Ruthenium-based dimensionally stable anode in chlorate electrolysis -: Effects of electrolyte composition on the anode potential

In this work the anodic reactions taking place on a dimensionally stable anode (DSA) in chlorate electrolyte have been investigated. Rotating disk electrodes were made from commercial RuO2-catalyzed DSAs and studied in steady-state polarization measurements, mainly IR-corrected polarization curves. Effects of varying pH and electrolyte concentrations of chloride, chlorate, chromium(VI), hypochlorite (ClO- + ClOH) as well as mass transport were studied. The kinetics for the chlorine evolution reaction, with a Tafel slope of 40 mV/decade of current, was not dependent on pH in the region 2-8, at potentials lower than 1.2 V vs. Ag/AgCl. The slope of the polarization curves increased at about 1.2 V vs. Ag/AgCl, a pH-dependent bend not due to mass-transport limitations in the electrolyte. At a pH of 6.5, typical for the chlorate process, oxygen evolution is an important side reaction favored by the dichromate buffer and by increased mass transport, both keeping down the pH at the anode. In the chlorine evolution region the potentials increased when adding Cr(VI) to the electrolyte, whereas no major effect was seen from additions of NaClO. (C) 2002 The Electrochemical Society.