## 3 Results and Discussion
### Structural Characterization and Morphology
On the other hand, SEM study of APC shows instead the presence of irregular-shaped porous particles due to chemical etching of the carbon matrix by KOH [8,9] (Figure 7). Based on the total nitrogen adsorbed amount during porosimetry measurements, which are depicted in Figure 8, it is obvious that APC exhibits around a four times higher pore volume compared to PC. The sharp knee in the low-pressure range (i.e., \(\mathrm{P/P_{o}}\) < 0.01) indicates the presence of microporosity (\(\mathrm{D_{micropore}}\)\(\leq\) 2 nm) in both cases. Figure 8: \(\mathrm{N_{2}}\) adsorption (filled circles)—desorption (empty circles) experimental data for PC and APC. The continuous lines show the CPSM model fitting comparison. The inset figures show the pore size distribution (PSD) according to the Horvath–Kawazoe (dash dots) and to the CPSM (line) model. Figure 7: SEM images of the APC sample. (**a**) Magnification \(\times\)1300, scale 10 μm, (**b**) magnification \(\times\)8000, scale 2 μm. More specifically, as it is reported in Table 1, according to the CPSM [10,11] and to the Dubinin-Raduskevitch (D-R) [19] method, the micropore volume fraction is 39% for PC and 73% for APC. The pore volume distribution, provided by the application of the CPSM or the Horvath-Kawazoe [20,31] model, indicates pores around D = 1.5 nm or 1.9 nm and D = 1.06 nm or 1.67 nm for PC and APC, respectively. One the other hand, the pore population distribution provided by the CPSM model indicates the major number of pores at around D = 1.38 nm for PC and at around D = 0.97 nm for APC. Moreover, according to Table 2, the perfect simulation of the nitrogen porosimetry process by the CPSM model provides specific surface area values at S\({}_{\text{CPSM}}\) = 105 m\({}^{2}\)/g for PC and S\({}_{\text{CPSM}}\) = 776 m\({}^{2}\)/g for APC. For the estimation of such values, this model takes into account the microporosity and the pore curvature effect on specific surface area calculations [12]. This effect is more intense in the case of APC, where the microporosity fraction is higher. The composition of PC and APC was analyzed via XPS, showing the dominant presence of carbon and oxygen (Figure 9). The oxidized state of carbon was further evidenced in the high-resolution C1s profiles, in which the characteristic peaks for oxidized carbon [29,32,33,34,29] were present in both samples (Figure 9). The position of the analysis peaks and the area percentiles of the deconvoluted carbon components were practically the same for both samples. More specifically, the C1s components were attributed to sp\({}^{2}\)/sp\({}^{3}\) carbons (284.8 eV, \(\sim\)70%), C-O (286.2 eV, \(\sim\)17%), C=O (287.5 eV, \(\sim\)7%) and O-C=O (289.1 eV, \(\sim\)4%). The component at 290.8 eV (\(\sim\)2%) is the characteristic satellite feature ascribed to \(\pi\)-\(\pi^{\ast}\) transitions within sp\({}^{2}\) conjugated carbons [35, 36, 37]. The presence of oxygen-containing groups on the surface of the solids provides active sites for further post-functionalization [38] or strong molecular interactions pertaining to adsorption/removal processes. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Sample** & **V\({}_{\text{pore}}\)** & **V\({}^{\text{D-R}}\)\({}_{\text{micro}}\)** & **V\({}^{\text{CPSM}}\)\({}_{\text{micro}}\)** & **D\({}^{\text{CPSM}}\)\({}_{\text{Nmean}}\)** & **D\({}^{\text{CPSM}}\)\({}_{\text{Vmean}}\)** & **D\({}^{\text{HK}}\)\({}_{\text{Vmean}}\)** \\
**Code** & **(cm\({}^{3}\)/g)** & **(cm\({}^{3}\)/g)** & **(\% cm\({}^{3}\)/g)** & **(nm)** & **(nm)** & **(nm)** \\ \hline PC & 0.080 & 0.034 & 0.030 & 1.38 & 1.50 & 1.90 \\ APC & 0.306 & 0.24 & 0.21 & 0.97 & 1.06 & 1.67 \\ \hline \end{tabular}
\end{table} 
Table 1: Pore structure parameters derived from the CPSM model simulation of the overall N\({}_{2}\) adsorption–desorption hysteresis loop, the Dubinin-Raduskevitch (D–R) method and the Horvath–Kawazoe (H–K) model.