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Methanol and formic acid oxidation in zinc electrowinning under process conditions
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.ORCID iD: 0000-0002-2268-5042
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.ORCID iD: 0000-0001-9203-9313
2008 (English)In: Journal of Applied Electrochemistry, ISSN 0021-891X, E-ISSN 1572-8838, Vol. 38, no 1, 17-24 p.Article in journal (Refereed) Published
Abstract [en]

The possibility of using methanol or formic acid oxidation as the anode process in zinc electrowinning was examined. The activity for methanol and formic acid oxidation on Pt coated high surface area electrodes was investigated over 36 h, at a current density used in industry. The activity could be maintained at a constant potential level in a synthetic electrowinning electrolyte if the current was reversed for short periods. During the tests, the anode potential was, more than 1.2 V below the potential for the oxygen evolving lead anodes used in modern zinc electrowinning. The lowered anode potential would lead to a significant energy reduction. However, tests in industrial electrolyte resulted in a very low activity for both methanol and formic acid oxidation. The low activity was shown to be caused mainly by chloride impurities. A reduction of the chloride content below 10(-5) M is needed in order to obtain sufficient activity for methanol oxidation on Pt for use in zinc electrowinning. Pt and PtRu electrodes were compared regarding their activity for methanol oxidation and the latter was shown to be more affected by chloride impurities. However, at a potential of 0.7 V vs NHE, with a chloride content of 10(-4) M, formic acid oxidation on PtRu gives the highest current density.

Place, publisher, year, edition, pages
2008. Vol. 38, no 1, 17-24 p.
Keyword [en]
anode reaction, zinc electrowinning, formic acid, methanol
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-24975DOI: 10.1007/s10800-007-9387-0ISI: 000251370900003Scopus ID: 2-s2.0-36749007865OAI: oai:DiVA.org:kth-24975DiVA: diva2:354753
Note
QC 20101004Available from: 2010-10-04 Created: 2010-10-04 Last updated: 2010-10-14Bibliographically approved
In thesis
1. Methanol and formic acid oxidising anodes in zinc electrowinning
Open this publication in new window or tab >>Methanol and formic acid oxidising anodes in zinc electrowinning
2006 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

Electrowinning is an energy demanding process, in which the electrolysis accounts for the major part of the energy consumption. The anodic reaction in the electrolysis of zinc from sulphate based electrolytes is oxygen evolution, which has a standard potential of 1.23 V at 25 ºC. The lead anodes used in zinc electrowinning today have a high overpotential for oxygen evolution and the operating anode potential is as high as 2 V vs NHE. Since the lead anodes are not stable some of the lead can dissolve and incorporate in the zinc metal on the cathode. The lead anodes are also undesired because of health concerns. By introducing formic acid or methanol oxidation as anode reaction, the anode potential could be lowered significantly. This would reduce the energy consumption and also enable the use of new types of electrodes.

The general aim of this thesis was to investigate how depolarised anodes for methanol and formic acid oxidation can be used in electrowinning processes.

Several electrodes were shown to be suitable as anodes in zinc electrowinning. A high activity was obtained with porous electrodes, whereas a platinum coated titanium was the most stable electrode. The choice of electrode will however always be a compromise between activity, stability and costs. All electrodes were deactivated with time and the best method for reactivation tested in this study, was periodic current reversal (PCR).

An operating procedure was established for methanol and formic acid oxidation on high surface area electrodes, Pt-TySAR®. With the use of PCR at process current density, a potential lower than 0.7 V vs NHE could be maintained for 36 hour in synthetic electrolyte. The use of formic acid oxidation resulted in lower potentials than the use of methanol oxidation.

