Hydrothermal synthesis of NiMn(PO4)2/electrochemically exfoliated graphene composites and wastewater sludge-derived activated carbon for hybrid supercapacitors

This study reports the synthesis and electrochemical performance of a sustainably designed nickel‑manganese phosphate/electrochemically exfoliated graphene (NiMn(PO 4 ) 2 /EEG) composite, paired with activated carbon (AC) derived from wastewater sludge, for hybrid supercapacitor applications. Unlike conventional phosphate/graphene systems that rely on chemically reduced graphene oxide and biomass-derived carbons, electrochemically exfoliated graphene is employed to enhance electrical conductivity and interfacial charge-transfer kinetics, while sludge-derived activated carbon enables a fully sustainable device architecture. The NiMn(PO 4 ) 2 /EEG composite was synthesised via a hydrothermal route, and the AC was obtained through phosphoric acid activation of wastewater sludge. Structural and spectroscopic analyses confirmed the formation of a mixed-metal phosphate phase anchored on a few-layer graphene sheets. The optimised NiMn(PO 4 ) 2 /50 mg EEG electrode delivered a high specific capacity of 822.1C g −1 at 1 A g −1 , outperforming pristine NiMn(PO 4 ) 2 . When assembled in an asymmetric configuration (NiMn(PO 4 ) 2 /EEG//AC), the hybrid supercapacitor achieved an energy density of 40.0 Wh kg −1 , and a maximum power density of 6538 W kg −1 , while retaining 80% of its initial capacitance after 15,000 charge-discharge cycles at 5 A g −1 . These results demonstrate the synergistic role of EEG in promoting rapid redox kinetics and efficient electron transport, alongside wastewater sludge-derived AC as a viable route for sustainable waste valorisation in high-performance energy storage devices. • NiMn(PO 4 ) 2 /EEG nanocomposite synthesised via a simple hydrothermal method. • Wastewater sludge is valorized into activated carbon electrodes. • Hybrid supercapacitor achieved 40.0 Wh kg −1 energy and 6538 W kg −1 power density. • Device retained 80% capacitance after 15 000 charge-discharge cycles. • Synergistic redox and EDLC behaviour enables sustainable, high-performance energy storage.