ELECTROCHEMICAL
SYSTEMS FOR ELECTRIC
POWER GENERATION
Dzmitry Malevich
Depatrment of Chemistry and Biochemistry
University of Guelph
Electric power conversion in electrochemistry
Electrolysis / Power consumption
Electric
Power
Chemical
Reactions
Electrochemical battery / Power generation
Volta’s battery (1800)
Alessandro Volta
1745 - 1827
Paper moisturized with
NaCl solution
Cu
Zn
Principles of power generation in the electrochemical systems
Me2n+ - ne- = Me20
Me20 - ne- = Me2n+
CATHODE
ANODE
Me2
Me1
Me1n+
SO42-
Salt Bridge
Me2n+
SO42-
IMPORTANT NOTICE !
Electrolysis
Battery
System consumes energy
System releases energy
G>0
G<0
ANODE
+
(oxidation process)
CATHODE
-
(reduction process)
ANODE
-
(oxidation process)
CATHODE
+
(reduction process)
Principles of power generation in the electrochemical systems
Me2n+ - ne- = Me20
Me20 - ne- = Me2n+
CATHODE
ANODE
Me1
Me1n+
Diaphragm
or
Membrane
SO42-
Me2
Me2n+
SO42-
Primary batteries
Modern ZincManganese
battery
Leclanché’s
battery
(1866)
Georges Leclanché
(1839-1882)
Anode: Zn  Zn2+ + 2eCathode: 2MnO2 + 2H2O +2e-  2MnOOH + 2OH-
Seal
Zn-container
MnO2 paste
(cathode)
Carbon rod
NH4OH
electrolyte
Gas space
Electrolyte:
Zn2+
2NH4Cl
+2OH-
 Zn(NH3)Cl2 + 2H2O
Zn-container
2MnO2 + Zn + 2NH4Cl  2MnOOH + Zn(NH3)Cl2
MnO2 paste
(cathode)
Carbon rod
Gel electrolyte
Primary batteries
Zinc-Manganese alkaline battery
MnO2 paste (cathode)
Gel electrolyte
Porous Zn (anode) Anode: Zn + 2OH - 2e  Zn(OH)2
Cathode: MnO2 + H2O +1e-  MnOOH + OHaaaaaaaaa MnOOH + H2O +e-  Mn(OH)2 + OH-
Zinc-Air battery
Anode: Zn + 2OH- - 2e-  Zn(OH)2
Cathode: 1/2 O2 + H2O + 2e-  Zn(OH)2
Secondary (rechargeable) batteries
Lead-acid battery
battery
Lead-acid
E=2.06 V
Pb
PbO2
Safety valve
36% H2SO4
Pb+(2H++SO42-)-2e-
discharge
charge
PbSO4+ 2H+
PbSO4
PbO2 + Pb + H2SO4
PbO2+(2H++SO42-)+2H++2ePbSO4+H2O
discharge
charge
Lead dioxide paste in
Pb-mesh (cathode)
PbSO4
discharge
2PbSO4 + 2H2O
Lead paste in
Pb-mesh
(anode)
Porous separator
Secondary (rechargeable) batteries
Lithium-ion battery
Cathode:
Discharge
LiMeO2 - xe-
CHARGE
DISCHARGE
Li1-xMeO2 + xLi+
Anode:
C + xLi+ + xe-
Charge
CHARGE
CLix
DISCHARGE
Anode (CLix)
Cathode (LiMexOy)
Negative terminal
LiCoO2 -utilized for commercial batteries
LiNiO2, LiMn2O4-prospective
Separator
Aluminum can
Positive terminal
Secondary (rechargeable) batteries
Nickel-Metal Hydride battery
Cathode:
NiOOH + H2O - e-
CHARGE
DISCHARGE
Ni(OH)2 + OH-
Anode:
Me +
OH-
+
e-
CHARGE
Me + H2O
DISCHARGE
Picture from: T. Takamura / Solid State
Ionics 152-153(2002)19
Types of the electrochemical system for electric
power generation
Reductant
(fuel)
Primary
batteries
Secondary
batteries
POWER
POWER
Recharge
POWER
Oxidant
Fuel cells
POWER
Reaction
products
(exhaust)
Grove’s fuel cell (1839)
O2
Sir William Grove
1811–1896
4H+ + 4e- 2H2
2H2O - 4e- O2 + 4H+
H2
Fuel Cells performance improving
Raising the voltage:
Raising the current:
Connection
of cells
Cell
stackin series
• Increasing the temperature
• Increasing the area of
eelectrode electrolyte interface
• The use of catalyst
Cathode catalyst
Anode catalyst
Bipolar
electrode
H2
O2
Stack of several hundred
Electrolyte frame
Bipolar plate
ANODE
ELECTROLYTE
CATHODE
ANODE
ELECTROLYTE
ANODE
CATHODE
ELECTROLYTE
ANODE
CATHODE
ELECTROLYTE
CATHODE
ANODE
ANODE
ELECTROLYTE
ELECTROLYTE
CATHODE
CATHODE
ANODE
ELECTROLYTE
CATHODE
Phosphoric Acid Fuel Cell (PAFC)
Electrolyte in SiC porous matrix
O2
Pt-particles catalysts
(anode or cathode)
Gas (H2 or O2)
PACF parameters:
current density - 200- 400 mA cm-2
At atmospheric
single cell voltage - 600-800 mV
pressure
temperature - 220 oC
H2
Gas Diffusion Electrode
Electrode
Dry zone (no reaction)
Reaction zone
H2
Gas
ee-
Electrolyte
Reaction zone
Dip zone (reaction is
slow because diffusion
limitation)
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!