The activities for methanol and formic acid oxidation in industrial electrolyte were very low and chloride impurities were shown to cause the major part of the activity reduction. Even small amounts of chloride (10-6-10-5 M) affected the activity of the electrodes. The chloride impurities are considered as the main problem to overcome in order to introduce the oxidation of formic acid or methanol in the zinc electrowinning process.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. 36 p.
Series
Trita-KET, ISSN 1104-3466 ; 230
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-4201 (URN)
Presentation
2006-11-28, Salongen, KTHB, Osquars Backe 31, Stockholm, 10:15
Opponent
Supervisors
Note
QC 20101004Available from: 2006-12-01 Created: 2006-12-01 Last updated: 2010-10-04Bibliographically approved
2. Electrochemical Reactions in Polymer Electrolyte Fuel Cells
Open this publication in new window or tab >>Electrochemical Reactions in Polymer Electrolyte Fuel Cells
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The polymer electrolyte fuel cell converts the chemical energy in a fuel, e.g. hydrogen or methanol, and oxygen into electrical energy. The high efficiency and the possibility to use fuel from renewable sources make them attractive as energy converters in future sustainable energy systems. Great progress has been made in the development of the PEFC during the last decade, but still improved lifetime as well as lowered cost is needed before a broad commercialization can be considered. The electrodes play an important role in this since the cost of platinum used as catalyst constitutes a large part of the total cost for the fuel cell. A large part of the degradation in performance can also be related to the degradation of the porous electrode and a decreased electrochemically active Pt surface.

In this thesis, different fuel cell reactions, catalysts and support materials are investigated with the aim to investigate the possibility to improve the activity, stability and utilisation of platinum in the fuel cell electrodes.

An exchange current density, i0, of 770 mA cm-2Pt was determined for the hydrogen oxidation reaction in the fuel cell with the model electrodes. This is higher than previously found in literature and implies that the kinetic losses on the anode are very small. The anode loading could therefore be reduced without imposing too high potential losses if good mass transport of hydrogen is ensured. It was also shown that the electrochemically active surface area, activity and stability of the electrode can be affected by the support material. An increased activity was observed at higher potentials for Pt deposited on tungsten oxide, which was related to the postponed oxide formation for Pt on WOx. An improved stability was seen for Pt deposited on tungsten oxide and on iridium oxide. A better Pt stability was also observed for Pt on a low surface non-graphitised support compared to a high surface graphitised support. Pt deposited on titanium and tungsten oxide, displayed an enhanced electrochemically active surface area in the cyclic voltammograms, which was explained by the good proton conductivity of the metal oxides. CO-stripping was shown to provide the most reliable measure of the electrochemically active surface area of the electrode in the fuel cell. It was also shown to be a useful tool in characterization of the degradation of the electrodes. In the study of oxidation of small organic compounds, the reaction was shown to be affected by the off transport of reactants and by the addition of chloride impurities. Pt and PtRu were affected differently, which enabled extraction of information about the reaction mechanisms and rate determining steps.

The polymer electrolyte fuel cell converts the chemical energy in a fuel, e.g. hydrogen or methanol, and oxygen into electrical energy. The high efficiency and the possibility to use fuel from renewable sources make them attractive as energy converters in future sustainable energy systems. Great progress has been made in the development of the PEFC during the last decade, but still improved lifetime as well as lowered cost is needed before a broad commercialization can be considered. The electrodes play an important role in this since the cost of platinum used as catalyst constitutes a large part of the total cost for the fuel cell. A large part of the degradation in performance can also be related to the degradation of the porous electrode and a decreased electrochemically active Pt surface.

In this thesis, different fuel cell reactions, catalysts and support materials are investigated with the aim to investigate the possibility to improve the activity, stability and utilisation of platinum in the fuel cell electrodes.

An exchange current density, i0, of 770 mA cm-2Pt was determined for the hydrogen oxidation reaction in the fuel cell with the model electrodes. This is higher than previously found in literature and implies that the kinetic losses on the anode are very small. The anode loading could therefore be reduced without imposing too high potential losses if good mass transport of hydrogen is ensured. It was also shown that the electrochemically active surface area, activity and stability of the electrode can be affected by the support material. An increased activity was observed at higher potentials for Pt deposited on tungsten oxide, which was related to the postponed oxide formation for Pt on WOx. An improved stability was seen for Pt deposited on tungsten oxide and on iridium oxide. A better Pt stability was also observed for Pt on a low surface non-graphitised support compared to a high surface graphitised support. Pt deposited on titanium and tungsten oxide, displayed an enhanced electrochemically active surface area in the cyclic voltammograms, which was explained by the good proton conductivity of the metal oxides. CO-stripping was shown to provide the most reliable measure of the electrochemically active surface area of the electrode in the fuel cell. It was also shown to be a useful tool in characterization of the degradation of the electrodes. In the study of oxidation of small organic compounds, the reaction was shown to be affected by the off transport of reactants and by the addition of chloride impurities. Pt and PtRu were affected differently, which enabled extraction of information about the reaction mechanisms and rate determining steps.

