Primary Cells
A primary cell is a cell that can be used
once only and cannot be recharged.
The reactants cannot be regenerated
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Primary cells non-rechargeable
These cells are not rechargeable.
․Zinc-carbon cells
Recharging is dangerous as it produces H2 and heat
which results in an explosion.
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Primary cells
These cells are not rechargeable.
․Alkaline manganese cells
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Primary cells
These cells are not rechargeable.
․Silver oxide cells (button cells)
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Primary cells
These cells are not rechargeable.
․Lithium primary cells (button cells)
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Zinc-carbon Cells
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Zinc-carbon Cells
At anode:
Zn(s)
Zn2+(aq) + 2e–
At cathode:
2MnO2(s) + 2NH4+(aq) + 2e–
Mn2O3(s) + 2NH3(aq) + H2O(l)
Overall reaction:
+(aq)
Ecell = +1.50 V
Zn(s) + 2MnO2(s) + 2NH4
 Zn2+(aq) + Mn2O3(s) + 2NH3(aq) + H2O(l)
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Zinc-carbon Cells
The cell diagram for the zinc-carbon cell is:
Zn(s) | Zn2+(aq) [2MnO2(s) + 2NH4+(aq)],
[Mn2O3(s) + 2NH3(aq) + H2O(l)] | C(graphite)
Overall reaction:
Zn(s) + 2MnO2(s) + 2NH4+(aq)
 Zn2+(aq) + Mn2O3(s) + 2NH3(aq) + H2O(l)
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At anode,
Zn(s) + 2OH(aq)
ZnO(s) + H2O(l) + 2e
At cathode,
Ag2O(s) + H2O(l) + 2e
2Ag(s) + 2OH(aq)
Overall reaction :Zn(s) + Ag2O(s)  ZnO(s) + 2Ag(s)
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Q.20(a)
HgO(s)
At anode,
Zn(s) + 2OH(aq)
ZnO(s) + H2O(l) + 2e
At cathode,
HgO(s) + H2O(l) + 2e
Hg(l) + 2OH(aq)
Overall reaction : Zn(s) + HgO(s)  ZnO(s) + Hg(l)
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Q.20(b)
HgO(s)
Ecell  Ecathode  Eanode
= +0.098V – (1.216V) = 1.314V
P. 11 / 99
Secondary Cells
Electrochemical cells that can be recharged.
Examples : Lead-acid accumulators
Nickel-cadmium cells (NiCad)
Nickel-Metal hydride(NiMH) cells
Lithium-ion cells
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Lead grids coated with PbSO4(s)
Pb(s) + H2SO4(aq)  PbSO4(s) + H2(g)
P. 13 / 99
Lead grids coated with PbSO4(s)
During charging
Negative electrode :
PbSO4(s) + 2e
Pb(s) + SO42(aq)
spongy lead
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Lead grids coated with PbSO4(s)
During charging
Positive electrode :
PbSO4(s) + 2H2O(l)
PbO2(s) + 4H+(aq) + SO42(aq) + 2e
spongy PbO2
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Lead grids coated with PbSO4(s)
During discharging
Anode :
Pb(s) + SO42(aq)
2e
PbSO4(s) +
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Lead grids coated with PbSO4(s)
During discharging
Cathode :
PbO2(s) + 4H+(aq) + SO42(aq) + 2e
PbSO4(s) + 2H2O(l)
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Overall reaction : Pb(s) + PbO2(s) + 2H2SO4(aq)
discharge
charge
2PbSO4(s) + 2H2O(l)
The cell diagram for the lead-acid
accumulator is:
Pb(s) | PbSO4(s) [PbO2(s) + 4H+(aq) +
SO42–(aq)], [2PbSO4(s) + 2H2O(l)] | Pb(s)
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Overall reaction : Pb(s) + PbO2(s) + 2H2SO4(aq)
discharge
charge
2PbSO4(s) + 2H2O(l)
PbSO4 is coated on the electrodes,
The reversed processes are made possible.
