Batteries:
Two main kinds - rechargeable and disposable.
Rechargeable:
1.
Lead-Acid battery (car batteries)
2.
Nickel -Cadmium ,Ni-MH
(mp3 players, digital cams, power tools)
Disposable Batteries:
1.
Leclanche batteries
2.
Alkaline batteries
3.
Mercury batteries
4.
Lithium batteries (pacemakers)
23-1
A note on practice quiz questions
+ve input
-ve input
-ve
Anode releases
(loses) electrons
Xe-
+ve
M+
Cathode uses
electrons
-becomes +ve
If meter gives a positive deflection then the anode is
connected to the negative terminal of the meter
23-2
Lead-Acid Car batteries
Pb 2+
Pb 4+
Pb
Pb2+
Schematic view of a lead storage cell. Lead is oxidized and
lead(IV)oxide reduced during the operation of this battery.
23-3
Cathode reaction
Anode reaction
Full (net) reaction
written as a red potential
PbO2 + Pb + HSO4- + 4 H+ ----> 2 PbSO4 + 2 H2O
E = 1.69 - (- 0.35) = 2.05 (six cells used to give 12V)
HSO4- is used up at anode and cathode: needs recharging
E = E0 - RT/nF ln 1/ [H+][HSO4-]
So as [H+] and {HSO4-] decrease RTlnQ is more negative and E decreases
23-4
Leclanche Cells: first “dry” cell
Each has a voltage E of 1.5 volts
when new and the voltage
gradually falls off during use as the
battery reactants are used up.
Anode: A ZINC cup to hold the
reactants
This is the Negative connection usually at the bottom of the battery
Cathode: An INERT GRAPHITE
rod - the Positive terminal (usually
top centre)
Leclanche cell, otherwise known as the
"dry cell battery," was
invented by Georges Leclanche' in 1866
and is
often called the Leclanche' cell in his
honor.
From the outside it looks like the common
household
batteries in use today for the most part.
However,
the chemicals inside are different.
The Zinc cup is filled with a paste
of MnO2(s), NH4CR(s) and water.
23-5
Reactions are quite complex
Anode Reaction:
Zn(s) 6 Zn2+ + 2e
Cathode Reaction:
2MnO2(s) + 2(NH4+)(aq) + 2e
6 Mn2O3(s) + 2NH3(aq) +
H2O(R)
Disadvantages: Not very good shelf life and when the battery is
'dead', real chance of corrosion.
Zn | Zn(NH3)4CR2(s), NH4CRa, MnO2(s) | C(s)
23-6
Alkaline batteries
alkaline aqueous solutions as electrolytes. first introduced
in the early ’60s.. Alkaline cells have many advantages over
zinc-carbon, higher energy density, longer shelf life,
superior leakage resistance,
Zinc powder increases anode surface area This lowers the internal resistance and
increases the power density., electrolyte, KOH, allows high ionic conductivity.
23-7
Very similar to the Leclanche cell, but replacing NH4CR with NaOH. More
expensive, has a steel casing. Anode usually a ZINC rod.
Anode Reaction:
Zn(s) + 2(OH-)(aq) 6 ZnO(s) + H2O(R) + 2e
Cathode reaction:
2MnO2(s) + H2O + 2e 6 Mn2O3(s) + 2(OH-)
In this case OH- is produced at the cathode and consumed at the anode, so its
concentration is effectively constant 6 steady voltage.
Zn | Zn(OH)2(s), NaOH, MnO2(s) | C(s)
23-8
Nickel Cadmium (rechargeable) Commonly called nicad cell
1.2 Volt cells using.
Ni(OH)2 cathode, Cd anode and alkaline KOH electrolyte.
small size and high capacity made portable tools and other consumer
applications practical for the first time.
Once the battery of choice for low power portable products they have lost market
share to the newer Nickel Metal Hydride batteries. (later)
Used in series
for power tools
•
•
•
•
can be recharged up to 500 times
Chemical contents:
– Anode: Cadmium (Cd)
– Cathode: Hydrated nickel oxide (NiOOH)
– Electrolyte: Potassium Hydroxide (KOH)
Half-cell reactions:
−
−
Cd
(
s
)
+
2
OH
(
aq
)
→
Cd
(
OH
)
(
s
)
+
2
e
2
– Anode:
NiOOH ( s) + H 2O(l ) + e − → Ni (OH ) 2 ( s ) + OH − (aq )
– Cathode:
usually give 1.2V
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Nickel-Metal Hydride (Ni-MH)
– Cathode: Metal Hydrides
– Anode: Nickel
– Electrolyte: Potassium hydroxide (KOH)
• Half equations:
– Charging: Alloy + H O + e − → Alloy[ H ] + OH −
2
• Anode:
−
−
Ni
(
OH
)
+
OH
→
NiOOH
+
H
O
+
e
2
2
• Cathode:
– Discharging
• Anode:
• Cathode:
Alloy[ H ] + OH − → Alloy + H 2O + e −
NiOOH + H 2O + e − → Ni (OH ) 2 + OH −
23-10
Mercury batteries: “button” batteries
Widely used, cameras, watches, hearing aids,
pacemakers, etc. Small size and constant
voltage.
Constant
voltage until it
wears out
Zn/Hg amalgam
Overall reaction:
Zn(s) + HgO(s) 6 ZnO(s) + Hg(R)
[all solids or liquids, so Q in the Nernst equation is 1 and
E = EE
E = 1.35 volts for the life of the battery
23-11
In a mercury battery, zinc is oxidized and mercury is reduced, but no
net change occurs in the concentrations of any species in solution.
The migration of OH- from the Hg electrode to the Zn electrode carries
current and maintains a uniform concentration.
23-12
Lithium batteries
Cathode material varies. Anode has Li metal and a solution of LiCRO4 in and
organic solvent. They yield a high voltage and have a long life.
Applications: Pacemakers, defibrillators, watches, meters, cameras,
calculators, portable, low-power use
Commonly used in notebook / laptop computers
Lithium is the lightest of metals and it floats on water. It also has
the greatest electrochemical potential which makes it one of the
most reactive of metals. These properties give Lithium the potential
to achieve very high energy and power densities in high power
battery applications such as automotive and standby power.
-many shapes
and sizes
available
15 year shelf life
23-13
Various types and designs Lithium Manganese Dioxide,
Lithium-Ion and Lithium-Ion-Polymer
One example: Cathode MnO2.
Anode reaction: Li
Li+ + e
Cathode MnO2 + Li+
Net: Li + MnO2
+ e
Mn(III)O2 (Li)+
MnO2 + Li+
23-14