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Synthesis and characterization of palladium based carbon nanostructure-composites and their clean-energy application
Umeå University, Faculty of Science and Technology, Department of Physics. (Thomas Wågberg)
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Carbon nanostructures are a wide field with many applications. The use of carbon nanostructures as support in heterogeneous catalysis is a key development that led together with the use of nanoparticles to a significant cost reduction of catalysts. Catalysts designed in this way are widely applied in fuel cell technologies. For portable devices especially low temperature fuel cells are desirable with low hazards for the user. One technology which fulfills these requirements is the direct formic acid fuel cell (DFAFC). DFAFC have many promising characteristics, such as high electromotive force and easy fuel handling. However, they still suffer from too low power output and lifetime for commercialization.

This thesis focusses on two main aspects: the synthesis of carbon nanostructures by chemical vapor deposition (CVD) and their application as catalyst support. The materials are investigated by many different techniques ranging from transmission electron microscopy (TEM) to fuel cell tests.

Different carbon nanostructures could be synthesized by catalytic CVD on palladium (Pd) nanoparticles. Multi-walled carbon nanotubes (MWCNTs), carbon nanofibers (CNFs) and helical carbon nanofibers (HCNFs) were grown, selectively, dependent on temperature, using acetylene as carbon precursor. Especially HCNF raised further interest due to their unique structure. A growth model for HCNFs was developed based on an anisotropic extrusion model. The synthesis conditions for HCNFs were optimized until an almost 100 % purity with very high efficiency was obtained.

The unique helical but fiber-like structure made the material very interesting as support for heterogeneous catalysis. Several catalysts based on Pd nanoparticle decorated HCNFs were developed. The synthesis methods ranged from standard methods like the polyol method to phase-transfer methods. The catalysts showed very promising results for the electro-oxidation of methanol, ethanol and formic acid. This makes them highly attractive for fuel cell applications. The catalysts were tested in DFAFC. The superiority of HCNF-based catalysts is attributed to the good attachment of nanoparticles to the defect-rich and easy to functionalize surface of HCNFs in combination with adequate film forming properties during electrode preparation.

Abstract [sv]

Nanostrukturerat kol är ett mycket brett fält med ett stort antal tillämpningar. Användning av kolnanostrukturer som support för heterogena katalysmaterial har tillsammans med utvecklingen av nanopartiklar lett till en avsevärd minskning av kostnaden för katalysatorer. Katalysatorer designade på detta sätt används frekvent i bränsleceller. För portabla tillämpningar är utvecklingen av säkra och miljövänliga lågtemperaturceller mycket viktig. En teknologi som uppfyller dessa kriterier är bränsleceller som drivs med myrsyra (DFAFC). Sådana bränsleceller har många önskvärda egenskaper, såsom en hög elektromotorisk kraft och en enkel hantering av bränslet. Trots dessa goda egenskaper har de också en del nackdelar som hindrar en full kommersialisering. De två mest problematiska är en för låg genererad effekt samt en för kort livslängd på katalysatorerna.

Denna avhandling fokuserar på två huvudpunkter som adresserar dessa problem; tillverkning och karaktärisering av kolnanostrukturer producerade med CVD, och deras tillämpningar som support för katalysatorer. Materialen karaktäriseras med en rad olika tekniker, allt från transmission-elektronmikroskopi till bränslecellstester.

Olika kolnanostrukturer har syntetiserats med katalytisk CVD på palladium (Pd) nanopartiklar. Produktionen av flerväggiga kolnanorör, kolfibrer och heliska kolnanofibrer har tillverkats med acetylen som kolkälla och genom att variera temperaturen kunde innehållet av olika typer av nanostrukturerat kol kontrolleras. Särskilt stort intresse har de heliska kolnanofibrerna rönt på grund av deras unika struktur. Vi beskriver en tillväxtmekanism baserad på en anisotrop diffusionsmodell. Genom att justera produktionsparametrarna visar vi att heliska kolnanofibrer kunde tillverkas med nära 100 %-ig renhet och hög effektivitet.