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Overall reaction : Pb(s) + PbO2(s) + 2H2SO4(aq)
discharge
charge
2PbSO4(s) + 2H2O(l)
The cell should be charged soon after complete
discharge
Otherwise, fine ppt of PbSO4 will become
coarser and inactive, making the reversed
process less efficient.
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Overall reaction : Pb(s) + PbO2(s) + 2H2SO4(aq)
discharge
charge
2PbSO4(s) + 2H2O(l)
Pb(s) and PbO(s) are on different electrodes
Direct reaction is not possible
Porous partition is not needed
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Overall reaction : Pb(s) + PbO2(s) + 2H2SO4(aq)
discharge
charge
2PbSO4(s) + 2H2O(l)
During discharging, H2SO4 is being used up
The density of electrolyte solution 
The charging/discharging status can be
monitored by a hydrometer.
P. 22 / 99
Q.21
Ecell = Eocathode – Eoanode
= (1.69V) – (0.35V) = 2.04V
P. 23 / 99
Nickel-cadmium cells – Nicad cells
Q.22(a)
At anode,
Cd(s) + 2OH(aq)
Cd(OH)2(s) + 2e
At cathode,
NiO(OH)(s) + H2O(l) + e
Ni(OH)2(s) + OH(aq)
P. 24 / 99
Nickel-cadmium cells – Nicad cells
Q.22(b)
Overall reaction : 2NiO(OH)(s) + Cd(s) + 2H2O(l) 
2Ni(OH)2(s) + Cd(OH)2(s)
P. 25 / 99
Nickel metal hydride cell
(NiMH)
Cathode : NiO(OH)
Anode : MH(s)
where M is a hydrogenabsorbing alloy.
More environmentally
friendly than NiCad
cell due to the absence
of Cd.
P. 26 / 99
Nickel metal hydride cell (NiMH)
On discharging,
Anode : 0
MH(s) + OH(aq)
+1
M(s) + H2O(l) + e
At cathode,
+3
NiO(OH)(s) + H2O(l) + e
+2
Ni(OH)2(s) + OH(aq)
P. 27 / 99
Nickel metal hydride cell
(NiMH)
Voltage : 1.2 V
Electrolyte : KOH
2 to 3 times the
capacity of an
equivalent NiCad cell
From 1100 mAh up to
8000 mAh.
P. 28 / 99
Lithium ion cell
Anode is graphite into
which Li+ are inserted
P. 29 / 99
Cathode is metal
oxide into which Li+
are inserted.
Lithium ion cell
P. 30 / 99
During charging,
Li+ moves from
cathode to anode
Lithium ion cell
P. 31 / 99
P. 32 / 99
During discharge, Li+ moves from anode to cathode
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P. 34 / 99
Voltage is 3.6/3.7V
Three times that of
NiCad or NiMH
Much higher
P. 35 / 99
voltage
si ze
Q.23
The anode of lithium cell is made of
reactive lithium metal.
If the lithium anode is exposed to
moisture and air, vigorous reactions
will occur.
Thus, lithium ion cell is safer to use.
P. 36 / 99
Continuous supply of
fuel, H2
P. 37 / 99
Continuous supply of
oxygen
No need for recharging
P. 38 / 99
Other fuels such as
hydrocarbon, alcohol, or
glucose are possible
P. 39 / 99
Ni(s) and NiO(s) are
catalysts for the halfcell reactions
P. 40 / 99
Fuel Cells
• At anode:
H2(g) + 2OH–(aq)  2H2O(l) + 2e–
• At cathode:
O2(g) + 2H2O(l) + 4e–  4OH–(aq)
• Overall reaction:
2H2(g) + O2(g)  2H2O(l)
P. 41 / 99
Q.24
Maximum energy that can be used to do useful
work
 nFE
o
cell
= (2)(96485)(1.22) = 235 kJ mol1
235
% efficiency 
 100%  82.2%
286
P. 42 / 99
Q.25
1. To increase the mobility of OH/K+ to
balance the extra charges built up in halfcells.
[OH(aq)]  quickly at anode
[OH(aq)]  quickly at cathode
2. To increase the solubility of KOH
P. 43 / 99
Q.26
Anode : CH4 + 2H2O
CO2 + 8H+ + 8e
Cathode : 2O2 + 8H+ + 8e
4H2O
Overall reaction : CH4 + 2O2  CO2 + 2H2O
P. 44 / 99
Supplementary notes from HKDSE
Chemistry
P. 45 / 99
What is a fuel
cell?
