new developments in electrochemical cells

New Developments in Electrochemical Cells

Science Update Programme

Education Bureau, HKSAR &

Department of Chemistry The University of Hong Kong

June 2002

June 2002 Electrochemical Cells, K.Y. Chan, HKU

2

References

Batteries

www.nec-tokin.net

www.duracell.com

Fuel Cells

www.fuelcells.com

chem..hku.hk/~fuelcell

Books:

A.J. Bard, L. Faulkner, “Electrochemical Methods”, 2001, Wiley.

Derek Pletcher and Frank C. Walsh, “Industrial Electrochemistry”, Chapman and Hall, 1990.

C.A. Vincent and B. Scrosati, “Modern Batteries : An Introduction to Electrochemical Power Sources”, Butterworth-Heinemann, 1998.

James Larminie and Andrew Dicks, “Fuel Cell Systems Explained”, Wiley, 2000.

Utilities

www.ifc.com

www.gepower.com

Portable Power Sources

www.nokia.com

www.motorola.com

Capacitors

www.nec-tokin.net

www.faradnet.com

Green Energy

www.greenenergy.org.uk

www.greenenergyohio.org

Electric Vehicles

Evworld.com


3

•Electrochemistry, General Chemistry

•Physical Chemistry:Thermodynamics, Kinetics, Transport

•Organic Chemistry

•Inorganic, Solid State Chemistry

•Materials Science

•Basics Physics, Energy, Electricity

•Environmental Science and Ecological/Biological Issues

Can be discussed with different emphasis, at different levels, and platforms.

Multidisciplinary and Integrated Science


4

1. Fundamental Theories and Concepts

2. Batteries

3. Fuel Cells

4. Applications


5

Fundamentals

Thermodynamics

•Relate Reactivity to Electrode Potential

•Nernst Equation accounts for concentration(activity) effects

•Calculate Electrode Potential from Free Energy

nF

GEEE

oocell

oanode

ocathode

][

[Re]log

0591.0ln

Oxnaa

aa

nF

RTEE

bB

aA

dD

cCo


6

-1.66 -0.76 0.0 0.52 1.23 V

Al/Al+3 Zn/Zn+2 H2/H+ Cu/Cu2+ H2O/O2

Electrochemical Activity Series

nF

GEEE o

celloanode

ocathode

oanodeE

ocathodeE


7

Fundamentals

Kinetics

•Current Rate of reaction (Faraday’s law)

•Rate (current) described by Tafel Equation

oeq consti

nF

RTEE .ln

RT

EEnF

C

C

RT

EEnF

C

Cii

eq

R

Req

O

Oo

)()1(exp

)(exp

**

or Butler-Volmer Equation (Bard and Faulkner, Wiley 2001)


8

Fundamentals

Kinetics

RT

EEnF

C

C

RT

EEnF

C

Cii

eq

R

Req

O

Oo

)()1(exp

)(exp

**

n F E

Free energy

G

Reaction co-ordinate

nF E

R

O + n e-

O*

from Absolute Rate Theory


9

E=Eeq

i

E or

Current into electrolyte

Electrons out of electrode

RT

EEnF

C

C

RT

EEnF

C

Cii

eq

R

Req

O

Oo

)()1(exp

)(exp

**


10

Concentration

or pH effect

i

E


11

i- i

Ecell CathodeAnode

E


12

i- i

Ecell CathodeAnode

E

Ref. electrode

E-Eref


13

Fundamentals

Transport and Interfaces

•Rate of supply of raw materials : diffusion of active materials

• Rate of removal of: products including ions, electrons

ionic vs ohmic resistance

•Change of solid interfaces: dentritic growth

•Wetting/non-wetting affects gas transport into electrolyte

•Selectivity of transport, e.g. cationic membrane


14

concentration

10 M

KOH

H2SO40.6

ohm-1 cm-1

CH3COOH

KCl


15

E

iLim

iLim

i


16

EG

nF

Ideal Voltage

Activation

Ohmic Mass-Transfer

Current Density

Cell

Voltage


17

Open Circuit Voltage

Equilibrium potential, Standard Potential

Overpotential, underpotential

Polarization (activation, ohmic, concentration)

Capacity mA hr

Energy Density W hr kg-1 , W hr l-1

Power Density W kg-1 , W l-1 , W cm-2

Current Density mA cm-2

Some Terminologies


18

•Anode: Oxidation reaction, release electrons to external circuit, negative terminal (galvanic cell)

•Cathode: Reduction reaction, receive electrons from external circuit, positive terminal (galnanic cell)

•Current Collector: continuous electronic conducting solid phase to collect electrons (in anode) and to distribute electrons (in cathode)

•Electrolyte: ionic conducting but electronic insulating, transfer ions from/to electrodes

•Separator: hydrophilic porous sheet material to hold a thin layer of electrolyte, electronic insulation


19

•Polymer Electrolyte: polymeric backbone with fixed charge to allow transport of either cation or anion

•Porous Matrix to hold electrolyte: Ceramic, asbestos, “polymers”.

