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
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
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
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)
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
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