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Trang 3DNIVERSITY OF CALGARY Hạ Oxidation at Solid Oxide Fuel Cell Anodes: Mechanistic, Kinetic and Structural Studies Peter George Keech A THESIS
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Trang 4ivi Archives Canada Published Heritage Branch 395 Wellington Street Ottawa ON K1A 0N4 Canada Canada NOTICE:
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Trang 5Abstract
Solid oxide fuel cells (SOFCs) are environmentally clean, high efficiency devices that operate at >600°C Present-day SOFCs consist of Ni-based anodes, 8% yttria stabilized zirconia (YSZ) electrolytes, and manganite-based cathodes The large-scale commercialization of SOFCs is hindered by several factors, including anode and cathode performance This work has focused on the kinetics and mechanism of hydrogen oxidation and water reduction at Ni-based anodes and at Pt, for comparative purposes
The electrochemical methods used to obtain this information were developed first at single phase Pt and Ni, and then applied later to all other electrode materials Experiments were carried out using a three-electrode half-cell at 750-950°C (typically at 800°C) in 97:3 H»:H20 Using the low-field, high-field and Allen-Hickling approximations, as well impedance spectroscopy, the exchange current density (ip ), the activation energy (130 kJ/mol), and the anodic and cathodic transfer coefficients were determined In general, the rate determining step of H2 oxidation at Ni was found to be the second electron transfer step, while water reduction is significantly slower, with the first step being the slowest
While the anodic transfer coefficient at Ni is larger than at Pt, the ig values cannot be compared as the reactive areas remain unknown Therefore, Ni point electrodes,
pressed against a YSZ disc, were examined to correlate the ig values with the known
electrode perimeter Unfortunately, the Ni/YSZ contact region was found to be porous, so that the true area remained unknown Efforts to use coke deposition as a means of identifying the active anode area also proved to be unsuccessful
Trang 6derived (SD) materials were studied Compared to ceramic grade, micron-sized NiO and YSZ, deposited using the same methods, the SD materials are very promising, being 40 to 50 times more active
A new method to establish the NiO-YSZ electrode porosity, involving monitoring the Ni?** oxide redox response in aqueous alkaline solutions was developed As long as the sweep rate was sufficiently slow, a good correlation between the measured redox charge and the amount of NiO in the composite electrodes was seen
Trang 7Acknowledgements
I am grateful to my advisor Dr Viola Birss for the many hours she spent with me on this project, and for the personal growth opportunities she has given me I have learned a lot under her advisement, within our field of research and about the cooperative nature of research
Mark Cassidy (formerly of Global Thermoelectric Inc.) and Tony Wood (Versa Power Systems) were instrumental in getting this project started, with their valuable experimental instructions, and by supplying materials in the early stages
My advisory committee has contributed in many ways to this project They are Dr J Hill (core member), Dr G Shimizu (core member initially), Dr V Thangadurai (core member at the end), Dr W Shaw (during candidacy and final examinations), Dr K Thurbide (during my candidacy examination), and finally, Dr H White of the University of Utah, the external examiner for my final defence
I would like to thank Drs Scott Paulson and Shen Jiang Xia for their help and advice pertaining to our experimental apparatus, and for assistance in the preparation of thin film electrodes Mark Toonen, Andy Read and Jose Lopez were invaluable for their parts in the construction of the electrochemical test cell Other department members, especially Bonnie King have made this project go smoothly as well Imaging was accomplished with the help of Rick Humphrey and John McGovern of the Microscopy and Imaging Facility and Rob Marr of Geology and Geophysics at the U of C
Trang 8flagging Karlie Haynes also gave significant experimental help on the aqueous work Our diverse research group has contributed in many ways to my time at this university, whether through group meeting discussions, paper editing or taking time away from the lab Harry Tsaprailis arrived when I did, and has worked alongside me the entire time I’ve been here, and our mutual support has benefited us both as we’ve gone through the Ph.D process Rudolf Potucek taught me much about Linux and networking issues, and assisted in writing scripts for data analysis Anne Co helped me with sputtering and imaging, while within our research group, Dr Jingbo Liu and Tyler Smith contributed greatly to interpretation of imaging results Joseph Fournier assisted during the early stages of our work with sols Other contributing group members include Heather Andreas, Erfan Abu Irhayem, Aislinn Sirk, Jeff Soderburg, Eric McLeod, Amit Jhas, Peyman Khalafpour, Ana Mani, Jason Young, and Sun Li, as well as Melanie Paulson Undergraduate researchers not mentioned above, that have made the lab a fun place, include Kerry Holmes and Trinh Nguyen
Alberta Ingenuity Fund and the Natural Science and Engineering Research Council (NSERC) have generously supported this project with scholarship money, along with the University of Calgary, and additional thanks go to Dr Yeong Yoo at the National Research Council (NRC) in Ottawa for supplying us with Pt paste
Finally, there are people that have made my time in Calgary fun, and are not included above: Kim Samkoe, Jude Hannaford and Nicole Hildebrand My family and especially my wife, Andrea, have supported me throughout this process, so I am grateful to these people as well
Trang 9List of Symbols Symbol Value Unit A A cm? œ Ga Oe B B 0<B<1 Ba Be C C F E V E(t) Eạ kJ/mol Emax V f Hz F 96485 c/mol 6 9 T Vv Ne V Na V H:O¿a; Vai i A/cm? ip Alem? I A Ip A I(t) A ip Alem? Tmax A IR V K K k L H À m n Nn) hạ Nx Description combined pre-exponential term area transfer coefficient
anodic transfer coefficient cathodic transfer coefficient line broadening (XRD) symmetry coefficient
anodic symmetry coefficient cathodic symmetry coefficient concentration of the reactant capacitance potential potential with respect to time activation energy Potential amplitude frequency Faraday constant phase angle (angle between the impedance vectors) reflection angle at maximum peak height overpotential cathodic overpotential anodic overpotential adsorbed water species imaginary unit current density exchange current density current exchange current
