Critical potential and oxygen evolution of the chlorate anode

Abstract: In the chlorate process, natural convection arises thanks to the hydrogen evolving cathode. This increases the mass transport of the different species in the chlorate electrolyte. There is a strong connection between mass transport and the kinetics of the electrode reactions. A better knowledge about these phenomena and their interactions is desirable in order to understand e.g. the reasons for deactivation of anode coatings and what process conditions give the longest lifetime and the highest efficiency. One of the aims of his work was to understand how the chlorate process has to be run to avoid exceeding the critical anode potential (Ecr) in order to keep the potential losses low and to achieve a long lifetime of the DSAs. At Ecr anodic polarisation curves in chlorate electrolyte bend to higher Tafel slopes, causing increasing potential losses and accelerated ageing of the anode. Therefore the impact on the anode potential and on Ecr of different electrolyte parameters and electrolyte impurities was investigated. Additionally, the work aimed to investigate the impact of an addition of chromate on oxygen evolution and concentration profiles under conditions reminiscent of those in the chlorate process (high ionic strength, 70 °C, ruthenium based DSA, neutral pH), but without chloride in order to avoid hypochlorite formation. For this purpose a model, taking into account mass transport as well as potential- and concentration-dependent electrode reactions and homogeneous reactions was developed. Water oxidation is one of the side reactions considered to decrease the current efficiency in chlorate production. The results from the study increase the understanding of how a buffer/weak base affects a pH dependent electrode reaction in a pH neutral electrolyte in general. This could also throw light on the link between electrode reactions and homogeneous reactions in the chlorate process. It was found that the mechanism for chloride oxidation is likely to be the same for potentials below Ecr as well as for potentials above Ecr. This was based on the fact that the apparent reaction order as well as αa seem to be of the same values even if the anode potential exceeds Ecr. The reason for the higher slope of the polarisation curve above Ecr could then be a potential dependent deactivation of the active sites. Deactivation of active ruthenium sites could occur if ruthenium in a higher oxidation state were inactive for chloride oxidation. Concentration gradients of H+, OH-, CrO4 2- and HCrO4 - during oxygen evolution on a rotating disk electrode (RDE) were predicted by simulations. The pH dependent currents at varying potentials calculated by the model were verified in experiments. It was found that an important part of the chromate buffering effect at high current densities occurs in a thin (in the order of nanometers) reaction layer at the anode. From comparisons between the model and experiments a reaction for the chromate buffering has been proposed. Under conditions with bulk pH and chromate concentration similar to those in the chlorate process, the simulations show that the current density for oxygen evolution from OH- would be approximately 0.1 kA m-2, which corresponds to about 3% of the total current in chlorate production.