Temperature-Dependent Kinetics of Outer-Sphere Electrochemical (Non-Catalytic) and Electrocatalytic Reactions

An experimental framework based on transition state theory (TST) is presented to establish the electrocatalysis of redox reactions. Ferri-/ferrocyanide ([Fe(CN)6]3−/4−) redox reactions on nitrogen-doped carbon (N/C-900, with ≤1% N) and carbon (C) electrodes, and the oxygen reduction reaction (ORR) on 30 wt.% platinum-cobalt on carbon (30% Pt3Co/C) and 20 wt.% platinum on carbon (20% Pt/C) catalysts are investigated as illustrative examples. The estimated kinetic current (iK) and corrected overpotentials (η) are used to extract the kinetic parameters using the Tafel and Eyring analyses. For the [Fe(CN)6]3−/4− system, the kinetic parameters are found to be independent of the electrode, yielding evenly spaced parallel Eyring lines for the same increment in η values. On the other hand, the estimated apparent enthalpy of activation (ΔH#) at the same η is lower on 30% Pt3Co/C for ORR as compared to that of 20% Pt/C. Furthermore, ΔH# decreases more rapidly with increasing η on 30% Pt3Co/C, resulting in converging Eyring lines. This behavior reflects the strong dependence of electrocatalytic reactions on electrode-intermediate interactions. This study provides a quantitative framework for differentiating electrocatalytic reactions from outer-sphere electrochemical (non-catalytic) reactions, essential to the rational design of electrode materials/electrolytes for energy conversion and storage devices.