Green-Synthesized Activated Carbon and CNT-Wrapped NiFePO₄ Nanocomposite for Advanced Energy Storage and Hydrogen Evolution Applications

Green-synthesized nanomaterials have attracted considerable attention owing to their potential to reduce environmental impact while offering high performance in energy-related applications. In this study, nickel–iron phosphate (NiFePO₄) was synthesized via a hydrothermal method and combined with carbon nanotubes (CNTs) to exploit the synergy between the high theoretical capacity of NiFePO₄ and the excellent conductivity of CNTs. Activated carbon (AC) was prepared through a green synthesis route using biomass (date seeds) and employed as the negative electrode. The optimized NiFePO₄@CNT@25% composite exhibited a high specific capacity (Q_S) of 744.77 C g⁻¹ with 95% retention, while the asymmetric NiFePO₄@CNT@25%//AC device operated within a 1.6 V window and achieved 258.56 C g⁻¹. The supercapattery device demonstrated a superior energy density of 50 Wh kg⁻¹ at a power density of 2837 W kg⁻¹, and retained 90% capacity with 95% coulombic efficiency after 10,000 cycles, underscoring its long-term durability. Beyond energy storage, the composite also showed promising catalytic activity toward the hydrogen evolution reaction (HER), requiring only 43.93 mV overpotential to achieve 10 mA cm⁻² with a Tafel slope of 60.41 mV dec⁻¹, indicative of efficient reaction kinetics. These findings establish NiFePO₄@CNT-25% as a versatile material with dual functionality for next-generation sustainable energy storage and conversion systems.