1. Fuel Cell Technology 1

2. 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 2

3. 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) 3

4. 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 4

5. Fuel Cell Basics fuel cell “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:
fuel
cell
H2O O2 H2
heat
work
Figure 2 5

6. 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.
69.91 J/mol·K
J/mol·K
J/mol·K
Entropy (S)
kJ/mol
Enthalpy (H)
H2O (l) O2 H2
Table 1 Thermodynamic properties at 1Atm and 298K
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. 6

7. Fuel Cell Basics Thermodynamics
Enthalpy of the chemical reaction using Hess’ Law:
ΔH = ΔHreaction = ΣHproducts – ΣHreactants
= (1mol)( kJ/mol) – (0)
= kJ
Entropy of chemical reaction:
ΔS = ΔSreaction = ΣSproducts – ΣSreactants
= [(1mol)(69.91 J/mol·K)] – [(1mol)( J/mol·K) + (½mol)( J/mol·K)]
= J/K
Heat gained by the system:
ΔQ = TΔS
= (298K)( J/K)
= kJ 7

8. Fuel Cell Basics Thermodynamics
The Gibbs free energy is then calculated by:
ΔG = ΔH – TΔS
= ( 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 = kJ
More simply stated: The chemical reaction can do 237 kJ of work and produces 48.7 kJ of heat to the environment. 8

9. 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. 9

10. Fuel Cell Basics
Putting it together.
Figure 3 10

11. Types of Fuel Cells The five most common types: Alkali
Molten Carbonate
Phosphoric Acid
Proton Exchange Membrane
Solid Oxide 11

12. Types of Fuel Cells SOFC
Vorteil: Keine aufwendige Brenngas-Aufbereitung
Nachteil: Hohe Betriebstemperaturen = Hohe System-Kosten
 Starke Material-Beanspruchung 12

13. 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 13

14. 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 14

15. 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 15

16. 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 → $$ 16

17. 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 17

18. 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) 18

19. Material‘s challenges of the PEM Fuel Cell19

20. Proton Conduction Process Water Transport and Interface Reactions
Review of Membrane (Nafion) Properties
Chemical Structure
Proton Conduction Process
Water Transport and Interface Reactions
08/28/12
Fuel Cell Fundamentals 20 20

21. Chemical structures of some membrane materials
PSSA
poly(styrene-co-styrenesulfonic acid) (PSSA)
Nafion,TM
Membrane C
Dow
PESA (Polyepoxy-
succinic Acid)
,,-Trifluorostyrene grafted onto poly(tetrafluoro-ethylene) with post-sulfonation)
PE (polymer electrolyte) FCs utilize a polymeric electrolyte. NafionTM, a perfluorinated polymer with sidechains terminating in sulfonic acid moieties, and its close perfluorosulfonic acid (PFSA) relatives, are currently the state-of-the-art in membranes for PEFCs, satisfying an array of requirements for effective, long-term use in fuel cells.
They combine well the important requirements for a membrane in a PEFC, namely: high protonic conductivity, high chemical stability under typical operating conditions, and low gas permeabilities. Typically, thickness of PFSA membranes for PEFCs range between 50 and 175 m. The main source of PFSA membranes is DuPont (USA), where these membranes were invented in the 1960’s and made into a commercial product for the chlor-alkali industry. Other sources of developmental PFSA membranes have been Dow Chemical (USA), Asahi Glass (Japan), and Asahi Chemicals (Japan).
Poly – AMPS
Poly(2-acrylamido- 2-methylpropane sulfonate) 21

22. Chemical Structure Nafion Membrane
The most important property of ionomeric membranes employed in polymer electrolyte fuel cells is the high protonic conductivity they provide at the current densities typically required in PEFCs.
The specific conductivity of fully hydrated PFSA (immersed) membranes is about 0.1 S/cm at room temperature, and about 0.15 S/cm at the typical cell operation temperature of 80ºC.
These high protonic conductivities provide the basis for the high power densities achievable in PEFCs.
The dependence of proton mobility in PFSA membranes on water content is, however, quite critical, and demands effective cell and stack design to maintain a high level of water through the thickness of the membrane for the complete range of dynamic operation. 22

23. Proton Conduction Process
Nafion Membrane
Proton Conduction Process
The number of water molecules carried through the membrane per proton is a central factor in determinating the water profiles in the membrane of an operating PEFC.
There is an important difference between the electroosmotic drag coefficient, (), a characteristic of an ionomeric membrane with fixed water content and flat water profile, and the net water flux through an operating fuel cell. The latter is the resultant of several water transport modes in the cell.
For fully hydrated and (immersed) Nafion 1100 membranes, a drag coefficient of 2.5 H2O/SO3H is measured, whereas for a membrane equilibrated with vapor-phase water the drag coefficient is close to 1.0 H2O/H+ over a wide range of water contents. The lack of dependence of the drag coefficient on membrane nanostructure suggests that the drag coefficient is determined by the basic elements of the proton transport process; I.e.; via the hydronium ion or complex.. 23

