+ All Categories
Transcript
Page 1: Master Thesis SOFC

SOLID OXIDE FUEL CELL:

DESIGN IMPROVEMENTS OF THE ELECTROSTATIC SPRAY

DEPOSITION SYSTEM AND CELL TEST STATION

DEVELOPMENT

BY

ARTURO VEIGA LÓPEZ

Submitted in partial fulfillment of the

requirements for the degree of

Master in Mechanical and Aerospace Engineering

in the Graduate College of the

Illinois Institute of Technology

Chicago, Illinois

October 2013

Page 2: Master Thesis SOFC

Master Project. Arturo Veiga López 2

Acknowledgements

Foremost, I would like to express my sincere gratitude to my advisor Prof. Philip Nash

for the continuous support during my stay in the Illinois Institute of Technology and

for giving me this opportunity; for his patience, motivation, and knowledge. His

guidance has helped me during my research. I would also like to thank Prof. Robert

Selman for his encouragement, insightful comments, and hard questions.

I thank my fellow labmate Quanzhi He, I am grateful to for his support and guidance in

the laboratory. I also would like to thank Russ Janota for his assistance throughout my

experiments.

Last, but not least, I would like to thank my sister Almudena for all her help and advice

during these years, for her support and encouragement, without you this wouldn´t be

possible. And also to my brother in law Carlos for his continuous encouragement.

My parents, Encarnación and Alfonso, receive my deepest gratitude and love for their

dedication and the many years of support during my undergraduate and college

studies that provided the foundation for this work.

Page 3: Master Thesis SOFC

Master Project. Arturo Veiga López 3

Table of contents

Acknowledgement………………………………………………………………………………………………. 2

List of figures………………………………………………………………………………………………………. 5

List of symbols…………………………………………………………………………………………………..... 8

Abstract……………………………………………………………………………………………………………… 10

Chapter

1. Introduction and background………………………………………………………………… 11

1.1. Introduction…………………………………………………………………………….. 11

1.1.1. Planning and work breakdown structure (WBS)………….. 12

1.1.2. Cost analysis……………………………………………………………….. 14

1.2. General overview and background………………………...…………………. 17

1.2.1. Principles of SOFC……………………………………………………….. 17

1.2.2. Fuel Cell Performance ………………………………………..……….. 20

2. Development of a SOFC electrolyte with ESD technique…………………..…........ 25

2.1. Introduction and objectives……………………………………………………… 25

2.2. Development of an enhanced spray system……………………………...... 28

2.2.1. Spray stability…………………..…………………..………………………….……. 29

2.2.1.1. Design requirements……………………………………………..…. 29

2.2.1.2. Experiment methodology…………………………………………. 32

2.2.1.3. Experiment results………………….……………..…………………. 34

2.2.2. Increasing the powder concentration of the suspension …………. 39

2.2.2.1. Electrostatic stabilization.…………………..…………………….. 42

2.2.2.2. Steric stabilization…………………..………………………………... 42

Page 4: Master Thesis SOFC

Master Project. Arturo Veiga López 4

3. SOFC Design, operation and results…………………………………..……………………. 44

3.1. Design of the SOFC test station…………………..…………………………....... 44

3.1.1. Principles of design of the SOFC test station…………………. 44

3.1.2. Parts of the SOFC test station………………….. ……………..……. 45

3.2. Preparation of the experiment ………………………………………………..... 54

3.2.1. Assembly process of the fuel cell …………………..…………...... 54

3.2.2. Experimental AC impedance procedure ……………..………... 58

3.3 Results …………..…………………………..…………………………..……………..…..61

3.3.1. Open circuit potential …………..…………………………..…...……. 61

3.3.2. Voltage and current curve (i-V) ……………..…………….……… 62

3.3.3. Electrochemical impedance spectroscopy ……………..…….. 65

4. Conclusion ..……………..………………………..………………………..………………………… 68

References ……………..………………………..…………………………..…………………………………..…. 70

Page 5: Master Thesis SOFC

Master Project. Arturo Veiga López 5

List of figures

Figure 1.1. Work Breakdown Structure of the project (WBS).

Figure 1.2. Gant Chart of the project.

Figure 1.3. Schematic of a SOFC.

Figure 1.4. i-V curve with Tafel fitting .

Figure 1.5. Application of a voltage perturbation creates a current response

maintaining the linearity.

Figure 1.6. Physical sketch, equivalent circuit and Nyquist plot of an interface.

Figure 1.7. Physical sketch, equivalent circuit and Nyquist plot of an entire fuel cell.

Figure 2.1. Schematic of Sketch of the Electrostatic Spray Deposition technique.

Figure 2.2. Schematic of Coulomb force between two particles. Particles with the same

charge repel each other.

Figure 2.3. Electrostatic Spray Deposition System developed by He, 2013. Syringe and

pump (top left are connected to the nuzzle (top right). Note detail of heated substrate

(top right).

Figure 2.4. Improved spray system.

Figure 2.5. Types of spray based on distance to voltage combinations. Flow rate and

temperature used were 1ml/h and 230C, respectively. Note that four modes of spray

can be achieved: sputtering, squirt, cone jet, and dripping. Refer to text for an in depth

explanation of each mode.

Figure 2.6. Left: Cone Jet Spray: Ideal spray mode. Uniform spray guarantees best

results. Right: Cone Jet Spray deposition result. Note uniformity of deposit.

Figure 2.7. Different samples of YSZ sprayed onto a silicon substrate.

Figure 2.8. SEM micrograph of the sample obtained with a spray time of 30 min using

original spray system.

Figure 2.9. SEM micrograph of the sample obtained with a spray time of 30 min using

new spray system without vacuum.

Figure 2.10. SEM micrograph of the sample obtained with a spray time of 1 hour using

new spray system without vacuum.

Page 6: Master Thesis SOFC

Master Project. Arturo Veiga López 6

Figure 2.11. SEM micrograph of the sample obtained with a spray time of 14 hours

using new spray system without vacuum.

Figure 2.12. SEM micrograph of the sample obtained with a spray time of 14 hours

using new spray system without vacuum.

Figure 2.13. SEM micrograph of the sample obtained with a spray time of 17 hours

using new spray system without vacuum.

Figure 2.14. Schematic of forces that a particle is subjected to when in a fluid.

Gravitational and drag forces are opposite.

Figure 2.15. Schematic representation of potentials involved in a particle (surface

charge , Stern potential and Zeta potential).

Figure 2.16. Schematic representation of electrostatic stabilization.

Figure 2.17. Schematic representation of steric stabilization.

Figure 3.1. Schematic sketch the Solid Oxide Fuel Cell (SOFC) system and its parts.

Figure 3.2. Schematic of the hot temperature gas system.

Figure 3.3. Zirconia tube.

Figure 3.4. Anode-supported fuel cell characteristics model ASC-2.0.

Figure 3.5. Schematic of the assembly of the fuel cell.

Figure 3.6. SEM of silver wire in contact with Aremco Cement at 800C.

Figure 3.7. Composition of the surface of the Ag in contact with Aremco Cement at

800C.

Figure 3.8. SEM of silver wire near Aremco Cement at 800C.

Figure 3.9. Composition of the surface of the Ag near Aremco Cement at 800C.

Figure 3.10. Image of the Zirconia and inconel tubes with the furnace open.

Figure 3.11. Detailed view of the silver mesh impregnated with the ink and connected

to the wire.

Figure 3.12. Detailed view of the cathode side of the fuel cell mounted in the Zirconia

tube.

Figure 2.13. Final placement of the fuel cell before the test.

Figure 3.14. Curing curves for a layer of cement

Figure 3.15. Schematic of the two electrode connection diagram.

Page 7: Master Thesis SOFC

Master Project. Arturo Veiga López 7

Figure 3.16. Open circuit potential curve. At 800C and 97% H2 and 3% H2O.

Figure 3.17. i-V curve. At 800C and 97% H2 and 3% H2O.

Figure 3.18. Power density curve. At 800C and 97% H2 and 3% H2O.

Figure 3.19. Fuel cell performance curves. Fuel Cell Materials data sheet for model

ASC-2.0.

Figure 3.20. Results AC impedance spectrocopy. At 800C and 97% H2 and 3% H2O.

Figure 3.21. Equivalent circuit of the fuel cell.

Figure 3.22. Experimental results (red); Fitting results (green).

Page 8: Master Thesis SOFC

Master Project. Arturo Veiga López 8

List of symbols

ASI Specific Area Impedance

C Capacitance

Ethermo Thermodynamically predicted voltage of fuel cell.

EIS Electrochemical Impedance Spectroscopy

ESD Electrostatic Spray Deposition

Fd Drag force

Fg Gravity force

FC Fuel Cell

FRA Frequency Response Analyzer

g gravity

i-V Current-Voltage

OPC Open Circuit Potential

R Radious of the particle

R Resistance

Rf Faradic Resistance

RΩ Ohmic Resistance

SEM Scanning Electron Microscope

SOFC Solid Oxide Fuel Cell

V Settling velocity

V Voltage

Vp Volume of the particle

WBS Work Breakdown Structure

YSZ Yttria-stabilized Zirconia

Z Impedance

Zreal Real part of the Impedance

Zimag Imaginary part of the impedance

act Activation losses due to reaction kinetics.

ohmic Ohmic losses from ionic and electronic losses.

conc Concentration losses due to mass transport.

