ER100/200 & Public Policy 184/284 Lecture 21: Fuel Cell Systems November 12, 2015
ER100/200 & Public Policy 184/284
Lecture 21: Fuel Cell Systems
November 12, 2015
Topics
1. 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
Fuel Cells
6
2
Fuel Cells provide flexible Voltage & Current
5
8
4
3
7
7
Applications
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)
Jules Verne, Mysterious Island, 1874“l believe that water will one day be employed as a fuel, that hydrogen and oxygen will furnish an inexhaustible source of heat and light.”
Energy Density kWh/kgGasoline 14
Lead Acid Batteries 0.04
Flywheel (silica) 1
Hydrogen 38
Compressed air 2/m3
Dams (hydrostorage) 0.3/m3
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).
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 Operation
• Pressurized hydrogen gas (H2) enters cell on anode side. • Gas is forced through catalyst by pressure.
– When H2 molecule comes contacts platinum catalyst, it splits into two H+ ions and two electrons (e-).
• Electrons are conducted through the anode– Make their way through the external circuit (doing useful work
such as turning a motor) and return to the cathode side of the fuel cell.
• On the cathode side, oxygen gas (O2) is forced through the catalyst– Forms two oxygen atoms, each with a strong negative charge. – Negative charge attracts the two H+ ions through the membrane, – Combine with an oxygen atom and two electrons from the external
circuit to form a water molecule (H2O).
Fuel cell thermodynamics (extra)The first law of thermodynamics:• The energy of a system is conserved
ΔQ - ΔW = ΔE
•The energy is a relative quantity Q – W = ΔE
• For a closed system (control mass system), such as a piston
ΔE = ΔU + ΔK + ΔP
(The total energy change equals the sum of the change in internal energy, the change in kinetic energy, and the change in potential energy
Change of heat provided to the
system
Change of work provided by the system
Change of system’s total
energyChange of
system energy
Fuel cell thermodynamics (extra)
From:
ΔE = ΔU + ΔK + ΔP
ΔE = ΔU + ΔK + Δ(pV)
H = U + pV
Or ΔH = ΔE – ΔK - Δ(pV)
If there is no change in volume so the kinetic and potential energies are constant:
H = Q – W
Enthalpy is the difference between the heat and the work involved in a system
Change of system’s total
energy
Fuel Cell Basics
fuel
cell
H2O
O2
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 ThermodynamicsH2(g) + ½O2(g) H2O(l)
Enthalpy is defined as the energy of a system plus the work needed to make room for it in an environment with constant
pressure.
Enthalpy = H = U + PV
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.
ΔS = Q/T
Entropy gain > Entropy Loss
(Q/T) + Sproducts ≥ Sreactants
Q ≥ T [Sproducts - Sreactants]
Gibbs Free Energy, G = H – Qliberated, = H – TΔS
Ηmax = ΔG / ΔH
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 ThermodynamicsH2(g) + ½O2(g) H2O(l)
Fuel Cell BasicsThermodynamics
Enthalpy of the chemical reaction:
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 BasicsPutting it together.
Figure 3
• Anode:2H2 => 4H+ + 4e-
• Cathode: O2 + 4H+ + 4e- => 2H2O
• Net Reaction:2H2 + O2 => 2H2O
• Exact opposite of electrolysis
Energy Conversion: PEM Fuel Cell
Reduction-Oxidation Rxn (redox)
−+ +→ eHH 442 2
OHeHO 22 244 →++ −+
ProductOxidant →+ −e
−+→ eProductReductant
energyOHOH +→+ 222 22
Anode Half-Reaction
Cathode Half-Reaction
Hydrogen and oxygen are combined in a non-combustion process Electricity, heat and water are produced
Proton Exchange Membrane Fuel Cell(Polymer Electrolyte Membrane)
Between the reduction and oxidation stages, the
electrons are routed through a circuit
−+ +→ eHH 442 2
Hydrogen ions (protons) permeate through the
electrolyte membrane
Reduction reaction
Oxidation reaction facilitated by a catalyst -
typically Pt ($$$)
OHeHO 22 244 →++ −+
1.23 V
Power Produced – Watts/m2
• Activation Loss– potential difference above
the equilibrium value required to produce a current (depends on activation energy of the reaction)
– energy is lost as heat• Ohmic Loss
– voltage drop due to resistance of the cell components and interconnects
• Mass Transport Loss– depletion of reactants at
catalyst sites under high loads
PEM (Polymer Electrolyte Membrane)
• Polymers such as polyphenylenes, Nafion are used
• Water is a crucial participant in the process
•absorption of water increases the proton conductivity
•membrane is confined – not free to swell – pushes electrodes
Platinum needs to be placed to maximize surface area
Needs to be encased in engineered components
• Thickness of the membrane and catalyst in the PEM can vary …
