JEE Mains Chemistry · Electrochemistry

Batteries, Fuel Cells and Corrosion

The named cells of NCERT, with their electrodes, reactions and uses; the hydrogen–oxygen and methanol fuel cells; and rusting as a small galvanic cell on the surface of iron.

All Electrochemistry notes JEE Mains Chemistry strategy

Why this matters

Eleven PYQs, every one of them multiple choice. Seven match a battery to its electrodes, reaction or use, or ask what happens on charging and discharging; four test fuel cells and the rusting of iron. It is a recall page: learn the two tables and it is done.

Concept 1 of 2: Primary and secondary batteries

Every battery is a galvanic cell with a named anode and cathode. A primary cell is used once and thrown away. A secondary cell is recharged by pushing current through it backwards, which runs its reaction as an electrolysis.

Definition

Dry (Leclanché) cell: at the cathode $\mathrm{MnO_2+NH_4^++e^-\to MnO(OH)+NH_3}$ . Mn goes from +4 to +3. About 1.5 V.
Mercury cell: $\mathrm{Zn(Hg)+HgO\to ZnO+Hg}$ . Its voltage stays at about 1.35 V, because no ion's concentration changes during its life.
Lead storage battery, discharge: anode $\mathrm{Pb+SO_4^{2-}\to PbSO_4+2e^-}$ ; cathode $\mathrm{PbO_2+SO_4^{2-}+4H^++2e^-\to PbSO_4+2H_2O}$ . Net: $\mathrm{Pb+PbO_2+2H_2SO_4\to 2PbSO_4+2H_2O}$ . The acid is used up.
On charging, NCERT keeps the discharge names: $\mathrm{PbSO_4}$ on the anode becomes Pb, and $\mathrm{PbSO_4}$ on the cathode becomes $\mathrm{PbO_2}$ . Both start at +2.
Nickel–cadmium cell, discharge: $\mathrm{Cd+2Ni(OH)_3\to CdO+2Ni(OH)_2+H_2O}$ . Rechargeable, with a longer life than the lead battery.
Metals used by the battery industry: Mn (as $\mathrm{MnO_2}$ ), Zn, Ni, Cd, Pb, Hg.

Cell	Anode	Cathode	Electrolyte	Type and use
Dry (Leclanché) cell	Zn container	Graphite rod in $\mathrm{MnO_2}$ and carbon	Paste of $\mathrm{NH_4Cl}$ and $\mathrm{ZnCl_2}$	Primary; clocks, transistors, torches
Mercury cell	Zn–Hg amalgam	Paste of HgO and carbon	Paste of KOH and ZnO	Primary; hearing aids, watches; steady voltage
Lead storage battery	Pb	$\mathrm{PbO_2}$ packed on a lead grid	About 38% $\mathrm{H_2SO_4}$	Secondary; cars and inverters
Nickel–cadmium cell	Cd	$\mathrm{Ni(OH)_3}$	KOH	Secondary; long life, rechargeable devices
$\mathrm{H_2}$ – $\mathrm{O_2}$ fuel cell	Porous carbon with $\mathrm{H_2}$ fed in	Porous carbon with $\mathrm{O_2}$ fed in	Concentrated aqueous NaOH or KOH	Continuous feed; Apollo space programme

Primary: used once. Secondary: recharged. Fuel cell: reactants fed in continuously.

Practice this conceptself-check · 5 quick reps

The same idea in a real exam question:

JEE Mains · 2025 · 29 Jan 2025 · Q110Moderate

Example 1 · Electrochemistry · Batteries, Fuel Cells and Corrosion

Match List-I with List-II :

	List-I (Application)		List-II (Battery/Cell)
(A)	Transistors	(I)	Anode -Zn/Hg; Cathode -HgO + C
(B)	Hearing aids	(II)	Hydrogen fuel cell
(C)	Invertors	(III)	Anode - Zn ; Cathode - Carbon
(D)	Apollo space ship	(IV)	Anode - Pb ; Cathode - $PbO_{2}$ on a lead grid

Choose the correct answer from the options given below :

Renaming the plates on charging

When a lead battery is charged, NCERT still calls the Pb plate the anode and the

\mathrm{PbO_2}

plate the cathode. Keys follow that naming.

Why the mercury cell is steady

Its overall reaction has only solids on both sides. Nothing in solution changes, so the Nernst term never moves.

Drill 6 more on primary and secondary batteries

Concept 2 of 2: Fuel cells and corrosion

A fuel cell burns its fuel electrochemically, so the energy comes out as electricity instead of heat. Rusting is the same chemistry running unwanted: one spot on the iron is an anode, another is a cathode, and the water film is the electrolyte.

