Fuel cell

Fuel Cell Technology

Topics

1. A Very Brief History2. Electrolysis3. 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. Benefits6. 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.”

fuelcell

H2OO2

H2

heat

work

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:

Figure 2

Fuel Cell BasicsThermodynamics

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·K205.14 J/mol·K130.68 J/mol·KEntropy (S)

-285.83 kJ/mol00Enthalpy (H)

H2O (l)O2H2

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.

Fuel Cell BasicsThermodynamics

Enthalpy of the chemical reaction using Hess’ Law:ΔH = ΔHreaction = ΣHproducts – ΣHreactants

= (1mol)(-285.83 kJ/mol) – (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 BasicsThermodynamics

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:

The heat transferred to the reaction by the environment is:

W = ΔG = -237 kJ

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 BasicsComponents

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 BasicsPutting 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

Vorteil: Keine aufwendige Brenngas-AufbereitungNachteil: Hohe Betriebstemperaturen = Hohe System-Kosten Starke Material-Beanspruchung

SOFC

Alkali Fuel Cell

compressed hydrogen and oxygen fuel

potassium hydroxide (KOH) electrolyte

~70% efficiency

150˚C - 200˚C operating temp.

300W to 5kW output

requires pure hydrogen fuel and platinum catylist → ($$)liquid filled container → corrosive leaks

Figure 4

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

electrolyte will not leak or crack

temperature good for home or vehicle use

platinum catalyst on both sides of membrane → $$

Figure 7

Solid Oxide Fuel Cell (SOFC)

hard ceramic oxide electrolyte

~60% efficient

~1000˚C operating temperature

cells output up to 100 kW

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

Figure 8

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

04/11/23 Fuel Cell Fundamentals 20

Review of Membrane (Nafion) Properties

• Chemical Structure• Proton Conduction Process• Water Transport and Interface Reactions

PSSApoly(styrene-co-styrenesulfonic acid) (PSSA)

Nafion,TM Membrane C

Dow

PESA(Polyepoxy-succinic Acid)

,,-Trifluorostyrene grafted onto poly(tetrafluoro-ethylene) with post-sulfonation)

Poly – AMPSPoly(2-acrylamido-2-methylpropane sulfonate)

Chemical structures of some membrane materials

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/ zwhere 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

SOFCSolid Oxide Fuel Cell

Air side = cathode: High oxygen partial pressure

1conductance

d

Fuel side= anode: H2 + H2O= low oxygen partial pressure

H2 + 1/2O2 H2O

H2

O2

H2O

SOFCElectromotive Force (EMF)

Chemical Reactions in 2 separated compartements:- Cathode (Oxidation): - Anode (Reduction):

½O2 + 2e- O2-

H2 + O2- H2O + 2e-

EMF of a galvanic Cell:

EMF = Gr /-z F

G = Free Enthalpie

z = number of charge carriers

F = Faraday Constant

G0= Free Enthalpie in standart state

R = Gas Constant

SOFC: ½O2 + H2 H2O 2

0 0.52 2

ln( ) ( )

a H OG G RT

a H a O

difference of G between anode und cathode

2

2

ln4

p ORTEMK

F p O

K

A

Nernst Equation:

SOFCElektrochemische Potential

Oxygen ions migrate due to an electrical and chemical gradient

2 2( ) ( ) 2O O F

2( )O

ChemicalPotential

ElectricalPotential

Electrochemichal Potential

Driving force for the O2- Diffusion through the electrolyte are the different oxygen partial pressures at the anode and the cathode side:

2( )2

iij O

F

ji = ionic current

i= ionic conductivity

SOFCengl. Open Circuit Voltage (OCV)

2( )2

iij O

F

2 2( ) ( ) 2O O F

2( ) 0O

What happems in case :

0ij No currentElectrical potential difference = chemical potetialOCV

SOFCLeistungs-Verluste

Under load decrease of cell voltageand internal losses

U(I) = OCV - I(RE+ RC+RA) - C - A

(RE+ RC+RA)OCV

C

A

cell current I [mA/cm2]

cell

volta

ge U

(I)

