Nov. 20, 2007 Solid Oxide Fuel Cells (SOFC)ndjilali/MECH549/Lecture9_10.pdf · Carbon monoxide (CO)...

University of Victoria Department ofMechanical Engineering IESVic

Nov. 20, 2007

Nov. 20, 2007

Mech 549

Fuel Cell Technology


Nov. 20, 2007

Solid Oxide Fuel Cells (SOFC)

High T Fuel Cells

Operating principles

Overview of Reforming

SOFC Brief History

Components and Materials

SOFC Challenges

Architectures: Tubular; Planar

Performance


Nov. 20, 2007

Low, Medium, & High Temp. FCs


Nov. 20, 2007

High T Fuel Cells

- Expanded Fuelling Options-


Nov. 20, 2007

Efficiency…

• The Ideal Reversible Potential of a Fuel

Cell decreases with increasing T

h

g

r

r=

sThg rrr =


Nov. 20, 2007

-183.1-0.0561-248.8900

-194.2-0.0549-247.6700

-205.0-0.0533-246.2500

-215.4-0.0507-244.5300

-225.2-0.0466-242.6100

rgrsrhT [oC]

sThg rrr =2.5%Change


Nov. 20, 2007

The Efficiency Issue


Nov. 20, 2007

Why Use High T, then ?

• Noble Metal catalyst

not needed

• Better reaction Kinetics higher io

• Fuel Flexibility

• ‘Waste Heat’ can be useful– Reforming

– CHP, CCP

• Bottoming Cycles


Nov. 20, 2007

SOFC Operating Principle

For H2-O2 operation

Anode: H2 + O2- H2O + 2e

-

Cathode: O2 + 2e- O2- O

Overall reaction: H2 + O2 H2O

Carbon monoxide (CO) and hydrocarbons such as methane (CH4) can be

used directly as fuels in SOFCs. In the high temperature environment of

SOFCs steam reforming and water gas shift can take place:

CH4 + H2O 3H2 + CO

CO + H2O H2 + CO2


Nov. 20, 2007

Direct oxidation of CO contained in reformed hydrogen is possible.

• Anode: H2 + O2- H2O + 2e

-

CO + O2- CO2 + 2e-

____________________________________________________________

aH2 + bCO +(a+b)O2- aH2O + bCO2 + 2(a+b)e

-

• Cathode: (a+b)O2 + 2(a+b)e- (a+b)O2-

• Overall cell reaction: (a+b)O2 +aH2 + bCO aH2O + bCO2

e-

e-

~


Nov. 20, 2007

SOFC Applications

Source: Subhas Singhal, 2003

Boston University Talk


Nov. 20, 2007

Reforming Technology

A

EC

O2 AirDepleted Air

CH4, H2O

H2, H2O,CO2,CO

IIR

Indirect Internal

Reforming

A

EC

Fuel

800°C

H2, CO

H2O, CO2

AirDepletedAir

O2 O2

ER

External Reforming

A

EC

CH4,

H2O

H2,H2O,CO2,CO

AirAir

DIR

Direct Internal

Reforming

Technological Evolution


Nov. 20, 2007

Steam Reforming

• CH4 + H20 CO + 3H2 H = 206 kJ/mol

• CnHm + nH20 nCO + (m/2+n)H2

Water Gas Shift

• CO + H2O CO2 + H2 H = -41 kJ/mol


Nov. 20, 2007

Steam Reforming Cont:

• One mole of steam is needed for each mole of

carbon

– More and more steam is needed as higher hydrocarbons

are reformed

• Carbon Dioxide is produced by these reactions.

• These are equilibrium reactions

some CO will always be present in the reformate


Nov. 20, 2007


Nov. 20, 2007

High Temperature Fuel Cells & Reforming:

Thermodynamics


Nov. 20, 2007

Reforming Cont’d

Important points:

– CO is a poison to the noble metal catalysts Fuel for PEM/AFC/PAFC must be treated

– Reforming is an endothermic process, and as such

requires heat input (this heat is less available with a low

temperature system)

– At the higher operating temperatures of SOFC/MCFC,

reforming can be carried out on Nickel catalysts used in

the Anode Internal reforming possible for SOFCs


Nov. 20, 2007

Reforming Cont’d

– In both MCFC and SOFC, water is formed at the

Anode

• Benefit needed for steam reforming

• Detriment H2 partial pressure decreases


Nov. 20, 2007

• Recall the Nernst Equation

• Isolating the effect of a change in H2 partial

pressure

Hydrogen Concentration

+=OH

OHo

P

PP

F

RTEE

2

22

2/1

ln2

=1

2ln

2 P

P

F

RTV


Nov. 20, 2007

Hydrogen Concentration

• As H2 is consumed, and water/CO2 is produced, the

H2 partial pressure drops, and the V term is

negative (P2 < P1)

