Fuel Cells Reaction Session 4 (2)

Course Overview

  • CH5013: Principles of Fuel Cells

    • Prof. Raghuram Chetty

    • Department of Chemical Engineering, IIT Madras

    • Focus on reaction kinetics of fuel cells.

Fundamental Concepts

Energy Levels in Atoms

  • Energy E = 0 corresponds to the vacuum level.

  • Small Molecules and Large Molecules differ in their energy states:

    • HOMO (Highest Occupied Molecular Orbital)

    • LUMO (Lowest Unoccupied Molecular Orbital)

  • Fermi Level and Vacuum Level are key indicators of electron energy.

  • Chemistry is influenced by states determining filled and empty transitions and their distribution in bulk materials.

Band Structure and Material Types

  • Three types of materials based on their band structure:

    • Conductors (Metals): Infinitesimal spacing between filled and empty states.

    • Semiconductors: Small, non-zero spacing between filled and empty states.

    • Insulators: Large spacing between filled and empty states (Band Gap).

Fermi Level Concepts

  • The Fermi Level (EF) represents the minimum energy required to remove an electron from a material.

  • Affects the binding strength of electrons to the nuclear structure; electrons closer to the vacuum level are less bound.

Electron Flow and Charge Transfer

  • When two conductors meet, electron flow generates a charge separation leading to a Contact Potential Difference.

  • Electron transfer phenomena are crucial in defining the equilibrium conditions for electrochemical reactions.

Ion in Solution

  • Ionization process presented in terms of HOMO, LUMO, and the corresponding energy gap.

  • The electronic structure of ions in electrochemical contexts (e.g. in a solution).

Metal in Solution

  • When metals align with electrolytes, there is charge transfer to balance Fermi levels, leading to potential disparities.

  • This principle applies to metal contacts in electrolytic solutions.

Junction Potentials

  • Junction potentials occur when dissimilar materials contact, essential in electrochemical cell design.

Electrochemical Thermodynamics

  • Chemical Potential: Reflects a substance's contribution to energy based on its proportion.

  • Electrochemical Potential includes this chemical behavior and the response to electric fields, mathematically expressed as:

    • ( m = m_0 + zFf )

  • Fundamental measures in comprehending thermodynamics in system energy interactions.

Gibbs Free Energy

  • Essential for predicting spontaneous chemical reactions.

  • Variables remain constant (T and P) during evaluations.

Chemical vs. Electrochemical Reactions

  • Chemical Reactions: Charge transfers occur between species without free electron liberation.

  • Electrochemical Reactions: Direct charge transfer occurs between an electrode and species, influencing the product's energy recovery.

Hydrogen Combustion Reaction

  • Exothermic reaction represented as:

    • ( 2H2 + O2 ightarrow 2H2O, ext{ } ext{ } ext{ } \Delta H = -482 ext{ kJ/mol} )

  • Characterized by its energy profile over the reaction progression.

Activation Energy & Reaction Profiles

  • Activation energy is crucial in controlling the kinetics of both chemical and electrochemical reactions.

  • Energy profiles depict the energy expenditure for breaking bonds and the energy release upon bond formation.

The Electrode Reaction Dynamics

  • The coupling of oxidation and reduction processes at the electrode is governed by reaction rate constants (kt).

  • Electrochemical reaction dynamics entail both mass transfer and energy processes at the interface.

The Role of the Electrical Double Layer

  • Functions like a capacitor with distinct layers enabling charge separation and movement.

Influences of Potential on Electrochemical Systems

  • Altering electrode potential influences electron energy levels, guiding reactive pathways.

  • Ongoing influence: Increased negativity favors reduction; increased positivity favors oxidation.

Current Measurement Techniques

  • Current (i) directly indicates the electrochemical reaction rate, linked through Faraday's law:

    • ( i = \frac{dQ}{dt} )

  • Integration of Faraday's law expresses total charge as proportional to moles of materials transferred.

Electrochemical Kinetics and Arrhenius Expression

  • Reaction rate constants linked to activation energy by the Arrhenius equation.

  • Influencing factors include temperature aspects expressed as:

    • ( k = f e^{-\frac{G}{RT}} )

Net Rate of Reaction

  • The net reaction rate takes both forward and reverse kinetics into account, expressed mathematically.

Butler-Volmer Equation

  • Core equation linking current density to activation overvoltage, fundamental in fuel cell functionality.

  • Current relationships can be simplified under certain conditions to mark major transitions in electrochemical processes.

Tafel Equation

  • Provides an approximation of Butler-Volmer under specific activation voltage ranges.

  • Offers clarity in interpreting electrochemical kinetics and efficiency.

Applications of Tafel Equation

  • Used to distinguish between activation-controlled and diffusion-controlled reactions based on their respective Tafel slopes.

  • Critical tool for operational optimization within fuel cell technologies.

Catalyst Performance Comparison

  • Exchange current density significantly varies across catalysts (e.g., Pt vs. lead), affecting efficiency.

Tafel & Linear Sweep Data

  • Data from rotating disk electrodes demonstrate the kinetics of oxygen reduction reactions.

Further Study

  • Directive references for deeper implications in fuel cell development and electrical engineering applications.