A Few Case Studies of CO2 and O2 Electrocatalysis in Energy Storage

Abstract

Li-ion batteries has been a resounding success in the domain of portable applications such as electronic devices and are being increasingly explored in electric vehicles and grid storage. Conventional Li-ion batteries work on intercalation chemistry in which Li-ion shuttle into and out of the cathode and anode. However, intercalation chemistry offers limited scope for upscale in terms of both size and energy output. As the global energy requirements are increasing, there is a need to develop battery systems with higher energy density at low cost. So, one must look beyond the state-of-the-art LIBs. Among beyond Li-ion battery chemistries, an increasingly important and promising strategy has been to explore the redox properties of simple gaseous molecules e.g., O2, CO2, N2 in energy storage. The very high specific energy associated with gaseous cathodes make them frontline candidates for the next generation batteries. Despite the numerous advantages, the electrochemistry of gases poses several fundamental challenges, which need to be addressed prior to their wide commercialization. The foci of research in gaseous cathode-based batteries are primarily to tackle the detrimental issues tending to undermine the cyclability and efficiency of the system. Among gaseous batteries, Li-CO2 batteries are one of the promising candidates for the next generation high energy batteries. It not only fixes CO2 (a greenhouse gas) in an electrochemical system leading to a novel energy storage system, but additionally constitutes an alternate strategy of CO2 reduction to value added products like Li2CO3. The energy density of Li-CO2 battery is 1876 Whkg-1 which is nearly one-order higher than the advanced Li-ion battery. Typical non-aqueous Li-CO2 battery configuration involves the negative Li anode separated by the electrolyte-soaked separator from the positive porous substrate loaded with electrocatalyst. During discharge, CO2 diffuses through porous channels of substrate and reduces at the electrolyte/electrocatalyst interface to Li2CO3 and C. This reaction proceeds via the formation of metastable Li2C2O4 which eventually converts to Li2CO3. During charge, theoretically, CO2 should fully evolve back. However, discharge product Li2CO3 is a wide band gap insulator with very high Gibbs free energy of formation (−1132.12 kJ mol−1 ) which makes charging energy intense. High potential charging results in sluggish CO2 evolution reaction with incomplete conversion of Li2CO3 to Li and CO2. This incomplete dissociation leads to the depletion of the electrochemical performance of the battery with time. All these detrimental matters have been tackled via the design and development of electrocatalysts, which are majorly immobilized on the porous-gaseous cathode. In the present thesis, various strategies are employed to address these challenges with primary emphasis on efficient electrocatalysis for a rechargeable and viable CO2 battery. Chapters 2-5 are entirely dedicated to the Li-CO2 battery system. Here, we have explored various kinds of homogeneous liquid (soluble Cu-compound) as well as solid heterogenous electrocatalyst (MXene, CNT) to enhance the sluggish kinetics of the CO2 redox process. The formation and decomposition of the conversion products have been probed extensively by ex-situ electrochemical, structural (including synchrotron) and spectroscopic techniques. Additionally, CO2 crossover towards Li anode and its effect on the solid electrolyte interface has been investigated using various spectroscopic and microscopic techniques. In addition to this, we have investigated the performance of dimethyl formamide (DMF) as an alternate solvent for the CO2 battery with Li free battery.