Fuel Cell Lecture

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Transcript Fuel Cell Lecture

Fuel Cell Technology
Topics
1. A Very Brief History
2. Electrolysis
3. Fuel Cell Basics
- Electrolysis in Reverse
- Thermodynamics
- Components
- Putting It Together
4. Types of Fuel Cells
- Alkali
- Molten Carbonate
- Phosphoric Acid
- Proton Exchange Membrane
- Solid Oxide
5. Benefits
6. Current Initiatives
- Automotive Industry
- Stationary Power Supply Units
- Residential Power Units
7. Future
A Very Brief History
Considered a curiosity in the 1800’s. The first fuel cell was built in 1839 by Sir William Grove, a
lawyer and gentleman scientist. Serious interest in the fuel cell as a practical generator did not begin
until the 1960's, when the U.S. space program chose fuel cells over riskier nuclear power and more
expensive solar energy. Fuel cells furnished power for the Gemini and Apollo spacecraft, and still
provide electricity and water for the space shuttle.(1)
Electrolysis
“What does this have to do with fuel cells?”
By providing energy from a battery, water (H2O)
can be dissociated into the diatomic molecules of
hydrogen (H2)
and oxygen (O2).
Figure 1
Fuel Cell Basics
“Put electrolysis in reverse.”
The familiar process of electrolysis requires work to proceed, if the process is put in reverse, it should be able to do
work for us spontaneously.
The most basic “black box” representation of a fuel cell in action is shown below:
work
Figure 2
O2
H2
fuel
cell
heat
H2 O
Fuel Cell Basics
Thermodynamics
H2(g) + ½O2(g)
H2O(l)
Other gases in the fuel and air inputs (such as N2 and CO2) may be present, but as they are not involved in the
electrochemical reaction, they do not need to be considered in the energy calculations.
Table 1 Thermodynamic properties at 1Atm and 298K
H2
O2
H2O (l)
Enthalpy (H)
0
0
-285.83 kJ/mol
Entropy (S)
130.68 J/mol·K
205.14 J/mol·K
69.91 J/mol·K
Enthalpy is defined as the energy of a system plus the work needed to make room for it in an environment with constant
pressure.
Entropy can be considered as the measure of disorganization of a system, or as a measure of the amount of energy that is
unavailable to do work.
Fuel Cell Basics
Thermodynamics
Enthalpy of the chemical reaction using Hess’ Law:
ΔH = ΔHreaction =
ΣHproducts
= (1mol)(-285.83 kJ/mol)
–
–
ΣHreactants
(0)
= -285.83 kJ
Entropy of chemical reaction:
ΔS = ΔSreaction =
ΣSproducts –
ΣSreactants
= [(1mol)(69.91 J/mol·K)] – [(1mol)(130.68 J/mol·K) + (½mol)(205.14 J/mol·K)]
= -163.34 J/K
Heat gained by the system:
ΔQ
= TΔS
= (298K)(-163.34 J/K)
= -48.7 kJ
Fuel Cell Basics
Thermodynamics
The Gibbs free energy is then calculated by:
ΔG =
ΔH
– TΔS
= (-285.83 kJ) – (-48.7 kJ)
= -237 kJ
The external work done on the reaction, assuming reversibility and constant temp.
W = ΔG
The work done on the reaction by the environment is:
W = ΔG = -237 kJ
The heat transferred to the reaction by the environment is:
ΔQ = TΔS = -48.7 kJ
More simply stated:
The chemical reaction can do 237 kJ of work and produces 48.7 kJ of heat to the environment.
Fuel Cell Basics
Components
Anode: Where the fuel reacts or "oxidizes", and releases electrons.
Cathode: Where oxygen (usually from the air) "reduction" occurs.
Electrolyte: A chemical compound that conducts ions from one electrode to the other inside a fuel cell.
Catalyst: A substance that causes or speeds a chemical reaction without itself being affected.
Cogeneration: The use of waste heat to generate electricity. Harnessing otherwise wasted heat boosts the
efficiency of power-generating systems.
