Fuel Cells and Hydrogen Polymer electrolyte membrane …wu/mae493/2-fuel cell-2.pdf · O2 + 4H+ +...

Fuel Cells and Hydrogen

MAE 493R/593V- Renewable Energy Devices

http://www.fueleconomy.gov/feg/tech/fuelcell.gif

Outline

• Basics of electrochemistry

• Polymer electrolyte membrane (PEM) fuel cells

• Solid oxide fuel cells (SOFCs)

• Hydrogen production and storage

•Coal-fired plants and integrated gasifier fuel cell (IGFC) systems

1839 Sir William Grove discovers the principle of the fuel cell

1889 Mond and Langer uses air and coal gas reactants andphosphoric acid as electrolyte in a “stack”

1959 Francis Bacon demonstrates an alkaline 5kW system

1960’s NASA begins alkaline fuel cell utilization on space flights -Mercury through current shuttle missions

1970’s UTC 250kW phosphoric acid technology developed and employed

1990’s – Stationary and automotive PEM-based field service Present demos

History of Fuel Cells

4

Polymer Electrolyte Membrane Fuel Cells

(PEMFCs)

5

Fuel Cell

http://www.fueleconomy.gov/feg/tech/fuelcell.gif

Fuel Cell is a device that converts chemical energy of fuel and oxygen into electricity. It usually consists of anode, electrolyte, cathode and interconnect.

Anode side: 2H2 →4H+ + 4e-

Cathode side:O2 + 4H+ + 4e- →2H2O

Net reaction:2H2 + O2 →2H2O

Image from: http://en.wikipedia.org/wiki/File:Fc_diagram_pem.gif

Operating temp: around 80 oC

Cathode ReactionO2 + 4H+ + 4e- 2H2O

Air

Anode

Cathode

e-

Membrane Electrode Assembly

Gas Diffusion Layer

Gas Diffusion Layer

H2

Bipolar plate

Bipolar plate

Load

H+

Anode Reaction2H2 4H+ + 4e-

NIST

PlatinumCatalyst

Carbonblack

Source: Ajay K. Prasad

Configuration of PEM fuel cell

7

Polymer electrolyte membrane fuel cellsPEM fuel cell components:

Electrolyte: Nafion manufactured by DuPontFuel (H2) and oxidant (O2) are separated by the electrolyteOnly ions transports in the electrolyteNo electrons migrate in the electrolyte

Anode and cathode: Carbon supported Pt particles –as the electro-catalysts

Journal of Power Sources 130 (2004) 61–76

8

Polymer electrolyte membrane fuel cells

Source: Chris Yang

- Nafion@

9

Fuel cell thermodynamics

Hydrogen

Energy = Electricity Energy = V·I·tOxygen ejected Heat

Water (byproduct)Fuel cell

The energy conversion of a fuel cell is described as:Chemical energy of fuel = Electrical energy + Heat energy

The fuel cell system can be considered as a control volume

10

The first law of thermodynamics for a control volume:


For a stationary control volume (i.e. the kinetic and potential energies are constant in time, then

Thus, the enthalpy is the difference between the heat and the work involved in a system

ΔH = Q - W


The maximum possible efficiency for an ideal, reversible fuel cell is:

An alternative derivation involves using “Gibbs Free Energy”

consider a fuel cell as an ideal device (reversible), thus behaves as a perfect electrochemical apparatus (Gibbs):“If no changes take place in the cell except during the passage ofcurrent, and all changes which accompany the current can be reversed byreversing the current, the cell may be called a perfect electrochemicalapparatus.”For a reversible process, heat transferred was expressed as:

Q = T ΔS

HSTH

HQW

ΔΔ−Δ

=Δ

−=ΔΗ

= minmax 1η

12

The enthalpy change of reaction (ΔH) captures the notion of energy changes for chemical reactions.

the change in Gibbs energy (ΔG) of reaction discounts the changeof entropy, retaining only the usable energy.

The two are related by the following equation:

ΔG = ΔH −TΔS

where T is the temperature in Kelvin and ΔS is the change in entropy.

Because absolute temperature is always positive and entropy increases with every reaction, the above equation tells us that the change in Gibbs energy is always less than the change in enthalpy, which is precisely what we would expect.


