Catalysts in metal-air batteries
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Prospective Article
Catalysts in metal–air batteries Qi Dong and Dunwei Wang, Department of Chemistry, Merkert Chemistry Center, Boston College, 2609 Beacon St., Chestnut Hill, MA 02467, USA Address all correspondence to Dunwei Wang at [email protected] (Received 16 January 2018; accepted 2 April 2018)
Abstract Metal–air batteries promise higher energy densities than state-of-the-art Li-ion batteries and have, therefore, received significant research attention lately. The most distinguishing feature of this technology is that it takes advantage of reversible conversion reactions of O2 or other air components (such as N2 or CO2) at the cathode. To promote these reactions, catalysts are often needed. A large number of materials have been studied for this purpose. In the present paper, we discuss the roles played by catalysts in metal–air battery systems. In particular, we choose to focus the discussions on the Li–O2 batteries as they are most intensely studied in the literature. Within this context, catalysts are often shown effective to facilitate the oxygen (O2) reduction reactions and/or O2 evolution reactions. The overall cell performance as measured by the round-trip efficiencies and charge/discharge rates can be significantly improved by the incorporation of catalysts. However, the presence of catalysts is also found to complicate the chemical reactions as they often exhibit activities toward parasitic chemical reactions such as electrolyte and electrode decompositions. The issue is especially acute in aprotic Li–O2 batteries, where organic electrolytes and reactive O2 species are mixed. In addition to heterogeneous catalysts, we also discuss the roles played by homogeneous catalysts as redox mediators, which are effective to promote redox reactions that are critical to energy storage applications.
Introduction Modern society has benefited tremendously from the invention and popular utilization of Li-ion batteries.[1] One signature characteristic of Li-ion batteries is the outstanding rechargeability, with state-of-the-art commercial cells capable of thousands of cycles of repeated charge/discharge. This remarkable feat is made possible by the chemical nature of the charge/discharge processes: it is essentially an intercalation one. Both the anode and the cathode materials feature ionic channels that permit Li+ ions to go in and/or out at ease with minimum impacts to the structural and chemical integrity of the host materials. This feature, however, also unfortunately sets a limit to another important metric of battery performance, the maximum achievable capacities.[2,3] Significant price has to be paid in terms of structural complexities in order for the reversible intercalation to take place. Consider high-performance cathode material such as LiFePO4 as an example.[4] Heavy transition metals like Fe are necessary but do not directly contribute to the chemical conversion upon Li+ insertion and extraction. As a result, Li-ion batteries have almost reached their theoretical limit in terms of performance metrics (e
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