An electrocatalyst lowers the activation barrier for the electrode
half-reaction (here, 2H⁺ + 2e⁻ → H₂ at the cathode). The applied
overpotential η drives the reaction rate; the catalyst material sets how
efficiently that driving force converts into current — expressed by the
Tafel slope. Gold/teal spheres are catalyst nanoparticles decorating the
electrode surface; bubbles nucleate faster on more, better, hotter sites.
j = j0 · exp(η / b) (Butler–Volmer, high-η limit)
b = Tafel slope (mV/decade)
j0 = exchange current density (catalyst-dependent)
- Overpotential η — extra voltage beyond the thermodynamic minimum needed to drive the reaction at a useful rate.
- Catalyst loading — fraction of the electrode surface covered by active nanoparticles.
- Electrolyte temperature — raises ion mobility and reaction kinetics, boosting current for the same η.
- Material — Pt-group metals have the lowest Tafel slope (fastest kinetics) but are costly; oxides and single-atom catalysts trade some activity for cost/durability/selectivity.