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

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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.