## 3 Results and Discussion
### Characterization of the Char-Based Materials
The activation with Na\({}_{2}\)CO\({}_{3}\) leads to a paramount decrease in O, i.e., from 20.8% (char) to 4.4% (AC), due to the reaction of Na\({}_{2}\)CO\({}_{3}\) with the oxygenated groups on the surface of the char and the associated dehydration reactions. The CA post-treatment in the CA post-treated AC samples raised the O content, probably due to the cross-linking effect of CA. The surface composition was analyzed by the XPS technique. The quantification results are shown in Table 3. Different trends regarding the surface carbon content (C\({}_{\text{XPS}}\)) were observed compared to the bulk percentages reported by elemental analysis. However, C\({}_{\text{XPS}}\) also decreased after activation with Na\({}_{2}\)CO\({}_{3}\), and the samples functionalized with CA also displayed a growth in C\({}_{\text{XPS}}\) content. The O\({}_{\text{XPS}}\) content should be taken under caution since the presence of oxygenated metals hinders the calculation of the specific O content linked to the carbonaceous matrix. Furthermore, the presence of some metals was registered, specifically Si and Ti, which are commonly dosed in plastic polymers as additives. Silica, silicon dioxide, and CaCO\({}_{3}\) are commonly added as reinforcing fillers while Ti is dosed as TiO\({}_{2}\) due to its whiteness and brightness to modify the color of the final polymeric product [59]. The surface contents of these inorganic components increase from the char to the char-CA and the AC-CA-X samples as they are concentrated in the solid. In addition, the nature of the chemical bonding of C and O was assessed by the deconvolution of C\({}_{\text{1s}}\) and O\({}_{\text{1s}}\) spectra. The high-resolution C\({}_{\text{1s}}\) spectrum was interpreted by the deconvolution of the following contributions: sp\({}^{2}\) C=C (284.6 eV), sp\({}^{3}\) C-C (285.3 eV), C-O (286.6 eV), C=O (288.0 eV), and O=C-O-R (289.9 eV) [60; 61]. As an example, Figure 3 illustrates the deconvolution for the samples char, AC, and AC-CA-1.0. The contributions of each of the samples are available in Table 3 and depicted in Figure 3. The process of activation increases the percentage of sp\({}^{2}\) carbon to the detriment of sp\({}^{3}\), probably due to the thermal dehydration reactions during the Na\({}_{2}\)CO\({}_{3}\) activation. The CA modification of the AC led to a decrease in the sp\({}^{2}\) C=C bonds and positively impacted the formation of oxygenated groups, especially C=O and C-O. Concretely, if the percentage of oxygenated carbon is accounted for (Sum C\({}_{\text{ox}}\)), the performance of the materials is described in the following order: char (4.5%) < char-CA-1.0 (17.2%) < AC (15.6%) < AC-CA-0.5 (18.0%) < AC-CA-1.0 (30.1%) < AC-CA-1.5 (24.0%). Hence, a positive effect from the activation with Na\({}_{2}\)CO\({}_{3}\) is deduced, especially with CA functionalization to enhance the presence of oxygenated carbonaceous groups. The activated and modified AC-CA-1.0 displays the highest value, i.e., double the value of the unmodified AC sample. In this sample, although all the oxygenated contributions were raised, the C-O outweighed the rest. The high-resolution O\({}_{\text{1s}}\) region was adjusted into three plausible contributions, i.e., quinone (530.7 eV), C=O (531.5-532.0 eV), and C-O (-533 eV) [62]. From the evolution of their relative contributions, we ascertained a rise in C=O to the detriment of the quinone groups as the samples were treated with CA, with the AC-CA-1.0 being the sample in which this effect is most noticeable. The nature of the oxygenated groups was further qualitatively evaluated by TPD. The TPD profiles are depicted in Figure 4. A tentative deconvolution was conducted based on the temperature at which the oxygenated functional groups decompose.