Separating kinetics and mass transfer in formic acid and formate oxidation on boron doped diamond electrodes

This paper describes the electrochemical oxidation of formic acid (pH 2) and formate (pH 6 and pH 10) on boron doped diamond electrodes and separates kinetic and mass transfer information using RDE and flow cell experiments. The voltammetry experiments show that the oxidation of formic acid and formate takes place before water oxidation, which indicates that the oxidation takes place via a direct electron transfer (DET) mechanism. The current densities measured in the linear sweep voltammograms were in line with the expected limiting current densities confirming mass transfer limitations. During chrono-amperometric experiments current densities were significantly lower, indicating a decrease in the kinetic rates. This is probably related to a surface modification at the electrode. This modification is reversible as bringing the electrode to low potentials restores the activity. The stable currents observed in the chronoamperometry experiments in the region before water oxidation indicate that there is probably a second DET mechanism, which has a much higher overpotential and a different pH dependency than the first DET mechanism. Koutecky-Levich plots showed clear mixed kinetic and mass transfer control and were used to deduce the Tafel slopes of this second mechanism. The observed high Tafel slopes of 240–300 mV/dec are in line with values reported for other reactions on BDD. Experiments in a parallel plate electrolyser show that the current efficiency is approximately 100% at 2.3 V for all pHs. At higher potentials current efficiency decreases due to the water oxidation. Interestingly at these potentials the formic acid oxidation exceeds the limiting current density, possibly related to the diffusion of ·OH radicals or other oxidation mediators to the bulk solution or the formation of oxygen bubbles on the electrode.