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Nanoscaled Structures of Chlorate Producing Electrodes
Mid Sweden University, Faculty of Science, Technology and Media, Department of applied science and design.
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.
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 134
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:miun:diva-17206ISBN: 978-91-87103-35-3 (print)OAI: oai:DiVA.org:miun-17206DiVA: diva2:561085
Public defence
2012-11-09, M102, 13:15 (Swedish)
Opponent
Supervisors
Available from: 2012-10-18 Created: 2012-10-17 Last updated: 2012-11-30Bibliographically approved
List of papers
1. Nanoscale characterisation of crystallinity in DSA coating
Open this publication in new window or tab >>Nanoscale characterisation of crystallinity in DSA coating
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2008 (English)In: PROCEEDINGS OF THE 17TH INTERNATIONAL VACUUM CONGRESS/13TH INTERNATIONAL CONFERENCE ON SURFACE SCIENCE/INTERNATIONAL CONFERENCE ON NANOSCIENCE AND TECHNOLOGY, Institute of Physics (IOP), 2008, Vol. 100, no 052026, 4-4 p.Conference paper, Published paper (Refereed)
Abstract [en]

Dimensionally Stable Anodes (DSA (R)) are used for industrial production of e. g. chlorine and chlorate. It is known that the superior electrocatalytical properties of DSA (R) is due to the large effective area of the porous coating. However, this knowledge is mainly found from in situ electrochemical measurements. Here, we used ex situ methods, AFM, TEM and gas porosimetry, for characterization at the nanoscale. The DSA (R) coating was found to consist of mono-crystalline grains with a size of 20-30 nm and with pores of about 10 nm in diameter. Using a simple geometrical model an effective area was calculated. For a typical coating thickness, an increase of about 1000 times in the effective surface area was found, which is consistent with in situ estimations. These results suggest that the dominating source of surface enlargement is due to nano-crystallinity.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2008
Series
Journal of Physics Conference Series, ISSN 1742-6588 ; 100
Keyword
nanoteknik nanotechnology
National Category
Natural Sciences
Identifiers
urn:nbn:se:miun:diva-4291 (URN)10.1088/1742-6596/100/5/052026 (DOI)000275655200122 ()5088 (Local ID)5088 (Archive number)5088 (OAI)
Conference
17th International Vacuum Congress/13th International Conference on Surface Science/Internatinal Conference on Nanoscience and Technology, Jul 02-06, 2007, Stockholm, Sweden
Available from: 2008-09-30 Created: 2009-03-19 Last updated: 2016-09-22Bibliographically approved
2. Nanocrystallinity in RuO2 coatings - influence of precursor and preparation temperature
Open this publication in new window or tab >>Nanocrystallinity in RuO2 coatings - influence of precursor and preparation temperature
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2010 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 518, no 14, 3615-3618 p.Article in journal (Refereed) Published
Abstract [en]

The effects of precursor and calcination temperature on the nano morphology of ruthenium dioxide on titanium, prepared from thermal decomposition of aqueous salt solutions were investigated. Transmission electron microscopy. X-ray diffraction, gas porosimetry and cyclic voltammetry showed that lower calcination temperature yielded smaller crystallites. The crystallites were between 6 and 22 nm in diameter. When using ruthenium nitrosyl nitrate the firing temperature had a large impact on the grain size, but for chloride there was only a minor effect in the temperature range 350-550 degrees C.

Keyword
Materials Science, Multidisciplinary; Materials Science, Coatings & Films; Physics, Applied; Physics, Condensed Matter
National Category
Natural Sciences
Identifiers
urn:nbn:se:miun:diva-8725 (URN)10.1016/j.tsf.2009.09.065 (DOI)000278064600004 ()2-s2.0-77950543416 (Scopus ID)
Available from: 2009-03-19 Created: 2009-03-19 Last updated: 2013-08-23Bibliographically approved
3. Spin coated titanium-ruthenium oxide thin films
Open this publication in new window or tab >>Spin coated titanium-ruthenium oxide thin films
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2013 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 536, 74-80 p.Article in journal (Other academic) Published
Abstract [en]

Substrates of different roughness spin coated with Ti0.7Ru 0.3O2 films have been evaluated as model system for fundamental studies of the industrially and scientifically interesting (Ti,Ru)O2 based electrodes. The approach allowed for much more accurate control over the material synthesis than the traditionally used brush-, dip-, or spray-coating, on titanium-metal substrates. It moreover yielded well-defined samples suitable for basic studies of the surface properties that are of fundamental importance for understanding the electrochemical functionality of the electrode. We have compared the films on silicon substrates to films prepared by spin coating the same material on titanium-metal substrates. Samples have been characterized using atomic force microscopy (AFM), X-ray diffraction, scanning electron microscopy (SEM), and cyclic voltammetry. The samples displayed a uniformity of the films appropriate for AFM characterization. The smoother the substrate the less cracks in the coating. Using easily broken silicon wafers as substrate, a straightforward sample preparation technique was demonstrated for cross-section SEM. In addition, using high spinning velocities we have deposited the oxide films directly on silicon-nitride grids, thin enough to allow for studies with transmission electron microscopy without further sample preparation.

