primary cells

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Primary CellsPrimary 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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․Zinc-carbon cells

Primary cellsPrimary cells

These cells are not rechargeable.

Recharging is dangerous as it produces H2 and heat which results in an explosion.

non-rechargeable

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․Alkaline manganese cells

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․Silver oxide cells (button cells)

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․Lithium primary cells (button cells)

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Zinc-carbon CellsZinc-carbon Cells

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Zinc-carbon CellsZinc-carbon CellsAt anode:Zn(s) Zn2+(aq) + 2e–

At cathode:

2MnO2(s) + 2NH4+(aq) + 2e–

Mn2O3(s) + 2NH3(aq) + H2O(l)

Overall reaction:

Zn(s) + 2MnO2(s) + 2NH4+(aq)

Zn2+(aq) + Mn2O3(s) + 2NH3(aq) + H2O(l)

Ecell = +1.50 V

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The cell diagram for the zinc-carbon cell is:

Zinc-carbon CellsZinc-carbon Cells

Overall reaction:

Zn(s) + 2MnO2(s) + 2NH4+(aq)

Zn2+(aq) + Mn2O3(s) + 2NH3(aq) + H2O(l)

Zn(s) | Zn2+(aq) [2MnO2(s) + 2NH4+(aq)], [Mn2

O3(s) + 2NH3(aq) + H2O(l)] | C(graphite)

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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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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)

HgO(s)

Q.20(a)

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HgO(s)

anodecathodecell EEE

= +0.098V – (1.216V) = 1.314V

Q.20(b)

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Secondary Secondary CellsCellsElectrochemical 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)

Negative electrode :

PbSO4(s) + 2e Pb(s) + SO42(aq)

During charging

spongy lead

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Positive electrode :

PbSO4(s) + 2H2O(l)

PbO2(s) + 4H+(aq) + SO42(aq) + 2e

spongy PbO2

During charging

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Anode :

Pb(s) + SO42(aq) PbSO4(s) + 2e

During discharging

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Cathode :

PbO2(s) + 4H+(aq) + SO42(aq) + 2e

PbSO4(s) + 2H2O(l)

During discharging

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Overall reaction : -

Pb(s) + PbO2(s) + 2H2SO4(aq) 2PbSO4(s) + 2H2O(l)dischargecharge

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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PbSO4 is coated on the electrodes,

The reversed processes are made possible.



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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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charge

Pb(s) and PbO(s) are on different electrodes

Direct reaction is not possible

Porous partition is not needed


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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 – Eo

anode

= (1.69V) – (0.35V) = 2.04V

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Nickel-cadmium cells – Nicad cellsQ.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)

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Nickel-cadmium cells – Nicad cellsQ.22(b)


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)

On discharging,

Anode : -

MH(s) + OH(aq) M(s) + H2O(l) + e

At cathode,

NiO(OH)(s) + H2O(l) + e

Ni(OH)2(s) + OH(aq)

+3

+2

+1

0

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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

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Cathode is metal oxide into which Li+ are inserted.

Lithium ion cell

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Lithium ion cellDuring charging, Li+ moves from cathode to anode

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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

sizevoltage

Much higher

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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.

Q.23

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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

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Ni(s) and NiO(s) are catalysts for the half-cell reactions

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Fuel CellsFuel 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)

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Q.24

Maximum energy that can be used to do useful work

ocellnFE

= (2)(96485)(1.22) = 235 kJ mol1

82.2%100%286235 efficiency %

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Q.25

1. 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

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Q.26

Anode : -

CH4 + 2H2O CO2 + 8H+ + 8e

Cathode : -

2O2 + 8H+ + 8e 4H2O


CH4 + 2O2 CO2 + 2H2O

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Supplementary notes from HKDSE Chemistry

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What is a fuel What is a fuel cell?cell?

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It converts the chemical energy of a continuous supply of reactants (a fuel and an oxidant) into electrical energy.

Fuel cellFuel cell

It is a primary cell.

The products are removed continuously.

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How a fuel cell worksHow a fuel cell works

anode (−) cathode (+)porous Ni electrodes

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e−e−

Anode : H2(g) + 2OH(aq) 2H2O(g) + 2e

H2

hydrogen

Fuel : H2hot KOH electrolyte ( 200°C)

steam (exhaust)

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e−e−

Cathode : O2(g) + 2H2O(g) + 4e 4OH(aq)

H2

hydrogen

Oxidant : O2

O2

oxygen

steam (exhaust)

H2

hydrogen

Fuel : H2

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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 cathodeO2(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 cellsApplications of fuel cellsFor remote locations, such as spacecraft, remote weather stations…

Fuel cells are used in space shuttle to provide electricity for routine operation.

Continuous supply of fuel No need to be replaced frequently

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high efficiency

Non-polluting

e.g. hydrogen-oxygen fuel cells : 70%much higher than internal combustion engines ( 20%) in motor cars.

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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A fuel cell car developed by DaimlerChrysler in Germany.

Fuel cells can also be used in electrical and hybrid vehicles.

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Fuel cells can be used in portable electronic products.

An MP3 player runs on methanol fuel cell in which methanol is used as fuel.

methanol

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A portable fuel cell charger for mobile phones.