Trang 10R 8.31451 Selectrode Selectrolyte V o 2nf cm J/Kmol @ or cm Q or Qem Q or Qcm Q or Qem Q or Q em Q or OQ cm Q or Qcem Nu B2 NY NY NY NY k2 K/°C rad/s
oxide species at the triple phase boundary hydroxyl species adsorbed at triple phase boundary
adsorbed hydroxide species perimeter
number of electrons transferred during the rate determining step
gas constant
resistance
resistance element in EIS fit resistance element in EIS fit resistance element in EIS fit series resistance
polarization resistance charge transfer resistance
sweep rate
surface vacancy on the electrode surface vacancy on the electrolyte
stoichiometric coefficient, the number of
times the rds occurs for one occurrence of the full reaction time temperature doubly charged oxygen vacancy in the electrolyte total impedance
resistance of the resistor
resistance due to out-of-phase reactance from the capacitor or inductor
angular frequency
Trang 11AC AFC AH CE CG CGA CGB CPE CV dia DC DMFC EDX EIS HF LF LSM MCFC Ni- YSZ NiO-YSZ OCP PAFC PEMFC rds RE rms SD SEM SG SOFC TEM TPB UHV WE XRD YSZ or 8YSZ List of Abbreviations alternating current alkaline fuel cell
Allen and Hickling method counter electrode
ceramic grade
ceramic grade electrode synthesis “A” ceramic grade electrode synthesis “B” constant phase element/capacitance cyclic voltammetry
diameter direct current
direct methanol fuel cell
energy dispersive X-ray spectroscopy electrochemical impedance spectroscopy high field (region of Butler-Volmer) low field (region of Butler-Volmer)
Lad.) Sr,.Mn03
molten carbonate fuel cell nickel-yttria stabilized zirconia nickel oxide-yttria stabilized zirconia open circuit potential
phosphoric acid fuel cell
proton exchange membrane fuel cell rate determining step reference electrode root mean square sol-derived scanning electron microscopy sol-gel
solid oxide fuel cell
Trang 12Chapter 1: IntroductiOn càng ng TH HH TH TH Hi TH TH hm 1 1.1 ProJect BackgrOUunnd - - si TH Tờ 1
ĩ9) 5 ee 2
Chapter 2: BacKgTOUnid - ‹ s9 TH TH HH T370 5 2.1 Advantages and disadvantages of fueÌ ceÌÌs ác ghe 6 2.2 Fuel Cell TYp€S LH HT TH TH HH TH HH 9 2.3 Solid Oxide Fuel Cells (SOFCS) cee 11 2.3.1 S920 TT 13 2.3.2 }9)J002(-v J0 0 14 2.3.3 N90 nh 15 2.3.4 The SOFC triple phase bournd4ry sư ry 16 2.3.5 h9)206.1 1011 6 -“- 17 2.4 Literature review ofthe hydrogen oxidation mechanism under SOFC COTỞIEIOTNS - TH TH HH HT TT TT kí 18 2.5 Electrochemical techniques utilized in this theS1S Son ssssisey 19 2.5.1 0 /9JìiÀ/J12ii› 0077 19
2.5.2 Electrochemical mpedance SpeC{TOSCODY cay 20 2.6 Principles of sol-gel cher1SẨTV - Ăn TT HH ng ngư 25 2.6.1 S DTOC€SSITE Án TH TH TT Họ HH 26 2.6.2 SG synth€S1S TOU{€S HH HH TH HH Tu nh 27 2.6.3 Forms of SƠ DrOdUCS - -Q HH HH HT ng ngu 28 Chapter 3: General Experimental Methods c1 SH ng ng nhiệt 29 3.1 Maferlals Ă SH nh ng HT TT HT gu TH 29 3.2 Physical and chemical character1zation methods cv sssvsss 30 3.2.1 X-ray diffraction charaCf€T1Z2f1OT - HH HH ng HH 30 3.2.2 Transmission electron microscopy and energy dispersive x-ray spectroscopy Characterization .:.cccccescccssccssecsscessceeseeeeeecessesseseesnecessersesesaesessees 30 3.2.3 Scanning electron microscopy analysis and energy dispersive x-ray Spectroscopy charaC†€T1ZAE1OTN ác HT TH ng 31 3.2.4 Microprobe anaÏySIS - cuc ch HH HH ng Hè 32 3.3 Electrochernical measurern€niS - «siết 32 3.3.1 Three electrode half-cells for high temperature WOrẨK c «c- 32 3.3.2 Conditions used during high temperature half cell experiments 34
Trang 134.3.1 Electrochemical methods for determining the kinetic and mechanistic parameters of Hạ oxidatlon/H;O reduction at SOFC electrodes - 42 4.3.2 Electrochemical testing of single phase Pt on YSZ - cc<°⁄ 48 4.3.2.1 CV results obtained during H, oxidation/H2O reduction at single phase PUYSZ mnterfaces at 8Ư °C LH HT ng HH kg 48 4.3.2.2 High field (HF) analysis of Hạ oxidation/H;O reduction at single phase PƯYSZ interfaces at S0 C - HH ng HH nh no 50 4.3.2.3 Allen-Hickling (AH) analysis ofH; oxidation/H;O reduction at single phase PU/YSZ Interfaces at 800 °C - Án HH TH ni nh kưh 57 4.3.2.4 Low Field (LF) analysis of Hạ oxidation/HaO reduction at single phase Pt/YSZ interfaces at 800°, TH HH ngờ 59 4.3.2.5 EIS analysis ofH; oxidation/HaO reduction at single phase Pt/YSZ interfaces at 8ƯÚC - cv T TH TH ng HH 62 4.3.2.6 Summary ofelectrochemical results obtained for Hạ oxidation/HạO reduction at single phase Pt/Y5Z Interfaces at 800°C .- - co sec 67 4.3.2.7 SEM analysis of single phase P† electrodes - - 2c Ặ {se 68 4.3.2.8 Electrochemical performance of single phase Pt electrodes at other LEMMPETALULES 2.0 eee eeeeeeeeeeneceeneeeeteeceaeeeseessseeesneessecenaseeseeesseesssesessseneusenessessasessess 70 4.3.2.9 Hp» oxidation/H2O reduction at ceramic grade Pt-YSZ/YSZ composite S235 74
4.3.3 Ha oxidation/H¿O reduction at single phase N¡ electrodes 76
4.3.3.1 CV results obtained during Hạ oxidation/H;O reduction at single phase Ni electrodes at 80 °C, LH HH TH HH TT TH HT 76
4.3.3.2 — EIS results for Hạ oxidation/H;O reduction at Ni/YSZ interfaces at 800°C 83
4.3.3.3 Summary ofH; oxidation/H2O reduction at single phase Ni/YSZ
1nterfaces at 800°C, and comparison with Pt/YSZ, Interfaces ‹ -«- 85 4.3.3.4 SEM analysis of single phase Ni electrodes co 87 4.4 — COMCIUSIONS nh 88 Chapter 5: Ni anodes with controlled triple phase boundaries -. -<c<< «<< 91 5.1 Literature Review on Controlled Triple Phase Boundary SOFC Anodes 92 5.2 Chapter Specific Experimental Methods Ăn ng 94 5.2.1 Preparation and characterization of Ni point anodes for SOFC applications
94
5.2.1.1 Fabrlcation ofNI poInf anod§ ĩc HH ng Hiệp 94 5.2.1.2 SEM analysis ofNi point anodes and YSZ electrolytes 96 5.2.1.3 Microprobe analysis of Ni point anodes and YSZ discs 97 5.2.1.4 Electrochemical Evaluation of Activity of Ni Point Anodes in 97:3
H;:HạO 97
5.2.1.5 Regeneration of Ni Point Surfaces via Electropolishing 99 5.2.1.6 Depositlon of Carbon on Ni point anodes - «xe, 100 5.3 Results and Discussion SH HT ng Tờ 100 5.3.1 Physical characterization of Ni Points and YSZ Electrolytes 100 5.3.1.1 SEM analysis ofNI point electrodes cư 100
Trang 14point 102 5.3.1.3 Microprobe analysis of YSZ electrolyte surfaces after Ni point removal 105 5.3.2 Electrochemical performance of Ni point anodes in H2/3% H20 ALMOSPHETE 00 eeseesessceseecssseesceesnecsseeecseeceeeeseeseseceeeenneceeecseecsseesseereteeeneeseneeenaeenses 109
5.3.2.1 CV analysis of Hz oxidation/H20O reduction at Ni point electrodes 109
5.3.2.2 EIS analysis oFNi point anodes in 97:3 H;:H;O -+.- 112
5.3.3 Correlation between SEM determined for Ni point/YSZ contact areas/ perimeter and measured electrochemical parameters at 97:3 H2:H2O and 800°C 116
