# Structural Evolution of Polyimide-Derived Carbon during Phosphoric Acid Activation
## 3 Results and Discussion
### Xps  
XPS is a powerful tool for studying the elemental composition and chemical state of phosphorus in carbon materials. An XPS survey scan revealed that both H\({}_{3}\)PO\({}_{4}\) and the Thermal series of carbons contain C, O, and N (Table 2). The Thermal series has a greater carbon content at all temperatures due to the higher oxygen and phosphorus content in carbons activated with phosphoric acid (H\({}_{3}\)PO\({}_{4}\) series), which decreases the proportion of carbon. Phosphorus is only found in carbons that have been activated with phosphoric acid. The phosphorus content increases to 13.2% when carbonization temperature rises to 800 \({}^{\circ}\)C and decreases at higher temperatures. This phenomenon is explained by the acceleration of the formation of phosphorus-carbon compounds when the activation temperature is raised to 800 \({}^{\circ}\)C and the breakdown of these compounds at higher temperatures [12]. The  
Figure 2: Nitrogen adsorption-desorption isotherms: (**a**) and pore size distributions; and (**b**) for parent BM-DVB copolymer and carbons obtained by carbonization at 600 \({}^{\circ}\)C of polyimide copolymer in the presence of phosphoric acid (H\({}_{3}\)PO\({}_{4}\)) and without acid (Thermal). thermodynamic study discovered that phosphorus compound volatilization is favorable at temperatures greater than 750 \({}^{\circ}\)C [12,58]. Furthermore, the thermal gravimetric analysis showed that mass loss in the 800-900 \({}^{\circ}\)C temperature range is proportional to phosphorus content [12,59]. The C 1s peak was deconvoluted into the six components: C-C sp\({}^{2}\) bonds at 284.7 eV; C-C sp\({}^{3}\) at 285.2 eV; C-O at 286.3 eV; C=O at 287.4 eV; O=C-O- at 288.7 eV and shake-up peak that corresponds to \(\pi\)\(\rightarrow\)\(\pi^{\ast}\) type transitions at 290.5 eV (Figure 3). The content of sp\({}^{2}\)-hybridized carbon, which belongs to graphite-like fragments of activated carbon, may be obtained by such deconvolution [60, 61, 62, 63]. Carbonization in the presence of phosphoric acid results in a significant increase in the content of sp\({}^{2}\)-hybrid  
rises from 400 to 500 \({}^{\circ}\)C, as seen in Figure 4a. On the contrary, when the BM-DVB copolymer is carbonized without phosphoric acid, the sp\({}^{2}\)-hybridized content increases over a larger temperature range--from 500 to 800 \({}^{\circ}\)C. This demonstrates that phosphoric acid facilitates the formation of the polyaromatic structure of carbon at lower temperatures. The changes in concentration of \(\pi\)-electrons were also measured by XPS using the \(\pi\)\(\rightarrow\)\(\pi^{\bullet}\) shake-up signal [64; 65]. It is seen that the shake-up signal is consistently higher for phosphoric acid-activated carbons (Figure 4b). However, it should be noted, that the shake-up peaks are superimposed on the broad emission of inelastically scattered electrons, which occurs starting in about the same energy region [64]. Deconvolution of the P 2p envelope revealed only one peak with P 2p 3/2 binding energy of 133.1 \(\pm\) 0.2 eV (Figure 5a). As the carbonization temperature increases, the binding energy shows some tendency to decrease from 133.4 to 132.8 eV. This peak is relatively  
Figure 4: Temperature evolution of sp\({}^{2}\)-hybridized carbon content: (**a**) and \(\pi\)\(\rightarrow\)\(\pi^{\bullet}\) shake-up signal (**b**) of polyimide-derived carbons obtained with (H\({}_{3}\)PO\({}_{4}\)-series) and without (Thermal-series) phosphoric acid at different temperatures. Figure 3: High-resolution X-ray photoelectron spectrum of C 1s peak of polyimide-derived carbon obtained with phosphoric acid at 800 \({}^{\circ}\)C.  
narrow (FWHM 1.9 \(\pm\) 0.1 eV), confirming the existence of only one type of phosphorus species. Analysis of the NIST X-ray Photoelectron Spectroscopy Database [49] shows that the detected range of binding energies is typical for phosphates and pyrophosphates (Figure 6).