Calculation of the Tafel Plot for H2 Oxidation on Pt(100) from Potential-Dependent Activation Energies

Quantum mechanically determined electrode potential dependent activation energies for hydronium ion discharge over Pt−H (Heyrovsky reaction) and the reverse reaction have been used to predict Tafel plots. The calculated Tafel plot for H2 oxidation is similar in shape to an experimental plot from the literature for a Pt(100) electrode and will overlap it when an appropriate preexponential factor is chosen in the Arrhenius expression. This provides strong theoretical support for the first electron-transfer step being rate limiting during H2 oxidation over the potential range 0 to 0.15 V, and the second electron-transfer step being rate limiting during H2 evolution over this electrode. The exchange current density is determined from the calculated oxidation and reduction currents and is found to overestimate experiment primarily because the predicted activation energy at the reversible potential underestimates the experimental value. This study illustrates that curvature in nonlinear Tafel plots may stem from the potential dependence of the activation energies or transfer coefficients as well as diffusion and concentration gradient effects. The observed current density and its increase, leveling off, and then decrease at potentials greater than the activation energy-controlled region are attributed to removal of under potential deposited H, passing through the double layer region, and then site blocking by water and its oxidation product OH(ads).