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On the suppression of cathodic hypochlorite reduction by electrolyte additions of molybdate and chromate ions
Mid Sweden University, Faculty of Science, Technology and Media, Department of applied science and design.
Mid Sweden University, Faculty of Science, Technology and Media, Department of applied science and design.
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2012 (English)In: Journal of electrochemical science and engineering, ISSN 1847-9286, Vol. 2, no 4, 185-198 p.Article in journal (Refereed) Published
Abstract [en]

The goal of this study was to gain a better understanding of the feasibility of replacing Cr(VI) in the chlorate process by Mo(VI), focusing on the cathode reaction selectivity for hydrogen evolution on steel and titanium in a hypochlorite containing electrolyte. To evaluate the ability of Cr(VI) and Mo(VI) additions to hinder hypochlorite reduction, potential sweep experiments on rotating disc electrodes and cathodic current efficiency (CE) measurements on stationary electrodes were performed. Formed electrode films were investigated with scanning electron microscopy and energy-dispersive X-ray spectroscopy. Cathodic hypochlorite reduction is hindered by the Mo-containing films formed on the cathode surface after Mo(VI) addition to the electrolyte, but much less efficient compared to Cr(VI) addition. Very low levels of Cr(VI), in the mM range, can efficiently suppress hypochlorite reduction on polished titanium and steel. Phosphate does not negatively influence the CE in the presence of Cr(VI) or Mo(VI) but the Mo-containing cathode films become thinner if the electrolyte during the film build-up also contains phosphate. For a RuO2-TiO2 anode polarized in electrolyte with 40 mM Mo(VI), the anode potential increased and increased molybdenum levels were detected on the electrode surface

Place, publisher, year, edition, pages
2012. Vol. 2, no 4, 185-198 p.
Keyword [en]
Current efficiency, hydrogen evolution, in-situ additives, cathode, electrolysis, EDS, SEM, potential sweeps, galvanostatic polarization
National Category
Natural Sciences
URN: urn:nbn:se:miun:diva-17213DOI: 10.5599/jese.2012.0021OAI: diva2:561321
Available from: 2012-10-18 Created: 2012-10-18 Last updated: 2016-10-17Bibliographically approved
In thesis
1. Nanoscaled Structures of Chlorate Producing Electrodes
Open this publication in new window or tab >>Nanoscaled Structures of Chlorate Producing Electrodes
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Sodium chlorate is mainly used for production of chlorine dioxide (a pulp bleaching agent). Sodium chlorate is produced by an electrochemical process where chloride ions (from sodium chloride dissolved in water) are oxidized to chlorine on the anodes and hydrogen is evolved on the cathodes. The anode of this process consists of a metal plate coated with a catalytically active metal oxide film. The electrocatalytic properties of the anode coating film have been widely investigated due to the great importance of these electrodes in the electrochemical industry. The material properties are, however, not as well investigated, and the studies described in this thesis are an attempt to remedy this.

Several standard material characterization methods were used, such as SEM, TEM, AFM, EDX, XRD, porosimetry and DSC. Also, a novel model system based on spin coated electrode films on smooth substrates was developed. The model system provided a way to design samples suitable for e.g. TEM, where the sample thickness is limited to maximum of 100 nm. This is possible due to the ability to control the film thickness by the spinning velocity when using the spin coating technique.

It was shown here that the anode coating has a nanostructure. It consists of grains, a few tens of nanometers across. The nanostructure leads to a large effective area and thus provides an explanation of the superior catalytic properties of these coatings. The grains were also shown to be monocrystalline. The size of these grains and its origin was investigated. The calcination temperature, the precursor salt and (if any) doping material all affected the grain size. A higher calcination temperature yielded larger grains and doping with cobalt resulted in smaller grains and therefore a larger real area of the coating. Some preparation conditions also affected the microstructure of the coating; such as substrate roughness. The microstructure is for example the cracked-mud structure. A smoother substrate gave a lower crack density.

The cathode of chlorate production is usually an uncoated metal plate, therefore 'less catalytically active'. It is, however, possible to activate the cathode by for example in situ additions to the electrolyte. It was shown here that sufficient addition of molybdate to the electrolyte resulted in a molybdenum film deposited on the cathode and thereby an increase of its surface area and an activation the hydrogen evolution reaction.

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden University, 2012
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 134
National Category
Natural Sciences
urn:nbn:se:miun:diva-17206 (URN)978-91-87103-35-3 (ISBN)
Public defence
2012-11-09, M102, 13:15 (Swedish)
Available from: 2012-10-18 Created: 2012-10-17 Last updated: 2012-11-30Bibliographically approved

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Hummelgård, ChristineBäckström, Joakim
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