Electrochemistry, Cells, Batteries & Metal Finishing – Comprehensive Bullet-Point Notes

Introduction to Electrochemistry

• Studies inter-conversion of chemical ⇌ electrical energy.

• Two broad phenomena:

  • Electrolysis / electrometallurgy / electroplating: electricity drives chemical change.

  • Galvanic / batteries / fuel cells: spontaneous reaction drives electricity.

• Conductors:

  • Electronic – metals & alloys; current by electron flow, no chemical change.

  • Electrolytic – molten salts / solutions; current by ion migration, chemical change.

• Industrial relevance: electroplating, metal extraction & refining, synthesis of compounds.

Electrochemical Cells

• Basic unit: two electrodes + electrolyte(s).

• Each “electrode compartment” = electrode + its electrolyte.

• Cells convert energy in either direction:

  • Galvanic (voltaic): chemical → electrical (spontaneous).

  • Electrolytic: electrical → chemical (non-spontaneous).

• Components when electrolytes differ:

  • Salt bridge (agar + $\text{KCl / KNO}<em>3 / \text{NH}</em>4\text{NO}_3$) closes circuit & minimises junction potential.

Galvanic Cell Essentials

• Electron flow: anode ⟶ cathode (external circuit).

• Ion flow: anions ⟶ anode, cations ⟶ cathode (internal circuit).

• Anode (–): oxidation; Cathode (+): reduction.

Daniell Cell (Cu|Cu²⁺║Zn²⁺|Zn) – paradigm

• Half-reactions

  • $\text{Zn(s)} \rightarrow \text{Zn}^{2+}(aq)+2e^-$ (anode)

  • $\text{Cu}^{2+}(aq)+2e^- \rightarrow \text{Cu(s)}$ (cathode)

• Overall $\text{Zn}+\text{Cu}^{2+}\rightarrow\text{Zn}^{2+}+\text{Cu}$ (redox).

Electrolytic Cell Essentials

• Driven by external source; polarity reversed from galvanic:

  • Anode (+), oxidation.

  • Cathode (–), reduction.

• Uses: purification, electroplating, electro-synthesis.

Cell Notation (IUPAC conventions)

• Left = anode | right = cathode.

• Phase boundary: single vertical line │ ; salt bridge: double ║.

• Include concentration/pressure.

  • Example: $\text{Zn}|\text{ZnSO}<em>4(1\,M)║\text{CuSO}</em>4(1\,M)|\text{Cu}$

Liquid Junction Potential (LJP)

• Potential at interface of two electrolytes due to unequal ion mobilities.

• Symbol $E<em>j=\phi</em>{R}-\phi_{L}$ ; typically a few mV.

• Minimized by salt bridge with ions of similar transference numbers (K⁺/Cl⁻ etc.).

Salt Bridge Functions

• Completes circuit without mixing bulk solutions.

• Maintains electroneutrality via counter-ion flow.

• Reduces LJP to ≈1–2 mV.

Electromotive Force (emf) of Cells

• Definition: potential difference driving electron flow.

• $E<em>{cell}=E</em>{cathode}-E_{anode}$

• Standard emf $E_{cell}^0$: all reactants/products at unit activity, 298 K, 1 atm.

• Thermodynamic link: $\Delta G=-nF E<em>{cell}$ ; spontaneous if E{cell}>0.

Measurement (Potentiometric / Poggendorff compensation)

• Use high-resistance potentiometer – no current draw at null point.

• Unknown emf determined relative to standard cell (e.g.
  Weston 1.0183 V @ 293 K).

Requirements for a Standard Cell

1. Very constant emf.

2. Highly reproducible.

3. Negligible $\frac{\partial E}{\partial T}$.

4. Reversible reaction.

5. No permanent damage on current draw.

Energetics Relationships

• $\Delta G=-nFE$ (work potential)

• $\Delta H = nF\,[T(\partial E/\partial T)_P - E]$

• $\Delta S = nF(\partial E/\partial T)_P$

• Sample calc (Weston cell 298 K): $\Delta G=-196.5\,\text{kJ},\;\Delta H=-198.8\,\text{kJ},\;\Delta S=-7.72\,\text{J K}^{-1}$.

Single Electrode Potential

• Potential difference between a metal & its ion solution.

• Determined only relatively (cannot measure absolute); reference = Standard Hydrogen Electrode (SHE, $E^0=0$ V).

• Sign convention:

  • Electrode reduced vs SHE → $E$ positive.

  • Electrode oxidized vs SHE → $E$ negative.

• Standard electrode potential $E^0$: 1 M, 1 atm, 298 K.

• Electrochemical series (selected): \text{Li}^+/\text{Li}\,(-3.03) <\cdots< \text{H}^+/\text{H}_2\,(0) < \text{Cu}^{2+}/\text{Cu}\,(+0.34) < \text{Ag}^+/\text{Ag}\,(+0.80) V.

Nernst Equation

• Half-cell: $E = E^0 + \frac{0.0592}{n}\log\,[\text{Ox}/\text{Red}]$ at 298 K.

• Full cell: $E<em>{cell}=E</em>{cell}^0-\frac{0.0591}{n}\log\,K_Q$.

• Uses: non-standard potentials, equilibrium constants (at $E=0$: $K= e^{nFE^0/RT}$).

Reference Electrodes

Calomel Electrode

• Construction: $\text{Hg(l)}|\text{Hg}<em>2\text{Cl}</em>2(s)|\text{KCl (x M)}$ with Pt contact.

• $E = E^0 - 0.0591\log[\text{Cl}^-]$ ; typical $E=+0.242\,\text{V}$ (sat’d KCl).

• Pros: simple, stable, low $\partial E/\partial T$; Cons: Hg toxicity, ≤50 °C limit.

