Sustainable hard carbon from Chlorella sp. for high-performance supercapacitors and sodium-ion batteries

This work highlights the promise of hard carbon derived from Chlorella sp. as an environmentally sustainable electrode material for next-generation energy storage systems. Thermogravimetric analysis revealed a three-stage thermal decomposition process, corresponding to the breakdown of proteins, carbohydrates, and lipids. The calculated activation energies using Model-free kinetic methods yielded in the range 71.46–135.04 kJ/mol, reflecting complex degradation mechanisms. Evolved gas analysis identified the release of light volatiles, hydrocarbons, nitrogen–sulfur species, and aromatics during pyrolysis. The resulting hard carbon (HC) and its chemically activated form (AHC) were characterised by SEM, XRD, Raman spectroscopy, and BET surface area analysis. AHC exhibited a porous microstructure, high surface area (231 m²/g), and increased structural disorder. Electrochemical tests confirmed that AHC outperformed HC, achieving a specific capacitance of 232.5 F/g (0.5 A g⁻¹) in supercapacitors and a reversible capacity of 336 mAh g⁻¹ in sodium-ion batteries. These enhancements are attributed to the optimized porosity, high surface area, and disordered carbon structure, which collectively facilitate rapid ion transport and efficient charge storage. This work highlights microalgae-derived hard carbon as a viable, eco-friendly alternative for high-performance electrochemical energy storage devices.