Keyword
Atomic force microscopy; Cross-section scanning electron microscopy; Ruthenium dioxide-titanium dioxide; Spin coating; Transmission electron microscopy; X-ray diffraction
National Category
Natural Sciences
Identifiers
urn:nbn:se:miun:diva-17210 (URN)10.1016/j.tsf.2013.03.044 (DOI)000318974800010 ()2-s2.0-84877696954 (Scopus ID)
Note

Published online 2 April 2013.

Available from: 2012-10-18 Created: 2012-10-18 Last updated: 2013-08-23Bibliographically approved
4. Physical and electrochemical properties of cobalt doped (ti,ru)O2 electrode coatings
Open this publication in new window or tab >>Physical and electrochemical properties of cobalt doped (ti,ru)O2 electrode coatings
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2013 (English)In: Materials Science & Engineering: B. Solid-state Materials for Advanced Technology, ISSN 0921-5107, E-ISSN 1873-4944, Vol. 178, no 20, 1515-1522 p.Article in journal (Refereed) Published
Abstract [en]

The physical and electrochemical properties of ternary oxides Ti 0.7Ru0.3-xCoxO2 (x = 0.093 and x = 0) have been investigated and compared. Samples of three different thicknesses were prepared by spin-coating onto polished titanium to achieve uniform and well-defined coatings. The resulting electrodes were characterized with a variety of methods, including both physical and electrochemical methods. Doping with cobalt led to a larger number of micrometer-sized cracks in the coating, and coating grains half the size compared to the undoped samples (10 instead of 20 nm across). This is in agreement with a voltammetric charge twice as high, as estimated from cyclic voltammetry. There is no evidence of a Co 3O4 spinel phase, suggesting that the cobalt is mainly incorporated in the overall rutile structure of the (Ti,Ru)O2. The doped electrodes exhibited a higher activity for cathodic hydrogen evolution compared to the undoped electrodes, despite the fact that one third of the active ruthenium was substituted with cobalt. For anodic chlorine evolution, the activity was similar for both electrode types.

Keyword
Chlorine evolution, Cobaltdoping, DSA®, Hydrogen evolution, Polarization curves, XRD
National Category
Natural Sciences
Identifiers
urn:nbn:se:miun:diva-17211 (URN)10.1016/j.mseb.2013.08.018 (DOI)000327830000024 ()2-s2.0-84887055267 (Scopus ID)
Available from: 2012-10-18 Created: 2012-10-18 Last updated: 2016-12-16Bibliographically approved
5. In-situ activated hydrogen evolution by molybdate addition to neutral and alkaline electrolytes
Open this publication in new window or tab >>In-situ activated hydrogen evolution by molybdate addition to neutral and alkaline electrolytes
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2012 (English)In: Journal of electrochemical science and engineering, ISSN 1847-9286, Vol. 2, no 3, 105-120 p.Article in journal (Refereed) Published
Abstract [en]

Activation of the hydrogen evolution reaction (HER) by in-situ addition of Mo(VI) to the electrolyte has been studied in alkaline and pH neutral electrolytes, the latter with the chlorate process in focus. Catalytic molybdenum containing films formed on the cathodes during polarization were investigated using scanning electron microscopy (SEM), energy-dispersive X ray analysis (EDS), X-ray photoelectron spectroscopy (XPS), and X ray fluorescence (XRF). In-situ addition of Mo(VI) activates the HER on titanium in both alkaline and neutral electrolytes and makes the reaction kinetics independent of the substrate material. Films formed in neutral electrolyte consisted of molybdenum oxides and contained more molybdenum than those formed in alkaline solution. Films formed in neutral electrolyte in the presence of phosphate buffer activated the HER, but were too thin to be detected by EDS. Since molybdenum oxides are generally not stable in strongly alkaline electrolyte, films formed in alkaline electrolyte were thinner and probably co-deposited with iron. A cast iron molybdenum alloy was also investigated with respect to activity for HER. When polished in the same way as iron, the alloy displayed a similar activity for HER as pure iron.

Keyword
Molybdate, molybdenum dioxide, electrodeposition, electrolysis
National Category
Natural Sciences
Identifiers
urn:nbn:se:miun:diva-17212 (URN)10.5599/jese.2012.0015 (DOI)
Available from: 2012-10-18 Created: 2012-10-18 Last updated: 2016-10-17Bibliographically approved
6. On the suppression of cathodic hypochlorite reduction by electrolyte additions of molybdate and chromate ions
Open this publication in new window or tab >>On the suppression of cathodic hypochlorite reduction by electrolyte additions of molybdate and chromate ions
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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

Keyword
Current efficiency, hydrogen evolution, in-situ additives, cathode, electrolysis, EDS, SEM, potential sweeps, galvanostatic polarization
National Category
Natural Sciences
Identifiers
urn:nbn:se:miun:diva-17213 (URN)10.5599/jese.2012.0021 (DOI)
Available from: 2012-10-18 Created: 2012-10-18 Last updated: 2016-10-17Bibliographically approved

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