Den unika heliska och fiberlika strukturen är mycket intressant for tillämpningar som support för heterogena katalysatorer. Ett flertal kompositer för katalytiska tillämpningar har utvecklats baserade på heliska kolnanofibrer, dekorerade med heterogena katalysatorer genom en rad olika kemiska/fysikaliska tekniker. De syntetiserade materialen visar mycket goda katalytiska egenskaper för att oxidera metanol, etanol och myrsyra. Därigenom blir de mycket attraktiva för användning i bränsleceller. Vi korrelerar de goda katalytiska egenskaperna med en bra vidhäftning av nanopartiklarna på de heliska kolnanofibrerna defekter, deras goda ledningsförmåga, bra egenskaper för att förbereda elektroder, samt deras stora yta i förhållande till deras volym och vikt.

Place, publisher, year, edition, pages
Umeå: Umeå University , 2013. , 66 p.
Keyword [en]
Carbon nanostructures, chemical vapour deposition, electro catalysts, transmission electron microscopy, direct formic acid fuel cells
National Category
Condensed Matter Physics
Research subject
Solid State Physics
Identifiers
URN: urn:nbn:se:umu:diva-68852ISBN: digital version: 978-91-7459-632-8 ISBN: printed version:978-91-7459-631-1 OAI: oai:DiVA.org:umu-68852DiVA: diva2:618336
Public defence
2013-05-31, Naturvetarhuset, N450, Umeå Universitet, Umeå, 13:00 (English)
Opponent
Supervisors
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council
Available from: 2013-05-07 Created: 2013-04-26 Last updated: 2013-04-29Bibliographically approved
List of papers
1. Easy synthesis of Pd fullerene polymer structures from the molten state of tris(dibenzylideneacetone)dipalladium(0)
Open this publication in new window or tab >>Easy synthesis of Pd fullerene polymer structures from the molten state of tris(dibenzylideneacetone)dipalladium(0)
2012 (English)In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 249, no 12, 2588-2591 p.Article in journal (Refereed) Published
Abstract [en]

Pd fullerene composites were first synthesized and studied in the early 90s by for example Nagashima et al. In this study we present a novel and rapid approach to synthesize Pd fullerides based on direct reaction of C60 with Pd2dba3. We show that the Pd fullerene polymer phase forms at temperatures around the melting point of Pd2dba3 (150 degrees C) and that it proceeds upon further annealing while releasing dba. The synthesis reactions were studied in TGA/DSC. TEM revealed that the material easily collapses under the electron beam into nanoparticles. Under very low doses almost no particles can be found. Similarly, Raman spectroscopy confirmed the formation of Pd fulleride polymers but also supported the collapse of the Pd fulleride phase when irradiated by high laser power. CVD experiments have been conducted on directly coated Si substrates showing similar results to previous reports, namely that Pd2C60 is an efficient catalysts for the growth of helical carbon nanofibers. Our study gives both insights into the formation of nanoparticles as well as the synthesis of C60 polymers. The method is also compatible with direct coating processes making it useful for a broad spectrum of CVD and catalysis applications. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Keyword
fullerenes, palladium, Pd fullerides, synthesis
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:umu:diva-64056 (URN)10.1002/pssb.201200084 (DOI)000312215300068 ()
Available from: 2013-01-15 Created: 2013-01-14 Last updated: 2017-12-06Bibliographically approved
2. Carbon nanotubes and helical carbon nanofibers grown by chemical vapour deposition on C60 fullerene supported Pd nanoparticles
Open this publication in new window or tab >>Carbon nanotubes and helical carbon nanofibers grown by chemical vapour deposition on C60 fullerene supported Pd nanoparticles
2011 (English)In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 49, no 4, 1101-1107 p.Article in journal (Refereed) Published
Abstract [en]

Chemical vapour deposition (CVD) represents a cheap and versatile method to produce carbon nanostructures. Here we present how we by using a standard CVD setup together with Pd nano particles as a catalyst can produce helical fibers with very periodic pitch, helicity, and narrow diameter distribution. The C60 supported Pd catalyst particles are produced by a wet chemistry process and applied to silicon substrates. By raising the growth temperature from 550 °C to 800 °C we can tune the growth products from helical carbon fibers to straight hollow carbon fibers and finally to carbon nanotubes at the highest temperatures. In the intermediate temperature region of 650 °C a mixture of all three components appears. At 550 °C the efficiency of the process is optimized by the amount of water during the growth. Different from most previous studies we can detect most of the catalyst particles embedded in the grown structures. In all fibers the catalyst particles are situated exactly in the middle of the fibers suggesting a two-directional growth. From the shape of the catalyst particles and by adopting a simple model we conclude that the fibers coil due to blocked carbon diffusion pathways on or through the catalyst particles.

