Effect of Pyrolysis Temperature on Acidic Oxygen-Containing Functional Groups and Electron Storage Capacities of Pyrolyzed Hydrochars

Hydrothermal carbonization (HTC) is a thermochemical process, where biomass is treated with subcritical water. Hydrochar, the solid product of HTC, is a carbon-rich material containing acidic functional groups. In order to increase total surface area, one of the common practices is to pyrolyze it. However, dehydration occurs during pyrolysis, which may affect the acidic functional groups on hydrochar. In addition, biochars have recently been shown to possess significant electron storage capacities (ESC), but it was unknown whether pyrolyzed hydrochars also possess ESC and to what extent. In this article, the effect of pyrolysis temperature on acidic oxygen-containing functional groups and ESC of pyrolyzed hydrochars is evaluated. Hydrochars were prepared from cellulose and wood at 220 and 260 °C. These hydrochars were then pyrolyzed at 400, 500, and 600 °C under an N2 atmosphere. Afterward, the changes in functional groups were evaluated by BET analysis, ultimate analysis, ESC measurement, pH, pH at point of zero charge (pHPZC), Boehm titration, and FTIR analysis. The hydrochars showed relatively low surface areas mostly due to the lack of pores or pores being clogged with volatiles. The surface area was increased by an order of magnitude after pyrolysis; however, acidic oxygen-containing functional groups decreased significantly with increasing pyrolysis temperature. ESC was also decreased with increasing pyrolysis temperature, ranging from 1.44 (cellulose at 600 °C) to 3.25 (wood at 400 °C) mmol/g. This result suggests that a portion of the ESC of the pyrolyzed wood hydrochars originated from cellulose. A linear correlation between ESC and lactonic group was observed for pyrolyzed hydrochars.