The mechanism of Li2O2-film formation and reoxidation – Influence of electrode roughness and single crystal surface structure

Abstract

In this work, the role of the electrode surface structure on oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in lithium ion containing dimethyl sulfoxide (DMSO) was elucidated for Pt single crystal electrodes and deliberately roughened polycrystalline gold and platinum electrodes. Both rotating ring disk electrode (RRDE) measurements and results obtained by differential electrochemical mass spectrometry (DEMS) show that the amount of insoluble Li 2 O 2 formed is proportional to the roughness factor and corresponds to one or two monolayers if referenced to the true surface area. The amount of soluble LiO 2 is independent of roughness, although the formation of the passivating Li 2 O 2 layer takes longer. On gold electrodes less side products are formed as compared to platinum electrodes. It was furthermore found that ORR on single crystalline Pt(100) leads to two monolayers of Li 2 O 2 , while in case of Pt(111)/Pt(pc) only one monolayer was reoxidized. The shape of the reoxidation peaks in the anodic potential scan is independent of roughness, but it depends not only on the electrode material (Au or Pt) but also on the atomic surface structure. Since Tafelslopes cannot be determined directly by usual methods for such surface confined reactions, we used ac voltammetry to determine the apparent transfer coefficient ( α′ ) and gain some insight into the reoxidation mechanism; the obtained α ′ of 0.3 hints to a rate determining first charge transfer. This agrees well with the detection of superoxide during oxidation of Li 2 O 2 on the surface. • Strong effect of electrode surface structure on electrochemistry in Li + -containing DMSO. • Superoxide as intermediate during lithium peroxide oxidation in DMSO. • Apparent transfer coefficient of lithium peroxide oxidation in Li + -containing DMSO. • Direct correlation of lithium peroxide formation and surface roughness.