N, P-codoped porous carbon derived from chitosan with hierarchical N-enriched structure and ultra-high specific surface Area toward high-performance supercapacitors

•A facile and cost-effective strategy was explored to synthesize porous carbon.•Ultra-high specific surface area and high N-doping level were achieved.•The carbon material shows an ultrahigh capacitance.•Synergic effects between N and P atoms provide more active sites and wettability.•The assembled symmetric supercapacitor shows extremely high energy density.