Fluorine-guided synthesis of copper nickel compounds with 2-methylimidazole and temperature control for battery supercapacitor hybrids

Abstract

Nickel-based compounds are widely studied for battery supercapacitor hybrids (BSHs) due to their redox activity and high theoretical capacitance, but their limited conductivity and structural instability remain challenges. Incorporating copper into nickel-based systems is a practical strategy to enhance electronic conductivity and influence phase formation. The use of fluorine-containing structure-directing agents (SDAs) such as NH₄BF₄ and NH₄HF₂ can regulate crystal growth, interlayer distance, and surface properties through the formation of fluorine complexes. In addition, 2-methylimidazole is introduced to coordinate with metal ions during the initial solution process, which helps stabilize precursor complexes and support uniform nucleation. These chemical agents collectively guide the formation of a porous nanostructure with tailored phase composition. In this work, nickel copper compounds are synthesized by a solution process using NH₄BF₄, NH₄HF₂ and 2-methylimidazole, followed by oxidation at varied temperatures. The optimal Cu-Ni compound synthesized at 300 °C (CuNi300) exhibits a multiphase composition of hydroxides and oxides with favorable morphology, delivering a specific capacitance (C_F) of 1026.0 F/g at 10 mV/s. A BSH assembled with CuNi300 and carbon electrodes achieves a maximum energy density of 82.95 Wh/kg at 350 W/kg, and a C_F retention of 93.2% and Coulombic efficiency of 89.4% after 10,000 cycles.