Page 9: Master Thesis SOFC

Master Project. Arturo Veiga López 9

µ Dynamic viscosity

ρf Fluid density

ρp Particle density

ω Frequency

Page 10: Master Thesis SOFC

Master Project. Arturo Veiga López 10

1. Abstract

The current project work focuses on the principles of fuel cells and the parameters

that measure the performance of a solid oxide fuel cell. The first two aims of the study

were to enhance the efficiency of the Electrostatic Spray Deposition (ESD) spray by

improving the spray stability. This can be achieved by increasing a more

homogeneous supply of particle by the ESD spray. Two main factors were studied; the

settling of the particles and the tendency of the particles to stick to the container

walls. The finished system was tested spraying layers of YSZ up to 14 hours and 70

microns of thickness. From our results we can conclude that this system is

appropriate to produce a stable spray system for YSZ and that the desired thickness

can be obtained varying the deposition time. The third aim of the study was to assess

the performance of a solid oxide fuel cell in order to improve its efficiency. For that, a

solid oxide fuel cell test station was designed and manufactured and the performance

of a commercially available fuel cell was evaluated. In conclusion, the fuel cell testing

system was validated and showed a fairly good approximation to the values provided

by the manufacturer, validating the functionality of the test station. Additional

improvements and research are warranted to achieve a fully reproducible and

commercially available solid oxide fuel cell.

Page 11: Master Thesis SOFC

Master Project. Arturo Veiga López 11

1. Introduction and background

1.1. Introduction

This project was part of the research focus of Professor Philip Nash and in collaboration with

Professor J. Robert Selman in the field of Solid Oxide Fuel Cells. The ultimate goal is to build a

Fuel Cell using the Electrostatic Spray Deposition technique in one step. The objective is to

achieve a sensible more efficient Fuel Cell in terms of electricity produced per unit of fuel, but

also developing a simple, low cost and scalable manufacturing process.

This research first started 2011 with the Master’s student Quanzhi He and I continued on the

project starting in September of 2012 as part of my research training during my Master in

Mechanical and Aerospace Engineering.

From my perspective this was a great opportunity to learn a new technology. The challenges

of this project were beyond the simply material science knowledge. This project combines

disciplines such as electrochemistry, material science, and fluid dynamics with nanoparticles.

This was a broad field that allowed to broaden my knowledge and challenged me every day.

Because most of the equipment used was design and build with the help of the faculty of the

Mechanical, Materials and Aerospace Engineering department, it also gave me the opportunity

of being part of designing and prototyping the equipment that will be used to perform those

tests and that will be used in the future by future colleagues.

Page 12: Master Thesis SOFC

Master Project. Arturo Veiga López 12

1.1.1. Planning and work breakdown structure (WBS)

This project was developed from September 2012 to October 2013 and was subjected to

changes of scope due to time and resource constraints, or due to unplanned challenges and

pitfalls along the process.

The following diagram shows the work breakdown structure of the project. In brief, the

structure has three components: The study of the Electrochemical Impedance Spectroscopy

(EIS) which is the technique that will be used for test the performance of a SOFC. The

Electrostatic Spray Deposition that is the technique used to build the SOFC and here a new

system was design build and tested. And the SOFC test station which is the system that checks

the performance of the SOFC, and here includes the design, build and test with a commercial

fuel cell.

3. SOFC

2. ESD

1. Study EIS

WBS

1.1. Principles

1.2. Test

with batteries

3.1. Design

Test station

3.2. Build

Test station

2.2. Build

New ESD system

2.3. Spray

YSZ

2.1. Design

New ESD system

3.3 Test

Commercial FC

Figure 1.1. Work Breakdown Structure of the project (WBS).

Page 13: Master Thesis SOFC

Master Project. Arturo Veiga López 13

A simplified Gant chart is showed in the figure 1.2. in order to give a basic time frame of the project and the activities executed during the

project.

Work breakdown structure

2012 2013

9 10 11 12 1 2 3 4 5 6 7 8 9

EIS

Study principles FC

Study EIS technique

Battery tests

Equivalent circuit

Test Station

Design

Build

Sealing Tests

Leakage repair

Improve system

ESD

Learn technique

Prototyping

Design final

Build final design

Repair Voltage control

Spray new system

SOFC Test

Build test station

OCP

I-V

EIS

Write thesis

Figure 1.2. Gant Chart of the project.

Page 14: Master Thesis SOFC

Master Project. Arturo Veiga López 14

1.1.2. Cost analysis

The current project has been fully funded by Professor Philip Nash, and my advisor during

this project.

Although a detailed breakdown of all the costs involved in this project has not been

conducted, we have estimated that the total cost of the project is ~ $100,000. However, many

items are difficult to estimate, such as work hours, specialists consultation, amortization of

high specialized-equipment, software used, maintenance and hours of labor to build the

prototypes.

Only a fraction of the real cost of the project is shown here (see Tables 1.1 and 1.2), namely

the materials needed to build a test for a Solid Oxide Fuel Cell (SOFC) station and the

improved in the existing Electrostatic Spray Deposition (EDS) system. It important to consider

that also these materials are part of the ones used in other projects, specially the equipment

like the inductance oven and heaters for the SOFC test station. In the EDS system the High

Voltage power supply, syringe pump, and substrate heater were reused from other research

projects.

The following itemized list of materials is only to be used as a guide for the total cost of the

materials to facilitate a better understanding of the whole project.

Page 15: Master Thesis SOFC

Master Project. Arturo Veiga López 15

Table 1.1. New EDS system materials.

Line Quantity Product Unit Price Total Price

1 1 8486K545 Optically Clear Cast Acrylic Tube, 2-1/2" OD X 2" ID, 1' Length 34.94 34.94

2 10 9685T3 High-Pressure White Nylon Tubing, .15" ID, 1/4" OD, .05" Wall Thickness 0.85 8.5

3 3 5012K38 Acetal Quick-Disconnect Coupling, Socket, 1/8 Coupling, for 1/8" Tube ID 6.91 20.73

4 3 5012K46 Acetal Quick-Disconnect Coupling, Plug, 1/8 Coupling, 1/8" Male NPT 7.01 21.03

5 1 51525K221 Plastic Quick-Turn (Luer Lock) Coupling, Nylon, Male X Male Thread, 1/4" 5.25 5.25

6 2 2930T63 Type 316 Stainless Steel Wire Cloth Disc, 100 X 100 Mesh, 2-9/16" 6.45 12.9

7 1 9319T183 Super-Corrosion-Resistant 316 Stainless Steel, 100X100 Mesh 8.59 8.59

8 10 8359K14 High-Strength Clear Nylon Tubing, .190" ID, 1/4" OD, .030" Wall Thickness 0.36 3.6

9 1 8560K358 Optically Clear Cast Acrylic Sheet, 1/4" Thick, 6" X 6" 5.49 5.49

10 2 5392K11 Rigid White Polypropylene Tubing, 1/8" ID, 1/4" OD, 1/16" Wall Thickness 0.48 0.96

11 2 53945K111 Hard-Wall Rigid Clear PVC Tubing, .170" ID, 1/4" OD, .04" Wall Thickness 0.84 1.68

12 1 1901K11 Chemical-Resistant PVDF Tube Fitting, 90 Degree Elbow for 1/4" Tube OD 4.5 4.5

13 1 51055K984 White Acetal Instant Tube Fitting, 90 Degree Elbow for 1/4" Tube OD 2.73 2.73

14 1 8560K275 Optically Clear Cast Acrylic Sheet, 1/8" Thick, 6" X 12" 5.07 5.07

15 1 8547K62 Tube Made of Teflon(R) PTFE, 1/2" OD X 5/16" ID, 1' Length 7.89 7.89

16 3 5128A62 Low-Profile Hold-Down Toggle Clamp, Standard, Steel 8.58 25.74

17 1 8486K555 Optically Clear Cast Acrylic Tube, 3" OD X 2-1/2" ID, 1' Length 35.73 35.73

18 2 5678K111 Magnetic Stirring Bar, Ocatagonal, 1/2" Length, 5/16" OD 3.96 7.92

19 2 5678K112 Magnetic Stirring Bar, Octagonal, 5/8" Length, 5/16" OD 4.41 8.82

20 5 52705K31 Ultra-Clear PFA Tubing, 1/16" ID, 1/8" OD, 1/32" Wall Thickness, Clear 1.52 7.6

21 1 91772A165 18-8 Stainless Steel Pan Head Phillips Machine Screw, 6-32 Thread 6.66 6.66

22 1 9545K27 Push-in Tapered Round Rubber Plug, Through Hole 8.33 8.33

23 1 9545K18 Push-in Tapered Round Rubber Plug, Solid, Size 6, Fits 1-1/4" 15.91 15.91

260.57TOTAL

New ESD System

Page 16: Master Thesis SOFC

Master Project. Arturo Veiga López 16

Table 1.2. SOFC test station materials.