– Hydrogen Utilization is an important consideration

– This is worse as T

=1

2ln

2 P

P

F

RTV


Nov. 20, 2007

The Efficiency of a Combined Cycle

Hydrogen

Octane

Methane

Fuel

23843079-141,788

22773108-48,254

22273054-55,496

Flame T,

Air [K]

Flame T,

O2

[K]

HHV

[KJ/kg]

ICE’s Reach ~ 1700 K


Nov. 20, 2007

Bottoming Cycles

Heat used to Raise Steam Temperature

which then drives a turbine

The whole system is pressurized, and the

high pressure exhaust drives a gas turbine

Maximum Efficiency gTA/ h

– TA –> Ambient Temperature


Nov. 20, 2007

Bottoming Cycles


Nov. 20, 2007

“Thus a high temperature fuel cell combined with, for

example, a steam turbine is a ‘perfect’

thermodynamic engine. The two components of this

perfect engine have the advantage of practically

attainable technologies. The thermodynamic losses

of the high temperature fuel cell are low, and a

thermal engine can be easily designed to operate at

typical heat source temperatures”

– A. J. Appleby (one of the Gurus of FC Technology)


Nov. 20, 2007

Combined Heat & Power Generation

• Traditional power generation: 60-70% of fuel energy is in

the form of “waste heat”

• Co-generation recovers heat and can achieve overall

efficiencies of up to 80-90%.


Nov. 20, 2007

SOFC: History• In 1897, Nernst made this observation:

“The conductivity of pure oxides rises very slowly with temperatureand remains relatively low, whereas mixtures possess anenormously much greater conductivity, a result in completeagreement with the known behaviour of liquid electrolytes'‘.

• Early 1900s:

Many mixed oxide combinations (at high temperatures) wereidentified, including, 85% zirconia, 15% yttria

• In 1937, Baur & Preis:

reported the operation of first ceramic fuel cell at 1000°C

• 1940s, Davtyan :

Increased conductivity and mechanical strength by adding monazitesand to mix of sodium carbonate, tungsten trioxide, and soda glass


Nov. 20, 2007

SOFC: History• 1950s:

Research by Central Technical Institute (Holland), Consolidation Coal

Company (USA), General Electric (USA)

• 1960s:

Solid electrolyte, 100 W fuel cell

power system

(Westinghouse Res. Lab)

– 20 batteries of 20 cells each

– Open circuit voltage = 200 V

– Current = 1.2 A

– H2/O2 system


Nov. 20, 2007

Viability of high temperature fuel cells due to advances in materials and

engineering and some key operational advantages:

+ solid state system with no liquid electrolyte with its attendant materialcorrosion and electrolyte management problems and with two-phase (gas-

solid) contacts that reduce corrosion

+ operating temperature of ~1000°C allows internal reforming, promotesrapid kinetics circumventing the need for noble metal electrocatalysts, and

produces high quality byproduct heat for cogeneration

– The free energy of formation is less negative at high temperature,resulting in lower OCV (~0.9 V)

– The high temperature of SOFCs is very demanding on materials. Theidentification and development of suitable low cost materials, low cost

manufacturing of ceramic structures and lowering of operating temperature

are the key challenges being addressed currently.

SOFC Technology: Pro’s and Con’s

The solid state nature of all SOFC components provides more freedom for the

cell architecture. Developments have taken place around two distinct

configurations: Tubular cell (Siemens Westinghouse Corporation),and

Flat plate design (AlliedSignal, SOF Co. etc.)

Flat Plate

Tubular


Nov. 20, 2007

SOFC Components

Material constraints

– High temperature (~1000oC)

– Chemical stability in reducing and oxidizing environment

– Contact compatibility

– Conductivity

– Thermomechanical properties capable of sustaining thermal

cycling

Fabrication is currently based on use of thin film concepts where

films of electrode, electrolyte, and interconnect material are deposited

one on another and sintered, forming a cell structure.


Nov. 20, 2007

SOFC Electrolyte

• Material: Yttria-stabilized Zirconia (8-10 mol%

yttria) or YSZ

• Conductivity: 0.1 S/cm at 1000oC

– Low conductivity at lower temperature


Nov. 20, 2007

Solid Oxide Electrolyte Conductivities

Stainless Steel

La (Ca) Cr O3


Nov. 20, 2007

SOFC Electrodes

Main functions are to

a) promote oxygen reduction/fuel oxidation,

b) serve as a current collector.

Electrode material should be electrochemically and thermomechanically

stable and of course ideally inexpensive!

