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Form 4 / Chemistry / Reading Assignment 3 / 1
Heep Woh College
F.4 Chemistry Reading Assignment 3
Group Name: ______________________________
Class: 4 (
)
Group Member: 1. _______________________ (
) 2. _______________________ (
)
3. _______________________ (
) 4. _______________________ (
)
Part I:
Read the following passage and answer the questions below.
New Developments in Fuel Cell Technology
Introduction
Fuel cells can convert the chemical energy in reactants to electrical energy
directly. They can generate electricity continuously if the supply of the reactants,
hydrogen and oxygen, is unlimited. Oxygen can be obtained from air directly while
the supply of hydrogen is a problem. Hydrogen can be produced from water and
other hydrogen-rich chemicals like hydrocarbons and alcohols.
Different Types of Fuel Cells
The following types of fuel cells are newly developed and currently used. They use
different chemicals as electrolytes and each has their own advantages, limitations
and special applications.
 Polymer Electrolyte Membrane Fuel Cells (PEMFC)
 Alkaline Fuel Cells (AFC)
 Phosphoric Acid Fuel Cells (PAFC)
 Molten Carbonate Fuel Cells (MCFC)
 Solid Oxide Fuel Cells (SOFC)
 Direct Methanol Fuel Cells (DMFC)
 Regenerative Fuel Cells (RFC)
Except direct methanol fuel cells, the other fuel cells use hydrogen directly as a fuel.
However, it is difficult and dangerous to store and transport a large amount of
hydrogen gas. These fuel cells are usually equipped with a reformer to produce
hydrogen gas from liquid hydrogen-rich chemicals.
Polymer electrolyte membrane fuel cells use a solid polymer as an electrolyte. This
makes the management of electrolyte easy and also reduce corrosion. Since they
use precious metals (e.g. platinum) as a catalyst, their costs are high. Moreover, the
catalyst is easily poisoned by impurities. On the other hand, they operate at
relatively low temperature compared with other fuel cells. This reduces its cost of
operation. In addition, for producing the same power, they have smaller weight
and size than other fuel cells.
S.Mo & C.K.Lau
Form 4 / Chemistry / Reading Assignment 3 / 2
Alkaline fuel cells use potassium hydroxide solution as an electrolyte. The alkaline
medium favours reaction at cathode. However, they are easily poisoned by
carbon dioxide. Hence, it is necessary to remove carbon dioxide in the reactant
gases, hydrogen and oxygen.
Phosphoric acid fuel cells use phosphoric acid as an electrolyte. They are
expensive because platinum is used a catalyst. They are have higher tolerance to
impurities in hydrogen gas compared with other fuel cells. They have high
efficiency in converting chemical energy to electrical energy and heat. However,
with the same weight and size, they provide less electrical energy than other fuel
cells.
Molten carbonate fuel cells use a mixture of lithium, sodium and / or potassium
molten carbonates as electrolytes, which are soaked in a porous and chemically
inert ceramic lithium aluminium oxide (LiAlO2) matrix. They can use non-precious
metals as a catalyst and thus they are not expensive. However, they must operate
at high temperature and this speeds up corrosion and even the breakdown of the
cell components. Anyway, they have high efficiency.
Molten carbonate fuel cells do not need a reformer to convert the hydrogen-rich
fuels to hydrogen because at high temperature, these fuels are converted to
hydrogen inside the fuel cell by a process called internal reforming. On the other
hand, since the cell reaction involves carbon dioxide, molten carbonate fuel cells
do not have the problem of “carbon dioxide poisoning”. Moreover, they are
resistant to other impurities especially those formed in the production of hydrogen
from carbon and steam.
Solid oxide fuel cells use solid zirconium oxide as an electrolyte. This makes the
management of electrolyte easy and also reduces corrosion. Since they can use
non-precious metals as a catalyst, they are not expensive. However, they must
operate at very high temperature and this speeds up the corrosion and even the
breakdown of the cell components. But they have high efficiency. Like molten
carbonate fuel cells, they can reform hydrogen-rich fuels internally to produce
hydrogen and are resistant to impurities.
