CNT-Based Slurry Electrodes Towards Electrochemically Mediated Carbon Dioxide Capture/Release

The alarming rise in global temperature mandates the reduction of carbon emissions. Along with the usage of clean energy technologies, removal of the existing/produced CO 2 in the atmosphere is also equally important to mitigate climate change. Amine-based sorbents are most popular for absorbing CO 2 from the atmosphere as well as industrial flue gas. However, the regeneration of amines is an energy-intensive process that limits their applicability [1]. Electrochemically mediated CO 2 capture/release (EMCCR) offers a significant advantage in terms of energy requirement and design considerations. Anthraquinone (AQ) derivatives are commonly used as a redox mediator in EMCCR. Reduced AQ has an affinity to bind with CO 2 and facilitate its release upon oxidation, allowing the shuttling of CO 2 between reduced and neutral AQ [2]. In all the reported studies, either dissolved form of CO 2 in an electrolyte or gaseous CO 2 that must diffuse through a porous electrode is captured (released) via redox active molecule reduction (oxidation) [3]. This poses problems regarding the solubility of CO 2 in electrolytes and the mass transfer of CO 2 through electrodes, which are key limiting parameters for EMCCR. In addition, the solubility of AQ in the electrolyte also restricts the CO 2 -capturing capability. Against this background, fresh impetus was given with CNT-based slurry electrodes that have the potential to bypass the solubility issue of CO 2 and also its diffusion limitation , thereby resulting in efficient EMCCR. Slurry electrodes are commonly composed of conductive molecules for electrons to flow, redox species that get reduced/oxidized upon electron transfer, and the electrolyte for ionic mobility [4]. In the present work, CNT is a conductive part, AQ molecules are redox active species, and the ionic liquid (IL), 1-butyl-3-methylimidazolium bis(trifluoromethyl sulfonyl) imide is the electrolyte. The concept is demonstrated by subjecting the CNT slurry electrode to electrochemical characterization in a three-electrode setup under N 2 and CO 2 environments. Figure 1 shows the cyclic voltammogram (CV) of the CNT slurry electrode in a potential window of -1.25 to 1.25 V vs. FC + /FC at a scan rate of 50 mV/s. Two reduction/oxidation peaks for a CV under N 2 are observed. In contrast, merged reduction/oxidation peaks with increased current in cathodic peak under CO 2 reveal that CO 2 is captured during reduction and released during oxidation [2]. Further, three different weight ratios of CNT to AQ were studied to understand the effect of CNT solid content on the electrochemical performance of slurry electrodes for EMCCR. Accordingly, 50:1, 100:1, and 150:1 ratios of CNT to AQ were chosen, which resulted in solid contents of 2.1, 4.2 and 6.3%. Lower solid content showed improved electrochemical reversibility for AQ molecules. In practice, one must consider the required viscosity and conductivity of the slurry to make it productive as an efficient flow ele