5.3.4 Coke Deposition as a Means of Establishing Ni/YSZ Contact Area and Perimeter 123 5.3.5 Analysis of Ni point anode after exposure to methane ‹› 123
5.4 CONCIUSIONS 0 130
Chapter 6: The Development of High Performance Ni-YSZ Catalysts for SOFCs 132
6.1 Literature Review of Nanoparticulate NIO — YSZ Materials 132
6.2 Chapter Spectfic Experimental Methods - - Ăn ng 134 6.2.1 Sol and sol-derived powder synthe€SIS HH, 134 6.2.1.1 Synthesis o£N¡O sol and sol-derived powders - sec 134 6.2.1.2 Synthesis of 8% YSZ sol and sol-derived poWders «- 135
6.2.1.3 Synthesis of NiO-YSZ composite sol and sol-derived powders 136
6.2.2 Electrochemical Cells .- cv Tnhh gọn HH ng cư 136 6.2.2.1 Preparation of YSZ electroÏyf€S LH ng rey 136 6.2.2.2 Application of sol-derived NiO-YSZ electrodes and other electrodes onto YSZ electrolyte SUTÍAC€S - HH ng gu ng ng Hư 136 6.2.2.3 Preparation of ceramic grade NiO-YSZ electrodes and other electrodes 137 6.2.2.4 Preparation of screen printed Ni-YSZ electrodes -.- 138
6.2.2.5 Preparation of Reference and Counter electrodes - 138
6.2.3 Physical and Chemical Characterization Methods - 139
6.2.3.1 TEM character1ZzatiOn - ch HH ng nư 139 6.2.3.2 SEM analySIS LH HH HT HH TH th ng 139 6.2.3.3 Microprobe nh 139
6.2.4 Electrochemical perforrmance testing of Ni-YSZ electrodes 140
6.2.4.1 H> oxidation/H,O reduction measurements of Ni-YSZ in half-cell 9014016151000 140
6.3 Results and DisCuSSIOn - Sàn TT HT HH TH Hà HH 140 6.3.1 Physical and chemical characterization of sol-derived NiO, YSZ and NiO- YSZ POWETS 4 140
6.3.1.1 XRD characfer1zation - SH HH HH kg kg 140 6.3.1.2 TEM examination of NiO, YSZ and NiO-YSZ powders 142
6.3.2 Electrodes fabricated using sol-derived NIO-YSŠZ” cà 146 6.3.2.1 Improving NiO-YSZ/VYSZ, Interfacial properties « 146
6.3.2.2 SEM characterization of YSZ and NiO-YSZ electrode materials 147
6.3.2.3 Microprobe analysis of sol-derived NIO-YSZ electrodes 151
Trang 156.3.3 Electrochemrical Evaluation ofNI-YSZ Electrodes - «+ 154 6.3.3.1 Electrochemical performance of ceramic grade (CG) Ni- YSZ electrodes
154
6.3.3.2 Electrochemical performance of sol-derived (SD) Ni-YSZ electrodes 164
6.3.4 Comparison of the H2/H2O electrochemistry at ceramic grade vs sol-
derived NI-YSZ elecfrO@S G G9 TH ng KT 168
6.3.4.1 Reaction Mechanism Considerations .cccccccccccccccsessssssssseseessseeseseens 169 6.3.4.2 Kinetic Considerations .cccccccccccssccsessssscenscescessesessessssssssesessceseeeens 171
6.3.5 Enhancing performance of Ni- YSZ composite electrodes through screen- printing 174 6.3.5.1 Electrochemical and mechanistic results for screen-printed Ni- YSZ electrodes in H;/H;O at §00°C 0 TQ HS HH ng ng kg xế 174 6.3.5.2 Comparison ofscreen-printed CG vs spray-coated CG (CGB) 324002) 176 6.3.5.3 Comparison of screen-printed SD vs spray-coated SD electrodes 176 6.3.5.4 Electrochemical performance screen printed ceramic grade Ni-YSZ electrode in full cell config1UIfat1OT - c- sư niey 177 6.4 ConcÌuSIOTAS - HH TT 180 Chapter 7: Aqueous testing of sol-derived NIO/Y SZZ LH HH xe, 184 7.1 Literature review on aqueous electrochemistry of sol-related NiO-YSZ 186 7.2 Chapter Specific Experimental Methods 0 ceccesssscsssseseceesseeessessesseesseees 188 7.2.1 Deposition of sol-gel NiO and NiO-YSZ on Pt -.-c c2 188 7.2.1.1 Preparation ofPt subsfraf€S -L cQ SH HH 1 ng 188 7.2.1.2 Application of NiO on Pt substrates for electrochemical evaluation 188 7.2.1.3 Application of NiO-YSZ sols on Pt substrates for electrochemical 1011500100077 3 190 7.2.2 Electrochemical evaluation of Ni and NiO-YSZ on Pt in aqueous solution
190
7.2.2.1 Electrochemical cell setup 0 ce ceceeseseesseeececeseceeesteceaeceaetenesaeenaees 190 7.3 Results and disCusSion ĩc HH HH HH HH ghe 191
7.3.1 Transmission electron microscopy analysis of NiO and YSZ sols 191 7.3.1.1 TEM Of NIO SOI Gà nrkn 19] 7.3.1.2 Transmission electron microscopy analysis of YSZ sol 193 7.3.2 The Deposition of sols onto Pt substraf€S ĩ5 àc series 195
7.3.2.1 Selection of Pt substrates for electrochemical evaluation: adhesion issues 195
7.3.3 The electrochemical behaviour of NiO and YSZ sols on Pt substrates 197
7.3.3.1 Dip-coated NiO and NIO-YSZ composIte soÌS - 22s 197 7.3.3.2 Electrochemistry of NiO deposited by Aliquot on Pt substrates 199 7.3.3.3 Electrochemistry ofNIO-YSZ deposited by aliquot - 204 “x92 o5 209 Chapter 8: Conclusions and future WOTK Ăn SH ng HH ng vu 211
8.1 Conclusions pertaining to hydrogen oxidation/water reduction at single phase Pt and Ni anOۤ - - kh HT HH TT TT TH TT HH ng 211
Trang 168.3 8.4 8.5
Conclusions pertaining to Ni- YSZ composite anodes se 214 Conclusions pertaining to the aqueous evaluation of NiO-YSZ sols 216 Proposed future Work .cccecesscsseceecsceeseeeaeeeseesseeseeeseesseeeseseeesaeensesenernersaee 218
Trang 17List of Tables
Table 2-1 Percentage Total System Emissions for Gasoline Combustion (GC), Methanol Fuel Cells (MF), and Natural Gas Fuel Cells (NGFC) Driven Passenger Vehicles 7 Table 2-2 Percentage Total System Emissions for Combined Cycle Gas Turbine
(CCGT), Solid Oxide Fuel Cell (SOFC), and Solid Oxide Fuel Cell Gas Turbine (SOFCGT) Power Generation? 0 c.ccccssssesseessesssesssessessecsssessesssestesstssessesseesstsseeaeen 8 Table 2-3 Common Fuel Cells, Electrolyte Types, Transporting Ion, and Typical
Operating Tempe€rfUT€S - - - <9 4H TH nọ TH HH HH nu nà 10 Table 4-1 Calculated Tafel Slopes (mV/decade) for Typical Transfer Coefficients 45
Table 4-2 LF, HF, AH, and EIS determined values of œ and ¡¿ for Hạ oxidation/H;O
reduction at sputtered Pt/YSZ at 800” C ch HT ng nàn 54 Table 4-3 Proposed mechanism for H2 oxidation/H2O reduction at sputtered Pt electrodes Iin60)000.0.)0)0 90h 56
Table 4-4 Values of elements from best-fit 2CPE equivalent circuit for H2 oxidation/H,O reduction at sputtered Pt at §ƯOC .- HH ng HH ng HH rên 65 Table 4-5 ig and a values for Hz oxidation/H20 reduction at Pt/YSZ interfaces as a
10910000) 19812i1-x1101 N8 71 Table 4-6 Arrhenius parameters for Hz oxidation/H2O reduction at sputtered Pt at 750 to
950°C, all calculated from the ig ValUeS cccccccccssccscccesececesccescssescessssucasevsestneescs 73 Table 4-7 LF, HF and EIS ig and o values for Hz oxidation/H2O reduction at single
Ji 280 N3 cuivvv 68.00200107 80 Table 4-8 Values of elements from best-fit 2CPE equivalent circuit at Ni- YSZ in 97:3
H;:H;O at 800°C Q0 Họ ni Họ ng gu ng HH net 85
Table 5-1 Geometry and EIS data for four Ni point anodes in 97:3 H2:H20 at 800°C 119 Table 5-2 Area / perimeter corrected EIS for four Ni point electrodes in 97:3 H2:H20 at
1000001755 119
Table 5-3 Area and Perimeter Corrected ig Values Calculated from EIS, HF and LF 122 Table 6-1 Comparison of electrochemical parameters for H, oxidation/H2O reduction at
