Batteries & Fuel Cells
Practical Electrochemistry. Master the charging cycles of secondary batteries, the constant voltage trap of the mercury cell, and the thermodynamics of fuel cells.
Module Focus: Portable Power
A battery is essentially one or more Galvanic cells connected in series. For a battery to be of practical use, it should be reasonably light, compact, and its voltage should not drop significantly during use. We categorize them into Primary Batteries (non-rechargeable, reaction occurs only once) and Secondary Batteries (rechargeable by passing current in the opposite direction).
1. Primary Batteries (Non-Rechargeable)
- Anode: Zinc container ($Zn$).
- Cathode: Carbon (graphite) rod surrounded by powdered $MnO_2$ + Carbon.
- Electrolyte: Paste of $NH_4Cl$ and $ZnCl_2$.
- Voltage: ~ 1.5 V
Cathode: $MnO_2 + NH_4^+ + e^- \rightarrow MnO(OH) + NH_3$
- Used in hearing aids, watches.
- Anode: Zinc-Mercury Amalgam ($Zn(Hg)$).
- Cathode: Paste of $HgO$ and Carbon.
- Electrolyte: Paste of $KOH$ and $ZnO$.
2. Secondary Batteries (Rechargeable)
Secondary batteries can undergo a reversible reaction. The most critical NEET concept here is the Lead Storage Battery, used in automobiles and inverters. You must master both its discharging and charging phases.
Anode (Oxidation):
$Pb + SO_4^{2-} \rightarrow PbSO_4(s) + 2e^-$
Cathode (Reduction):
$PbO_2 + SO_4^{2-} + 4H^+ + 2e^- \rightarrow PbSO_4(s) + 2H_2O$
Overall: $Pb + PbO_2 + 2H_2SO_4 \rightarrow 2PbSO_4 + 2H_2O$
To recharge, a DC voltage greater than the cell voltage (2V per cell) is applied in the opposite direction. The reactions are exactly reversed.
- $PbSO_4$ on the anode is reduced back to $Pb$.
- $PbSO_4$ on the cathode is oxidized back to $PbO_2$.
- $H_2SO_4$ is regenerated, increasing the specific gravity of the acid.
3. Fuel Cells ($H_2 - O_2$)
Galvanic cells that are designed to convert the energy of combustion of fuels like hydrogen, methane, or methanol directly into electrical energy. They never go "dead" as long as fuel and oxidant are continuously supplied.
- Electrodes: Porous carbon containing suitable catalysts (like finely divided Pt or Pd).
- Electrolyte: Concentrated aqueous $KOH$ or $NaOH$.
- Efficiency: Operates at ~70% efficiency (much higher than thermal plants at ~40%).
- By-product: Pure water! The water vapors produced were condensed and added to the drinking water supply for astronauts.
$2H_2(g) + 4OH^-(aq) \rightarrow 4H_2O(l) + 4e^-$
$O_2(g) + 2H_2O(l) + 4e^- \rightarrow 4OH^-(aq)$
4. Corrosion (Rusting of Iron)
Corrosion is an electrochemical phenomenon where a metal acts as an anode and undergoes oxidation. Rusting requires both oxygen and water.
- Anode Spot: Iron oxidizes to $Fe^{2+}$.
$2Fe(s) \rightarrow 2Fe^{2+} + 4e^-$ - Cathode Spot: Electrons flow through the metal to another spot where $O_2$ is reduced in the presence of $H^+$ (from dissolved $CO_2$).
$O_2(g) + 4H^+ + 4e^- \rightarrow 2H_2O(l)$
The $Fe^{2+}$ ions are further oxidized by atmospheric oxygen to $Fe^{3+}$, which precipitates as hydrated ferric oxide (Rust).
Prevention: Galvanization (coating with a more active metal like Zinc acting as a sacrificial anode).
NEET Grand Test: Batteries
15 High-Yield Questions testing the Lead-Acid cycle, Mercury cell constants, and Fuel Cell thermodynamics.
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