Tunable energy storage in acidic and alkaline electrolytes using a NiO-embedded N,P-Doped biomass-derived electrode

Abstract

In this study, a novel biomass-derived composite electrode material was developed from walnut shell waste for high-performance supercapacitor applications. The raw walnut shells were chemically activated using potassium hydroxide to obtain activated carbon (WSAC), followed by phosphorus (P) doping via hydrothermal treatment with phosphoric acid, resulting in P-doped WSAC. To further enhance the electrochemical performance, nickel oxide (NiO) nanoparticles and nitrogen (N) species were incorporated through a secondary hydrothermal process using urea, yielding NiO@N,P-doped WSAC. The composite was systematically investigated as an electrode material in both symmetric and asymmetric supercapacitor configurations using acidic (H2SO4) and alkaline (NaOH) electrolytes. Cyclic voltammetry (CV) analysis revealed electric double-layer capacitive behavior in acidic media and a pronounced redox response under alkaline conditions, indicating pseudocapacitive contributions. The device demonstrated superior performance in alkaline electrolyte, achieving a maximum power density of 2380 W/kg and an energy density of 6.02 Wh/kg, compared to 840 W/kg and 2.15 Wh/kg in acidic conditions. Furthermore, the electrodes retained 91.15 % and 83.10 % of their initial capacitance after 5000 charge–discharge cycles in acidic and alkaline environments, respectively. This study highlights the potential of engineered biomass-derived hybrids as sustainable and efficient materials for advanced energy storage systems.