Batteries & Fuel Cells | Electrochemistry Class 12

Batteries (Commercial Cells)

Primary, Secondary & Fuel Cells | Electrochemistry

1. Introduction

A battery is a galvanic cell (or a series of cells) used as a source of electrical energy. There are two main types:

• Primary Batteries: Reaction occurs only once. They become dead after use and cannot be recharged.
• Secondary Batteries: Can be recharged by passing current through them in the opposite direction.

2. Primary Batteries

A. Dry Cell (Leclanché Cell)

Used in transistors and clocks. Potential $\approx 1.5V$.

• Anode: Zinc container ($Zn$).
• Cathode: Carbon (Graphite) rod surrounded by powdered $MnO_2$ and Carbon.
• Electrolyte: Paste of $NH_4Cl$ and $ZnCl_2$.

Anode: $Zn(s) \rightarrow Zn^{2+} + 2e^-$
Cathode: $MnO_2 + NH_4^+ + e^- \rightarrow MnO(OH) + NH_3$

Note: Ammonia produced forms a complex $[Zn(NH_3)_2]^{2+}$ with $Zn^{2+}$.

B. Mercury Cell

Used in low-current devices like hearing aids. Potential $\approx 1.35V$.

• Anode: Zinc-Mercury Amalgam ($Zn-Hg$).
• Cathode: Paste of $HgO$ and Carbon.
• Electrolyte: Paste of $KOH$ and $ZnO$.

Overall Reaction: $Zn(Hg) + HgO(s) \rightarrow ZnO(s) + Hg(l)$

Since the overall reaction involves no ions in solution whose concentration can change, the cell potential remains constant throughout its life.

3. Secondary Batteries

A. Lead Storage Battery

Used in automobiles and inverters. It consists of 6 cells connected in series (~12V).

• Anode: Lead ($Pb$).
• Cathode: Grid of lead packed with Lead Dioxide ($PbO_2$).
• Electrolyte: 38% solution of Sulphuric Acid ($H_2SO_4$).

Discharging Reactions:
Anode: $Pb(s) + SO_4^{2-}(aq) \rightarrow PbSO_4(s) + 2e^-$
Cathode: $PbO_2(s) + SO_4^{2-}(aq) + 4H^+(aq) + 2e^- \rightarrow PbSO_4(s) + 2H_2O(l)$
Overall: $Pb + PbO_2 + 2H_2SO_4 \rightarrow 2PbSO_4 + 2H_2O$

Recharging: The reactions are reversed. $PbSO_4(s)$ on anode and cathode is converted back to $Pb$ and $PbO_2$ respectively.

B. Nickel-Cadmium (Ni-Cd) Cell

Longer life than lead storage but more expensive. Output $\approx 1.4V$.

Reaction: $Cd(s) + 2Ni(OH)_3(s) \rightarrow CdO(s) + 2Ni(OH)_2(s) + H_2O(l)$

4. Fuel Cells

Galvanic cells that convert the energy of combustion of fuels (like $H_2, CH_4, CH_3OH$) directly into electrical energy.

Hydrogen-Oxygen Fuel Cell

Used in the Apollo space program.
Electrodes: Porous carbon containing catalysts (Pt/Pd).

Anode: $2H_2(g) + 4OH^-(aq) \rightarrow 4H_2O(l) + 4e^-$
Cathode: $O_2(g) + 2H_2O(l) + 4e^- \rightarrow 4OH^-(aq)$
Overall: $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$

Advantages: Pollution-free, high efficiency (~70% compared to ~40% for thermal plants), continuous source of energy.

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