Ionic Conductivity of Pine Needle Biochar-Derived Carbon Quantum Dots (CQDs) in Aqueous Medium for Sustainable Energy Applications

Carbon quantum dots (CQDs) have recently gained attention for applications in electrochemical and energy storage systems as a green nanomaterial. Due to their unique surface chemistry, charge transport properties, and low cytotoxicity, CQDs are suitable for use in solar cells, photocatalysis, and drug delivery systems. In this study, CQDs were derived from Pine needle biochar at different carbonization temperatures (300 ℃ (CQD300), 500 ℃ (CQD500), and 700 ℃ (CQD700)) and investigated for their effect on ionic conductivity. The ionic conductivity of CQDs dispersed in aqueous medium was evaluated using a two-electrode configuration with stainless steel blocking electrodes. The sonicated CQD dispersion was placed between the electrodes, and electrochemical impedance spectroscopy (EIS) was carried out using an Autolab PGSTAT 302N in the frequency range of 0.1 Hz to 1 MHz at room temperature (26 ℃). The bulk resistance (R_b) obtained from the high-frequency intercept of the Nyquist plot was used to calculate the ionic conductivity. The CQD300 sample exhibited the highest ionic conductivity (9.98 × 10⁻⁴ S cm⁻¹), followed by CQD500 (7.59 × 10⁻⁴ S cm⁻¹) and CQD700 (5.00 × 10⁻⁴ S cm⁻¹). According to the results, the pyrolysis temperature variations of the precursor biochar can significantly influence ion transport within the CQD matrix. The CQDs derived from lower-pyrolysis temperature biochar possess a higher density of oxygen-containing surface functional groups, which leads to ion transport and enhances ionic conductivity. In contrast, the CQDs derived from higher-pyrolysis temperature biochar have graphitic structures and reduce the availability of polar functional groups, leading to increased resistance and lower ionic conductivity. These findings demonstrate the potential of biochar-derived CQDs as sustainable nanomaterials for electrochemical and energy-related applications, such as electrolyte additives, ionic conductive fillers, and supercapacitor electrode modifiers. This will also contribute to the development of environmentally friendly energy technologies.