Specific issues for electrode material:

– stability over wide range of partial pressures of O2

– non-reactive with other cell components (during operation or

fabrication)

– low contact resistance with electrolyte layer

– thermal expansion coef. very similar to that of electrolyte layer


Nov. 20, 2007

SOFC Anode

• Material: CERMET Composite of ceramic electrolyte and

Nickel (Metal)

• Porosity: 20-40 %

• Thickness: up to 1.0 mm for anode supported cells

• Fabrication Method: Conventional ceramic processing methods

such as tape casting, screening printing

• Issues: Nickel can sinter at high temperatures leading to reduced

surface area


Nov. 20, 2007

SOFC Cathode

• Material: Lanthanum Strontium Manganate (LSM);

also, composites of or mixtures of electrolyte (YSZ)

and electro-catalyst (LSM)

• Porosity: ??

• Thickness: 20-40 microns

• Fabrication Method: Conventional screening printing, slurry method


Nov. 20, 2007

• Metal cathodes considered in early days. But only noble metals werepractical because of highly oxidizing environment too costly

• Oxide matrix with wire grid metallic conductor: Porous zirconia + Pt (or

Pd)

• Electronically conducing oxide

[Source:Blomen,

Fig. 10.2, p469]


Nov. 20, 2007

Micro-Structure of Porous

Cathode(Source: SOFC Research Group Queen’s University)

20μm


Nov. 20, 2007

Triple Phase Boundary in

Composite Anodes & Cathodes

O2-

e-

Triple-Phase Boundary

e-

O2-

e-

Triple-Phase Boundary

e-

O2-

Particle of pure

electronic conductivityParticle of

mixed conductivity

O2-

O2-


Nov. 20, 2007

Effect of Temperature


Nov. 20, 2007

SOFC Component Issues




Nov. 20, 2007

SOFC - Different Cell Geometries

• Seal-less Tubular Design

• Segmented-cell-in-series Design

• Planar Design

• Monolithic Design

Low power density

High power density


Nov. 20, 2007

Comparative Performance

Cell Performance of Major SOFC Companies

0

200

400

600

800

1000

1200

600 650 700 750 800 850 900 950 1000

Operating Temperature ( oC)

Po

wer

Den

sit

y (

mW

/cm

2)

Westinghouse

CFCL

AlliedSignal

Siemens

Tokyo Gas Co

Juelich

Global 1stgeneration

Global SC-1

Global SC-2

Humidified H2 fuel

Power at 0.7 V


Nov. 20, 2007

Planar Design

Rectangular shape Circular shape


Nov. 20, 2007

F F F

F F F

O

O

O

O

Anode

Cathode

Electrolyte

Interconnect

Bell and Spigot

Configuration

Monolithic

Configuration

Stack Configurations


Nov. 20, 2007

Seal-less Tubular Design

Tubular Westinghouse SOFCs

Tubular Cells

Tubular Cell Manufacturing

Westinghouse Seal-less Tubular Design

Seal-less Tubular Design


Nov. 20, 2007

Single-Cell/ Cell Module/ Stack

Tube Diameter = 2.2 cm


Nov. 20, 2007


Nov. 20, 2007

Siemens Westinghouse SOFC Unit


Nov. 20, 2007

Siemens Westinghouse SOFC System


Nov. 20, 2007

Hybrid System - Fuel Cell + Gas Turbine


Nov. 20, 2007

Demonstration Units




Nov. 20, 2007

Solid Oxide Electrolyte Conductivities

Stainless Steel

La (Ca) Cr O3

Planar SOFCs: microstructure

Single-Cell Manufacturing


Nov. 20, 2007

Planar Stack


Nov. 20, 2007

Cell Performance

Long-term Performance

T= 750C, 556 mA/cm2; 3000 mlpm air; air utilization = 27%

636 mlp H2, 630 mlpm N2, 3% H2O fuel utilization = 54 %


Nov. 20, 2007

Gen.4 Stack Assembly


Nov. 20, 2007

Stack Performance

16-cell stack test data (4 Jan 2003)

10 x10 cm2 (81 cm2 active area)

750 C, 0.21 A/cm2, 60% fuel utilization, 25% air utilization)

Cell Voltage = 0.75 – 0.85 V


Nov. 20, 2007

Global’s RP2 System

Reformer

20-cell stack

Internal Volume = 1.9 m3


Nov. 20, 2007

RP2 Stack Performance


Nov. 20, 2007

Tubular vs. Planar Architectures

RequiredNot necessaryHigh Temperature Seals )

LowHighManufacturing Cost ($/kW)

HighLowVolumetric Power (W/cm3)

High (0.6-2.0)Low (0.2-0.25)Specific Power (W/cm2)

Planar

Cells

Tubular

Cells

Nov. 20, 2007 Solid Oxide Fuel Cells (SOFC)ndjilali/MECH549/Lecture9_10.pdf · Carbon monoxide (CO)...

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