Synthesis of low dimensional hierarchical transition metal oxides via a direct deep eutectic solvent calcining method for enhanced oxygen evolution catalysis

Abstract

Transition metal oxides (TMOs) are regarded as important materials due to their wide applications in catalysis, sensors, energy storage and conversion devices owing to their advantages of facile synthesis, low cost, and high activity. Here we develop a direct deep eutectic solvent (DES) calcining method to prepare low-dimensional and highly active TMOs for the electrochemical oxygen evolution reaction (OER). Glucose monohydrate and urea can form a glucose-urea DES, which was calcined under a N₂ atmosphere to produce 2D N,O-doped graphene. When metal precursors were introduced into the glucose-urea DES and calcined together, the TMOs were templated by graphene flakes and exhibited low-dimensional morphologies. With this method, 2D nanonet-shaped La_0.5Sr_0.5Co_0.8Fe_0.2O₃ (LSCF), Co₃O₄, NiCo₂O₄, and RuO₂ and 1D nanowire-shaped Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃ (BSCF) were readily synthesized, and their thickness and porosity can be conveniently tuned by adjusting the concentrations of metal salts. Our nanostructured TMOs were further applied for the OER, and they showed quite competitive activities over their counterparts obtained from other methods. The 2D porous LSCF₂₀-DES exhibited the largest specific surface area (28.9 m² g⁻¹) and the highest OER electrocatalytic activities (0.304 V overpotential at a current density of 10 mA cm⁻²). These results demonstrate that the DES calcining method is a comprehensive approach to synthesize hierarchical TMOs as highly active OER catalysts.