Abstract [sv]

Polymerelektrolytbränslecellen omvandlar den kemiska energin i ett bränsle, exv. vätgas eller metanol, och syrgas  till elektrisk energi. Den höga verkningsgraden samt möjligheten att använda bränsle från förnyelsebara källor gör dem attraktiva som energiomvandlare i framtida hållbara energisystem. En enorm utveckling har skett under det senaste årtiondet men för att kunna introducera polymerelektrolytbränslecellen på marknaden i en större skala måste livstiden öka och kostnaden minska. Elektroderna har en central del i detta då den platina som används som katalysator står för en stor del av kostnaden för bränslecellen. En stor del av prestandaförsämringen med tiden hos bränslecellen kan också relateras till en degradering av den porösa elektroden och en minskad elektrokemiskt aktiv platinayta.

I denna avhandling studeras olika bränslecellsreaktioner samt olika katalysatorer och supportmaterial med målet att undersöka möjligheten att förbättra platinakatalysatorns aktivitet, stabilitet och utnyttjandegrad i bränslecellselektroder.

Utbytesströmtätheten, i0, för vätgasoxidationen i bränslecell bestämdes till 770 mA cm-2Pt genom försök med modellelektroderna. Denna var högre än vad som framkommit tidigare i litteratur, vilket visar att de kinetiska förlusterna på anoden är mycket små. Katalysatormängden på anoden borde därför kunna minskas utan några större potentialförluster så länge masstransporten av vätgas är tillräcklig. Den elektrokemiskt aktiva ytan, aktiviteten och stabiliteten hos elektroden visade sig kunna påverkas av supportmaterialet. Platina deponerad på volfram oxid hade en högre aktivitet vid höga potentialer vilket relaterades till den förskjutna oxidbildningen på ytan. Elektroder med platina på volframoxid och iridiumoxid var mer stabila än elektroder med platina på kol. Det var även platina på ett icke grafitiserat kol med låg yta jämfört med platina på grafitiserade kol med en hög yta. Platina på metalloxidskikt av volfram och titan visade en högre elektrokemiskt aktiv yta i de cykliska voltamogrammen än platina på kol, vilket förklarades med att båda metalloxiderna har en bra protonledningsförmåga. CO-stripping gav det säkraste måttet på den elektrokemiskt aktiva ytan i en elektrod i bränslecell. CO-stripping visade sig även vara användbart för karaktärisering av degraderingen av en elektrod. Oxidationen av små organiska föreningar påverkades av borttransporten av intermediärer samt av kloridföroreningar. Pt aoch PtRu påverkades olika vilket gjorde det möjligt att få fram information om reaktionsmekanismer och hastighetsbestämmande steg.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. 55 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2010:40
Keyword
Fuel cell, model electrodes, oxygen reduction, methanol oxidation, formic acid oxidation, hydrogen oxidation, CO oxidation, degradation, tungsten oxide, carbon support, Bränslecell, modellelektroder, syrgasreduktion, metanoloxidation, myrsyraoxidation, vätgasoxidation, CO oxidation, degradering, wolfram oxid, kolsupport
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-25267 (URN)978-91-7415-747-5 (ISBN)
Public defence
2010-10-25, F3, Lindstedts väg 26, KTH, Stockholm, 10:00 (English)
Opponent
Note
QC 20101014Available from: 2010-10-14 Created: 2010-10-14 Last updated: 2010-12-10Bibliographically approved

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Lagergren, CarinaLindbergh, Göran

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