13

The maximum possible efficiency for an ideal, reversible fuel cell is:


HG

ΔΔ

=maxη

For the reaction in the fuel cells:

83.08.2852.237

max ==ΔΔ

=HGη

Thus, the maximum possible efficiency for an ideal, reversible fuel cell is:

14

Fuel cell thermodynamicsThe Nernst equation

ΔG = ΔG° + RT ln QΔG0 for a redox reaction that is reversible

ΔGo = −nFE0

where n is the number of moles of electrons transferred, and F is a constant, the Faraday.1 F = 96,485 C/mol = 96,485 J/V-molΔG = -237.2 kJ/molen=2

The maximum output voltage of an ideal fuel cell:

However, additional energy loosing mechanisms further reduce this voltage.

Activation Polarization: These losses are caused by the slowness of the reaction taking place on the surface of the electrodes. A proportionof the voltage generated is lost in driving the chemical reaction that transfers the electrons.

Ohmic Polarization : The voltage drop due to the resistance to the flow of electrons through the material of the electrodes. This loss varieslinearly with current density.

Concentration Polarization : Losses that result from the change in concentration of the reactants at the surface of the electrodes as the fuel is used.

Actual Efficiency of PEM fuel cells: ~ 0.5

Energy loss in the PEM fuel cells:

Therefore,

PEM Fuel Cells

16

The actual output voltage:

Rayment and Sherwin, Introduction to Fuel Cell Technology

PEM Fuel Cells

17

The actual output voltage:

Maximum output at the ideal, reversible case

Ohmic drop

Activation loss

Mass diffusion loss

Rayment and Sherwin, Introduction to Fuel Cell Technology

PEM Fuel Cells PEM fuel cell stack

PEM fuel cell stack

3kW, 48Vfuelcellstore.com

PEM fuel cell stack

A small stack of about 10 cells

NMSEA

NREL

21

Comparison of two engines for vehicles

• Only <33% efficient at • best 80-90% efficient• Air emissions • Zero direct emissions• Peaky torque-rpm curve • Broad torque-rpm curve(needs a transmission) (does not need a transmission)• Power loss in idle • No idle• Irreversible energy conversion • Regenerative braking• Big and heavy • Small and light(250 hp in 600 lbs = 0.7 kW/kg) (75 kW in 13 kg = 5.8 kW/kg)• Noisy • Quiet

Internal-combustion engine Electric motor

why don’t we have electric motors in our automobiles today?We do not have an electric power supply source

From Benoit R Roisin

PEM fuel cell is the option for supply electric power in a vehicle

Fuel Cell VehiclesSeries Hybrid:•Small fuel cell, large battery bankBattery drives motor at all times•Fuel cell operates continuously and keeps battery charged•Fuel cell is not “load-following”

Parallel Hybrid:•Large fuel cell, small battery bank•Fuel cell drives motor at all times, it is “load-following”•Battery provides boost power as and when required

Ajay K. Prasad

Engine: 100 kW, 57 liters, 148 lbFuel economy: 74 mpgRange: 280 milesH2 Storage: 4.1 kg at 5000 psi Ajay K. Prasad

Fuel Cell Vehicles

24

Vehicles Driven by Ideal EnginesThe Carnot heat engine is the most efficient of all heat engines operatingbetween the same high- and low-temperature reservoirs.

The maximum efficiency of Carnot heat engine

Therefore, a Carnot engine would have to have a high temperatureof 1753 K, with a corresponding low temperature of 298 K, to achieve an efficiency of 83%, which is comparable to the maximum efficiency of fuel cells.

Why Fuel Cells?

PEMFC combined with motor: 50%Combustion engine: 20‐28%

Vehicles driven by actual Engines

– Unburned hydrocarbons• Ozone formation

– CO (incomplete combustion)– NOx (both from fuel and air

combustion)– SOx (from sulfur in fuel)– Particulates

• Sulfate aerosols, carbonanceous

Comparison of two engines for vehicles

Internal-combustion engine PEM fuel cell

Water is the only emission and exhaust product

Environmental Impacts

Fuel cell/internal combustion engine hybrid vehicles

Hybrid increase the fuel economy and horsepower

ToshibaManhattan Scientifics Smart Fuel Cells Ballard Power Systems

Portable

Military applications

PEM fuel cell applications

PEM fuel cell applications

30

Fuel CellsSummary of fuel cell value points:• Reduced weight

• Extended run times

• Reduced size

• Lower cost of ownership

• Greater efficiency

• Reduced emissions

• Low noise level

• Lower heat signatures

31

PEM Fuel Cells

Advantages of PEM Fuel Cells:

• Higher Power density→ Fewer cells needed in stack

• No electrolyte corrosion or safety concerns

• Lower operating temperature, 50 ~120 oC→ In a car, it allows for instant start-up.