24. 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. 24

25. 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. 25

26. Water Transport (& Interface Reactions) in Nafion Membrane of the PEM Fuel Cell26

27. Material‘s challenges of the SOFC27

28. Solid Oxide Fuel Cell Air side = cathode: High oxygen partial pressure
H2 + 1/2O2  H2O H2
H2O
Fuel side= anode: H2 + H2O= low oxygen partial pressure 28

29. Electromotive Force (EMF)
Chemical Reactions in 2 separated compartements:
- Cathode (Oxidation):
- Anode (Reduction):
½O2 + 2e-  O2-
H2 + O2-  H2O + 2e-
G = Free Enthalpie
z = number of charge carriers
F = Faraday Constant
G0= Free Enthalpie in standart state
R = Gas Constant
EMF of a galvanic Cell:
EMF = Gr /-z F
SOFC:
½O2 + H2  H2O

difference of G between anode und cathode 
Nernst Equation: K A 29

30. Elektrochemische Potential
Oxygen ions migrate due to an electrical and chemical gradient
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 side:
ji = ionic current
i= ionic conductivity 30

31. OCV What happems in case : engl. Open Circuit Voltage (OCV) No current
Electrical potential difference = chemical potetial
OCV 31

32. Non ohmic resistances= over voltages
Leistungs-Verluste
Under load decrease of cell voltage
and internal losses
U(I) = OCV - I(RE+ RC+RA) - C - A
cell voltage U(I) [V]
OCV
(RE+ RC+RA)
Ohmic resistances C
Non ohmic resistances= over voltages A
cell current I [mA/cm2] 32

33. Over voltages exist at interfaces of Elektrolyte - Cathode
Überspannungen
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 33

34. Reduce electrolyte thickness
Ohm‘s losses
Past
Future
800nm
Kathode
Anode
Reduce electrolyte thickness 34

35. Leistungs-Verluste 1 2 3 Open circuit voltage (OCV), I = 0
SOFC under Load  U-I curve
(3) Short circuit, Vcell = 0
(2)
(1)
(3) 35

36. Electrical resistance:
How to determine the electrical conductance
Electrical resistance:
Iinput
Umeasured
Electrical conductivity:
U : voltage [V]
I : current [A]
R : resistivity [ohm]
L : distance between both
inner wires [cm]
A : sample surface [cm2]
: conductivity [S/m]
Ea : activation energy [eV]
T : temperature [K]
K : Boltzmann constant 36

37. SOFC-Designs 37

38. SOFC Design Tubular design i.e. Siemens-Westinghouse design
Segment-type tubular design
Planar design
i.e. Sulzer Hexis, BMW design 38

39. Tubular Design – Siemens-Westinghouse
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 design !
Drawback of tubular design:
Electric current flows along circumference of anode and cathode  high cell losses
cathode
interconnection
cathode
(air)
air flow
anode (fuel) 39

40. Tubular Design – Siemens-Westinghouse
To overcome problems new Siemens-Westinghouse „HPD-SOFC“ design:
New: Flat cathode tube with ligaments
Advantages of HPD-SOFC:
Ligaments within cathode  short current pathways  decrease of ohmic resistance
High packaging density of cells compared to tubular design
anode (fuel)
cathode
(air)
electrolyte
Siemens-Westinghouse shifted from basic technology to cost reduction and scale up.
Power output: Some 100 kW can be produced. 40

41. Planar Design – Sulzer Hexis
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 h  needs to be improved by optimization of mechanical and electrochemical stability of used materials.
Power output: 1 kW is aimed.
anode (fuel)
electrolyte
cathode (air)
interconnect 41

42. Planar Design – BMW Air channel bipolar plate
Cathode current collector
cathode
electrolyte
anode
porous metallic substrate
Fe-26Cr-(Mo, Ti, Mn, Y2O3) alloy
bipolar plate
Application
Batterie replacement in the BMW cars of the 7-series.
Power output: 135 kW is aimed.
Fuel channel
20-50 m
Plasma spray
5-20 m
Plasma spray
15-50 m
Plasma spray 42

43. 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 43

44. Current Initiatives Automotive Industry
Fords Adavanced Focus FCV (2002)
fuel cell battery hybrid
85 kW PEM
~50 mpg (equivalent)
4 kg of compressed 5000 psi
Figure 10
Approximately 40 fleet vehicles are planned as a market introduction for Germany, Vancouver and California for 2004.
Figure 11 44

45. 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 45

46. 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. 46

47. 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 47

48. 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 $ $
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. 48

49. 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
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.
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. 49

50. Future internal combustion obsolete? solve pollution problems?
common in homes?
better designs?
higher efficiencies?
cheaper electricity?
reduced petroleum dependency?
...winning lottery numbers? 50

51. 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, (3) Proceedings of the 2000 DOE Hydrogen Program Review: Analysis of Residential Fuel Cell Systems & PNGV Fuel Cell Vehicles,
Figures 1, – 8
Table 1
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 51