Exploring novel nickel schiff-base complexes: One-pot green synthesis, density functional theory studies, and structural investigations toward energy storage applications

Four new nickel Schiff-base complexes, [Ni(L1)₂(H₂O)₂]Cl₂ꞏ2H₂O (NiL1), [Ni(L2)₂]·2H₂O (NiL2), [Ni₂(L3)₂]Clꞏ5.5H₂O (NiL3), and [Ni₂(L4)₂]Clꞏ5.5H₂O (NiL4), have been synthesized via the condensation of diaminoguanidine with glyoxylic acid (L1), pyruvic acid (L2 and L3), and α-ketobutyric acid (L4). The complexes have been characterized using various physicochemical techniques, confirming octahedral geometries around the nickel ions. Their structural, redox, and electronic properties have been investigated through DFT calculations. Stable structures were optimized using the hybrid B3LYP method with a split basis set: LANL2DZ for nickel and 6-31G∗ for all other atoms. The complexes demonstrate promising supercapacitor (SC) performance, with NiL4 achieving the highest capacitance of 202 C/g, 95 % capacitance retention over 2000 cycles, and the lowest charge-transfer resistance of 0.8 Ὼ, facilitating efficient high-current charge–discharge operations. An asymmetric device is constructed using activated carbon as the negative electrode and the NiL4 complex as the positive electrode. This device delivers a specific capacity of 128 F/g and an outstanding energy density of 61 Wh/kg at a power density of 5467 W/kg at 2 A/g. The device exhibits remarkable stability, retaining 87 % of its capacity after 5000 consecutive galvanostatic charge–discharge cycles. These exceptional characteristics highlight the NiL4 complex as a promising material for SC electrode applications.