Line Quantity Product Description Unit Price Total Price

1 1 4757K61 Miniature PVC Ball Valve Diverting, 3-Port, Push-to-Connect 20.61$ 20.61$

2 50 5195T64 Water-Resistant Clear Polyurethane Tubing 3/16" 0.39$ 19.50$

3 1 89495K151 Stainless Steel Round Tube Type 304, 3" OD, 2-1/2" 90.05$ 90.05$

4 2 5182K256 Type 316 SS Yor-Lok Tube Fitting Adapter for 1/4" Tube NPT 19.23$ 38.46$

5 2 50775K403 Tube Support for 1/4" Tube ID Durable Nylon Compression Tube Fitting 6.09$ 12.18$

6 1 6811A22 Super-High Temperature Wrap for Wire and Cable Protection 5.16$ 5.16$

7 30 5195T64 Water-Resistant Clear Polyurethane Tubing 3/16" ID, 1/4" OD 0.39$ 11.70$

8 1 89495K151 Stainless Steel Round Tube Type 304, 3" OD, 2-1/2" 90.05$ 90.05$

9 1 50775K403 Tube Support for 1/4" Tube OD Durable Nylon Compression Tube Fitting 6.09$ 6.09$

10 1 4093T21 Diverting 3-Port Brass Ball Valve Any-Direction, 1/4" NPT Female 41.64$ 41.64$

11 1 92001A291 18-8 Stainless Steel Wing Nut 8-32 Thread Size, 29/32" Wing Spread 6.58$ 6.58$

12 1 92141A011 18-8 SS General Purpose Flat Washer NO. 10 Screw Size, 7/16" OD 2.33$ 2.33$

13 2 47065T142 Std Zinc-Plated STL End-Feed Fastener, for 1" Aluminum Inch T-Slotted 2.30$ 4.60$

14 1 87175K93 NON-Porous High Alumina Ceramic Tube 4 Bore, .188" OD 27.88$ 27.88$

15 1 5182K269 Type 316 SS Yor-Lok Tube Fitting Adapter for 1/2" NPT Female Pipe 20.04$ 20.04$

16 1 87175K91 NON-Porous High Alumina Ceramic Tube 4 Bore, .188" OD 20.91$ 20.91$

17 1 92001A291 18-8 Stainless Steel Wing Nut 8-32 Thread Size, 29/32" Wing Spread 6.58$ 6.58$

18 1 92141A011 18-8 SS General Purpose Flat Washer NO. 10 Screw Size, 7/16" OD 2.33$ 2.33$

19 2 47065T142 Std Zinc-Plated STL End-Feed Fastener, for 1" Aluminum Inch T-Slotted 2.30$ 4.60$

20 1 5182K269 Type 316 SS Yor-Lok Tube Fitting Adapter for 1/2" Tube OD X 1/4" NPT 20.04$ 20.04$

21 1 9922K121 Corrosion-Resistant Steel Tubing 1/4" OD, .18" ID 18.97$ 18.97$

22 3 5182K577 Type 316 SS Yor-Lok Tube Fitting Front and back Sleeve for 1/2" Tube OD 4.50$ 13.50$

24 3 5182K557 Type 316 SS Yor-Lok Tube Fitting Nut for 1/2" Tube OD 5.25$ 15.75$

25 1 5182K129 Type 316 SS Yor-Lok Tube Fitting Adapter for 1/2" NPT Male Pipe 16.32$ 16.32$

26 1 5182K269 Type 316 SS Yor-Lok Tube Fitting Adapter for 1/2" NPT Female Pipe 20.04$ 20.04$

27 2 47065T206 Aluminum Inch T-Slotted Framing System Single Horiz Tube Holder 36.12$ 72.24$

28 1 47065T101 Aluminum Inch T-Slotted Framing System Four-Slot Single 14.20$ 14.20$

29 8 47065T223 Aluminum Inch T-Slotted Framing System 90 Degree Bracket 3.98$ 31.84$

30 1 9657K302 Steel Compression Spring Zinc-Pltd Music Wire, 1.00" L,.300" OD 9.80$ 9.80$

31 1 91792A209 18-8 SS Pan Head Slotted Machine Screw 8-32 Thread, 3" Length 7.01$ 7.01$

32 1 9435K43 302 SS Precision Compression Spring 1.0" Length, .24" OD, 5.02$ 5.02$

33 1 9246K11 Multipurpose Aluminum (Alloy 6061) 1/4" Thick, 8" X 8" 17.38$ 17.38$

34 1 8746K541 NON-Porous High-Alumina Ceramic Tube .875" OD, 5/8" ID, 12" Length 67.60$ 67.60$

35 1 8746K542 NON-Porous High-Alumina Ceramic Tube .875" OD, 5/8" ID, 24" Length 122.90$ 122.90$

36 1 9396K104 Silicone O-Ring AS568A Dash Number 020, Packs of 25 5.27$ 5.27$

37 4 3854T81 Clear Plastic Petri Dish with Cover, 3-1/2" Diameter, 5/8" Height 6.62$ 26.48$

38 1 5943K232 Cleaned and Bagged Yor-Lok SS Tube Fitting Branch Tee for 1/2" NPT 56.42$ 56.42$

39 3 5182K755 Type 316 SS Yor-Lok Tube Fitting Tube Stem Reducing Cplg for 1/2" 13.33$ 39.99$

40 5 5182K574 Type 316 SS Yor-Lok Tube Fitting Front & Back Sleeve for 1/4" Tube OD 2.56$ 12.80$

42 1 5182K242 Type 316 SS Yor-Lok Tube Fitting Reducing Coupling for 1/4" 15.90$ 15.90$

43 6 2572K26 Load-Rated Bumper Polyurethane, Med-Hard, 1/4" 7.94$ 47.64$

44 3 5182K577 Type 316 SS Yor-Lok Tube Fitting Front & Back Sleeve for 1/2" Tube OD 4.50$ 13.50$

46 3 5182K557 Type 316 SS Yor-Lok Tube Fitting Nut for 1/2" Tube OD 5.25$ 15.75$

47 1 5182K129 Type 316 SS Yor-Lok Tube Fitting Adapter for 1/2" NPT Male Pipe 16.32$ 16.32$

48 1 5182K269 Type 316 SS Yor-Lok Tube Fitting Adapter for 1/2" NPT Female Pipe 20.04$ 20.04$

49 1 SSR330DC10 Solid State Relay 21.00$ 21.00$

50 3 TJ50 Thermocuple TJ50-CAIN-14U-6-SB-OSTW-M 51.25$ 153.75$

51 1 TJ70 Thermocouple TJ70-CAIN-116U-30-SB-OSTW-M 51.45$ 51.45$

52 1 TJ50 Thermocouple TJ50-CAIN-116U-12-SB-OSTW-M 43.25$ 43.25$

53 1 INC-12-30OPEN Miniature protection tube INC-12-30OPEN 28.70$ 28.70$

54 1 AG-M40-100 Silver mesh 205.00$ 205.00$

55 1 Ag-I Silver ink 95.00$ 95.00$

56 1 ASC-2.0 Anode supported fuel cell 155.00$ 155.00$

57 1 C-885 Zr-based Cement - Ceramabond 885 93.00$ 93.00$

2,000.16$ TOTAL

Test Station

Page 17: Master Thesis SOFC

Master Project. Arturo Veiga López 17

1.2. General overview and background

1.2.1. Principles of SOFC

A Fuel Cell is a device that transforms the chemical energy of a fuel into electric

current through an electrochemical process. It was first discovered in 1839 by Sir

William Grove.

The focus of this project is the Solid Oxide Fuel Cell (SOFC, see figure 1.3. for details),

one of many types of fuel cells. This fuel cell is in solid state in its operation and it is

made with special ceramics with key characteristics that will be covered in the

current chapter.

All fuel cells have 3 main parts: an anode, a cathode and the electrolyte, which is the

layer in between. Both anode and cathode have to be electric conductors in contrast

with the electrolyte that should be an insulator. The electrolyte is made by a material

that allows the anions of Oxygen (O2-) to go through, but at the same time, forces the

electrons to circulate through the electric circuit producing a current that powers it.

Page 18: Master Thesis SOFC

Master Project. Arturo Veiga López 18

Figure 1.3. Schematic of a SOFC.

The electrolyte used for the SOFC is made of 8mol% Yttria-stabilized Zirconia

(Y2O3 Fully Stabilized ZrO2), which is the most common electrolyte material in SOFC.

The electrolyte main requirement is that it has to conduct the Oxygen anions and at

the same time it has to be an electrical insulator. The ionic movement occurs through

the crystal lattice. This phenomenon is thermally activated so it has to be heated to a

temperature of 800 C. This is the biggest challenge in this type of Fuel Cell and most

of the research is focused in attempting to lower this temperature by making several

layers between the anode/cathode and the electrolyte, using other materials, or like in

this research project, lowering the electrolyte thickness.

Fuel = H2

H2O

Air = O2

Anode Cathode

Electrolyte

O2-

e-e-

Page 19: Master Thesis SOFC

Master Project. Arturo Veiga López 19

The anode is made of Nickel Ytttria-stabilized Zirconia (Ni/YSZ). Here is where the

fuel is oxidized to produce H2O. The main characteristics of a good anode are:

Good Hydrogen catalyst

Large specific area, commonly is a porous material

Good electronic conductor

The reaction that takes place in the anode is:

The cathode is made of Lanthanum Strontium Manganite (LSM). Here is where the

Oxygen molecule breaks down and the result atoms are ionized. The main

characteristics of a good anode are:

Good Oxygen catalyst

Large specific area normally is a porous material

Good electronic conductor

The reaction that takes place in the cathode is:

Both anode and cathode reactions combine lead us to the overall reaction:

There are also other requirements for all of these 3 materials like thermal stability,

same expansion coefficient, chemical compatibility but that goes further out of the

scope of this introductory chapter.

The main advantages of this type of fuel cell is its great efficiency, theoretically up to

60%, and that it can be a good candidate for a combined heat and power application

that could increase it further.

Page 20: Master Thesis SOFC

Master Project. Arturo Veiga López 20

1.2.2. Fuel Cell Performance

The performance of a fuel cell is measured by identifying and calculating the losses

that lowers the potential from the thermodynamic ideal case. One of the objectives of

to test the fuel cell and for that a test station needs to be built.

