1. Nanomaterials for Renewable Energy: Solid Oxide Fuel Cells
Group 11:
Youmean Lee
Ethan Licon
Sokkwan Leong
William Liang
Figure 1: Solid Oxide Fuel Cells

2. Outline Purpose Introduction Background Information Applications
Comparison Between Other Fuel Cells
Advantages
Disadvantages
Cost & Lifetime Targeting
Structure
Planar
Tubular
Experimental Structures
Electrolyte-Supported vs. Electrode- Supported
Advances Intermediate- and Low- Temperature SOFCs
Operations
Materials
Anode
Cathode
Polarization
Fuels
Hydrocarbons
Renewable
Environmental Impact
Summary
References

3. Purpose
The objective of this presentation is to discuss Solid Oxide Fuel Cells, including
Advantages
Disadvantages
The Structure of Solid Oxide Fuel Cells
How an Solid Oxide Fuel Cells Works
Materials used in Solid Oxide Fuel Cells
We will discuss the challenges and problems with current Solid Oxide Fuel Cell production. As well as the long term benefits and the importance of material science in the development of Solid Oxide Fuel Cells.
Solid Oxide Fuel Cells provide a promising lead for green energy. If developed fully, it can bring major benefits to societies energy future.
Figure 2: Cross section of SOFC

4. Introduction
Fuel Cell: A device that oxidizes fuel to produce a continuous electric current. A fuel cell consists of an anode, a cathode, and an electrolyte.
Solid Oxide Fuel Cell (SOFC): A fuel cell that uses a solid oxide or ceramic electrolyte that conducts oxygen ions. To conduct these ions, a SOFC operates at very high temperatures ( oC).
Figure 3: A stack of fuel cells

5. Introduction: What Makes SOFC Renewable?
While the incoming fuel is still a hydrocarbon, the efficiency is about 45-60%
The overall reaction also involves hydrogen and oxygen such that the main waste component is water.
Hydrogen can be produced from renewable fuels and electrolysis while oxygen is readily available from the atmosphere.
Figure 4: Animated representation of SOFC operation

6. Background Information and Applications
The concept of the first fuel cell was derived in the early 19th century by Cornish chemist, Sir Humphry Davy.
By the end of the 19th century German physicist, Walther Nernst, discovered that Y2O3 and ZrO2 could be added to other solid oxides to reduce their electrical resistivity.
In the late 1950s, the development period for the SOFC began.
Today, some applications for the SOFC include:
Stationary power generation
Mobile\transportation power
Auxiliary power (power not used for propulsion)
Figure 5A: Microscopic structure of a SOFC
Figure 5B: Power generation from SOFC

7. Applications
Used in large and small stationary power generations for example:
Planar: SOFCs with output of few kW are tested for smaller cogeneration applications, such as domestic combined heat and power
The only fuel cell that has the potential for a wide range of applications:
Considered for portable devices (500W battery chargers)
Distributed generation power plants ( kW systems)
Integrated with gas turbine to form large pressurized hybrid systems
Figure 6: Portable SOFC system

8. Comparison Between Other Fuel Cells
Compared to other fuel cells, the SOFC operates at much higher temperatures, uses short hydrocarbons as fuel, and operates in a relatively high efficiency range.
Types of fuel cell
Electrolyte
Operating T
Fuel
Oxidant
Efficiency
Alkaline (AFC)
potassium hydroxide (KOH)
50-200oC
pure hydrogen, or hydrazine
O2/Air
50-55%
Direct methanol (DMFC)
polymer
60-200oC
liquid methanol
40-55%
Phosphoric acid (PAFC)
phosphoric acid oC
hydrogen from hydrocarbons and alcohol
40-50%
Sulfuric acid (SAFC)
sulfuric acid
80-90oC
alcohol or impure hydrogen
Proton-exchange membrane (PEMFC)
polymer, proton exchange membrane
50-80oC
less pure hydrogen from pure or methanol
Molten carbonate (MCFC)
molten salt such as nitrate, sulphate, carbonates... oC
hydrogen , carbon monoxide, natural gas, propane, marine diesel
CO2/O2/Air
50-60%
Solid Oxide (SOFC)
ceramic as stabilized zirconia and doped perovskite oC
natural gas or propane
45-60%
Protonic ceramic (PCFC)
Thin membrane of barium cerium oxide oC
hydrocarbons
Table 1: Comparison of different fuel cell types

9. Advantages High fuel flexibility; allows internal reforming
High efficiency due to high thermal process (45-60% conversion efficiency; >90% in cogeneration)
Low emission as carbon monoxide is converted to carbon dioxide
Elimination of electrode loss maintenance and corrosion due to solidity of components in the cell.
Tolerance of impurities
Long-term stability
Longer life span (40,000-80,000 hr)
Relatively low cost
Electrolyte is ductile; can be casted into various shapes
No noise pollution due to modular and solid state construction
Table 2: Advantages of SOFC plants
Compared to other types of energy production, SOFC has much higher efficiency and is cleaner.
The DEMOSOFC is a current project to create a SOFC plant in Turin, Italy.