Unlike other fuel cells, direct methanol fuel cells use methanol as fuel instead of
hydrogen. In operation, methanol and steam are mixed together and then fed into
the anode. Since methanol exists as a liquid at room temperature and pressure, it is
convenient to store, transport and supply to the market. Owing to these
characteristics, direct methanol fuel cells are suitable for portable electronic
devices such as mobile phones and notebook computers.
Similar to other fuel cells, regenerative fuel cells also generate electricity through
the reaction between hydrogen and oxygen. However, they have an electrolyzer
to decompose water into hydrogen and oxygen for storage. The electricity
generated from alternative energy sources such as solar energy powers the
electrolyzer.
S.Mo & C.K.Lau
Form 4 / Chemistry / Reading Assignment 3 / 3
Limitations of Fuel Cells
Fuel cells are still not widely used because of several reasons. Firstly, the catalyst
used, which is usually platinum, is expensive. To minimize the amount used,
powdered platinum is coated on one side of the carbon paper on the electrodes.
Secondly, the cost of generating electricity by the current fuel cell power plants is
too high compared with other common ways of power generation. Thirdly, fuel
cells need to have better designs including size and weight. Fourthly, storage of
fuels, ease of use and maintenance also need to be improved.
Questions:
1.
State one similarity and one difference between fuel cells and zinc-carbon
cells.
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2.
Why are alkaline fuel cells easily poisoned by carbon dioxide? Write the
chemical equation(s), with state symbols, to show any chemical reaction(s)
caused by carbon dioxide.
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3.
To deliver the same power, which type of fuel cells is usually large and heavy?
Why?
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4.
Which type(s) of fuel cells does / do not have a reformer? Why?
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5.
Write ionic half equations, with state symbols, for the reactions that occur at the
anode and cathode of a molten carbonate fuel cell. Using these half
equations, write an overall chemical equation, with state symbols, showing the
cell reaction.
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S.Mo & C.K.Lau
Form 4 / Chemistry / Reading Assignment 3 / 4
6.
Write ionic half equations, with state symbols, for the reactions that occur at the
anode and cathode of a direct methanol fuel cell. Using these half equations,
write an overall chemical equation, with state symbols, showing the cell
reaction.
____________________________________________________________________________
____________________________________________________________________________
End of Part I
Part II:
Read the following passage and answer the questions below.
New Developments in Rechargeable Lithium Cells
Introduction
Lithium ion cells are the most common rechargeable lithium cells. However, they
have higher internal impedance than nickel cadmium cells. For high power
applications, their prices are higher than those of lead acid accumulators.
Nevertheless, the situation is improving. Their cycle life and charge capacity are
becoming longer and larger respectively, especially for high power applications.
Different Types of Rechargeable Lithium Cells
1.
Lithium Ion Polymer Cells
Lithium ion polymer cells produce a voltage of 2.70 to 4.23 V. Their energy
density by mass is over 20% higher than that of lithium cobalt cells. They are
more stable than lithium ion cells. They do not degrade at high temperatures,
when overcharged or crushed. This is mainly because their electrolytes are
solids. Hence, they do not need protective circuits and special safety
precautions.
Lithium ion polymer cells operate as lithium ion cells do. However, the
electrolytes plasticize the polymers to form ion conductive polymers (or solid
electrolytes), which are safe and do not leak. Therefore, they are called solid
state cells.
Since lithium ion polymer cells do not contain liquid, they do not need
protective cases as lithium ion cells do. Therefore, the cells can be made into
ultra-thin (i.e. 0.64 mm) and different shapes to fit any space. Moreover, the
manufacturing process is simpler and the cells can be packaged in foil. This not
only lowers its cost but also allows more flexibility in design.
Lithium ion polymer cells have long shelf life but low discharge rate. They have
thicker separator than lithium ion cells. This decreases the effective surface
area of the electrodes and thus the current carrying capacity of the cells.
S.Mo & C.K.Lau
Form 4 / Chemistry / Reading Assignment 3 / 5
Fortunately, the relatively larger volume of the electrolyte helps to increase
energy storage. This facilitates their use in high capacity low power
applications.
On the other hand, some manufacturers produce cells designated as lithium
polymer cells that actually contain liquid or gel. These cells are more vulnerable
to swelling than genuine solid lithium ion polymer cells.
2.
Variants on Positive Electrodes
Researches are centred on the positive electrodes of lithium ion cell chemistry.
