Effect of Trialkyl Borates as Lewis Acids on Quinones for Electrochemical Carbon Dioxide Capture and Concentration

Abstract

Electrochemical CO2 capture and concentration (eCCC) systems have the potential to operate at efficiencies higher than those of thermal systems. Quinones have been the focus of many eCCC studies as redox carriers, as they can capture or release CO2 depending on their oxidation states. Quinones (Q) with higher cathodic potentials bind CO2 more strongly when they are reduced; however, higher cathodic potentials result in O2 sensitivity. Intra- and intermolecular noncovalent interactions have been explored to anodically shift the second reduction potential (E1/2(Q–1/Q–2)) while maintaining the CO2 binding affinity (log(KCO2)). In this report, we investigate the effects of four trialkyl borate additives, B(OMe)3, B(OEt)3, B(OiPr)3, and B(OBu)3, on the second reduction potential (E1/2(Q–1/Q–2)) and CO2 binding affinity (log(KCO2)) of three quinones, 2-chloro-1,4-benzoquinone (2-ClBQ), 2,3,5,6-tetrafluoro-1,4-benzoquinone (TFQ), and 2,3,5,6-tetrachloro-1,4-benzoquinone (TCQ). Changes in the second reduction potential ranged from 0 to 430 mV with up to 300 equiv of these additives. The magnitude of the anodic shift of the quinones is correlated to the Lewis acidity of the trialkyl borate additive and the Lewis basicity of the quinone. We then investigated the effect of trialkyl borate additives on log(KCO2), which decreases for TFQ and 2-ClBQ but remains constant for TCQ. These results underscore the importance of understanding the additive and quinone interactions in eCCC systems.