• Uses: secondary reference, pH cells, measuring unknown electrode potential.

Glass Electrode (Ion-Selective for H⁺)

• Thin glass membrane encloses 0.1 M HCl + internal $\text{Ag}/\text{AgCl}$.

• Boundary potential $E_b = K-0.0592\,\text{pH}$.

• Overall $E<em>G = E</em>G^0 - 0.0592\,\text{pH}$ ; combine with SCE to get pH.

• Advantages: chemical robustness, quick response, tiny samples; Disadvantages: fragile bulb, alkaline/acid errors, needs hydration & high-impedance meter.

Batteries

Classification

• Primary (non-rechargeable) – e.g. dry cell, Li–MnO₂.

• Secondary (rechargeable) – e.g. Pb-acid, Ni–Cd, Li-ion.

Desired Traits

• Primary: light, cheap, long shelf-life, high energy density, constant voltage.

• Secondary: long cycle & shelf life, high P/W ratio, fast recharge, high energy density.

Lithium-Ion Battery

• Anode (during discharge): lithiated graphite $\text{Li}<em>x\text{C}</em>6$.

• Cathode: layered $\text{LiCoO}_2$ (or LiMn₂O₄ etc.).

• Electrolyte: organic carbonate + $\text{LiPF}<em>6$/$\text{LiBF}</em>4$.

• Discharge net: $\text{Li}<em>x\text{C}</em>6 + \text{CoO}<em>2 \rightarrow \text{Li}</em>x\text{CoO}_2 + 6\text{C}$.

• Advantages: high V (≈3.6 V/cell), light, high cycle life, wide T-range.

• Drawbacks: cost, 10 %/month self-discharge, charge retention.

• Applications: phones, laptops, medical implants, EVs.

Fuel Cells

• Continuous feed of fuel + oxidant; products removed; emf ≈0.7–1 V per cell.

Alkaline Fuel Cell (AFC)

• Electrolyte: aqueous KOH.

• Electrodes: porous Pt/C.

• Reactions:

  • Anode $\text{H}<em>2+2\text{OH}^-\rightarrow2\text{H}</em>2\text{O}+2e^-$

  • Cathode $\tfrac12\text{O}<em>2+\text{H}</em>2\text{O}+2e^-\rightarrow2\text{OH}^-$

  • Net $\text{H}<em>2+\tfrac12\text{O}</em>2\rightarrow\text{H}_2\text{O}$.

• Pros: high efficiency, used in NASA; Cons: CO₂ sensitive, needs pure gases.

Proton Exchange Membrane Fuel Cell (PEMFC)

• Electrolyte: Nafion polymer membrane.

• 40–80 °C operation; fast start.

• Reactions:

  • Anode $\text{H}_2\rightarrow2\text{H}^++2e^-$

  • Cathode $\tfrac12\text{O}<em>2+2\text{H}^++2e^-\rightarrow\text{H}</em>2\text{O}$.

• Advantages: high power density, low T, compact.

• Issues: Pt cost, CO poisoning, water management.

Fuel Cell vs Galvanic Cell

• Fuel cell: external reactant supply, no storage, long continuous power, higher density, products removed.

• Galvanic: reactants integral, limited capacity, can be primary or secondary.

Metal Finishing

Electroplating

• Electrolytic deposition of metal layer for aesthetics, corrosion protection, wear resistance.

• Essential factors:

  • Decomposition potential $E_D$ – minimum V for sustained electrolysis.

  • Overvoltage $\eta$ – extra V above theoretical, esp. for gas evolution.

  • Polarisation – concentration & activation; mitigated by agitation, temp, depolarisers.

• Characteristics of good deposit: bright, smooth, adherent, fine-grained, ductile.

• Influencing variables: current density, metal-ion concentration, temperature, pH, additives (complexers, brighteners, levellers, wetting agents), throwing power.

• Surface preparation: solvent degreasing, alkaline cleaning, mechanical abrasion, acid pickling, electropolishing.

Chromium Plating

• Always over Ni/Cu undercoat (Cr micro-porous, cracks).

• Bath: $\text{CrO}<em>3$ 250 g + $\text{H}</em>2\text{SO}_4$ 2.5 g + $\text{Cr}^{3+}$ 1 g.

• Decorative: 145–430 A ft⁻², 10–15 % cathode efficiency.

• Hard Cr: 290–580 A ft⁻², 17–21 % efficiency; used for gauges, dies, piston rings.

• Inert Pb-Sn-Sb anodes; cathodic reaction $\text{Cr}^{3+}+3e^-\rightarrow\text{Cr}$.

Electroless Plating

• Autocatalytic redox deposition; works on metals & non-conductors.

• Bath constituents: metal salt, reducing agent (e.g. $\text{HCHO},\;\text{NaH}<em>2\text{PO}</em>2$), complexer (EDTA), stabiliser, accelerator, buffer.

• Advantages: uniform thickness on complex shapes, no power supply, hard & wear-resistant.

• Downsides: bath instability, chemical cost, waste treatment.

Electroless Copper Example (for PCBs)

• Bath: $\text{CuSO}_4,\;\text{HCHO},\;\text{NaOH},\;\text{EDTA}$.

• Optimum: pH ≈11, 25 °C.

• Reactions:

  • Cathode $\text{Cu}^{2+}+2e^-\rightarrow\text{Cu}$

  • Anode (solution) $2\text{HCHO}+4\text{OH}^-\rightarrow2\text{HCOO}^-+\text{H}_2+2e^-$

  • Net $\text{Cu}^{2+}+2\text{HCHO}+4\text{OH}^-\rightarrow\text{Cu}+2\text{HCOO}^-+\text{H}_2$.

• Uses: through-hole metallisation, plastic metallising.

Electroplating vs Electroless

(End of detailed bullet-point notes)