Reformer: A device that extracts pure hydrogen from hydrocarbons.
Direct Fuel Cell: A type of fuel cell in which a hydrocarbon fuel is fed directly to the fuel cell stack,
without requiring an external "reformer" to generate hydrogen.
Fuel Cell Basics
Putting it together.
Figure 3
Types of Fuel Cells
The five most common types:
•
•
•
•
•
Alkali
Molten Carbonate
Phosphoric Acid
Proton Exchange Membrane
Solid Oxide
Types of Fuel Cells
SOFC
Vorteil: Keine aufwendige Brenngas-Aufbereitung
Nachteil: Hohe Betriebstemperaturen = Hohe System-Kosten
 Starke Material-Beanspruchung
Alkali Fuel Cell
compressed hydrogen and oxygen fuel
potassium hydroxide (KOH) electrolyte
~70% efficiency
150˚C - 200˚C operating temp.
300W to 5kW output
Figure 4
requires pure hydrogen fuel and platinum catylist → ($$)
liquid filled container → corrosive leaks
Molten Carbonate Fuel Cell (MCFC)
carbonate salt electrolyte
60 – 80% efficiency
~650˚C operating temp.
cheap nickel electrode catylist
up to 2 MW constructed, up to 100 MW designs exist
Figure 5
The operating temperature is too hot for many applications.
carbonate ions are consumed in the reaction → inject CO2 to compensate
Phosphoric Acid Fuel Cell (PAFC)
phosphoric acid electrolyte
40 – 80% efficiency
150˚C - 200˚C operating temp
11 MW units have been tested
sulphur free gasoline can be used as a fuel
Figure 6
The electrolyte is very corrosive
Platinum catalyst is very expensive
Proton Exchange Membrane (PEM)
thin permeable polymer sheet electrolyte
40 – 50% efficiency
50 – 250 kW
80˚C operating temperature
Figure 7
electrolyte will not leak or crack
temperature good for home or vehicle use
platinum catalyst on both sides of membrane → $$
Solid Oxide Fuel Cell (SOFC)
hard ceramic oxide electrolyte
~60% efficient
~1000˚C operating temperature
cells output up to 100 kW
Figure 8
high temp / catalyst can extract the hydrogen from the fuel at the electrode
high temp allows for power generation using the heat, but limits use
SOFC units are very large
solid electrolyte won’t leak, but can crack
Benefits
Efficient:
in theory and in practice
Portable:
modular units
Reliable:
few moving parts to wear out or break
Fuel Flexible: With a fuel reformer, fuels such as natural gas, ethanol,
methanol,
propane, gasoline, diesel, landfill gas,wastewater,
treatment digester gas, or even ammonia can be used
Environmental:
produces heat and water (less than combustion in both cases)
near zero emission of CO and NOx
reduced emission of CO2 (zero emission if pure H2 fuel)
Material‘s challenges of the PEM Fuel Cell
Review of Membrane (Nafion) Properties
• Chemical Structure
• Proton Conduction Process
• Water Transport and Interface Reactions
7/7/2015
Fuel Cell Fundamentals
20
Chemical structures of some membrane materials
PSSA
poly(styrene-costyrenesulfonic acid)
(PSSA)
Nafion,TM
Membrane C
Dow
PESA
(Polyepoxysuccinic Acid)
,,-Trifluorostyrene grafted onto
poly(tetrafluoro-ethylene) with postsulfonation)
Poly – AMPS
Poly(2-acrylamido2-methylpropane sulfonate)
Nafion Membrane
Chemical Structure
Nafion Membrane
Proton Conduction Process
The water transport through Nafion Membrane
Water flux due to electroosmotic drag (mol/cm2 s) is: Nw, drag = I()/F.
Where: I is the cell current, () is the electroosmotic drag coefficient at a
given state of membrane hydration (=N(H2O)/N(SO3H) and F is the Faraday
constant. This flux acts to dehyddrate the anode side of a cell and to
introduce additional water at the cathode side.
