Solid Oxide Fuel Cells

Rodger McKain, PhD

Ion transport observed by William Grove in 1839…Based on hydrogen-oxygen, sulfuric acid electrolyte, and platinum electrodes

“I cannot but regard the experiment as an important one…”

William Grove to Michael Faraday October 22, 1842

Fuel Cell

– An energy conversion device that directly converts chemical energy into electrical energy (dc power).

– Analogous operation to a natural gas fueled electric generator: energy in fuel and oxygen are converted to electric power as long as fuel and air are supplied.

– Six types, each suited for specific applications +

Heat, H2O

Fuel Cell Types

Source: U.S. Fuel Cell Council

Incr

easi

ng

Tem

per

atu

re

Attributes of Fuel Cells

AFC PACF AFC PACF PEM MCFC PEM MCFC SOFCSOFC

ElectrolyteElectrolyte KOH KOH Phosphoric Phosphoric SulfonicSulfonic Molten Molten YY22OO33-ZrO-ZrO22 AcidAcid Acid Acid Carbonate Carbonate CeramicCeramic

Polymer SaltPolymer Salt

TemperatureTemperature 10010000CC 200 20000CC 100 10000CC 650 65000CC 800-1000800-100000CC FuelFuel H H22 H H22 H H22 H H22/CO/CO H H22/CO/CO

Efficiency (HEfficiency (H22 fuel) fuel) 60% 60% 55% 55% 60% 60% 55% 55% 55% 55%

(NG fuel) -- (NG fuel) -- 40% 40% 35% 35% 50% 50% 50% 50%

PollutionPollution Very lowVery low Very low Very low Very lowVery low Low Low Low Low

HydrocarbonHydrocarbon No No Difficult Difficult Difficult Difficult Yes Yes Yes YesFuel UseFuel Use

Start-UpStart-Up Fast Fast Moderate Moderate Fast Slow Fast Slow Slow Slow

Fuel Cell Stacks

• Operating voltage of a single cell is ~0.7 volts• Cells are “stacked” in series to increase voltage

to useful levels:

Source: U.S. Fuel Cell Council

Fuel Cell Power System

Fuel cell StackFuel cell StackSub AssemblySub Assembly

Useful heat

AirAirAirAir

FuelFuelFuelFuel10 kW

HeatHeat

ManagementManagement

PowerPowerConditionerConditioner

FuelFuelProcessorProcessor

ControlsControls

High Efficiency

High Efficiency at Part Load

Contaminant

Average U.S. Utility

Emissions(lbs per megawatt-hour)

ONSI PC25 200 kW NG Fuel Cell

(lbs per megawatt-hour)

Nitrogen Oxides 7.65 0.016

Carbon monoxide 0.34 0.023

Reactive organic gases

0.34 0.0004

Sulfur oxides 16.1 0

Particulates (PM10) 0.46 0

Low Emissions

Solid Oxide Fuel Cells

• Based upon ion conductivity of certain ceramic materials at elevated temperatures (>600 C)– First observed by Nernst in 1890’s – Fluorite Structures (e.g. yttria stabilized zirconia)– Face Centered cubic arrangement– Transport through crystal lattice vacancies and oxide ions

located between crystal faces – First SOFC constructed in 1937 by Baur and Preis

• Requires porous electrodes and dense electrolyte, low electronic conductivity, and high strength

Pt Ink

Fuel/CH4

Effluent

CH4 + CO2 2CO + 2H2

CH4 + H2O CO2 + 3H2

CO + H2O CO2 + H2

CH4 + 0.5 O2 CO + 2H2

O2-

CH4 + 3O2- CO2 + H2O + 2e-

Electrolyte Disc

Cathode catalyst layer

Anode catalyst layer

Pt Wire

Yttrium-stablized Zirconia (>950 °C)Galladium-doped Ceria (>600°C)

O2 + 4e- 2O2-

A Oad Products T (°C)

CH4 Oad CO, H2, CO2, H2O 600-1200

CnH2n Oad CnH2nO, CO2, H2O

C Oad CO2 550-950

vA

RL

Gasification

SOFCThermal integrated

Reformer

CO SelectiveOxidation

Shift ReactionH2/CO

Conversion toH2/CO

S-removal

HC-basedSOFC

Coal-basedSOFC

Gas Cleaning

PAFC(CO<5 %)

PEMFC(CO<10 ppm)

Solid FuelCoal, Pet Coke

Liquid Fuel

Natural Gas

Fuel Cell TypesUnder development

500 to 1000 oC

600 oC

500 to 1000 oC

500 to 1000 oC

200 oC

80 oC

Hydrogen

Ref: N.F. Brandon, S. Skinner, B.C.H. Steele, Annu. Rev. Mater. Res. 2003. 33:183-213

