Is Thermodynamics a Good Descriptor for the Activity? Re-Investigation of Sabatier’s Principle by the Free Energy Diagram in Electrocatalysis

The computational hydrogen electrode (CHE) approach has spurred ab initio investigations in the field of electrocatalysis, since the underlying concept enables to quantify free energy changes, ΔG (thermodynamics), for the formation of reaction intermediates on an electrocatalyst surface. The connection between thermodynamics and kinetics (activity) is achieved by Sabatier's principle: the optimum situation to realize an active electrocatalyst is ascribed to reaction intermediates that are thermoneutrally bound (ΔG = 0 eV) at zero overpotential. In order to validate the linkage between thermodynamics and kinetics at zero overpotential for two-electron processes, free energy diagrams as a function of the applied electrode potential are compiled. Herein, the chlorine evolution reaction (CER) over RuO2(110), one of the best understood model systems in electrocatalysis, is used as a starting point for this investigation. It turns out that the connection between thermodynamics and kinetics at zero overpotential does not reproduce activity trends correctly if the Tafel slope is overpotential dependent. Therefore, it appears expedient to include the applied overpotential into the thermodynamic framework: for electrocatalysts with a change in the Tafel slope, it is suggested to employ the absolute free energy change for the formation of a reaction intermediate at respective overpotential η, |ΔG(η)|, as thermodynamic descriptor for the kinetics of two-electron processes, which may aid the construction of overpotential-dependent Volcano plots for improved material screening.