The rapid growth of global energy demand and the expanded adoption of intermittent renewable sources like solar and wind have intensified the need for safe, efficient, and low-cost energy storage devices. Zinc-ion hybrid capacitors have emerged as a promising technology because they combine the high power density and long lifespan of traditional supercapacitors with the high energy density of zinc-ion batteries. However, the practical performance of these hybrid energy devices depends heavily on the structural and chemical design of their carbon cathodes. Biomass resources offer a sustainable alternative to fossil-based carbon precursors, but raw biomass carbons often suffer from limited internal pore accessibility, low electrical conductivity, and non-uniform surface chemistry. To address these structural bottlenecks, recent research published in iScience by Mei Wang and colleagues at Changzhou University reports a collaborative synthesis strategy to convert Weigela japonica waste into high-performance porous carbon.

The synthesis approach combines hydrothermal pretreatment, potassium hydroxide chemical activation, and controlled nitrogen doping using melamine. Hydrothermal treatment at 180 degrees Celsius breaks down hemicellulose and disrupts the compact plant structure, allowing potassium hydroxide to penetrate deep into the carbon framework during high-temperature activation. Subsequent thermal activation and melamine treatment generate a three-dimensional hierarchical network rich in micropores, mesopores, and macropores. The resulting material, designated as NWJPC-2, achieves a high specific surface area of 2,916 square meters per gram. Micropores within the carbon matrix maximize the available surface area for ion adsorption, while larger mesopores and macropores serve as low-resistance channels for rapid electrolyte transport.

Chemical characterization confirmed that nitrogen and oxygen heteroatoms were successfully incorporated throughout the carbon framework. X-ray photoelectron spectroscopy revealed high concentrations of pyridinic, pyrrolic, and graphitic nitrogen, alongside oxygen functional groups. Pyridinic and pyrrolic nitrogen species act as active chemisorption and redox sites that lower the binding energy for zinc ions and introduce pseudocapacitance. Meanwhile, graphitic nitrogen enhances overall electrical conductivity, and surface oxygen species improve electrolyte wettability at the electrode interface. In a three-electrode testing setup, the NWJPC-2 electrode achieved a high specific capacitance of 318.8 Farads per gram at a current density of 1 Ampere per gram.

When assembled into a symmetric supercapacitor using an aqueous potassium hydroxide electrolyte, the device operated stably across a 0 to 1 Volt window. It delivered an energy density of 24.7 Watt-hours per kilogram at a power density of 245.0 Watts per kilogram, maintaining 99.8% of its initial capacitance with nearly 100% Coulombic efficiency after 10,000 continuous cycles. Kinetic analysis confirmed that the charge storage mechanism was dominated by fast surface-controlled processes rather than slow internal diffusion, enabling rapid charge and discharge rates.

To demonstrate its practical viability in advanced energy storage systems, NWJPC-2 was evaluated as a cathode material in an aqueous zinc-ion hybrid capacitor paired with a metallic zinc anode and zinc sulfate electrolyte. Operating within a voltage window of 0 to 1.8 Volts, the zinc-ion hybrid capacitor achieved a specific capacity of 109.5 milliamp-hours per gram at 0.5 Amperes per gram. The device delivered a peak energy density of 44.7 Watt-hours per kilogram at a power density of 225.0 Watts per kilogram. Even when pushed to a high power density of 8,727.0 Watts per kilogram, it maintained an energy density of 19.6 Watt-hours per kilogram. Furthermore, the hybrid device demonstrated extraordinary long-term durability, retaining 99.9% of its initial capacity over 10,000 charge-discharge cycles at 5 Amperes per gram. These results highlight the conversion of forestry and grass waste into functional porous carbon materials as an effective pathway toward sustainable, high-performance electrochemical energy storage.

Source: Wang, M., Lei, T., Li, Y., Liu, S., Tang, J., Liu, Y., & Yang, Y. (2026). Research on N-doped modified Weigela japonica-based porous carbon and its zinc ion hybrid capacitors energy storage. iScience, 29(9), 117248.