P. 46 / 99
Fuel cell
It is a primary cell.
It converts the chemical energy of a
continuous supply of reactants (a fuel
and an oxidant) into electrical energy.
The products are removed continuously.
P. 47 / 99
How a fuel cell works
anode (−)
porous Ni
electrodes
cathode (+)
P. 48 / 99
How a fuel cell works
Fuel : H2
e−
e−
hot KOH
electrolyte
( 200°C)
steam (exhaust)
H2
hydrogen
Anode : H2(g) + 2OH(aq)  2H2O(g) + 2e
P. 49 / 99
How a fuel cell works
Fuel : H2
Oxidant : O2
e−
e−
steam (exhaust)
H2
O2
hydrogen
oxygen
Cathode : O2(g) + 2H2O(g) + 4e  4OH(aq)
P. 50 / 99
Functions of nickel electrodes:
act as electrical conductors that
connect the fuel cell to the external
circuit
act as a catalyst for the reactions
P. 51 / 99
The reactions involved are:
At anode
H2(g) + 2OH–(aq)  2H2O(l) + 2e–
At cathode
O2(g) + 2H2O(l) + 4e–  4OH–(aq)
Overall reaction
2H2(g) + O2(g) ⇌ 2H2O(l)
P. 52 / 99
Overall reaction
2H2(g) + O2(g) ⇌ 2H2O(l) + electrical energy
Direct reaction : Heat energy, light energy and sound
energy (pop sound) will be released.
Other possible fuels include :
ethanol, methanol, glucose solution…
But the cells have to be redesigned.
P. 53 / 99
Applications of fuel cells
For remote locations, such as spacecraft,
remote weather stations…
Continuous supply of fuel
 No need to be replaced frequently
Fuel cells are used in
space shuttle to provide
electricity for routine
operation.
P. 54 / 99
high efficiency
e.g. hydrogen-oxygen fuel cells : 70%
much higher than internal combustion
engines ( 20%) in motor cars.
Non-polluting
The only waste product of hydrogenoxygen fuel cells is water. No greenhouse
gases like CO2 or acidic gases like SO2
and NOx are emitted.
In fact, water vapor is a greenhouse gas
due to its high specific heat capacity.
P. 55 / 99
Fuel cells can also be used in electrical
and hybrid vehicles.
A fuel cell car developed by
DaimlerChrysler in Germany.
P. 56 / 99
An MP3 player runs on methanol fuel
cell in which methanol is used as fuel.
methanol
Fuel cells can be used in portable electronic
products.
P. 57 / 99
A portable fuel cell charger for mobile
phones.
fuel cell charger
Fuel cells can be
used in portable
electronic
products.
P. 58 / 99
Application Features of fuel cells
Power source
for remote
locations
high efficiency
high reliability
non-polluting
able to work
continuously
Examples
spacecraft
remote
weather
stations
large parks
rural
locations
The features of fuel cells and their applications.
P. 59 / 99
Application
Features of fuel cells
Examples
Backup power
source
high reliability
non-polluting
able to work
continuously
hospitals
hotels
office
buildings
Transportation
quiet
high efficiency(70%)
non-polluting
able to work
continuously
electric
vehicles
boats
But
expensive
The features of fuel cells and their applications.
P. 60 / 99
Application
Portable
electronic
products
Features of fuel
cells
high efficiency
non-polluting
lightweight
can be refilled
conveniently
Examples
notebook
computers
mobile phones
MP3 players
handheld
breathalyzers
The features of fuel cells and their applications.
Class practice 32.4
P. 61 / 99
Class practice 32.4
The fuel cells used to power
mobile phones and
notebook computers are not
hydrogen-oxygen fuel cells.
Instead, they are called
‘direct methanol fuel cells
(DMFC)’. The DMFC uses
replaceable methanol
cartridges for refilling. The
fuel, methanol, is a liquid
and can be fed directly in
the cell for power generation.