SD, CGA and CGB Ni-YSZ electrodes (at 800°C) ccc eeesecstscesneeesreseneessseesees 169 Table 7-1 Drying temperature dependence of initial and final cathodic CV charges (uC)
arising from 4.6 pmol of NiO sol on Pt foil at 100 mÝV/§ cv 201 Table 7-2 Film composition dependence of 1nitial and final cathodic CV charges (nC)
arising from 4.6 pmol of NiO sol on Pt foil at 100 mÝV/§ cccccc<sererers 205 Table 7-3 Sweep rate dependence of cathodic CV charges (nC) arising from 4.6 mol of NiO sol on Pt foil at 1 to 10Ơ mV/S Q1 v ng HH HH nườn 207
Trang 18Fig 2-1 Schematic ofa single solid oxide fuel cell (SOFFC) .-c c2 5 Fig 2-2 Schematic of triple phase boundary for hydrogen oxidation at Ni in SOFC
Elements are shown without charge for sITnpÏICIfV - .- 5-5 ssssssexseeseesssss 16 Fig 2-3 Cartesian and polar (|Z|, Ð) coordinates for impedance «s« «+ 22 Fig 2-4 Equivalent circuit used to model simple electrochemical systems 23 Fig 2-5 Simulated (a) Nyquist and (b) Bode plots, corresponding to equivalent circuit
shown as Fig 2-4, using values of R; = 10 Q, R2 = 40 Q, CPE = 10 pF, andn=1
(green squares) or n = 0.9 (blue CirCÏ€S) c1 v9 1111111 1 xe 24 Fig 3-1 Schematic of 3 electrode SOFC test cell, illustrating positions and dimensions of
working, counter and reference electrodes, as well as of current collectors and Clectrical leads eee csssssssssscesceseeeeeeseesecsecescesecsessecseceeessesessesseseeessecaseneseeseeeeasaes 33 Fig 3-2 Schematic of high temperature electrochemical half celÏ -«‹ 35 Fig 3-3 Assembled high temperature electrochemical half cell with shielding wire 36
Fig 4-1 CV of H» oxidation/H2O reduction at Pt paste at 800°C and 100 mV/s at 18, 48 and 110 ml H›/mI1 - S99 0 T550 5 SE g5 6k ki 48 Fig 4-2 CV (10, 50, and 100 mV/s) ofH; oxidation/H;O reduction at sputtered Pt at
L000001ẼẺẼ.8® 49 Fig 4-3 (a) Anodic and (b) cathodic Tafel plot of Hạ oxidation/H20 reduction at
sputtered Pt at 800°C and 10 mV/s, with i in MA/CM? wo ese sssesssesteseessesstessenes 51 Fig 4-4.(a) Anodic and (b) cathodic Allen-Hickling plots of H2 oxidation/H2O reduction
at sputtered Pt at 800°C and 10 mV/s with ¡ in mA/CIỂ 5-5 56 ccccccczeri 58
Fig 4-5 LF region of CV for H2 oxidation/H2O reduction at sputtered Pt at 800°C and 10 ¡0 1n 60 Fig 4-6 OCP EIS response of of Hạ oxidation/HaO reduction at 800°C showing: (a)
decrease in arc diameter following periods of polarization and with 1CPE EC inset, with ECs and fit data shown as insets; (b) no change in response seen over period of days at 800°C in absence of polarization ::csccsscssscssscetsceesscessscesecesscnnecneeeneeees 63 Fig 4-7 Top down SEM views of as deposited (a) sputtered Pt Anode on YSZ, and
following heat treatment at 800°C in H2/3% HạO for 10h of (b) sputtered Pt on YSZ and (c) Pt paste on SZ so HH HH 69 Fig 4-8 Arrhenius plot for Hz oxidation/H2O reduction at sputtered Pt using ig values
from (top down): Anodic high field Tafel Region #1, anodic Allen-Hickling Tafel Region #1, cathodic high field, cathodic Allen-Hickling, low field, electrochemical impedance spectroscopy, anodic high field Tafel Region #2, and anodic Allen- Hickling Tafel Region #/2 - cv HH g9 HH uc Hà ng nà 72 Fig 4-9 (a) CV and (b) anodic Tafel plot (IR corrected) for H, oxidation/H,O reduction
at Pt-YSZ composite electrode at 800°C with ¿ in mA/CTrỶ - - 5c 5cccscsc5¿ 75 Fig 4-9 (c) EIS data for Hạ oxidation/H;O reduction at Pt-YSZ composite electrode at
00001 76 Fig 4-10 CV (10 mV/s) of Hz oxidation/H2O reduction at vapour deposited Ni at 800°C
(a) as acquired and (b) following IR compensation .:ccccccssessecssessrseseeeseesteeees 77 Fig 4-11 Hạ oxidation/H2O reduction at vapour deposited Ni at 800°C and 10 mV/s
with i in mA/cm’ showing anodic: (a) Tafel plot (b) Allen-HIckling plot 78
Trang 19Fig 4-12 Cathodic branch ofa CV (10 mV/s) for H; oxidation/H;O reduction at vapour
deposited Ni at 800°C demonstrating infÏlexion r€g1on - «cv 81 Fig 4-13 Simulated Butler-Volmer CV for reactions at 800°C described by (a) & = 1.5
(maroon), O = 0.5 (blue dash), and sum of these (black), and by o, = 0.5 (red), O& = 0.5 (blue dash), and sum of these (solid dark blue) and (b) blow-up of cathodic region of these showing predicted inflexion region (red) -ccccsssscsee 82 Fig 4-14 OCP EIS data for Hạ oxidation/HO reduction at vapour deposited Ni at 800°C
t1911 01111 TT HT TT TT TH TH HH HT TT TH TT Ti HT g1 84 Fig 4-15 Top down SEM views of vapour deposited Ni (a) as deposited and (b)
following heat treatment at 800°C in H2/3% H2O for 10 Beewee eee cceeetseneceneeeeeeeaes 88 Fig 5-1 (a) Side view of Ni point electrode pressed on YSZ electrolyte, and (b) top view
of Ni point electrode, showing contact area to YSZ, and two possible (simulated) textures of the Contact TEQION ee ceceescesccsseeceseceesceeseeseescseeeceseceesecesaserseeeesreneas 95 Fig 5-2 SEM end-on view of Ni points electrode that were (a) newly machined, and (b)
following heat treatment 800°C for 3 days and electrochemical testing for several
5ì PP 101
Fig 5-3 Secondary SEM electron image of end-on view of electropolished Ni point is 102 Fig 5-4 (a) Secondary and (b) Backscattered (SEM) images of YSZ discs after removal
of Ni point electrode Dark region in (a) and (b) examined for NiO and found to be
H200 PP 103
Fig 5-5 (a) Backscattered image of YSZ disc surface following cell disassembly, showing Ni traversing the YSZ surface, likely due to the Ni point being dragged across the disc during disassernbÌy - «kg HH ng ng ng ke 105 Fig 5-6 (a) Backscattered electron image and element map for (b) Ni and on YSZ disc
following cell disassembly, (Ni is white region near centre of (b)) - 107 Fig 5-7 Protocol # 1 CV (10 mV/s) of H2 oxidation/H20 reduction at Ni point electrode
at 800°C showing (a) effect of DC polarization of 300 mV (with inset shows Tafel Plot) and (b) effect of several days at 800°C and testing sec 110 Fig 5-8 (a) Protocol # 2 CV (10mV/s) of H2 oxidation/ H2O reduction at Ni point
electrode at 800°C and 10 mV/s and (b) corresponding Tafel plot of same data with J 0Ú 111 Fig 5-9 Nyquist plots for two different EIS of Ni point electrodes in (97:3) H2:H20 at
800°C showing (a) one apparent time constant, and (b) two apparent time constants, with equivalent circuits Shown As insets eeescsecenecesetereeeeceneteeeteseaeessereeeeaaee 113 Fig 5-10 SEM images of Ni point electrodes surfaces, after cell disassembly that
produced electrochemistry of Fig 5-9 revealing (a) non-porous point and (b) porous 09511017 114 Fig 5-11 (a) and (c) SEM images of end-on view of two Ni points electrode with (b) and
