Electrochemical Cells

Introduction

Electrochemical cells convert chemical energy to electrical energy or vice versa via redox reactions. Components: two electrodes, electrolyte, external circuit. Key function: spontaneous electron transfer drives current or applied voltage induces chemical change.

"Electrochemical cells underpin modern energy storage and conversion technologies, bridging chemistry and electricity." -- Allen J. Bard

Basic Concepts

Electrodes

Conductors facilitating electron exchange. Types: anode (oxidation), cathode (reduction). Material choice impacts kinetics and stability.

Electrolyte

Ionic medium enabling ion migration to maintain charge neutrality. Can be liquid aqueous, molten salts, or solid polymer electrolytes.

Redox Reactions

Oxidation: loss of electrons. Reduction: gain of electrons. Occur simultaneously; total electrons conserved. Reaction couples define cell function.

Electromotive Force (emf)

Voltage produced between electrodes due to redox potential difference. emf > 0: spontaneous reaction. emf < 0: non-spontaneous, requires external energy.

Types of Electrochemical Cells

Galvanic (Voltaic) Cells

Spontaneous redox reactions produce electrical energy. Example: Daniell cell. Function: power sources, sensors.

Electrolytic Cells

External voltage drives non-spontaneous reactions. Application: metal plating, electrorefining, water splitting.

Concentration Cells

Electrodes of identical material but different ion concentrations. emf arises from concentration gradient, used in ion selective electrodes.

Fuel Cells

Continuous fuel and oxidant supply; convert chemical energy to electricity with high efficiency. Example: hydrogen fuel cells.

Redox Reactions in Cells

Oxidation Half-Reaction

Electron loss at anode. Example: Zn → Zn²⁺ + 2e⁻.

Reduction Half-Reaction

Electron gain at cathode. Example: Cu²⁺ + 2e⁻ → Cu.

Overall Cell Reaction

Sum of half-reactions; electrons cancel. Determines cell emf and direction.

Electron Flow

From anode to cathode externally; internally, ions migrate to maintain charge balance.

Cell Notation and Diagrams

Cell Notation

Standardized shorthand for reactions. Format: Anode | Anode electrolyte || Cathode electrolyte | Cathode.

Example: Daniell Cell

Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s).

Phase Boundaries

Single vertical bar (|) indicates phase boundary. Double vertical bar (||) denotes salt bridge.

Cell Diagram Interpretation

Order: oxidation half-cell left, reduction half-cell right. Ion concentrations specified if non-standard.

Electrode Potentials

Definition

Potential difference between electrode and electrolyte, measured in volts. Indicates tendency to gain or lose electrons.

Standard Electrode Potential (E°)

Measured under standard conditions (1 M, 1 atm, 25°C) relative to standard hydrogen electrode.

Measurement

Using reference electrode (typically SHE). Difference in potentials yields cell emf.

Significance

Positive E°: strong oxidizing agent. Negative E°: strong reducing agent.

Standard Hydrogen Electrode

Definition

Reference electrode with assigned potential 0.00 V. Consists of Pt electrode in H₂ gas at 1 atm, immersed in 1 M H⁺ solution.

Reaction

2H⁺(aq) + 2e⁻ ⇌ H₂(g).

Usage

Baseline for measuring electrode potentials of other half-cells under standard conditions.

Limitations

Fragility, gas handling, alternative reference electrodes (Ag/AgCl, calomel) often used.

Nernst Equation

Purpose

Calculates electrode potential under non-standard conditions. Accounts for ion concentrations, temperature.

Equation

E = E° - (RT/nF) ln Q

Where: E = electrode potential, E° = standard potential, R = gas constant, T = temperature (K), n = electrons transferred, F = Faraday constant, Q = reaction quotient.

At 25°C Simplification

E = E° - (0.0592/n) log Q

Applications

Predict cell emf, equilibrium constants, pH measurement, sensor design.

Electrolysis

Definition

Driving non-spontaneous chemical reactions by applying external voltage. Process converts electrical energy to chemical energy.

Electrolytic Cell Components

Anode: site of oxidation. Cathode: site of reduction. Electrolyte: ion conductor.

Faraday’s Laws

Mass of substance deposited proportional to charge passed. Quantitative relation using Faraday constant.

Examples

Water splitting to H₂ and O₂, metal refining, electroplating.

Applications of Electrochemical Cells

Batteries

Primary and secondary cells convert chemical energy to electrical energy for portable power.

Fuel Cells

Continuous fuel supply systems for clean energy. Types: PEMFC, SOFC.

Electroplating

Deposition of metals on surfaces for corrosion resistance, aesthetics, electronics.

Sensors

Ion selective electrodes for pH, gas detection, biomedical sensing.

Corrosion Prevention

Cathodic protection via sacrificial anodes or impressed current.

Limitations and Challenges

Overpotential

Extra voltage needed beyond theoretical emf due to kinetics, surface effects.

Electrode Degradation

Material corrosion, passivation reduce cell lifespan.

Ion Transport Limitations

Concentration polarization, limited electrolyte conductivity.

Energy Efficiency

Losses due to internal resistance, side reactions.

Recent Advances

Nanostructured Electrodes

Increased surface area, improved kinetics, enhanced catalytic activity.

Solid-State Electrolytes

Improved safety, stability in batteries and fuel cells.

Bioelectrochemical Systems

Microbial fuel cells, enzymatic sensors integrating biology and electrochemistry.

Energy Storage Integration

Hybrid systems combining batteries and supercapacitors for grid applications.

References

• Bard, A.J., Faulkner, L.R., Electrochemical Methods: Fundamentals and Applications, Wiley, 2001, pp. 1-850.
• Atkins, P., de Paula, J., Physical Chemistry, 10th Edition, Oxford University Press, 2014, pp. 578-615.
• Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE International, 1974, pp. 1-390.
• Schmickler, W., Santos, E., Interfacial Electrochemistry, 2nd Edition, Springer, 2010, pp. 45-210.
• Gerischer, H., "Electrochemical energy conversion," Electrochimica Acta, vol. 45, 2000, pp. 2751-2760.