Dual Role of Deep Eutectic Solvent as a Solvent and Template for the Synthesis of Octahedral Cobalt Vanadate for Oxygen Evolution Reaction

Abstract

In this study, the deep eutectic solvent (DES)-aided synthesis of gamma-CoV2O6 under modest reaction conditions using 1:1 choline chloride malonic acid was reported. In the presence of DES, the reaction occurred at a lower temperature (500 degrees C) compared with that of the respective conventional solid-state synthesis of metal oxides and also with the calcination process involving the metal salts, thereby decreasing the overall formation energy. Differential scanning calorimetry revealed a 2-fold decrease in the endo effect of the thermodestruction of DES, which was observed at a low temperature. The structural pathway taken during the phase and morphology formation was investigated by in situ and ex situ X-ray diffraction (XRD) analysis, Raman spectroscopy, and scanning electron microscopy. The in situ XRD results confirmed the presence of high-temperature alpha-CoV2O6, which was finally converted to alpha-CoV2O6. On account of the metal speciation in DES similar to the biotemplate, well-defined octahedral CoV2O6 nanocrystals were obtained, which exhibited a remarkable OER catalytic activity at current density of 10 mA/cm(2) with an overpotential of a mere 324 mV, with excellent durability for greater than 24 h. Thus, a designer DES solvent with characteristics of biodegradability, cost-effectiveness, and renewability demonstrates potential for the synthesis of noble-metal-free oxygen evolution reaction (OER) catalysts, which allows the present synthesis route to fit well within the merits of green chemistry.