Activating biomass carbon with metallurgical slag by pyrolysis in molten salt for high-performance supercapacitors

Pyrolysis of sustainable biomass to advanced carbon materials for energy storage is key-enabling in energy and environmental sustainability. However, obtaining carbon materials with well-defined microstructure and composition for high-performance energy storage is extremely challenging. Herein, efficient activation of biomass carbon is realized by introducing extra metallurgical slag during pyrolysis of coconut shell in Na₂CO₃-K₂CO₃ molten salt. The molten salt guides the formation of carbon with a hierarchical honeycomb-like nanostructure, while the metallurgical slag facilitates enhanced doping of the heteroatom species, conjointly contributing to the increase of the specific surface area of carbon materials from 424 m² g^-1 to 1451 m² g^-1 and the extension of the single N dopant to multiple dopants of N, P, Zn and Co. Such adequate tuning of the microstructure and composition in the pyrolysis product increases the capacitance for supercapacitors from 30 F g^-1 to 135 F g^-1 at 0.5 A g^-1. The results can provide new insights for the controllable upgradation of both biomass and waste industrial slag toward enhanced energy storage.