Synthesis, fabrication and characterization of 2-naphthyloxy group-substituted bis(2-pyridylimino)isoindoline and its derivatives as a positive electrode for vanadium redox flow battery applications

In recent years, tridentate nitrogen donor ligands have played a vital role in inorganic chemistry. The ease of synthesis, readily modifiable structure and high stability of 1,3-bis(2-pyridylimino)isoindole (BPIs) compounds make them suitable candidates for many potential applications. In this study, a 1,3-bis(2-pyridylimino)isoindoline derivative bearing a naphthoxy unit and its palladium complex (PdBPI) were synthesized and characterized by single crystal X-ray diffraction, NMR, FT-IR, UV-Vis, and mass spectroscopic methods. The BPI- or PdBPI-modified pencil graphite electrodes were clarified via cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), EDX, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The efficiency of these substances in a vanadium redox flow battery (VRB) system was investigated for the first time. The behaviors of the BPI-modified carbon felt electrode (BPI-CF) and PdBPI-modified carbon felt electrode (PdBPI-CF) were investigated in the redox flow battery (RFB) applications. These modified electrodes were obtained by the electrodeposition method. The respective charge potentials of BPI-CF and PdBPI-CF reached 1.63 V and 1.88 V, respectively. The discharge capacity maxima obtained were ∼301 mA h (1204 mA h L⁻¹) and ∼303 mA h (1212 mA h L⁻¹) for BPI-CF and PdBPI-CF at the VRB system under a charge current density of 4.0 mA cm⁻² and discharge current density of 0.4 mA cm⁻², respectively.