## 5 Conclusions  
Ball-milling and pyrolysis strongly affect electrochemical surface area (ECSA), catalytic activity, and stability of Fe-N-C electrocatalysts prepared using Iron(II) phthalocyanine, urea and carbon black pearls as precursors. Ball-milling increased the ECSA, ORR peak potential (Epr) and peak current density (Jpr) as compared to the FeNC sample obtained without any ball-milling treatments. Different ball-milling times (1 h, 3 h, and 6 h) were explored, and ORR performance of all the three catalysts prepared (FeNC_BM1, FeNC_BM3_FeNC_BM6) displayed a good catalytic activity toward ORR. In particular, 3 h and 6 h of ball-milling time allowed for the production of samples with the highest ECSA, Jp and Epr values due to its beneficial effect in enhancing the interaction between FePc  
Figure 8: Current density retention (Jretention) @ 0.26 V vs. RHE over time for (**a**) nonpyrolyzed FeNC, FeNC_BM1, FeNC_BM3 and FeNC_BM6 and (**b**) pyrolyzed FeNC_P, FeNC_BM1_P, FeNC_BM3_P and FeNC_BM6_P.  
and carbon support. Ball-milling also affected catalytic stability of the Fe-N-C catalysts, as increasing ball-milling time led to an increase of current density retention up to 65% for the FeNC_BM6 sample. The effect of a subsequent pyrolysis treatment after ball-milling on ORR catalytic activity and stability was also investigated. We found that pyrolysis after ball-milling had a detrimental effect on ECSA, JP and Epr, with such a negative effect being more relevant for the samples obtained with higher ball-milling time (FeNC_BM3_P and FeNC_BM6_P). In fact, pyrolysis combined with long ball-milling time caused the increase in metal iron and iron oxide in the samples, which are less effective as ORR active sites as compared to nitrogen-coordinated iron. On the other hand, pyrolysis further improved electrocatalytic stability of the materials, allowing retention up to 80% of initial current density for FeNC_BM3_P and FeNC_BM3_P samples.  
Overall, considering the requirements of high ORR catalytic activity and good stability under operative conditions, 6 h of ball-milling without any further pyrolysis was effective to prepare a Fe-N-C material with a good compromise between ORR activity and stability (FeNC_BM6). Also, 1 h of ball-milling followed by pyrolysis was effective in producing a Fe-C-materials with high ORR activity and stability (FeNC_BM1_P). Hence, FeNC_BM6 and FeNC_BM1_P can be good candidates for use at the cathode side of microbial fuel cells operating in neutral pH.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/15/5278/s1](http://www.mdpi.com/2076-3417/10/15/5278/s1)]([http://www.mdpi.com/2076-3417/10/15/5278/s1](http://www.mdpi.com/2076-3417/10/15/5278/s1)), Figure S1: FTIR spectra of FePc, FeNC-BM1 catalyst, and urea.; Figure S2: XRD patterns of BPO, urea, and FePc.; Figure S3: XRD patterns of FeNC_BM1 and FeNC_BM6.; Table S1: Characteristic IR bands of iron (II) phthalocyanine.; Table S2: Characteristic IR bands (cm\({}^{-1}\)) of urea.  
Conceptualization, B.M. and A.D.; methodology, B.M., W.J.S.F and M.A.C.d.O.; investigation, B.M., A.D., P.P.M.P., W.d.S.F. and M.A.C.d.O.; resources, B.M., A.D., and M.A.C.d.O.; data curation, B.M., W.d.S.F, P.P.M.P., and M.A.C.d.O.; writing--original draft preparation, B.M. and M.A.C.d.O.; writing--review and editing, B.M.; supervision, B.M.; project administration, B.M.; funding acquisition, A.D. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the University of Roma Tor Vergata (Grant \"BEYOND BORDERS 2019\": Project Name: AEROBIC).  
The authors acknowledge [PERSON] (Dept. Chemical Science and Technologies of the University of Rome Tor Vergata) for her valuable technical support.  
The authors declare no conflict of interest.