Download 3 Batteries

3-1
1999 Chem 121
3 Batteries
3.1 Leclanché and alkaline dry cells
Common Dry Cell
Graphite
+
cathode
MnO2 +
Graphite
Zn anode
moist paste
of ZnCl2 +
NH4Cl +
graphite
-
Steel
casing
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Dry cell: Acid version (simplified)
1999 Chem 121
“Leclanché element”
Zn(s)  Zn2+ + 2 e-
-E0 = -(-0.76V)
2 MnO2 (s) + 2 NH4+(aq) + 2 e-  Mn2O3(s) + NH3 (aq) + H2O
E0 = +0.75 V?
2 MnO2 (s) + 2 NH4+(aq) + Zn(s)  Mn2O3(s) + NH3 (aq) + H2O + Zn2+
E0cell = +1.5 V
 Formation of Zn2+ at anode
 decrease of [NH4+]
 spontaneous reaction of Zn with electrolyte
 does not last long!
3-3
Dry cell: Alkaline version (simplified)
Zn(s) + 2 OH-  Zn(OH)2 + 2 e-
1999 Chem 121
“common household battery”
-E0 = -(-1.246 V)
2 MnO2 (s) + 2 H2O + 2 e-  Mn2O3(s) + 2 OH-
E0 = +0.118 V
2 MnO2 (s) + H2O + Zn(s)  Mn2O3(s) + Zn(OH)2
E0cell = +1.364 V
 longer life
 voltage does not drop by much when current is drawn
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1999 Chem 121
Recharging of dry cell?
Problem: Zn(OH)2 and Mn2O3(s) do not stick to the electrodes. It is not possible
to reverse the reaction by applying external voltage, i.e. no recharging
Also electrolysis of water causes produces explosive mix of H2 and O2
2 x 2 H2O +2 e-  H2 + 2 OH – (aq)
4 OH-(aq)  O2 + H2O + 4e2 H2O  2 H2 + O2
E0 = -0.83 V
-E0 = -(+0.40 V)
E0cell = -1.23 V
Note E0cell <0 V ( G>0J) when recharging, since work is done on the system
3-5
1999 Chem 121
3.2 Nickel-Cadmium Cell
“Rechargeable Battery”
Anode: Cd(s) + OH-  Cd(OH)2(s) + 2 eCathode NiO(OH)(s) + 2 H2O + 2 e-  Ni(OH)2(s) + 2OHNiO(OH)(s) + 2 H2O + Cd (s)  Ni(OH)2(s) + Cd(OH)2(s) + OH-
-E0 = -(-0.83 V)
E0 = 0.52 V
E0 = 1.35 V
3-6
1999 Chem 121
3.3 Aluminum Air Battery
Cathode (+) 3 { O2(g) + 2 H2O + 4 e-  4 OH- (aq) }
Anode: (-) 4 { Al(s) 4 OH-(aq)  [Al(OH)4]-(aq) +3 e- }
4 Al(s) + 3 O2(g) + 6 H2O + 4 OH-  [Al(OH)4]-(aq)
 Very high power per unit weight (high “energy density”)
 [Al(OH)4]-(aq) can be recycled to Aluminum
 Not rechargeable
E0 = 0.401 V
-E0 = -(-2.310 V)
E0 = 2.711 V
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1999 Chem 121
3.4 Car battery (Lead storage battery)
Lead Storage Battery
+
-
Lead Grid
Filled with
Spongy Lead
(anode)
Lead Grid
Filled with
PbO2
(cathode)
H2SO4 Electrolyte
Trick question: What is the voltage of this battery?
3-8
1999 Chem 121
Half reactions for lead storage battery
Anode (-)
Pb + HSO4-
 PbSO4 + H+ + 2 e- -E0 = -(-0.356 V)
Cathode (+) PbO2 + HSO4- + 3 H+ + 2 e-  2 H2O + PbSO4
E0 = 1.690 V
PbO2 + Pb + 2 HSO4- + 2 H+ + 2 e-  2 H2O + 2 PbSO4
E0 = 2.046 V
6 galvanic cells in series produce a battery of 12 V
! electrode reactions can be reversed many times  rechargeable !
3-9
1999 Chem 121
3.5 Things to consider when matching a battery to an application
Chemistry -- Which kind of battery chemistry is best for the application? Different
chemistries will generate different voltages and currents.
Primary or Secondary -- Primary batteries are most appropriate for applications
where infrequent, high-energy output is required. Secondary batteries are most
appropriate for use in devices that see steady periods of use and non-use
(pagers, cellular phones, etc.).
Standardization and Availability -- Is there an existing battery design that meets
the application needs? Will replacement batteries be available in the future?
