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Solid Oxide Fuel Cells Presentation
- 1. Solid Oxide Fuel Cells: The Future Of Energy By Farbod Moghadam
- 2. What is a Fuel Cell? • An electrochemical cell that converts chemical energy to electrical energy without being consumed • Most often used for stationary purposes (especially SOFCs) • Can run on either H₂, more abundant natural gas (CH₄), or virtually any gaseous fuel • Many types: SOFC, PEMFC, DMFC, AFC, PAFC, MCFC, RFC, ZAFC, MFC
- 3. SOFC-Solid Oxide Fuel Cell • Use solid electrolytes to transport ions, either proton-conducting or oxygen ion- conducting • Perovskites as electrode components, sometimes electrolyte • Usually use either hydrogen gas or natural gas as fuel • Current Operating Temperature- 700- 1000°C
- 4. Perovskites-The Fundamentals • Often found in electrodes • Modeled after the structure of CaTiO₃ • ABO₃ general formula • A= large cation, B= smaller cation, A-site cation is structural, B-site is electronic • Commonly generated with two “B” cations; many deviations and substitutions from the general structure • Dopants are often added • Structures: cubic, orthorhombic, tetragonal, trigonal • Virtually endless mole ratios within each compound, so near-infinite number of perovskites possible • Ionic conductivity comes from oxygen vacancies, formed by reduction of valences (oxidation states) and dopants/substitutions
- 5. • Configuration: cubic structure with frequent, disordered oxygen vacancies (Goldshmidt tolerance factor of 0.9 to 1) • Characteristics: MIEC (electrodes), ionic conductor (electrolyte) extremely porous and/or thin (if not ionically conducting), structurally/chemically stable and functional throughout a wide range of oxygen partial pressures, temperatures, and conditions, nonreactive with other components, no destructive phase transitions between room and operating temperatures, resistant to CO₂, CO, or S poisoning and solid carbon formation • Dopants/Substitutions: Minimal • Operating Temperature: ~500°Celsius • Irony: The best materials for use as components are somehow unusable. The Optimal Perovskite
- 6. Double and Triple Phase Boundaries Courtesy of Professor Chueh & Sossina M. Haile • Triple-phase boundary obviously more complicated, want to avoid it. • 2PB has entire boundary to react along • 3PB as greater interfacial resistance (non-material related resistance), material needs to be porous to allow gas flow • Anode always has a 3PB, cathode can have either a 2PB or a 3PB
- 7. MIEC-Mixed Ionic & Electronic Conductors • Found in the cathode • Very unique in that they are not insulators as most ceramics are, but still share many of the same properties • More effective at conducting both - simpler interface with electrolyte and higher efficiency • While ionic conduction is not specific to elements, electronic conduction is intrinsic to certain substances
- 8. Cathode-LSM/LSCF • MIECs • LSM is more expensive (manganese), LSCF has unstable surface properties, otherwise very similar • Solution: LSM nanoparticles/film on LSCF, high surface area of LSM • LSCF-Strontium adds to the structural stability of the crystal and creates oxygen vacancies due to lower oxidation state than iron. Cobalt is easily reducible and thus an excellent electronic conductor. Iron is less so, but is abundant, and adding cobalt increases the TEC greatly due to said reducibility.
