2.1.3. Experimental Procedure

The HTC experiments were performed at 200 ◦C, with a constant water biomass ratio 7:1, and residence time equal to 0 and 120 min. The substrate was ground in a mortar and sieved. The fraction below 1 mm was recovered and used for the reaction after drying until constant weight in an oven, at 105 ◦C. Demineralized water and dry biomass were loaded into the reactor in the chosen weight ratio. Then, the reactor was sealed, evacuated for about 15 min, and heated up to the process temperature. The warm-up lasted for about 20–25 min. Residence time was measured once it reached the temperature set point. At the end of the run, the reactor was quenched with air until 160 ◦C, and then with water until room temperature.

After the quenching step, the gas phase was recovered and weighed. Solid and liquid products were recovered and separated by filtration. The filter and solid phases were put in an oven at 105 ◦C for at least 24 h for dry weight determination. After drying, the hydrochar was milled and sieved (certified ISO 3310–1 & 2 and ASTM E11, Endecott sieves). The fraction between 106 and 355 μm was recovered and stored in vials before the CHNS characterization (data reported in Supplementary Material) and the activation step. Then, 15 grams of 1:2 solid mixture of hydrochar and KOH was put in an oven under nitrogen gas flow and warmed up to 600 ◦C (3 ◦C/min). The dwell step lasted 1h[40]. The samples were washed with 10 wt% HCl to remove any inorganic salts, and then with demineralized water until neutral pH. Finally, the samples were dried in an oven at 105 ◦C for 24 h.

## 2.1.4. Characterization Methods

The nitrogen sorption isotherms of activated and non-activated hydrochars were determined with a high-speed surface area and pore size analyzer NOVA 1200e Alfates Quartachrome. Brunauer-Emmett-Teller (BET) [42] and Barrett-Joyner-Halenda (BJH) [43] methods were utilized for a standard determination of surface area, pore volume, and pore size distribution and the average pore diameter [44]. Each sample was outgassed for 3 h at 100 ◦C.

Textural properties were investigated by means of scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS). The analyses were performed with a Field Emission Zeiss Gemini500SEM analyser (acceleration voltage from 0.02 to 30 KV—store resolution up to 32 k × 24 k pixels—0.6 nm at 15 KV, in back-scattering electron mode—pressure from 5 to 500 Pa), equipped with EDS (energy dispersive spectroscopy) microanalyzer OXFORD Aztec Energy with INCA X-ACT PELTIER COOLED ADD detector. All samples were coated with 5 nm of Cr in an automatic sputter coater Q 150T ES.

#### *2.2. CO2 Adsorption Test*
