Nanoarchitectonics of hierarchical porous biochars from heavy bio-oil by coupling calcium salts assisted template carbonization and K2CO3 activation for high-performance supercapacitors

Biochar derived from biomass has shown potential as electrode materials for supercapacitors, but controlling its pore structure is a challenge. Additionally, coking issues have limited the industrial application of heavy bio-oil, which could be used to prepare carbonous biochar as a substitute for biomass. Herein, to widen the utilization of bio-oil and to control the pore structure, this study introduces a novel approach to utilize heavy bio-oil and regulate pore structure by combining the hard template method and activation method, in which calcium citrate tetrahydrate (CCT), calcium acetate and calcium carbonate act as templates and K₂CO₃ acts as activator. Through CCT-assisted template carbonization at 700 °C and K₂CO₃ activation with a mass ratio of 1:2, a hierarchical porous biochar (CCT-700–1:2) with a specific surface area of 2213.09 m² g⁻¹ was successfully synthesized. The CCT-700–1:2 electrode shows outstanding capacitive performance with a specific capacitance of 213.86 F g⁻¹ at 0.5 A. The assembled solid-state symmetric supercapacitor displays impressive rate capability, maintaining the capacity retention ratio being 69.7 % at 20 A g⁻¹. Furthermore, it demonstrates exceptional cycle stability and the capacitance retention is around 98 % even after 60,000 cycles. The symmetric supercapacitors also show the high energy density being 16.36 W h kg⁻¹ at 250 W kg⁻¹. This study presents a hopeful method for producing sustainable carbon materials by bio-oil used as energy storage devices, enhancing the potential utilizations of biomass derived products in the area of supercapacitors.