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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.

Black carbon such as biochar has been shown to possess electron storage capacity (ESC), which enables black carbon to store and reversibly exchange electrons with its surroundings. However, the origin of black carbon ESC remains unknown to date. To answer this question, we measured the ESC of cellulose, lignin, hemicellulose, and biochars prepared from these biopolymers, their mixture, and a pinewood, through pyrolysis at 350−650 °C. Mediated electrochemical analysis (MEA) with ABTS and diquat as mediators and chemical redox titration (CRT) with O 2 and titanium(III) citrate were used to quantify ESC. MEA could measure lignin ESC but significantly underestimated biochar ESC; CRT could measure the total reversible ESC of biochars but not that of lignin. Lignin was the only biopolymer that possessed ESC, which was largely destroyed during pyrolysis at 350 °C. After pyrolysis at ≥450 °C, the three biopolymers, their mixture, and pinewood all yielded biochars that possessed a highly reversible ESC of 1−2 mmol e − /g. This indicates that pyrolysis created reversible ESC of biochar from lignocellulosic biomass, presumably by converting oxygen-containing moieties of the biopolymers into (hydro)quinones in biochar. The implications of our findings for biogeochemistry, climate, contaminant fate, and engineering applications are discussed.

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