A comparison of hydrothermal carbonization versus pyrolysis-activation for sludge-derived carbon materials on physiochemical properties and electrochemical performance

Abstract

Sludge-derived carbons were prepared by hydrothermal carbonization and pyrolysis-activation. In this work, the physiochemical properties and electrochemical performance of these carbons were compared in detail. The structure of pyrochar (PC) has been fully developed that layered porous skeleton was formed during activation process. Thus, obtained the pyrolysis-activated carbon (PAC) had excellent specific surface area (463.2 m2/g) and pore volume (0.36 m3/g), which were higher than these of hydrothermal carbonization-activated carbon (HCAC-300) (364.1 m2/g, 0.18 m3/g). Due to the high nitrogen content of sludge, there were abundant N functional groups (N-5, N-6, N-Q) on the surface of PAC and HCAC. The self-doping of nitrogen was beneficial for improving the carrier concentration and charge transport capacity of carbons, and forming the pseudocapacitance via the redox reaction. Owing to the remarkable physiochemical properties, PAC exhibited highest specific capacitance of 114.7 F/g at 1 A/g, which led to a large amount of charge storage and rapid diffusion of electrons/ions. However, hydrothermal carbonization performed a high economic advantage in disposing the high-humidity feedstock such as sludge. Therefore, the exploration on improving the pore structure in hydrothermal carbonization process thus obtaining high performance carbon material would be of significance and of prospects.