Low concentrated green NaClO: influence of cathode material on kinetic regularities of electrolysis

Abstract

Kinetic regularities of cathode processes during electrolysis of low concentrated NaCl solutions have been investigated in this work. It was shown that the reduction of hypochlorite ion on platinum in the concentration range up to 0.015 M is limited by the diffusion stage as evidenced by linear dependence of the reciprocal limiting current density versus reciprocal square root of the electrode rotation rate. The diffusion coefficient of ClO– ions to the Pt-RDE surface was calculated from the Koutecki-Levich equation and it was 6.610–6 cm2 s–1. It was stated that the cathode material has a significant effect on the overvoltage of hypochlorite ion reduction. For example, on platinum, the beginning of the current increase corresponding to the reduction of ClO– is observed at +0.3 V, whereas it is –0.25 V on steel and nickel and –0.8 V on titanium. The preparation of a platinum electrode, which affects the texture and condition of the surface, also affects the reduction potential of the hypochlorite ion. Thus, the current increase is observed at +0.1 V on a molded and machined Pt-RDE, while the growth of the current is observed at +0.3 V on a flat Pt electrode etched in aqua regia. The potentials of the beginning of hydrogen evolution are practically indistinguishable. It was shown that the electrolysis of NaCl solutions in an undivided cell should be performed at the maximum possible cathode current densities in order to minimize sodium hypochlorite losses due to its reduction at the cathode and reduce the rate of chlorate accumulation.