Challenges for PEMFC ApplicationsLack of H2 fuel infrastructure

H2 safety concern

Complex bulky H2 storage system (DOE requirement; 6.5 wt%)High-pressure containers (4 wt%)

High cost: NEED to obtain higher catalytic activity than the standard carbon-supported Pt particle catalysts

CO poisoning of Pt catalysts

Hot research fields:Hydrogen storage in carbon nanotubes 1-8 wt% reversible storageHydrogen storage by metal hydrides;LaNi5H6 (1.37 m%, 2 bar, 298k)6-7 Mg2NiH4 (3.59 m%, 1 bar, 555k) ZrV2H5.5 (3.01 m%, 10-8 bar, 323K)Low Pt catalyst loading

Pow er Density (W/kg)10 100 1000

Ener

gy D

ensi

ty (W

h/kg

)

1

10

100

100010 h 1 h

0.1 h

0.01 h

Fuel Cells

Lead-AcidBattery

NiCdBattery

Li-IonBattery

Li Battery

NiMHBattery

Highest energy density in all batteries

Fords Advanced Focus FCV fuel cell battery hybrid

High cost of fuel cells

10

100

1,000

10,000

100,000

2000 2010 2020

Sys

tem

Cos

t ($/

kW)

Innovators / Early Adopters

Backup & Standby

Remote Stationary Stationary Mass

Markets

Automotive

Peak Shaving

Residential CHP

Fuel cell cars are not ready yet

Fuel cell/battery hybrid systems for EV/HEV/PHEV

35

Fuel Cells

Fuel Cell Types:• Solid Oxide Fuel Cell (SOFC)

• Molten Carbonate (MCFC)

• Phosphoric Acid (PAFC)

•Proton Exchange Membrane Fuel Cell (PEMFC)

• Alkaline Fuel Cell (AFC)

Source: U.S. Fuel Cell Council

36

Fuel Cells

37

Solid Oxide Fuel Cells(SOFCs)

38

Solid Oxide Fuel Cells

High efficiency: Theoretical efficiency reaches 70%. Currently 51% (Mitisubishi) and 54% (Kyocea) have reported as compared to 36 % for gas turbines.

Utilization of ejected heat: The high operating temperature makeit utilize the high-temp steam, which could be coupled to gas turbines system. The SOFC/gas turbine hybrid system can achieve an efficiency of 70%.

Fuel flexibility: directly reform hydrocarbon fuels at the anodeside.

No precious metal

Advantages of SOFCs:

39

OHOH 222 22 →+

2224 22 COOHOCH +→+

2242 284 COOHCHeO +→+−−

OHHeO 222 2242 →+−−

−→+ 22 24 OeO

Operating Mechanism of SOFCCathode reaction

Anode reaction

Overall reaction

Source: cool fuel cells by NASA

Overall reaction

Anode reaction

−→+ 22 24 OeO

Cathode reaction

40

Operating Mechanism of SOFC

YSZ: yttria-stablized zirconiaSOFC animation: http://people.bu.edu/pzink/sofc-1.html

41

SOFC ConfigurationSOFC Configuration

Most popular

Anode-Supported SOFC

Micrograph of anode-supported thin electrolyte cell structure

On either side of the dense electrolyte is a fine-structured “functional layer” for the electrocatalytic promotion of the electrode reactions

electrolyte

cathode

Anode

SOFC Testing Rig SOFC Testing

SOFC Testing

46

Key Elements in SOFCs

• Good ion conduction in the electrolyte

• Efficient electrocatalysts at the anode and the cathode

• High triple-phase interfacial contact

47

SOFC Electrolyte Materials

Requirements of electrolyte• Stability in both reducing and oxidizing environments

• No chemical reaction with the electrodes

• High ionic conductivity and negligible electronic conductivity at operating temperature

• High mechanical strength

• Coefficient of thermal expansion (CTE) that matches to the electrode materials

48

Fluorite Structure of Electrolyte

Paul Bristowe

Fluorite Structure

Paul Bristowe50

Phase Diagram of ZrO2

Fluorite Structure –Yttria Stabilized Zirconia (YSZ)

Pure ZrO2: monoclinic at room temperature, tetragonal above 1170 oC and cubic fluorite above 2370 oC.

