Tuning the course of the oxygen reduction reaction at a carbon electrode using alkaline electrolytes based on binary DMSO–water solvent mixtures

We studied the oxygen reduction reaction (ORR) at a glassy carbon electrode in KOH electrolytes based on DMSO–water solvent mixtures of various compositions (ranging from 0.0 to 93.3 vol% DMSO). The results of RDE (rotating disk electrode) experiments show that by changing the composition of the DMSO–water solvent mixture, one can control the course of oxygen electroreduction. Specifically, in the absence of dimethyl sulfoxide, the ORR at −1.5 V vs. Hg/HgO proceeds mainly as a two-electron process (n = 2.28 at 400 rpm), whereas with increasing DMSO content the ORR shifts gradually towards complete four-electron reduction. This tendency continues until the maximum electron transfer number is reached (n = 3.70 at 400 rpm) in an alkaline electrolyte based on a 1:1 H2O:DMSO (v/v) solvent mixture. The effect of DMSO addition on the course of the ORR process at the carbon electrode is thought to arise from the unique physicochemical properties of DMSO–water systems, i.e. their high viscosity and polarity as a result of H-bonded intermolecular complexes formed between water and DMSO molecules.