Place, publisher, year, edition, pages
Elsevier, 2011
Keyword
carbon nanotubes, helical carbon fibers, synthesis, fullerenes, palladium, catalysts
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:umu:diva-39631 (URN)10.1016/j.carbon.2010.11.015 (DOI)
Funder
Swedish Research Council, 2010-3973
Available from: 2011-02-09 Created: 2011-02-03 Last updated: 2017-12-11Bibliographically approved
3. Synthesis of palladium nanoparticles decorated helical carbon nanofiber as highly active anodic catalyst for direct formic acid fuel cells
Open this publication in new window or tab >>Synthesis of palladium nanoparticles decorated helical carbon nanofiber as highly active anodic catalyst for direct formic acid fuel cells
Show others...
2012 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 63, 323-328 p.Article in journal (Refereed) Published
Abstract [en]

We present a single metal approach to produce highly active catalyst materials based on Pd-decorated helical carbon nanofibers. Helical carbon fibers are synthesized by a chemical vapor deposition process on a C-60 supported Pd catalyst and the obtained fibers are functionalized by H2O2 followed by a decoration with Pd nanoparticles. Although transmission electron microscopy images show that the decoration is relatively inhomogeneous the electrocatalytic activity for formic acid oxidation is very high. Cyclic voltammetry measurements (CV) show that the generated current peak value for Pd-decorated helical carbon nanofibers is 300 mA/mg(Pd) for a scan rate of 10 mV/s. This is significantly higher than the corresponding value of a reference sample of multiwalled carbon nanotubes decorated with Pd nanoparticles by the same process. Fuel cell tests for our Pd-decorated helical carbon nanofibers also displayed a high power density, although not as superior to Pd-decorated multiwalled nanotubes as measured by CV. Our results show that helical carbon nanofibers have several good properties, such as a rigid anchoring of catalyst nanoparticles and a suitable structure for creating functionalization defects which make them an interesting candidate for electrochemical applications. (C) 2012 Elsevier Ltd. All rights reserved.

Place, publisher, year, edition, pages
Oxford: Pergamon Press, 2012
Keyword
Helical carbon fibers, Electrochemistry, Transmission electron microscopy, Catalyst, Palladium, Fuel cells, Formic acid
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-54138 (URN)10.1016/j.electacta.2011.12.104 (DOI)000301468800046 ()
Available from: 2012-04-20 Created: 2012-04-17 Last updated: 2017-12-07Bibliographically approved
4. Palladium nanocrystals supported on helical carbon nanofibers for highly efficient electro-oxidation of formic acid, methanol and ethanol in alkaline electrolytes
Open this publication in new window or tab >>Palladium nanocrystals supported on helical carbon nanofibers for highly efficient electro-oxidation of formic acid, methanol and ethanol in alkaline electrolytes
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2012 (English)In: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 209, 236-242 p.Article in journal (Refereed) Published
Abstract [en]

We present the synthesis of palladium nanocrystals self-assembled on helical carbon nanofibers functionalized with benzyl mercaptan (Pd-S-HCNFs) and their electrocatalytic activity toward the oxidation of formic acid, methanol and ethanol. Helical carbon nanofibers (HCNFs) were first functionalized with benzyl mercaptan based on the pi-pi interactions between phenyl rings and the graphitic surface of HCNFs. Palladium nano crystals (PdNC) were fixed on the surface of functionalized HCNF by Pd-S bonds in a simple self-assembly method. The as-prepared materials were characterized by high resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), cyclic voltammetry (CV), and fuel cell tests. CV characterization of the as-prepared materials shows a very high electrocatalytic activity for oxidation of formic acid, ethanol and methanol in strong alkaline electrolyte. In comparison to commercial catalyst Vulcan XC-72 decorated with Pd nanoparticles, the proposed Pd-S-HCNFs nano composite material shows oxidation currents for formic acid, ethanol and methanol at the Pd-S-HCNF-modified electrode that are higher than that at the Pd/XC-72 modified electrode with a factor of 2.0, 1.5, and 2.3, respectively. In a formic acid fuel cell the Pd-S-HCNF modified electrode yields equal power density as commercial Pd/XC-72 catalyst. Our results show that Pd-decorated helical carbon nanofibers with diameters around 40-60 nm have very high potential as active material in fuel cells, electrocatalysts and sensors. (C) 2012 Elsevier B.V All rights reserved.

