Engineered B, N Co-Doped Biochar: Theoretical and Experimental Insights into Dual-Sided Dye Adsorption and High-Performance Supercapacitor Applications

The present work addresses two pressing global challenges: (i) Water purification and (ii) Eenergy storage by developing a multifunctional, sustainable material aligned with key Sustainable Development Goals. We report the synthesis of boron and nitrogen co-doped biochar (B-BC), obtained via a scalable, single-step carbonization (700-900 °C) of Bambusa Tulda bamboo using boric acid as a dopant precursor. Comprehensive physicochemical characterization like XRD, SEM, FTIR, XPS, and BET were performed to understand the biochar properties. Boron and nitrogen co-doping effectively modulate surface polarity, enhances electronic conductivity, and increases the surface area of the biochar matrix, enabling dual adsorption capabilities. The optimized B-BC800 sample exhibited exceptional adsorption capacities of 492.61 ± 39.37 mg/g for Crystal Violet (cationic dye) and 178.56 ± 17.46 mg/g for Congo Red (anionic dye). Meanwhile, the B-BC700 sample demonstrated a high specific capacitance of 296.0 ± 6.6 F/g at 0.5 A/g and excellent cycling stability (89% retention after 5000 cycles), underscoring its potential in energy storage. Density Functional Theory (DFT) calculations, including band structure, density of states (DOS), and Bader charge analyses indicates the mechanistic insights into how heteroatom doping tailors the electronic structure and surface charge distribution, supporting the observed adsorption and capacitive behavior. This study highlights the 2 effect of rational tuning of the doping levels and carbonization temperature to optimize the surface functionality and multifunctional performance, paving the way for nextgeneration materials in environmental and energy applications.