•Gel/Paste electrolyte: immobilize electrolyte but allow ionic transport

•Molten Salt Electrolyte:e.g. Carbonates

•Solid Oxide Electrolyte: oxide ion mobiliity at elevated temperature

Batteries

A. Volta, 1880


21

Primary Batteries: Zn/C

Alkaline

Zn/HgO

Li metal

Secondary Batteries: Lead Acid

(Rechargeable) Ni-Cd

Ni-MH

Li ionHybrid of Battery and Fuel Cell: Zn-Air

Al-Air

(Regenerative Fuel Cells)


22

Batteries

Zinc/Carbon (Leclanché 1880s)

•Cathode: 2 MnO2 + H2O + 2e- Mn2O3 + 2OH-

•Anode: Zn Zn2+ + 2e-

•Overall: 2 MnO2 + Zn + H2O Mn2O3 + Zn2+ + 2OH-

G=-257 kJ mol-1 , Eo = 1.55 V

• electrolyte: moist NH4Cl/ZnCl2/MnO2/C powder

• current collectors: graphite rod and zinc

•Capacity 6 A hr, energy density 80 Whr kg-1


23

Batteries

Zinc/Carbon (Leclanché 1880s)

Zinc can anode (-ve)

Carbon rod current collector (+ve)

MnO2 based positive paste

separator


24

Batteries

Lead/Acid

•Cathode: PbO2 + 4H+ +SO42- + 2e- 2H2O + PbSO4

•Anode: Pb + SO42- PbSO4 + 2e-

•Overall: PbO2 + Pb + 4H+ + 2SO42- 2PbSO4 + 2H2O

G= -394 kJ mol-1 , Eo = 2.05 V

• electrolyte: aqueous H2SO4

• current collectors: both Pb

•Capacity: 2.7 Ahr, Energy density 30 Whr kg-1

•cell voltage> 1.23 V, Electrolysis of water kinetically hindered

Discharge reactions


25

-0.3505 0.0 1.23 V 1.698

Pb/PbSO4 H2/H+ H2O/O2 PbSO4/PbO2

Possible Electrode Pairs?


26

Batteries

Nickel/Cadmium

•Cathode: 2NiO(OH) + 2H2O + 2e- 2Ni(OH)2 + 2OH-

•Anode: Cd + 2OH- Cd(OH)2 + 2e-

•Overall: 2NiO(OH) + Cd + 2H2O 2Ni(OH)2 + Cd(OH)2

G= -283 kJ mol-1 , Eo = 1.48 V

• electrolyte: aqueous KOH

• current collectors: Ni foam and peforated nickel sheet

•Capacity: 4 Ahr, energy density: 33 Whr kg-1


27

Batteries

Nickel/Metal hydride

•Cathode: NiO(OH) + H2O + e- Ni(OH)2 + OH-

•Anode: MH + OH- M + H2O + 2e-

•Overall: MH + NiO(OH) M + Ni(OH)2

•Metal hydride: AB5 e.g. LaNi5 or AB2, e.g. TiMn2 , ZnMn2

• electrolyte: aqueous KOH

• current collectors: Ni foam and peforated nickel sheet

•Capacity: 4 Ahr, energy density: 80 Whr kg-1


28

Batteries

Nickel/Metal Hydride

Overcharging

•Cathode: 2 OH- H2O + ½O2 + 2e-

•Anode: charge reserve M + H2O + 2e- MH + OH-

•Oxygen dissolves to Anode: 2MH + ½ O2 2M + H2O

Prevent gassing and build up of pressure


29

Batteries

Lithium Ion

•Cathode: xLi+ + LiM2O4 + xe- Li1+xM2O4

M=Mn,Ti

xLi+ + LiMO2 + xe- Li1+xMO2

M=Co, Ni

•Anode: LiC6 x Li+ + x e- + Li1-xC6

•Overall: C6 + LiMO2 LixC6 + Li1-xMO2

• LiMn2O4 G= -287 kJ mol-1 , Eo = 2.97 V

•Energy density > 100 Whr/kg


30

Batteries

Lithium Ion

•Aprotic Solvent

•Gel

•Polymer (lower weight)

Electrolyte

•Li in graphite lattice

•Lower activity but safer than Li metal

Anode

•Solid Structures for storing Li

•Spinels, Olivines, rhombohedral NASICON

Cathode


31

Batteries and Fuel Cells

Batteries Recharge Intermittent Closed system Mostly solid High power density

Fuel Cells ReFuel Continuous Open system Mostly Gas/Liquid Fuel High energy density Micro to Mega Watts


