How many electrons are transferred during the electrochemical O2 reduction in a Mg2+-free / Mg2+-containing ionic liquid?

Abstract

Aiming at a better understanding of the air electrode processes in Mg-air batteries, the influence of Mg2+ ions is studied on the oxygen reduction (ORR) and oxygen evolution (OER) reactions in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (BMP-TFSI) by differential electrochemical mass spectrometry (DEMS). The numbers of electrons transferred per reduced O2 molecule are determined in cyclic voltammetry (CV) and potential step measurements, both in absence and presence of Mg2+. The numbers of transferred electrons indicate that in neat BMP-TFSI a transition from superoxide to peroxide formation takes place when applying more negative potentials. In the presence of Mg2+, the formation of superoxide can be found in potential step measurements to potentials ≥ −0.4 V (vs. Mg/MgO) and in CV measurements on largely passivated electrode surfaces. In contrast, mainly peroxide is formed when stepping to −1.2 V (vs. Mg/MgO) and at the beginning of the ORR in CV measurements. The formation of superoxide in the presence of Mg2+, on the passivated electrode, is found to be partly reversible, as indicated by the evolution of O2 detected via DEMS.