In this study, FeS2 (FS) was synthesized via the hydrothermal technique. The various percentages of FS were subsequently incorporated into polyaniline (PANI) through an in-situ chemical process, aiming to improve the performance of the vanadium redox flow battery (VRFB). The synthesized nanocomposites (NPFS) were characterized using various techniques such as spectroscopy, microscopy, chemical, thermal study and electrochemical analysis (CV, EIS and Tafel) and charge discharge cycles study to understand their structure and performance. Morphological, chemical and structural analyses revealed the successful integration of FS into the PANI matrix, forming a hierarchical with highly porous nanocomposite structure. The results of the electrochemical analysis significantly improved the redox behavior and charge transfer kinetics compared to PANI. Based on the characterization and electrochemical characterizations, the best performing composition (NPFS15) sample (3 mg/cm2) was coated on 132 cm2 graphite felt (GF) and inserted in VRFB to examine the charge–discharge cycles. At a current density 70 mA/cm2, the selected nanocomposite with the specified loading on GF exhibited superior electrochemical performance due to the sufficient availability of active sites. Besides, the integration of NPFS nanocomposite materials in VRFB technology may pave the way for more efficient and sustainable energy storage solutions in the future.