(d) showing estimated contact areas (whife T€BØ1OTIS) - án nghiệt 117 Fig 5-12 SEM images showing (a) higher perimeter and (b) lower perimeters of Ni on
YSZ, but with approximately equivalent N/YSZ conftact areas - ‹«- 120 Fig 5-13 (a) Secondary image, (b) backscattered image, (c) carbon element map, and (d)
Ni map of end of Ni point electrode that was attached to YSZ and exposed to
methane at 800°C Warmer colours indicate higher element concentrations 125
Trang 20was attached to YSZ and then exposed to methane at 800°C Images show an
4900101318081) 000001007877 126 Fig 5-15 (a) and (b) Two possible contact region after exposure to methane at 800°C,
X28 ÄJ[ i10) 0ui n6 128 Fig 5-16 (a) and (b) Two possible contact regions for methane exposed Ni point at
800°C, same electrode as in Fig Š- Í4 Ăn HH HH Hiệp 129 Fig 6-1 XRD pattern for (a) NiO, (b) 8% YSZ, and (c) 1:1 NiO:8% YSZ sol-derived
powders, with data standard (Jade, 6.5), shown below each set 142 Fig 6-2 (a) TEM image of NiO powder on C-coated Cu grid, and (b) Histogram of NiO
particle sizes (diameters) cccsesssessccesessscesseesecssessecseeseesssasssesseseaseesecssesseseesans 143 Fig 6-3 TEM image of 8% YSZ SD powder on C-coated Cu grid - ‹ 144 Fig 6-4 (a) TEM image of 1:1 NiO: 8% YSZ sol-derived powder and EDX data for (b)
Spot 1 (NiO) and (c) Spot 2, (8% YSZ) particles ccccscssscessecesteceseeceteeesseeeees 145 Fig 6-5 SEM Image of YSZ, anchoring layer on YSZ electrolyte disc 147 Fig 6-6 SEM analysis of ceramic grade NiO-YSZ produced using synthesis, method
“CGA” showing (a) YSZ-rich region with embedded NiO particles and (b) NiO-rich T€Đ]OHI HH HT TH HH TH TT HT TT HT TH H0 HH ch 148 Fig 6-7 SEM (backscattered) images of NiO-YSZ electrode on YSZ electrolyte disc (a)
low resolution top-down view, (b) cross-section, (c) top-down view showing large agglomerates within electrode, and (d) low resolution top-down view, showing agglomerates on electrolyte and within electrOde - 5 St ssxrerres 150 Fig 6-8 Top-down microprobe analysis of sol-derived NiO-YSZ anode showing (a)
backscattered image, and (b-e) element maps for (b) Ni, (c) Zr, (d) Y and (e) O Warmer colours indicate higher element concerifraf1OnS 5 55s ssssss 152 Fig 6-9 Cross-sectional microprobe analysis of sol-derived NiO-YSZ functional anode
showing (a) backscattered image, and (b-c) element maps for (b) Ni, (c) Zr Warmer colours indicate higher element concentrations cccssesssseeseetesssctseereeseeesesees 153 Fig 6-10 (a) IR uncompensated CV of H2 oxidation at CG Ni-YSZ composite prepared
by synthesis method “CGA” showing high series resistance at sweep rate of 10mV/s and at 800°C, (b) Tafel plot for same data illustrating effect of high series resistance With 7 in MA/CI eeeeccsssecscsessesssssessesscsrcsscarssesssssesseseesussecasssesaesussucaucaesateaseueteeeees 155 Fig 6-11 (a) Anodic Tafel plot derived from IR compensated CV for H2 oxidation/H2O
90100 157 Fig 6-12 (a) IR uncompensated CV of H2 oxidation/H20 reduction at CG Ni-YSZ
composite prepared by synthesis method “CGB” showing Butler-Volmer kinetics at 10 mV/s and at 800°C eee ceeeseencssscteceeseseeseesecseseesescesesssesecsusesesssesessesesseaseaseneseees 160 Fig 6-13 (a) IR uncompensated CV of H2 oxidation/H20 reduction at SD Ni-YSZ
composite prepared by spray-coating method showing Butler-Volmer kinetics at 800°C and at 10 MV/S 164 Fig 6-14 Anodic Allen-Hickling plot calculated from CV of H2 oxidation at CG Ni-YSZ
composite using Value n/v = 1.25 instead of n/v = 2 showing distortion to Tafel region when compared to Flg Ĩ-]2C - c2 1v E111 81118111 11111111 xee 172
Trang 21Fig 6-15 IR corrected CV of H2 oxidation at Ni-YSZ composite prepared by screen- printing on roughened YSZ discs for (a) ceramic grade Ni- YSZ anode and (b) sol- derived Ni-YSZ anode 0.0 ccccecsesccesesseeseeseeseeseesessececseessececsecesevaeenessecneesronseeesenees 175 Fig 6-16 (a) CV of H> oxidation at a screen printed Ni- YSZ composite anode, on
COMME CIAal YSZ oo eescscsseessesseceseseeseccsseeeseceseeseceeesseeceeeeesseeetaeseseecseessneeseaeeenatenas 178 Fig 7-1 Schematic of NiO-YSZ composite on Pt substrate, illustrating a non-active
structure (on left), arising from lack of electrolyte or independent electrical
connections between NiO solution or Pt, and an active structure (on right) with good CONMUCHION PAthWAYS cccsccesescesccssscessetscssecsssesseessceseeeneseaseseeteesaesesseeeeeseaesnees 185 Fig 7-2 TEM image of 3 mM NiO sol, aliquoted onto a C-coated Cu grid, and air dried
ẨOT S€V€FA] Ủ SG HH HT HT HH TT TH TH HH 192 Fig 7-3 TEM Images of 9 mM (a) YSZ gel and (b) YSZ sol, both aliquoted onto C-
coated Cu grids and dried in air for several he oo eee eeeecesesscenecseeseeeeeneeeaeeneeaeens 193 Fig 7-4 CV of Pt foil substrate in 1 M NaOH before NiO sol application (squares) and
after cleaning with H2SO, (circles), showing hydrogen adsorption /desorption peaks used for real area and roughness calculÏafIOTnS - -ĩ- 5 ngay 195 Fig 7-5 CV (20 mV/s) showing Ni(OH)2 S NIOOH oxidation/reduction for dip-coated
NiO sol (with Triton), in 1 M NaOH, formed by withdrawal from sol 4 cm/s, and dried at 200°C for l5 mmim -.- - << 11 119v nh 197 Fig 7-6 CV (2mV/s) showing Ni(OH)2 S NiOOH oxidation/reduction reaction of dip-
coated NiO-YSZ sol (with Mazawet), in 1 M NaOH for withdrawal rate at 4 cm/s, and dried at temperature of 350°C for 1Š TmIT - ĩ5 «cv cư 198 Fig 7-7 CV (100mV/s) showing an Ni(OH)2 S NiOOH oxidation/reduction for 15 pL
aliquot of NiO sol , showing effect of drying temperature for 100°C, 200°C, 500°C ANd 800°C for lŠ mIm - 0 5 2s s13 99 1g TH TT Hà Hư 200 Fig 7-8 Dependence of the cathodic peak current on the sweep rate for a 15yL aliquot of Ni Sol on Pt foil, dried at 100°C for 15 Min ccc cccccccssssceesssecessseceeseeeceeecessaes 203 Fig 7-9 CVs showing an NiOOH > Ni(OH), reaction for 20:80 NiO: YSZ dried at
200°C sols, for 15 min, showing effect o sweep rafe co seo 206 Fig 7-10 Dependence of the cathodic peak current on the sweep rate for a 30:70
NiO: YSZ film on Pt foil, dried at 200°C for 15 min c- 5 5S 55123 s++ 208
Trang 221.1 Project Background
In recent years, there has been an increased interest in the development of efficient electricity generators Demand for electricity is at an all-time high, partly due to an exponential increase in its global consumption (4% per year between 1970 and 1990)’ Greenhouse gas reduction targets associated with the Kyoto Protocol require improvements in energy conversion efficiency, reductions in energy consumption, or a combination of the two Conventional fossil fuel electricity plants convert about 40% of chemical energy to electrical energy through heat turbines, not including losses incurred from transportation of power’ On the other hand, fuel cells convert chemical energy to electricity in one step, making them fundamentally different This one-step conversion ensures that they are not bound by Carnot cycle limitations, and are, in principle, able to achieve a higher energetic conversion between chemical and electrical energies’ than combustion sources
