Waste-derived porous carbon/hematite composites as efficient supercapacitor electrode material

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According to the circular economy concept, including emerging materials, carbon/hematite composites are recovered entirely from wastes and investigated in view of sustainable energy storage. The pyrolysis and chemical activation of peanut shell waste yield a highly microporous carbon structure (PEAC) with a high specific surface area of 3041 m 2 /g, while acid-leaching, precipitation, and hydrothermal treatment of red mud waste allows to obtain hematite nanoflakes (IO). The materials and their composites with varying mass ratios were tested as supercapacitor electrodes in a three-electrode setup. Most of the composites demonstrated an improved performance compared to the individual components, exploiting the synergistic charge storage mechanism of electric double-layer capacitors and redox-active pseudocapacitors. The optimum composite containing 10 % hematite (PEAC/IO-10) exhibits a notable specific capacitance of 337 F/g at 10 mV/s and 269 F/g at 2 A/g on Ni foam. On a glassy carbon substrate, it demonstrates superior performance with capacitance up to 567.9 F/g at 10 mV/s and 284.3 F/g at 2 A/g in 1 M KOH electrolyte, outperforming 1 M Na 2 SO 4 and 1 M H 2 SO 4 . A significant contribution from both diffusive and capacitive charge storage was confirmed by Dunn's analysis. The symmetrical coin cell device test reveals a specific capacitance of 152 F/g at 1 A/g with an energy and power density of 7.6 Wh/kg and 720 W/kg, demonstrating 106 % retention of initial capacitance after 1000 cycles. The composite is competitive or superior to the best-known carbon/hematite systems obtained mostly from commercial precursors, while giving an advantage of waste management for a sustainable future. • High surface area, porous carbon (PEAC) is prepared from peanut shell waste • 2D α-Fe 2 O 3 (IO) flakes are obtained from alumina industry red mud waste • PEAC/IO composites with varying mass ratios studied as supercapacitor electrodes • 10 % α-Fe 2 O 3 composite with stable, specific capacitance up to 568 F/g in 1 M KOH • High capacitance with 106 % of retention after 1000 cycles in symmetrical cell

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