*2.1. Starting Samples: Grape Seeds and Hydrochars*

The starting substrate consists of grape seeds and their corresponding hydrochars produced and characterized in previous work by our group [32]. Grape seeds are a residue of the winemaking industry and were provided by an alcohol producer company in the North of Italy (province of Trento). Hydrochars were obtained at 180, 220, and 250 ◦C through a solar HTC reactor, at a residence time of two hours, and a dry biomass to water weight ratio (B/W) of 0.3 [32]. Table 1 summarizes the main properties of the samples.

**Table 1.** Grape seeds and hydrochar (HC) properties, from the literature [32] (C: carbon, H: hydrogen, O: oxygen, N: nitrogen, S: sulfur, HHV: higher heating value).


#### *2.2. Sample Characterization*

Grape seeds and hydrochars were characterized through thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and Fourier-transform infrared spectroscopy (FTIR).

TGA was performed in a thermobalance Mettler TG50 under non-isothermal conditions. 15 mg of whole dried samples (i.e., the entire raw and carbonized grape seeds) were placed into a 70 μL alumina crucible. Runs were performed between 40 and 700 ◦C at a heating rate of 1, 3, and 10 ◦C/min, under 100 mL/min of nitrogen or air (20.95% O2 and 78.08% N2). The thermobalance has a sensitivity of ±<sup>1</sup> × <sup>10</sup> <sup>−</sup><sup>3</sup> mg and 0.1 ◦C. The extent of conversion α at any time t was computed as:

$$\alpha(\mathbf{t}) = \frac{\mathbf{m}\_0 - \mathbf{m}(\mathbf{t})}{\mathbf{m}\_0 - \mathbf{m}\_\mathbf{f}} = \frac{\mathbf{v}(\mathbf{t})}{\mathbf{v}\_\infty} \tag{1}$$

where m0, m(t), and mf are the mass at time zero (dry), at a given time t, and final time, respectively. This corresponds to the ratio between v(t), which represents the amount of volatiles evolved up to a certain time t, and v∞, which represents the amount of volatiles that evolved up to the end of the process.

DSC was performed in a Mettler DSC20 calorimeter between 40 and 600 ◦C, at a heating rate of 10 ◦C/min, under nitrogen or air at a flow rate of 100 mL/min. About 15 mg of material was placed into a 40 μL aluminum crucible. The heat released/absorbed was computed by integrating the measured heat flow over time in the 40–600 ◦C range, while the sample mass variation was recorded after every run.

FTIR spectra of samples were acquired using PerkinElmer FT-IR spectrometer Spectrum One. Spectra were obtained over a wavenumber range of 4000–650 cm−1. Dried samples were ground to be compacted into a thin film and a flat top-plate Zinc Selenide crystal was used for the detection.