Trang 232 This project was related to a collaboration between our group and Versa Power Systems (formerly Global Thermoelectric Inc.) in Calgary and the Alberta Energy Research Institute (AERI) through the Coordination of University Research for Synergy and Effectiveness (COURSE) program Goals at the project outset involved the development of reliable electrochemical methods for the evaluation of the performance of individual electrodes in SOFCs Subsequently, it has developed into a fundamental study of anodes, at which hydrogen is oxidized to protons, and has also led to the development of new anode materials for SOFCs
The performance of fuel cell systems is often measured using two-electrode electrochemical techniques, from which the overall cell performance is assessed The individual effects from the anode, cathode and electrolyte have to be subsequently
extracted from a combination of data In the present work, a three electrode, half-cell
was employed, which allowed the analysis of just the anode data, separate from the rest
of the cell
1.2 Project Objectives
The goals of this project were as follows: to gain an understanding of the mechanism and kinetics of the anode reaction under typical SOFC operating conditions, thereby contributing to the overall development of SOFCs for power sources; and to synthesize novel Ni catalyst structures using sol-gel chemistry and to evaluate them as SOFC anode materials For these purposes, the specific focus of the research presented
Trang 241.3
To establish reliable methods for measuring the reaction kinetics and mechanism of the hydrogen oxidation reaction, and the reverse reaction (water reduction), at primarily single phase Ni- and Pt-based anodes on yttria stabilized zirconia (YSZ) electrolytes
To utilize sol-gel chemistry to produce small particle NiO-YSZ composite materials as possible high performance Ni- YSZ anodes in SOFCs and to apply the electrochemical analysis methods established using single phase Ni and Pt to these anodes
To restrict and quantitatively establish the triple phase boundary length, where the electrochemistry occurs, and to correlate it with the observed electrochemistry To establish the porosity and stability of sol-gel derived NiO-YSZ materials using aqueous electrochemistry approaches
Thesis Organisation
Trang 254 triple phase boundary length The electrochemical analysis methods developed in Chapter 4 for single phase metallic anodes were then applied to H»2 oxidation at composite Ni-YSZ anodes in Chapter 6, including to new anode nanomaterials synthesized through sol-gel based chemistry Chapter 7 documents the continued exploration of the sol-gel based materials; in this work, the materials are tested in an aqueous environment to assess the accessibility of Ni within the Ni-YSZ structure to external reactants and to provide a measure of the internal porosity In Chapter 8, the conclusions reached in this thesis work are presented, and recommendations for future
Trang 26A fuel cell is a device that electrochemically converts chemical energy to
electrical energy It consists of three core components, an anode, where fuel is oxidized;
a cathode, where oxygen is reduced; and an electrolyte, which separates the two electrodes, allows charge balance through the passage of ions, and prevents the cell from electrically shorting Fig 2-1 shows these components for a solid oxide fuel cell (SOFC)
Fuel H, /
hydrocarbons
Fig 2-1 Schematic ofa single solid oxide fuel cell (SOFC)
Trang 276 the time consuming process of recharging required for batteries is eliminated for fuel cells In addition, fuel cells are easily stacked, creating a system of high energy density, when compared to battery technologies
The work described in this thesis is directed towards solid oxide fuel cells (SOFCs) and the focus is primarily on the anode reaction As such, this chapter summarizes the advantages and disadvantages of fuel cells, specifically SOFCs Particular attention is paid to the state of the anodes in SOFCs, as well as the kinetics and mechanisms of the reactions relevant to SOFC anodes The electrochemical techniques used in this investigation, such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), are also described in brief
2.1 Advantages and disadvantages of fuel cells
Two key driving factors for the development of fuel cells are their high energy conversion capability and their low environmental impact For fuel cells operating on hydrogen, water is the only chemical product, making them true zero emission energy devices Fuel cells that operate on carbon fuels produce the greenhouse gas, carbon dioxide, but their high efficiency means that this occurs to a lower extent than in combustion devices, where Carnot limitations’ significantly reduce efficiency In
addition, the emission of the pollutant NO,, which occurs through the combination of Na with O2 during conventional fuel combustion, is much lower in the case of fuel cells Particulate matter, SO, and CO are also much lower in the emissions from fuel cells
Trang 28fuelled by gasoline combustion (GC), a methanol fuel cell (MFC); and a natural gas fuel cell (NGFC) The emissions are normalized to that from the GC vehicle
Table 2-1 Percentage Total System Emissions for Gasoline Combustion (GC), Methanol Fuel Cells (MF), and Natural Gas Fuel Cells (NGFC) Driven Passenger Vehicles°® NO, SO, co CO; PM Energy Vehicle (%GC) (%GC) (%GC) (%GC) (%GC) (%GC) MFC GC 100 15 100 3 100 0.6 100 62 100 14 83.3 100 NGFC 9.1 3.2 0.32 40 0.08 53.4
Trang 298 those for SOFC and SOFC-gas turbine (SOFCGT) generation; the data are normalized to
the emissions from the CCGT®
Table 2-2 Percentage Total System Emissions for Combined Cycle Gas Turbine (CCGT), Solid Oxide Fuel Cell (SOFC), and Solid Oxide Fuel Cell Gas Turbine
(SOFCGT) Power Generation® NO, SO, CO CO; PM Energy Vehicle (%GC) (%GC) (%GC) (%GC) (%GC) (%GC) CCGT SOFC 100 4 100 65 100 2 100 81 100 0 100 79 SOFCGT 4 51 1 64 0 62
Besides increased efficiency, there are several other advantages that fuel cells possess over traditional combustion energy sources Unlike combustion sources, chemical energy is not converted to a mechanical form in fuel cells, as they have no moving parts Fuel cells are also much more scalable than conventional sources, due to the ease of stacking multiple fuel cell units together to achieve the required power
