In Situ Growth of Iron Phosphate on Orange Peel-Derived Activated Carbon: Strategic Approach toward the Design of a Prototype Symmetric Solid-State Supercapacitor

This study explores an in situ hydrothermal approach for the synthesis of iron phosphate with orange peel-activated carbon (OPAC) composite, wherein OPAC was prepared through K 2 CO 3 -assisted pyrolysis. The growth kinetics not only improved the electrochemical performance but also demonstrated an efficient waste valorization strategy by transforming agricultural waste into high-performance electrode material. Surface morphological analysis confirmed the homogeneous dispersion of spherical FePO 4 ·2H 2 O microflakes within the highly porous OPAC matrix, with a specific surface area of 102.01 m 2 /g at a gas adsorption volume of 23.43 cm 3 /g, which helped to enhance the charge storage efficiency. Powder XRD and FTIR spectroscopy further validate the structural modifications, crystallinity changes, and functional group interaction in the composite compared to those of pristine materials. The structural features were comprehensively confirmed through HR-TEM and SAED analysis. Electrochemical investigation revealed that the OPAC/FePO 4 ·2H 2 O composite electrode exhibited a high specific capacitance ( C s ) of 769.67 F/g (areal capacitance, C a = 769.67 mF/cm 2 ) at a scan rate of 5 mV/s, along with excellent cycling stability, retaining 80.1% capacitance after 5000 CV cycles. Furthermore, a symmetric solid-state supercapacitor device fabricated using OPAC/FePO 4 ·2H 2 O composite electrode and a PVA-KOH gel electrolyte demonstrated a wide operating potential window of 1.6 V. The device successfully powered a VNIT LED panel and a small DC fan, highlighting its potential for prototype scale energy storage and future portable electronic application.