- 9. Electrolyte-YSZ • Optimal Dopant Concentration: 8 mol %, lower or a lot higher does not ensure stable cubic phase, can cause stresses on the system • Pure Ionic Conductor (Insulator) • Y₂O₃ introduces oxygen vacancies to ZrO₂ and stabilizes the structure • Ceramic, but not perovskite
- 10. Anode- Ni/YSZ Cermet • Ni = electronically conducting element, YSZ = ionically conducting element • YSZ helps to match TEC of other components
- 11. Machines I Worked With • PLD (preparation of substrates and guided use) • ICP-MS (observed preparation of standards and samples) • Profilometer • XRD (observed + data analysis w/Bragg’s law-lattice spacing) • Optical Microscope • SEM (2) • Javier • Wanda
- 12. My Experiments-LCF Conductivity • Started with oxide nitrate hydrates and added urea • Baked gel twice with intermediate grinding to get optimal cubic- crystal powder • XRD used to analyze samples, calculated lattice spacing to see if it matches previous data • Prepared platinum stripes on YSZ substrate with the PLD (Pt sputtering with capton tape down the middle of the substrate) • Looked at Nyquist Diagram for the impedance measurement • Objective: Assess the substrate’s resistance to subtract it out from overall value when LCF electrodes are measured
- 14. Surface Impurities Before and After Heating
- 15. Residue & Uneven Edges from Capton Tape Before and After Heating
- 16. XRD Powder Analysis First diffraction: not cubic phase. Second diffraction: cubic phase
- 17. Nyquist Diagram • Data was unintelligible noise • Indicates that there is a scratch or some other major obstacle in the measurement which opens the system • Microscope image shown previously verifies this hypothesis and shows that it is in fact scratches from the probe causing this issue
- 18. Problems and Prospective Solutions • Challenges: • The electronic resistance of the YSZ substrates was difficult to measure with Javier-severe scratching of the platinum surface short circuited the impedance test • This is a long-term issue that cannot be solved immediately • Impurities and residue were not burnt off during heating - convolutes data • Recommendations: • Organizationally: creating a more orderly system of reservations for Phoenix, have an organized training schedule to get people acquainted • Mechanically: designing a probe on the impedance device for Javier that causes less harm to the sample (gold foil fitting to contact point), fitting the probe to a spring (like Phoenix) to simplify the process of placing the probe and prevent scratching
- 19. Why Does this Experiment and its Results Matter? • Energy is and will always be a necessity in society • This need grows correspondingly as the population does, and the search for “green” energies intensifies due to climate change • Along with solar/wind, fuel cells can provide a sustainable source of energy that is both extremely reliable and clean • However, to get to this point, we must perfect fuel cell technology first, and this always begins at the small scale, with experiments of different materials just like this one
- 20. Anode Materials Species Advantages Disadvantages Ni/YSZ cermet -low cost -thermal expansion match to most components -lower ionic conductivity (impedes on efficiency of cell, greater overpotential) -carbon deposition problem (excess extremely harmful) -sulfur poisoning -microstructural changes in oxidizing + reducing environments (causes decreased triple-phase boundary area) -many perovskites reactive with Ni at high temperatures (can be positive w/ Ni nanoparticles) Ni/GDC or Ni/SDC cermet -high ionic conductivity (ensures effective triple-phase boundary) -high cost (Sm or Gd) -slight thermal expansion mismatch Cu/YSZ cermet -resistant to carbon buildup -not well-reasearched Titanates, Chromites, YTZ, etc. -homogenous -dual-phase boundary, MIEC -low cost -resistant to sulfur poisoning and other impurities -electrical behavior dominated by defects -naturally electronic/ionic conductors, must be doped -even after doping still not as effective -not studied at low temperatures Honorable mention: apatites
- 21. Cathode Materials Species Advantages Disadvantages SSC -very high ionic conductivity -high cost (Sm) LSCF -possibly high conductivity (dependent on stoichiometry, especially of B-site) -low cost -unstable surface properties at electrolyte interface LSM -thoroughly researched -dependable electronic and ionic conductivity -high cost (Mn)
- 22. Electrolyte Materials Species Advantages Disadvantages LSGM (Gallate-based) -high ionic conductivity at low temperatures -low cost -Ni + Co + Fe doping improves conductivity (LSGMC & LSGMF) -max structural stability -reactive with Nickel (only at temperatures above 1000°C) (can be positive) -has to be given more attention in the U.S. -requires greater electrolyte thickness for proper function (200+ µm in thin film), causes greater overpotential due to ionic resistivity and potential stress on system GDC/SDC (Ceria-based) -very high ionic conductivity at low temperatures -chemically and structurally stable -high cost (for Sm and Gd) -slightly higher thermal expansion coefficient (may require modifications if used in bulk)- 13.5x10^-6 K^-1 [not major issue] -at times can require thick electrolyte YSZ/ScSZ (Zirconia-based) -low cost and abundant -decent chemical and structural stability throughout varying temperatures and oxygen partial pressures -ScSZ has high initial conductivity -medium conductivity at low temperatures -YSZ reactive at dual-phase boundary, requires SDC buffer layer (phase segregation) -high cost and geopolitical issues (Sc) -Sc performance degradation over time (metastable)
- 23. Advantages of the Fuel Cell • Uses electrochemical reactions to convert chemical energy directly to electricity without degrading internal components • Exothermic Reaction- “thermally self-sustaining”- Prof. Chueh • Perovskite structure and other functional structures can be found in phase transformations of non-perovskite substances, such as apatites, brownmillerites, and LAMOX • Put it in reverse and you can create H₂ and O₂ from water, producing fuel and making fuel cells suitable mediums for energy storage • GE has now entered the SOFC industry again - possible turning point in SOFC technology and commercialization • The solid electrolyte enables minimal gas crossover, is less reactive with other components, and is less corrosive and damaging to the cell • Infrastructure for natural gas already existent 2𝐻2 𝑂 ↔ 2𝐻2 + 𝑂2
- 24. Disadvantages of the Fuel Cell • Stacks add immense complications and energy loss to the system • We have no idea how these thin films will translate into functional fuel cells 1000s of times their current size • SOFCs are currently not portable due to their high operating temperature (even 500°C) →
- 25. “Hydrogen Economy Dream” Concept courtesy of Fuel Cell Fundamentals by Ryan O’Hayre
- 26. Thanks to… Dr. David MuellerProfessor William Chueh AND THE REST OF THE CHUEH GROUP!
