Electrochemical CO2 conversion reactions have a growing role to play in mitigating rising emissions and identifying novel end-uses for CO2. Recent years have witnessed increasing scientific interest in CO2 transformations in nonaqueous media, where typical end products are solids such as mineralized carbonates, carbon monoxide, and/or carbon. Such transformations could find utility in future nonaqueous energy storage and conversion devices, such as electrochemical reactors, as well as alkali metal-based O2 /CO2 and -CO2 batteries. Electrochemical reduction of largely unreactive gases like CO2 by alkali metals (typically Li) in nonaqueous electrolytes presents a promising approach because the highly reducing nature of Li, coupled with the wide voltage window in nonaqueous electrolytes, enables access to a wide energetic landscape for accessing activity and controlling reaction pathways. However, the large kinetic barriers associated with CO2 activation have presented significant hurdles such that only limited systems – namely those utilizing glyme solvents or ionic liquids - have reported any reduction activity of CO2 to date. In this work, we report an amine-mediated CO2 capture and conversion process for enhancing the kinetics of CO2 discharge reactions in nonaqueous electrolytes. Our approach utilizes an alkyl amine in a Li+-containing electrolyte to activate CO2 via chemisorption prior to electrochemical conversion. Our results indicate that electrochemical reduction of CO2-loaded amines proceeds at significantly higher discharge potentials (~3 V vs. Li) compared to physically dissolved CO2, which exhibits negligible activity without the amine present. Moreover, the discharge reaction also yields high discharge capacities (&gt; 1000 mAh/gc). We attribute the improved kinetic performance of CO2 reduction in the presence of amines to the chemical absorption step which by creating new electroactive species in the form of an amine-CO2 complexes, successfully avoids the rate-limiting step in classical electrochemical CO2 reduction (electron-coupled formation of the CO2- radical). To elucidate the reaction mechanism, we analyze all discharge phases present within the Li-CO2 cell. We also study the variation in cell discharge potentials and capacities as a function of discharge rate and amine concentration. Based on our results, we discuss the coupling of CO2 capture chemistry to non-aqueous electrochemistry as a new avenue for tailoring CO2 conversion reactions.

Tailoring the discharge reaction in Li-CO2 batteries through incorporation of CO2 capture chemistry

256th National Meeting (2018)

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