The electrochemical reduction of oxygen plays a significant role in many critical applications such as gas sensors, hydrogen peroxide electrosynthesis, and electrochemical energy storage. Oxygen reduction reaction (ORR) drives the operation of fuel cells and metal-air batteries. The latter potentially can provide the highest specific energy among energy storage devices.
To increase the ORR efficiency, a catalyst immobilized on (or mixed with) conductive support is introduced to the positive electrode composition. Usually, porous sp2-carbon materials, like graphene, serve as such supporting materials. Its electronic configuration (sp2) provides the sufficient electric conductivity to the positive electrode. Nevertheless, ORR proceeds too slowly on the neat surface of ideal sp2-carbon in the absence of a catalyst.
The role of graphene imperfections (vacancies, impurity atoms, and functional groups) on catalyzing ORR (mainly in aqueous media) has been under intense investigation during the last decades. However, little is known about the effect of oxygen functionalization of carbon onORR in aprotic media (lacking the acidic protons). The interest in this process, especially in the presence of metal ions in the electrolyte, is relevant for various aprotic metal-oxygen batteries (lithium, sodium, magnesium, etc.) which are now considered as the most promising electrochemical power sources due to their outstanding theoretical performance. For such devices carbon electrodes are highly attractive due to their light weight and low cost, and the effect of carbon surface chemistry on the processes occurring upon battery operation is of great importance.
The present research shows for the first time that oxygenation of carbon electrode surface does not affect the rate of one-electron oxygen reduction in aprotic media. At the same time, in Li+-containing electrolytes, oxygen groups enhance both the rate of electrochemical Li2O2 formation and carbon electrode degradation due to faster oxidation by lithium superoxide (LiO2) intermediate yielding carbonate species as a product.
The research is led by scientists from Lomonosov Moscow State University and the Semenov Institute of Chemical Physics, in collaboration with Friedrich–Alexander–Universität Erlangen-Nürnberg, IFW Dresden, Saint Petersburg State University, Donostia International Physics Center and Massachusetts Institute of Technology.
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Image: C 1s core level spectra of a) pristine and b) oxidized graphene electrodes before and after discharge. C) Model spectroelectrochemical Li-O2 cell. D) Evolution of C 1s components’ ratios upon discharge for pristine and oxidized graphene.