Glossary

19.3 Electrochemical reactions and fuel cells

3 min read•july 23, 2024

Electrochemical cells and fuel cells are game-changers in energy conversion. They turn chemical energy into electrical power through redox reactions, with electrons moving between electrodes. The magic happens thanks to the and .

Fuel cells take this concept further, offering clean power for various applications. From hydrogen-powered cars to natural gas-fueled power plants, these devices are reshaping our energy landscape. Their efficiency depends on factors like temperature, pressure, and catalysts.

Electrochemical Cells

Principles of electrochemical cells

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  • Convert chemical energy into electrical energy through redox reactions
    • Involve transfer of electrons between species
    • Oxidation releases electrons at (zinc electrode)
    • Reduction accepts electrons at (copper electrode)
  • Gibbs free energy (ΔG\Delta G) determines spontaneity and maximum electrical work
    • Negative ΔG\Delta G indicates spontaneous reaction and positive electrical work ()
    • Positive ΔG\Delta G indicates non-spontaneous reaction and negative electrical work ()
  • Relationship between ΔG\Delta G and cell potential (EcellE_{cell}) given by ΔG=−nFEcell\Delta G = -nFE_{cell}
    • nn is number of electrons transferred per mole of reaction
    • FF is (96,485 C/mol)

EMF calculation using Nernst equation

  • Relates cell potential (EcellE_{cell}) to (Ecell∘E_{cell}^{\circ}) and concentrations of reactants and products
    • Ecell=Ecell∘−RTnFln⁡QE_{cell} = E_{cell}^{\circ} - \frac{RT}{nF} \ln Q
    • RR is universal gas constant (8.314 J/mol·K)
    • TT is absolute temperature (K)
    • QQ is reaction quotient, ratio of product concentrations to reactant concentrations raised to stoichiometric coefficients
  • Standard cell potential (Ecell∘E_{cell}^{\circ}) determined by difference between standard reduction potentials of half-reactions
    • Ecell∘=Ecathode∘−Eanode∘E_{cell}^{\circ} = E_{cathode}^{\circ} - E_{anode}^{\circ}
    • Standard reduction potentials listed in reference tables for various half-reactions (, )

Fuel Cells

Fuel cell types and applications

  • Electrochemical devices convert chemical energy of fuels directly into electrical energy
  • Common types
    • Proton exchange membrane fuel cells (PEMFCs)
      • Use hydrogen as fuel and oxygen as oxidant
      • Applications in transportation (vehicles) and portable power (laptops)
    • Solid oxide fuel cells (SOFCs)
      • Use hydrocarbons (natural gas) or hydrogen as fuel and oxygen as oxidant
      • Applications in stationary power generation (power plants)
    • Molten carbonate fuel cells (MCFCs)
      • Use hydrocarbons as fuel and oxygen as oxidant
      • Applications in large-scale power generation (industrial facilities)
  • Consist of anode, cathode, and
    • Fuel oxidized at anode, releasing electrons
    • Oxidant reduced at cathode, accepting electrons
    • Electrolyte allows transfer of ions between electrodes (proton exchange membrane, solid oxide, molten carbonate)

Thermodynamics of fuel cell reactions

  • Efficiency determined by ratio of to
    • Efficiency = Electrical energy outputChemical energy input\frac{Electrical \: energy \: output}{Chemical \: energy \: input}
  • Thermodynamic efficiency limited by Gibbs free energy change of reaction
    • Maximum thermodynamic efficiency = ΔGΔH\frac{\Delta G}{\Delta H}
    • ΔH\Delta H is of reaction
  • Factors affecting efficiency
    1. Operating temperature
      • Higher temperatures improve efficiency by increasing reaction rates and reducing activation losses
    2. Pressure
      • Higher pressures increase efficiency by improving mass transport and reducing concentration losses
    3. Catalyst
      • Effective catalysts (platinum) reduce and improve reaction kinetics
    4. Fuel and oxidant composition
      • Impurities in fuel or oxidant reduce efficiency by causing side reactions or poisoning catalyst

Key Terms to Review (24)

