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Overview

🌳 Concept tree

Lesson 2→Commercial Batteries, Fuel Cells and Metal Corrosion

├── Kinds of galvanic cells

├── Primary cells

├── Secondary cells (storage cells
accumulators)

├── Why corrosion matters

├── Mechanism→corrosion is a galvanic cell

├── Factors that cause corrosion

└── Protection of iron

Lesson 2: Commercial Batteries, Fuel Cells and Metal Corrosion · 1 / 9

Kinds of galvanic cells

According to whether the cell can be recharged, galvanic cells are classified as primary cells and secondary cells.
(The fuel cell is a special case that is supplied continuously with its reactants.)

Lesson 2: Commercial Batteries, Fuel Cells and Metal Corrosion · 2 / 9

Primary cells

A primary cell stores energy as chemical energy and releases it through a spontaneous, irreversible redox reaction.
It stops working when the anode material is used up or the ions of the cathode half-cell are consumed.
It cannot be recharged, because its reaction cannot be reversed in a practical way.
For convenience, especially in portable devices, primary cells are made in the "dry" form, which gives a steady potential for a long time and allows small sizes.
(A) The mercury cell. Made as a small cylinder or button, so it is used in hearing aids, watches and cameras.

Comparison aspect	Main idea	Keywords
Negative pole (anode)	zinc	zinc
Positive pole (cathode)	mercury(II) oxide HgO with graphite	mercuryoxidegraphite
Electrolyte	potassium hydroxide in a tightly sealed case	potassiumhydroxidetightlysealed
Cell reaction	Zn + HgO → ZnO + Hg
•	Emf ≈ 1.35 V practically constant until the cell is exhausted	practicallyconstantuntilcell
•	Mercury is poisonous so used cells must be disposed of safely	Mercurypoisonoususedcells
(B) The fuel cell.	Hydrogen burns in oxygen: 2H₂ + O₂ → 2H₂O + energy. In a fuel cell this reaction is carried out in a controlled way so that the energy appears directly as electricity instead of heat and light	Hydrogenburnsoxygen2H₂O

Comparison aspect	Main idea	Keywords
•	Two porous carbon electrodes (hollow lined with porous carbon with a catalyst) dip in hot concentrated potassium hydroxide solution hydrogen is fed to one oxygen to the other	porouscarbonelectrodeshollow
Anode (oxidation)	2H₂(g) + 4OH⁻(aq) → 4H₂O(l) + 4e⁻ (E°ox = +0.83 V)	4H₂O
Cathode (reduction)	O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq) (E°red = +0.40 V)	2H₂O
Total	2H₂(g) + O₂(g) → 2H₂O emf = 1.23 V	2H₂O

Features it is not consumed like other cells because fuel is supplied from outside and the product (water) is removed continuously; it does not store energy — it needs continuous supply of reactants.
It works at high temperature, so the water leaves as vapour, which can be condensed and used as drinking water by astronauts.
It is preferred in spacecraft because its fuel (hydrogen and oxygen) is the same fuel used to launch rockets, and it is light and clean.

Lesson 2: Commercial Batteries, Fuel Cells and Metal Corrosion · 3 / 9

Secondary cells (storage cells, accumulators)

In a secondary cell the chemical reactions are reversible.
During use (discharge) the cell works as a galvanic cell; it is recharged by passing a direct current from an outside source through it in the direction opposite to the discharge current, so it then works as an electrolytic cell and the chemical energy is stored again.
(A) The lead–acid battery (car battery).

