Electrochemistry Fundamentals: Cells, Potential, and Reactions

Posted on Oct 21, 2024 in Chemistry

Fundamental Concepts of Electrochemistry

What is Electrochemistry?

Electrochemistry is the study of the relationship between chemical reactions and electricity. Advancements in this field are driving new developments in energy storage, energy conversion, catalysis, synthesis, separations, and instrumentation. Electroanalytical chemistry, a branch of analytical chemistry, uses electrochemical techniques to study and analyze substances in solution.

Common Electroanalytical Methods

• Potentiometry
• Controlled Potential
  • Amperometry
  • Voltammetry
    • Cyclic Voltammetry
    • Pulse Voltammetry
    • Stripping Voltammetry
  • Controlled Current
  • Impedance Measurement

Fundamental Concepts

Electrochemistry primarily involves measuring current at a fixed or controlled potential, or measuring potential with a fixed/controlled current or no current.

• Electrochemical cells: basic components, galvanic and electrolytic cells
• Potential in electrochemical cells with no current: Nernst Equation, concentration and activity
• Electrochemical cells with current: ohmic drop, polarization, overvoltage

Electrochemical Cells

• A galvanic cell
  • Spontaneous reaction
  • Produces electrical energy
• An electrolytic cell
  • Non-spontaneous reaction
  • Consumes electrical energy
• Half cells
• Anode (oxidation) and cathode (reduction)
• Electrolyte
• Liquid junction
  • Results from an unequal distribution of cations and anions across the boundary due to differences in their diffusion rates.

Potentials in Electrochemical Cells with No Current

• Key Equations:
  • E_cell = E_cathode – E_anode
  • ΔG = -nFE_cell
• Standard reduction potential measures the individual potential of a reversible electrode at standard state (1 M solute concentration, 1 atm gas pressure, 25 °C) with respect to the standard hydrogen electrode (SHE).

Nernst Equation

The Nernst equation relates the reduction potential of an electrochemical reaction (half-cell or full cell) to the standard electrode potential, temperature, and activities (often approximated by concentrations) of the chemical species undergoing reduction and oxidation.

Activity vs. Concentration

Activity (a) is the “effective concentration (c)”:

Electrochemical Cells with Current

• Most electroanalytical methods involve current flow.
• Faradaic and Non-Faradaic Current
  • Faradaic current: direct electron transfer via redox reactions, following Faraday’s Law.
  • Non-faradaic current (capacitive or double-layer current): involves the accumulation/removal of charges on the electrode and in the nearby electrolyte solution, without chemical reactions.
• The measured cell potential with current flow usually differs from the calculated thermodynamic potential.
• Ohmic resistance and polarization effects reduce galvanic cell voltage or increase the voltage needed for electrolytic cell current.

Ohmic Potential

• Ohmic potential: voltage needed to overcome solution resistance (resistance of ions moving towards electrodes).

E_cell = E_right – E_left – IR

• Net effect: reduces galvanic cell voltage or increases electrolytic cell voltage requirement.
• Randles circuit: an equivalent electrical circuit.

Polarization and Overpotential

• Polarization: deviation of electrode potential from equilibrium.
• Overpotential (η): potential difference between the potential at a certain current and the equilibrium potential.
• Polarized electrode: infinitely slow reaction, zero exchange current density, behaves like a capacitor.
• Non-polarized electrode: infinitely fast reaction, infinite exchange current density, behaves like a short circuit.
• Ideal polarized electrode
  • Current independent of potential over a range.
  • No charge-transfer (no Faradaic current).
• Ideal non-polarized electrode
  • Potential independent of current.
  • Current passes freely.
  • No overpotential.

Sources of Polarization

• Possible steps in an electrochemical reduction: Ox + ne^– ↔ Red
• Mass transfer (diffusion, migration, convection): concentration polarization
• Chemical reactions (decomposition, dimerization, etc.): reaction polarization
• Adsorption: adsorption polarization
• Charge transfer kinetics: charge transfer polarization

Mass Transfer

• Diffusion: analyte depletion near the electrode creates a concentration gradient.
• Migration: movement of ions under an electric field.
• Convection: mass transport due to temperature/density differences or mechanical disturbance (e.g., stirring).

Charge-Transfer Polarization

• Occurs when the redox reaction rate is insufficient to produce theoretically predicted currents.
• Characteristics of overpotential
  • Increases with current density.
  • Usually decreases with increasing temperature.
  • Varies with electrode composition (especially with mercury).
  • Significant for reactions producing gaseous products (e.g., H₂, O₂).
  • Cannot be precisely predicted.
• Overpotential for H₂ and O₂ evolution is particularly significant.
  • Difference between smooth and platinized surfaces.
  • Sometimes useful: prevents interference with metal deposition.