ELECTROCHEMICAL SYSTEMS FOR ELECTRIC

POWER GENERATION

Dzmitry Malevich

Depatrment of Chemistry and Biochemistry

University of Guelph

Electric power conversion in electrochemistry

Chemical Reactions

Electric Power

Electrolysis / Power consumption

Electrochemical battery / Power generation

Volta’s battery (1800)

Alessandro Volta 1745 - 1827

Paper moisturized with NaCl solution

Cu

Zn

Me1n+ SO4

2- Me2n+ SO4

2-

Me2

Me20 - ne- = Me2

n+

ANODE

Me2n+ - ne- = Me2

0

CATHODE

Salt Bridge

Me1

Principles of power generation in the electrochemical systems

IMPORTANT NOTICE !

Electrolysis

System consumes energy

G>0

ANODE +

CATHODE -

Battery

System releases energy

G<0

ANODE -

CATHODE +(oxidation process) (oxidation process)

(reduction process) (reduction process)

Me1n+ SO4

2- Me2n+ SO4

2-

Me2

Me20 - ne- = Me2

n+

ANODE

Me2n+ - ne- = Me2

0

CATHODE

Me1 Diaphragm

Membraneor

Principles of power generation in the electrochemical systems

Anode: Zn Zn2+ + 2e-

Cathode: 2MnO2 + 2H2O +2e- 2MnOOH + 2OH-

Electrolyte: Zn2+ 2NH4Cl +2OH- Zn(NH3)Cl2 + 2H2O

2MnO2 + Zn + 2NH4Cl 2MnOOH + Zn(NH3)Cl2

Modern Zinc-Manganese

battery

Zn-container

Carbon rod

MnO2 paste (cathode)

Gas space

Gel electrolyte

Georges Leclanché (1839-1882)

Primary batteries

Leclanché’s battery (1866)

Seal

Zn-container

MnO2 paste (cathode)

Carbon rod

NH4OH electrolyte

Primary batteries

Zinc-Air battery

Anode: Zn + 2OH- - 2e- Zn(OH)2

Cathode: 1/2 O2 + H2O + 2e- Zn(OH)2

Anode: Zn + 2OH- - 2e- Zn(OH)2

Cathode: MnO2 + H2O +1e- MnOOH + OH- aaaaaaaaa MnOOH + H2O +e- Mn(OH)2 + OH-

MnO2 paste (cathode)

Gel electrolytePorous Zn (anode)

Zinc-Manganese alkaline battery

Lead-acid batteryLead-acid battery

Lead paste in Pb-mesh (anode)

Lead dioxide paste in Pb-mesh (cathode)

Porous separator

Safety valve

Secondary (rechargeable) batteries

Pb PbO2

E=2.06 V

36% H2SO4

PbSO4 PbSO4

discharge

chargePbSO4+H2O

PbO2+(2H++SO42-)+2H++2e-

discharge

PbSO4+ 2H+

Pb+(2H++SO42-)-2e-

charge

PbO2 + Pb + H2SO4 2PbSO4 + 2H2Odischarge

Secondary (rechargeable) batteries

Lithium-ion battery

Discharge

Charge

Cathode (LiMexOy)

LiCoO2 -utilized for commercial batteries

LiNiO2, LiMn2O4-prospective

Anode (CLix)

Cathode:

LiMeO2 - xe- Li1-xMeO2 + xLi+

Anode:

C + xLi+ + xe- CLix

CHARGE

DISCHARGE

CHARGE

DISCHARGE

Separator

Aluminum can

Positive terminal

Negative terminal

Secondary (rechargeable) batteries

Nickel-Metal Hydride battery

Cathode:

NiOOH + H2O - e- Ni(OH)2 + OH-

Anode:

Me + OH- + e- Me + H2O

CHARGE

DISCHARGE

CHARGE

DISCHARGE

Picture from: T. Takamura / Solid State Ionics 152-153(2002)19

Types of the electrochemical system for electric power generation

Primary batteries

POWER

Fuel cells

Reaction products (exhaust)

Reductant (fuel)

Oxidant

POWER

Secondary batteries

Recharge

POWER

POWER

Sir William Grove 1811–1896

Grove’s fuel cell (1839)

O2 H2

4H+ + 4e- 2H2

2H2O - 4e- O2 + 4H+

Electrolyte frame Bipolar plate

Fuel Cells performance improving

Raising the current:

• Increasing the temperature

• Increasing the area of eelectrode electrolyte interface

• The use of catalyst

Raising the voltage:

ANODE

CATHODEELECTROLYTE

ANODE

CATHODEELECTROLYTE

ANODE

CATHODEELECTROLYTE

ANODE

CATHODEELECTROLYTE

Connection of cells in seriesCell stack

ANODE

CATHODEELECTROLYTE

ANODE

CATHODEELECTROLYTE

ANODE

CATHODEELECTROLYTE

ANODE

CATHODEELECTROLYTE

Bipolar electrode

Anode catalystCathode catalyst

O2

H2

Stack of several hundred

Phosphoric Acid Fuel Cell (PAFC)

O2

H2

Electrolyte in SiC porous matrix

Pt-particles catalysts (anode or cathode)

Gas (H2 or O2)

PACF parameters:

current density - 200- 400 mA cm-2

single cell voltage - 600-800 mV

temperature - 220 oC

At atmospheric pressure

Gas Diffusion Electrode

Ele

ctro

de

e-

e-

Gas

Electrolyte

H2

Reaction zone

Dry zone (no reaction)

Dip zone (reaction is slow because diffusion limitation)

Reaction zone

Disadvantages of liquid electrolyte fuel cell

Low operation temperature ! (reaction is slow, expensive catalysts are needed to produce valuable current)

Difficulties in three-phase interface maintaining !