The buildup of water at the cathode (including the product water
from the cathode reaction) is reduced, in turn, by diffusion back down the
resulting water concentration gradient (and by hydraulic permeation of water
in differentially pressurized cells where the cathode is held at higher overall
pressure). The fluxes (mol/cm2 s) brought about by the latter two mechanisms
within the membrane are:
Nw,diff = -D()c/ z,
Nw,hyd = -khyd()P/ z
where D is the diffusion coefficient in the ionomer at water content , c/ z
is a water concentration gradient along the z-direction of membrane thickness,
khyd is the hydraulic permeability of the membrane, and P/ z is a pressure
gradient along z.
The water transport through Nafion Membrane
Many techniques have been introduced to prevent the dehydration of the anode (including the introduction
of liquid water into the anode and/or cathode, etc. – which, however, can lead to “flooding” problems that
inhibit mass transfer).
However, the overall question of “water management,” including the issue of drag as a central component,
has been solved to a very significant extent by the application of sufficiently thin PFSA membranes (<100
µm thick) in PEFCs, combined with humidification of the anode fuel gas stream.
An example of a development specifically enabling this to an extreme degree is the developmental composite
membrane introduced W. L. Gore that provides usable mechanical properties for very thin (20 µm and less)
perfluorinated membranes with high protonic conductivity.
Water Transport (& Interface Reactions)
in Nafion Membrane of the PEM Fuel Cell
Material‘s challenges of the SOFC
Solid Oxide Fuel Cell
SOFC
Air side = cathode: High oxygen partial pressure
O2
H2 + 1/2O2 D H2O
conductance   
H2
1
d
H 2O
Fuel side= anode: H2 + H2O= low oxygen partial pressure
Electromotive Force (EMF)
SOFC
Chemical Reactions in 2 separated compartements:
- Cathode (Oxidation): ½O2 + 2e- D O22- Anode (Reduction): H2 + O D H2O + 2e
G = Free Enthalpie
z = number of charge carriers
F = Faraday Constant
EMF of a galvanic Cell:
(1)
SOFC:
G0= Free Enthalpie in
standart state
EMF = Gr /-z F
½O2 + H2 D H2O
R = Gas Constant
a ( H 2O )
(2) G  G0  RT ln
a( H 2 )a(O2 )0.5
difference of G between anode und cathode 
Nernst Equation:
RT p ( O2 )
EMK 
ln
A
4F p ( O2 )
K
Elektrochemische Potential
SOFC
Oxygen ions migrate due to an electrical
and chemical gradient
 (O2 )   (O2 )  2F 
Electrochemichal
Potential
Chemical
Potential
Electrical
Potential
Driving force for the O2- Diffusion through the electrolyte are the
different oxygen partial pressures at the anode and the cathode
2
side:
 (O )
ji  
i
2F
 (O )
2
ji = ionic current
i= ionic conductivity
engl. Open Circuit Voltage (OCV)
SOFC
 (O )   (O )  2F 
2
2
ji  
i
2F
 (O 2 )
What happems in case :
 (O )  0
2
ji  0
OCV
No current
Electrical potential difference = chemical potetial
Leistungs-Verluste
SOFC
Under load decrease of cell voltage
and internal losses
U(I) = OCV - I(RE+ RC+RA) - hC - hA
cell voltage U(I) [V]
OCV
(RE+ RC+RA)
hC
hA
cell current I [mA/cm2]
Ohmic resistances
Non ohmic resistances=
over voltages
Überspannungen
SOFC
Over voltages exist at interfaces of
•
Elektrolyte - Cathode
•
Elektrolyte - Anode
Reasons:
•
•
•
Kinetic hindrance of the electrochemical reactions
Bad adheasion of electrode and electrolyte
Diffusion limitations at high current densities
Ohm‘s losses
SOFC
Past
Future
800nm
Kathode
Reduce electrolyte thickness
Anode
Leistungs-Verluste
SOFC
cell voltage U(I) [V]
OCV
1
(RE+ RC+RA)
hC
hA
2
cell current I [mA/cm2]
3
(1) Open circuit voltage (OCV), I = 0
(2) SOFC under Load  U-I curve
(3) Short circuit, Vcell = 0
(2)
0.5
1.0
0.8
Leistung [W/cm ]
0.3
0.6
0.2
0.4
2
Zellspannung [V]
0.4
0.1
0.2
(1)
0.0
0.0
900°C
in Luft/Wasserstoff
0.5
1.0
1.5
2
Stromdichte [A/cm ]
(3)
0.0
2.0
How to determine the electrical conductance
SOFC
Iinput Umeasured
Electrical resistance:
U
L
R  f (T )  
I
A *
Electrical conductivity:
0
Ea
 
log( 
)
T
kT
1
 T vs.
