Course Notes in
Fuel Cell Engineering
Spring 2003
A course devoted to all aspects of fuel
cells: types, operation, design, and safety.
(c) 1998-2003 by Eric M. Stuve. “Course Notes in Fuel Cell Engineering” will be distributed in
multiple segments throughout the course. All segments are copyrighted. This material is
provided to registered students of CHEM E 445 and intended for their use only. Registered
students may keep an electronic version and a printed version of this material. All other uses
are prohibited. Whether in part or in its entirety, this material may not be reproduced or
retransmitted by any means, nor stored in electronic form, without the expressed written
consent of the copyright owner. Parts of these course notes appeared earlier as “Fuel Cell
Engineering Chem E 498.”
Some parts of the course notes are intentionally left blank. Students are expected to fill in
these parts during the course.
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
1
Course Web Site:
http://courses.washington.edu/cheme445
Instructors:
Prof. Eric M. Stuve
Dept. of Chem. Engr.
Univ. of Washington
Box 351750
Seattle, WA 98195-1750
Loew 206
stuve@u.washington.edu
Yan Zhu
yanzhu@u.washington.edu
bagley 336A
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
2
•Course Mechanics
- Lecture/homework/exams
- Grading percentages:
Homework (due Wed.)
Exam I (April 30)
Exam II (May 28)
Design Project (June 9)
25
25
30
20
•Texts
- Course Notes in Fuel Cell Engineering
- Fuel Cell Technology Handbook
(Gregory Hoogers, Ed., CRC Press,
2002)
- Perry’s Chemical Engineers’ Handbook,
7th edition, Sections 2, 5, 6, and 2755.
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
3
•Some history…
- Electrocatalysis research at UW
since ‘91
- Ugrad. research in PEM fuel cells
since ‘92
- Fuel Cell Locomotive project
started 9/96
- New courses in fuel cells
Intro. to Fuel Cells (2002)
Solid Oxide Fuel Cells (2003)
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
4
What is a Fuel Cell?
• Device that converts the chemical
energy stored in a fuel directly to
electrical energy.
• Fuel cell is an open system.
–
H2
O2
Fuel
Cell
+
H2O
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
5
• Some similarity to a battery
except that energy must be
stored or built into a battery
• Batteries are closed systems.
–
Batt
ery
+
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
6
Two Principles of the Course
1. Chemoelectricity
- Chemistry must occur before
energy flows
- F/C system like an entire
chemical plant
2. Match Energy Source to
Application
- Stationary / Vehicle / Portable
- Sometimes F/Cs won’t work
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
7
Design and Technology
• Replacement technology
- Replaces an existing product
electric vs. gas light
- New product must cost less
• Enabling technology
- Provides new capability
airplanes ––> flight
- Cost not so important
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
8
• Fuel cells can fall into either
category
- Energy efficiency =>
- Environ. regulations =>
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
9
Fuel Cell Reactions
Let’s look at some possible
reactions (energies in kJ/mol)…
H2 + 1/2 O2 ––> H2O
–∆Ho –∆Go
286 237
CH4 + 2 O2 ––> CO2 + 2 H2O
890
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
818
10
General oxyhydrocarbon reaction:
CxHyOz + (4x + y – 2z)/4 O2 ––>
x CO2 + y/2 H2O
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
11
Select a Fuel
Fuel \ HHV kJ/mol
MJ/kg
MJ/liter*
kJ/mol CO2
H2
286
142
1.73
CH4
890
55.5
0.04 / 24.0
890
CH3OH
638
19.9
15.8
638
C2H5OH
1235
26.8
21.2
618
Glucose
2814
15.6
24.3
469
Gasoline
46.8
34.1
≈ 600
Kerosene
45.9
37.6
≈ 600
Coal, bit.
27
21
< 600
∞
*H2: at 2200 psi; CH4: at STP and as LNG; Glucose: solid
HHV (LHV): Higher (lower) heating value [water as liquid
(vapor)]
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
12
Energy Conversion
• Combustion: the time honored
way
CH4
O2
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
13
Oxidation and reduction at same
place & time
CH4 + 2 O2 ––> CO2 + 2 H2O
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
14
• Direct energy conversion: fuel
cells
CO2
–
CH4
Oxidn
Redn
O2
+
H2O
Half reactions:
oxidn:
redn:
Oxidation and reduction separated
in space
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
15
Cathode? / Anode?
• Definitions
- Cathode: electrode to which
cations migrate
- Anode: electrode to which
anions migrate
• Mnemonic
- Reduction occurs at the
cathode (redcats)
- Oxidation occurs at the anode
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
16
What Makes a Fuel Cell?
• Because oxidation and reduction
are physically separate, two
things must happen to complete
the reaction in a fuel cell:
1. Ions travel through e-lyte:
- acidic f/c:
- alkaline f/c:
2. Electrons travel anode to cathode
Electrons fall through “potential
gradient” and thus do work.
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
17
Cell Potential
• Cell potential U or Uo is the
difference between the anode
potential Ua and the cathode
potential Uc.
• Fuel cells: Cathode (+); Anode
(–) ; U,Uo < 0
• Electrochem. cells: Cathode (–);
Anode (+); U,Uo > 0
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
18
Reversible Cell Potential
Look at H2/O2 fuel cell:
Anode:
H2 ––> 2 H+ + 2 e–
Uoshe*/ V
Cathode:
O2 + 4 H+ + 4 e– ––> 2 H2O
0
(def.)