Strong fuel crossover!

H2 O2

Anode Liquid electrolyte Cathode

Recombination (no electron transfer through outer socket - energy loss)

- +

H2

H2

Air (O2)

H2O +Air (O2)

H+

Current collector /

gas distributor

Catalyst support

(carbon cloth)

Nafion®

membrane

Proton Exchange Membrane Fuel Cell (PEMFC)

H2 crossover

Proton Exchange Membrane (PEM)

C

H

C

H

HH

Ethylene

CCC CC C

H H H H H H

H H H H H H

Polymerization

Polyethylene

CCC CC C

F F F F F F

F F F F F F

Fluorination

Polytetrafluoroethylene (PTFE, Teflon®)

S

F C F

O O

CCC CC C

F F F F F F

F O F F F FF C FF C F

O

F C F

O

Grafting

Nafion® (DuPont)- H+

Fuel reforming

CnHm + nH2O = nCO + (m/2 + n)H2

CH4 + H2O = CO + 3H2

CO + H2O = CO2 + H2

CH3OH + H2O = 3 H2 + CO2

T~ 500 oC, Ni-catalyst

T~ 250 oC, Ni-catalyst

no CO

Stainless still

Catalyst

Catalyst

CH4 + H2O H2 + COx

CH4 + O2 CO2 + H2O

HEAT

- +

CH3OH + H2O

CH3OH + H2O + CO2

Air (O2)

H2O +Air (O2)

H+

Current collector /

fuel distributor

Catalyst support

(carbon cloth)

Nafion®

membrane

Direct Methanol Fuel Cell (DMFC)

CH3OH crossover

Pt Pt

Methanol oxidation mechanism

carbon

oxygen

hydrogen

+

ê

+

ê

+

ê

+

ê

+

ê

+

ê

0.046 1.23Potential vs. HRE, V

Current

Theoretical voltage = 1.182 V

CH3OH + H2O = CO2 + 6H+ + 6e- 3/2O2 + 6H+ + 6e- = 3H2O

Real voltage

Direct Methanol Fuel Cell (DMFC)

Ru

Pt

Carbon monoxide tolerant anode

carbon

oxygen

hydrogen

Methanol crossover through Nafion

Temperature oC Current density, A cm-2 Crossover rate, A cm-2

90 0.1 0 .32

90 0.2 0.30

90 0.3 0.27

S. R. Narayanan, DOE/ONR Fuel Cell Workshop, Baltimore, MD, Oct 6-8 1999

From M.P. Hogharth and G.A. Hards, Platinum Metals Rev. 40 (1996) 150

Nm = jc·S·t/n·F, where j - current density(crossover rate) , S - membrane area, t - time, n-number of electrons (n=6 for methanol oxidation), F - Faraday constant

Number of methanol moles (Nm) transported by crossover can be calculated by Faraday low:

Catalysts for fuel cells with polymer electrolyte

PEMFC DMFC

Anode: Pt or PtRu (~50% Pt) black 1-10 nm Cathode: Pt (~50% Pt) black 1-10 nm

Catalysts are supported on carbon nanoparticles (50-200 nm, for example Vulcan XC72)

Anode: usually PtRu (~50% Pt) black 1-10 nm Cathode: Pt (~50% Pt) black 1-10 nm

Catalysts are usually unsupported

Precious metals load is 0.2 - 0.5 mg cm-2 for both electrodes

Precious metals load is 1.0 - 10.0 mg cm-2 for both electrodes

Power density - 100 mW cm -2 at cell voltage 0.5 V (t=90 oC, CH3OH concentration - 0.75 M)

Power density - 500 mW cm -2 at cell voltage 0.5 V (t=80 oC, CO-free hydrogen)

Catalysts cost ~ 0.8 g per kW ( ~140 CAN$ per kW)

Catalysts cost ~ 10 g per kW ( ~1750 CAN$ per kW)

Molten Carbonate Fuel Cell (MCFC)

Anode Porous electrolyte support CathodeNiCr alloy LiNiO2 or

LiCoO2Alkali metal carbonates in LiAlO2 matrix

H2 O2 +CO2

0.2 - 1.5 mm 0.5 - 1.0 mm 0.5 - 1.0 mm

O2 +CO2 H2 +CO2 + H2O

2H2 + 2CO32- - 4e- = 2H2O + 2CO2

O2 + 2CO2 + 4e - = 2CO32-

T= 600-700 oC

CO32-

Solid Oxide Fuel Cell (SOFC)

Anode Electrolyte Cathode

H2

H2 + H2O

O2

O2

2H2 + 2O2- - 4e - = 2H2O O2 + 4e - = O2-

O2-

T= 800-1100 oC

Air Air

Electrolyte

Anode

Cathode Fuel

Sr doped La-manganite

Ni+YSZ

YSZ

Types of Fuel Cells

Phosphoric Acid Fuel Cell (PAFC)

Proton Exchange Membrane Fuel Cell (PEMFC)

Direct Methanol Fuel Cell (DMFC)

Molten Carbonate Fuel Cell (MCFC)

Solid Oxide Fuel Cell (SOFC)

Mobile ion

Operating temperature

Power range

H+ ~220 oC 10 - 1000 kW

H + 50 - 100 oC 1 - 100 kW

H + 50 - 100 oC 1 - 100 kW

CO32- ~650 oC 0.1 - 10 MW

O2- 500 - 1000 oC 0.01 - 10 MW