Activation Energy: Activation energy is the minimum energy required for a chemical reaction to occur. It plays a crucial role in determining the rate of a reaction, as reactions with higher activation energies proceed more slowly than those with lower activation energies. Understanding activation energy helps in analyzing how factors like temperature and catalysts affect chemical reactions and equilibrium.
Anode: An anode is the electrode in an electrochemical cell where oxidation occurs, meaning it is the site where electrons are released. This makes the anode a critical component in the function of batteries and fuel cells, as it plays a vital role in the flow of electrical current through the system. In fuel cells, the anode facilitates the reaction with the fuel, helping to generate energy.
Cathode: A cathode is an electrode where reduction occurs in an electrochemical cell, meaning it gains electrons from the external circuit. In this context, it plays a crucial role in facilitating electrochemical reactions, serving as the site for the chemical transformation of reactants into products, particularly in fuel cells and batteries. Understanding the function of the cathode is essential for comprehending how energy conversion takes place within these systems.
Cell Potential: Cell potential, also known as electromotive force (emf), is the measure of the ability of an electrochemical cell to generate electrical energy from chemical reactions. It represents the difference in electric potential between the two electrodes of a cell, influencing how efficiently a reaction can proceed and how much work can be performed by the system. This concept is crucial in understanding the functionality of electrochemical reactions and fuel cells, where cell potential directly correlates with the energy produced during these processes.
Chemical energy input: Chemical energy input refers to the energy supplied to a system through the breaking and forming of chemical bonds during a reaction. This energy is crucial for driving electrochemical reactions, as it influences how well a system can convert chemical energy into electrical energy, particularly in devices like fuel cells that rely on these processes to generate power.
Electrical energy output: Electrical energy output refers to the amount of electrical energy produced by a system, such as an electrochemical cell or fuel cell, as a result of chemical reactions. This output is crucial in determining the efficiency and effectiveness of energy conversion processes, impacting everything from portable electronics to large-scale power generation.
Electrochemical cell: An electrochemical cell is a device that converts chemical energy into electrical energy or vice versa through electrochemical reactions. These cells consist of two electrodes, an anode and a cathode, immersed in an electrolyte solution that facilitates the movement of ions. The reactions occurring at the electrodes generate electrical current or consume electricity, making them crucial for applications like batteries and fuel cells.
Electrolyte: An electrolyte is a substance that, when dissolved in water or melted, produces ions that can conduct electricity. This ability to dissociate into charged particles is crucial for various electrochemical processes, including those involved in fuel cells and other electrochemical reactions.
Electrolytic cell: An electrolytic cell is an electrochemical device that uses an external power source to drive a non-spontaneous chemical reaction. In this setup, electrical energy is converted into chemical energy, facilitating processes like electrolysis, where compounds are broken down into their components. This contrasts with galvanic cells, which generate electricity through spontaneous reactions, showcasing the versatility of electrochemical systems in various applications.
Enthalpy Change: Enthalpy change is the amount of heat absorbed or released by a system during a chemical reaction or physical process at constant pressure. It reflects the energy transfer that occurs when reactants transform into products, influencing the spontaneity and equilibrium of reactions, as well as the performance of systems like batteries and fuel cells.
Faraday's Constant: Faraday's constant is the electric charge carried by one mole of electrons, approximately equal to 96485.33289 coulombs per mole. This constant is crucial in understanding electrochemical reactions as it relates the amount of substance transformed at an electrode during electrolysis to the electric charge passed through the system, making it a key concept in fuel cells and various electrochemical applications.
Galvanic cell: A galvanic cell is an electrochemical device that converts chemical energy into electrical energy through spontaneous redox reactions. It consists of two electrodes, an anode and a cathode, immersed in electrolyte solutions, allowing for the flow of electrons from the anode to the cathode, which generates electrical current. This process highlights the fundamental principles of electrochemistry and plays a crucial role in various applications, including batteries and fuel cells.
