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1
Primary Cells
A primary cell is a cell that can be used once
only and cannot be recharged. The reactants
cannot be regenerated
2
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.
3
Primary cells

• These cells are not rechargeable.

Alkaline manganese cells
4
Primary cells

• These cells are not rechargeable.

Silver oxide cells (button cells)
5
Primary cells

• These cells are not rechargeable.

Lithium primary cells (button cells)
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Zinc-carbon Cells
7
Zinc-carbon Cells
Overall reaction Zn(s) 2MnO2(s) 2NH4(aq)?
Zn2(aq) Mn2O3(s) 2NH3(aq) H2O(l)
Ecell 1.50 V
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Zinc-carbon Cells
The cell diagram for the zinc-carbon cell is
Overall reaction Zn(s) 2MnO2(s) 2NH4(aq)?
Zn2(aq) Mn2O3(s) 2NH3(aq) H2O(l)
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Overall reaction Zn(s) Ag2O(s) ? ZnO(s)
2Ag(s)
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Q.20(a)
HgO(s)
Overall reaction Zn(s) HgO(s) ? ZnO(s) Hg(l)
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Q.20(b)
HgO(s)
0.098V (?1.216V) 1.314V
12
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)
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Lead grids coated with PbSO4(s)
During charging
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Lead grids coated with PbSO4(s)
During charging
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Lead grids coated with PbSO4(s)
During discharging
17
Lead grids coated with PbSO4(s)
During discharging
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Overall reaction -
19
Overall reaction -
PbSO4 is coated on the electrodes, The reversed
processes are made possible.
20
Overall reaction -
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.
21
Overall reaction -
charge
Pb(s) and PbO(s) are on different
electrodes Direct reaction is not possible Porous
partition is not needed
22
Overall reaction -
During discharging, H2SO4 is being used up The
density of electrolyte solution ? The
charging/discharging status can be monitored by a
hydrometer.
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Q.21
Ecell Eocathode Eoanode (1.69V)
(?0.35V) 2.04V
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Nickel-cadmium cells Nicad cells
Q.22(a)
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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)
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Nickel metal hydride cell (NiMH)
Cathode NiO(OH) Anode MH(s) where M is a
hydrogen-absorbing alloy. More environmentally
friendly than NiCad cell due to the absence of Cd.
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Nickel metal hydride cell (NiMH)
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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.
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Anode is graphite into which Li are inserted
Lithium ion cell
30
Cathode is metal oxide into which Li are
inserted.
Lithium ion cell
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During charging, Li moves from cathode to anode
Lithium ion cell
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During discharge, Li moves from anode to cathode
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Voltage is 3.6/3.7V Three times that of NiCad or
NiMH
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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.
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Continuous supply of fuel, H2
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Continuous supply of oxygen
No need for recharging
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Other fuels such as hydrocarbon, alcohol, or
glucose are possible
40
Ni(s) and NiO(s) are catalysts for the half-cell
reactions
41
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)

42
Q.24
Maximum energy that can be used to do useful work
(2)(96485)(1.22) 235 kJ mol?1
43
Q.25

• To increase the mobility of OH?/K to balance the
  extra charges built up in half-cells.
• OH?(aq) ? quickly at anode
• OH?(aq) ? quickly at cathode
• 2. To increase the solubility of KOH

44
Q.26
Overall reaction - CH4 2O2 ? CO2 2H2O
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Supplementary notes from HKDSE Chemistry
46
What is a fuel cell?
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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.

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How a fuel cell works
49
How a fuel cell works
e-
e-
Anode H2(g) 2OH?(aq) ? 2H2O(g) 2e?
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How a fuel cell works
Oxidant O2
e-
e-
H2
hydrogen
Cathode O2(g) 2H2O(g) 4e? ? 4OH?(aq)
51

• Functions of nickel electrodes

• act as electrical conductors that connect the
  fuel cell to the external circuit

• act as a catalyst for the reactions

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• 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)
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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.
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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.
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• 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 hydrogen-oxygen 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.
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• Fuel cells can also be used in electrical and
  hybrid vehicles.

A fuel cell car developed by DaimlerChrysler in
Germany.
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• An MP3 player runs on methanol fuel cell in which
  methanol is used as fuel.

Fuel cells can be used in portable electronic
products.
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• A portable fuel cell charger for mobile phones.