A car battery has six cells in series, each giving about 2 V, so total emf = 6 × 2 = 12 V.
Anode (negative plate) lead grid filled with spongy lead, Pb.
Cathode (positive plate) lead grid filled with a paste of lead(IV) oxide, PbO₂.
Plates are separated by insulating sheets and dip in dilute sulphuric acid (the electrolyte), all in a case of hard rubber or plastic that the acid does not attack.
Discharge (galvanic cell):

AnodePb(s) + SO₄²⁻(aq) → PbSO₄(s) + 2e⁻ (E°ox = +0.36 V)

CathodePbO₂(s) + 4H⁺(aq) + SO₄²⁻(aq) + 2e⁻ → PbSO₄(s) + 2H₂O(l) (E°red = +1.69 V)

TotalPb + PbO₂ + 4H⁺ + 2SO₄²⁻ → 2PbSO₄ + 2H₂O emf ≈ 2.05 V per cell.

During discharge both plates are slowly covered with lead(II) sulphate, sulphuric acid is used up and water is produced, so the acid becomes more dilute and its density falls.
The state of charge is checked with a hydrometer for a fully charged battery the acid density is about 1.28–1.30 g/cm³; if it falls below about 1.20 g/cm³ the battery needs recharging.

Charging (electrolytic cell) a d.c.
source of slightly higher voltage than the battery’s is connected so that its negative pole goes to the battery’s negative plate and its positive pole to the positive plate.
The discharge reaction is reversed:
2PbSO₄(s) + 2H₂O(l) → Pb(s) + PbO₂(s) + 4H⁺(aq) + 2SO₄²⁻(aq)

Lead sulphate is turned back into lead at one plate and lead(IV) oxide at the other, and the acid concentration is restored.
In a car the dynamo/alternator recharges the battery continuously while the engine runs.
(B) The lithium-ion battery.

A dry, rechargeable battery used in mobile phones, laptops and modern (electric) cars in place of the lead–acid battery, because it is lighter, stores more energy per unit mass, and is small in size.
It is based on lithium, which is the lightest metal and has a very low reduction potential (about −3.04 V), so it can give a high cell voltage.

Inside the metal case three thin layers are rolled in a spiral: the anode (negative): lithium–graphite, LiC₆; the cathode (positive): cobalt(IV) oxide, CoO₂, which becomes lithium cobalt oxide, LiCoO₂, as the cell discharges; and a very thin plastic separator that keeps the electrodes apart but lets Li⁺ ions pass.
The layers are soaked in an anhydrous (water-free) electrolyte, a solution of lithium hexafluorophosphate, LiPF₆, in an organic solvent.
Discharge at the anode LiC₆ → C₆ + Li⁺ + e⁻; at the cathode CoO₂ + Li⁺ + e⁻ → LiCoO₂.
Total LiC₆ + CoO₂ → C₆ + LiCoO₂ emf ≈ 3 V.
On recharging, the reactions run in the opposite direction and lithium ions move back through the separator.

Lesson 2: Commercial Batteries, Fuel Cells and Metal Corrosion · 4 / 9

Why corrosion matters

Corrosion is the chemical (electrochemical) attack on a metal by its environment.
It causes huge economic losses; it is estimated that about a quarter of the world’s yearly iron production is lost to corrosion.
The corrosion of iron is called rusting.
This is why preventing it is considered a basis of protecting the world economy.

Lesson 2: Commercial Batteries, Fuel Cells and Metal Corrosion · 5 / 9

Mechanism: corrosion is a galvanic cell

Very pure iron does not rust easily.
Industrial iron and steel contain impurities (for example carbon) and are alloys that are never perfectly uniform.
Wherever a more active region (iron) touches a less active region (carbon or another metal) in the presence of moisture, a tiny local galvanic cell forms.
The more active metal is the anode and is corroded; the less active one is the cathode.
For iron in damp air:

Comparison aspect	Main idea	Keywords
Anode	(the iron itself): 2Fe(s) → 2Fe²⁺(aq) + 4e⁻. The Fe²⁺ passes into the water film which containing dissolved ions acts as the electrolyte the electrons travel through the metal to the cathode regions (carbon impurities)	ironitself2Fe²passes
Cathode	(carbon/impurity regions): O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)	carbonimpurityregions2H₂O
Ions meet	2Fe²⁺ + 4OH⁻ → 2Fe(OH)₂(s) (iron(II) hydroxide)	2Fe²ironhydroxide
Further oxidation by dissolved oxygen	2Fe(OH)₂ + ½O₂ + H₂O → 2Fe(OH)₃ (iron(III) hydroxide after losing water it becomes the brown rust Fe₂O₃·xH₂O)	ironhydroxideafterlosing
Overall	2Fe + 3/2 O₂ + 3H₂O → 2Fe(OH)₃	3H₂O
•	Rusting is slow in pure water because very few ions are present but it is much faster in sea water or salty water because the dissolved salts make the electrolyte a much better conductor	Rustingslowpurewater

Lesson 2: Commercial Batteries, Fuel Cells and Metal Corrosion · 6 / 9

Factors that cause corrosion

Comparison aspect	Main idea	Keywords
Factors in the metal itself	Heterogeneity of alloys: industrial alloys are rarely uniform so many local cells form and the more active component corrodes. Contact between different metals: at welds rivets or screws made of a different metal a galvanic cell forms and the more active metal corrodes first (aluminium corrodes when in contact with copper iron corrodes when in contact with copper)	Heterogeneityalloysindustrialrarely
Factors in the surrounding medium	water (moisture) oxygen and dissolved salts (and acids)	watermoistureoxygendissolved

Lesson 2: Commercial Batteries, Fuel Cells and Metal Corrosion · 7 / 9

Protection of iron

The aim is usually to cover the iron so that it is isolated from air and water, or to make it the cathode of a cell.

Comparison aspect	Main idea	Example	Keywords
Painting or oiling	coating with paint oil grease or varnish. It is simple but not effective for the long term because the coating wears or cracks	—	coatinggreasevarnishsimple
Covering with a metal	Less active metal (	tin plating of food cans): protects only while the coating is whole. If it is scratched iron (the more active metal) becomes the anode and tin the cathode so the iron corrodes faster than unprotected iron. More active metal (e.g. zinc "galvanising" – dipping steel in molten zinc): if scratched zinc is higher in the series so it is the anode and corrodes first while iron is the cathode and is protected. Corrosion of the iron begins only after all the zinc nearby is consumed	Lessactiveplatingfood
Cathodic protection with a sacrificial anode	buried iron pipes and ships’ hulls (always in contact with salty water or damp soil) are connected to blocks of a more active metal	magnesium (or zinc). The magnesium acts as the anode and is corroded instead of the iron so it is called a sacrificial electrode and is replaced from time to time	buriedironpipesships

Lesson 2: Commercial Batteries, Fuel Cells and Metal Corrosion · 8 / 9

Part 9

📖

🔍 In-Depth Explanations, Deductions & Multi-Book Insights:

Chemistry Thanaweya Amma (2026/2027)

Lesson 2 – Commercial Batteries, Fuel Cells and Metal Corrosion
Detailed Academic Lesson Overview:
Batteries store chemical energy for on-demand electrical delivery.
Primary cells (Mercury cell, Fuel cell) are irreversible.
Secondary rechargeable cells (Lead-acid car battery, Lithium-ion battery) reverse their chemistry during charging.
We also examine electrochemical metal corrosion and cathodic/anodic protection.

🌳 Hierarchy Classification & Conceptual Roadmap

Commercial Cells and Corrosion

├── 1. Primary Batteries (Non-rechargeable)

│ ├── Mercury Cell (1.35 V): Zn anode, HgO cathode, KOH electrolyte; compact for hearing aids
│ └── Hydrogen Fuel Cell (1.23 V): Porous carbon cylinders with H₂ and O₂; operates at high temp, provides drinking water in space

├── 2. Secondary Batteries (Rechargeable)