Notas del editor
- Today I will present on a field I knew nothing about 7 weeks ago, and hopefully you all will agree by the end of this presentation that I made great progress. That being said, my presentation will be more of a holistic, informative review of Solid Oxide Fuel Cells than my personal experiments. As I have taken great interest in the materials aspect of SOFCs, that will make up the majority of my presentation.
- combustion engine extremely complicated- I don't know how it works and I frankly don't want to Too many moving parts = bound for failure Not portable- high operating temperatures, dangerous fuel, poor energy density, and bad scalability for high temp. cells PEMFC=proton exchange membrane DMFC=direct methanol AFC=alkaline
- “CH₄ often used, but usually changed into CO and H₂ through steam reforming before its reaction”-point 3
- Modeled after the structure of CaTiO₃ ABO₃ general formula A=large cation (such as actinide), B=smaller cation (transition metal), A-site cation is structural, B-site is electronic Commonly generated with two “B” cations; many deviations and substitutions from the general structure Various dopants used to stabilize structures, increase ionic conductivity thorugh oxygen vacancies, and/or prevent reactivity with other components (transitional metal oxides and rare earth metal oxides)-often common components of perovskites themselves, often aliovalent to create oxygen vacancies (cubic, orthorhombic, tetragonal, trigonal) Virtually endless mole ratios within each compound, so near-infinite number of perovskites possible [one of he things I find most interesting about perovskites. Not only most elements can be substituted, but this can be done in infinite proportions too, fuel cell industry can never die] Ionic conductivity comes from oxygen vacancies, formed by reduction of valences (oxidation states) and dopants/substitutions, as the charges must always balance, either a metal must oxidize or oxygen vacancy must occur (manufacturing crystallographic defects to use in our advantage) [most interesting thing from HS Chemistry student perspective, creating defects and using them to transport ions]
- Configuration: cubic structure with frequent, disordered oxygen vacancies (but not enough to decrease structural or chemical stability)-allows varied, “3d”, oxygen ion movement rather than restricted to one-directional or linear [𝑡=𝑅_(𝐴−𝑂)/√2(𝑅_(𝐵−𝑂))] [Goldshmidt tolerance factor of 0.9 to 1 = cubic] Characteristics: MIEC (for electrodes), ionic conductor (for electrolyte) extremely porous and/or thin if not ionically conducting, structurally/chemically stable(in respect to interface as well) and functional throughout a wide range of oxygen partial pressures, temperatures, and conditions (reducing, oxidizing), nonreactive with other components, no destructive phase transitions between room and operating temperatures, resistant to CO₂, CO, or S poisoning and solid carbon formation + deposition (carbon and carbonates detrimental to cell efficiency and lifespan) Dopants/Substitutions: Minimal (generally tend to have some major disadvantage-thermal expansion coefficient, conductivity, stability, etc.) Operating Temperature: ~500°Celsius (reactions slower and catalysts less effective, but less cross-reaction, portable, long lifespan) Irony: The best materials for use as catalysts and conductors, even perovksites and dopants, are often the rarest, most expensive, toxic ones out there, or are somehow unusable. (H₂-fuel, Platinum-electrocatalyst, Gadolinium, Samarium, Scandium-dopants)
- Triple-phase boundary obviously more complicated, want to avoid it. 2PB has entire boundary to react along 3PB as greater interfacial resistance (non-material related resistance), material needs to be porous to allow gas flow Anode always has a 3PB, cathode can have either a 2PB or a 3PB
- Found in the cathode Very unique in that they are not insulators as most ceramics are, but still share many of their properties More effective at conducting both=less complicated interface with electrolyte and higher efficiency While ionic conduction is not specific to elements (dependent on reducibility instead) electronic conduction is intrinsic to certain elements (Co, Fe used in MIECs as electronic conductors, Co and Sr reduce to create oxygen vacancies)
- MIECs LSM more expensive (manganese), LSCF has unstable surface properties, otherwise very similar Solution: LSM nanoparticles/film on LSCF, high surface area of LSM, low amount=low cost LSCF-why so many TMs? Strontium adds to the structural stability of the crystal and creates oxygen vacancies due to lower oxidation state than Iron. Cobalt is easily reducible and thus an excellent electronic conductor, Iron is less so, but is abundant and cobalt creates high TEC because of reducibility (changes in ionic radius immense).
