Nickel-manganese phosphate/electrochemical exfoliated graphene nanocomposites for hybrid supercapacitors

Abstract

The global shift toward sustainable energy technologies has increased the demand for advanced energy storage systems that deliver both high energy and power densities. Conventional supercapacitors provide excellent power output, rapid charge-discharge capability, and superior cycling stability; however, their low energy density restricts practical applications. In contrast, rechargeable batteries offer higher energy densities but are limited by slower charge rates and reduced cycling stability. Hybrid configurations that integrate the complementary advantages of both systems have therefore emerged as a promising approach to achieving balanced performance. Nickel-manganese phosphate/electrochemically exfoliated graphene NiMn(PO4)2/EEG) nanocomposite was synthesized via a hydrothermal method. Structural and morphological analyses, including X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), high-resolution scanning electron microscopy (HRSEM), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy, confirmed the formation of well well-crystallised composite with homogeneous morphology. The NiMn(PO4)2/EEG composite leverages the multiple redox-active sites and structural robustness of bimetallic phosphate combined with the high conductivity of electrochemically exfoliated graphene, resulting in enhanced charge transport efficiency and electrochemical performance. Additionally, this work presents an investigation of heteroatom-doped EEG thin film synthesised via a two-step process involving graphite intercalation and electrochemically exfoliation in sulphuric-phosphoric acid medium, enabling in-situ doping with nitrogen (N), phosphorus (P), and sulphur (S). The exfoliation products were vacuum filtered to form porous films. Raman spectroscopy revealed Fermi-level shifts of approximately 0.5 eV and heterogeneous defect distributions. The doped EEG films exhibited enhanced electrical conductivity (~10,000 S·m-1) and improved interfacial properties, as evidenced by reduced adhesion forces in force-distance measurements. Electrochemical analyses demonstrated that Fermi-level modulation facilitated rapid interfacial charge transfer by lowering the electrode-electrolyte potential barrier. The doped EEG films achieved a specific capacitance of 150.5 F·g-1 at 1.0 A·g-1, confirming their potential as highly conductive supports for hybrid electrode configurations. Furthermore, a NiMn(PO4)2/EEG composite was integrated with activated carbon (AC) derived from wastewater sludge to fabricate energy storage devices. The NiMn(PO4)2/EEG composite was synthesised hydrothermally, while AC was produced via phosphoric acid activation. Characterisation verified the formation of a mixed-metal phosphate phase anchored on few-layer graphene. The optimised NiMn(PO4)2/50 mg EEG electrode achieved a high specific capacity of 822.1 C·g-1 at 1 A·g-1, significantly outperforming pristine NiMn(PO4)2. In an asymmetric configuration (NiMn(PO4)2/EEG//AC), the device delivered an energy density of 40.0 Wh·kg-1, a peak power density of 6538 W·kg-1, and retained 87% of its capacitance after 5000 cycles at 5 A·g-1. These results underscore the synergistic contribution of EEG towards improved electrical conductivity and redox kinetics, while demonstrating the potential of wastewater sludge-derived AC for sustainable electrode development. Overall, this research integrates synthesis, characterisation, and electrochemical evaluation to elucidate the role of heteroatom-doped EEG in enhancing the electrochemical behaviour of NiMn(PO4)2-based electrodes. By uniquely combining NiMn(PO4)2, heteroatom-doped graphene, and waste-derived activated carbon, this study presents an innovative, sustainability-oriented approach to device design. These findings contribute to the advancement of scalable, sustainable energy storage technologies.