Effect of the Ionic Liquid Additive-[BMIM]HSO4 on the Kinetics of Oxygen Evolution during Zinc Electrowinning

The effect of the ionic liquid additive 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM]HSO4) on the kinetics of oxygen evolution during zinc electrowinning from an acidic sulfate solution was investigated. We used potentiodynamic polarization, electrochemical impedance spectroscopy, scanning electron microscopy, and X-ray diffraction for this study. Potentiodynamic polarization curves and the corresponding kinetic parameter analysis show that [BMIM]HSO4 has a catalytic effect on oxygen evolution by stimulating the reaction rate constant. Impedance data reveal that [BMIM]HSO4 can markedly reduce the oxygen evolution charge transfer resistance. The addition of 5 mg . L-1 [BMIM]HSO4 obviously decreased the resistance value by at least 50% over the studied potential range from 1.85 to 2.10 V. In addition, the results of the impedance measurements also suggest an inhibition effect of [BMIM]HSO4 on the secondary reactions and this is due to the adsorption of the additive on the anode surface, which decreased the amount of active sites for anion adsorption. All electrochemical results were corroborated with a morphological and orientation analysis of the anodic surface after 120 h of anodic polarization. The addition of [BMIM]HSO4 inhibited the generation of the intermediate product beta-PbO2 and it promoted the generation of larger, loose, and porous alpha-PbO2, which benefited the oxygen evolution reaction.