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Yttria-Stabilized Zirconia and Gadolinia-Doped Ceria Thin Films for Fuel Cell Applications
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Solid oxide fuel cells convert chemical energy directly into electrical energy with high efficiency and low emission of pollutants. However, before fuel cell technology can gain a significant share of the electrical power market, the operation temperature needs to be reduced in order to decrease costs and improve the durability of the cells. Application of thin film electrolytes and barrier coatings is a way of achieving this goal.

In this thesis, I have investigated film growth and microstructure of yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (CGO) thin films deposited by physical vapor deposition. The aim is to make industrially applicable coatings suitable for application in solid oxide fuel cells (SOFCs). For this purpose, the coatings need to be thin and dense. YSZ coatings were prepared by pulsed direct current (DC) magnetron sputtering and high power impulse magnetron sputtering (HiPIMS) in both laboratory- and industrial-scale setups.

Industrial-scale pulsed DC magnetron sputtering of YSZ showed that homogenous coating over large areas was possible. In order to increase film density of the YSZ, HiPIMS was used. By tuning deposition pressure, peak power density and substrate bias voltage it was possible to deposit noncolumnar thin films without voids and cracks as desired for SOFC applications.

CGO coatings were deposited by pulsed DC magnetron sputtering with the purpose of implementing diffusion barriers to prevent reactions between Sr from the SOFC cathode and the electrolyte. A model system simulating a SOFC was prepared by depositing thin CGO and YSZ layers on cathode material. This setup allowed the study of Sr diffusion by observing SrZrO3 formation using X-ray diffraction while annealing. Electron microscopy was subsequently performed to confirm the results. The study revealed Sr to diffuse along column/grain boundaries in the CGO films but by modifying the film thickness and microstructure the breaking temperature of the barrier could be increased.

CGO thin films were implemented in metal-based SOFC and the influence of film microstructure and thickness on the electrochemical performance of the cell was studied. Cell tests showed that an area specific resistance (ASR) down to 0.27 Ωcm2 could be obtained 650 °C with sputtered CGO barrier layers in combination with a lanthanum strontium cobaltite cathode. In comparison a spin-coated CGO barrier resulted in an ASR value of 0.50 Ωcm2. This shows the high effectiveness of the sputtered barrier in obtaining state-of-the-art performance.

In summary, this work provides fundamental understanding of the deposition and growth of YSZ and CGO thins films and proves the prospective of employing thin film barrier coating in order to obtain high-performing SOFCs.  

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. , 63 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1564
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-102513DOI: 10.3384/diss.diva-102513ISBN: 978-91-7519-441-7 (print)OAI: oai:DiVA.org:liu-102513DiVA: diva2:678657
Public defence
2014-02-25, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2013-12-12 Created: 2013-12-12 Last updated: 2015-01-13Bibliographically approved
List of papers
1. Reactive magnetron sputtering of uniform yttria-stabilized zirconia coatings in an industrial setup
Open this publication in new window or tab >>Reactive magnetron sputtering of uniform yttria-stabilized zirconia coatings in an industrial setup
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2012 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 206, no 19-20, 4126-4131 p.Article in journal (Refereed) Published
Abstract [en]

Yttria-stabilized zirconia (YSZ) thin films were deposited by reactive magnetron sputtering in an industrial scale setup on silicon wafers as well as commercial NiO-YSZ fuel cell anodes. The texture, morphology, and composition of the deposited films were investigated as a function of deposition parameters. Homogeneous coatings could be deposited over large areas within the coating zone, which is important for industrial applications. The use of substrate bias during film growth was identified as a key parameter to promote less columnar coatings and made it possible to tailor the texture of films deposited on Si. Bias voltages less than= - 40V resulted in highly less than 200 greater than textured YSZ films, intermediate bias voltages of - 50 V to - 70 V in less than 220 greater than textured films and high bias voltages (greater than= - 90 V) in a mixed orientation. In contrast, films grown on NiO-YSZ were seen to be randomly orientated when deposited at substrate bias voltages less than= - 30 V. When bias was further increased the film took over the orientation of underlying substrate due to substrate template effects.