Trang 30be overcome for the continued development of fuel cells, including: durability issues, such as thermal cycling of high temperature fuel cells or catalyst deactivation; catalyst poisoning by fuel impurities or reaction products; material incompatibility within fuel cells or fuel cell stacks; limitations in infrastructure for the delivery of fuels suitable for many fuel cells; as well as the acceptance of their integration into the existing marketplace as safe energy devices Some of the technological barriers remain poorly understood, relegating many fuel cells to the development stage
2.2 Fuel Cell Types
As anode and cathode reactions are often common between different types of fuel cells, their classification is generally done according to the nature of the electrolyte Some common fuel cells are the proton exchange membrane (PEMFCs) and the related direct methanol fuel cells (DMFCs); alkaline fuel cells (AFCs); phosphoric acid fuel cells (PAFCs); molten carbonate fuel cells (MCFCs); and solid oxide fuel cells (SOFCs) These fuel cells, as well as their electrolyte/charge carriers and typical operating temperatures, can be found in Table 2-3
Trang 3110 Table 2-3 Common Fuel Cells, Electrolyte Types, Transporting Ion, and Typical Operating Temperatures!
FC Type Electrolyte Ion T (CC)
PEMEC / DMEC NafionTM H 60-80 AFC KOH OH 65-220 PAFC H;POa H 205 MCFC Na;COz/K;CO; CO3;" 650 SOFC Y;Oz/ZrO; O7 600-1000
Trang 322.3 Solid Oxide Fuel Cells (SOFCs)
SOFCs require elevated temperatures (600 to 1000°C) to ensure suitable conductivity of the charged species through the solid electrolyte, which typically is composed of 3-8% yttria stabilized zirconia (YSZ) This high temperature also promotes the reaction kinetics at both anode and cathode and can allow for the direct oxidation of carbonaceous fuel!* Owing to the large amounts of heat required for and generated by SOFCs, their primary use is likely to be for the generation of power at stationary locations, where heat can be managed most efficiently Two types of SOFC are typically seen, planar and tubular; both types are described below
Planar SOFCs can be classified according to the component material that gives the FC strength In general, the classifications includes electrolyte-, anode-, cathode- or interconnect-supported When the support is the anode, cathode or electrolyte, the supporting component is commonly tape-casted and the remaining materials are screen- printed onto the support For electrolyte-supported SOFCs, the ohmic resistance can be quite high, so a great deal of research is focused on anode-or cathode-supported cells
Trang 3312 An alternate design for SOFCs that negates the need for gas sealants is based on a tubular structure In these structures, one electrode is on the inside of the tube, the other is on the outside, and they are separated by an electrolyte layer Gas separation is achieved by flowing one feed gas through tube and the other feed gas on the outside of the tube Tubular SOFCs are currently being developed by Siemens-Westinghouse and assembled by Fuel Cell Technologies in Kingston; the latter has produced the Alaskan prototype that has been operable for more than one year
Tubular SOFCs, made by Siemens-Westinghouse and which are cathode- supported, are synthesized using a cathode tube support (ca 1.4 mm thick), upon which the YSZ electrolyte is deposited as a dense, thin film (40 pm) The Ni-YSZ anode is subsequently added as a slurry, prior to the cells being fired at high temperatures (> 1000°) The Acumentrics tubular SOFCs, on the other hand, are anode-supported In this case, the electrolyte is added as a slurry to the anode tube support prior to the initial firing, which densifies the YSZ The cathode layer is subsequently added and the cell undergoes a second firing
As this project was focussed on the performance of anode materials, the mechanisms used in practice to separate the gas streams are not relevant to the experimental parameters we employed in this work Here, the electrochemical experiments were performed using a three-electrode “half-cell’”’ configuration (Fig 3-1) This configuration negates the need for the passage of different gases over each electrode, as each electrode is under a common atmosphere in this configuration Chapter 3
Trang 34two-electrode “full cell” methodology (Fig 2-1), where leads are attached to the anode and cathode and the overall fuel cell performance is measured
2.3.1 SOFC cathodes
One difficulty that arises with respect to SOFC cathode materials is that the nature of the atmosphere, e.g., hot air/oxygen, can be very harsh on many electrochemically useful materials, particularly metals Virtually all non-noble metals oxidize rapidly in this atmosphere, eliminating most of them as oxygen reduction catalysts However, Ag has a relatively low melting point, 962°C'*, and Au is not well-known for its oxygen reduction electrochemistry Of the noble metals, Pt has been explored with the most success'*!’; however, Pt has proven not to be stable during O reduction, giving inductive features in electrochemical measurements In addition, the cost of Pt is prohibitive for use as commercial SOFC cathodes
Most SOFC cathodes are based on a conductive ceramic, which overcomes the
Trang 3514 LSM based cathodes containing various noble metal dopants are also of recent interest, and have been characterized by Haanappel, ef al*° While Pd, Pt and Ag additives seemed to show no positive influence on the cathode electrochemistry, Pd on activated carbon proved to enhance the oxygen reduction reaction rate significantly, particularly at lower operating temperatures
2.3.2 SOFC electrolytes
SOFC electrolytes must be ionically conductive, electrically insulating, non- porous to gases, and stable at high temperatures for long periods of time The most
common electrolyte material is ZrO, doped with 8 mole % Y203 (8YSZ, or YSZ in this
thesis) As mentioned previously, the charge carrier in SOFCs is O*; however, the ionic conductivity of YSZ is only 0.1 @''em'” at 1000°C and much lower at the typical SOFC operating temperatures of 700 to 800°C The relatively low ionic conductivity of YSZ is partially overcome by making it very thin, typically only a few pm, within an electrode- supported cell It is the other characteristics of YSZ, specifically its durability and very high electronic resistance, which make it the primary option for SOFCs at this time
Specifically, YSZ does not react with conventional anode or cathode materials, even after
very many hours at high temperatures
Trang 36conductivity than YSZ, but has not yet demonstrated enough stability to be considered a viable replacement for YSZ in SOFC technology