10. Disadvantages
The high operating temperature of the SOFC makes the cells take longer to start up
SOFC can be expensive because the cell must be constructed of robust, heat-resistant materials, and must be shielded to prevent heat loss.
Thermal expansion mismatches between materials causes difficulty in sealing, especially in planar configurations
Sealing is done to prevent mixing of the fuel and oxidant
This sealing is often design specific.
Figure 7: Possible seal configurations
v/File%20Library/Rese arch/Coal/energy%20s ystems/fuel%20cells/p roceedings/OverviewSt evenson.pdf

11. Cost & Lifetime Targeting
The majority of the cost can be explained by the complex manufacturing of the fuel cell’s electrode/electrolytic plates as well as the lower power density (W/cm2) and is therefore dependent on the stack size and area of the SOFC.
The US Department of Energy recommends a factory cost of about $40/kW and has guidelines for the lifetime of SOFC:
Table 3: Lifetime comparison of fuel cells
Stationary Fuel Cells
Transportation Fuel Cells
Lifetime
40,000 hours (~ 4.5 yrs)
5,000 hours (~ 6 months)

12. Structure
SOFC consists of a Fuel Cell Unit and a Hot Storage Tank/Supplementary Boiler Unit:
Fuel Cell Unit: Generates electricity and recovers the exhaust heat as hot water simultaneously
Hot Storage Tank/Supplementary Boiler Unit: Stores the recovered hot water in order to reuse the hot water
Figure 8: Map of a SOFC system

13. Structure Table 5. Fuel Cell Unit
Reformer
Converts city gas, fuel, into hydrogen (H) and carbon monoxide (CO)
Cell Stack
Generates power using oxygen in the air and city gas (reformed to H and CO)
Inverter
Converts power generated in the cell stack into alternating current and supplies it to homes
Exhaust Gas Heat Exchanger
Recovers the heat generated during power generation as hot water
Table 6: Hot Storage Tank/Supplementary Boiler Unit
Hot Water Storage Tank
Tank for storing hot water recovered by the exhaust gas heat exchanger
Supplementary Boiler
When there is not enough hot water in the Hot Water Storage Tank, the Supplementary Boiler is used to supply hot water
Figure 9: Map of a SOFC system

14. Structure A single cell consists of four layers stacked together:
Interconnect: electrically connects the anode of one cell to the cathode of another
Electrolyte: dense material that provides pure ionic conductivity and physically keep separated fuel and oxidant
Cathode: air electrode; positive electrode associated with reduction of the oxidant that gains electrons from the external circuit
Anode: fuel electrode; negative electrode associated with fuel oxidation and release of electrons into the external circuit
Figure 10: Cross section of layers of a tubular SOFC

15. Traditional Structure: Planar
Two types of SOFCs in terms of cell structure: planar and tubular
Planar
Cell is made into a flat disk, square, or rectangular plane
Cells placed in series and connected by interconnected plates
Fuel is fed through the anode and interconnect
Air through the Cathode and interconnect
Stack of anode, interconnect, cathode, and electrolyte stack are repeated
Figure 11: Unit Stack of a Planar SOFC

16. Traditional Structure: Tubular
In the tubular structure, the electrode is made into a long tube with porous walls
Outside the electrode tube, the electrolyte and another electrode are wrapped around
Cells connected in series through interconnect placed along the edge of the tube
Advantages
Rapid start-up and cool-down times
Good for smaller applications
Figure 12: Structure of a tubular SOFC

17. Traditional Structures: Comparison of Planar and Tubular
Higher in power area and volume
Easier to manufacture
More expensive
Tubular
Lower in power area and volume
Harder to manufacture
Cheaper
Better long term stability
Better sealing
Table 7: Comparison of planar and tubular SOFC structures

18. Experimental Structures
Modified Planar
Features a wave-like structure in a flat configuration
The cells are said to be in “flat-configuration” when each solid oxide fuel cell is stacked on top of each other
Shares the benefits of both tubular and planar cells
Increased surface area allows rapid cooling and rapid starting
The flat configuration allows for lower manufacturing costs
Figure 13: Diagram of a Modified Planar Cell

