Ionic Liquid Electrolytes for Metal-Air Batteries: Interactions between O₂, Zn²⁺ and H₂O Impurities

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 O 2 and Zn(TFSI) 2 as well as the influence of H 2 O 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 O 2 molecule. In O 2 saturated neat BMP-TFSI the ORR and OER were found to be reversible, in Zn 2+ containing IL Zn deposition/stripping proceeds reversibly as well. Simultaneous addition of O 2 and Zn 2+ suppresses Zn metal deposition, instead ZnO 2 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 O 2 and Zn 2+ in the water containing electrolytes. Consequences for the operation of Zn-air batteries in these electrolytes are discussed.

Influence of Additives on the Reversible Oxygen Reduction Reaction/Oxygen Evolution Reaction in the Mg2+‐Containing Ionic Liquid N‐Butyl‐N‐Methylpyrrolidinium Bis(Trifluoromethanesulfonyl)imide

“…[36,40,41] Between À0.6 and À1.2 Vasecond plateau appeared with approximately twice the current density,w hich corresponds to at wo-electron transfer and thus related to the formation of ap eroxide. [25,34,42] Decreasingthe potential furtherresulted in an increaseinthe current density with unchanged oxygen consumption.T herefore, we attribute the current increase to side reactions such as the decomposition of the IL or reduction of residual water impurities in the electrolyte. [25,29,34] Comparing the ORR in pure [BMP][TFSI] with the ORR in Mg(TFSI) 2 -containing IL, am uch lower ORR current density was observedi nt he first cycle.…”

“…[25,34,42] Decreasingthe potential furtherresulted in an increaseinthe current density with unchanged oxygen consumption.T herefore, we attribute the current increase to side reactions such as the decomposition of the IL or reduction of residual water impurities in the electrolyte. [25,29,34] Comparing the ORR in pure [BMP][TFSI] with the ORR in Mg(TFSI) 2 -containing IL, am uch lower ORR current density was observedi nt he first cycle. However,t here seemed to be two current plateaus, one between 0a nd À0.5 Vand the other appears to start at À0.9 V. However,i nt his case the current quickly decayed at more negative potentials.T he oxygen consumption stayed approximately constant in the ORR potential window.I nt he presence of Mg(TFSI) 2 ,t he number of electrons transferredw as approximately 1.5 in the potential region of the first plateau, indicating that the product at least partly consisted of superoxides.…”

“…Complexing Mg 2 + on the cathode side might help "masking" of the magnesium ions for the ORR, mimicking aM g 2 + -free electrolyte and, thus, hindering or even suppressing the formation of ap assivation layer.T his would allow ar eversible ORR/OER, as observedi nn eat [BMP][TFSI]. [25,28,34,36] In that electrolyte, the ORR proceeds in two reaction steps, formings uperoxide first, followed by the formation of peroxide at lower potentials.…”

“…This allowed us to determine the amount of O 2 consumed and evolvedi nt he ORR and the OER, respectively,a nd calculate the number of electrons transferred per O 2 molecule in the reaction, which provides detailed insights into the processes on the electrode. [25,28,34] The solubility of Mg(TFSI) 2 in THF-based electrolytes has been improved with the addition of 18-crown-6, which also leads to an increasei nt he ion conductivity. [35] Using the IL Nmethyl-N-propylpiperidinium-TFSI ([PMP][TFSI]),apromoting effect on the Mg platingw as reportedb yS agane et al fora Mg(TFSI) 2 /[PMP][TFSI] = 1:5mol electrolyte.…”

“…Considering the effects of trace impurities of water,r esidual water in the electrolyte was shown to have ap romoting effect on ORR in aprotice lectrolyte. [34,[37][38][39] It also enhances the formation of non-oxidizable magnesium peroxides and oxidesi n Mg 2 + -containing electrolyte in the ORR, thus passivating the electrode. [14] Therefore, we also explored the effect of boron hydride as aw ater removing additive in the electrolyte.…”