## 3 Results and Discussion  
Figure 2a shows TGA curves of FePc, urea, carbon black pearls treated with nitric acid (BPO), and pristine carbon black pearls (BP). Whereas BP were thermally stable over the whole temperature range investigated, BPO had a total weight loss of 33.5% due to the splitting off of oxygen containing surface groups introduced with the treatment with nitric acid [6]. As far as urea is concerned, three main weight losses can be identified. In the first temperature region (room temperature to 230 \({}^{\circ}\)C), mass loss can be associated with urea decomposition to ammonium cyanate, cyanic acid (HNCO) and ammonia. As the temperature increased (230 \({}^{\circ}\)C-400 \({}^{\circ}\)C), HNCO can further react with undecomposed urea to produce different decomposition products that eventually decomposed to carbon dioxide and ammonia up to 400 \({}^{\circ}\)C [53]. In the case of FePc, the first weight loss (188 \({}^{\circ}\)C) did not involve a degradation step due to evaporation of adsorbed water. The second weight loss (200-450 \({}^{\circ}\)C) was the first degradation stage, leading to phthalocyanine cores being decomposed, followed by a second (550-650 \({}^{\circ}\)C) and third (650-800 \({}^{\circ}\)C) degradation stage, overall corresponding to breakdown of the three-dimensional polymer structure and following decomposition of the breakdown products [54]. Weight loss in the prepared Fe-N-C electrocatalys reflected weight losses of the precursors, and no substantial changes were observed among FeNC_BM1, FeNC_BM3 and FeNC_BM6 in terms of thermal stability. The first weight loss in Fe-N-C electrocatalys can be ascribed to a combination of the splitting off of oxygen containing surface groups in BPO, the evaporation of adsorbed water in both BPO and FePc, and the first step of urea decomposition. The second weight loss corresponds to the first thermal degradation step of FePc combined with further decomposition of unreacted urea. After 350 \({}^{\circ}\)C, the weight loss in Fe-N-C electrocatalys was less pronounced and followed the decomposition of the FePc and urea breakdown products. Table 2 lists thermal parameters extrapolated from TGA curves for both precursors and electrocatalys. Figure 2: TGA curves of (**a**) precursors: FePc, urea, carbon black pearls treated with nitric acid (BPO), and pristine carbon black pearls (BP). (**b**) the prepared electrocatalys before pyrolysis: FeNC, FeNC_BM1, FeNC_BM3 and FeNC_BM6. A structural analysis of Fe-N-C samples was carried out by FTIR spectroscopy and XRD. Figure 3a shows the FTIR spectrum of FeNC_BM1, FeNC_BM3 and FeNC_BM6 in a broad range of wavenumber (400-4000 cm\({}^{-1}\)), while Figure 3b shows a portion of the same spectra in a reduced range of wavenumber (500-1800 cm\({}^{-1}\)). As a general comment, the FTIR spectrum of the three catalysts showed all the vibration bands of urea, and two main contributions of FePc at 1335 cm\({}^{-1}\) and 727 cm\({}^{-1}\), corresponding to the C=C stretching vibration, and the C\({}_{\text{\scriptsize{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{ \rm{\rm{\rm{\rm{\rm{\rm{\rm{\  
This finding suggests a higher vibrational activity of the isoinde groups on FeNC-BM1, indicating that increasing ball-milling time enhanced the interaction between FePc macrocycle and the carbon support. Figure 4a shows XRD patterns of FeNC_BM1, FeNC_BM3 and FeNC_BM6. The XRD patterns of FePc, urea and BPO are shown in Figure S2. In the XRD pattern of the three catalysts, the major contribution of urea can be observed, while the more intense peak of graphitic carbon (from BPO support around 25 20\({}^{\circ}\)) and the peaks of FePc were hidden due to the greater crystalline contribution of urea. However, as shown in Figure S3, the contribution of graphitic carbon was greater for FeNC-BM6, especially compared to FeNC_BM1. The contribution in the amorphous fraction was comprised in the 2\(\theta\) range in which FePc exhibited a greater number of peaks (i.e., 15-85 20\({}^{\circ}\)).