Synthesis of N-doped carbon material via hydrothermal carbonization: Effects of reaction solvent and nitrogen source

This work examined the effects of reaction solvent (water/ethanol) and nitrogen source (pyridine/pyrrole) on synthesising N-doped carbon material via a hydrothermal process. The presence of water favored the carbonization of cellulose and doped the nitrogen of pyridine in the form of pyridinic-N and graphitic-N via Maillard reaction, while almost no carbonization and nitrogen doping were observed either in pure pyridine or in pyridine and ethanol mixture. Increasing temperature and the water/pyridine ratio enhanced the carbonization of cellulose and the Maillard reaction between cellulose and pyridine. The N-doped carbon (NC1) derived from the activation of the hydrochar produced at 240 °C for 1 h with a pyridine:water ratio of (5 mL:35 mL) exhibited a significant specific surface area of 544.91 m²/g and a specific capacitance of 214.1 F/g (at 1 A/g) under the three-electrode system. With employing the aqueous phase derived from the above reaction, the derived N-doped carbon (NC2) exhibited a specific surface area of 534.29 m²/g and a specific capacitance of 189.7 F/g. The time constant of NC1 (0.5 s) was higher than that of NC2 (0.28 s), suggesting NC2 operates more quickly in the process of reversible charge-discharge. Over 10,000 cycles at 5 A/g, NC1 maintained 97.82 % of the capacitance, while for NC2, the capacitance retention reached 106 %. Exploring pyrrole in the place of pyridine provided a high yield and high N content of the hydrochars and NCs, but the electrochemical performances were significantly lowered. This work suggests a low-cost and sustainable method for the preparation of N-doped carbon materials for supercapacitors.