Harnessing sustainable N-doped activated carbon from walnut shells for advanced all-solid-state supercapacitors and targeted Rhodamine B dye adsorption

Abstract

This research introduces a novel approach to repurposing walnut shells, an abundant agricultural waste, to synthesize sustainable nitrogen-doped activated carbon (N@AC). The resulting material exhibits remarkable properties suitable for dual applications in high-performance all-solid-state supercapacitors and efficient Rhodamine B dye (RhB) adsorption. In a three-electrode setup, the N@AC electrode exhibits an impressive specific capacitance of 484.6 Fg⁻¹ at 1 Ag⁻¹ and remarkable long-term stability, maintaining 97.4% of its initial performance even after 5000 charge-discharge cycles. Simultaneously, the all-solid-state symmetric supercapacitor configuration (N@AC//N@AC) demonstrates outstanding specific capacitance, registering at 168.8 Fg⁻¹ at 1 Ag⁻¹, accompanied by a favourable rate capability of 67.3% at 10 Ag⁻¹. Notably, the N@AC//N@AC configuration attains a high energy density of 39.8 WhKg⁻¹ at 1 Ag⁻¹. Furthermore, N@AC//N@AC exhibits favourable cyclic stability, retaining 83.91% of its initial capacitance even after 10,000 charge-discharge cycles. Moreover, the adsorption efficiency of N@AC toward RhB is scrutinized, highlighting its efficacy in addressing environmental remediation challenges. The porous architecture and nitrogen functionalities of N@AC play a crucial role in expeditiously eliminating organic pollutants from aqueous solutions, offering a sustainable approach to treating wastewater. Optimal conditions for the highest RhB adsorption are identified: pH 7.2, a contact duration of 180 min, and an initial dye concentration of 20 mgL⁻¹. Thermodynamic evaluations, encompassing the determination of ΔH^◦, ΔH^◦, and ΔS^◦, signify the endothermic and spontaneous nature of the adsorption process. In desorption investigations, it is noted that H₂O, employed as an eluting agent, proficiently releases 87.35% of the adsorbed RhB dye.