General losses

In a fuel cell there are several losses that lower its thermodynamic potential.

Where:

V: Operating voltage.

Ethermo: Thermodynamically predicted voltage of fuel cell.

act: Activation losses due to reaction kinetics.

ohmic: Ohmic losses from ionic and electronic losses.

conc: Concentration losses due to mass transport.

For measuring the performance of a fuel cell basically 2 tests are needed: The i-V

curve and the Electrochemical Impedance Spectroscopy (EIS).

The current voltage curve (i-V)

The i-V curve is a curve that shows the potential at different current densities. From

the curve itself the losses can not be determined, but using the Tafel equation it can be

identified the activation loss and the Ohmic and concentration loss (see Figure 1.4).

Page 21: Master Thesis SOFC

Master Project. Arturo Veiga López 21

Activation loss

Ohmic +

concentration loss

Vo

lta

ge

Current density

i-V curve

Tafel fitting

Figure 1.4. i-V curve with Tafel fitting .

Electrochemical Impedance Spectroscopy.

The other test that helps us to evaluate the performance of the Fuel Cell is the

Electrochemical Impedance Spectroscopy (EIS). This technique allows us to

differentiate the majority of sources of loss in a Fuel Cell.

The EIS principles are based on measuring the impedance of the Fuel Cell when a

small AC voltage perturbation superposes to the DC voltage of the polarization applied

to the cell.

This perturbation needs to be small enough for the i-V curve to be pseudolinear, but

large enough to yield a current response that can be measured (see Figure 1.5.).

Page 22: Master Thesis SOFC

Master Project. Arturo Veiga López 22

Vo

lta

ge

Current density

Voltage

perturbation

Creates current

response

Pseudolinear

portion of i-V curve

Figure 1.5. Application of a voltage perturbation creates a current response

maintaining the linearity.

The impedance measured with this technique and at different frequencies is plotted in

a Nyquist diagram like in the bottom of the figure 1.6.

An electrochemical reaction can be represented by a resistor and a capacitor in

parallel. The resistor reflects the Faradic resistance, while the capacitor reflects the

capacitive nature of the interface due to the charge separation between 2 layers.

Page 23: Master Thesis SOFC

Master Project. Arturo Veiga López 23

C1

CathodeElectrolyte

↓ ω

-Zimag

Zreal

Rf,1

Rf,1

Figure 1.6. Physical sketch, equivalent circuit and Nyquist plot of an interface.

As shown in figure 1.6., the diameter of the circle reflects the resistance. The

capacitance can be calculated from the following equation:

In an equivalent circuit of a fuel cell there are 2 interfaces, anode-electrolyte and cathode

electrolyte and an Ohmic resistance that is mainly due to the resistance that the anions have

to go through the electrolyte. This equivalent circuit is shown in the figure 1.7.

Page 24: Master Thesis SOFC

Master Project. Arturo Veiga López 24

-Zimag

Zreal

RΩ Rf,2Rf,1

C1 C2

Anode CathodeElectrolyte

Rf,1

Rf,2

Rf,2

Figure 1.7. Physical sketch, equivalent circuit and Nyquist plot of an entire fuel cell.

Page 25: Master Thesis SOFC

Master Project. Arturo Veiga López 25

2. Development of a SOFC electrolyte with ESD technique

2.1. Introduction and Objectives

The current section will briefly describe the rationale behind the selection of the

Electrostatic Spray Deposition (ESD) technique to build a Solid Oxide Fuel Cell and the

basic concepts of this methodology.

Based on the results of the Thesis called “Electrostatic spray deposition of Solid Oxide

Fuel Cell electrolyte” [He, 2013] it is feasible to achieve a 100% dense YSZ layer using

the ESD technique. Moreover, the control of the porosity of the sprayed layer with

other materials provides with the opportunity to build an anode and cathode with the

same technique, which should be porous to maximize the surface in contact with the

gases in the fuel cell. However, this is not without challenges. We will cover those

challenges and how to overcome them in the following sections.

The overall objective of these experiments is to advance the knowledge of the

properties of the sprayed layer and to face the challenges in long-time sprays and

other materials. The ultimate goal of this research is the development of a complete

fuel cell with the solely use of the ESD technique in several steps of spray and sinter

the several layers of a fuel cell.

ESD is essentially a coating technique that uses a liquid to create a suspension where

nanoparticles of the material which we want to coat our surface with are dissolved

into (see Figure 2.1. for details).

Page 26: Master Thesis SOFC

Master Project. Arturo Veiga López 26

Figure 2.1. Schematic of Sketch of the Electrostatic Spray Deposition technique.

An ESD system consists of a suspension system with a syringe pump and a nozzle, a

high voltage generator, and a heated substrate. The suspension is sprayed onto a

substrate using a high electric field (8 kV – 13 kV) between the nuzzle (where the

suspension is pumped) and the substrate. The substrate is heated to a certain

temperature that helps evaporate the excess of liquid in the flight time of those drops.

Flight time refers to the time that elapses between the time that the suspension leaves

the heated surface and that it reaches the nuzzle. The remaining of the liquid

evaporates in contact with the substrate allowing a slight movement of the particles in

the substrate. This helps to achieve a more uniform layer.

The electric field charge the drops when they go through the nuzzle (that drives the

particles to stick to the substrate), but also this charge is responsible for the Columbus

Film

Drops

Flow: 1ml/h

Voltage8-13 kV

Nuzzle

Heated substrate

Page 27: Master Thesis SOFC

Master Project. Arturo Veiga López 27

force that prevents agglomeration between them. As a result they form a uniform

layer (see Figure 2.2.).

Figure 2.2. Schematic of Coulomb force between two particles. Particles with the same

charge repel each other.

The system developed by He, 2013 is shown on Figure 2.3. This system requires a

series of improvements to enhance robustness and reproducibility of the

methodology, which will be the focus of this work.

The main advantage of this technique is that it not only provides with the opportunity

to create a thin film (as thin as 1 µm), but that it also allows with a high control over

the porosity of the layer. Other advantages include the low equipment cost and the

easy scalability for manufacturing. Overall, this technique is highly competitive over

others, such as physical vapor deposition, screen printing, or slurry coating.

+

++

+

+

+

+

++

++

+

+

+

+

+

Drop

YSZ particles

Page 28: Master Thesis SOFC

Master Project. Arturo Veiga López 28

Figure 2.3. Electrostatic Spray Deposition System developed by He, 2013. Syringe and

pump (top left are connected to the nuzzle (top right). Note detail of heated substrate

(top right).

2.2. Development of an enhanced spray system

The former system developed by He, 2013 although innovative, lacked of the

repeatability and robustness required. One of the main challenges with this system is

that the ESD technique is very slow, which in turns leads to a high variability of the

final outcome. The amount of suspension sprayed is about 1 ml per hour, with an

average of 3 µm per hour. Assuming the final anode thickness should be over 100

microns, this speed of growth to generate the layer is too slow and requires a very

long spraying phase (up to 30 hours).

Nuzzle Heated substrate

Syringe + pump

Page 29: Master Thesis SOFC

Master Project. Arturo Veiga López 29

Maintaining the spray constant and stable for this amount of time is quite difficult due

to the sensitivity of the technique. Note that any change on the suspension or the

ambient conditions can change the spray behavior. Therefore, it is essential to 1)

achieve complete spray stability (described in section 2.2.1.) and 2) reduce the spray

time (described in section 2.2.2.).

To achieve complete spray stability, a new design and improvements to the system

have been implemented.

To reduce the spray time, the flow rate and/or the powder concentration in the

suspension need to be evaluated. These improvements are studied in detail in this

chapter.

2.2.1. Spray stability

2.2.1.1. Design requirements

To achieve a complete spray stability and repeatability we identified the underlying

causes that may affect the spray behavior. It was determined by experimental

observation that the main problem for having a stable spray was the supply to the

spray with a constant concentration of particles. Two critical factors of the design

were responsible for the heterogeneous supply of particles, namely the settling of the

particles and the tendency of the particles to stick to the container walls. These will be

covered in the next sections.

Settling of the particles

Settling refers to the process by which particles settle on the bottom of a liquid and

form a sediment. The main problem is the settling of particles due to the different

density between the YSZ nanopowders and the liquid where those particles are

Page 30: Master Thesis SOFC

Master Project. Arturo Veiga López 30

dissolved. Later in this chapter we will study if it is possible to modify the suspension

itself to prevent this settling phenomenon. In this section, we will discuss how to

minimize this effect by solely improving the design.

We also observed that the particles have a tendency to stick to the walls of the tubes.

The solution proposed and tested is to incorporate a reservoir as close to the nuzzle as

possible. This will allow to have the highest amount of suspension next to the nuzzle.

We also implemented a magnetic stirring system in it so we can stir the suspension at

the same time that spraying is occurring. This will minimize the tendency of the

particles to stick to the container walls by increasing the ratio volume/area and

assuring that the suspension sprayed is not in contact with any tubes before is being

sprayed. This solution was tested through different prototypes before the

implementation into the final design.

The magnetic stirring system has to be capable to operate at a very low speed (~ 90

rpm). To achieve this speed, the voltage and frequency regulator has to be connected

in series with a dimmer that “cuts” part of the AC current in each cycle. This was a fast

and very cost effective solution to lower the speed of the stirring motor.

Another possible approach could be the implementation of an ultrasound system that

constantly induces a vibration on the particles that theoretically prevents the

agglomeration that causes the settling. This could be a solution not only for the

reservoir, but also for the syringe and the remaining tubes. It was not incorporated in

this design because of its complexity, but it is a solution worth exploring in future

design improvements.

