Synthesis of nickel, calcium and magnesium naphthalene diimide complexes as supercapacitor materials

Three novel coordination complexes [Ni(H₂L1)(DMF)] (Complex 1), [Ca(H₂L2)(DMF)] (Complex 2), and [Mg(H₂L2)] (Complex 3) have been synthesized hydrothermally using naphthalene diimide (NDI) ligands. Additionally, these complexes have been studied by single-crystal XRD, IR spectroscopy, TGA analysis, UV–visible spectroscopy, and Hirschfeld surface analysis. Complexes 1 and 2 are 2D polymers of coordination, whereas complex 3 is a 3D coordination framework. The electrochemical properties of the three complexes are determined using cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) analysis, and their suitability as supercapacitor electrode materials is determined by fabricating working electrodes in 0.5 M TBAPF₆ electrolytes in a three-electrode cell. The capacitances of the three complexes are gravimetric 214 Fg⁻¹, 141 Fg⁻¹, and 127 Fg⁻¹ at 1 Ag⁻¹, respectively, with a voltage window of − 2.5 to 1.0 V and outstanding stability up to 6000 cycles with retentions of around 80%, 78%, and 77%, respectively. We assembled a symmetric device of complex 1 only (because complex 1 has a higher specific capacitance than the other two complexes) that exhibited remarkable electrochemical performance, with a high specific capacitance of 102 Fg⁻¹ at 0.5 Ag⁻¹ and an excellent energy density of 28 Whkg⁻¹. Additionally, the symmetric device demonstrated excellent cyclic stability over 5000 charge–discharge cycles at 8 Ag⁻¹ with capacitive retention of 98%. Moreover, DFT study has been carried out to support the results. The results indicate that the three coordination complexes with superior electrochemical characteristics have the potential to operate as capacitors.