Definition

Fuel cell: a galvanic cell fed with fuel and oxygen continuously. $\mathrm{H_2}$ – $\mathrm{O_2}$ : about 70% efficient, with platinum or palladium catalyst on porous carbon; its only product is water.
In any fuel cell the fuel is oxidised at the anode and $\mathrm{O_2}$ is reduced at the cathode. So $E^\circ(\text{fuel couple})=E^\circ(\mathrm{O_2/H_2O})-E^\circ_{cell}$ , with $E^\circ(\mathrm{O_2/H_2O})=1.229$ V.
Rusting: at an anodic spot $\mathrm{Fe\to Fe^{2+}+2e^-}$ . At a cathodic spot $\mathrm{O_2+4H^++4e^-\to 2H_2O}$ . $\mathrm{Fe^{2+}}$ is then oxidised to hydrated $\mathrm{Fe_2O_3}$ .
Acid speeds rusting; so do dissolved acidic oxides ( $\mathrm{CO_2}$ , $\mathrm{SO_2}$ , $\mathrm{NO_2}$ ). Above pH 9 to 10 it practically stops.
Zinc ( $-0.76$ V) is oxidised before iron ( $-0.44$ V), so galvanised iron is protected even when scratched. Tin ( $-0.14$ V) is not: once the tin coat breaks, the iron corrodes faster.

Statement	Verdict	Reason
The H₂–O₂ fuel cell was used in the Apollo space programme	True	Its water was drunk by the crew
The H₂–O₂ fuel cell is about 40% efficient	False	About 70%, far above a thermal power plant
Its electrodes use aluminium as a catalyst	False	Finely divided Pt or Pd on porous carbon
Reactants are fed in at one go	False	They are fed in continuously
A fuel cell is a galvanic cell	True	A spontaneous reaction gives electricity
In a methanol fuel cell, methanol is oxidised at the anode	True	The fuel is always the anode's reactant
Rusting is an electrochemical process	True	Anodic and cathodic spots on one piece of iron
Rusting is faster in alkaline water than in acid	False	$\mathrm{H^+}$ drives the cathode reaction; above pH 9 to 10 rusting stops
A tin coat protects iron even after it peels	False	Iron is below tin in the series, so exposed iron corrodes faster
A scratched zinc coat still protects iron	True	Zinc is oxidised first, as a sacrificial anode

Practice this conceptself-check · 5 quick reps

The same idea in a real exam question:

JEE Mains · 2025 · 3 Apr 2025 · Q109Moderate

Example 2 · Electrochemistry · Batteries, Fuel Cells and Corrosion

The standard cell potential

\left( E_{\text{cell~}}^{\ominus} \right)

of a fuel cell based on the oxidation of methanol in air that has been used to power television relay station is measured as 1.21 V . The standard half cell reduction potential for

O_{2}\left( E_{O_{2}/H_{2}O}^{o} \right)

is 1.229 V . Choose the correct statement:

Oxygen at the anode

In a fuel cell

\mathrm{O_2}

is consumed at the cathode. It is neither formed nor consumed at the anode.

Tin protects like zinc

Tin is a barrier only. Once it breaks, iron is the anode of the tin–iron couple and rusts faster than bare iron.

Drill 3 more on fuel cells and corrosion

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (2)

Primary and secondary batteries5 rows

Cell	Anode	Cathode	Electrolyte	Type and use
Dry (Leclanché) cell	Zn container	Graphite rod in $\mathrm{MnO_2}$ and carbon	Paste of $\mathrm{NH_4Cl}$ and $\mathrm{ZnCl_2}$	Primary; clocks, transistors, torches
Mercury cell	Zn–Hg amalgam	Paste of HgO and carbon	Paste of KOH and ZnO	Primary; hearing aids, watches; steady voltage
Lead storage battery	Pb	$\mathrm{PbO_2}$ packed on a lead grid	About 38% $\mathrm{H_2SO_4}$	Secondary; cars and inverters
Nickel–cadmium cell	Cd	$\mathrm{Ni(OH)_3}$	KOH	Secondary; long life, rechargeable devices
$\mathrm{H_2}$ – $\mathrm{O_2}$ fuel cell	Porous carbon with $\mathrm{H_2}$ fed in	Porous carbon with $\mathrm{O_2}$ fed in	Concentrated aqueous NaOH or KOH	Continuous feed; Apollo space programme

Primary: used once. Secondary: recharged. Fuel cell: reactants fed in continuously.

Fuel cells and corrosion10 rows

Statement	Verdict	Reason
The H₂–O₂ fuel cell was used in the Apollo space programme	True	Its water was drunk by the crew
The H₂–O₂ fuel cell is about 40% efficient	False	About 70%, far above a thermal power plant
Its electrodes use aluminium as a catalyst	False	Finely divided Pt or Pd on porous carbon
Reactants are fed in at one go	False	They are fed in continuously
A fuel cell is a galvanic cell	True	A spontaneous reaction gives electricity
In a methanol fuel cell, methanol is oxidised at the anode	True	The fuel is always the anode's reactant
Rusting is an electrochemical process	True	Anodic and cathodic spots on one piece of iron
Rusting is faster in alkaline water than in acid	False	$\mathrm{H^+}$ drives the cathode reaction; above pH 9 to 10 rusting stops
A tin coat protects iron even after it peels	False	Iron is below tin in the series, so exposed iron corrodes faster
A scratched zinc coat still protects iron	True	Zinc is oxidised first, as a sacrificial anode

Watch out for (4)

Renaming the plates on charging
→ Primary and secondary batteries
Why the mercury cell is steady
→ Primary and secondary batteries
Oxygen at the anode
→ Fuel cells and corrosion
Tin protects like zinc
→ Fuel cells and corrosion

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