9.1 Fundamentals of Fuel Cell Operation

Fuel cells are ingenious devices that convert chemical energy into electricity. They work by oxidizing fuel at the anode and reducing oxygen at the cathode, with electrons flowing through an external circuit to generate power.

The key components of a fuel cell are the electrolyte, electrodes, and catalyst. These work together to facilitate the electrochemical reactions, with the anode oxidizing fuel and the cathode reducing oxygen to produce water as the main byproduct.

Fuel Cell Components and Operation

Principles of fuel cell operation

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• Convert chemical energy directly into electrical energy through electrochemical reactions
• Fuel (hydrogen) oxidized at anode releases electrons and produces protons (H+)
• Oxidant (oxygen from air) reduced at cathode consumes electrons and combines with protons forming water
• Electrochemical reactions occur at electrode-electrolyte interface
  • Anode reaction: $H_2 \rightarrow 2H^+ + 2e^-$
  • Cathode reaction: $\frac{1}{2}O_2 + 2H^+ + 2e^- \rightarrow H_2O$
  • Overall reaction: $H_2 + \frac{1}{2}O_2 \rightarrow H_2O$
• Electrons flow through external circuit from anode to cathode generating electric current
• Protons migrate through electrolyte from anode to cathode completing the circuit

Components of fuel cells

• Electrolyte
  • Conducts ions (protons) between anode and cathode
  • Separates fuel and oxidant preventing direct mixing and combustion
  • Common electrolytes include polymer electrolyte membranes (PEM) and solid oxide electrolytes (SOFC)
• Electrodes (anode and cathode)
  • Provide sites for electrochemical reactions
  • Anode: site of fuel oxidation reaction, releases electrons
  • Cathode: site of oxidant reduction reaction, accepts electrons
  • Typically made of porous materials with high surface area (carbon, metal foams)
• Catalyst
  • Facilitates and accelerates electrochemical reactions at electrodes
  • Lowers activation energy barrier allowing reactions at lower temperatures
  • Common catalysts include platinum, palladium, and their alloys
  • Often dispersed as nanoparticles on electrode surface maximizing active surface area

Anode vs cathode reactions

• Anode reaction
  1. Oxidation of fuel (hydrogen)
  2. Releases electrons to external circuit
  3. Produces protons that migrate through electrolyte to cathode
  • Example: $H_2 \rightarrow 2H^+ + 2e^-$
• Cathode reaction
  1. Reduction of oxidant (oxygen from air)
  2. Accepts electrons from external circuit
  3. Combines protons from electrolyte with oxygen forming water
  • Example: $\frac{1}{2}O_2 + 2H^+ + 2e^- \rightarrow H_2O$

Fuel and oxidant supply

• Continuous supply of fuel and oxidant essential for sustained fuel cell operation
  • Fuel (hydrogen) must be continuously fed to anode
  • Oxidant (oxygen from air) must be continuously fed to cathode
• Fuel and oxidant flow rates affect fuel cell performance
  • Insufficient flow rates lead to mass transport limitations and reduced power output
  • Excessive flow rates lead to inefficient fuel utilization and increased system complexity
• Fuel and oxidant purity crucial for optimal fuel cell performance
  • Impurities can poison catalyst reducing effectiveness and lifetime
  • Fuel impurities (carbon monoxide) adsorb onto catalyst surface blocking active sites
  • Oxidant impurities (sulfur compounds) degrade catalyst and electrolyte
• Fuel and oxidant humidification may be necessary for certain fuel cell types
  • Polymer electrolyte membrane fuel cells require proper humidification maintaining ionic conductivity
  • Insufficient humidification leads to membrane dehydration and increased ohmic losses
  • Excessive humidification leads to flooding of electrodes and mass transport limitations