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4 CHAPTER1. INTRODUCTION as well as decomposition of hypochlorites (HOCl or OCl–) according to 2HOCl → 2HCl + O2 . (1.10) The decomposition of hypochlorites might be due both to bulk electrolyte and elec- trode surface reactions. The reduction in current efficiency due to these parasitic reactions ranges from 1-5%. While measures such as usage of selective electrodes (see the following section) and anolyte acidification can reduce the oxygen side reaction to enable a current efficiency of close to 99% in the chlor-alkali process, this is not possible in sodium chlorate production, where 5% losses in current efficiency are common. Research also indicates that the oxygen evolution side- reaction not only lowers the current efficiency of the processes, but that it is also directly connected to the rate of deactivation of the DSA.[1] The cathode reaction in both chlor-alkali and sodium chlorate production is hydro- gen evolution, reaction 1.2. The interest in this reaction has been rising in recent years, as it is a way of producing hydrogen for use as fuel (or in production of other fuels, e.g. by Fischer-Tropsch synthesis) in a renewable fashion[11]. In both indus- trial electrosynthesis of chlor-alkali and sodium chlorate, and in water electrolysis for production of hydrogen, the hydrogen evolution takes place in a strongly al- kaline solution, which places limitations on the cathode material. Although early industrial electrosynthetic hydrogen production was carried out already in 1927, electrosynthetic hydrogen makes up only a very small part (about 5%) of the total hydrogen produced today. Production of hydrogen based on fossil hydrocarbons is the dominant production method, as it requires less energy. The theoretical en- ergy consumption required for production using methane is 41 MJ/kmol H2 while the theoretical energy consumption for electrolytic hydrogen is about six times higher at 242 MJ/kmol H2[12]. Today, industrial electrolytic cells for chlor-alkali and water electrolysis often use steel; Ni or Ni alloys; or RuO2 deposited on Ni as hydrogen evolution reaction (HER) cathodes[3, 5, 12, 13]. However, due to the more demanding conditions, mainly steel or Ti cathodes are used in chlorate production[5]. Ni-based cathodes are not acceptable in chlorate production as Ni is a well-known catalyst for decomposition of HOCl[1]. As interest in electrochem- ical hydrogen production has risen, so has the research into “activated cathodes”, which are cathodes with an activity exceeding that of Ni[13] . A primary goal of the present thesis has been to further the understanding for how the electrode composition determines the selectivity, activity and stability in these processes. To this end, the first comprehensive literature review of the selectivity issue in industrial chlor-alkali and sodium chlorate production has been carried out as a part of the present thesis project[1]. As the review discusses the present knowledge regarding the effect of anode structure and composition, electrolyte contamination, and process parameters on the selectivity in great detail, the reader is advised to read the Discussion section of that review for a thorough discussion of these aspects. A brief overview will still be provided in Section 1.2 of thisPDF Image | Studies of Electrode Processes in Industrial Electrosynthesis
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