19. Experimental Structures
Monolithic
Design laminates the air electrode, electrolyte, fuel, and interconnect
Uses co-sintering to bring the stack together.
Sintering is compacting and forming a solid mass using heat and pressure
Avoid making the solids into a liquid
Eliminates the need for high temperature seals in planar configurations
Figure 14A: 3D View Monolithic Structure
Figure 14B: 2D View of Monolithic Structure

20. Experimental Structures
Modified Tubular Shapes
The biggest challenge faced by SOFC is the lengthy start up and cool down times
Modified tubular designs increase the surface area of the tubular base design. This increases heat transfer and allows for a quicker start up and cool down
Quicker start up and cool down allow for more applications of SOFC
A disadvantage is that the modified tubular shapes may be more difficult to manufacuter
Figure 15: Modified Tubular Shapes

21. Structure: Electrolyte-Supported vs. Electrode-Supported
The SOFC can be supported by either using the electrolyte or the electrodes (anode or cathode)
The supporting electrolyte or electrodes (anode or cathode) will be thicker than the other layers
This can be seen in Figure 16
The left SOFC is supported by the electrolyte
The right SOFC is supported by the anode
Electrolyte-supported cells are harder to produce
Electrode-supported cells are now commonly used throughout industry
Electrode-supported are typically thinner and have less internal resistance
Electrode-supported can operate at slightly lower temperatures
Figure 16: Electrolyte-supported SOFC (Left) compared with an Electrode-supported SOFC (Right)

22. Structure: Advanced Intermediate- and Low-Temperature SOFCs
Current challenge with SOFC is lowering the operating temperature
Lowering the operating temperature will decrease the cost of materials and operation
The triple-phase boundary (where the electrodes and electrolyte come together) must be in electrical contact and exposed to the reactant
There are two solutions to help achieve a low-temperature SOFC
Ideal would be a porous substance with fine particle size for a high surface area (as seen in the figure)
Reducing the thickness would also reduce the resistive loss
Currently the thickness of the electrolyte is several hundred micrometers
Target is to decrease the thickness to ten micrometers
Figure 17: Comparison of large particle size (a) and small particle size (b) for the electrolyte

23. Operation
Hydrogen containing fuel is fed into the anode of the fuel cell
Example: Natural Gas
Oxygen is fed into the cathode
Step 1
On the Anode side, hydrogen containing fuel is combined with oxygen to produce carbon monoxide and hydrogen gas (CO and H2) and 2 e-
This decreases the concentration of oxygen on the anode side, creating a demand across the electrolyte
Figure 18: Operation of SOFC
Figure 19: Operations of SOFC
H2 + O2-
H2O + 2e-

24. Figure 20: The O2- conversion within SOFC
Operation
Step 2
The free electrons travel around from the anode to the cathode to produce a current
Oxygen is being used up on the Anode side, leading to a decreased concentration
The decreased concentration pulls oxygen from the oxygen-rich cathode side through the electrolyte
The oxygen-rich cathode side combines with the O2 to create O2(2-)
The solid electrolyte is an oxygen ion conductor
The electrolyte will only let oxygen ions through, thus the oxygen has to be ionized prior to traveling to the anode to react
O2 + 4e-
2O2-
Figure 21: Ionization of oxygen

25. 4e- + O2 2O2- H2 + O2- H2O + 2e- Operation Step 3: Anode Side
The O2- Ion combines with freed H2 to produce water and 2e- on the anode (reaction below)
2 electrons are freed by the reactions
The electrons will travel to the cathode side
This flow of electrons produces a current
Electrons flowing onto the cathode side recombine with oxygen to form oxygen ion
The electrolyte will conduct oxygen ions only
Figure 22: Diagram of SOFC Operation
4e- + O2
2O2-
H2 + O2-
H2O + 2e-

26. CO + O2- CO2 + 2e- Operation Step 4
On the anode side, CO is created through the incomplete combustion of CH4
This excess CO combines with O2- to make Carbon Dioxide and electrons
Electrons travel around the wire to the cathode, producing a current
Electrons deposited on the cathode combine with Oxygen to create oxygen ion
Oxygen ion travels through the solid electrolyte to react more
Figure 23: The CO + O2- converstion to CO2
CO + O2-
CO2 + 2e-

27. CH4+ 2O22- 2H2O + CO2 + 8e- Operation Step 5 - Anode
Uncombusted CH4 is sometimes left in the anode
This uncombusted CH4 reacts with oxygen ion (O2-) to create Water, Carbon Dioxide, and Electrons
The free electrons travel around to electron-deficient cathode side
This flow of electron create a current
Electrons entering the cathode recombine with Oxygen to form Oxygen ion
Figure 24: Overall conversion within the cell
CH4+ 2O22-
2H2O + CO2 + 8e-

