32][33][34] This tunability of potential is a powerful tool not only to increase the respective energy density in a battery application, but also to mitigate problems related to interface instability, safety issues and stability limits associated with most organic electronic devices. 35,36 In addition, AQ and its derivatives have shown great promise as an electrode material in the electrochemical capture and release of CO 2 .…”