Electrode Systems, Batteries, and Solar Energy Notes

Electrode Systems and Analytical Techniques

Electrode System

• Electrodes are conductors making electrical contact with the non-metallic circuit.
• Types of electrodes:
  • Metal / Metal ion electrode (M / Mn+ ): Metal immersed in its own salt solution. Example: $Cu(s) | Cu^{++}(aq)$
  • Gas Electrode: Gas in contact with a solution containing ions derivable from the gas. Example: Hydrogen electrode.
  • Metal insoluble metal salt/ common ion electrode: Metal covered with insoluble metal salt. Example: Calomel electrode $(Hg/Hg<em>2Cl</em>2/Cl^-)$
  • Redox electrode: Inert metal immersed in a solution containing ions of the same metal in two different oxidation states. Example: $Pt/ Fe^{2+} /Fe^{3+}$
  • Ion selective electrode: Sensing part made of an ion-specific membrane. Example: Glass electrode.

Ion Selective Electrode

• Selective towards particular ions, developing a potential proportional to concentration.
• Working Mechanism: Based on galvanic cell principle; potential difference created by ion transport.
• Applications:
  • Clinical chemistry: Analysis of electrolytes like $Na^+, K^+$
  • Environmental chemistry: Analysis of $CN^-, F^-, NH<em>3, NO</em>3$ in water samples
  • Agricultural chemistry: Analysis of $K^+, NH_4^+$ in soil samples

Glass Electrode

• Ion-selective electrode with a doped glass membrane sensitive to specific ions, commonly for pH measurement.
• Construction: Ag/AgCl internal reference electrode in a glass bulb with 0.1M HCl solution.
• Working: Boundary potential $(E_b)$ develops across the gel layers of the glass membrane due to H+ concentration differences.
  • $E_b = K - 0.0591pH$
  • $E<em>G = L - 0.0591pH$ where $L=K+E</em>{Ag/AgCl} + E_{sy}$
• pH Determination: Coupled with calomel electrode and connected to potentiometer.
  • $E<em>{Cell} = (L - 0.0591pH) - E</em>{SCE}$

Reference Electrode

• Electrodes with known, stable potentials used to determine potentials of unknown electrodes.
• Example: Standard Hydrogen Electrode (SHE), Saturated Calomel Electrode (SCE).
• Saturated Calomel Electrode (SCE):
  • Used as secondary reference electrode.
  • Used in pH measurement

Concentration Cells

• Electrode material is the same, but electrolyte solutions have varying concentrations.
• Components: Anode, cathode, salt bridge, voltmeter.
• Cell Representation: Two half-cells separated by a salt bridge.
• Nernst Equation:
  • $E<em>{cell} = \frac{RT}{nF} \log \frac{C</em>2}{C_1}$

Batteries

• Compact device of galvanic cells storing chemical energy and converting it to electrical energy via redox reactions.
• Components: Anode, cathode, electrolyte, separator.
• Classification:
  • Primary Batteries: Non-rechargeable, irreversible reactions. Example: Dry cell.
  • Secondary Batteries: Rechargeable, reversible reactions. Example: Li-ion battery.
  • Reserve Batteries: Active materials isolated, brought into contact when high energy is needed. Example: Magnesium-water activated batteries.

Lithium-Ion Battery (LIB)

• Composition:
  • Anode: Li/Graphite
  • Cathode: $LiCoO_2$
  • Electrolyte: Lithium salt
  • Separator: Polypropylene
  • Output Voltage: 3.6V
• Working:
  • Discharge: Lithium ions move from anode to cathode
    • Anodic Reaction: $xLiC_6 → xLi^+ + xe^- + 6C$
    • Cathodic Reaction: $(1-x)CoO<em>2 + xLi^+ + xe^- → LiCoO</em>2$
    • Overall Reaction: $(1-x)CoO<em>2 + xLiC</em>6 ↔ LiCoO_2 + 6C$
  • Charge: Lithium ions move from cathode to anode
    • Cathodic Reaction: $LiCoO<em>2 → L(1-x)CoO</em>2 + xLi^+ + xe^-$
    • Anodic Reaction: $xLi^+ + xe^- + 6C → xLiC_6$
    • Overall Reaction: $LiCoO<em>2 + 6C ↔ Li(1-x)CoO</em>2 + xLiC_6$

Sodium-Ion Battery (SIB)

• Energy conversion devices using sodium-ions.
• Components:
  • Anode: Hard carbon
  • Cathode: Layered oxides (e.g., $FePO<em>4, CoO</em>2$
  • Electrolyte: Sodium salts in organic solvents
  • Separator: Polymer or fiber-based
  • Cell Potential: 1.85 to 3.45 V.
• Reactions:
  • Charging:
    • Cathodic Reaction: $NaCoO<em>2 → Na(1-x)CoO</em>2 + xNa^+ + xe^-$
    • Anodic Reaction: $xNa^+ + xe^- + 6 → xNaC_6$
    • Overall Reaction: $NaCoO<em>2 + 6C ↔ Na(1-x)CoO</em>2 + xNaC_6$
  • Discharging:
    • Anodic Reaction: $xNaC_6 → xNa^+ + xe^- + 6C$
    • Cathodic Reaction: $Na(1-x)CoO<em>2 + xNa^+ +xe^- → NaCoO</em>2$
    • Overall Reaction: $Na(1-x)CoO<em>2 + xNaC</em>6 ↔ NaCoO_2 +6C$

Solar Energy

• Photovoltaic cells convert sunlight into direct current.
• Importance: Environmentally friendly, reliable, cost-effective.
• Photovoltaic Cell (PV Cell): produces direct electric current when exposed to sunlight based on the photovoltaic effect.
• Principle: Certain elements emit electrons when electromagnetic radiation of sufficient energy falls on it.
  • $E = \frac{hc}{λ}$
• Construction:
  • Thin wafer of n-type silicon on top and p-type silicon at the bottom forming a p-n junction.
• Working:
  • Electrons move to the n-type region, driving through an external circuit for applications.