Effect of Pyridine on Zinc Electrolysis Parameters at Different Current Densities

The effect of pyridine on the parameters of electrolysis in acidic and neutral solutions of zinc sulfate was studied in a wide range of potentials and current densities. The effect of pyridine additives on the parameters of cationic discharge from the zinc sulfate electrolytes, including sulfuric acid, was analyzed. The zinc-to-sulfuric acid mass ratio in one of the three electrolytes corresponded to the composition of the commercial solutions utilized for zinc electrolysis. It was shown that an increase in the electrolyte acid content and cathode potential results in higher cation discharge rate, while an addition of pyridine causes it to decrease. The obtained experimental data evidence the preferential effect of pyridine on hydrogen cation discharge in zinc-containing electrolyte solution. It was noted that an increase in overvoltage accelerates quicker in the solution containing pyridine additives at a current density (j) in excess of 300 A/m² , while without such additives it actually slows down. This is associated with the ability of pyridine additive to protonate and absorb a significant amount of hydrogen causing a stronger inhibition of hydrogen discharge compared to zinc in the areas with high current density. It was shown that within certain regions of the voltammetric curves with the current density exceeding 300 A/m² , the exchange current (i_ex ) increases by about nine times at elevated cathode potentials and increased pyridine concentrations to 0.3 mg/liter. However, when the pyridine concentration increases to 0.6 mg/liter, the exchange current, same as the rate of ionic discharge, decreases, which is consistent with the Butler-Volmer equation for a cathodic process at high overvoltage values associated with increased current densities. The calculations of the transfer coefficients made it possible to conclude that at current densities of 600 A/m² and higher, the transfer coefficient approaches zero, and the process of cationic discharge from acidic electrolytes with the addition of pyridine proceeds in an activationless mode. The results of the conducted study have led to the following important conclusion in terms of practical use of zinc electrolysis: in order to exclude the negative effect of pyridine, the process should be conducted below the current densities of 300–350 A/m².