9.3 Fuel Cell Components and Materials

Fuel cells are electrochemical devices that convert chemical energy into electricity. Their components work together to facilitate this process, with each part playing a crucial role in efficiency and performance.

Electrolyte materials are the heart of fuel cells, enabling ion transport and separating reactions. Proton exchange membranes and solid oxide electrolytes serve different purposes, influencing cell design, operating conditions, and potential applications.

Fuel Cell Components

Functions of fuel cell components

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• Membrane efficiently separates anode and cathode compartments while allowing proton transport and preventing mixing of reactant gases (hydrogen and oxygen)
  • Commonly made of proton exchange membrane (PEM) materials (Nafion) that enable selective proton transport
• Electrodes (anode and cathode) provide sites for electrochemical reactions to occur
  • Anode oxidizes hydrogen fuel, releasing electrons and protons
  • Cathode reduces oxygen, consuming electrons and protons to form water
  • Typically made of porous carbon materials with catalysts (platinum) to enhance reaction kinetics and improve performance
• Bipolar plates distribute reactant gases (hydrogen and oxygen) evenly to the electrodes
  • Remove product water from the cell to maintain optimal performance
  • Conduct electrons between adjacent cells in a fuel cell stack, enabling electrical current flow
  • Provide mechanical support and help maintain cell integrity
  • Usually made of graphite, metal, or composite materials that are electrically conductive and mechanically robust

Materials for fuel cell components

• Membrane materials require high proton conductivity, low electronic conductivity, chemical and thermal stability, and mechanical strength
  • Enable efficient proton transport while preventing short-circuiting and maintaining durability under fuel cell operating conditions
• Electrode materials need high surface area, porosity, electrical conductivity, and stability
  • Maximize reaction sites, facilitate electron transport, and remain compatible with catalyst materials (platinum) in the fuel cell environment
• Bipolar plate materials must have high electrical conductivity, low gas permeability, chemical and corrosion resistance, mechanical strength, and cost-effectiveness
  • Minimize ohmic losses, prevent reactant gas crossover, withstand the fuel cell environment, and provide durability while being lightweight and affordable for practical applications

Electrolyte Materials

Importance of proton exchange membranes

• PEM materials (Nafion) enable proton transport from anode to cathode, which is essential for completing the electrochemical reaction
  • Allows for the separation of electrochemical reactions at the anode and cathode, preventing mixing of reactants and products
• PEMs operate effectively at low temperatures (typically below 100°C), enabling faster startup times and simplified system design compared to high-temperature fuel cells
  • Suitable for applications such as transportation (fuel cell vehicles) and portable power (fuel cell generators)
• PEMs require humidification to maintain proton conductivity, making water management crucial for optimal performance
  • Balancing water production at the cathode and water removal is necessary to prevent flooding or drying out of the membrane

Role of solid oxide electrolytes

• Solid oxide electrolytes (yttria-stabilized zirconia, YSZ) conduct oxygen ions ($O^{2-}$) from cathode to anode, completing the electrochemical reaction
  • Allows for the separation of electrochemical reactions at the anode and cathode, preventing mixing of reactants and products
• Solid oxide fuel cells (SOFCs) operate at high temperatures (typically 600-1000°C), enabling the use of non-precious metal catalysts and reducing cost
  • Allows for direct internal reforming of hydrocarbon fuels (natural gas) without the need for an external reformer
  • Suitable for stationary power generation (power plants) and combined heat and power (CHP) applications (industrial facilities)
• Challenges associated with solid oxide electrolytes include the need for thermally stable materials, thermal expansion compatibility between components, and slow startup times
  • High operating temperatures require careful material selection and design to prevent cracking and delamination
  • Thermal cycling can limit applications due to the time required for heating and cooling the system