Eco-Friendly and Sustainable Production of Bifunctional Carbon Quantum Dots from Pine Cones: Utilizing Dual Fractions and Exploring Polarity-Dependent Electrochemical Properties for Energy Storage Applications

Carbon quantum dots (CQDs) have gained recognition as promising nanomaterials for energy storage applications, owing to their distinctive attributes including high surface area, tunable surface chemistry, superior electrical conductivity, and cost-effective production from renewable sources. Unlike traditional carbon electrodes which typically operate at only one polarity, an ideal energy storage material should exhibit electrochemical activity across both positive and negative voltage windows to enable high-energy and high-power asymmetric supercapacitors. However, such dual-range functionality remains uncommon in carbonaceous materials. Here, we report a green, one-step hydrothermal synthesis of fluorescent CQDs from Pine cones—an abundant forest waste—and demonstrate, a dual-range electrochemical behavior that, to the best of our knowledge, has rarely been reported for biomass-derived carbons. The CQDs deliver a remarkable specific capacitance of 237 F/g in the positive window (0 to +0.6 V) through mixed pseudocapacitive-EDLC contributions, while simultaneously exhibiting predominantly EDLC behavior with faster ion transport ( apparent D ≈ 10⁻⁷ cm²/s) in the negative window (–0.6 to 0 V). This unexpected polarity-dependent switching of the charge-storage mechanism is attributed to the core–shell architecture of the CQDs: an oxygen-rich shell drives pseudocapacitance under oxidizing conditions, whereas the graphitic sp² core ensures rapid EDLC under reducing potentials. This intrinsic bifunctionality—uncommon in biomass-derived carbons—offers a promising route for designing single-electrode materials capable that operates across the entire voltage spectrum for advanced asymmetric supercapacitors.