Evolution of pore structure in cypress-derived carbon during hydrothermal treatment and subsequent carbonization

Hydrothermal carbonization (HTC) is a promising method for converting wet biomass into carbon materials; however, the mechanisms underlying pore formation during HTC remain poorly understood. This study examined how reaction temperature affects pore development in cypress hydrochar. Hydrochars were prepared at 170–240 °C and characterized by nitrogen adsorption and thermogravimetric analysis. While samples prepared at 170 and 200 °C showed almost no porosity, the hydrochar produced at 240 °C exhibited a significant increase in mesopore volume despite a low surface area, indicating the formation of relatively large pores (~50 nm). Thermogravimetric analysis revealed extensive hydrolysis of hemicellulose and cellulose at 240 °C, yielding a thermally stable structure enriched in lignin-derived components. To further develop the pore structure, the hydrochar was carbonized at 900 °C. This carbonization greatly increased microporosity while partially maintaining larger pores, creating a hierarchical pore structure. In contrast, raw cypress char exhibited mainly microporosity, with little mesoporosity. These findings suggest that higher HTC temperatures play a key role in mesopore formation, likely through enhanced hydrolysis and heterogeneous solid formation, offering insights into the design of biomass-derived porous carbon materials.