Electrolysis of sodium chloride solutions on Ti/Pt anodes under current reversal conditions

Abstract

An increase in the current efficiency of hypochlorite up to 80-85% and a possible decrease in the current efficiency of chlorate up to 5-8% are observed if the electrolysis of NaCl occurs on a pre-reduced Ti/Pt electrode surface. However, the time of the electrolysis on a reduced surface is limited and significantly decreases with increasing current density. The need for the reduction of the surface every time before the electrolysis almost negates the practical value of the above results. The duration of the transition from the reduced surface to the oxidized state can be appreciably decreased in low-concentrated solutions of NaCl (0.15-0.45 M NaCl solutions are commonly used in practice to prepare hypochlorite solutions by direct membraneless electrolysis). For instance, the duration of the transition is 1200 s and only 80 s in 0.3 M and in 0.15 M solutions, respectively (at 20 mA cm-2). Moreover, it becomes possible to refuse the procedure of the pre-treatment of the anode and conduct the electrolysis at any duration on the reduced surface under current reversal conditions. If the electrolysis of 0.15 M NaCl is performed in a «usual» regime at 20-40 mA cm-2, then the current efficiency of hypochlorite does not exceed 26% and the current efficiency of chlorate is at least 21%. It is shown that carrying out the electrolysis under current reversal conditions allows increasing the current efficiency of hypochlorite up to 39% while reducing the current efficiency of chlorate to 5%. An increase in the concentration of NaCl to 0.3 M improves the performance of reverse electrolysis even more significantly. In this case, the current efficiency of ClO- formation increases to 78% and the current efficiency of ClO3- formation decreases to 2%.