Using existing battery types is almost always preferable to specifying a custommade battery design.
Flexibility -- Can the battery provide high or low currents over a wide range of
conditions?
Temperature Range -- Can the battery provide adequate power over the
expected temperature range for the application?
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1999 Chem 121
Good Cycle Life -- How many times can the rechargeable battery be discharged
and recharged before it becomes unusable?
Costs -- How expensive is the battery to purchase? Does the battery require
special handling?
Shelf Life -- How long can the battery be stored without loss of a significant
amount of its power?
Voltage -- What is the voltage of the battery? [Most galvanic cells produce
voltages of between 1.0 and 2.0 V.]
Safety -- Battery components range from inert, to mildly corrosive, to highly toxic
or flammable. The more hazardous components will require additional safety
procedures.
Hidden Costs -- Simpler manufacturing processes result in lower cost batteries.
However, if a battery contains toxic or hazardous components, extra costs will be
incurred to dispose of the battery safely after its use.
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3.5.1
1999 Chem 121
Energy density of different batteries
12% means that a 100 kg battery has the same energy content as a 12 kg petrol.
(Check p. 482 Zumdahl on “specialty batteries”)
 Hybrid EV
3-12
1999 Chem 121
3.6 Fuel cells
3.6.1
Hydrogen fuel cell
(Apollo and Gemini missions, Ballard EV, Chicago bus lines, etc.)
2 x H2 + 2 OH – (aq)  2 H2O +2 eO2 + H2O + 4e-  4 OH-(aq)
2 H2 + O2  2 H2O
-E0 = -(-0.83 V)
E0 = +0.40 V
E0cell = 1.23 V
3-13
1999 Chem 121
llustration provided by http://www.ballard.com/01in/in03.html
Advantages:
 Efficiency > 50% (compared to 20-35% for combustion engine)
 Pollution
 Noise
 Handling
EV Problems:
 Hydrogen storage and handling
 Cheap production of hydrogen
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3.6.2
1999 Chem 121
Other fuel cells
Methanol fuel cell (under development)
- balance O using water
- balance H using H+
- balance charges using electrons
2 x H2O + CH3OH  CO2 + 6 H+ + 6 e-
E0 = -0.007 V
3 x O2 + 4 H+ + 4e-  2 H2O
E0 = +1.229 V cathode
anode
2 H2O + 2 CH3OH + 3 O2  2 CO2 + 6 H2O
2 CH3OH + 3 O2  2 CO2 + 4 H2O
E0cell = +1.222 V
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3.6.3
η
Maximum efficiency of fuel cells

r H
r H

rG  n F E
e.g. for methanol fuel cell
with
rH = -1533 kJ/mol
and
E0cell = +1.222 V
It follows
rG = -1415 kJ/mol
and


1999 Chem 121
3-16
1999 Chem 121
Links to interesting Websites
Table of standard reduction potentials (in acid and basic solution)
http://www.prenhall.com/~bookbind/pubbooks/hill2/medialib/tools/standard.html
Scientific American Article about fuel cells
(excellent overview; many links outdated)
http://www.sciam.com/explorations/122396explorations.html
New technologies Battery guide
http://www.nlectc.org/oles/batteryguide/ba-cont.htm
Space shuttle power generation
http://www.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-eps.html#sts-fuel-cell
Hybrid electric vehicles presented by the DOE
http://www.hev.doe.gov/components/component.html
Environmental newsnetwork
http://www.enn.com/enn-news-archive/1998/03/031298/hcell.asp
Ballard web site (very good!)
http://www.ballard.com/01in/in03.html