8 at.% Y2O3 can stabilize the fluorite structure down to room temperature.

M. Yashima, et al, solid state ionic, 86/88 (1993), 1131

Ionic Conductivity of Doped ZrO2

Sc doped ZrO2 has the highest ionic conductivity because Sc3+ has similar ionic radius with Zr4+. But it has the problem of ageing

••++→ oxoZr VOYZrOOY 32)( '

232

Conductivity of ZrO2-M2O3 at 1000 oCY. Arachi, et al, solid state ionic, 121 (1999),133

52

Temperature Dependence of Electrical Conductivity for Oxide Ion Conductors

Ionic Conductivity of ElectrolyteDoped ZrO2 is widely used as SOFC electrolyte because of its stability in both reducing and oxidizing environments and compatibility with other components of fuel cell

Other oxides have disadvantages such as electronic conductivity, high cost, or difficulties in processing.

The electronic conductivity is negligible at the normal SOFC operating range of 0.21-10-20 atm

53

Properties of Doped Zirconia

S.C. Singhal, K. Kendall, Book, “High Temperature Solid Oxide Fuel Cells”

54

Fluorite Structure of Electrolyte

The electronic conductivity is negligible at the normal SOFC operating range of 0.21-10-20 atm W. Weppner, J. Solid State Chem, 20 (1977), 305

55

Fluorite Structure -- Doped Ceria

Concentration dependence of electrical conductivity for CeO2-Sm2O3

Total electrical conductivity of Ce0.8Sm0.2O1.9-δ

H. Yahiro, et al, J. Appl. Electrochem, 18 (1988),527

500 oC

900 oC


56

Perovskite Phase

A cations

B cations

Oxygen anions

Formula: ABO3

A site: Ca2+, Sr2+, Ba2+, La3+, Pr3+, Nb3+

B site: Ga3+, Mg2+, Ti3+, V3+, Cr3+, Mn3+, Fe3+, Co3+,Ni3+, Ti4+, Mn4+

57

Perovskite-Structured Electrolyte Lanthanum Gallate, LaGaO3

Effect of alkaline earth cation in La site of La0.9M0.1GaO3

Electrical conductivity of the LaGaO3-based oxide doped with transitional metal cations at Ga site

The highest ionic conductivity of doped LaGaO3 was found at composition of La0.8Sr0.2Ga0.85Mg0.15O3

T. Ishihara, et al, JACS, 116 (1994), 3801 S.C. Singhal, K. Kendall, Book, “High Temperature Solid Oxide Fuel Cells”

58

Conductivity: doped LaGaO3 > doped ceria >YSZ

Disadvantages

Weaker mechanical strength of LaGaO3 than YSZ

Electronic conductivity under low Po2 because of the partial reduction of Ce4+ ions

LaGaO3 can be a mixed proton/oxide ion conductor

Lanthanum Gallate Electrolyte

59

Anode of SOFC

Requirements for anode

• High electrical conductivity• High catalytic activity for fuel oxidation• Close CTE match with the electrolyte• Stability in reducing environment at high temperature• Chemical compatibility with the other cell components• Corrosion resistance to the fuel and impurities

Fabrication of anode cermets

Nickel distributionOverall morphology Zirconia skeleton

Typical anode material: Ni + YSZ

Where YSZ stands for yttria-stabilized zirconia


61

Anode Reaction & Materials

Triple Phase Boundary (TPB)

62

YSZ

Ni

Anode reaction

OHHeO 222 2242 →+−−

2242 284 COOHCHeO +→+−−

Necessary conditions• Catalytic properties

• Ionic conductivity

• Electronic conductivity

• Porosity

Anode materialsPt, Au (peeling off problem)Ni (aggregation)Ni+YSZ, sensitive to S impurity