Place, publisher, year, edition, pages
Elsevier, 2012
Keyword
Physical vapor deposition (PVD); Solid oxide fuel cell (SOFC); X-ray diffraction (XRD); Electron microscopy; Elastic recoil detection analysis (ERDA)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-79708 (URN)10.1016/j.surfcoat.2012.04.007 (DOI)000305662400043 ()
Available from: 2012-08-13 Created: 2012-08-13 Last updated: 2017-12-07
2. Deposition of yttria-stabilized zirconia thin films by high power impulse magnetron sputtering and pulsed magnetron sputtering
Open this publication in new window or tab >>Deposition of yttria-stabilized zirconia thin films by high power impulse magnetron sputtering and pulsed magnetron sputtering
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2014 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 240, 1-6 p.Article in journal (Refereed) Published
Abstract [en]

Yttria-stabilized zirconia (YSZ) thin films were reactively sputter-deposited by high power impulse magnetron sputtering (HiPIMS) and pulsed direct current magnetron sputtering (DCMS). The use of substrate bias voltage was studied in both modes of deposition as a process parameter to promote the growth of dense and less columnar films. Films were deposited on both Si(100) and NiO-YSZ fuel cell anodes. The texture, morphology and composition of the deposited films were investigated with regard to their application as thin electrolytes for solid oxide fuel cells (SOFCs). Independent of the deposition mode the films were found to be stoichiometric. The application of substrate bias voltage had opposite effects on texture and crystallinity of films deposited by pulsed DCMS and HiPIMS. Films deposited by pulsed DCMS became highly crystalline and <220> textured at high bias voltage whereas bias applied to HiPIMS deposited films disrupted crystal growth leading to deterioration of crystallinity. Comparing film morphology, it was found that pulsed DCMS films were columnar and contained voids regardless of the applied substrate bias. When depositing by HiPIMS a window of operation at a bias voltage of -25 V to -50 V was found in which it is possible to deposit non-columnar thin films without voids and cracks as desired for SOFC applications. 

Keyword
HiPIMS, HPPMS, pulsed DCMS, SOFC, YSZ, Substrate bias
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-102516 (URN)10.1016/j.surfcoat.2013.12.001 (DOI)000331989900001 ()
Available from: 2013-12-12 Created: 2013-12-12 Last updated: 2017-12-06
3. Industrial-scale high power impulse magnetron sputtering of yttria-stabilized zirconia on porous NiO/YSZ fuel cell anodes
Open this publication in new window or tab >>Industrial-scale high power impulse magnetron sputtering of yttria-stabilized zirconia on porous NiO/YSZ fuel cell anodes
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2015 (English)In: SURFACE & COATINGS TECHNOLOGY, ISSN 0257-8972, Vol. 281, 150-156 p.Article in journal (Refereed) Published
Abstract [en]

Yttria-stabilized zirconia (YSZ) thin films are reactively sputter-deposited by high power impulse magnetron sputtering (HiPIMS) in an industrial setup on porous NiO/YSZ fuel cell anodes. The influence of deposition pressure, peak power and substrate bias on the deposition rate and film morphology is studied. It is seen that depositing at increasing the deposition pressure from ~370 mPa to ~750 mPa results in a 64 % increase in the deposition rate and denser film. Films are deposited at peak power densities ranging from 0.4 kW/cm2 to 1.1 kW/cm2. By increasing the peak power density the degree of ionization degree of both Ar and sputtered metallic species is significantly increased which results in denser films as open column boundaries are removed. The increase in peak power also results in a significant drop in deposition rate. By combining a peak power density of ~0.6 kW/cm2 with the application of -180 V substrate bias voltage a homogenous and essentially columnless coating can be deposited. These results demonstrate HiPIMS deposition is capable of producing dense, YSZ coatings on porous substrates as needed for solid oxide fuel cell application. 

Place, publisher, year, edition, pages
Elsevier, 2015
Keyword
Physical Vapor deposition (PVD), Solid Oxide Fuel Cell (SOFC), YSZ, HPPMS
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-102515 (URN)10.1016/j.surfcoat.2015.09.058 (DOI)000366072200019 ()
Available from: 2013-12-12 Created: 2013-12-12 Last updated: 2016-06-01
4. Strontium diffusion in magnetron sputtered gadolinia-doped ceria thin film barrier coatings for solid oxide fuel cells
Open this publication in new window or tab >>Strontium diffusion in magnetron sputtered gadolinia-doped ceria thin film barrier coatings for solid oxide fuel cells
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2013 (English)In: Advanced Energy Materials, E-ISSN 1614-6840, Vol. 3, no 7, 923-929 p.Article in journal (Refereed) Published
Abstract [en]

Strontium (Sr) diffusion in magnetron sputtered gadolinia-doped ceria (CGO) thin films is investigated. For this purpose, a model system consisting of a screen printed (La,Sr)(Co,Fe)O3−δ (LSCF) layer, and thin films of CGO and yttria-stabilized zirconia (YSZ) is prepared to simulate a solid oxide fuel cell. This setup allows observation of Sr diffusion by observing SrZrO3 formation using X-ray diffraction while annealing. Subsequent electron microscopy confirms the results. This approach presents a simple method for assessing the quality of CGO barriers without the need for a complete fuel cell test setup. CGO films with thicknesses ranging from 250 nm to 1.2 μm are tested at temperatures from 850 °C to 950 °C which yields an in-depth understanding of Sr diffusion through CGO thin films that may be of high scientific and technical interest for implementation of novel fuel cell materials. Sr is found to diffuse along column/grain boundaries in the CGO films but by modifying the film thickness and microstructure the breaking temperature of the barrier can be increased.