 Ea
T
U :
I :
R :
L :
A

Ea
T
K
voltage [V]
current [A]
resistivity [ohm]
distance between both
inner wires [cm]
: sample surface [cm2]
: conductivity [S/m]
: activation energy [eV]
: temperature [K]
: Boltzmann constant
SOFC
SOFC-Designs
SOFC Design
SOFC
Tubular design
i.e. Siemens-Westinghouse design
Segment-type tubular design
Planar design
i.e. Sulzer Hexis, BMW design
Tubular Design – Siemens-Westinghouse
cathode
interconnection
cathode
(air)
air flow
anode (fuel)
SOFC
Why was tubular design
developed in 1960s by
Westinghouse?
• Planar cell: Thermal
expansion mismatch
between ceramic and
support structures leads to
problems with the gas
sealing  tubular design
was invented
Advantages of tubular
design:
• At cell plenum: depleted air
and fuel react  heat is
generated  incoming
oxidant can be pre-heated.
• No leak-free gas
manifolding needed in this
Tubular Design – Siemens-Westinghouse
SOFC
To overcome problems new
Siemens-Westinghouse „HPDSOFC“ design:
cathode
(air)
New: Flat cathode tube with
ligaments
anode (fuel)
electrolyte
Advantages of HPD-SOFC:
• Ligaments within cathode  short
current pathways  decrease of
ohmic resistance
• High packaging density of cells
Siemens-Westinghouse
shifted from
compared to tubular design
basic technology to cost reduction and
scale up.
Power output: Some 100 kW can be
produced.
Planar Design – Sulzer Hexis
SOFC
interconnect
cathode (air)
electrolyte
anode (fuel)
Advantages of planar
design:
• Planer cell design of bipolar
plates  easy stacking  no
long current pathways
• Low-cost fabrication
methods, i.e. Screen printing
and tape casting can be
used.
Drawback of tubular
design:
• Life time of the cells 30007000h  needs to be
improved by optimization of
mechanical and
electrochemical stability of
used materials.
Planar Design – BMW
SOFC
Air channel
bipolar plate
Cathode current collector
cathode
electrolyte
anode
porous metallic substrate
Fe-26Cr-(Mo, Ti, Mn, Y2O3) alloy
bipolar plate
Fuel channel
20-50 m
Plasma spray
5-20 m
Plasma spray
15-50 m
Plasma spray
Application
Batterie replacement in the
BMW cars of the 7-series.
Power output: 135 kW is
aimed.
Current Initiatives
Automotive Industry
Most of the major auto manufacturers have fuel cell vehicle (FCV) projects currently under way, which involve all sorts of fuel cells and
hybrid combinations of conventional combustion, fuel reformers and battery power.
Considered to be the first gasoline powered fuel cell vehicle is the H20 by GM:
GMC S-10 (2001)
fuel cell battery hybrid
low sulfur gasoline fuel
25 kW PEM
40 mpg
112 km/h top speed
Figure 9
Current Initiatives
Automotive Industry
Fords Adavanced Focus FCV (2002)
fuel cell battery hybrid
85 kW PEM
~50 mpg (equivalent)
4 kg of compressed H2 @ 5000 psi
Figure 10
Approximately 40 fleet vehicles are planned as a market
introduction for Germany, Vancouver and California for
2004.
