Spatial Engineering of Ru Sites in Rare‐Earth High‐Entropy Oxides for Selective Chlorine Electrocatalysis

ABSTRACT Chlorine is primarily produced through the chlorine evolution reaction (CER) in the chlor‐alkali process. However, the higher equilibrium potential of CER leads to lower selectivity for Cl 2 , primarily because of the competing oxygen evolution reaction (OER) during water electrolysis. Therefore, developing highly efficient and selective electrocatalysts for CER over OER remains a significant challenge. Herein, we present a customized protocol to fabricate 2D, ultrathin high‐entropy rare earth oxides (HE‐REOs) with tunable grain boundaries (GBs), which serve as robust supports for anchoring RuO 2 nanoislands. The integrated HE‐REOs/RuO 2 heterostructure increases GB density through the multi‐metal entropy effect, optimizes the electronic structure of HE‐REOs, and establishes an intrinsic electron activation pathway that facilitates the delocalization of RE 4f electrons. The multiple sites in HE‐REOs result in excessive adsorption of OH species and inhibit oxygen evolution, while the unique selective oxygen preferential adsorption effect of the carrier enhances the efficient precipitation of Cl species from Ru sites. Notably, the optimal CeZrCoNiZnO/RuO 2 catalyst exhibits near‐100% selectivity for Cl 2 , a high mass activity of 38 850 A g −1 Ru , and exceptional durability exceeding 1000 h at 80°C. This work provides a promising strategy for optimizing the activity, selectivity, and stability of RE‐based catalysts through grain boundary engineering.