Enhanced performance of nickel-doped porous carbon derived from soybean hulls for supercapacitor applications

Abstract

Bio-based activated carbon and transition metal oxides, as electrode materials, exhibit excellent electrochemical performance and have broad prospects for application in supercapacitor devices. Soybean husks are a renewable, inexpensive, and widely utilized biomass waste, making them an ideal choice for biomass-activated carbon. Nickel can undergo reversible multivalent state transformation (such as Ni²⁺/Ni³⁺/Ni⁴⁺) during redox reactions and has high electrochemical activity. In this study, Ni(NO₃)₂·6H₂O was used as the nickel source and loaded onto soybean hulls (SBH) activated by K₂CO₃ to prepare SBH@Ni-x. The composite material achieved a maximum specific capacitance of 513.9 F g⁻¹ at 1 A g⁻¹ during the three-electrode test with 6 M KOH. When forming symmetrical SCs with SBH@Ni-0.05//SBH@Ni-0.05, the energy density reaches 28.35 Wh kg⁻¹ in a 1 M Na₂SO₄ electrolyte, whereas in 6 M KOH electrolyte it remains at 14.24 Wh kg⁻¹ (1 A g⁻¹). Additionally, after 10 000 cycles, it retains 83.3% of its original capacitance and has a coulombic efficiency of nearly 100%. These results indicate that nickel loaded on biomass-activated carbon is a promising continuous, efficient, and eco-friendly material for supercapacitor devices.