## 3 Results and Discussion
Ball milling is indeed a very well-known method to increase materials surface area and porosity, contributing to breakdown of larger solid clusters into smaller ones, and optimizing ball-milling time is crucial to achieve desirable surface area and morphology [39]. \begin{table}
\begin{tabular}{c c c c c} \hline
**Sample** & **ECSA (m\({}^{2}\)g\({}^{-1}\))** & **E\({}_{\text{pr}}\) (V)** & **J\({}_{\text{pr}}\) (m\(\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{ \text{\text{\text{\text{\text{\text{\text{\text{\text{      }}}}}}}}}}}}}}}\))** & **J\({}_{\text{pr}}\) (m\(\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{{ \text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{      }}}}}}}}}}}}}}}}\))** & **J\({}_{\text{pr}}\) (m\(\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{{ \text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\texttext{      }}}}}}}}}}}}}}}}}\))** & **J\({}_{\text{pr}}\)** (m\(\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{{ \text{\text{\text{\text{\text{\text{\text{\text{\text{\texttext{\texttexttexttext {      \texttexttexttexttexttexttexttexttext {  }}}}}}}}}}}}}}}}\) \\ & 19 & 0.58 & 0.21 & 12 \\ FeNC\_P & 146 & 0.65 & 0.38 & 30 \\ FeNC\_BM1 & 54 & 0.63 & 0.22 & 12 \\ FeNC\_BM1\_P & 69 & 0.60 & 0.31 & 50 \\ FeNC\_BM3 & 80 & 0.61 & 0.37 & 20 \\ FeNC\_BM3\_P & 70 & 0.51 & 0.35 & 75 \\ FeNC\_BM6 & 46 & 0.73 & 0.32 & 65 \\ FeNC\_BM6\_P & 24 & 0.40 & 0.26 & 80 \\ \hline \end{tabular}
\end{table} 
Table 4: Electrochemical surface area (ECSA) and electrochemical parameters (oxygen reduction reaction (ORR) peak potential, E\({}_{\text{pr}}\), peak current density, J\({}_{\text{pr}}\), and current density retention (@ 0.26 V vs. RHE) after 10 h, J retention) of the prepared electrocatalysts. Figure 7: Trend of electrochemical surface area (ECSA), ORR peak potential (Epr) and peak current density (Jpr) of the prepared FeNC-based electrocatalyst before and after ball-milling (**a**) and pyrolysis (**b**) (symbols: experimental data; dash lines: guide for eyes). ORR peak current density (ORR) followed the same trend as ECSA due to an increased density of active sites for higher ECSA samples. By contrast, Epr values increased as the ball-milling time increased, FeNC_BM6 sample having the more positive Epr value (Epr = 0.73 V vs. RHE). The samples were further subjected to a pyrolysis treatment to overcome a possible heterogeneity in porosity and modification in surface chemistry induced by ball-milling, which may have contributed to decrease the active site utilization [59]. The pyrolysis step after ball-milling led to an increase in ECSA only for the sample prepared with no ball-milling (146 m\({}^{2}\)g\({}^{-1}\) for FeNC_P vs. 19 m\({}^{2}\)g\({}^{-1}\) for FeNC) and the sample obtained with 1 h of ball-milling time (69 m\({}^{2}\)g\({}^{-1}\) for FeNC_BM1_P vs. 54 m\({}^{2}\)g\({}^{-1}\) for FeNC_BM1). By contrast, the pyrolysis treatment led to a decrease in ECSA for ball-milling time higher than 1 h. This can be ascribed to an agglomeration of solid clusters and pore growth between them, as previously reported by [PERSON] et al., who demonstrated that ESCA of Fe-N-C materials is reduced by a pyrolysis step after ball milling due to changes in porosity [60]. Epr and Jpr values were affected by pyrolysis with a similar trend as compared to that described for ECSA and illustrated in Figure 7.  
The body of ECSA, Epr and Jp results indicated that ball-milling time in the range of 1-3 h allowed for the production of electrocatalysts with high ECSA, high density of ORR active sites (Jp), and good catalytic activity toward ORR (Epr). Longer ball-milling time (6 h) led to a catalyst (FeNC_BM6) with lower ECSA and Jp, but the lowest ORR overpotential (as can be seen from Epr value) still pointed at a good electrocatalytic activity toward ORR. The good catalytic activity of FeNC_BM6 can be explained by considering the enhanced FePc/carbon support interaction, as indicated by FTIR and XRD data.