Electrocatalytic Oxygen Reduction and Oxygen Evolution in Mg‐Free and Mg–Containing Ionic Liquid 1‐Butyl‐1‐Methylpyrrolidinium Bis (Trifluoromethanesulfonyl) Imide

Abstract

Aiming at a better understanding of the air electrode processes in Mg‐air batteries, we have investigated the activity of different electrode materials, viz., Pt, Au, glassy carbon (GC) and manganese (III)‐ and (IV)‐oxides (Mn2O3 and MnO2) for the electrocatalytic oxygen reduction (ORR) and oxygen evolution (OER) reactions in the ionic liquid 1‐butyl‐1‐methylpyrrolidinium bis (trifluoromethanesulfonyl) imide (BMP‐TFSI) and the influence of Mg2+ thereon. Employing planar model electrodes in a rotating ring disk electrode (RRDE) setup, we used cyclic voltammetry at different rotation rates to test the reversibility of these reactions and quantify the number of electrons transferred during the ORR. Reversible ORR and OER are observed for all electrodes in neat BMP‐TFSI electrolyte, while in the Mg2+ containing electrolyte (Mg‐TFSI2) the OER is strongly hindered for all electrode materials. This goes along with the rapid build‐up of a passivation layer during cycling, which increasingly inhibits also the ORR. The morphology and chemical composition of the passivation layer were characterized by scanning electron microscopy (SEM) and X‐ray photoelectron spectroscopy (XPS), indicating that MgO2 is the main product formed by the ORR.

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

“…In non-aqueous electrolytes, the role of the electrocatalyst on the reaction mechanism is less understood. Comparing GC, Au, 32 Pt and manganese oxide catalysts, 33 no significant enhancement of the ORR/OER has been found in dry BMP-TFSI with and without the addition of Mg 2+ . This observation may be explained by very weak interactions between the electrode surface and the superoxide anion, resulting in an outer sphere reaction mechanism.…”

“…A Mg wire (Goodfellow, 99.99+, diameter 0.25 mm) with a native oxide film (Mg/MgO; −1.0 vs. Fc/Fc + ) served as a quasi-reference electrode. 33 The sensitivity factor k * of 3.4 × 10 −7 of the DEMS measurements was determined assuming a one electron transfer (n = 1) for the first reduction step (E > −0.4 V) of the ORR in neat BMP-TFSI, using the ratio between the ion current of the m/z 32 signal (I 32 ) and the faradaic current (I F ) according to the equation k * = nI MS /I F . 25 Note that the ion currents plotted in following figures are corrected for the time delay due to mass transport of the gaseous species from the working electrode to and through the membrane.…”

“…A similar shift of the ORR onset potential was already observed previously upon the addition of Mg 2+ addition on a GC electrode. 25,32,33 The lower currents in the second cycle of each potential window indicate that there is some electrode passivation. During the ORR, two electrons per O 2 molecule are transferred, which points to the formation of O 2 2− and, in the presence of Zn 2+ , to the formation of deposited ZnO 2 .…”

“…This is different from the addition of Mg 2+ (0.1 M), where MgO 2 was found as the main product of the ORR. 33 However, also side processes which do not consume O 2 , such as the Zn deposition (which at least in the absence of O 2 occurs at these potentials) or electrolyte decomposition could contribute to the higher number of transferred electrons. We can rule out, however, H 2 formation, which could be expected from reduction of water traces.…”

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

“…[18][19][20][21][22] However,a pplying ILs as electrolytes for Mg-air batteries (IL containing Mg 2 + )h as been shown to result in as low deposition/dissolution of Mg on the anode and as trong passivation on the cathode, whereby the latter resultsi nastrongly limitedO RR/OER reversibility. [23][24][25][26][27][28] Furthermore, on the anode side, strong interactions between Mg 2 + and bis(trifluoromethanesulfonyl)imide (TFSI À )w ere speculated to cause ah igh overpotential for Mg deposition, resultingi nt he decomposition of the electrolyte, which in turn leads to the formation of ap assivating film on the anode. [23,29] It is well knownf rom pre-…”