Comparison aspect	Main idea	Keywords
•	│ ├── Lead-Acid Car Battery (12 V / 6 cells × 2 V)	LeadAcidBatterycells
│ │ ├── Discharging	Pb(s) + PbO2(s) + 2 H2SO4 → 2 PbSO4(s) + 2 H2O (acid density drops)	PbO2H2SO4PbSO4acid
│ │ └── Recharging	Connect to DC source with voltage slightly higher than 12 V	Connectsourcevoltageslightly
•	│ └── Lithium-Ion Battery (3.0 V)	LithiumBattery
│ ├── Anode	Lithium-graphite (LiC6) Cathode: Lithium cobalt oxide (LiCoO2)	LithiumgraphiteLiC6Cathode
│ └── Electrolyte	Non-aqueous anhydrous LiPF6 (Lithium hexafluorophosphate)	aqueousanhydrousLiPF6Lithium

└── 3. Metal Corrosion and Rusting of Iron

Comparison aspect	Main idea	Example	Keywords
├── Electrochemical mechanism	Fe anode impurities/carbon cathode moisture electrolyte	—	anodeimpuritiescarboncathode
├── Overall rust reaction	2 Fe + 3/2 O2 + 3 H2O → 2 Fe(OH)3 (Hydrated Iron(III) oxide)	—	HydratedIronoxide
•	└── Corrosion Protection	—	CorrosionProtection
├── Galvanic / Anodic Protection	Coating iron with more active metal (Zinc / Sacrificial anode Mg)	—	Coatingironactivemetal
└── Cathodic Protection	Coating iron with less active metal (Tin Sn dangerous if scratched!)	—	Coatingironlessactive
•	📋 Master Teacher's Blackboard Rules & Exam Secrets	—	MasterTeacherBlackboardRules
RULE 1	Fuel cell DOES NOT store energy unlike other galvanic cells it must be continuously supplied with fuel (H2) and oxidant (O2) from external sources	—	FuelcellDOESstore
RULE 2	Fully charged lead-acid battery has electrolyte density 1.28 to 1.30 g/cm³. If density drops below 1.20 g/cm³ it requires recharging because sulfuric acid is consumed to form water	—	Fullychargedleadacid
RULE 3	During recharging connect the battery's positive pole to the external source's positive pole and negative to negative with voltage slightly exceeding the battery voltage	—	Duringrechargingconnectbattery
EXAM TRAP	Anodic coating (	galvanizing with Zn) protects iron even if scratched because zinc oxidizes preferentially. Cathodic coating (e.g. tin-plating with Sn) causes iron to rust FASTER if scratched because iron becomes the anode!	Anodiccoatinggalvanizingprotects
📊 Detailed Comparison Matrix	Comparative Synthesis of Primary and Secondary Battery Cells	—	ComparativeSynthesisPrimarySecondary

Battery Type	Anode (-)	Cathode (+)	Electrolyte	Total Cell EMF
Mercury Cell	Zinc amalgam (Zn/Hg)	Mercuric oxide (HgO)	KOH (Alkaline)	1.35 V
Hydrogen Fuel Cell	Porous carbon (H₂ gas)	Porous carbon (O₂ gas)	Hot KOH solution	1.23 V
Lead-Acid Accumulator	Spongy Lead (Pb)	Lead dioxide (PbO₂)	Dilute H₂SO₄ (1.28 g/cm³)	12.0 V (6 × 2.0 V)
Lithium-Ion Battery	Lithium Graphite (LiC₆)	Lithium Cobalt Oxide (LiCoO₂)	LiPF₆ in non-aqueous solvent	3.00 V

Lesson 2: Commercial Batteries, Fuel Cells and Metal Corrosion · 9 / 9

Curriculum

Chemistry

Chapter 1 - Transition Elements and Iron Metallurgy

Chapter 2 - Chemical Analysis

Chapter 3 - Chemical Equilibrium

Chapter 4 - Electrochemistry

Chapter 5 - Organic Chemistry

More lessons

Lesson 2: Commercial Batteries, Fuel Cells and Metal Corrosion