Solid Oxide Fuel Cells: The Future Of

Energy

By Farbod Moghadam

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

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

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

• 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

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

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

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.

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

Anode- Ni/YSZ Cermet

• Ni = electronically conducting element, YSZ = ionically conducting element• YSZ helps to match TEC of other components

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

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

Substrate Defects/Damage

Surface Impurities Before and After Heating

Residue & Uneven Edges from Capton Tape Before and After Heating

XRD Powder Analysis

First diffraction: not cubic phase.

Second diffraction: cubic phase

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

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

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

Anode MaterialsSpecies Advantages DisadvantagesNi/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-reasearchedTitanates, 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

Cathode Materials

Species Advantages DisadvantagesSSC -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)

Electrolyte Materials Species Advantages DisadvantagesLSGM (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)

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

2

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)

→

“Hydrogen Economy Dream”

Concept courtesy of Fuel Cell Fundamentals by Ryan O’Hayre

Thanks to…

Dr. David Mueller

Professor William Chueh

AND THE REST OF THE CHUEH GROUP!