Abstract
Motivated by the potential of ionic liquids (ILs) to replace traditional aqueous electrolytes in Zn-air batteries, we investigated the effects arising from mutual interactions between O2 and Zn(TFSI)2 as well as the influence of H2O impurities in the oxygen reduction/oxygen evolution reaction (ORR/OER) and in Zn deposition/dissolution on a glassy carbon (GC) electrode in the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide (BMP-TFSI) by differential electrochemical mass spectrometry. This allowed us to determine the number of electrons transferred per reduced/evolved O2 molecule. In O2 saturated neat BMP-TFSI the ORR and OER were found to be reversible, in Zn2+ containing IL Zn deposition/stripping proceeds reversibly as well. Simultaneous addition of O2 and Zn2+ suppresses Zn metal deposition, instead ZnO2 is formed in the ORR, which is reversible only after excursions to very negative potentials (−1.4 V). The addition of water leads to an enhancement of all processes described above, which is at least partly explained by a higher mobility of O2 and Zn2+ in the water containing electrolytes. Consequences for the operation of Zn-air batteries in these electrolytes are discussed.
Highlights
The major advantage of primary Zn-air batteries is their high energy density, which is about five times higher than the one of Li-ion batteries.[1,2,3] primary Zn-air batteries are already commercially used in applications where their low power output capability (
The reversibility of the oxygen reduction reaction (ORR) and of the oxygen evolution reaction (OER) during discharge and charge of the battery is a key issue for improving Zn-air batteries.[2,6,7,8,9,10]
Katayama et al observed an anodic peak in cyclic voltammograms (CVs) in O2-saturated, humidified (0.5 vol% of water) EMIM-TFSI after the reduction reaction and attributed this to the re-oxidation of either HO2 or H2O2.22 This anodic peak occurred at higher potentials compared to dry conditions, indicating that the addition of water hinders the OER.[22]
Summary
The major advantage of primary Zn-air batteries is their high energy density, which is about five times higher than the one of Li-ion batteries.[1,2,3] primary Zn-air batteries are already commercially used in applications where their low power output capability (
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