Anode Reaction & Materials


Anodes for Hydrocarbon Fuels



65


Current-potential characteristics of copper cermets at 800 oC showing effect of hydrocarbon fuel and electrocatalysis by ceriaTriangle: CeriaCircle: Cu/YSZ


W. Thiefelder , et al, in Book “Solid Oxide Fuel Cells V”, eds. U. Stimming et al

Stability of ceria-catalyzed copper cement under different fuel conditions contrasts with rapid irreversible deactivation of nickel cement


W. Thiefelder , et al, in Book “Solid Oxide Fuel Cells V”, eds. U. Stimming et al

Cathode of SOFC

Requirements for cathode material

• High electrical conductivity

• High catalytic activity for oxygen reduction

• Close CTE match with electrolyte

• Stability in oxidizing environment at high temperature

• Chemical compatibility with other cell components

Cathode Materials

343

123

13 OMnMnSrLaLaMnO xxxxxSrO ++

−++

−⎯⎯→⎯Typical cathode materials• Doped LaMnO3 Doped LaCoO3, LaFeO3 Doped SmCoO3

70

Cathode Materials

Source: Georgia Tech

71

Cathode Materials


Cathode Materials


Mechanism of Cathode ReactionMechanism of Cathode Reaction

LSM

YSZ

O2-

O2-

O2-

Air(O2)xOVeO 0

..2 224

0→++ −

)(,)(221

LSMsadsLSM OsO ⇔+

)(,)(, TPBsadsLSMsads OO ⇔

)()(..

)()(, 2 TPBTPBxoTPBTPBsads sOeVoO +⇔++

..)()(

..)()( TPB

xYSZoYSZTPB

xo VoOVoO +⇔+

)(,)(221

LSMsadsLSM OsO ⇔+

sOeVoO LSMxoLSMLSMsads +⇔++ )(

..)()(, 2

La0.9Sr0.1MnO3

74

Cathode of SOFC

Interconnect for SOFC

interconnectcathodeelectrolyteanode

Requirements of Interconnect

• Stability in both oxidizing and reducing environments• Chemical compatibility with other cell components

• High electronic conductivity and negligible ionic conductivity

• High density/negligible gas leakage

• Close Coefficient of thermal expansion (CTE) match to other cell elements

Ceramic InterconnectSo far, only LaCrO3 is found to satisfy the requirements at high temperature from 850-1000 C.

Thermal Expansion of Doped LaCrO3

Close CTE match with YSZCTE increase with increasing doping contentDifferent CTE in reducing and oxidizing environment, which causes warping of the material.


AdvantagesRelatively high electronic conductivityStability at high temperaturesClose CTE to that of YSZ electrolyteGood compatibility with cathode and anode

DisadvantagesDifferent CTE in oxidizing and reducing environmentsHigh material and fabrication costLow thermal conductivity

Ceramic Material - LaCrO3

Metallic Interconnect for Intermediate Temperature SOFC

Alloys which have a self-protection mechanism can satisfy the requirements. Only alloys containing Al, Si or Cr have this property.Al and Si---oxides have too low electrical conductivityOnly Cr-containing alloys can be used for this purpose.

Compared to ceramic materials, metals have the following advantages:

High thermal conductivity

High electronic conductivity

Ease to fabricate and machine---low cost

Ferritic steels are intensively studied because of its suitable CTE.

Problems of metallic interconnectOxidationPoisoning of cathode

Effect of different surface coating on the oxidation rate at 800 C

Comparison of ASR with different surface coating as a function of temperature

0

20

40

60

80

100

120

(La,Sr)CrO3coating

AISI 446 HNA DIN ODS

Red

uctio

n of

Cr

evap

orat

ion LaCrO3 Coating or Spinel

Phase Formation on Alloy Surfaces will significantly reduce Cr evaporation

Coating on metallic interconnect

Two approaches are used to form a thin layer of oxide:

Alloy design

Coating

Synthesizing LaCrO3 Coatings

Step 1: Formation of Dense, Adherent Cr2O3TemplateHydrogen-Low Oxygen Partial Pressure Anneal

Step 2: Deposition of La2O3 Layer-Physical Vapor Deposition

Step 3: Reactive Formation of LaCrO3Post-Deposition Anneal in Air

FerriticSteel

Cr2O3

Cr2O3

LaCrO3La2O3

FerriticSteelFerriticSteel

2μmAfter interdiffusion

(1000°Cx1hr)

