Phosphorus-doped carbon/carbon nanotube hybrids as high-performance electrodes for supercapacitors

Abstract

P-doped carbon hybrids were prepared by hydrothermal carbonization of glucose in the presence of CNT followed by chemical activation with phosphoric acid. The role played by phosphoric acid on the final textural and chemical properties of the samples was thoroughly investigated by varying the temperature of activation. Temperatures higher than 700°C were needed to chemically activate the samples, while 800°C is the optimum temperature to generate the largest amount of micropores. In addition to oxygen functional groups such as carboxylic acids, anhydrides, lactones, phenols and carbonyl quinones, several phosphorus functional groups are also detected. At 700°C, only COPO3 and CPO3 groups are observed; meanwhile, C3P=O and C3P groups appear at 800 °C and 900 °C, respectively. The oxygen moieties bonded to carbon and the phosphates (COPO3) contribute significantly to the pseudocapacitance; the oxygen groups linked to phosphorus (CPO3 and C3P=O) enhance the electrostatic charge and the non-oxygen functional group (C3P) increases the capacitance retention. The P-doped hybrid treated at 800 °C resulted in a porous carbon material with 806 m2 g−1 active surface area and high phosphorus content (6.6%) that increases the total capacitance from 26 (non-doped hybrid treated at 700°C) to 124 F g−1 at 1 A g−1. Besides, its high content of phosphorus increases the rate capability (115 F g−1 at 10 A g−1), resulting in an electrode material with higher performance at large current density than that of commercial activated carbon Supra50.