Vacuum

A vacuum can be created during the spray phase and that can affect the stability when

spraying occurs during a long period of time. To solve this problem, a tube open to the

air was attached in the top part of the container.

Page 31: Master Thesis SOFC

Master Project. Arturo Veiga López 31

With this tube we cannot apply a pressure to the system so the flow rate will be only

imposed by the electric field and not by the syringe. That also poses the following

challenge; to maintain the suspension level by guessing the flow rate sprayed. Because

of this phenomena and the poor results obtained trying to prevent this vacuum, this

system was not implemented.

Electric field

Another problem that directly affects the spray stability is the uniformity of the

electric field. The electric field is the driving force of the spray; any distortion of the

electric field affects directly shape of the spray. This represents a complex challenge,

because the electric permittivity is directed affected by the concentration,

distribution, and nature of the particles sprayed. In the original design [He, 2013], a

plastic disk was placed in the middle of the nuzzle. This disk allows to concentrate the

electric field in the upper part of the nuzzle, that is a perfect cylinder, blocking the rest

of the possible electric field of the rest of the nuzzle.

To improve this approach a grounded Faraday cage was built to ensure the electric

field, that comes from any other part that is not the last 0.5 inches of the nuzzle, is

cancelled.

All of the above descried systems were designed and built specifically for this project

(see improved design in Figure 2.4.). They were made as compact as possible and met

the requirements and needs of material compatibility. The system adapted the design

of the original design [He, 2013] in terms of the chamber, the positioning, and the

heating system. The design was developed using the software PRO-E a CAD software.

Page 32: Master Thesis SOFC

Master Project. Arturo Veiga López 32

Figure 2.4. Improved spray system.

2.2.1.2. Experiment methodology

Several experiments were made to spray a YSZ layer with different thicknesses. The

results were observed in the SEM to evaluate the microstructure of the deposited

layer. The parameters controlled during the experiment were: voltage, distance, flow

rate, and temperature of the substrate.

To get a stable spray, and based on previous experiments, the flow rate was set to 1

ml/h. The temperature is a critical parameter to achieve a completely dense layer and

it was set at 250 C.

Page 33: Master Thesis SOFC

Master Project. Arturo Veiga López 33

To achieve a good spray shape, the combination of distance and voltage is essential.

This spray mode is called cone-jet and it is the operation mode that allows to have a

perfect and uniform layer. To determine these parameters it is needed to calibrate

using different distances and voltages. Figure 2.5. represents the relationship between

these two parameters and the type of spray that will provide with.

Figure 2.5. Types of spray based on distance to voltage combinations. Flow rate and

temperature used were 1 ml/h and 230C, respectively. Note that four modes of spray

can be achieved: sputtering, squirt, cone jet, and dripping. Refer to text for an in depth

explanation of each mode.

Four modes of spray can be achieved based on the distance to voltage relationship:

1) Sputtering: This mode occurs when the voltage is very high in relationship to

the distance. In this case, multiple sprays come out of the nuzzle and cannot be

easily controlled.

2) Squirt: This mode produces a single spray, but it is not uniform. The center of

the spray has more drops, that cause a little hill in the middle of the deposited

layer.

Page 34: Master Thesis SOFC

Master Project. Arturo Veiga López 34

3) Cone Jet: This is the ideal mode of spray; it is completely uniform and best

results are achieved when formed (Figure 2.6.).

4) Dripping: In this mode, the voltage is too low for the selected distance and

instead of having a continuous spray, only drops are formed.

Figure 2.6. Left: Cone Jet Spray: Ideal spray mode. Uniform spray guarantees best

results. Right: Cone Jet Spray deposition result. Note uniformity of deposit.

2.2.3. Experiment results

A series of experiments were conducted to test the spray stability. SEM technique was

used to compare results with the previous system and to achieve sprays for up to 17

hours and evaluate its microstructure.

In Figure 2.7. different YSZ samples sprayed onto a silicon substrate are shown. The

different of color within each half of each sample is due to the fact that a more obscure

side was sintered.

Page 35: Master Thesis SOFC

Master Project. Arturo Veiga López 35

Figure 2.7. Different samples of YSZ sprayed onto a silicon substrate.

To evaluate the results in terms of density and uniformity after the sinterization, the

samples obtained were compared to the original system [He, 2013]. Figure 2.8. shows

a SEM micrograph obtained after the use of the original system. Some pores are

evident, but most of them will disappear with the sinter process.

Page 36: Master Thesis SOFC

Master Project. Arturo Veiga López 36

Figure 2.8. SEM micrograph of the sample obtained with a spray time of 30 min using

original spray system.

The new spray system without the vacuum was tested using the following conditions:

Flow rate: 1 ml/h

Concentration of YSZ in the suspension: 1 wt %

Solvent of the suspension: Butyl Carbitol

Temperature of the substrate 230 C

The voltage and distance were adjusted to achieve the best possible Cone Jet

Figure 2.9. SEM micrograph of the sample obtained with a spray time of 30 min using

new spray system without vacuum.

Page 37: Master Thesis SOFC

Master Project. Arturo Veiga López 37

Figure 2.10. SEM micrograph of the sample obtained with a spray time of 1 hour using

new spray system without vacuum.

Thickness of the layer is proportional to the time (thicker in 1 hour vs. 30 min

exposure) as the rest of parameters remain constant. We can notice that there is still

some porosity, but it has improved compared to the original system design. Longer

exposure times, up to 17 hours were tested (see Figures 2.11. and 2.12.).

Figure 2.11. SEM micrograph of the sample obtained with a spray time of 14 hours

using new spray system without vacuum.

Page 38: Master Thesis SOFC

Master Project. Arturo Veiga López 38

Figure 2.12. SEM micrograph of the sample obtained with a spray time of 14 hours

using new spray system without vacuum.

As seen in the micrographs after 14 of exposure (Figures 2.11. and 2.12.)one can

obtain a layer of up to 69 microns and with good properties. The cracks along the

surface are caused by the cutting process, since the layer is too thick to have a perfect

cross section.

The above described experiments were performed without the vacuum tube that will

theoretically allow the achievement of a more stable spray. The type of spray achieved

is slightly of squirt mode, as shown in Figure 2.13., the center has more particles

sprayed than on the sides.

Page 39: Master Thesis SOFC

Master Project. Arturo Veiga López 39

Figure 2.13. SEM micrograph of the sample obtained with a spray time of 17 hours

using new spray system without vacuum.

We can conclude that this system is appropriate to produce a stable spray system for

YSZ and to achieve the desired thickness varying the deposition time.

2.2.2. Increasing the powder concentration of the suspension

As mentioned earlier, reducing the spray time is critical to achieve a complete stable

spray. Changes in the composition of the actual suspension will allow to produce a

thicker layer in less time.

In order to limit the particle settling it is needed to understand the principles that

regulate the particles settling. A particle suspended in a fluid is subjected to two

applied forces, gravity and drag. The drag force is a function of the particle velocity, so

when the gravity accelerates the particle, its velocity increases until both forces are

equal. This is called the terminal velocity. The terminal velocity is affected by

numerous parameters like the shape, size of the particles, the density, and viscosity of

the fluid.

Figure 2.14. Schematic of forces that a particle is subjected to when in a fluid.

Gravitational and drag forces are opposite.

Page 40: Master Thesis SOFC

Master Project. Arturo Veiga López 40

We assume that there is a smooth spherical particle in a fluid, hence no interferences

among particles. Based on the Reynolds number we can have 3 different scenarios:

Stokes drag

Newtonian drag

Transitional drag

In this case, and due to the small particles involved, the Reynolds number is very

small. This indicates that the main dragging force is due to the viscous forces.

Applied force (gravity)

Where:

ρp : Particle density

ρf : Fluid density

g: gravity

Vp: Volume of the particle

Drag force

Where:

µ: dynamic viscosity of the fluid (6 Pa*s)

R: radius of the particle (20 nm)

v: settling velocity

Then: ρ

For our suspension, the velocity is 2.7∙10-3 mm/h which is equivalent to ~ 1 mm/year.

Theoretically, with this particle size (20 nm), the suspension should remain stable for

years. But, experimentally, it is observed that the suspension settles within hours.

Page 41: Master Thesis SOFC

Master Project. Arturo Veiga López 41

That observation leads to the assumption that the particles agglomerate to produce

much bigger particles in the order of magnitude of 10 microns.

Also, another phenomenon that was observed is that with the time, even if we agitate

the suspension every hour the velocity of settling is higher. That means that the

particles are agglomerating and even after an agitation process they still remain

together, making the settling faster. Two methods are available to avoid the

agglomeration phenomenon, electrostatic stabilization and steric stabilization.

Before explaining those two methods, it is important to review that particles may

carry electrical charges in their surfaces, which in this case is the main factor of

agglomeration (electric charge). There are three types of potential involved in a

particle (surface charge, Stern potential and Zeta potential), and each of them is

predominant in a specific region around the particle. We will focus on the Zeta

potential, which is the one that drives the agglomeration of small particles. Zeta

potential is defined as the potential difference between the dispersion medium and

the stationary layer of fluid near a single particle.

Figure 2.15. Schematic representation of potentials involved in a particle (surface

charge, Stern potential, and Zeta potential).

