Facet‐Dependent Selectivity of Rutile IrO2 for Oxygen and Chlorine Evolution Reactions

Rutile iridium dioxide (IrO2) is a benchmark catalyst for heterogeneous electrooxidation reactions including the oxygen and chlorine evolution reactions (OER, CER), with broad applications such as water splitting, water treatment, chlor–alkali electrolysis, and chlorine‐mediated electrosynthesis reactions. Intrinsic OER activities and mechanisms are well established for low‐index crystallographic facets of IrO2. However, corresponding understanding of CER activity and selectivity is not yet well understood beyond its reactivity on the most thermodynamically stable (110) surface. Here, we experimentally show that the (110) and (100) facets have higher OER activity and lower CER selectivity compared to (101) and (001) IrO2 epitaxial thin films. We compute unique facet dependence in the binding energies between reactive intermediates, where OER adsorbates are stabilized by bridging oxygen atoms on the least CER‐selective facets (110) and (100). The potential dependence and concentration dependence of experimental combined reaction rate order suggest that dynamic intermediate coverage changes in this regime drive differences in CER selectivity across facets. These facets show unique differences in activity between perchloric and sulfuric acid electrolytes, highlighting differences in competitive anion adsorption. These results indicate that facet engineering manipulates the local reaction environment to independently tailor OER and CER rates, providing control over selectivity in competitive electrooxidation conditions.