Place, publisher, year, edition, pages
John Wiley & Sons, 2013
Keyword
Ce0.9Gd0.1O2−δ; CGO; GDC; Sr diffusion; X-ray diffraction
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-84610 (URN)10.1002/aenm.201300003 (DOI)000327698200015 ()
Note

On the day of the defence date the status of this article was Manuscript.

Available from: 2012-10-15 Created: 2012-10-15 Last updated: 2017-04-10Bibliographically approved
5. Magnetron sputtered Gadolina-doped Ceria Diffusion Barriers for Metal-supported Solid Oxide Fuel Cells
Open this publication in new window or tab >>Magnetron sputtered Gadolina-doped Ceria Diffusion Barriers for Metal-supported Solid Oxide Fuel Cells
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2014 (English)In: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 267, 452-458 p.Article in journal (Refereed) Published
Abstract [en]

Gadolinia-doped ceria (GDC) thin films are deposited by reactive magnetron sputtering in an industrial-scale setup and implemented as barrier layers between the cathode and electrolyte in metal-based solid oxide fuel cells consisting of a metal support, an electrolyte of ZrO2 co-doped with Sc2O3 and Y2O3 (ScYSZ) and a Sr-doped lanthanum cobalt oxide cathode. In order to optimize the deposition of GDC to obtain high electrochemical performance of the cells, the influence of film thickness and adatom mobility is studied. The adatom mobility is varied by tuning the deposition temperature and substrate bias voltage.

A GDC layer thickness of 0.6 µm is found to effectively block Sr diffusion when bias voltage and deposition temperature is tuned to promote dense coatings. The adatom mobility has a large influence on the film density. Low temperature and bias voltage result in underdense column boundaries which function as channels for Sr to diffuse to the GDC-ScYSZ interface. By tuning deposition temperature, bias voltage and film thickness area specific resistances down to 0.34 Ωcm2 are achieved at cell tests performed at an operating temperature of 650 °C.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
Physical Vapor deposition (PVD); Solid Oxide Fuel Cell (SOFC); GDC; CGO
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-102517 (URN)10.1016/j.jpowsour.2014.05.101 (DOI)000339601800054 ()
Available from: 2013-12-12 Created: 2013-12-12 Last updated: 2017-12-06
6. High performance metal-supported solid oxide fuel cells with Gd-doped ceria barrier layers
Open this publication in new window or tab >>High performance metal-supported solid oxide fuel cells with Gd-doped ceria barrier layers
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2011 (English)In: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 196, no 22, 9459-9466 p.Article in journal (Refereed) Published
Abstract [en]

Metal-supported solid oxide fuel cells are believed to have commercial advantages compared to conventional anode (Ni-YSZ) supported cells, with the metal-supported cells having lower material costs, increased tolerance to mechanical and thermal stresses, and lower operational temperatures. The implementation of a metallic support has been challenged by the need to revise the cell fabrication route, as well as electrode microstructures and material choices, to compete with the energy output and stability of full ceramic cells. less thanbrgreater than less thanbrgreater thanThe metal-supported SOFC design developed at Riso DTU has been improved, and an electrochemical performance beyond the state-of-the-art anode-supported SOFC is demonstrated possible, by introducing a CGO barrier layer in combination with Sr-doped lanthanum cobalt oxide (LSC) cathode. Area specific resistances (ASR) down to 0.27 Omega cm(2), corresponding to a maximum power density of 1.14 W cm(-2) at 650 degrees C and 0.6 V. were obtained on cells with barrier layers fabricated by magnetron sputtering. The performance is dependent on the density of the barrier layer, indicating Sr(2+) diffusion is occurring at the intermediate SOFC temperatures. The optimized design further demonstrate improved durability with steady degradation rates of 0.9% kh(-1) in cell voltage for up to 3000 h galvanostatic testing at 650 degrees C and 0.25 A cm(-2).

Place, publisher, year, edition, pages
Elsevier, 2011
Keyword
SOFC, Metal-supported, Barrier layer, Magnetron sputtering, Durability
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-71631 (URN)10.1016/j.jpowsour.2011.07.014 (DOI)000295602400048 ()
Note

Funding Agencies|Topsoe Fuel Cell A/S||Danish National Advanced Technology Foundation||EU|FP7-211940 (METSOFC)|NordForsk|9346|

Available from: 2011-10-27 Created: 2011-10-27 Last updated: 2017-12-08

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