Page 42: Master Thesis SOFC

Master Project. Arturo Veiga López 42

2.2.2.1. Electrostatic stabilization

This method is based in changing the zeta potential of the suspension. If the zeta

potential is greater than +30 mV or smaller than -30 mV, the agglomeration is

avoided. It is important to remember that the settling time for nanoparticles is in the

range of years. To change the zeta potential, the most common method is to change

the pH of the suspension to increase or decrease the zeta potential of the particle in

the medium.

This method works better in mediums that have a high electric dielectric constant.

The solvent used to spray YSZ is Diethylene Glycol Monomethyl and has a dielectric

constant of 10.15, which makes it not very suitable, but possible.

Figure 2.16. Schematic representation of electrostatic stabilization.

2.2.2.2. Steric stabilization

Another approach is to use a polymer chain that sticks to the surface of the particles

using the electric charge of the particles. The chains of the polymer interact among

them before the particles are close enough to be attracted by electrostatic forces. It is

Page 43: Master Thesis SOFC

Master Project. Arturo Veiga López 43

required that the medium may be a good solvent for the polymer chain. In addition,

the charge of these polymers has to be opposite to the zeta potential of the particles.

Figure 2.17. Schematic representation of steric stabilization.

In order to use these methods the zeta potential of the particles used in the medium

needs to be determined experimentally. The equipment required to determined the

zeta potential of these particles is highly specialized and currently will not represent a

cost-effective measure to implement.

Theoretically, the improvement on the amount of particles is limited by the viscosity,

from the suspension point of view, apart other factors that should be tested

experimentally like the stability of the spray, which can be up to 30 % of volume of

powders. That allows to work with 70 wt % of powders in a stable suspension, which

is 70 times higher than the actual powder concentration.

There are other factors involved that have the potential to reduce the powder

concentration. It is likely that the spray system cannot operate with such amount of

powder concentration, but the current limitation factor (the suspension) will not be a

problem in the future, so there is potential for improvement by increasing the powder

concentration. This means that this technique can be accelerated to achieve same

thickness in a fraction of the time required.

Page 44: Master Thesis SOFC

Master Project. Arturo Veiga López 44

3. SOFC Design, operation and results

3.1. Design of the SOFC test station

In order to assess the performance of a Solid Oxide Fuel Cell (SOFC), a test station was

built. This chapter is going to focus on the design and manufacture of the test station

and on the evaluation of the performance of the fuel cell.

3.1.1. Principles of design of the SOFC test station

The fuel cell design is based on a traditional test equipment for SOFC button cells

using a cylindrical induction furnace to achieve the operation temperature using a

Zirconia tube as support for the SOFC. Some critical factors were to be taken into

account to design this test station:

Working temperature: 800 C.

Fragility of the materials used to build the SOFC and the SOFC per se.

Minimize the difference among the expansion coefficients of the materials used.

Achieve a 100 % seal system due to the nature of the gases and temperatures

involved.

Minimize the electric interferences from the furnace.

Among these factors, the high temperature (~ 800 C) is the most important design

factor to take into account.

The fuel cell (see Figure 3.1. for detailed design) has to not only provide with both fuel

and air in the necessary conditions, but has also to control the temperature of system,

collect the current and voltage, and process them.

Page 45: Master Thesis SOFC

Master Project. Arturo Veiga López 45

Figure 3.1. Schematic sketch of a Solid Oxide Fuel Cell (SOFC) system and its parts.

3.1.2. Parts of the SOFC test station

The test station can be divided up into several subsystems that include the following:

Low temperature gas system

The furnace and the temperature control system

The hot temperature gas system

SOFC button cell

The AC impedance system

Gas Valve

N2

H2Heated

water bath

To the

outside

Low temperature gas system

Hot temperature gas system

Low temperature gas system

Blower

AC impedance systemPotentiostat /

Freq. Response Analyser

Ag wires

Zirconia

tubes

Page 46: Master Thesis SOFC

Master Project. Arturo Veiga López 46

Electrical insulator

Sealing

Low temperature gas system

This system includes the bottles of Hydrogen and Nitrogen and runs up to the

entrance of the Zirconia tube system. The gas control system (Diavac 885, Leybold)

has the flow meters and valves to adjust the flow of the gas to perform the test.

The incoming gas is bubbled into a 500 ml flask with three openings, two for the gas

and one for the thermocouple that will register its temperature. This flask

incorporates a heater (Omega LHM0500) that allows to increase the humidity of the

gas, until the saturation point at any given temperature. This is especially important

because the level of humidity of the gas (saturated of water) depends on the

temperature of the gas. The heater is controlled by the same system as the furnace. It

is important to keep the distance between the flask and the entrance of the Zirconia

tube as short as possible, in order to prevent condensation in the tube. Otherwise, the

gas may be less humid than what it should be.

Once the gases pass through the hot gas system, the exhausted gases are bubbled

through another device, which induces a little over pressure into the system. Its

function is to check the presence of leakages throughout the experiment and to

provide with a steady flow along the process. Finally, the gases are safely extracted to

the exterior of the building.

The furnace and the temperature control system

The furnace is based on a cylindrical furnace (ATS series 3210) with a maximum

output of 4,010 W and a series of three thermocouples (model Omega TJ50)

connected to a digital controller (model Omega Multizone controller), that allows to

control and maintain the needed temperature in the furnace.

Page 47: Master Thesis SOFC

Master Project. Arturo Veiga López 47

The hot temperature gas system

The hot temperature gas system (see Figure 3.2.) is made of two Zirconia tubes (see

Figure 3.3.) and one inconel tube that is placed inside one of the Zirconia tubes and

the connection system required to connect all three tubes to the low temperature gas

system.

Figure 3.2. Schematic of the hot temperature gas system.

Air

H2 N2

H2ON2

H2ON2

Oven

Ag wires

To potentiostat

Inconel tube

Stainless

steel tube

Alumina tube

Alumina tube

Inconel tube

Page 48: Master Thesis SOFC

Master Project. Arturo Veiga López 48

Figure 3.3. Zirconia tube.

The gases go through the inconel tube that ends in the fuel cell where the gases react

with the SOFC. The products of this reaction go through the space in between the

Zirconia and the inconel tube. Knowing the temperature close to the fuel cell is critical,

because is this will be the operating temperature, which drives the ion mobility

through the YSZ. To measure the temperature, a thermocouple TJ70-CAIN was placed

inside the Zirconia tube and as close as possible to the fuel cell.

The structure to hold the Zirconia tube was made using an extrusion aluminum frame,

commonly used in this type of constructions because of its great flexibility and

robustness.

Page 49: Master Thesis SOFC

Master Project. Arturo Veiga López 49

SOFC button cell

The button cell used is the model ASC-2.0, an anode supported cell supplied by fuel

cell materials with the Ni-YSZ/YSZ/LSM structure (20 mm of diameter). Other details

of the cell are the following: a 10 μm thick YSZ electrolyte, LSM as cathode with a

thickness of 50μm and NiO-YSZ anode with 220-260μm of thickness (see Figure 3.4.

for details). Both anode and cathode are porous to increase the reactive area surface

with the gases. This type of fuel cell requires a reduction of the anode before use.

Specifics of the assembly of the button fuel cell are shown in Figure 3.5.

Figure 3.4. Anode-supported fuel cell characteristics model ASC-2.0.

Cathode

Electrolyte

Anode

Page 50: Master Thesis SOFC

Master Project. Arturo Veiga López 50

Figure 3.5. Schematic of the assembly of the fuel cell.

The AC impedance system

The system used to analyze the AC impedance data is the PARSTAT 4000, Princeton

Applied Research, a potentiostat / galvanostat coupled with a frequency response

analyzer (FRA) contained in a single unit. The impedance system is connected and

controlled a computer using the VersaStudio electrochemistry software that allows

the design of the experiments and the analysis of the data obtained. Silver mesh and

wires (silver wire diameter: 0.25 mm) are used to the transport of the voltage and

current to be analyzed. Note that when using this of equipment at these temperatures,

platinum is the ideal element of choice because it has a melting point much higher

than the silver (Ag: 961 C vs. Pt: 963 C).

To test the integrity of the silver under experimental conditions, an experiment was

conducted to determine if degradation in the silver may affect the results. The

integrity of the silver wire was evaluated by analyzing the structure and composition

of the silver wire (see Figures 3.6. to 3.9).

Ag wires

Cement

Cathode

Ag mesh

(current collector)

Page 51: Master Thesis SOFC

Master Project. Arturo Veiga López 51

Figure 3.6. SEM of silver wire in contact with Aremco Cement at 800C.

Figure 3.7. Composition of the surface of the Ag in contact with Aremco Cement at

800C.

Page 52: Master Thesis SOFC

Master Project. Arturo Veiga López 52

Figure 3.8. SEM of silver wire near Aremco Cement at 800C.

Figure 3.9. Composition of the surface of the silver near Aremco Cement at 800C.

During this experiment it was noticed that the phosphorous included in the cement

forms a layer of Ag3PO4 in the surface (see Figure 3.6.), which makes the silver wire

more brittle. After 5 hours, the conductivity of the silver wires starts to be affected. To

minimize the silver wire degradation, as well as to decrease the ohmic resistance and

make a better electrical connection with the potentiostat, a thicker silver wire was

selected (new diameter: 0.25 mm).

Page 53: Master Thesis SOFC

Master Project. Arturo Veiga López 53

Electrical insulator

To achieve the best measurement of the impedance and other electrical signals the

fuel cell and the silver wires have to be fully isolated from the noise coming from the

induction furnace. A 316 stainless steel alloy tube was used to protect the fuel cell. An

inconel tube was used to protect the silver wires that collect the current and that are

connected to the potentiostat.

