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

Abstract

The influence of different additives on the oxygen reduction reaction/oxygen evolution reaction (ORR/OER) in magnesium‐containing N ‐butyl‐ N ‐methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([BMP][TFSI]) on a glassy carbon electrode was investigated to gain a better understanding of the electrochemical processes in Mg–air batteries. 18‐Crown‐6 was used as a complexing agent for Mg ions to hinder the passivation caused by their reaction with ORR products such as superoxide and peroxide anions. Furthermore, borane dimethylamine complex (NBH) was used as a potential water‐removing agent to inhibit electrode passivation by reacting with trace impurities of water. The electrochemical processes were characterized by differential electrochemical mass spectrometry to monitor the consumed and evolved O 2 in the ORR/OER and determine the number of transferred electrons. Crown ether and NBH efficiently masked Mg 2+ . A stochiometric excess of crown ether resulted in reduced formation of a passivation layer, whereas at too high concentrations the reversibility of the ORR/OER was diminished.

Mechanistic Aspects and Side Reactions during Reversible Mg Deposition and Oxygen Reduction on a Pt Film Electrode in BMP‐TFSI‐Based Electrolytes: A DEMS Study

“…It also agrees with previous findings by Eckardt et al. for the ORR/OER on a glassy carbon electrode in the same electrolyte, though with a different additive [14] . In that work the ORR products in Mg 2+ ‐containing BMP‐TFSI were concluded to be MgO and MgO 2 , where only the latter ones can be re‐oxidized to form O 2 [14] .…”

“…In this work we present and discuss results of a systematic online differential electrochemical mass spectrometry (DEMS) study on the reversible Mg deposition/stripping (in O 2 ‐free electrolytes) and O 2 reduction (ORR)/evolution (OER) (in O 2 ‐saturated electrolyte) on a Pt film electrode during potential cycling in four different BMP‐TFSI based electrolytes of different composition, containing different concentrations of Mg 2+ (as Mg(TFSI) 2 and Mg(BH 4 ) 2 ), of BH 4 − (as Mg(BH 4 ) 2 ) and of the crown ether 18‐c‐6 (see Table 1). This mainly differs from our previous studies [10–15] either by the different additive used as water scavenger, with BH 4 − in the present case and NBH in Ref. [14], or by the enforced presence of O 2 in the electrolyte, which was not the case in Ref.…”

“…In contrast to the rather regular hemispheric structures formed in electrolyte II (Figure 2e), the electrodeposit obtained in electrolyte III exhibits relatively large, irregular agglomerates (Figure 2h). Some of them have cauliflower‐like morphologies, similar to those reported for the Mg electrodeposits from BMP‐TFSI based electrolyte in the presence of O 2 [11,14] . The structures resolved here differ from the mossy, non‐dendritic morphology of the electrodeposits reported previously for deposition from a mixed Mg(TFSI) 2 , 18‐c‐6 and MPPp‐TFSI electrolyte (molar ratio 0.32 : 0.32 : 1.6 M), [27] where the mossy shape was attributed to the inclusion of organic fragments and impurities in the plated Mg [27] .…”

“…The sharp peak in the m/z=2 ion current, which coincides with the equally sharp reduction current peak, indicates that both result from a reduction reaction associated with H 2 formation such as reduction of H 2 O trace impurities. We cannot exclude, however, contributions from a change in the ORR selectivity, from a 1‐electron process to a 2‐electron process, which would perfectly agree with our previous data [12,14,59] . Interestingly, a comparable H 2 evolution was not observed in the O 2 ‐free electrolyte in this potential region (Figure 1a), indicating that the presence of O 2 activates the reductive hydrogen evolution from traces of water and possibly also from electrolyte components.…”

“…Recently we started a systematic study on the deposition/stripping of Mg and the reduction/evolution of O 2 from/in a room temperature ionic liquid (RTIL), specifically in 1‐butyl‐1‐ methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (BMP‐TFSI), [10–15] which will be continued in the present study. RTILs are attractive candidates as solvent in Li and post‐Li ion batteries because of their wide electrochemical stability window, low flammability and low vapor pressure, [16–18] where the latter is especially important for metal‐air batteries [19–24] .…”