Highly functionalized carbonaceous materials were produced by hydrothermal carbonization of cellulose at 220-250°C. The process involves dehydration, similar to that observed in the hydrothermal transformation of saccharides. The resulting material consists of carbonaceous microspheres (2-5 μm) as seen by SEM. Chemical analysis using elemental analysis, infrared, Raman, and XPS spectroscopy indicates that the core contains stable oxygen groups (ether, quinone, pyrone), while the shell has reactive oxygen functionalities (hydroxyl, carbonyl, carboxylic, ester). The hydrothermal carbonization of cellulose was carried out by dispersing cellulose in water, heating in an autoclave, and filtering the resulting solid (hydrochar). The hydrochar was then washed and dried. The chemical and structural properties of the hydrochar were analyzed using various techniques, including SEM, XRD, FTIR, Raman, and XPS. The hydrochar samples showed a transition in morphology from irregular to microsphere aggregates at around 220°C. XRD analysis confirmed that the hydrochar is no longer crystalline at temperatures ≥220°C. FTIR and Raman spectroscopy revealed the presence of aromatic structures and oxygen functionalities in the hydrochar. XPS analysis showed that the core contains stable oxygen groups, while the shell contains more reactive oxygen functionalities. The hydrothermal carbonization of cellulose involves the hydrolysis of cellulose chains, dehydration and fragmentation of monomers, polymerization or condensation of soluble products, aromatization of polymers, and nucleation and growth of microspheres. The hydrochar is composed of micrometer-sized spheres with a high oxygen content in both the core and shell. The oxygen groups in the core are less reactive, while those in the shell are more reactive and hydrophilic. The hydrochar produced from cellulose has potential applications as a carbon source for electrocatalysts and as a material for the production of carbon nanocells. The process is effective in increasing the turnover time of carbon in biomass. The study highlights the potential of cellulose as a precursor for the production of highly functionalized carbonaceous materials via hydrothermal carbonization.Highly functionalized carbonaceous materials were produced by hydrothermal carbonization of cellulose at 220-250°C. The process involves dehydration, similar to that observed in the hydrothermal transformation of saccharides. The resulting material consists of carbonaceous microspheres (2-5 μm) as seen by SEM. Chemical analysis using elemental analysis, infrared, Raman, and XPS spectroscopy indicates that the core contains stable oxygen groups (ether, quinone, pyrone), while the shell has reactive oxygen functionalities (hydroxyl, carbonyl, carboxylic, ester). The hydrothermal carbonization of cellulose was carried out by dispersing cellulose in water, heating in an autoclave, and filtering the resulting solid (hydrochar). The hydrochar was then washed and dried. The chemical and structural properties of the hydrochar were analyzed using various techniques, including SEM, XRD, FTIR, Raman, and XPS. The hydrochar samples showed a transition in morphology from irregular to microsphere aggregates at around 220°C. XRD analysis confirmed that the hydrochar is no longer crystalline at temperatures ≥220°C. FTIR and Raman spectroscopy revealed the presence of aromatic structures and oxygen functionalities in the hydrochar. XPS analysis showed that the core contains stable oxygen groups, while the shell contains more reactive oxygen functionalities. The hydrothermal carbonization of cellulose involves the hydrolysis of cellulose chains, dehydration and fragmentation of monomers, polymerization or condensation of soluble products, aromatization of polymers, and nucleation and growth of microspheres. The hydrochar is composed of micrometer-sized spheres with a high oxygen content in both the core and shell. The oxygen groups in the core are less reactive, while those in the shell are more reactive and hydrophilic. The hydrochar produced from cellulose has potential applications as a carbon source for electrocatalysts and as a material for the production of carbon nanocells. The process is effective in increasing the turnover time of carbon in biomass. The study highlights the potential of cellulose as a precursor for the production of highly functionalized carbonaceous materials via hydrothermal carbonization.