Oxygen Stable Electrochemical CO2 Capture and Concentration through Alcohol Additives

Abstract

Current methods for CO2 capture and concentration (CCC) are energy intensive due to their reliance on thermal cycles, which are intrinsically Carnot limited in efficiency. In contrast, electrochemically driven CCC (eCCC) can operate at much higher theoretical efficiencies. However, most reported systems are sensitive to O2, precluding their practical use. In order to achieve O2 stable eCCC, we pursued the development of molecular redox carriers with reduction potentials positive of the O2/O2- redox couple. Prior efforts to chemically modify redox carriers to operate at milder potentials resulted in a loss in CO2 binding. To overcome these limitations, we used common alcohols additives to anodically shift the reduction potential of a quinone redox carrier, 2,3,5,6-tetrachloro-p-benzoquinone (TCQ), by up to 350 mV, conferring O2 stability. Intermolecular hydrogen-bonding interactions to the dianion and CO2-bound forms of TCQ were correlated to alcohol pKa to identify ethanol as the optimal additive, as it imparts beneficial changes to both the reduction potential and CO2 binding constant, the two key properties for eCCC redox carriers. We demonstrate a full cycle of eCCC in aerobic simulated flue gas using TCQ and ethanol, two commercially available compounds. Based on the system properties, an estimated minimum of 21 kJ/mol is required to concentrate CO2 from 10% to 100%, or twice as efficient as state-of-the-art thermal amine capture systems and other reported redox carrier-based systems. Furthermore, this approach of using hydrogen-bond donor additives is general and can be used to tailor the redox properties of other quinones/alcohol combinations for specific CO2 capture applications.

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

“…Combining these CV analyses with the approach we employ using DFT, one can investigate the thermodynamics of different potential capture mechanisms and see if such a trade-off still persists. We also note that very recent work has shown that electrolyte additives such as alcohols offer another interesting means to tune electrochemical CO 2 capture thermodynamics, 39 and it will be interesting to explore the trade-off between redox potential and capture strength with that approach. Overall, our work reveals the trade-off between redox potential and the strength of electrochemical CO 2 capture, and provides a foundation for the design of improved molecules and materials that can mitigate greenhouse gas emissions.…”

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

“…Overall, in the context of electrochemical CO 2 capture, the chemical steps are less thermodynamically favourable as more EWGs are added, supporting a trade-off between redox potential and CO 2 affinity, as suggested by previous experimental work on functionalised benzoquinones. 11,19 To explore this result experimentally, we measured the CVs of unsubstituted AQ, 1,4-F-AQ and AQ-F 8 under N 2 and under CO 2 (Figure 3(c)). For all cases, the re-duction wave positions of the first electron transfer (6) remain unchanged in the presence and absence of CO 2 .…”

“…In these mechanisms, two electron transfer (E) steps occur before any chemical (C) CO 2 capturing step, which has been supported in previous CV analysis of EWG-substituted quinones at low CO 2 concentration. 17,[19][20][21]37 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 any CO 2 capture. The electrochemical reduction occurs as:…”

“…In the EEC scheme previously proposed as the capturing mechanism for quinones with more than two EWG substituents, 11,19,20 a single capturing event occurs after electrochemical reduction:…”

“…11 They found a linear relationship between the CO 2 binding constant and the second reduction potential for a series of 4 functionalised benzoquinones and phenanthrenequinone. More recent experiments on benzoquinones have supported this trade-off, 19 while other studies have further explored how functionalisation impacts the chemistry of electrochemical CO 2 capture by quinones. 20,21 Despite this progress, a complete assessment of the relationship between redox potentials and CO 2 capturing abilities in a wide range of redox-active molecules has not been carried out.…”