An Experimental Study of Oxygen Evolution and Mass Transfer at Microelectrodes

Abstract

It is well known that electrolytic gas generation at electrodes in an electrochemical reactor can affect ohmic resistance, dispersion and mass transfer in the cell. Many published studies of these phenomena employed electrodes of a few cm in size. In this work, experiments were conducted to study oxygen evolution and mass transfer at both vertical and horizontal microelectrodes of 0.05 to 5 mm in diameter. Electrode orientation was found to have some effect on oxygen evolution, mainly at higher gas evolution. When there was mass transfer with co-evolution of oxygen at larger microelectrodes, generally higher mass transfer coefficients were observed in vertical than in horizontal position. However, orientation had no effect in the case of microelectrodes smaller than 0.5 mm. Also, mass transfer diminished drastically at gas current density ≥300 mA cm^–2 due to single-bubble shielding of the electrode surface completely. Experiments on single-bubble generation at a 0.05 mm microelectrode showed that bubble generation could be regular under certain conditions. Relatively smaller bubbles formed more frequently at a vertical than a horizontal electrode. Results of mass transfer enhancement by passing gas bubbles confirm that this effect at microelectrodes too is smaller than the enhancement due to co-evolution of gas.