Deep Eutectic Solvent Synthesis of Perovskite Electrocatalysts for Water Oxidation

Oxide perovskites have attracted great interest as materials for energy conversion due to their stability and structural tunability. La-based perovskites of 3d-transition metals have demonstrated excellent activities as electrocatalysts in water oxidation. Herein, we report the synthesis route to La-based perovskites using an environmentally friendly deep eutectic solvent (DES) consisting of choline chloride and malonic acid. The DES route affords phase-pure crystalline materials on a gram scale and results in perovskites with high electrocatalytic activity for oxygen evolution reaction. A convenient, fast, and scalable synthesis proceeds via assisted metathesis at a lower temperature as compared to traditional solid-state methods. Among LaCoO₃, LaMn_0.5Ni_0.5O₃, and LaMnO₃ perovskites prepared via the DES route, LaCoO₃ was established to be the best-performing electrocatalyst for water oxidation in alkaline medium at 0.25 mg cm^-2 mass loading. LaCoO₃ exhibits current densities of 10, 50, and 100 mA cm^-2 at respective overpotentials of approximately 390, 430, and 470 mV, respectively, and features a Tafel slope of 55.8 mV dec^-1. The high activity of LaCoO₃ as compared to the other prepared perovskites is attributed to the high concentration of oxygen vacancies in the LaCoO₃ lattice, as observed by high-resolution transmission electron microscopy. An intrinsically high concentration of O vacancies in the LaCoO₃ synthesized via the DES route is ascribed to the reducing atmosphere attained upon thermal decomposition of the DES components. These findings will contribute to the preparation of highly active perovskites for various energy applications.