Keratin-derived functional carbon with superior charge storage and transport for high-performance supercapacitors

This work reports a scalable method to synthesize hierarchically porous, hetero-atom doped activated carbon nanosheet from waste biomass−human hair, and demonstrates the use of this carbon as an ultra-high performance electrode material for supercapacitor applications. Microscopic analyses reveal that sheet size ranges from 50 to 200 nm having a thickness of 15–27 nm. As-synthesized, carbon nanosheets possess a hierarchical porous structure having a specific surface area of 1548 m² g⁻¹. Heteroatom concentration (nitrogen, oxygen, and sulfur) of around 25% is confirmed from XPS analysis. Therefore, the novel interconnected hierarchical porous nanosheets structure enables fast adsorption and transportation of ions during electrochemical processes. Also, the abundant chemically available electroactive heteroatom species in the material enhance the wettability of ions and contribute to pseudocapacitance. The electrochemical analyses through cyclic voltammetry and galvanostatic charge-discharge measurements in 6 M KOH reveal the quasi-EDLC behavior of the activated carbons due to the presence of heteroatoms. A reprensentative KOH activated carbon shows an excellent specific capacitance value of 999 F g⁻¹ at a current density of 1 A g^-1. The symmetrically assembled two-electrode device also delivers a maximum energy density of 32 W h Kg^-1 at a power density of 325 W Kg^-1. In addition, excellent cyclic stability of 98% capacitance retention is observed after 10,000 continuous GCD cycles even at a high current density of 5 A g⁻¹. Symmetric flexible supercapacitor device using KOH-PVA-K₃FeCN₆ as redox gel polymer electrolyte shows a synergistic specific capacitance of 145 mF cm⁻¹ at a current density of 0.8 mA cm⁻¹ exhibiting very less deviation in bending mode. Therefore, this waste biomass derived heteroatom doped hierarchical porous carbon nanosheets can be a promising material for cost-effective high- performance supercapacitor.