A Polarity Switching Technique for the Efficient Production of Sodium Hypochlorite from Aqueous Sodium Chloride Using Platinum Electrodes

A novel, periodic polarity-switching technique is developed for the electrolytic production of sodium hypochlorite from aqueous sodium chloride. The technique employs a flow-through cell having two identical platinum electrodes. Polarities of these electrodes are periodically swapped. The main purpose of this step is to prevent the growth of platinum oxide film and thereby retain high initial activity of the electrodes throughout the operation. The performance of this mode of operation is evaluated using the cumulative productivity and current efficiency of the cell as indicators. Typical ranges of parameters used are as follows: concentration of Cl- ions, 0.10.5 M; switching frequency, 0.011 Hz; and current density, 100700 A m(-2). The productivity of the switching mode is 56 times higher and its current efficiency is 15%20% higher than that of the DC mode. The switching frequency has a strong influence on both the cumulative production rate and the average current efficiency and an optimal switching frequency exists under the given operating conditions. At high frequencies, the major side reaction is oxidation of the adsorbed hydrogen. As the concentration of NaCl increases, the optimal switching frequency decreases, and both the cumulative productivity and the average current efficiency increase. At the optimal frequency, the current efficiency of the switching mode varies from 60% to 95% as the concentration of NaCl- is varied from 0.1 M to 0.5 M. The higher rate of circulation of the electrolyte improves the cell performance. The main cause for the loss in the current efficiency at high conversions of Cl ions is the reduction of the hypochlorite ions at the cathode, which occurs under diffusion limiting conditions.