28. Materials Requirements:
Chemically and physically stable for oxidizing and/or reducing reactions
Very high operating temperatures: oC
Chemically compatible with other components; absence of interface reaction/diffusion
Proper conductivity
Similar thermal expansion coefficients to other components
Dimensional stability in the presence of chemical gradients
Strong, yet easy to fabricate
Figure 25: Simple scheme of a SOFC, with the state-of-the-art materials used as anode, cathode and electrolyte

29. Materials: Anode
Ceramic layer that must be porous for fuel flow towards the electrolyte
Often Granular matter
Commonly the thickest and strongest layer
Must conduct oxygen ions, but not oxygen
Cermet made up of nickel mixed with the ceramic used for the electrolyte (YSZ, ScSZ, GDC) is most commonly used
The performance of the anode, in terms of minimal electrode polarization loss and minimal degradation during operation, depends strongly on its microstructure
Conducting oxygen ions allows the flow of the ionized oxygen but not oxygen
This is important because if diatomic oxygen is conducted, no electrons would need to flow to ionize the oxygen
Figure 26: SEM of a Ceramic Anode

30. Materials: Electrolyte
Dense ceramic layer that conducts oxygen ions
Electron conductivity must be kept as low as possible to prevent losses from leakage currents
High operating temperatures of SOFCs allow the kinetics of oxygen ion transport to be sufficient for good performance. As the operating temperature approaches the lower temperature, the electrolyte begins to have large ionic transport resistances and affect the performance
Common Examples:
Yttria-stabilized zirconia (YSZ),
Scandia stabilized zirconia (ScSZ)
Gadolinium doped ceria (GDC); highly conductive between oC
The ability of electrolyte to hold a large content of oxygen vacancies makes them a good candidate of electric conductor
Figure 27A: Yttria-stabilized zirconia (YSZ) 3h
Figure 27B: Peroxide structure
characterisation-properties-and-applications/designing-perovskite-oxides -for-solid-oxide-fuel-cells

31. Materials: Cathode
Only noble metals or electronic conducting oxide due to the high operating temperature of the SOFC.
Noble metals are extremely expensive and using them would be unsustainable
The material depends on the application the fuel cell will be used in
Common examples
LASrMnO3 (Lanthanum strontium manganite, LSM)
LaCaMnO3 (Lanthanum calcium manganite, LCM)
Both are excellent options for operating temperatures above 800oC
For lower temperatures ( oC)
For increased performance, electrolyte materials (YSZ, SDC) can be mixed with perovskite electrode materials (LSM, LCM).
The electrolyte material increases the amount active sites for electrochemical reactions, allowing good performance at lower temperatures
Figure 28: Microscopic view of Lanthanum Strontium Manganite

32. Materials: Interconnect
Interconnecting plates are used to allow the flow of hydrogen and oxygen to repeating units
Only a few oxides can be used for this application
Initially, doped CoCr2O4 was used as an interconnect
Commonly used now is YCrO3
Glass interconnects with a LaCrO3 coating is also being developed
The major difficulty (and cost) of the interconnect comes with sustaining the high temperatures of the SOFC
Most work is dedicated to bringing down the operating temperature of SOFC. If this can be achieved, material cost across the board will decrease
Figure 29: Cross section showing the layers of a SOFC

33. Anode
Porosity characteristics of anode must be maintained at high temperature to allows the fuel to flow towards the electrolyte
At the anode, hydrogen in the fuel is being electrochemically oxidized.
The protons formed during oxidation also combine with oxygen ions, conducted through the electrolyte from the cathode, to form byproduct water vapor in the anode
Besides porosity, the amount of reaction zones aka triple phase boundary(TPB) also plays an important role in the kinetics of the oxidation reaction
Figure 30: Ni-YSZ example of an anode

34. Anode
The relationship between the conductivity of the nickel/solid electrolyte cermet and nickel content at ambient temperature is shown on the figure:
The threshold for electrical conductivity is about 30 vol% nickel
< 30 vol% nickel, the conductivity of the cermet is similar to that of YSZ.
> 30 vol% nickel, the conductivity is about 3 orders magnitude higher, corresponding to a change in mechanism of the nickel phase
Nickel is also preferred over cobalt and precious metals due to its low cost
Figure 31: Conductivity vs. nickel content b105764m