Figure 11
Current Initiatives
Automotive Industry
Daimler-Chrysler NECAR 5 (introduced in 2000)
85 kW PEM fuel cell
methanol fuel
reformer required
150 km/h top speed
Figure 12
version 5.2 of this model completed a California to Washington DC drive
awarded road permit for Japanese roads
Current Initiatives
Automotive Industry
Mitsubishi Grandis FCV minivan
fuel cell / battery hybrid
68 kW PEM
compressed hydrogen fuel
140 km/h top speed
Figure 13
Plans are to launch as a production vehicle for Europe in 2004.
Current Initiatives
Stationary Power Supply Units
More than 2500 stationary fuel cell systems have been installed all over the world - in hospitals, nursing homes, hotels,
office buildings, schools, utility power plants, and an airport terminal, providing primary power or backup. In large-scale
building systems, fuel cells can reduce facility energy service costs by 20% to 40% over conventional energy service.
Figure 14
A fuel cell installed at McDonald’s restaurant, Long Island Power Authority to install 45 more fuel cells across Long Island,
including homes.(2) Feb 26, 2003
Current Initiatives
Residential Power Units
There are few residential fuel cell power units on the market but many designs are undergoing testing and should be
available within the next few years. The major technical difficulty in producing residential fuel cells is that they must be
safe to install in a home, and be easily maintained by the average homeowner.
Residential fuel cells are typically the size of
a large deep freezer or furnace, such as the
Plug Power 7000 unit shown here, and cost
$5000 - $10 000.
Figure 15
If a power company was to install a residential fuel cell power unit in a home, it would have to charge the homeowner at
least 40 ¢/kWh to be economically profitable.(3) They will have to remain a backup power supply for the near future.
Future
“...projections made by car companies themselves and energy and automotive experts concur that around 2010, and perhaps
earlier, car manufacturers will have mass production capabilities for fuel cell vehicles, signifying the time they would be
economically available to the average consumer.” Auto Companies on Fuel Cells, Brian Walsh and Peter Moores, posted on www.fuelcells.org
A commercially available fuel cell power plant would cost about $3000/kW, but would have to drop below $1500/kW to
achieve widespread market penetration. http://www.fuelcells.org/fcfaqs.htm
Technical and engineering innovations are continually lowering the capital cost of a fuel cell unit as well as the operating
costs, but it is expected that mass production will be of the greatest impact to affordability.
Future
internal combustion obsolete?
solve pollution problems?
common in homes?
better designs?
higher efficiencies?
cheaper electricity?
reduced petroleum dependency?
...winning lottery numbers?
References
(1) FAQ section, fuelcells.org
(2) Long Island Power Authority press release: Plug Power Fuel Cell Installed at McDonald’s Restaurant, LIPA to
Install 45 More Fuel Cells Across Long Island, Including Homes,
http://www.lipower.org/newscenter/pr/2003/feb26.fuelcell.html
(3) Proceedings of the 2000 DOE Hydrogen Program Review: Analysis of Residential Fuel Cell Systems & PNGV
Fuel Cell Vehicles, http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/28890mm.pdf
Figures
1, 3 http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/electrol.html
4 – 8 http://fuelcells.si.edu/basics.htm
10 http://www.moteurnature.com/zvisu/2003/focus_fcv/focus_fcv.jpg
11 http://www.granitestatecleancities.org/images/Hydrogen_Fuel_Cell_Engine.jpg
12 http://www.in.gr/auto/parousiaseis/foto_big/Necar07_2883.jpg
13 http://www3.caradisiac.com/media/images/le_mag/mag138/oeil_mitsubishi_grandis_big.jpg
14 http://www.lipower.org/newscenter/pr/2003/feb26.fuelcell.html
15 http://americanhistory.si.edu/csr/fuelcells/images/plugpwr1.jpg
Table 1 http://hyperphysics.phy-astr.gsu.edu/hbase/tables/therprop.html#c1
Fuel cell data from: Types of Fuel Cells, fuelcells.org
Fuel Cell Vehicle data primarily from: Fuel Cell Vehicles (From Auto Manufacturers) table, fuelcells.org