## 3 Results and Discussion
### Characterization of the Char-Based Materials
Regarding the micropore size distribution, due to the use of Na\({}_{2}\)CO\({}_{3}\) as an activating agent, the micropore size contribution registered in Figure 2B is addressed at 0.6-0.7 nm due to the CO\({}_{2}\) released during the thermal decomposition of Na\({}_{2}\)CO\({}_{3}\)[53]. The proximate analysis was used as a tool to predict the carbon content in all the samples, leading to the results presented in Table 2. The non-activated and untreated char exhibits relatively low moisture (2.9%) and high ash content (38.8%), highlighting a significant inorganic presence; the fixed carbon content remains as low as 28.3%, suggesting limited carbon retention, to the detriment of volatile matter (30.0%). After activation with Na\({}_{2}\)CO\({}_{3}\), the volatile matter content decreased and the ash content increased, an effect ascribed to the concentration of inorganic matter. The CA modification of the char increased the fixed carbon percentage, demonstrating the anchorage of CA molecules onto the surface. Following the CA functionalization of the activated char, a certain balance emerges. For instance, with 0.5 M citric acid, the fixed carbon remains high at 33.6%, while the ash content is somewhat controlled at 46.3%. At a higher CA concentration, i.e., 1.5 M citric acid, the fixed carbon declines slightly to 31.3%, and the ash content increases to 48.5%. This phenomenon suggests that the combination of Na\({}_{2}\)CO\({}_{3}\) activation and CA post-treatment enhances the contribution of carbon, and the citric acid concentration must be carefully optimized to balance carbon fixation and ash minimization. The carbon content in the bulk of the samples was estimated by elemental analysis, as shown in Table 2. All the samples exhibited moderate carbon content. The char exhibited 37.0%, whereas the activation with Na\({}_{2}\)CO\({}_{3}\) decreased the value to 32.8%, which is commonly observed during chemical activation with a wide range of different reagents [54]. The combination of a strong oxidizing medium and dehydrating effects justifies this increase in the C content [55]. The citric acid modification increased the carbon content either in the non-activated char or in that activated with Na\({}_{2}\)CO\({}_{3}\). Concretely, the CA modification of the non-activated char led to the highest carbon content, e.g., 44.3%, whereas the CA-modified activated carbon displayed \(\sim\)39% C. The CA functionalization of activated carbon has been \begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{4}{c}{**Proximate Analysis**} & \multicolumn{4}{c}{**Elemental Analysis *} \\ \cline{2-9}  & **Moisture** & **Volatile** & **Fixed Carbon** & **Ash** & **C\({}_{\rm EA}\)** & **H\({}_{\rm EA}\)** & **N\({}_{\rm EA}\)** & **O\({}_{\rm EA}\)** \\ \hline Char & 2.9 & 30.0 & 28.3 & 38.8 & 37.0 & 2.4 & 1.0 & 20.8 \\ char-CA-1.0 & 3.2 & 16.1 & 36.8 & 43.9 & 44.3 & 2.0 & 1.8 & 8.0 \\ AC & 8.6 & 9.4 & 20.9 & 61.1 & 32.8 & 1.3 & 0.7 & 4.4 \\ AC-CA-0.5 & 8.8 & 11.3 & 33.6 & 46.3 & 39.1 & 1.3 & 0.9 & 12.4 \\ AC-CA-1.0 & 7.9 & 12.4 & 32.4 & 47.3 & 39.4 & 1.3 & 0.8 & 11.2 \\ AC-CA-1.5 & 7.4 & 12.8 & 31.3 & 48.5 & 39.7 & 1.1 & 0.9 & 9.8 \\ \hline \hline \end{tabular}  
* No S was detected in any sample. The O is estimated as the difference after ashes subtraction.  
\end{table} 
Table 2: Proximate and elemental analysis (wt. %) after Na\({}_{2}\)CO\({}_{3}\) activation and citric acid modification. reported to increase the C content [56] due to the anchorage of CA leaving the formation of C=O species [57]. The H content remained relatively stable among all the samples but tended to slightly increase after CA treatment. The N content slightly decreased after Na\({}_{2}\)CO\({}_{3}\) activation, possibly due to chemical reactions that eliminate nitrogen-containing compounds during the activation step [58]. The O contribution was estimated by the difference after ash removal.