## 3 Results and Discussion
Therefore, we consider that the increase in ball-milling time led to an enhanced incorporation of FePc in BPO carbon support, in good agreement with FTIR results. Figure 4b shows the diffractograms of the catalysts obtained after heat treatment. The diffraction peak around 25 20 degrees was observed for all materials, indicating a strong contribution of graphitic carbon to the XRD pattern of all catalysts, which is different from what observed for nonpyrolyzed catalysts. This was due to a precursor decomposition and breakdown during pyrolysis, as pointed out by TGA. Diffraction peaks for metal iron and iron oxide were also evident [57], and their intensity increased as the ball milling-time increased. This was particularly evident in the case of the diffraction peaks at 45 20\({}^{\circ}\) and 82 20\({}^{\circ}\) for the FeNC_BM6_P sample, suggesting an increase in iron content in the samples with the ball-milling time, in accordance with FTIR results. To evaluate catalytic activity toward ORR of the prepared catalysts, cyclic voltammetry was carried out in either nitrogen- or oxygen-saturated phosphate buffer. No peaks were detected in N\({}_{2}\)-saturated PBS for both FeNC and FeNC_P (Figure 5), and the resulting capacitive current was much higher for FeNC_P as compared to FeN, because an expected effect of pyrolysis in increasing surface area of the catalyst. When the electrolyte was saturated with oxygen, a well-defined reduction peak around 0.6 V vs. RHE can be seen for both samples demonstrating the occurrence of ORR at the surface of Fe-N-C catalysts. The ORR peak was shifted toward more positive voltage values for  
Figure 4: Intensity-normalized X-Ray diffractograms of **(a)** FeNC_BM1, FeNC_BM3 and FENC_BM6, and **(b)** FeNC_BM1_P, FeNC_BM3_P and FeNC_BM6_P. FeNC_P, pointing at the beneficial effect of pyrolysis in enhancing ORR. ORR peak current density also increased, indicating an increased density of active sites after pyrolysis. The same characterization was carried out for all the prepared electrocatalysts: The samples prepared with different ball-milling times (FeNC_BM1, FeNC_BM3 and FeNC_BM6) and the samples obtained after a further pyrolysis treatment of the above-mentioned samples (FeNC_BM1_P, FeNC_BM3_P and FeNC_BM6_P). The corresponding CV curves are shown in Figure 6.  
CV analysis indicated that both ball-milling and pyrolysis affect capacitive current density values, ORR peak potential and peak current density. Electrochemical surface area (ECSA) was then  
Figure 5: Cyclic voltammetry (CV) curves in N\({}_{2}\)- (dash black lines) and O\({}_{2}\)-saturated (solid red lines) phosphate-buffered saline (PBS) for FeNC (**a**) and FeNC_P (**b**) samples. Figure 6: CV curves in N\({}_{2}\)- (dash black lines) and O\({}_{2}\)-saturated (solid red lines) PBS for FeNC_BM1 (**a**), FeNC_BM1_P (**b**), FeNC_BM3 (**c**), FeNC_BM3_P (**d**), FeNC_BM6 (**e**) and FeBM6_P (**f**) samples. calculated by integrating capacitive current, and the results are shown in Table 4, together with further electrochemical parameters extrapolated from CV analysis, such as ORR peak potential (Epr) and peak current density (Jpr). As a general comment, ECSA values changed after either ball-milling time and subsequent pyrolysis, and variations in Epr and Jpr also took place. To identify a trend in ECSA, Epr and Jp after ball-milling and pyrolysis, those parameters were plotted in Figure 7 for nonpyrolyzed (Figure 7a) and pyrolyzed catalysts (Figure 7b). In the case of nonpyrolyzed sample, ECSA increased as the ball-milling time increased up to 3 h (80 m\({}^{2}\)g\({}^{-1}\) for FeNC_BM3). After that, ECSA decreased for the sample prepared with 6 h of ball-milling time (46 m\({}^{2}\)g\({}^{-1}\) for FeNC_BM6). This finding indicates that increasing ball-milling is effective in increasing surface area of the electrocatalysts up to 3 h, but longer ball-milling time may reverse the effect as previously reported [44; 58].