Synthesizing novel NH4CoxNi1-xF3 as electroactive material for supercapacitors using 2-methylimidazole: Study of reaction durations

Abstract

Zeolite imidazolate framework-67 (ZIF67) with high surface area is one of promising metal–organic frameworks as electroactive material for supercapacitors (SC). Energy storage ability of electroactive material is primarily influenced by morphology, which is confined via structure-directing agent (SDA). It is the first time to use NH4F as SDA to develop novel bimetallic ZIF67 derivative, NH4CoxNi1-xF3 (NCNF), as electroactive material for SC. The polyhedron structure of ZIF67 is greatly evolved to particle-aggregated sphere (PAS) when NH4F is incorporated. Different reaction times are used for synthesizing NCNF to understand PAS formation mechanism. The NCNF electrode prepared using 24 h shows the highest specific capacitance (CF) of 490.4 F/g at 20 mV/s, due to less grain boundaries between smooth particles for assembling huge sphere, while bimetallic ZIF67 synthesized without NH4F merely shows CF value of 2.7 F/g. The largely enhanced CF value for NCNF electrode with NH4F incorporation is benefitted from novel PAS formation and high electronegativity of fluorine. The SC composed of optimized NCNF electrodes shows CF value of 150 F/g at 1 A/g, maximum energy density of 19.7 Wh/kg at 500 W/kg, and excellent cycling stability with CF retention of 100% and Coulombic efficiency of 76% after 5000 times charge/discharge process.