P. 62 / 99
Methanol and water react at the anode, producing H+.
Positive ions (H+) are transported across the proton
exchange membrane to the cathode where they
react with oxygen to produce water. The products of
the overall reaction are carbon dioxide and water.
(a) Write the equations for the reactions at the
anode and the cathode respectively.
-2
+4
At anode: CH3OH + H2O  6H+ + CO2 + 6e
0
-2
At cathode: O2 + 4H+ + 4e  2H2O
P. 63 / 99
(b) State one advantage of using methanol over
hydrogen as fuel in the fuel cell.
Methanol is a liquid which is easier to handle
than gaseous hydrogen during refilling. Or
Methanol poses a lower risk of explosion than
hydrogen. (Any ONE)
P. 64 / 99
(c) What are the potential dangers associated with
using methanol fuel cells?
Methanol is flammable, if carelessly handled,
it may catch fire.
Furthermore, methanol is a colourless liquid
like water, yet it is highly poisonous.
If it is not stored or labelled properly, there
is a danger of accidental poisoning.
P. 65 / 99
Different types of fuel cells
and their applications
The hydrogen-oxygen fuel cells discussed
in Ch.32 is a type of Alkaline Fuel Cells
(AFC).
The table below summarizes the main
features of some fuel cells.
P. 66 / 99
Fuel cell
type
Operating
Common
electrolyte temperature
Electrical
efficiency
System
output
Applications
Proton
Exchange
Membrane
(PEMFC)
50–100°C
Solid
organic
polymer
called polyperfluorosulphonic
acid
< 1 kW–
250 kW
53–58%
• Backup power
(transportation) • Portable power
25–35%
• Small
(stationary)
distributed
generation
• Transportation
Alkaline
(AFC)
below 80°C
Aqueous
solution of
potassium
hydroxide
soaked in a
matrix
10 kW–
100 kW
60%
• Military
applications
• Space projects
The summary of the main feature of some fuel cells.
P. 67 / 99
Fuel cell
type
Operating
Common
electrolyte temperature
Electrical
efficiency
System
output
Applications
150–200°C
Phosphoric Liquid
Acid (PAFC) phosphoric
acid
soaked in a
matrix
50 kW–
1 MW
(250 kW
module
typical)
> 40%
• Distributed
generation
600–700°C
Molten
lithium,
sodium,
and / or
potassium
carbonates,
soaked in a
matrix
< 1 kW–
1 MW
(250 kW
module
typical)
45–47%
• Electric utility
• Large
distributed
generation
Molten
Carbonate
(MCFC)
The summary of the main feature of some fuel cells.
P. 68 / 99
Fuel cell
type
Operating
Common
electrolyte temperature
Solid Oxide Solid
(SOFC)
zirconium
oxide to
which a
small
amount of
yttrium(III)
oxide is
added
650–1000°C
Electrical
efficiency
System
output
< 1 kW–
3 MW
35–43%
Applications
• Auxiliary
power
• Electric utility
• Large
distributed
generation
The summary of the main feature of some fuel cells.
P. 69 / 99
All of these fuel cells need fairly pure
hydrogen gas as fuel.
A reformer is usually used in these fuel
cells to generate hydrogen gas from
liquid fuel like petrol except MCFC and
SOFC. (refer to Example 34.1(c))
Class practice 34.1
P. 70 / 99
Article reading
Read the article below and answer the questions that
follow.
Microbial fuel cells−a greener and more
efficient source of electricity for tomorrow
Bacteria are very small (size ~1μm) organisms which
can convert a huge variety of organic compounds into
carbon dioxide, water and energy. The microorganisms use the produced energy to grow and to
maintain their metabolism.
P. 71 / 99
However, by using a microbial fuel cell (MFC), we can
collect a part of this microbial energy in the form of
electricity.
An MFC consists of an anode, a cathode, a proton
or cation exchange membrane and an electrical circuit.
P. 72 / 99
anode
cathode
wastewater
glucose
(bacteria)
CO2
e
−
H2O
H+
H+
membrane
The general layout of an MFC.
