Ru-Doped Co₃O₄ Nanoparticles as Efficient and Stable Electrocatalysts for the Chlorine Evolution Reaction

The electrochemical chlorine evolution reaction (CER) is one of the most important electrochemical reactions. Typically, iridium (Ir)- or ruthenium (Ru)-based mixed metal oxides have been used as electrocatalysts for the CER due to their high activities and durabilities. However, the scarcity of Ir and Ru has indicated the need to develop alternative earth-abundant transition-metal-based CER catalysts. In this study, we report a Co₃O₄ nanoparticle (NP) catalyst synthesized by a hydrothermal method. Furthermore, Ru was successfully incorporated into the Co₃O₄ NPs (RuCo_3-O₄ NPs) for further improvement of catalytic performance in chlorine generation. Electrokinetic analyses combined with in situ X-ray absorption near-edge structure (XANES) results suggested an identical CER mechanism for the Co₃O₄ NPs and RuCo_3-O₄ NPs. Various characterization techniques demonstrated that the homogeneous substitution of Ru⁴⁺ ions into the Co³⁺ octahedral sites enhanced the structural disorder and changed the electronic state of Co₃O₄, resulting in additional exposed active sites. Remarkably, the Ru_0.09Co_2.91O₄ NP electrode exhibited outstanding stability for more than 150 h even at a high current density of 500 mA/cm², which shows its commercial viability for active chlorine generation.