• Example: catalyst layers containing about 0.15 milligrams (mg) Pt/cm2
• thickness of the catalyst layer is close to 10 micrometers
•yields a MEA with a total thickness of about 200μm (or 0.2 mm or 20 sheets of paper)
•generates more than half an ampere of current per cm2 at a voltage of 0.7 volts
PEM (Polymer Electrolyte Membrane)
Parts of a Fuel CellBipolar Plates
• Serpentine channels for hydrogen and oxygen to flow through device
• Acts as a current collector –electrons enter and exit cell through the plate
Anode• Conducts electrons away from
catalyst to external circuit
• Channels to supply H2 evenly to the surface of the catalyst
Cathode• Channels to supply O2 evenly
to the surface of the catalyst
• Conducts electrons back to catalyst for recombining
Parts of a PEM Fuel CellMembrane Electrode Assembly• Anode
• Cathode
• PEM (Polymer Electrolyte Membrane)• conducts only positively charged ions
• blocks electrons and other substances
• Catalyst• thin coat of platinum powder applied to
carbon paper or cloth•maximizes surface area
• Backing Layers• porous carbon cloth conducts electrons
away from catalyst to external circuit• allows right amount of water vapor to
enter/exit• too much blocks the pores• membrane needs to be humidified
Schematic of Fuel Cell Operation
energyOHOH +→+ 222 21
1.2 V = theoretical maximum voltage generated by this reaction
Typical output = 0.7V – 0.9V ….. (1 W per cm2)
• Anode
Schematic of Fuel Cell Operation
Electron is stripped from Hydrogen as it makes contact with Pt catalyst which is embedded in a carbon nanoparticle
Electron conducted away through circuit
Hydrogen nucleus (proton) passes through PEM membrane to cathode
Hydrogen gas is circulated through ‘serpentine’channelsHydrogen from
channels passes through porous medium
(gas diffusion backing)
Types of Fuel Cells
The five most common types:
• Alkali
• Molten Carbonate
• Phosphoric Acid
• Proton Exchange Membrane
• Solid Oxide
A Wide Range of Types of Fuel Cells
Types of Fuel Cells
• Polymer Electrolyte Membrane (PEM) Fuel Cells
• Direct Methanol Fuel Cells • Alkaline Fuel Cells • Phosphoric Acid Fuel Cells • Molten Carbonate Fuel Cells • Solid Oxide Fuel Cells • Regenerative Fuel Cells
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
11/12/2015 Fuel Cell Fundamentals 40
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)
α,β,β-Trifluorostyrene grafted onto poly(tetrafluoro-ethylene) with post-
sulfonation)
Poly – AMPS
Poly(2-acrylamido-
2-methylpropane sulfonate)
Chemical structures of some membrane materials
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
1conductanced
σ= ∝
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:(1) 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∆ = ∆ +(2)
difference of ∆G between anode und cathode
( )( )
2
2
ln4
p ORTEMKF p O
=K
A
Nernst Equation:
SOFCElektrochemische Potential
Oxygen ions migrate due to an electrical and chemical gradient
2 2( ) ( ) 2O O Fµ µ ϕ− −∆ = ∆ − ∆
2( )Oµ −∆
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
Fσ µ −= − ∆
ji = ionic current
σi= ionic conductivity
SOFCengl. Open Circuit Voltage (OCV)
2( )2
iij O
Fσ µ −= − ∆ 2 2( ) ( ) 2O O Fµ µ ϕ− −∆ = ∆ − ∆
2( ) 0Oµ −∆ =
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) - ηC - ηA
(RE+ RC+RA)OCV
ηC
ηA
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
pann
ung
[V]
0.0
0.1
0.2
0.3
0.4
0.5
Leistung [W/cm
2]
(1)
(2)
(3)
(RE+ RC+RA)OCV
ηCηA
cell current I [mA/cm2]cell
volta
geU
(I) [V
] (RE+ RC+RA)OCV
ηCηA
cell current I [mA/cm2]cell
volta
geU
(I) [V
]
1
23
SOFC
( )*
U LR f TI A σ
∆= = =
0 log( )aET kTσσ = −
1. aT vs ET
σ ⇒
Electrical resistance:
Electrical conductivity: U : voltage [V]I : current [A]R : resistivity [ohm]∆L : distance between both
inner wires [cm]A : sample surface [cm2]σ : conductivity [S/m]Ea : activation energy [eV]T : temperature [K]K : Boltzmann constant
How to determine the electrical conductance
Iinput
Um
easured
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 !
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)electrolytecathode (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.
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 µm5-20 µm
15-50 µm
Plasma sprayPlasma 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 hybrid
low sulfur gasoline fuel
25 kW PEM
40 mpg
112 km/h top speed
Figure 9
Fords Adavanced Focus FCV (2002)
fuel cell battery hybrid
85 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 drive
awarded 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, 2013
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