One of the more interesting stack ideas for fuel cells is phosphoric acid based units.

AS all fuel cells these require special materials and the cooling of machines is a bit tricky!  Thermgym can really help out here.

Comparison of Fuel Cell Types: PAFC, MCFC, AFC, and SOFC

Advantages and Disadvantages

PAFC

Advantages:

• Good CO tolerance
• Stable electrolyte
• High-quality waste heat
• Long lifetime

Disadvantages:

• Expensive catalysts (Pt)
• Corrosive electrolyte
• Moderate efficiency

Thermogym can use Hastelloy C-276 for fin and tube heat exchangers that offer excellent heat transfer with excellent corrosion resistance.

Let’s do a deep dive into these to really understand.

Phosphoric Acid Fuel Cells (PAFCs): Advanced Analysis

Fundamental Principles

PAFCs operate on the electrochemical principle of separating the oxidation of hydrogen and reduction of oxygen into two half-reactions:

1. Anode reaction: 2H₂ → 4H⁺ + 4e⁻
2. Cathode reaction: O₂ + 4H⁺ + 4e⁻ → 2H₂O

The overall cell reaction is: 2H₂ + O₂ → 2H₂O

Electrolyte Characteristics

• Composition: Concentrated phosphoric acid (H₃PO₄), typically >95% concentration
• Operating temperature: 150-200°C
• Proton conductivity: σ ≈ 0.05 S/cm at 150°C, increasing with temperature
• Activation energy for proton conduction: Ea ≈ 16-20 kJ/mol

Electrode Kinetics

1. Hydrogen Oxidation Reaction (HOR):
   • Fast kinetics on Pt catalyst
   • Exchange current density: i₀ ≈ 1 mA/cm² (Pt, 180°C)
2. Oxygen Reduction Reaction (ORR):
   • Rate-limiting step
   • Exchange current density: i₀ ≈ 10⁻⁸ to 10⁻⁷ mA/cm² (Pt, 180°C)
   • Tafel slope: b ≈ 90-120 mV/decade

Thermodynamics and Efficiency

• Standard cell potential: E⁰ = 1.23 V at 25°C
• Actual cell potential: E ≈ 0.6-0.8 V under operating conditions
• Efficiency: η = ΔG/ΔH ≈ 40-50% (electrical)
• Combined heat and power efficiency: η_CHP ≈ 80-85%

Materials Science Aspects

1. Electrodes:
   • Anode and cathode: Porous carbon paper with Pt catalyst
   • Pt loading: typically 0.1-0.5 mg/cm²
   • Catalyst support: High surface area carbon black
2. Matrix:
   • Silicon carbide (SiC) particles bonded with PTFE
   • Porosity: 50-60%
   • Thickness: 100-200 μm

Advantages Over Other Fuel Cell Types

1. CO Tolerance:
   • Can tolerate CO concentrations up to 1-2% at 190°C
   • CO adsorption on Pt is weakened at higher temperatures
2. Thermal Management:
   • High-quality waste heat (150-200°C) suitable for cogeneration
   • Simplified cooling system compared to low-temperature fuel cells
3. Water Management:
   • Phosphoric acid has a low vapor pressure, minimizing electrolyte loss
   • Product water is easily removed as vapor, simplifying water management
4. Long-term Stability:
   • Demonstrated lifetimes >40,000 hours in stationary applications
   • Slow degradation rate: typically <10 μV/h

Challenges and Limitations

1. Phosphoric Acid Management:
   • Electrolyte volume changes with temperature and humidity
   • Potential for acid leaching at high current densities
2. Cathode Catalyst Degradation:
   • Pt dissolution and redeposition (Ostwald ripening)
   • Carbon support corrosion at high potentials
3. Start-up Time:
   • Requires 3-4 hours to reach operating temperature from cold start
4. Power Density:
   • Typical power density: 0.1-0.3 W/cm²
   • Lower than PEM fuel cells due to slower ORR kinetics in acid

Recent Advancements

1. Novel Cathode Catalysts:
   • Pt-Co and Pt-Ni alloys for enhanced ORR activity
   • Pyrolyzed Fe-N-C catalysts as potential Pt-free alternatives
2. High-Temperature Membranes:
   • Phosphoric acid-doped polybenzimidazole (PBI) membranes
   • Operating temperatures up to 200°C with reduced acid leaching
3. Carbon Support Materials:
   • Graphitized carbon blacks for improved corrosion resistance
   • Carbon nanotubes and graphene as high-conductivity supports

Thanks for your patience!  I hoped this helped to understand!

Back to Fuel Cells Generators