Current efficiency and mass transfer effects in electrochemical oxidation of C1 and C2 carboxylic acids on boron doped diamond electrodes

The oxidation of acetic, glycolic, glyoxylic, oxalic and formic acid has been studied on boron doped diamond electrodes. Our voltammetry study on rotating disk electrodes highlights clear differences between the carboxylic acids (formic, oxalic, glyoxylic and glycolic acid) that can be oxidized via a direct electron transfer (DET) and acetic acid which can presumably solely be oxidized by •OH radicals formed in the region where water oxidation takes place. In glycolic and glyoxylic oxidation oxalic acid is the main intermediate formed. Surprisingly, glyoxylic acid could not be detected as an intermediate in the glycolic acid oxidation. Chronoamperometric experiments confirm that all compounds except acetic acid react further to CO₂, which could be deduced from the electron balances. It was shown that formic, oxalic, glyoxylic and glycolic acid can be selectively oxidized at 2.3 V vs. Ag/AgCl with high current efficiencies, below or close to mass transfer limiting rates. At higher potentials (2.4 V and 2.5 V) simultaneous water electrolysis results in lower current efficiencies. At these potentials the conversion rates can exceed the limiting rates, which might be attributed to effects related to water oxidation (i.e. O₂ evolution and •OH radical formation).