P. 73 / 99
O2
The bacteria live in the anode compartment and
convert a substrate such as glucose and wastewater
into carbon dioxide, hydrogen ions and electrons. The
electrons then flow through an electrical circuit to the
cathode. The potential difference (Volt) between the
anode and the cathode, together with the flow of
electrons (Ampere) result in the generation of electrical
power (Watt). The hydrogen ions flow through the
proton or cation exchange membrane to the cathode. At
the cathode, an electron acceptor is chemically reduced.
Ideally, oxygen is reduced to water.
P. 74 / 99
Microbial fuel cells have a number of potential
uses. The first and most obvious is collecting the
electricity produced for a power source. Virtually any
organic material could be used to ‘feed’ the fuel cell.
MFCs could be installed in wastewater treatment
plants. MFCs are a very clean and efficient method of
energy production.
P. 75 / 99
Questions
1. Are microbial fuel cells (MFC) really fuel cells?
Why?
Yes.
This is because a fuel (organic material) and an
oxidant (oxygen) are used in MFC to generate
electricity.
P. 76 / 99
Questions
2. Why are microbial fuel cells (MFC) considered a
greener source of energy?
Microbial fuel cells use wastewater as the
source of fuel and produce CO2 and water
which are harmless.
3. Suggest TWO substances that can be used as the
‘fuel’ for microbial fuel cells (MFC).
Glucose and wastewater
P. 77 / 99
Write balanced ionic equations for the reactions
that occur at the cathode and the anode.
Cathode
O2(g) + 4H+(aq) + 4e  2 H2O(l)
(1)
Anode
C6H12O6(aq) + 6H2O (l)
 6 CO2(g) + 24 H+(aq) + 24e (2)
Overall reaction : 6(1) + (2)
C6H12O6(aq) + 6O2(g)  6CO2(g) + 6H2O(l)
P. 78 / 99
Class practice 34.1
1. One possible use of fuel cells with great potential
of becoming more and more common is as
‘combined heat and power systems’ (CHP). A
CHP is a small power station used to generate
both electric power and heat energy for use in a
block of flats, or in a factory.
Give three reasons to support the argument that
‘a fuel cell CHP is better than a diesel generator
for use as a CHP.’
Distributed generation
P. 79 / 99
P. 80 / 99
1. (1) A diesel generator has a lower efficiency
than a fuel cell system. In other words, a
diesel generator consumes more fuel to
produce the same quantity of heat and
electricity as compared to a fuel cell.
(2) A diesel generator causes pollution to the
environment, producing smoke, bad smell,
and a lot of NOx and SO2. A fuel cell system
is clean and the exhaust is non-polluting, so
it is more suitable for on-site energy
production for a block of flats.
P. 81 / 99
(3) A diesel generator is very noisy while a fuel
cell operates quietly. This again is better for
on-site power production.
(4) Renewable fuels such as glucose can be used
in CHP while diesel used in diesel generator
is non-renewable.
P. 82 / 99
2. Phosphoric acid fuel cells (PAFC) are a suitable
choice to be used in CHP. In this type of cells, the
electrolyte used is liquid phosphoric acid soaked
in a matrix.
(a) Write the ionic half equations at the cathode
and the anode of PAFC respectively
At cathode: O2(g) + 4H+(aq) + 4e  2H2O(l)
At anode: 2H2(g)  4H+(aq) + 4e
(b) Write the overall equation for the cell reaction.
2H2(g) + O2(g)  2H2O(l)
P. 83 / 99
There are two types of rechargeable
lithium cells:
P. 84 / 99
Lithium-ion rechargeable
batteries
Lithium-ion
rechargeable batteries
are commonly used in
portable electronic
devices.
P. 85 / 99
In a lithium-ion rechargeable battery, both
the positive electrode and negative
electrode contain lithium compounds.
