Solid Oxide Fuel Cells

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Solid Oxide Fuel Cells Rodger McKain, PhD
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• Ion transport observed by William Grove in
  1839Based on hydrogen-oxygen, sulfuric acid
  electrolyte, and platinum electrodes
• I cannot but regard the experiment
• as an important one
• William Grove to Michael Faraday
• October 22, 1842

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Fuel Cell

• An energy conversion device that directly
  converts chemical energy into electrical energy
  (dc power).
• Analogous operation to a natural gas fueled
  electric generator energy in fuel and oxygen
  are converted to electric power as long as fuel
  and air are supplied.
• Six types, each suited for specific applications

Heat, H2O
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Fuel Cell Types
Increasing Temperature
Source U.S. Fuel Cell Council
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Attributes of Fuel Cells
AFC PACF PEM
MCFC SOFC Electrolyte KOH
Phosphoric Sulfonic Molten Y2O3-ZrO2
Acid Acid Carbonate Ceramic
Polymer Salt Temperature 100
0C 2000C 1000C
6500C 800-10000C Fuel H2 H2
H2 H2/CO H2/CO Efficiency (H2
fuel) 60 55 60 55
55 (NG fuel) --
40 35 50
50 Pollution Very low Very low Very low
Low Low Hydrocarbon No
Difficult Difficult
Yes Yes Fuel Use Start-Up Fast
Moderate Fast Slow
Slow
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Fuel Cell Stacks

• Operating voltage of a single cell is 0.7 volts
• Cells are stacked in series to increase voltage
  to useful levels

Source U.S. Fuel Cell Council
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Fuel Cell Power System
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High Efficiency
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High Efficiency at Part Load
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Low Emissions
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The Fuel Cell Opportunity

• High efficiency Energy
  Independence
• Low regulated emissions
• Quiet
• Fuel flexibility
• High quality power
• High reliability Energy
  Security
• Widespread applications (transportation, power,
  medical, communications, military, aerospace,
  electronics)
• IF lt400/kW
  stationary power
• lt35/kW
  automotive
• New industry (250 billion per year)

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Solid Oxide Fuel Cells

• Based upon ion conductivity of certain ceramic
  materials at elevated temperatures (gt600 C)
• First observed by Nernst in 1890s
• Fluorite Structures (e.g. yttria stabilized
  zirconia)
• Face Centered cubic arrangement
• Transport through crystal lattice vacancies and
  oxide ions located between crystal faces
• First SOFC constructed in 1937 by Baur and Preis
• Requires porous electrodes and dense electrolyte,
  low electronic conductivity, and high strength

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v
RL
A
Anode catalyst layer
CH4 3O2- CO2 H2O 2e-
O2 4e- 2O2-
Pt Ink
O2-
Effluent
Pt Wire
Fuel/CH4
Cathode catalyst layer
CH4 CO2 2CO 2H2 CH4 H2O CO2 3H2 CO
H2O CO2 H2 CH4 0.5 O2 CO 2H2
Electrolyte Disc
Yttrium-stablized Zirconia (gt950
C) Galladium-doped Ceria (gt600C)
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Relationship between fuel processing and fuel
cells
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Basis for Fuel Cell Operation

• Electron transfer chemical reaction
• Voltage determined by difference in chemical
  potential of fuel and oxygen
• Current determined by area of cell
• Catalyzed conversion of oxygen and hydrogen into
  reactive species O and H
• H2 O2 H2O 2 electrons heat
• Electrons are separated from reactants by circuit
• Need to understand electrical circuit background
  as it relates to fuel cell

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Electric terms
6,240,000,000,000,000,000 electrons / sec 1 amp
Volts
Copper wire, 1/16 diameter, 10 amps, electrons
travel 1 cm In 28 seconds.
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Whats a watt?
Work involves height lifted and weight of ball,
ft-lbs
Power (height lifted times weight of ball)
times (balls per second), or Power voltage
times current, Watts volts times amps
Work has no time limit, power does
550 ft-lbs/sec 1 horsepower 746 watts
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Energy flow
Same story for electric system Food ? anode, Air
? cathode Stack produces power and heat
In a perfect system all the energy in the food
would be converted to power. Actually, only part
is converted which defines the efficiency.
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V-I scan
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V-I scan
10
8
6
1 of these 2 of these!
Height lifted or volts (V)
4
2
0
0
5
10
15
20
25
Balls lifted per hour, or amps (I)
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Micro view - Electric
A nonmetallic electric conductor in which
current is carried by the movement of ions.
Fuel utilization Air Stoics
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Complete micro view
Icon
Fuel Flow, H2OCO ? H2CO2
Via
-
H2
Anode
H2O
CO
Bond Layer
e -
Fuel layer 1
Electrolyte
CO2
CO
CO2
CO2
CO
CO
e -
e -
Cathode
CO2
CO
CO2
CO2
CO
CO
O
O
O
O
O
O H2 2e- H2O
Porous
N2
N2
N2
N2
N2
N2
½ O2 2e- O
N2
N2
N2
N2
N2
N2
e -
e -
e -
N2
N2
N2
N2
N2
Air layer 1
N2
N2
Bond Layer
e -
O2
O2

Air Flow, O2 N2
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Co-flow Design Concept Unit Cell
Multi-layer ceramic construction
Vias carry current
Cell
Fuel flow
Air flow
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Interconnect
Ink bumps printed on vias
Sealant
Thermocouples, Voltage taps
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Add a cell
Thermocouples, Voltage taps
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Manifold arrangement
Air inlets
Fuel inlets
Gasket
Manifold
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Vehicle ICE vs. Fuel Cell Direct Drive Efficiency
Comparison
40
100 Energy Units
IC Engine 40
Power Train 37.5
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60
20 Idling
5 Friction
20
40 Energy Units
Fuel Cell 50
Direct Drive 75
15
20
0 Idling
5 Friction
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Summary

• Fuel Cells have been around a long time
• They present the potential to be highly efficient
  because of direct conversion of chemical energy
  to electrical energy
• Solid oxide fuel cells are based upon ion
  conducting properties of ceramic materials like
  doped zirconia
• Temperatures above 600 C are required for
  operation
• To be viable fuel cells must have high power per
  area, and operate with low cost materials