Improvement of Electrochlorination Efficiency using Oxygen-Evolution-Suppressing Anodes in Dilute Chloride Solutions

Abstract

In this study, the IrOy∙FeOz and IrOa∙CoOb anodes were fabricated to improve the current efficiency of chlorine evolution reaction (CER) in dilute chloride solutions by lowering the activity of oxygen evolution reaction (OER). Dimensionally stable anodes (DSAs) are regarded to be optimized electrodes for electrochlorination owing to their excellent electrocatalytic activity for the CER and reliable stability. However, in dilute chloride solutions, DSAs preferentially produce oxygen rather than chlorine because of their low overpotential for OER. Considering the frequent use of electrochlorination in dilute conditions, the poor efficiency of DSAs severely limits their environmental and industrial applications. Although there are several attempts to maintain a high concentration of chloride such as a continuous supply of synthesized brine or seawater, these require additional facilities and costs. Furthermore, in consideration of the global trend of a small-scaled, decentralized water treatment system, electrochlorination in dilute conditions becomes increasingly important. Therefore, this study aims to improve the electrochlorination efficiency in dilute chloride solutions by suppressing the competitive reaction of CER, i.e. OER. Herein, iron oxide and cobalt oxide were used as an OER-suppressing catalyst to improve the CER efficiency of DSAs in dilute chloride solutions. Iron oxide (FeOx) is well-known to have an extremely slow reaction rate of OER with a high OER overpotential. Meanwhile, cobalt oxide (Co3O4) was reported to have a much higher working potential for OER than that for CER, which can be interpreted as Co3O4 has high selectivity for CER against OER. Furthermore, Co3O4 showed a relatively larger difference in the working potential between the OER and CER compared with other metal oxides. These characteristics of FeOx and Co3O4 can be an indication that they have the potential to improve the CER efficiency in dilute chloride solutions. Although there have been many trials to develop novel anodes with various transition metals, the lifetime of the anodes was too short. Therefore, a small amount of IrO2 was used as a co-catalyst to enhance the stability of FeOx and Co3O4, resulting in the IrOy∙FeOz and IrOa∙CoOb anodes fabricated by the thermal decomposition method. The IrOy∙FeOz and IrOa∙CoOb showed superior CER efficiency than DSAs not only in dilute chloride solutions but also in concentrated solutions (1 mM − 2000 mM). The improvement in CER efficiency of the anodes is attributed to the synergistic effect of suppressed OER (FeOx and Co3O4) and high CER activity of IrO2. The stability of the anodes also exceedingly improved compared with that of pristine FeOx and Co3O4. In addition, hazardous byproducts formation during eletrochlorination such as ClO2−, ClO3−, and ClO4− was also examined and confirmed to satisfy the standard for drinking water. These results suggest that the IrOy∙FeOz and IrOa∙CoOb have great potential to expand the scope of application of the electrochlorination system, particularly in dilute solutions