MCFCThermal Intergrated

Reformer

Relationship between fuel processing and fuel cells

Basis for Fuel Cell Operation

• Electron transfer – chemical reaction– Voltage determined by difference in chemical

potential of fuel and oxygen– Current determined by area of cell

• Catalyzed conversion of oxygen and hydrogen into reactive species O= and H– H2 + O2 = H2O + 2 electrons + heat

• Electrons are separated from reactants by circuit• Need to understand electrical circuit background as it

relates to fuel cell

Current is the flow of electrons

Electric terms

Volts

6,240,000,000,000,000,000 electrons / sec = 1 amp

ResistanceIf h is 1 volt and current is 1 amp

Resistance is 1 ohm

Copper wire, 1/16” diameter,10 amps, electrons travel 1 cmIn 28 seconds.

Fuel CellStack

Low resistance

High resistance

What’s a watt?

Work involves height liftedand weight of ball, ft-lbs

Work has no time limit, power does Power = (height lifted times weightof ball) times (balls per second), orPower = voltage times current,Watts = volts times amps

550 ft-lbs/sec = 1 horsepower = 746 watts

Energy flow

Same story for electric systemFood anode, Air cathode

Stack produces power and heat

In a perfect system all the energy inthe food would be converted to power.Actually, only part is converted whichdefines the efficiency.

Food

Air Work,power

Heat

Heat

All the energy in the food eventually appears as heat.

V-I scan

Balls lifted per hour, or amps (I)

0 10 20 30 40 500

2

4

6

8

10

Height liftedor volts (V)

ASR is the slopeof the dashed red line

V-I scan

Balls lifted per hour, or amps (I)

0 5 10 15 20 250

2

4

6

8

10

Height liftedor volts (V)

1 of these = 2 of these!

Micro view - Electric

++

ViaVia

AnodeAnode

Electrolyte*Electrolyte*

CathodeCathode

e e --

e e --

e e --

e e --

e e -- e e --

OO==OO== OO==

e e -- e e --

Air layer #1Air layer #1

Fuel layer #1Fuel layer #1

PorousPorous

½½ O O22 + 2e + 2e-- = O = O==e e --

OO== + H + H22 = 2e = 2e-- +H +H22OO

e e --

--

OO== OO==

IconIcon

Bond LayerBond Layer

Bond LayerBond Layer

e e --e e --e e --

e e --e e --e e --

*A nonmetallic electric conductor in which current is carried by the movement of ions.

Fuel utilizationAir Stoics

Complete micro view

Air Flow, OAir Flow, O22 + +

NN22

NN22

ViaVia

AnodeAnode

ElectrolyteElectrolyte

CathodeCathode

e e --

e e --

e e --

e e --

e e -- e e --

OO==OO== OO==

e e -- e e --

Air layer #1Air layer #1

Fuel layer #1Fuel layer #1

Fuel Flow, HFuel Flow, H22O+CO O+CO H H22+CO+CO22

NN22

NN22

NN22

NN22NN22

NN22

NN22

NN22

NN22

NN22NN22

NN22

NN22

NN22 NN22 NN22

NN22

NN22

OO22 OO22

PorousPorous

½½ O O22 + 2e + 2e-- = O = O==

OO== + H + H22 = 2e = 2e-- +H +H22OO

e e --

HH22HH22OO

COCO22

COCO

COCO22 COCO22

COCO22COCO22

COCO22COCO

COCO COCO

COCO

COCO

++

--

COCO

OO== OO==

IconIcon

Bond LayerBond Layer

Bond LayerBond Layer

Co-flow Design Concept – Unit Cell

Air flow Fuel flowCell

Multi-layerceramic construction

Vias carrycurrent

InterconnectInk “bumps” printed on viasSealant

Thermocouples,Voltage taps

Add a cell

Thermocouples,Voltage taps

Manifold arrangement

Fuel inletsAir inlets

GasketManifold

100Energy Units

IC Engine40%

Power Train37.5% 15

6020

Idling5

Friction

40

40Energy Units

Fuel Cell50%

Direct Drive75% 15

200

Idling5

Friction

20

Vehicle ICE vs. Fuel Cell Direct Drive Efficiency Comparison

Summary• Fuel Cells have been around a long time• They present the potential to be highly efficient

because of direct conversion of chemical energy to electrical energy

• Solid oxide fuel cells are based upon ion conducting properties of ceramic materials like doped zirconia

• Temperatures above 600 C are required for operation

• To be viable fuel cells must have high power per area, and operate with low cost materials