Electrochemical and Molecular Assessment of Quinones as CO₂-Binding Redox Molecules for Carbon Capture

Abstract

The complexation and decomplexation of CO2 with a series of quinones of different basicity during electrochemical cycling in dimethylformamide solutions were studied systematically by cyclic voltammetry. In the absence of CO2, all quinones exhibited two well-separated reduction waves. For weakly complexing quinones, a positive shift in the second reduction wave was observed in the presence of CO2, corresponding to the dianion quinone-CO2 complex formation. The peak position and peak height of the first reduction wave was unchanged, indicating no formation of complexes between the semiquinones and CO2. The relative heights of both reduction waves remained constant. In the case of strongly complexing quinones, the second reduction wave disappeared while the peak height of the first reduction wave approximately doubled, indicating that the two electrons transferred simultaneously at this potential. The observed voltammograms were rationalized through several equilibrium arguments. Both weakly and strongly complexing quinones underwent either stepwise or concerted mechanisms of oxidation and CO2 dissociation depending on the sweep rate in the cyclic voltammetric experiments. Relative to stepwise oxidation, the concerted process requires a more positive electrode potential to remove the electron from the carbonate complexes to release CO2 and regenerate the quinone. For weakly complexing quinones, the stepwise process corresponds to oxidation of the uncomplexed dianion and accompanying equilibrium shift, while for strongly complexing quinones the stepwise process would correspond to the oxidation of mono(carbonate) dianion to the complexed semiquinone and accompanying equilibrium shift. This study provides a mechanistic interpretation of the interactions that lead to the formation of quinone-CO2 complexes required for the potential development of an energy efficient electrochemical separation process and discusses important considerations for practical implementation of CO2 capture in the presence of oxygen with lower vapor pressure solvents.

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

“…When EWGs are substituted, as in the case of -Cl and -F, the reduction potentials become more positive for both CV and DFT results (Figures 2(c), 2(d)). This agrees with previous studies 19,20,23 and is consistent with the electron density on the reduced quinones being more delocalised, making reduction more thermodynamically favourable. Positional effects observed experimentally and predicted computationally for the case of -Cl also agree: substitution at the 2-position shifts E • EE to a more positive value than at the 1-position.…”

“…This is in line with the quinone in all cases needing to be first activated by reduction before capturing CO 2 , as observed in previous studies. 19 For both AQ and 1,4-F-AQ, the second reduction wave (2) appears to shift underneath the first reduction wave, in agreement with previous work. 14,16 In contrast, for AQ-F 8 a smaller shift of the first reduction wave is observed, and two redox processes can be clearly observed under CO 2 .…”

“…We employ this mechanism in our analysis for two reasons: (i) the EECC mechanism has been supported in previous CV analysis of EWG-substituted quinones at low CO 2 concentration. 16,19,26,27 Since our goal is to mitigate side-reactions of reduced quinones with O 2 , our main interest is to add EWGs to stabilise the reduced quinones, increasing the likelihood of both electrochemical reduction steps being required prior to CO 2 capture; (ii) this scheme also allows the combination of two reduction steps (1) and (2) to an "EE" step, and two capturing events (3) and (4) to a "CC" step according to:…”

“…We note that other electrochemical capture mechanisms are possible, notably the ECEC mechanism, where after the first reduction step, Qreacts with CO 2 to form the semicarbonate anion Q(CO 2 ) -, as suggested in studies of EDG-substituted quinones at high CO 2 concentration. 14,15,19,26 We first explored the CO 2 capture steps for anthraquinone using DFT calculations. The localised orbital forms representing the lone pairs (LPs) and twoelectron two-centre bonds (BDs) were decomposed from the DFT wavefunction by natural bonding orbital analysis (NBO) (Figure 1(c)).…”

“…However, by tuning the redox potentials, one might also change the CO 2 capturing ability of the quinones. While this has been hinted at by cyclic voltammetry (CV) studies of different quinone derivatives under CO 2 , 18,19 a complete assessment of the relationship between redox potentials and CO 2 capturing abilities has not been carried out. We believe this is because (i) quantifying the thermodynamic driving force of CO 2 capture through CV alone is challenging and (ii) a systematic electrochemical study of quinone derivatives with different number of substituents and different functional groups is not easily accessible experimentally.…”