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 Mg 2+ 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 O 2 molecule are determined in cyclic voltammetry (CV) and potential step measurements, both in absence and presence of Mg 2+ . 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 Mg 2+ , 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 Mg 2+ , on the passivated electrode, is found to be partly reversible, as indicated by the evolution of O 2 detected via DEMS.