## 3 Results
### Scanning Transmission Electron Microscope (STEM)  
To obtain more in-depth information on the morphology of the carbonized samples, an ultra-high-resolution scanning transmission electron microscopy (UHR-STEM) was performed. STEM presents several advantages when compared to conventional TEM. One of the most relevant is that it can deploy a focused beam to scan the specimen's surface and analyze the scattered signal. A high-angle dark-field detector (HAADF) almost eliminates diffraction contrast. It has become the most widely used imaging method in STEM because of its efficiency and ease in interpreting the obtained images [29]. Figures 4-6 present the UHR-STEM analysis, with EDX mapping analysis (a), SEM (b), Z-contrast (c), and STEM (d) modes, and HR-STEM with an interplanar distance graph (e) for the three carbonized samples. Figure 3: Overall XPS survey spectra for (**a**) blue shark chondro sulfate, (**b**) blue shark gelatine, and (**c**) commercial chondro sulfate, carbonized for 1 h at 1000 \({}^{\circ}\)C under N\({}_{2}\) flow. Figure 4: ZC image (**a**) with EDX mapping analysis at \(\times\)80 K magnification (**b**); (**c**) SEM mode \(\times\)150 k\({}_{2}\) (**d**) Z-contrast mode \(\times\)150 k\({}_{2}\) (**e**) STEM mode \(\times\)150 k; (**f**) HR-STEM at 200 kV and interplanar distance (inset), graph (inset) obtained from the analysis of the area inside the red box, of blue shark chondro sulfate carbonized at 1000 \({}^{\circ}\)C for 1 h.  
Figure 5: ZC image (**a**) with EDX mapping analysis at \(\times\)80 K magnification (**b**); (**c**) SEM mode \(\times\)150 k\(\times\) (**d**) Z-contrast mode \(\times\)150 k; (**e**) STEM mode \(\times\)150 k; (**f**) HR-STEM at 200 kV and interplanar distance graph (inset) obtained from the analysis of the area inside the red box, of commercial chondro sulfate carbonized at 1000 \({}^{\circ}\)C for 1 h.  
The STEM images reveal a three-dimensional porous network for the blue shark and commercial chondroi sulfate-based carbons at SEM, Z-contrast, and STEM images. On the other hand, the blue shark gelatine-based carbon presents a completely distinct morphology in which overlayed sheets are visible. Regarding the compositional analysis, the EDX mapping evidences the uniform distribution of calcium in the blue shark and commercial chondroi sulfate carbon-based samples. However, the presence of calcium  
Figure 6: ZC image (**a**) with EDX mapping analysis at \(\times\)80 K magnification (**b**); (**c**) SEM mode \(\times\)150 k; (**d**) Z-contrast mode \(\times\)100 k; (**e**) STEM mode \(\times\)100 k; (**f**) HR-STEM at 200 kV and interplanar distance graph (inset) obtained from the analysis of the area inside the red box, of blue shark gelatine carbonized at 1000 \({}^{\circ}\)C for 1 h.  
in the blue shark gelatine sample is not so evident. The results agree with the EDX analysis performed in SEM, wherein clearly, this sample showed the lowest calcium content (Figure S1). Figures 4-6 (e) present the HR-STEM analysis with the determination of the interplanar distance (d). The blue shark chondroit sulfate-based carbon presents a d value of 2.6 \(\pm\) 0.2 A, followed by the commercial chondroit sulfate-based carbon with a d value of 2.9 \(\pm\) 0.1 A, while the blue shark gelatine-based carbon presents a d value of 3.4 \(\pm\) 0.05 A. The interplanar distance is the distance between adjacent graphene layers in a stack of graphene sheets. In pure graphene, the interplanar distance is approximately 3.35 A [30]. This value is often called the \"ideal\" or \"equilibrium\" interplanar distance for graphene. However, it is worth noting that the interplanar distance in graphene can be influenced by various factors such as intercalation, strain, and doping. By introducing foreign atoms or molecules between graphene layers or applying mechanical strain, it is possible to modify the interplanar distance [31]. The blue shark gelatine-based carbon is the only material in the study with similar values (3.41 \(\pm\) 0.05 A).