2.3.3 SOFC Anodes
Ni is typically the anode catalyst used for Hz oxidation in SOFCs It has a high electrochemical activity, is relatively inexpensive, and when mixed with YSZ in a 1:1 mole ratio, it has a thermal expansion coefficient which is comparable to that of YSZ
(12.5 x 10° cm/cmK vs 10.8 x 10° cm/emK above 600°C) Currently, it is the material
of choice for Siemens-Westinghouse, Acumentrics, and Versa Power Systems
However, the oxidation of Ni can cause volumetric increases of up to 30%, a
problem that may arise during a failure of a gas seal or the rapid shutdown of an SOFC stack Rapid volumetric expanses are likely to cause fractures within the SOFC cell or detachment of the Ni anode In addition, Ni has a low tolerance for sulphur within fuel feeds, so it must be removed prior to fuel introduction into SOFCs Another ongoing issue with Ni-based anodes is their inability to directly oxidize carbonaceous fuels, such as natural gas, without pre-reforming them If these fuels are added directly to Ni-based anodes, they undergo cracking, leaving residual carbon deposits on the Ni-based catalysts’* and causing an irreversible drop in cell performance” Consequently, most fuel feed stocks for SOFC anodes undergo a pre-reforming step to convert the
carbonaceous fuels to H2, in addition to CO2 and CO, and minimize carbon deposition?
To avoid the difficulty of pre-reforming, substantial effort has been directed at developing catalysts that do not have these carbon deposition issues Some of the most
14,26
Trang 3716 electrochemical oxidation and does not go through an internal reforming process In related work’’, the authors have demonstrated that Cu-CeO anodes can electrochemically
oxidize either H, or CHg, and that H oxidation does not diminish following exposure to
CHg, unlike Ni-YSZ, which rapidly undergoes irreversible deactivation upon exposure to
CHạ Within this study’’, it is notable that the Ni-YSZ anode began with a significantly
higher activity than the Cu-CeO anode; this may partially indicate why Cu-CeO anodes have not developed more rapidly
2.3.4 The SOFC triple phase boundary
The electrochemically active region of an SOFC is often referred to as a triple phase boundary (TPB), as fuel, electrolyte and electrode all must be present for the electrochemical reaction to proceed The TPB is discussed extensively throughout Chapters 4-7, while Fig 2-2 pictorially demonstrates the reaction site (TPB) for Hz oxidation at Ni eH @0o e @ Zr O?- from cathode reaction -——_+ @ — ° Y @Ni YSZ Ni
Fig 2-2 Schematic of triple phase boundary for hydrogen oxidation at Ni in SOFC Elements are shown without charge for simplicity
Trang 382.3.5 SOFC fuels
As alluded to in Section 2.3.3, there are several fuels currently being used for 28,29 , carbon monoxide’, syngas”, ca 30 31 SOFCs These include hydrogen, natural gas”°, butane
and coal gas** Existing infrastructure for its delivery, as well as its relative abundance, make natural gas an attractive fuel for SOFCs However, as mentioned in Section 2.3.3, when carbonaceous fuels such as methane, butane or coal gas are supplied directly to Ni anodes, hydrocarbon cracking occurs, and carbon fibres are formed within the SOFC
anode Following this, Ni anodes deactivate, and in some cases, the SOFCs are damaged Cu-CeO anodes have shown a resistance to carbon formation, but the high activity of Ni for H> oxidation continues to attract most SOFC anode researchers
Consequently, significant efforts have been made to efficiently reform carbonaceous fuels to make them more suitable for Ni-based SOFC anodes One such method is steam reforming, combined with the water-gas shift reaction®®, which produces H, and CO, from H20 and CH, Steam reforming is generally performed on supported metal catalysts, such as NẺ, or Ni-Cu”” and temperatures typically exceed 500°C However, this reaction is very endothermic, requires low pressure (i.e large reactor volumes and constant removal of products), and a high H,O content must be maintained to prevent carbon deposition Partial oxidation of CHs produces CO and H), and minimizes carbon deposition, but uses chemical energy that could otherwise be used by the fuel cell, so this solution is not ideal either
Regardless of the specific method for making H2 from hydrocarbons, a mixture of gases is produced from any reforming process This mixture generally includes H2, CO:,
Trang 3918
the reformer Consequently, there is much demand for H; fuel cells that can function with these other gases in the fuel stream The current anode that gives the best activity is Ni, but C deposition from the trace CH, and S poisoning continue to be problematic This issue is linked with the development of modified Ni-based catalysts that reduce these problems, yet retain the high activity seen for H2 oxidation This is the focus of the latter half of this work: to develop active Ni-based catalysts, with the eventual goal being to test them within mixed gas streams However, within the scope of this thesis, H2/H2O
was selected as the fuel gas, to establish the activity of these new catalysts in the absence of C or S problems H>/H2O was also used in the early portion of this work, to establish kinetic and mechanism parameters for Ni- and Pt- based electrodes
Trang 40to vary between research groups In some cases, the reaction is presumed to be limited by gas diffusion®®, while sometimes it has been determined that adsorption”® or electron transfer?’ determine anode kinetics It is partly because of these discrepancies in the literature that we directed our work towards establishing reliable electrochemical methods to identify the rate determining step in the H» oxidation reaction at Ni and Pt anodes, as described in Chapter 4
2.5 Electrochemical techniques utilized in this thesis
As mentioned in Section 2.3, the electrochemical experiments performed in this thesis utilized a three-electrode set-up, where the kinetics and performance of only the working electrode (WE) are established In these experiments, a potentiostat is required, which controls the potential of the WE against a reference electrode (RE) Current that flows at the WE is matched in magnitude at the counter electrode (CE), which balances the charge within the electrochemical cell In this work, the CE was placed directly opposite to the WE, on the YSZ electrolyte, while the RE was located on the same side as the CE, several mm away, as described in Chapter 3
2.5.1 Cyclic voltammetry
In cyclic voltammetry (CV), the potential between the WE and RE is linearly
increased with time and then decreased, at specific sweep rates, and the current which