[V] Ohmic resistances

Non ohmic resistances= over voltages

SOFCÜberspannungen

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

SOFCOhm‘s losses

800nm

Kathode Anode

Reduce electrolyte thickness

Past Future

SOFCLeistungs-Verluste

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

0.0 0.5 1.0 1.5 2.00.0

0.2

0.4

0.6

0.8

1.0

900°Cin Luft/Wasserstoff

Stromdichte [A/cm2]

Zells

pa

nn

un

g [

V]

0.0

0.1

0.2

0.3

0.4

0.5

Le

istun

g [W

/cm 2]

(1)

(2)

(3)

(RE+ RC+RA)OCV

C

A

cell current I [mA/cm2]

cell

volta

geU

(I)

[V] (RE+ RC+RA)

OCV

C

A

cell current I [mA/cm2]

cell

volta

geU

(I)

[V]

1

23

SOFC

( )*

U LR f T

I A

0 log( )aE

T kT

1. aT vs E

T

Electrical resistance:

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

How to determine the electrical conductance

Iin

pu

tU

measu

red

SOFC

SOFC-Designs

SOFC

Tubular designi.e. Siemens-Westinghouse design

Planar designi.e. Sulzer Hexis, BMW design

Segment-type tubular design

SOFC Design

SOFCTubular Design – Siemens-Westinghouse

air flow anode (fuel)

cathode interconnection

cathode (air)

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

SOFC

anode (fuel)

cathode (air)

electrolyte

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 designSiemens-Westinghouse shifted from

basic technology to cost reduction and scale up.

Power output: Some 100 kW can be produced.

SOFCPlanar Design – Sulzer Hexis

anode (fuel)

electrolyte

cathode (air)

interconnect 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 3000-7000h needs to be improved by optimization of mechanical and electrochemical stability of used materials.

Power output: 1 kW is aimed.

SOFCPlanar Design – BMW

electrolyteanode

porous metallic substrateFe-26Cr-(Mo, Ti, Mn, Y2O3) alloy

cathodeCathode current collector

bipolar plate

bipolar plate

Air channel

Fuel channel

20-50 m

5-20 m

15-50 m

Plasma spray

Plasma spray

Plasma spray

ApplicationBatterie replacement in the BMW cars of the 7-series.

Power output: 135 kW is aimed.

Current InitiativesAutomotive 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 hybridlow sulfur gasoline fuel25 kW PEM40 mpg112 km/h top speed

Figure 9

Fords Adavanced Focus FCV (2002)fuel cell battery hybrid85 kW PEM~50 mpg (equivalent)4 kg of compressed H2 @ 5000 psi

Approximately 40 fleet vehicles are planned as a market introduction for Germany, Vancouver and California for 2004.

Current InitiativesAutomotive Industry

Figure 10

Figure 11

Daimler-Chrysler NECAR 5 (introduced in 2000)

85 kW PEM fuel cell

methanol fuel

reformer required

150 km/h top speed

version 5.2 of this model completed a California to Washington DC driveawarded road permit for Japanese roads

Current InitiativesAutomotive Industry

Figure 12

Mitsubishi Grandis FCV minivan

fuel cell / battery hybrid

68 kW PEM

compressed hydrogen fuel

140 km/h top speed

Plans are to launch as a production vehicle for Europe in 2004.

Current InitiativesAutomotive Industry

Figure 13

Current InitiativesStationary Power Supply Units

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

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

Current InitiativesResidential 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.

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.

Figure 15

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

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.

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

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

Figures1, 3 http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/electrol.html4 – 8 http://fuelcells.si.edu/basics.htm10 http://www.moteurnature.com/zvisu/2003/focus_fcv/focus_fcv.jpg11 http://www.granitestatecleancities.org/images/Hydrogen_Fuel_Cell_Engine.jpg12 http://www.in.gr/auto/parousiaseis/foto_big/Necar07_2883.jpg13 http://www3.caradisiac.com/media/images/le_mag/mag138/oeil_mitsubishi_grandis_big.jpg14 http://www.lipower.org/newscenter/pr/2003/feb26.fuelcell.html15 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