Facile synthesis of nitrogen-doped carbon nanosheets with hierarchical porosity for high performance supercapacitors and lithium-sulfur batteries

Magnesium citrate and potassium citrate are two commonly used food additives in our daily life. Herein, we prepared nitrogen-doped hierarchical porous carbon nanosheets (N-HPCNSs) through direct pyrolysis of their mixtures and subsequent NH₃ treatment. The as-prepared N-HPCNS shows hierarchical porosity (specific surface area of 1735 m² g^-1 and pore volume of 1.71 cm³ g^-1), and a moderate nitrogen doping of 1.7%. Moreover, it can be effectively applied in various energy storage/conversion systems. When used as supercapacitor electrodes, it shows a high specific capacitance of 128 F g^-1 in organic electrolytes and retains 45% of the original capacitance even at an ultrahigh current density of 100 A g^-1. It can also serve as an effective sulfur carrier in lithium-sulfur batteries. The N-HPCNS/sulfur cathode shows high discharge capacities of 1209 mA h g^-1 at 0.2C and 493 mA h g^-1 even at 4C. Over 500 charge/discharge cycles at 1C, it still retains a high discharge capacity of 486 mA h g^-1 with an ultralow capacity loss of 0.051% per cycle and a high average coulombic efficiency of 99.4%.