An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. These systems form the foundation of modern energy storage, metal extraction, and electroplating technologies.
Classification of Electrochemical Cells
Electrochemical cells are broadly categorized into two primary types based on whether the underlying chemical reaction is spontaneous or driven by an external power source.
| Feature | Galvanic / Voltaic Cell | Electrolytic Cell |
| Energy Conversion | Converts Chemical Energy into Electrical Energy. | Converts Electrical Energy into Chemical Energy. |
| Spontaneity | Based on spontaneous redox reactions (Δ G < 0). | Based on non-spontaneous redox reactions (Δ G > 0). |
| Anode Polar Character | Negative (-) electrode terminal. | Positive (+) electrode terminal. |
| Cathode Polar Character | Positive (+) electrode terminal. | Negative (-) electrode terminal. |
| Salt Bridge | Absolutely essential to maintain electrical neutrality. | Not required; both electrodes generally share one compartment. |
| Key Industrial Example | Daniell Cell, Lithium-ion battery (during discharging). | Down’s Cell for Sodium extraction, copper electrorefining. |
Components and Working of a Galvanic Cell
The classic example of a Galvanic cell is the Daniell Cell, which utilizes the spontaneous redox reaction between Zinc and Copper ions.
Half-Cells and Electrodes
A Galvanic cell is divided into two separate compartments called half-cells, each containing an electrode dipped in an electrolyte.
- Oxidation Half-Cell (Anode): Consists of a Zinc strip dipped in a ZnSO4 solution. Here, zinc atoms lose electrons: Fe → Fe2+ + 2e^- (Oxidation).
- Reduction Half-Cell (Cathode): Consists of a Copper strip dipped in a CuSO4 solution. Here, copper ions gain electrons: Cu2+ + 2e^- → Cu (Reduction).
The Role of the Salt Bridge
The salt bridge is a U-shaped tube filled with an inert electrolyte paste (like KCl, KNO3, or NH4NO3 mixed with agar-agar jelly). It serves two critical functions:
- It completes the electrical circuit by allowing the migration of ions from one semi-cell to another.
- It maintains electrical neutrality in both half-cells by neutralizing excess positive charge at the anode and excess negative charge at the cathode.
Cell Notation Convention
A cell is represented symbolically using standard IUPAC notation:
Thermodynamics of Electrochemical Cells
The driving force that pushes electrons through the external circuit is called the Cell Potential or Electromotive Force (EMF), measured in volts.
Standard Cell Potential
The standard cell potential (E°cell) is the difference between the standard reduction potentials of the cathode and the anode.
Gibbs Free Energy and EMF
The electrical work done by a galvanic cell is equal to the decrease in Gibbs free energy (Δ G) of the system. The relationship is expressed as:
- n is the number of moles of electrons transferred in the balanced reaction.
- F is Faraday’s Constant (≈ 96487 C mol-1).
- Ecell is the cell potential.
Condition for Spontaneity
- If Ecell is positive, Δ G is negative, meaning the cell reaction is spontaneous (Galvanic Cell).
- If Ecell is negative, Δ G is positive, meaning the reaction is non-spontaneous and requires external energy (Electrolytic Cell).
Electrolytic Cells and Electrolysis
An electrolytic cell uses direct electrical current (DC) from an external source to force a non-spontaneous chemical change to occur.
Mechanism of Electrolysis
When a voltage exceeding the opposing cell potential is applied, ions migrate toward the oppositely charged electrodes:
- Cations (M^+) move toward the negative electrode (Cathode) where they accept electrons and undergo reduction.
- Anions (X^-) move toward the positive electrode (Anode) where they release electrons and undergo oxidation.
Preferential Discharge Theory
When multiple ions are present in an electrolytic solution, they compete for discharge at the electrodes. The ion discharged is determined by its position in the electrochemical series:
- At the cathode, the cation with a higher reduction potential (lower in the reactivity series) is preferentially reduced.
- At the anode, the anion with a lower reduction potential (higher oxidation ease) is preferentially oxidized.
High-Yield Trivia for Civil Services Prelims
- Lithium-Ion Twin Action: A lithium-ion battery operates as a Galvanic cell when discharging to power a smartphone or Electric Vehicle, and switches to function as an Electrolytic cell when plugged into a charger.
- The Demarcation of SHE: The Standard Hydrogen Electrode (SHE) is assigned a potential of exactly 0.00 V at all temperatures, serving as the reference baseline for constructing the entire electrochemical series.
- Why Potassium Chloride is Preferred: KCl is chosen for salt bridges because the transport numbers (migration velocities) of K^+ and Cl^- ions are nearly identical, preventing the development of a liquid junction potential.