32

Fuel Cells Efficient conversion of Chemical

Energy to useful energy (without losing to heat, mechanical linkages)

Environmentally friendly Flexible: from micro to mega Materials and Nanotechnology


33

Fuel Cells Classification according to electrolyte Alkaline Fue Cells Proton Exchange Membrane (PEM) Phosphoric Acid Molten Carbonate Solid Oxide Electrolyte


34

燃料電池發電的原理

正極 ﹕氧氣 ( 氧化劑 )

負極﹕燃料 ( 氫氣﹐酒精﹐ 葡萄糖等 )

電能

CxHyOz ===> CO2 + H2O + e-

O2 + e- ===> H2O

負極

電解液

正極


35

Fuel Cells

Chemical Energy Electrical Energy

thermal

Work

Heat

G

H

H T S

H


36

EG

nF

Ideal Voltage

Activation

Ohmic Mass-Transfer

Current Density

Cell

Voltage


37

Diversity of Technology andMaterials Problems in Fuel Cells

Fuel Oxidant Catalyst Container Control Transport Storage


38

Fuels: HydrogenMetalsNatural GasSmall Hydrocarbons(methanol, glucose)

Oxidant: airoxygenhalidesoxides

Catalysts: platinummetalsmetal oxidesmacrocycles

Catalyst Support: Porous CarbonCeramic MatrixMolecular SievesPolymer

Container and Movable Parts:AlloysCeramicPolymers

Transport/Electrolyte:Proton Exchange MembranesPTFE (Teflon)Solid Electrolyte

Storage: Metal Hydride


39

Fuels Hydrogen H2+2OH- 2H2O +2e-

2e- +½ O2+H2O 2 OH-

Methanol CH3OH + H2O CO2 + 6H+ +6e-

6e- +1½ O2+6H+ 3H2O

Aluminium Al + 4OH- Al(OH)4- +3e-

4e- +O2+2H2O 4 OH-

Borohydride NaBH4 + 8 OH- NaBO2 + 6H2O + 8e-

Methane (natural gas) Octane : demonstrated in SOFC half cell


40

Thermochemistry

cH (kJmol-1)

cG (kJmol-1)

n E kJ/kg kJ/cm3

Hydrogen - 285 - 237 2 1.23 118500 0.011 Methane - 890 - 818 8 1.06 51125 17.31 Methanol - 726 - 702 6 1.21 21938 17.37 Glucose -2808 -2865 24 1.23 15916 24.57 Octane -5471 -5297 50 1.10 47907 66.10


41

Micro and Nanostructured Electrodes:

Catalyst Support: High Surface Carbon Size Effects of Catalysts Controlled Porosity Controlled Wetting Maxinum Gas-Liquid-Solid Interface Minimize ohmic resistance Minimize ionic resistance


42

Scanning Tunneling Spectroscopy


43

Catalysts

Platinum is the most important for both anode and cathode

Platinum can be replaced by Ag, Mn, Co, only for oxygen reduction in alkaline medium

Platinum subject to CO poisoning (impure H2) Binary/Ternary system, macrocycle,

bifunctional Stability/Life of nanometals


44

01020304050607080901000

20

40

60

Pe

ak C

urre

nt D

en

sity (mA

/cm

2 )

Co atom % in Co/Pt

Maximum peak current density at 52.5~77.6% Co, one order of magnitude higher than that of pure Pt particles. One possible role of cobalt in promoting the catalysis of platinum, is the removal of COad COOHad intermediates.

)2( eHCOOHHCOOH ad

)4(2 eHCOCOOH ad

)3( 22 eHOHCOCOCOOHHCOOH adad

)5(2 eHCOOHCO adad

Chi et al., Catalysis Letters, 71 (2001) 21.


45

Catalysts

Oxygen Cathode is most limiting and is present in most fuel cells

Non-platinum cathode catalyst can tolerant cross over effect.

At high temperature, no precious metal or no catalysts is needed in MCFC and SOFC


46

Performances of different air cathode

-10 0 10 20 30 40 50 60 70-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

10mA/cm2 constant current discharge(186mA CCD)Fuel: 95% ethanol/7M KOH 1:11 Membrane added

Performance Comparision of air cathodesAC-51, AC-65, AC-75

Po

ten

tia

l(V

)H

gO

Time(min.)

AC51KMnO4 AC65Ag AC75Co


47

Gas Diffusion Electrodes

H2H+

e-

Chan et al. , Electrochimica Acta, 32 (1987), 1227;33 (1988) 1767.

Tang and Chan, Electroanal. Chem. 334 (1992) 65.

