Biochar-based carbons with hierarchical micro-meso-macro porosity for high rate and long cycle life supercapacitors

The development of supercapacitors with high energy density and power density is an important research topic despite many challenging issues exist. In this work, porous carbon material was prepared from corn straw biochar and used as the active electrode material for electric double-layer capacitors (EDLCs). During the KOH activation process, the ratio of KOH/biochar significantly affects the microstructure of the resultant carbon, which further influences the capacitive performance. The optimized carbon material possesses typical hierarchical porosity composed of multi-leveled pores with high surface area and pore volume up to 2790.4 m(2) g(-1) and 2.04 cm(3) g(-1), respectively. Such hierarchical micro-meso-macro porosity significantly improved the rate performance of the biochar-based carbons. The achieved maximum specific capacitance was 327 F g(-1) and maintained a high value of 205 F g(-1) at a ultrahigh current density of 100 A g(-1). Meanwhile, the prepared EDLCs present excellent cycle stability in alkaline electrolytes for 120 000 cycles at 5 A g(-1). Moreover, the biochar-based carbon could work at a high voltage of 1.6 V in neutral Na2SO4, and exhibit a high specific capacitance of 227 F g(-1), thus giving an outstanding energy density of 20.2 Wh kg(-1).