- Optimal Dopant Concentration: 8 mol %, lower or higher = metastable cubic phase, causes stress in system Pure Ionic Conductor (Insulator) Y₂O₃ introduces oxygen vacancies to ZrO₂ and stabilizes the structure Ceramic, but not perovskite
- Heterogeneous distribution
- May not have gotten done a lot of experimental work per say, but I did get a great deal of valuable lab work/experience and exposure to important technologies of the Materials Science field SEM-”(2-good one and free one)”
- Started with oxide-hydrates, added urea (used stoichiometry for quantities) Baked gel twice with intermediate grinding to get optimal cubic-crystal powder (didn’t work first time) XRD used to analyze samples, calculated lattice spacing to see if it matches previous data Prepared platinum stripes on YSZ substrate (negligible resistance) with the PLD (Pt sputtering with capton tape down the middle of substrate) Looked at Nyquist Diagram for impedance Objective: assess substrate conductivity/resistance to subtract it out from overall value when LCF electrodes measured
- While we dodged a bullet on that one, the next bullet hit us right in the head
- heating did not help nor exaggerate
- as one can imagine, removing tape is not a perfect process and leaves both residue and edges that are uneven expected at least capton residue to burn away heating spread residue throughout
- Small peaks = impurities, noise large peaks = perovskite After 2nd one, data matched previous examples well, so we know we got cubic
- Data was unintelligible noise Indicates that there is a scratch or some other major obstacle in the measurement which opens the system Microscope image shown previously verifies this hypothesis and shows that it is in fact scratches from the probe causing this issue
- don't know what would go into fitting the spring, possibly too difficult gold foil softens contact pt + makes scratching minimal
- Energy is and will always be a necessity in society This need grows correspondingly as the populatiom does, and the search for “green” energies intensifies due to climate change Along with solar/wind, fuel cells can provide a sustainable source of energy that is both extremely reliable and clean However, to get to this point, we must perfect fuel cell technology first, and this always begins at the small scale, with experiments of different materials just like this one This all may same obvious, but it is always good to remember what we’re working for
- yttrium-titanium-zirconium, titanium introduces electronic conductivity without much effect on ionic apatites ionically conduct through intersitial ions rather than oxygen vacancies
- After going over advantages/disadvantages of individual components, let’s move on to the fuel cell in general… Uses electrochemical reactions to convert chemical energy directly to electricity (heat first in Combustion Engine), without degrading internal components [open system] (anode in Galvanic Cell)-electrodes are catalysts/mediums rather than consumables in the reaction, fuel is fed to system Exothermic Reaction- “thermally self-sustaining”- Prof. Chueh (in mobile fuel cell, heat dissipates, must be large to be self sustaining) Perovskite structure can be found in phase transformations of other substances, such as apatites, brownmillerites, and LAMOX Put it in reverse and you can create H₂ and O₂ from water, producing fuel and making fuel cells suitable mediums for energy storage GE has now entered the SOFC industry again - possible turning point in SOFC technology and commerialization Solid Electrolyte = minimal gas crossover, less reactive with other components, less corrosive and damaging to the cell (hence why we want to reduce temperature, high temp solids also reactive) Infrastructure for natural gas transport already existent (so if we get the cels going, people can just install and get started)
- Stacks add immense complications and energy loss to the system (interconnects) No idea how these thin films will translate into functional fuel cells 1000s of times current size SOFCs are currently not portable due to high operating temperature (even 500°C) Althoug people think bloom energy is successful, it’s not: lifespan of their cells are far less than anticipated and advertised
- This makes our lab unique, diff projects work in conjunction-water electrolysis = H2 = perfect fuel for fuel cells- at SU we have capability to create this cycle ourselves actually (solar, electrolysis, fuel cells)-shouldn't think of them as seperate or one without another, must be thought of together, SOFCs not suitable for mobile purposes (at least for now), hence the need for batteries it is impossible to stress enough that we are truly working on the future here, and our work can go a longer way than we can probably comprehend-world impact, amazingly large scope, can literally change world's energy production into this model; it's so much bigger than just us-I'm sure Professor Chueh would be ecstatic if a group at another university found the future of technology rather than us because it is not a competition, but then again we would all prefer this to happen in our group and that's why we have brought together a group of such great minds here at Stanford University