Sealing

The most important part of the cell assembly is the sealing between the test station

and the fuel cell, and it is not without challenges. If the system is not completely

sealed, it can result in complications such as loss of voltage, hot spots, and also

explosions. If the leakage is large enough, the hydrogen can be in contact with the

oxygen present in the air, which in a certain range of composition can lead to an

explosion. Appropriate safety measures were taken when conducting the

experiments.

Requirements to achieve the best sealing possible:

Gas tightness

Same thermal expansion between fuel cell and Zirconia tube

Chemical stability and low partial pressure at high temperatures

To match the thermal expansion of the materials involved a Zirconia-based cement

needs to be used, discarding other cements based on conventional glass-ceramic

materials or glass. Another potential solution may be the use of a gold ring. Applying

pressure to this gold ring at high temperatures, the gold will deform itself and seal the

system. This solution was rejected due to manufacture precision issues during the

cutting of the tubes and the higher cost involved, although the main advantage is the

easy and fast replacement of the cell.

Page 54: Master Thesis SOFC

Master Project. Arturo Veiga López 54

A Zirconia-based cement (Ceramabond 885) provided by Aremco was used. The main

problem encountered in this sealant is that it forms a porous layer. The easy diffusion

of H2 through the layer requires to apply several layers (up to 4) and to cure them

successively to achieve a good sealing and prevent any H2 leakage.

3.2. Preparation of the experiment

3.2.1 Assembly process of the fuel cell

The assembly of this cell is a critical step in the fuel cell testing and in order to achieve

a good result is crucial to have a perfect assembly of the system. The literature is

scarce regarding fuel cell assembly. This methodology used was based on the results

of the Thesis called “Characterization of porous SOFC Electrodes” [Hslao, 1996] and

was specifically adapted and improved for this design.

To make the process easier and time effective when it comes to test more than one

cell, 1 inch of the Zirconia tube was cut to mount the fuel cell. Then, the other side was

pasted with the same cement to the Zirconia tube. This also allows to have more space

to take the silver wire connected to the anode outside of the system. After that,

another collector is placed in the cathode side and pasted with the silver ink.

Page 55: Master Thesis SOFC

Master Project. Arturo Veiga López 55

Figure 3.10. Image of the Zirconia and inconel tubes with the furnace open.

The current collector is a silver mesh (AG-M40-100) that was cut to fit the shape of

the anode and the cathode. In order to mount the collector in the cathode side, the

anode should not be touched, because that can give us a deviation from the theoretical

potential. Two silver wires of 0.35 mm provided by Goodfellow are responsible for the

conduction of the current and voltage to the potentiostat.

In order to achieve a perfect contact between the collector, the anode, and the

cathode, silver ink (AG-i) is applied. This will keep the mesh in the same place

throughout the experiment. To cure the silver ink it should be fired up to 800 C for 1

hour.

Page 56: Master Thesis SOFC

Master Project. Arturo Veiga López 56

Figure 3.11. Detailed view of the silver mesh impregnated with the ink and connected to

the wire.

Although silver is not the most suitable material to be used as a collector, because of

the high temperatures reached during the experiment, silver was used due to the

better cost compared to platinum. Using silver allows 5 hours of tests at 800 C, time

after which, the silver starts a degradation process.

Figure 3.12. Detailed view of the cathode side of the fuel cell mounted in the Zirconia

tube.

Page 57: Master Thesis SOFC

Master Project. Arturo Veiga López 57

Figure 2.13. Final placement of the fuel cell before the test.

In order to increase the temperature of the cell it is extremely important in every

stage not to increase or decrease the temperature more than 1 C/min to prevent

cracks due to the thermal expansion and to allow the cement to expel the gases

produced in the cure process of the cement.

After applying the silver ink to stick the current collectors in the anode and cathode a

thin layer of cement is applied and the following temperature cycle was followed: 2

hours at room temperature, 2 hours at 93 C, 2 hours at 260 C, and 2 hours at 370 C,

and then the temperature was reduced to 20 C. Four layers should be applied and

cured one after another checking that there are no visible cracks after each curing

step.

Page 58: Master Thesis SOFC

Master Project. Arturo Veiga López 58

Figure 3.14. Curing curves for a layer of cement.

Thereafter, the silver ink is fired according to the manufacturer by increasing the

temperature up to 800 C for one hour. A sealing test is performed to check that there

is no leakage; a mixture of 5% H2 and 95% N2 is pumped through the system and some

overpressure is added to the system bubbling the exhausted gas. Then, with an

hydrogen detector, the entire system is inspected for possible leaks.

3.2.2. Experimental AC impedance procedure

To prepare the fuel cell, the manufacturer does not provide with sufficient

information and different research groups use their own procedure to make the set-

up. Three main aspects need to be taken into account:

1) The leakage is detected by comparison of the open circuit potential with the

theoretic obtained in the Nerst equation for the reaction:

0

50

100

150

200

250

300

350

400

450

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Tem

pera

ture

(C

)

Time (h)

Cement cure heat treatment

Real temperature

Theroretic temperature (1°C/min)

Page 59: Master Thesis SOFC

Master Project. Arturo Veiga López 59

Where:

E - The electromotive force (or reversible open circuit voltage), V

E0 - the EMF at standard pressure, V

R - The universal gas constant, 8.314 JK-1mol-1

F - Faraday constant, 96485 Coulombs

Pi - partial pressure, bar

which in our conditions of gas compositions and temperature (800C, 97% H2

3% H2O, Air) has a value of 1.10 V.

If the voltage is out of the ± 5 %, another layer of cement is recommended. Also,

if the OCP is less than the one predicted, this can be indicative that the

electrolyte layer is not continuous or that it is broken.

2) Anode reduction

Anode reduction is achieved by a mixture of 10% of Hydrogen and 90%

Nitrogen starting at 400 C. According to Xiao (2006), the complete reduction

of the anode will be achieved at 500 C.

3) Conditioning

New fabricated button cells are not in a full equilibrated state and they need a

conditioning; normally the time needed to reduce the anode is enough for

achieving thermal and electrochemical stability.

After having the silver and the cement cured, the procedure starts at room

temperature.

1) The temperature is increased until 400 C with a ramp of 1 C/min without gas

supply.

Page 60: Master Thesis SOFC

Master Project. Arturo Veiga López 60

2) When 400 C is reached 10 cc/min of N2 is pumped in for about 10 min to

ensure there is no O2 inside the system

3) Then, the anode is reduced according to the above-mentioned description

between the temperatures of 400 and 600 C. Gas supply is stopped after 600

C are reached.

4) At 750 C the gas supply switches to 90% N2, 10% H2 monitoring the open

circuit voltage. When it is close to the theoretical, the gas mixture is changed to

humidified Hydrogen (97% H2 , 3% H2O). Under our experimental conditions, 3

hours were needed to reach an open circuit potential close enough to the

theoretical (1.10 V).

Throughout the process, the temperature of the furnace needs to be monitored and

remain stable; that is, it should not be a difference of more than 5 C between the

highest and the lowest temperature inside it.

Page 61: Master Thesis SOFC

Master Project. Arturo Veiga López 61

3.3. Results

A two electrodes configuration is used for testing the fuel cell according to the

instructions manual of the PARSTAT 4000 unit.

Figure 3.15. Schematic of the two electrode connection diagram.

In order to test the performance of the SOFC, three electrochemical tests are

performed: open circuit potential, current voltage test, and AC impedance test.

3.3.1 Open circuit potential (OCP)

The OCP of the tested cell was in a range between 1.03 V and 1.12 V, which is close to

the theoretical predicted by the Nest equation: 1.10 V at 800 C. This means that a

good seal is accomplished and that there are no electrical leaks during the tests. It also

ensures the proper operation of the cell.

In Figure 3.16. the OPC trajectory of the fuel cell at 800 C and gas composition of 97%

H2 and 3% H2O are shown.

• WE: Working electrode

• CE: Counter electrode

• SE: Sense electrode

• RE: Reference electrode

Page 62: Master Thesis SOFC

Master Project. Arturo Veiga López 62

Figure 3.16. Open circuit potential curve at 800C and 97% H2 and 3% H2O.

Before each test, the OCP was checked to verify the integrity of the system. After 7

hours of testing at 800 C, the OCP revealed a drastic drop in the open circuit

potential. After cooling down the test station and examination the fuel cell, it was

observed that the silver mesh that works as current collector was displaced. It is likely

that the silver ink degraded and stopped sticking together the silver mesh and the

anode, due to the long sustained high temperature. The silver ink is recommended for

temperatures under 600 C. In this project, due to the extremely high cost of Platinum,

silver ink mesh and cables were used.

3.3.2. Current voltage curve (i-V)

The voltage and current curve is the main instrument to evaluate the performance of

the fuel cell, maintaining constant the fuel and the temperature. In order to calculate

the current density, the current is divided by the area of the fuel cell in contact with

the gas and the collector resulting in an area of 1.6 cm2.

1

1.02

1.04

1.06

1.08

1.1

1.12

1.14

1.16

1.18

1.2

0 5 10 15 20

E(V

olt

)

Time (minutes)

Open Circuit Potential

Page 63: Master Thesis SOFC

Master Project. Arturo Veiga López 63

The measurements were performed every 100 mA cm-2 except from the first 250mA

cm-2 that were measured every 50mA cm-2 to be more accurate. Between each

measure, a relaxation time of 10 min was allowed to achieve a steady state condition.

