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Nanoscaled Structures of Chlorate Producing Electrodes
Mittuniversitetet, Fakulteten för naturvetenskap, teknik och medier, Institutionen för tillämpad naturvetenskap och design.
2012 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Sundsvall: Mid Sweden University , 2012.
Serie
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 134
Nationell ämneskategori
Naturvetenskap
Identifikatorer
URN: urn:nbn:se:miun:diva-17206ISBN: 978-91-87103-35-3 (tryckt)OAI: oai:DiVA.org:miun-17206DiVA, id: diva2:561085
Disputation
2012-11-09, M102, 13:15 (Svenska)
Opponent
Handledare
Tillgänglig från: 2012-10-18 Skapad: 2012-10-17 Senast uppdaterad: 2012-11-30Bibliografiskt granskad
Delarbeten
1. Nanoscale characterisation of crystallinity in DSA coating
Öppna denna publikation i ny flik eller fönster >>Nanoscale characterisation of crystallinity in DSA coating
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2008 (Engelska)Ingår i: 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, nr 052026, s. 4-4Konferensbidrag, Publicerat paper (Refereegranskat)
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.

Ort, förlag, år, upplaga, sidor
Institute of Physics (IOP), 2008
Serie
Journal of Physics Conference Series, ISSN 1742-6588 ; 100
Nyckelord
nanoteknik nanotechnology
Nationell ämneskategori
Naturvetenskap
Identifikatorer
urn:nbn:se:miun:diva-4291 (URN)10.1088/1742-6596/100/5/052026 (DOI)000275655200122 ()2-s2.0-77954381591 (Scopus ID)5088 (Lokalt ID)5088 (Arkivnummer)5088 (OAI)
Konferens
17th International Vacuum Congress/13th International Conference on Surface Science/Internatinal Conference on Nanoscience and Technology, Jul 02-06, 2007, Stockholm, Sweden
Tillgänglig från: 2008-09-30 Skapad: 2009-03-19 Senast uppdaterad: 2017-10-09Bibliografiskt granskad
2. Nanocrystallinity in RuO2 coatings - influence of precursor and preparation temperature
Öppna denna publikation i ny flik eller fönster >>Nanocrystallinity in RuO2 coatings - influence of precursor and preparation temperature
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2010 (Engelska)Ingår i: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 518, nr 14, s. 3615-3618Artikel i tidskrift (Refereegranskat) 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.

Nyckelord
Materials Science, Multidisciplinary; Materials Science, Coatings & Films; Physics, Applied; Physics, Condensed Matter
Nationell ämneskategori
Naturvetenskap
Identifikatorer
urn:nbn:se:miun:diva-8725 (URN)10.1016/j.tsf.2009.09.065 (DOI)000278064600004 ()2-s2.0-77950543416 (Scopus ID)
Tillgänglig från: 2009-03-19 Skapad: 2009-03-19 Senast uppdaterad: 2017-12-13Bibliografiskt granskad
3. Spin coated titanium-ruthenium oxide thin films
Öppna denna publikation i ny flik eller fönster >>Spin coated titanium-ruthenium oxide thin films
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2013 (Engelska)Ingår i: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 536, s. 74-80Artikel i tidskrift (Övrigt vetenskapligt) 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.

Nyckelord
Atomic force microscopy; Cross-section scanning electron microscopy; Ruthenium dioxide-titanium dioxide; Spin coating; Transmission electron microscopy; X-ray diffraction
Nationell ämneskategori
Naturvetenskap
Identifikatorer
urn:nbn:se:miun:diva-17210 (URN)10.1016/j.tsf.2013.03.044 (DOI)000318974800010 ()2-s2.0-84877696954 (Scopus ID)
Anmärkning

Published online 2 April 2013.

Tillgänglig från: 2012-10-18 Skapad: 2012-10-18 Senast uppdaterad: 2017-12-07Bibliografiskt granskad
4. Physical and electrochemical properties of cobalt doped (ti,ru)O2 electrode coatings
Öppna denna publikation i ny flik eller fönster >>Physical and electrochemical properties of cobalt doped (ti,ru)O2 electrode coatings
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2013 (Engelska)Ingår i: Materials Science & Engineering: B. Solid-state Materials for Advanced Technology, ISSN 0921-5107, E-ISSN 1873-4944, Vol. 178, nr 20, s. 1515-1522Artikel i tidskrift (Refereegranskat) 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.

Nyckelord
Chlorine evolution, Cobaltdoping, DSA®, Hydrogen evolution, Polarization curves, XRD
Nationell ämneskategori
Naturvetenskap
Identifikatorer
urn:nbn:se:miun:diva-17211 (URN)10.1016/j.mseb.2013.08.018 (DOI)000327830000024 ()2-s2.0-84887055267 (Scopus ID)
Tillgänglig från: 2012-10-18 Skapad: 2012-10-18 Senast uppdaterad: 2017-12-07Bibliografiskt granskad
5. In-situ activated hydrogen evolution by molybdate addition to neutral and alkaline electrolytes
Öppna denna publikation i ny flik eller fönster >>In-situ activated hydrogen evolution by molybdate addition to neutral and alkaline electrolytes
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2012 (Engelska)Ingår i: Journal of electrochemical science and engineering, ISSN 1847-9286, Vol. 2, nr 3, s. 105-120Artikel i tidskrift (Refereegranskat) 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.

Nyckelord
Molybdate, molybdenum dioxide, electrodeposition, electrolysis
Nationell ämneskategori
Naturvetenskap
Identifikatorer
urn:nbn:se:miun:diva-17212 (URN)10.5599/jese.2012.0015 (DOI)
Tillgänglig från: 2012-10-18 Skapad: 2012-10-18 Senast uppdaterad: 2016-10-17Bibliografiskt granskad
6. On the suppression of cathodic hypochlorite reduction by electrolyte additions of molybdate and chromate ions
Öppna denna publikation i ny flik eller fönster >>On the suppression of cathodic hypochlorite reduction by electrolyte additions of molybdate and chromate ions
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2012 (Engelska)Ingår i: Journal of electrochemical science and engineering, ISSN 1847-9286, Vol. 2, nr 4, s. 185-198Artikel i tidskrift (Refereegranskat) 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

Nyckelord
Current efficiency, hydrogen evolution, in-situ additives, cathode, electrolysis, EDS, SEM, potential sweeps, galvanostatic polarization
Nationell ämneskategori
Naturvetenskap
Identifikatorer
urn:nbn:se:miun:diva-17213 (URN)10.5599/jese.2012.0021 (DOI)
Tillgänglig från: 2012-10-18 Skapad: 2012-10-18 Senast uppdaterad: 2016-10-17Bibliografiskt granskad

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