1.23
Reversible cell potential
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
19
Recall that the reversible cell
potential is defined consistently for
both fuel cells and electrochemical
cells.
*she = standard hydrogen
electrode; hydrogen electrode at
standard conditions (unit activity of
protons in solution and 1 atm of H2)
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
20
Electromotive Force (emf)
• For fuel cells: emf is output
• For e-chem cells: emf is input
• emf always reported as positive
number
Eo = |Uo|
or
E = |U|
• For fuel cell at equilibrium:
Eo = |–1.23 V| = 1.23 V
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
21
Gibbs Free Energy
• Basic thermodynamic relation
between Gibbs free energy and
cell potential (memorize!)
∆Go = nFUo
• ∆Go is always negative for a
spontaneous process
Fuel cells:
Electrochem. cells:
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
∆Go < 0
∆Go > 0
22
Ideal Thermodynamic Efficiency
• Thermal energy: ∆H (enthalpy)
• Elec. energy:
∆G (free energy)
• Thermodynamic efficiency is
measure of electrical output vs.
possible heat output (if fuel were
simply burned)
o
ηt
=
Fuel Cell Engineering Course Notes
∆G o
∆H o
© 1998-2003 Eric M. Stuve
23
Real Energy Conversion
(CO 2 )
(–)
Fuel
Anode
Electrolyte
Air
1. Reactant/product transport
2. Reaction at electrocatalyst
Cathode
(+)
H2O
3. Ion transport through e-lyte
4. Electron transport
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
24
Overpotentials
•
Transport and kinetic limitations lead to
reduced cell voltage
•
Voltage reduction ==> overpotential
Uao
εa
0
Fuel Cell Engineering Course Notes
Uco
Ua
Uc
0.5
© 1998-2003 Eric M. Stuve
εc
1.0
U
RHE
/V
25
• Overpotential: εi = Ui – Uio
Anode
Cathode
More oxidizing More reducing
εa > 0
εc < 0
• If cell were ideal, then
Uo = –1.23
• Real cell operates at lower emf
(
) (
U = U a − Uc = U ao + ε a − Uco + ε c
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
)
26
Polarization Curves
• Most common description of fuel
cell performance
• Measure cell voltage as a function
of current
o
U
U/ V
0
Fuel Cell Engineering Course Notes
0
j / mA cm–2
© 1998-2003 Eric M. Stuve
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Uo
U/ V
0
Fuel Cell Engineering Course Notes
0
j / mA cm–2 = current density
© 1998-2003 Eric M. Stuve
28
• Regions of polarization curve
(2) Kinetic limitations … need to
achieve sufficient
overpotential for reaction
(3) Ion transport limitation …
“ohmic” resistance of
electrolyte (or membrane)
(1) External mass transfer
limitations
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
29
• About ohmic losses
-
Behave Ohm’s Law
(A = electrode area)
-
Straight line on polarization
curve
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
30
Fuel Cell Efficiency
–
Fuel
Air
Fuel
Cell
Load
+
Water
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
31
Put quantities on an area basis
(area of fuel cell)…
.
N i = molar flux of species I
[=] mol cm–2 s–1
.
W e = rate of elec. work (power)
done on F/C
.
[=] W/cm2
Q e = rate of heat supplied to F/C
[=] W/cm2
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
32
• Electrical efficiency
ηe =
elec. work
=
max. elec. work
Fuel Cell Engineering Course Notes
.
We
.
W e,max
© 1998-2003 Eric M. Stuve
=
jU
jU o
33
• Thermodynamic efficiency
.
elec. work
=
ηt =
heat of rxn.
We
.
o
N f ∆Hrxn
=
jU
.
o
N f ∆Hrxn
Note that
(for no excess fuel):
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
34
which becomes
ηt =
ne FU
o
∆Hrxn
Fuel Cell Engineering Course Notes
 o
U  ne FU o
=
• 
o
o
 U  ∆Hrxn
 U  ∆G o
rxn
•
=
o
 U o  ∆Hrxn
© 1998-2003 Eric M. Stuve
U 
•

o
U 
35
• Applies to both fuel cells and
heat engines
but
heat engine must absorb heat at
the flame temperature (2000+
K) and reject at 298 K
• Carnot efficiency
T2 − T1 40 − 50% (power plants)
ηt =
≈
T2
< 25% (automotive)
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
36
Heat Dissipation
• Fuel cells not perfectly efficient, so
heat must be dissipated. Look at
energy balance…
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
37
.
.
.
o
Q = N f ∆Hrxn
−We
By definition of thermo.
efficiency…
.
Q = (1 −
.
o
ηt ) N f ∆Hrxn
 j 
o
= (1 − ηt )
∆Hrxn
 ne F 
Because the heat of combustion is
negative, heat must be removed
from the fuel cell.
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
38
Example
An H2/O2 fuel cell operates at 0.6 V
and supplying 1 A/cm2 at 298 K.
The area of the fuel cell is 1000
cm2. Determine the following:
(a)
electrical efficiency
(b)
thermodynamic efficiency
(c)
total electric power output
(d)
heat dissipation
(e)
flow rates of H2 and O2.
Fuel Cell Engineering Course Notes
© 1998-2003 Eric M. Stuve
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© 1998-2003 Eric M. Stuve
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© 1998-2003 Eric M. Stuve
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