Fuel Cell Electrode Kinetics: Butler-Volmer

About This Visualisation

This interactive tool shows how Butler-Volmer kinetics at the anode and cathode determine fuel cell performance. The key insight is the enormous asymmetry between the two electrode reactions:

• Anode — Hydrogen Oxidation (HOR): H₂ → 2H⁺ + 2e⁻ at E₀ = 0.00 V vs SHE. Extremely fast on Pt (i₀ ~ 0.1 A/cm²)
• Cathode — Oxygen Reduction (ORR): O₂ + 4H⁺ + 4e⁻ → 2H₂O at E₀ = 1.229 V vs SHE. Very sluggish, even on Pt (i₀ ~ 10⁻⁵ A/cm²)
• Open Circuit Voltage: 1.229 − 0.00 = 1.229 V

Try This: Select different catalyst materials for each electrode and increase the current. Watch the Overpotential Breakdown panel — nearly all the kinetic loss comes from the cathode!

What You're Seeing

• Anode curve (blue): Positive current — rate of hydrogen oxidation. Steep curve = fast kinetics, small overpotential
• Cathode curve (orange): Negative current — rate of oxygen reduction. Shallow curve = sluggish kinetics, large overpotential
• Operating points (dots): Where each electrode operates at the chosen current density
• Vertical dotted lines: Show how far each electrode deviates from E₀ — the overpotential
• Red line on x-axis: V_cell = E_cathode − E_anode

Key Learning Points

• The ORR is the bottleneck: Even on Pt, the cathode i₀ is ~10,000× smaller than the anode. Nearly all kinetic loss is at the cathode
• Try switching cathode to Carbon: Watch the voltage collapse — this is why catalysts are essential for the ORR
• The anode is almost "free": Even at high currents, the HOR overpotential on Pt is tiny
• Exchange current density (i₀): The single most important parameter distinguishing fast from slow electrode reactions. It depends on both the reaction and the catalyst material