Overpotential-Dependent Volcano Plots to Assess Activity Trends in the Competing Chlorine and Oxygen Evolution Reactions

The selectivity problem of the competing chlorine evolution (CER) and oxygen evolution (OER) reactions at the anode in chlor-alkali electrolysis is a major challenge in the chemical industry. The development of electrode materials with enhanced stability and CER selectivity could result in a significant reduction of the overall process costs. In order to gain an atomic-scale understanding of the CER versus OER selectivity, commonly, density functional theory (DFT) calculations are employed that are analyzed by the construction of a volcano plot to comprehend trends. Herein, the binding energy of oxygen, Delta E-O, has been established as a descriptor in such analyses. In the present article, it is demonstrated that Delta E-O is not suitable to assess activity trends in the OER over transition-metal oxides, such as RuO2(110) and IrO2(110). Quite in contrast, the free-formation energy of oxygen with respect to hydroxide, Delta G(O-OH), reproduces activity trends of RuO2(110) and IrO2(110) in the CER and OER correctly. Consequently, re-investigation of the CER versus OER selectivity issue, using Delta G(O-OH) as a descriptor, is strongly suggested.