Electrochemical Oxidation of Methanol and Small Polyols in Neutral Media: The Effect of the Interfacial pH on Dynamic Instabilities.

The electrochemical oxidation of alcohols and polyols is interesting to feed direct alcohol fuel cells or to provide both protons and electrons to H2 production in electrolyzers. While these small organic molecules are oxidized in a wide pH range, oscillatory phenomena are commonly observed only in acidic media, and the phenomenon is restricted for few molecules in neutral or alkaline media. Herein, the electrochemical oxidation of methanol, ethylene glycol, and glycerol was studied in unbuffered Na2SO4 solutions (pH 8.2) by means of cyclic voltammetry, chronoamperometry, and chronopotentiometry. Changes of the near-surface solution pH (NSSpH) were estimated by mathematical modeling, revealing changes up to 6.5 pH units and a 4 mm depletion layer after 100 s of polarization. NSSpH changes provide an acidic environment to alcohol/polyol oxidation as observed by means of the oscillations features. Facilitating the mass-transport condition, NSSpH becomes close to the bulk, in which both the low potential activity and the potential oscillations are no longer observed. Applying mathematical modeling to the galvanostatic timeseries, it was possible to evaluate the changes in NSSpH and to conclude that the observed oscillations are due to the pH decrease triggered by the alcohol/polyol oxidation.