Recombination (no electron transfer through
outer socket - energy loss)
H2
O2
Anode
Liquid electrolyte
Cathode
Proton Exchange Membrane Fuel Cell (PEMFC)
H2O +Air (O2)
H2
Nafion®
membrane
Catalyst support
(carbon cloth)
Current collector /
gas distributor
H+
H2 crossover
H2
-
Air (O2)
+
Proton Exchange Membrane (PEM)
Polyethylene
Ethylene
H
H
H
C
C
H
F F F F F F
C C C C C C
Polymerization
H H H H H H
C C C C C C
H H H H H H
Fluorination
F O F F F F
F F F F F F
F C F
Grafting
C C C C C C
F C F
F F F F F F
O
F C F
Polytetrafluoroethylene (PTFE, Teflon®)
F C F
O S O
Nafion® (DuPont)
+
O H
Fuel reforming
CnHm + nH2O = nCO + (m/2 + n)H2
CH4 + H2O = CO + 3H2
T~ 500 oC, Ni-catalyst
CO + H2O = CO2 + H2
CH3OH + H2O = 3 H2 + CO2
T~ 250 oC, Ni-catalyst
no CO
CH4 + O2
CO2 + H2O
Catalyst
Stainless still
HEAT
Catalyst
CH4 + H2O
H2 + COx
Direct Methanol Fuel Cell (DMFC)
H2O +Air (O2)
CH3OH + H2O + CO2
Nafion®
membrane
Catalyst support
(carbon cloth)
Current collector /
fuel distributor
H+
CH3OH
crossover
CH3OH + H2O
-
Air (O2)
+
Methanol oxidation mechanism
+
+
+
+
+
carbon
oxygen
hydrogen
ê ê ê ê ê ê
Pt
Pt
+
Direct Methanol Fuel Cell (DMFC)
Theoretical voltage = 1.182 V
Real voltage
Current
0.046
Potential vs. HRE, V
CH3OH + H2O = CO2 + 6H+ + 6e-
1.23
3/2O2 + 6H+ + 6e- = 3H2O
Carbon monoxide tolerant anode
carbon
oxygen
hydrogen
Ru
Pt
Methanol crossover through Nafion
From M.P. Hogharth and G.A. Hards, Platinum Metals Rev. 40 (1996) 150
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
Number of methanol moles (Nm) transported by crossover can be calculated by
Faraday low:
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
Catalysts for fuel cells with polymer electrolyte
PEMFC
Anode: Pt or PtRu (~50% Pt) black 1-10 nm
Cathode: Pt (~50% Pt) black 1-10 nm
DMFC
Anode: usually 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)
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 - 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)
Power density - 100 mW cm -2 at cell voltage 0.5 V
(t=90 oC, CH3OH concentration - 0.75 M)
Catalysts cost ~ 10 g per kW
( ~1750 CAN$ per kW)
Molten Carbonate Fuel Cell (MCFC)
Anode
NiCr alloy
Porous electrolyte support
Cathode
Alkali metal carbonates
in LiAlO2 matrix
H2 +CO2 + H2O
LiNiO2 or
LiCoO2
O2 +CO2
CO32-
H2
O2 +CO2
0.2 - 1.5 mm
0.5 - 1.0 mm
T= 600-700 oC
2-
2H2 + 2CO3 -
4e-
= 2H2O + 2CO2
0.5 - 1.0 mm
O2 + 2CO2 + 4e - = 2CO32-
Solid Oxide Fuel Cell (SOFC)
Anode
Electrolyte
H2 + H2O
Cathode
Sr doped La-manganite
O2
O2-
H2
O2
YSZ
2H2 + 2O2- - 4e - = 2H2O
Ni+YSZ
Electrolyte
Anode
Air
Cathode
Air
Fuel
T= 800-1100 oC
O2 + 4e - = O2-
Types of Fuel Cells
Mobile
ion
Operating
temperature
Power
range
Phosphoric Acid
Fuel Cell (PAFC)
H+
~220 oC
10 - 1000 kW
Proton Exchange
Membrane Fuel Cell
(PEMFC)
H+
50 - 100 oC
1 - 100 kW
H+
50 - 100 oC
CO32-
~650 oC
Direct Methanol
Fuel Cell (DMFC)
Molten Carbonate
Fuel Cell (MCFC)
Solid Oxide
Fuel Cell (SOFC)
O2-
500 - 1000 oC
1 - 100 kW
0.1 - 10 MW
0.01 - 10 MW