P. 86 / 99
Discharging
Load
electrons
negative
electrode
current
separator
electrolyte
P. 87 / 99
positive
electrode
Charging
Charger
electrons
current
negative
electrode
separator
electrolyte
P. 88 / 99
positive
electrode
Positive electrode
e.g. Li1-xCoO2
a metal oxide fitted with Li+ ion
e.g. cobalt dioxide CoO2, manganese
dioxide MnO2 or nickel dioxide NiO2
Negative electrode
graphite
lithium-carbon compound LixC6
Electrolyte
Prevents electrolysis
of water to give H2
a lithium salt in an organic solvent
P. 89 / 99
The chemical equations for the reactions are:
Positive electrode
+4
Li1-xCoO2 +
xLi+
+ xe
−
discharging
charging
+3
LiCoO2
Negative electrode
discharging
LixC6
charging
6C +
xLi+
+ xe
−
It is the graphite in the lithium compound that loses
electrons
P. 90 / 99
Overall reaction
Li1-xCoO2 + LixC6
(+)
()
discharging
charging
LiCoO2 + 6C
Note that lithium ions themselves are
neither oxidized nor reduced.
The voltage of a lithium-ion rechargeable
battery is 3.7 V.
P. 91 / 99
Feature
Comparison with other cells
High charge density
A lithium-ion rechargeable battery
weighs about half that of a NiCd or
NiMH cell of the same charge
capacity.
High voltage (3.6–3.7 V)
A voltage range more suitable for
many portable electronic devices
like mobile phones, MP3 players,
digital cameras, etc.
A summary of the comparison of lithium-ion
rechargeable batteries with other cell types.
P. 92 / 99
Feature
Comparison with other cells
High drain capacity
Lithium-ion rechargeable batteries
can discharge at much larger
currents than NiCd or NiMH cells
continuously for a longer period of
time. This is very important for
some applications such as the
steady conversation over the
mobile phones.
Environmentally preferred
Lithium-ion rechargeable batteries
do not contain mercury, lead or
cadmium, as the zinc-carbon cells,
lead-acid accumulators or nickelcadmium cells do.
A summary of the comparison of lithium-ion
rechargeable batteries with other cell types.
P. 93 / 99
Feature
Comparison with other cells
No lithium metal
Lithium-ion rechargeable batteries
contain lithium compounds instead
of the reactive lithium metal. This
makes lithium-ion rechargeable
batteries safer for use and for
transportation.
Long cycle life
Lithium-ion rechargeable batteries
can be recharged and discharged
for 1200 cycles within 3 years.
Low self-discharge rate
Lithium-ion rechargeable batteries
only lose about 5% of the charge
per month. NiCd and NiMH cells
lose about 1% of the charge per
day.
P. 94 / 99
Feature
Comparison with other cells
Fast charge possible
Lithium-ion rechargeable batteries
can be fast charged to 70–80% of
full capacity in one hour.
Wide range of operating
temperatures
Lithium-ion rechargeable batteries
can be discharged between the
temperature range from –20°C to
60°C, and can be recharged
between 0°C to 45°C.
A summary of the comparison of lithium-ion
rechargeable batteries with other cell types.
P. 95 / 99
Lithium-ion polymer
rechargeable batteries (Li-poly
/ LiPo)
Lithium-ion polymer
rechargeable
batteries are now
commonly used in
mobile phones.
P. 96 / 99
The lithium-salt electrolyte is not held in an
organic solvent as in the lithium-ion design,
but in a solid polymer composite such as
polyethene oxide or polyacrylonitrile.
Advantages of Li-poly/ LiPo
The battery can be made to any shape.
The rate of self-discharge is much lower
compared with that of nickel-cadmium
and nickel-metal hydride rechargeable
batteries.
Class practice 34.2
P. 97 / 99
Class practice 34.2
Lithium-ion rechargeable batteries use lithium
compound instead of lithium metal as the anode.
Explain why lithium metal should not be used in
batteries.
P. 98 / 99
Lithium metal, like other alkali metals (sodium,
potassium, etc.) reacts vigorously with water to
produce hydrogen and a corrosive, strongly
alkaline solution LiOH.
2Li(s) + 2H2O(l)  2LiOH(aq) + H2(g)
If the seal of a cell with a lithium metal anode is
broken, water or even moisture in the air may
react with lithium, causing hydrogen and alkaline
solution to leak out.
Hydrogen may cause explosion and the alkaline
solution can cause severe skin burns.
P. 99 / 99