La-O Layer

Cr2O3 Layer

444 Stainless Substrate

LaCrO3 Layer

444 Stainless Substrate

Kirkendall Voids

As-sputtered

• A LaCrO3 layer was formed after annealing of the two layers and some Kirkendall voids were present at the initial interface of the two layers due to interdiffusion

Synthesizing LaCrO3 Coatings Dip Coating

85

SOFC ConfigurationSOFC Configuration

TubularPlanar

Tubular configuration Planar configuration

http://www.msm.cam.ac.uk/doitpoms/tlplib/fuel-cells/high_temp_sofc.php

86

Typical Component Material

Interconnect …….... Doped- LaCrO3 or Cr AlloysAir Flow

RepeatingUnit

Fuel Flow

Current FlowEnd Plate

Anode …….......... Ni-YSZ CermetElectrolyte …….. YSZ

Cathode ………... LaMnO3

Solid Oxide Fuel Cell Structure

87

http://people.bu.edu/upal/SOFC_02.htm

SOFC for Stationary Power

UTC Fuel Cells: 5kW fuel cell power plants for backup power for telecommunications towers, power for small businesses, and residential use.

UTC Fuel Cells: (PureCell™ 200) 200kW of electricity and 900,000 BTUs of usable heat. This system provides power at locations including a New York City police station, a major postal facility in Alaska, a credit-card processing system facility in Nebraska, and a science center in Japan.

SOFC/Gas Turbine Hybrids


SOFC/Gas Turbine Hybrids• Operational Basics

– Air stream to SOFC pressurized by compressor and preheated by heat exchanger

– High temperature SOFC exhaust expanded through turbine for powergeneration

– Combustion of unutilized fuel in exhaust can boost power produced by turbine

Generator

M

Stack

Fuel

Air

M

Compressor Turbine

CompressedAir

PressurizedPreheated Air

Fuel CellExhaust and

Unutilized Fuel

ExhaustGases

PressurizedCombustion

Products

ExpandedCombustion

Products

HeatExchangers

Startup/PostCombustor

Steam

Reformeror

Gasifier

Ano

de

Elec

troly

te

Cat

hode

PowerConditionerM


SOFC/Gas Turbine Hybrids

Generator

M

Stack

Fuel

Air

M

Compressor Turbine

CompressedAir

PressurizedPreheated Air

Fuel CellExhaust and

Unutilized Fuel

ExhaustGases

PressurizedCombustion

Products

ExpandedCombustion

Products

HeatExchangers

Startup/PostCombustor

Steam

Reformeror

Gasifier

Ano

de

Elec

trol

yte

Cat

hode

PowerConditionerM

• Benefits– High efficiency (η > 60%)

• Common combined cycle plants η ~ 50% maximum– Lowered emissions for criteria pollutants– Depending on fuel carbon dioxide can be eliminated or at least sequestered


92

Solid Oxide Fuel CellsEfficiency of SOFCs for stationary power generation:

Source: Rodger McKain

93

Solid Oxide Fuel CellsDisadvantage of high-temp SOFCs:

• Material costs are high, particularly for interconnect and construction materials. For high operating temperature SOFC (> 850 oC) the interconnect may be a ceramic such as lanthanum chromite. The interconnect represents a major proportion of the cost of the stack. Stack construction materials and balance of plant also need to be refractory enough.

• The use of chromium containing ceramics and alloys is the volatility of the material, leading to contamination of the stack components.

• High temperature cause the thermal instability of electrode/electrolyte interface.

• High temperature shorten the lifetime of SOFCs

94

40,000 h of service for stationary fuel cell applications

5,000 h for transportation systems (fuel cell vehicles)

factory cost of $400/kW for a 10 kW coal-based system

overall degradation of 4.0% per 1,000 hs

DOE target of R&D:


95

SOFC market:

Global Solid Oxide Fuel Cells (SOFCs) Market to Reach $240.6 Million by 2015, According to New Report by Global Industry Analysts, Inc.


96

ResourcesOne website that we recommend is:http://www.fuelcells.org/ced/education.html

Fuel Cells and Hydrogen Polymer electrolyte membrane …wu/mae493/2-fuel cell-2.pdf · O2 + 4H+ +...

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