Keyword
Helical carbon fibers, Electrocatalysts, Palladium, Nanoparticles, Electron microscopy, Fuel cells
National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-56189 (URN)10.1016/j.jpowsour.2012.02.080 (DOI)000303698800031 ()
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council, dnr-2010 3973
Note
Funding Agency: Magnus Bergvalls stiftelse, Kempestiftelsen, Wenner-Gren stiftelsen, Polish Ministry of Science and Higher Education through NCN  2011/01/B/STS/03888, Ångpanneforeningen, Gustaf Richerts stiftelse.   Available from: 2012-06-12 Created: 2012-06-12 Last updated: 2017-12-07Bibliographically approved
5. Self-assembled palladium nanocrystals on helical carbon nanofibers as enhanced electrocatalysts for electro-oxidation of small molecules
Open this publication in new window or tab >>Self-assembled palladium nanocrystals on helical carbon nanofibers as enhanced electrocatalysts for electro-oxidation of small molecules
Show others...
2012 (English)In: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 22, no 17, 8541-8548 p.Article in journal (Refereed) Published
Abstract [en]

We present a novel approach to prepare helical carbon nanofibers homogeneously functionalized with single crystal palladium nanoparticles via a phase-transfer method. The materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA) and electrochemical measurements. We find that homogeneous and small single-crystal Pd nanoparticles can be easily functionalized with phenyl mercaptan, transferred into the toluene phase from the dimethyl sulfoxide (DMSO) phase and then non-covalently self-assembled onto the surface of helical carbon nanofibers with a very good dispersion and homogeneous diameters of 4.5 +/- 0.6 nm. The palladium-helical carbon nanofiber composite exhibits significantly higher electrochemical active area and electrocatalytic activity towards the electrooxidation of formic acid, ethanol and methanol than the commercial electrocatalyst Pd/Vulcan XC-72. Our results show that the prepared material can be potentially used as an advanced nano-electrocatalyst in a direct alkaline fuel cell system.

National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-55402 (URN)10.1039/c2jm16075g (DOI)000302367500053 ()
Available from: 2012-05-14 Created: 2012-05-14 Last updated: 2017-12-07Bibliographically approved
6. Phase-transfer synthesis of amorphous palladium nanoparticle-functionalized 3D helical carbon nanofibers and its highly catalytic performance towards hydrazine oxidation
Open this publication in new window or tab >>Phase-transfer synthesis of amorphous palladium nanoparticle-functionalized 3D helical carbon nanofibers and its highly catalytic performance towards hydrazine oxidation
Show others...
2012 (English)In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 543, 96-100 p.Article in journal (Refereed) Published
Abstract [en]

Amorphous palladium nanoparticles functionalized helical carbon nanofibers (ApPd-HCNFs) were synthesized using a phase-transfer method. Palladium nanoparticles (Pd-NP) were first prepared using n-dodecyl sulfide as reducing agent and stabilizing ligands in ethanol. The Pd-NPs were then modified with benzyl mercaptan and transferred into a toluene solution with HCNFs which were decorated with amorphous palladium. The materials were characterized with high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and cyclic voltammetry showing that amorphous palladium nanoparticles were uniformly anchored at the HCNFs surface and that the ApPd-HCNFs exhibit high electrocatalytic activity towards hydrazine oxidation. (C) 2012 Elsevier B.V. All rights reserved.

Place, publisher, year, edition, pages
Elsevier, 2012
National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-58907 (URN)10.1016/j.cplett.2012.05.042 (DOI)000306972300018 ()
Available from: 2012-09-07 Created: 2012-09-06 Last updated: 2017-12-07Bibliographically approved

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