Electrochemistry — Batteries, Electrolysis & Corrosion

Fundamentals of Electrochemistry

Electrochemistry studies the relationship between chemical reactions and electrical energy. Oxidation is loss of electrons; reduction is gain (OIL RIG). Redox reactions involve electron transfer between species. Each half-reaction has a standard electrode potential E° (measured vs. Standard Hydrogen Electrode, SHE = 0 V). The overall cell potential E°cell = E°cathode – E°anode. A positive E°cell indicates a spontaneous reaction (ΔG° = –nFE°, where n = electrons transferred, F = 96,485 C/mol).

Galvanic & Electrolytic Cells

Galvanic (voltaic) cells convert chemical energy to electrical energy spontaneously. Example: Daniell cell (Zn|Zn²⁺||Cu²⁺|Cu, E° = 1.10 V). The anode (oxidation) is negative, cathode (reduction) is positive. A salt bridge maintains electrical neutrality. Electrolytic cells use external electrical energy to drive non-spontaneous reactions: electrolysis of water (2H₂O → 2H₂ + O₂), aluminum extraction (Hall-Héroult process), electroplating (e.g., chromium plating). Faraday’s laws quantify electrolysis: m = (MIt)/(nF), where m = mass deposited, M = molar mass, I = current, t = time.

The Nernst Equation

The Nernst equation relates cell potential to non-standard conditions: E = E° – (RT/nF) ln Q, where Q is the reaction quotient. At 25°C: E = E° – (0.0592/n) log Q. At equilibrium E = 0 and Q = K, so ln K = nFE°/RT. The equation predicts how concentration, temperature, and pressure affect cell voltage. Applications: pH meters (glass electrode responds to H⁺ concentration via Nernst equation), concentration cells, and biological membrane potentials (Goldman equation).

Batteries & Fuel Cells

Primary batteries (non-rechargeable): alkaline (Zn-MnO₂, 1.5 V), lithium metal (Li-MnO₂, 3 V). Secondary batteries (rechargeable): lead-acid (12 V, 30–40 Wh/kg, cars), lithium-ion (3.7 V, 250–300 Wh/kg, LiCoO₂|C, electronics/EVs), lithium iron phosphate (LFP, longer life, safer). Emerging: solid-state (ceramic/polymer electrolyte, 400+ Wh/kg), sodium-ion (cheaper, Na₂Mn₃O₇), lithium-sulfur (500+ Wh/kg theoretical). Fuel cells : hydrogen PEMFC (2H₂ + O₂ → 2H₂O, η ≈ 50–60%), SOFC for stationary power. Fuel cells power Toyota Mirai, space shuttles, and grid backup systems.

Corrosion & Prevention

Corrosion is an electrochemical process where metals oxidize in the presence of oxygen and moisture. Iron rusts: Fe → Fe²⁺ + 2e⁻ (anode); O₂ + 2H₂O + 4e⁻ → 4OH⁻ (cathode). Fe²⁺ + 2OH⁻ → Fe(OH)₂ → Fe₂O₃·xH₂O (rust). Corrosion costs ~3.4% of global GDP ($2.5 trillion/year). Prevention methods: cathodic protection (sacrificial anode of more reactive metal, e.g., Zn on ships), coatings (paint, galvanization, chromium plating), passivation (stainless steel forms Cr₂O₃ layer), inhibitors (chemicals reducing corrosion rate), and material selection (corrosion-resistant alloys, titanium, polymers).

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❓ Frequently Asked Questions

How does a lithium-ion battery work?

During discharge, Li⁺ ions move from the graphite anode through the electrolyte to the cathode (LiCoO₂, NMC, or LFP), while electrons flow through the external circuit, providing power. During charging, an external voltage reverses the process. The electrolyte (LiPF₆ in organic solvent) conducts ions but not electrons. Energy density: 250–300 Wh/kg.

What is the difference between galvanic and electrolytic cells?

Galvanic cells produce electricity from spontaneous chemical reactions (E°cell > 0, ΔG < 0). Electrolytic cells use external electricity to drive non-spontaneous reactions (E°cell < 0, ΔG > 0). In galvanic cells, the anode is negative; in electrolytic cells, the anode is positive (connected to the battery’s positive terminal).

What is electrolysis of water?

Applying ≥ 1.23 V (theoretically; practically ~1.8–2.0 V) across electrodes in water decomposes it: cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻, anode: 2H₂O → O₂ + 4H⁺ + 4e⁻. An electrolyte (KOH or H₂SO₄) improves conductivity. This is how green hydrogen is produced using renewable electricity.

How does a fuel cell differ from a battery?

A battery stores chemical energy internally and is depleted (or recharged). A fuel cell continuously converts externally supplied fuel (hydrogen) and oxidant (oxygen) into electricity, water, and heat — it produces power as long as fuel is supplied. Fuel cells are more like engines; batteries are like tanks.

Why does iron rust?

Iron oxidizes electrochemically when exposed to oxygen and moisture. Small galvanic cells form on the metal surface: anodic regions oxidize Fe to Fe²⁺, cathodic regions reduce O₂. The Fe²⁺ ions react with water and oxygen to form hydrated iron(III) oxide (Fe₂O₃·xH₂O = rust). Unlike aluminum’s protective Al₂O₃ layer, rust is porous and doesn’t protect the underlying iron.

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