Growth of Single and Bimetallic Nickel and Cobalt Compounds for High Performance Supercapacitors via Fluoride-Based Structure-Directing Regulation

Abstract

Transition-metal compounds have attracted considerable attention as electroactive materials for supercapacitors because of their efficient reversible redox reactions. NH4HF2 and NH4BF4 have been demonstrated as effective structure-directing agents (SDAs) for regulating the growth and electrochemical properties of transition-metal compounds. However, systematic investigations of single and bimetallic systems under an NH4HF2/NH4BF4 environment remain limited. In this study, nickel- and cobalt-based single and bimetallic systems are synthesized in the solution containing NH4HF2 and NH4BF4 as mixed SDAs. The dual-SDA environment together with bimetallic interaction effectively modulates nucleation kinetics and crystal growth, inducing abundant oxygen vacancies and constructing a hierarchical porous architecture with improved ion-transport pathways. These structural advantages significantly enhance electroactive surface exposure and facilitate rapid faradaic redox reactions. The bimetallic electrode delivers a high specific capacitance (CF) of 1933.3 F/g at 20 mV/s, outperforming Ni- and Co-based electrodes with CF values of 778.7 and 132.4 F/g, respectively. The assembled symmetric supercapacitor exhibits a maximum energy density of 13.4 Wh/kg at 666.7 W/kg as well as a CF retention of 87.5% and Coulombic efficiency of 98.0% after 10,000 cycles. This work provides a viable strategy for coordinating defect engineering and porous structure optimization in bimetallic systems for advanced energy storage applications.