Electronic circuit: continuous solid phase

Ionic circuit: Continuous electrolyte phase

Materials flow circuit: feed of reactancts


48

Single air cathode


49

Electrolyte Alkaline electrolyte (first deployed

for Apollo mission) Phosphoric Acid 180 C Polymer Electrolyte Cross Over Stability (CO2 removal in alkaline) Solid Oxide (YSZ, doped Ceria) Shunt Current / Leak Current


50

SOFC Electrolyte

Ytrium Stabilized Zirconia Doped Ceria (Cerium Oxide) O2- conductivity at 600~800 C

ZrCe

Ce

Ce

Y Y

O2-

O2-

O2-

O2- O2-

O2-O2-

O2-

O2-

CeCe

Ce Ce

CeCe

Ce


51

Stack Design

Manifold for fuel feed Manifold for oxidant feed Electronic circuit Ionic circuit Water transport Temperature, humidity control


52H+

e-


53

Product Name: Fuel Cell StackFuels usable: Glucose, methanol, ethanol,NaBH4 No. of Fuel Cells: 10 in SerialOpen Circuit Voltage: 4.0-9.0VPower Output: 0.5-1.0WApplication: Stationary or Portable( Mobile phone or toy cars )

Applications Demonstrated:Radio(Voice of Glucose);Portable CD player;Mobile Phone(GSM).

Stack Design


54

Electronic circuit: continuous solid phase with minimum electrical resistance to electronically connect anode and cathode through external circuit.

•Ionic circuit: to complete the other half of the “charge circuit”. Continuous electrolyte phase connecting cathode and anode, but electronic insulating. Maintain balance of ions for anodic, cathodic reactions.

•Materials flow circuit: feed of reactancts to and removal of products from anode/cathode.

•Avoid shunt current, leak current in multiple cells

•Avoid short circuit of cathode and anode

•Avoid breaking electrochemical window of electrolyte


Stationery Power Utilities10~100 kW100~500 kWhrONSY (IFC), Fiji SOFC (Westing House, Honey Well)Load LevellingPower DistributionLife


56


Electric Vehicles10~100 kW

100~500 kWhr

Battery vs Fuel Cells

Hybrid with ICE and capacitor

Costs: 7 times normal costs

Startup time

Direct/Reformer

Fueling Station Infrastructure


58

Transportation Fuel CellBallard Power Systems

1st Generation Fuel Cell Transit Bus2nd Generation Fuel Cell Transit Bus

ChryslerFuel Cell Vehicle Model.

Coval H2 PartenersT-1000 Neighborhood Truck.

Daimler-BenzThe NECAR 3 Three Generations of NECAR VehiclesThe NEBUS

DaimlerChryslerJeep Commander Hybrid Fuel Cell Concept

Energy PartnersThe "Gator" Utility VehicleThe "Genesis" Golf CartThe "Green Car"


59

Ford Motor CompanyThe P2000 Prodigy Hydrogen Fuel Cell VehicleThe P2000 - Platform for a Fuel Cell Vehicle.

General MotorsFuel Cell Engine Model

H Power CorporationFuel Cell Bus50w PEM Fuel CellFuel Cell BicycleFuel Cell Wheelchair

Humboldt State University's Schatz Energy Research Center

The Kewet (Danish 2-Seater)Fuel Cell Golf Carts

International Fuel CellsThe Georgetown University Fuel Cell Bus


60

MazdaThe Fuel Cell DemioThe Demio - On the RoadUnder the Hood of the Demio

OpelThe Fuel Cell SintraThe Fuel Cell Zafira

Siemens AGPEM Fuel Cell Powered Forklift

ToyotaThe Fuel Cell RAV 4

Volkswagen/VolvoThe Fuel Cell Golf (coming soon)

ZevcoThe Fuel Cell Taxi Cab (London)

Updated January 8, 1999


61


Portable Power Sources10~100 kW

100~500 kWhr

Battery vs Fuel Cells

Safety (H2 , MeOH, caustic electrolyte), Open vs Closed System

Volume vs Weight

Refueling Vs Recharging


Special ApplicationsSpace/DefenceCommunicationEnergy Storage for SolarEnergy VectorBiomedicalEnery Recovery from WasteMarine and Remote Power Sources


64

Energy Vector


65

Fuel Cells Running on Biogas from Garbage

(Kajima Co. Japan)

67% CH4

33% CO2140kg/day


66


Demo Fuel Cells0.02 ~ 10 W

H2 , MeOH, Glucose, alcohols, NaBH4

PEM, Alkaline


68

World’s first glucose FC Demonstration Kit(HKU-002, Version 3)


69

Typical Performance of HKU-001

0 50 100 150 200 2500.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

Area of anode: 12cm2Area of cathode: 35cm2

Vo

ltag

e(V

)

Time(min)

Model: FC-001

2g Glucose, 30mL 1M NaOH

new developments in electrochemical cells

Documents

hkuelectrochemical cells

comelectrochemical cells

electrodeelectrochemical

power density w

electrochemical methods

electrochemical power

current density ma

comportable power