Nitrogen Surface Passivation of Biomass-Based Carbon Quantum Dots: A Pathway to Enhanced Colloidal Stability and Electrochemical Capacitance

Abstract

The conversion of organic biomass wastes into carbon quantum dots (CQDs) offers a sustainable pathway to reduce environmental pollution and valorize renewable resources. However, pristine biomass-derived CQDs typically exhibit low electrical conductivity, a wide band gap, and poor dispersion stability, limiting their practical applicability in energy storage systems. This study aims to address these challenges through nitrogen doping, which enhances charge transport, narrows the band gap, and improves colloidal stability. Two synthesis routes; direct carbonization and in-situ nitrogen modification were systematically compared to determine the most effective approach. Characterization using photoluminescence, UV–Vis spectroscopy, and zeta potential analysis confirmed that nitrogen incorporation effectively improved particle dispersion and surface passivation. The resulting nitrogen-doped CQDs (NCQDs) exhibited markedly higher conductivity of 1.7 mS cm⁻¹ and dispersion stability, leading to a substantial enhancement in the electrochemical performance when integrated into activated carbon (AC/NCQD) composite electrodes. The optimized in-situ modification route yielded the greatest improvement, achieving a specific capacitance increase exceeding 150 % relative to pristine AC. These findings highlight that the in-situ nitrogen modification route offers a promising strategy for enhancing the functionality of biomass-derived CQDs, paving the way for sustainable, high-performance materials in next-generation energy storage devices.