Fuel cells can be used in portable electronic
products.
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Application Features of fuel cells Examples
Power source for remote locations high efficiency high reliability non-polluting able to work continuously spacecraft remote weather stations large parks rural locations
The features of fuel cells and their applications.
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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.
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Application Features of fuel cells Examples
Portable electronic products high efficiency non-polluting lightweight can be refilled conveniently notebook computers mobile phones MP3 players handheld breathalyzers
The features of fuel cells and their applications.
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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.
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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.

1. 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 ? 2H2O
4e?
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(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)
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(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.
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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.

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Fuel cell type Common electrolyte Operating temperature System output Electrical efficiency Applications
Proton ExchangeMembrane(PEMFC) Solid organic polymer called poly-perfluoro-sulphonic acid 50100C lt 1 kW250 kW 5358 (transportation) 2535 (stationary) Backup power Portable power Small distributed generation Transportation
Alkaline (AFC) Aqueous solution of potassium hydroxide soaked in a matrix below 80C 10 kW100 kW 60 Military applications Space projects
The summary of the main feature of some fuel
cells.
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Fuel cell type Common electrolyte Operating temperature System output Electrical efficiency Applications
Phosphoric Acid (PAFC) Liquid phosphoricacid soaked in a matrix 150200C 50 kW 1 MW (250 kW module typical) gt 40 Distributed generation
Molten Carbonate (MCFC) Molten lithium, sodium, and / or potassium carbonates, soaked in a matrix 600700C lt 1 kW 1 MW (250 kW module typical) 4547 Electric utility Large distributed generation
The summary of the main feature of some fuel
cells.
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Fuel cell type Common electrolyte Operating temperature System output Electrical efficiency Applications
Solid Oxide (SOFC) Solid zirconium oxide to which a small amount of yttrium(III) oxide is added 6501000C lt 1 kW3 MW 3543 Auxiliary power Electric utility Large distributed generation
The summary of the main feature of some fuel
cells.
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• 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))

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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 micro-organisms use the produced energy to
grow and to maintain their metabolism.
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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.
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anode
cathode
wastewater
glucose
H2O
(bacteria)
H
O2
CO2 e- H
membrane
The general layout of an MFC.
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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.
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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.
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• Questions
• 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.
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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
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Write balanced ionic equations for the reactions
that occur at the cathode and the anode.
Cathode
O2(g) H(aq) ? H2O(l)
(1)
2
4
4e?
Anode
C6H12O6(aq) ? CO2(g)
H(aq) (2)
24e?
6H2O (l)
6
24
Overall reaction 6?(1) (2)
C6H12O6(aq) 6O2(g) ? 6CO2(g) 6H2O(l)
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Class practice 34.1

• 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
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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.
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(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.
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1. 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)
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• There are two types of rechargeable lithium cells

Lithium-ion rechargeable batteries
Lithium-ion polymer rechargeable batteries
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Lithium-ion rechargeable batteries
Lithium-ion rechargeable batteries are commonly
used in portable electronic devices.
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• In a lithium-ion rechargeable battery, both the
  positive electrode and negative electrode contain
  lithium compounds.

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Discharging
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Charging
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e.g. Li1-xCoO2
Positive electrode
a metal oxide fitted with Li ion e.g. cobalt
dioxide CoO2, manganese dioxide MnO2 or nickel
dioxide NiO2
Negative electrode
lithium-carbon compound LixC6
Electrolyte
a lithium salt in an organic solvent
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• The chemical equations for the reactions are

Positive electrode
discharging
4
3
charging
Negative electrode
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Overall reaction

• Note that lithium ions themselves are neither
  oxidized nor reduced.

• The voltage of a lithium-ion rechargeable battery
  is 3.7 V.

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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 (3.63.7 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.
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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 nickel-cadmium cells do.
A summary of the comparison of lithium-ion
rechargeable batteries with other cell types.
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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.
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Feature Comparison with other cells
Fast charge possible Lithium-ion rechargeable batteries can be fast charged to 7080 of full capacity in one hour.
Wide range of operating temperatures Lithium-ion rechargeable batteries can be discharged between the temperature range from 20C to 60C, and can be recharged between 0C to 45C.
A summary of the comparison of lithium-ion
rechargeable batteries with other cell types.
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Lithium-ion polymer rechargeable batteries
(Li-poly / LiPo)
Lithium-ion polymer rechargeable batteries are
now commonly used in mobile phones.
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• 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.

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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.
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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.