Gibbs Free Energy: Gibbs free energy is a thermodynamic potential that measures the maximum reversible work obtainable from a closed system at constant temperature and pressure. It is a crucial concept because it helps predict the direction of chemical reactions and phase transitions, determining whether a process will occur spontaneously based on changes in enthalpy and entropy.
Hydrogen electrode: The hydrogen electrode is a standard reference electrode used in electrochemistry, consisting of a platinum electrode in contact with hydrogen gas at a pressure of 1 atm, immersed in a solution of hydrogen ions (typically 1 M HCl). This electrode serves as a fundamental building block for measuring the electrode potential of other half-cells in electrochemical reactions and fuel cells, establishing a baseline for comparison.
John B. Goodenough: John B. Goodenough is an American physicist and engineer known for his groundbreaking work in the development of lithium-ion batteries, which are critical for modern portable electronics and electric vehicles. His innovations in electrochemistry and materials science have made a profound impact on energy storage technology, enhancing the efficiency and capability of energy systems that rely on electrochemical reactions.
Michael Faraday: Michael Faraday was a British scientist known for his pioneering work in electromagnetism and electrochemistry during the 19th century. His discoveries laid the foundation for the fields of electrochemistry and electromagnetic induction, significantly impacting technology and science, especially in the development of fuel cells and batteries.
Molten carbonate fuel cell: A molten carbonate fuel cell (MCFC) is a type of fuel cell that operates at high temperatures, typically around 600 to 700 degrees Celsius, using a molten carbonate salt as the electrolyte. This high-temperature operation allows for the efficient conversion of chemical energy from fuels such as natural gas into electrical energy, with the added benefit of producing water and carbon dioxide as byproducts.
Nernst Equation: The Nernst Equation is a mathematical formula used to calculate the electromotive force (EMF) of an electrochemical cell under non-standard conditions. It connects the cell potential to the concentrations of the reactants and products involved in the electrochemical reaction, which is crucial for understanding how fuel cells operate and how electrochemical reactions can be manipulated for energy production.
Proton exchange membrane fuel cell: A proton exchange membrane fuel cell (PEMFC) is an electrochemical device that converts chemical energy from hydrogen fuel and an oxidant, typically oxygen, directly into electrical energy through a series of reactions. This process is characterized by the use of a solid polymer electrolyte, which conducts protons while being impermeable to gases, thus facilitating efficient ion transport and enabling the generation of electricity with water as the only byproduct.
Reaction rate: The reaction rate refers to the speed at which reactants are converted into products in a chemical reaction. This rate can be influenced by various factors, such as temperature, concentration of reactants, and the presence of catalysts. Understanding the reaction rate is essential for analyzing electrochemical reactions and fuel cells, where the efficiency and performance can hinge on how quickly these reactions occur.
Redox reaction: A redox reaction is a chemical process involving the transfer of electrons between two species, leading to changes in their oxidation states. This type of reaction encompasses both oxidation, where an atom or ion loses electrons, and reduction, where an atom or ion gains electrons. Redox reactions are fundamental to various electrochemical processes, including those occurring in batteries and fuel cells, where the conversion of chemical energy into electrical energy happens through electron transfer.
Silver chloride electrode: A silver chloride electrode is a type of reference electrode that consists of a silver wire coated with silver chloride. It is widely used in electrochemical cells due to its stable potential and reliable performance, especially in various applications involving redox reactions and as a half-cell in potentiometric measurements.
Solid oxide fuel cell: A solid oxide fuel cell (SOFC) is an electrochemical device that converts chemical energy from fuel into electrical energy through electrochemical reactions at high temperatures, typically between 600°C to 1,000°C. This technology uses a solid ceramic electrolyte to conduct oxygen ions from the cathode to the anode, where they react with hydrogen or other fuels to produce electricity, water, and heat. SOFCs are known for their high efficiency and ability to utilize a variety of fuels.
Standard Cell Potential: Standard cell potential is the measure of the voltage that an electrochemical cell can produce under standard conditions, defined as a temperature of 25°C, 1 atm pressure, and 1 M concentration of all reactants and products. This potential indicates the driving force for an electrochemical reaction, reflecting the difference in electrical potential between the cathode and anode in a galvanic cell.
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