35. Cathode
Cathode must be highly conductive, stable and porous at high operating temperature
At the cathode, oxygen reduction takes places;
Oxygen combines with electrons from the cell externally, is reduced to oxygen ions through the following electrochemical reaction
Like anode, conduction in porous electrodes occurs through TPB, whereas in MIECs, this also occurs through DPB
Figure 32A: Example of a cathode; Lanthanum Nickel Cobalt Oxide Cathode Powder
Figure 32B: Schematic representation of conducting mechanism of cathode

36. Polarization
Polarization is the loss of voltage output due to imperfections in materials, microstructure, and design in SOFC.
Types of polarization include:
Ohmic polarization - Resistance in materials lead to voltage reduction (V = IR)
Concentration polarization - Occurs from inefficient transport of fuel to reaction sites
Activation polarization - A result of the energy requirement of the chemical reaction at the anode and cathode
To calculate the effect of polarization (Pe) on power output, the following equation is used:
Figure 33: Calculation of Effect of Polarization on Power Output

37. CH4+ O2- CO + H2 + 2e- CH4+ 4O2- CO2 + 2H2O + 8e- Fuels: Hydrocarbon
Requires a partial oxidation reformer to pre-process fuel.
Examples: Gasoline, Diesel, Natural Gas
Benefits of Hydrocarbons
Infrastructure readily available
Naturally stable for easy transport and storage
Complex hydrocarbons will be able to give off more electrons, since they are able to undergo more reactions
Methane is commonly used
Figure 34: Methane (CH4) is commonly used to fuel SOFC
CH4+ O2-
CO + H2 + 2e-
CH4+ 4O2-
CO2 + 2H2O + 8e-

38. Fuels: Renewable H2 can be used as a fuel source Extremely volatile
Costly to manufacture
Very difficult to handle and transport
Complete renewable
Biogas (a combination of methane and carbon dioxide) extracted from farms can be used to power the fuel cell
Sulfur must be extracted from the biogas prior to entering the SOFC
Varying differences in the amount of methane and CO2 is a challenge for directly using biogas as a fuel source
Biogas would have no carbon footprint
All carbon produced would be negated
Figure 35: Using carbon dioxide as a fuel source

39. Environmental Impact
Even though SOFC runs on hydrocarbons, it can still be considered a green technology
The main benefit from SOFC stems from the increased efficiency (45%-60%)
Increased electricity output from the same amount of hydrocarbon
The product gas reactions with O2- allow for the increased efficiency
In combustion engines product gases do not react further
Could be made to run of pure hydrogen
Liquid hydrogen is extremely unstable
Storage and Handling would be difficult in comparison to hydrocarbons
According to the figure below, SOFC is the most efficient source of power generation that is based on non-renewable resources
Figure 36: Comparison of SOFC efficiencies to other non-renewable power generation.

40. Environmental Impact Emissions
If biogas is used (methane and CO2), the net carbon emissions would be zero
A net reduction of 2 million kg of CO2 per year
Near-zero levels of NOx, SOx, and particulates
Only emissions: steam, trace amounts of NOx, trace amounts of SOx, and a small amount of CO2
SOFC provides the lowest emissions of any non-renewable power generation
Air Emissions
SOx
NOx CO
Particles
Organic compounds
CO2
Fossil Fuelled Plant
12,740
18,850
12,797
228
213
1,840,020
SOFC system 32
846,300
Table 8: Environmental Impact of Fossil Fuel Plant vs SOFC System

41. Summary
Solid Oxide Fuel Cells have the potential to provide major benefits as we work towards a more sustainable future.
Its primary advantages are its high efficiency and low emissions. It can run on both hydrocarbons and hydrogen fuel. If powered by Hydrogen or Biogas, it is a carbon neutral energy source.
A SOFC is composed of a cathode, electrolytic plate, an anode, and an interconnection between plates.
Its primary disadvantage involves high operating temperature. This high operating temperature prevents the use of conventional materials. The material must withstand high temperatures and extra measures must be taken to avoid heat loss and mixing of gases.
A SOFC is first heated to oC so that the ceramic electrolytic plate conducts oxygen ions. Oxygen is ionized at the cathode. Oxygen ions diffuse through the electrolytic plate while hydrogen comes in via the anode. Together they oxidize the fuel. The electrons given off in the process then flow through an external circuit to do work. The electrons then enter the cathode and the cycle repeats.
The most important material involved is the particular solid oxide used as the ceramic electrolytic plate, typically a variant of Yttria-stabilized zirconia (YSZ).
Future research could focus on finding ceramic materials that begin to conduct ions at lower temperatures.

42. References