Beyond the Traditional Volcano Concept: Overpotential-Dependent Volcano Plots Exemplified by the Chlorine Evolution Reaction over Transition-Metal Oxides

The chlorine evolution reaction (CER) over a single-crystalline RuO2(110) model electrode is one of the best understood model systems in the field of electrocatalysis, which is taken here as a benchmark system to advance the concept of activity-based Volcano plots. Volcano curves can be derived from linear scaling relationships, in which thermodynamic considerations based on Sabatier's principle and the Bronsted-Evans-Polanyi relation at zero overpotential are assumed to describe activity trends of electrocatalysts within a homologous series of materials. However, the underlying approach does not capture the influence of the applied overpotential on the activity, which is given by the Tafel slope. This may explain, why in certain cases the traditional Volcano analysis at zero overpotential does not reproduce activity trends of highly active catalytic materials with an overpotential-dependent Tafel slope correctly. Herein, a novel approach of overpotential-dependent Volcano plots is presented, which connects thermodynamics with kinetics at the respective target overpotential and includes the experimental Tafel slope into the analysis to describe the activity. This methodology is applied to the CER over transition-metal oxide electrodes, such as RuO2(110) and IrO2(110): while the traditional Volcano analysis at zero overpotential ascertains IrO2(110) to be more active in the CER, the overpotential-dependent Volcano plot reproduces the experimentally observed higher CER activity of RuO2(110) compared to IrO2(110) qualitatively as well as quantitatively. This result puts additional emphasis on the fact that the applied overpotential needs to be accounted for in material screening trend studies.