Fuel cells can be used in portable electronic products.

fuel cell charger

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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


hospitals

hotels

office buildings

Transportation quiet

high efficiency(70%)

non-polluting


electric vehicles

boats


But expensive

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ApplicationFeatures of fuel

cellsExamples

Portable

electronic

products

high efficiency

non-polluting

lightweight

can be refilled conveniently

notebook computers

mobile phones

MP3 players

handheld breathalyzers

Class practice 32.4Class practice 32.4


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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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(a) Write the equations for the reactions at the anode and the cathode respectively.

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.

At anode: CH3OH + H2O 6H+ + CO2

At cathode: O2 + 4H+ 2H2O

-2 +4+ 6e

0 -2+ 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. OrMethanol poses a lower risk of explosion than hydrogen. (Any ONE)

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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.

(c) What are the potential dangers associated with using methanol fuel cells?

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Different types of fuel cells and Different types of fuel cells and their applicationstheir 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

temperatureSystem output

Electrical

efficiencyApplications

Proton ExchangeMembrane(PEMFC)

Solid organic polymer called poly-perfluoro-sulphonic acid

50–100°C < 1 kW–250 kW

53–58% (transportation) 25–35% (stationary)

• Backup power

• Portable power

• Small distributed generation

• Transportation

Alkaline (AFC)

Aqueous solution of potassium hydroxide soaked in a matrix

below 80°C 10 kW–100 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


Electrical


Phosphoric Acid (PAFC)

Liquid phosphoricacid soaked in a matrix

150–200°C 50 kW– 1 MW (250 kW module typical)

> 40% • Distributed generation

Molten Carbonate (MCFC)

Molten lithium, sodium, and / or potassium carbonates, soaked in a matrix

600–700°C < 1 kW–

1 MW (250 kW module typical)

45–47% • Electric utility

• Large distributed generation


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Fuel cell type

Common electrolyte

Operating


Electrical


Solid Oxide (SOFC)

Solid zirconium oxide to which a small amount of yttrium(III) oxide is added

650–1000°C < 1 kW–3 MW

35–43% • Auxiliary power

• Electric utility

• Large distributed generation


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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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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 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 cancollect 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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The general layout of an MFC.

wastewaterglucose

(bacteria)

CO2 e− H+

anode cathode

membrane

H+

H2O

O2

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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

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.

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Questions

2. Why are microbial fuel cells (MFC) considered a greener source of energy?

3. Suggest TWO substances that can be used as the ‘fuel’ for microbial fuel cells (MFC).

Microbial fuel cells use wastewater as the source of fuel and produce CO2 and water which are harmless.

Glucose and wastewater

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Write balanced ionic equations for the reactions that occur at the cathode and the anode.

O2(g) + H+(aq) H2O(l) (1)Cathode

C6H12O6(aq) CO2(g) + H+(aq) (2)Anode

+ 6H2O (l)

6 24 + 24e

24 + 4e

Overall reaction : 6(1) + (2)

C6H12O6(aq) + 6O2(g) 6CO2(g) + 6H2O(l)

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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

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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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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

(b) Write the overall equation for the cell reaction.

At cathode: O2(g) + 4H+(aq) + 4e 2H2O(l)

At anode: 2H2(g) 4H+(aq) + 4e

2H2(g) + O2(g) 2H2O(l)

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There are two types of rechargeable lithium cells:

Lithium-ion Lithium-ion

rechargeable rechargeable

batteriesbatteries

Lithium-ion polymer Lithium-ion polymer rechargeable

rechargeablebatteriesbatteries

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Lithium-ion rechargeable Lithium-ion rechargeable batteriesbatteries

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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DischargingLoad

electronscurrent

positiveelectrode

negativeelectrode separator

electrolyte

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Charging

negativeelectrode

positiveelectrodeseparator

Charger

electrolyte

currentelectrons

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lithium-carbon compound LixC6

Negative electrodeNegative electrode

a metal oxide fitted with Li+ ion e.g. cobalt dioxide CoO2, manganese dioxide MnO2 or nickel dioxide NiO2

Positive electrodePositive electrode

a lithium salt in an organic solvent

ElectrolyteElectrolyte

graphite

e.g. Li1-xCoO2

Prevents electrolysis of water to give H2

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The chemical equations for the reactions are:

Positive electrodePositive electrode

discharging

chargingLi1-xCoO2 + xLi+ + xe− LiCoO2

Negative electrodeNegative electrode

discharging

chargingLixC6 6C + xLi+ + xe−

+4 +3

It is the graphite in the lithium compound that loses electrons

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Overall reactionOverall reaction

Li1-xCoO2 + LixC6 LiCoO2 + 6Cdischarging

charging

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 (3.6–3.7 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.

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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.


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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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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.


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Lithium-ion polymer rechargeable bLithium-ion polymer rechargeable batteries (Li-poly / LiPo)atteries (Li-poly / LiPo)

Lithium-ion polymerrechargeable 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/ LiPoAdvantages 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.

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.

2Li(s) + 2H2O(l) 2LiOH(aq) + H2(g)

primary cells

Documents

agsoverall reaction

ohaqoverall reaction

boverall reaction

neededoverall reaction

lithium anode

nimhthe anode of lithium

reactive lithium metal

metal oxide