Many corresponding cell variants have been developed but only lithium
cobalt cells and lithium manganese cells are being manufactured in large
quantities. The technologies for other variants are either not mature or their
patents still being held by certain companies without industry standardization.
Lithium cobalt cells (LiCoO2) produce a voltage of 3.7 V. They are the most
popular lithium ion cells because its corresponding technology is mature and
proven. However, cobalt and its compounds are expensive.
Lithium manganese cells (LiMn2O4) give a voltage of 3.0 V. However, their
energy density by mass is 20% less than that of lithium cobalt cells. In spite of this,
compared with lithium cobalt cells, lithium manganese cells have a lower cost,
a better performance at high temperature, are more stable and hence are
safer.
Lithium nickel cells (LiNiO2) have a voltage of 3.6 V. Their energy density by
mass is 30% higher than that of lithium cobalt cells. Their reactions are the most
exothermic. This brings cooling problems especially in high power applications.
Lithium nickel cobalt manganese cells (Li(NiCoMn)O2) contain equal quantities
of nickel, cobalt and manganese. This achieves a balance among high energy
density by mass, large cell voltage, safety and cost.
Lithium iron phosphate cells (LiFePO4) have the best thermal and chemical
stability among all lithium ion cells. They do not burn in case of overcharge and
short circuit. They resist high temperature without decomposition. They also
have the longest cycle life.
A range of new environmentally friendly cathodes have been developed on
the basis of lithium transition metal phosphate cells (e.g. LiFePO 4). Doping the
cathodes of lithium ion cells with different transition metals can regulate the
current carrying capacity of the active materials, the internal impedance and
even the voltage of cells. Therefore, the cell voltage can be regulated in the
range of 2.1 to 5.0 V.
S.Mo & C.K.Lau
Form 4 / Chemistry / Reading Assignment 3 / 6
Lithium transition metal phosphate cells have made a great improvement in
cost, safety and environmental protection. However, the expense is a
decrease of 140% in energy density by mass compared with lithium cobalt cells.
In spite of this variants with higher energy density by mass are being explored.
More importantly, since lithium transition metal phosphate cells are safer than
lithium cobalt cells, additional safety devices are not necessary. Hence, more
space is available to install larger cells with a larger charge capacity.
Lithium metal polymer cells use lithium metal as negative electrodes instead of
lithium carbon based ones and use organic compounds as an electrolyte. The
cell voltage is 4.0 V while the energy density by mass is over 30% less than that
of lithium cobalt cells. They usually operate at 80 to 120oC for maximum power
though they can still operate in lower power at room temperature.
Lithium sulphur cells (Li2S8) have a voltage of 2.1 V. Its energy density by mass is
more than two times higher than that of lithium cobalt cells.
3.
Variants on Negative Electrodes
Recently, lithium titanate spinel (Li4Ti5O12) has been used to make the negative
electrodes in rechargeable lithium cells. The material makes the cells provide
high power, more stable and have longer cycle life.
Developments in Technology
In 2005, the United States, Japan and France developed nano-materials to make
the electrodes of rechargeable lithium cells. They provide a larger active surface
area to give a larger current. Hence, the cells can deliver three times the power of
the existing lithium ion cells and they can be fully charged up in 6 minutes. In
addition, they have a longer cycle life. However, their charge capacity only
increases slightly.
In 2006, some American scientists figured out a method to use viruses to form
nano-sized wires. These wires can be used to build ultra-thin lithium ion cells, which
have three times the normal energy density by mass of the existing lithium ion cells.
Questions:
1.
Which lithium ion cell can deliver the largest voltage?
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S.Mo & C.K.Lau
Form 4 / Chemistry / Reading Assignment 3 / 7
2.
Arrange the following rechargeable lithium cells in decreasing order of energy
density by mass.
Lithium ion polymer cells, lithium cobalt cells, lithium manganese cells, lithium
nickel cells, lithium transition metal phosphate cells, lithium metal polymer cells,
lithium sulphur cells
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3.
What is the meaning of energy density by mass for rechargeable lithium cells?
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4.
Explain whether water can be used as a solvent in lithium ion cells.
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5.
Explain whether it is more dangerous to use lithium metal polymer cells or
lithium ion cells.
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6.
State two most recent developments of rechargeable lithium cells.
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The End
S.Mo & C.K.Lau