The measures were performed until the current upper limit was fixed; this happened

when the voltage reached 0.2 V. Another important benchmark of the performance of

the fuel cell using this test is the so called specific area impedance (ASI). ASI is

measured in the middle of the curve and a value of 0.5 Ω or less is accepted as a good

performance of the fuel cell. In our case the ASI achieved was 0.34 Ω.

In Figure 3.17., the i-V curve of the fuel cell at 800 C and fuel composition of 97%H2

and 3%H2O is shown. The curve does not display appreciable hysteresis.

Figure 3.17. i-V curve at 800C and 97% H2 and 3% H2O.

Figure 3.18. shows the power density; the maximum achieved was 0.6 W/cm2 at 1.12

A/cm2.

0

0.2

0.4

0.6

0.8

1

1.2

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

Vo

ltag

e (V

)

Current (A/cm2)

i-V

Page 64: Master Thesis SOFC

Master Project. Arturo Veiga López 64

Figure 3.18. Power density curve at 800C and 97% H2 and 3% H2O.

The performance measured by the manufacturer at different temperatures is shown

in Figure 3.19. In comparison with the manufacturer’s material datasheet, our results

are slightly smaller, which may be due to the precision of our system. Also, it is

important to remark, the high sensibility of the temperature, as we can observe in the

graph.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.5 1 1.5 2 2.5

Po

wer

den

sity

(W

/cm

2 )

Current (A/cm2)

Power density

Page 65: Master Thesis SOFC

Master Project. Arturo Veiga López 65

Figure 3.19. Fuel cell performance curves. Fuel Cell Materials data sheet for model ASC-

2.0.

3.3.3. Electrochemical impedance spectroscopy

As mentioned in Chapter 1, the AC impedance technique allows us to compare the

different resistances of the fuel cell associated to different phenomena occurring in

the fuel cell, as well as to compare different conditions of the fuel cell to understand

better its behavior.

To perform the AC impedance spectroscopy test, it is required to superimpose an AC

sine wave on the DC potential. One of the key parameters of this technique is the

amplitude of the AC sine wave. Unfortunately, the only option to choose the correct

amplitude is with trial an error. Too large of an amplitude will invalidate the linearity

of the response, which the impedance theory is based on. Too small of an amplitude

Page 66: Master Thesis SOFC

Master Project. Arturo Veiga López 66

will not create a sufficient strong signal to be analyzed. The values used of this

amplitude are about 5 mV. The frequency range applied is from 1 MHz to 0.5 Hz

In Figure 3.20., the real part of the impedance versus the imaginary part (note that is

minus the imaginary) has been plotted.

Figure 3.20. Results AC impedance spectrocopy. At 800C and 97% H2 and 3% H2O.

With a ZSimDemo3.2 software, a study of the equivalent circuit was made. This fuel

cell fits the equivalent circuit with the following parameters. From them we can say

that the Ohmic resistance is 0.126 Ω and the kinetic resistance is 0.677 Ω.

Ohmic resistance: RΩ = 0.126 Ω

High frequency resistance Rf1 = 0. 62 Ω

High frequency capacitor C1= 0.124 F

Low frequency resistance Rf2 = 0.057 Ω

Low frequency capacitor C2= 0.015 F

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

- Im

(Z),

Ω

Re(Z), Ω

Impedance spectra

Page 67: Master Thesis SOFC

Master Project. Arturo Veiga López 67

RΩ Rf,2Rf,1

C1 C2

Figure 3.21. Equivalent circuit of the fuel cell.

Figure 3.22. shows the alignment between the experimental results and the ones

resulting from the equivalent circuit.

Figure 3.22. Experimental results (red); Fitting results (green).

Other gas composition tests were planned to be assayed, but the silver degradation

prevented further testing.

In conclusion, the fuel cell testing system was validated and showed a fairly good

approximation to the values provided by the manufacturer, assuring that the test

station works correctly.

Page 68: Master Thesis SOFC

Master Project. Arturo Veiga López 68

4. Conclusions

The current research work focuses on the principles of fuel cells and specifically, the

parameters that measure the performance of a solid oxide fuel cell.

A rigorous study has been developed to improve the original Electrostatic Spray

Deposition (ESD) spray system, a new technology to develop a efficient solid oxide

fuel cell. This original EDS spray system was developed in the Thesis: “Electrostatic

spray deposition of Solid Oxide Fuel Cell electrolyte” [He, 2013]. The improvements

developed in the current project focused on enhancing the efficiency of the EDS spray

by achieving a better spray stability.

To increase the stability, and in turn the efficiency of the EDS spray, a more

homogeneous supply of particles is required. This is controlled by two main factors:

the settling of the particles and the tendency of the particles to stick to the container

walls. Several experiments were specifically designed to test these two aspects and

included improvements of the original design that targeted the following aspects:

- Include a reservoir under the nuzzle.

- Avoid the vacuum produced during the spray.

- Create a more uniform electric field.

Several prototypes described in Chapter 2 were developed to test the results of the

above-mentioned aspects. The finished system was tested spraying layers of YSZ up to

14 hours and 70 microns of thickness and we can conclude that this system is

appropriate to produce a stable spray system for YSZ and that the desired thickness

can be obtained varying the deposition time.

Because reducing the spray time is critical to achieve a complete stable spray, a

theoretical analysis was developed to increase the percentage of particles in the

suspension.

Page 69: Master Thesis SOFC

Master Project. Arturo Veiga López 69

This step is a major hurdle of the design of the EDS spray technology. This project has

only started to assess one of the main problems of this technique. The achievement of

faster deposition rates are still required to develop a future commercial method to

build solid oxide fuel cells.

On the other hand, assessing the performance of a solid oxide fuel cell is required to

improve its efficiency. The second part of this project focused on the design and

manufacture of a solid oxide fuel cell test station and on the evaluation of the

performance of a commercially available fuel cell. The test station was designed and

built focusing on simplicity and robustness.

The designed fuel cell test station was tested with a commercial fuel cell to evaluate its

proper operation and accuracy. Results from these tests were compared to the

specifications provided by the manufacturer. In conclusion, the fuel cell testing system

was validated and showed a fairly good approximation to the values provided by the

manufacturer, validating the functionality of the test station.

As mentioned above, the improvements implemented in the development of a solid

oxide fuel cells, especially the composition of the suspension, are only the tip of the

iceberg of a very complex engineering problem and leave open many unsolved

technical difficulties.

In order to achieve a fully functional and reproducible fuel cell with EDS technique it

will be necessary not only to successfully spray the anode and cathode, but also to find

a solution to support mechanically the fuel cell during the operation time. Future

research is required to target these unsolved questions.

Page 70: Master Thesis SOFC

Master Project. Arturo Veiga López 70

References

[He, 2013] He, Quanzhi. “Electrostatic Spray Deposition of Solid Oxide Fuel Cell

Electrolyte”. Thesis. Illinois Institute of Technology, 2013.

[Hslao, 1996] Hsiao, Yin-Chang. “Characterization of Porous SOFC Electrodes”. Thesis.

Illinois Institute of Technology, 1996.

O'Hayre, Ryan, Suk-Won Cha, Withey Colella, and Fritz B. Prinz. Fuel Cell Fundamentals.

Hoboken, NJ: John Wiley & Sons, 2006.

Varma, Ravi, and Robert Selman, eds. Techniques for Characterization of Electodes and

Electrochemical Processes.: Willey Interscience, 1991. The Electrochemical Society.

Nakajima, Hironori. "Electrochemical Impedance Spectroscopy Study of the Mass

Transfer in an Anode-Supported Microtubular Solid Oxide Fuel Cell." Mass Transfer -

Advanced Aspects. N.p.: Intech, 2011. 285-304.

Nomura, Hiroshi, Sandeep Parekh, Robert Selman, and Said Al-Hallaj. "Fabrication of

YSZ Electrolyte Using Electrostatic Spray Deposition (ESD):I – a Comprehensive

Parametric Study." Journal of Applied Electrochemistry (2005): 61-67.

Nomura, Hiroshi, Sandeep Parekh, Robert Selman, and Said Al-Hallaj. "Fabrication of

YSZ Electrolyte for Intermediate Temperature Solid Oxide Fuel Cell Using Electrostatic

Spray Deposition: II – Cell Performance." Journal of Applied Electrochemistry(2005):

1121-126.

Xiao, Haiming, and Thomas Reitz. "Anode-Supported Solid Oxide Fuel Cells with Thin

Film Electrolyte for Operation at Reduced Temperatures." ECS Transactions (2006).

Page 71: Master Thesis SOFC

Master Project. Arturo Veiga López 71

Muccillo, R., E.N.S. Muccillo, F.C. Fonseca, Y.V. França, T.C. Porfirio, D.Z De Florio, M.A.C.

Berton, and C.M. Garcia. "Development and Testing of Anode-supported Solid Oxide

Fuel Cells with Slurry-coated Electrolyte and Cathode." Journal of Power

Sources (2006): 455-60.

Minh, Nguyen Q. "Ceramic Fuel Cells." Journal of the American Ceramic Society 76, no. 3

(2005): 563-88.

Barsoukov, Evgenij, and J. Ross Macdonald. Impedance Spectroscopy: Theory,

Experiment, and Applications. Hoboken, NJ: Wiley-Interscience, 2005.

Bard, Allen J., and Larry R. Faulkner. Electrochemical Methods: Fundamentals and

Applications. New York: Wiley, 1980.

Yuan, Xiao-Zi. Electrochemical Impedance Spectroscopy in PEM Fuel Cells:

Fundamentals and Applications. London: Springer, 2010.


Top Related