## 2 Materials and Methods  
### Materials  
Treatment of carbon support: Carbon Black pearls 2000 (BP) were purchased by Cabot corporation (MA, US) and treated in refluxing concentrated HNO\({}_{3}\) 65 wt.% at 90 \({}^{\circ}\)C for 16 h. Then, BP was filteredusing a Whatman membrane filters nylon (pore size: 0.45 \(\upmu\)m, diameter: 47 mm) and a diaphragm vacuum pump (Vacuubrand GMBH). Then, the material was washed with distilled water until neutral pH was obtained, dried in an oven at 70 \({}^{\circ}\)C overnight and milled in mortar agate (treated BP was labelled as BPO). Preparation of electrocatalysts: 0.5 g of BPO, 0.5 g of iron (II) phthalocyanine (FePc, Aldrich) and 2 g of CH\({}_{4}\)N\({}_{2}\)O (Urea, VWR Chemicals) were mixed by hand using a mortar for 30 min, and part of this sample was then pyrolyzed in an Al\({}_{2}\)O\({}_{3}\)-tube furnace (Forni De Marco, Italy) at 900 \({}^{\circ}\)C for 1 h under an Ar atmosphere, obtaining two samples labelled as FeNC and FeNC_P, respectively. In addition, a further part of FeNC sample (2 g) was ball-milled in a Zirconia jar (V = 80 mL) with 100 Zirconia balls (5-mm diameter) in a planetary ball-miller (Pulverisette 6, Fritsch GmbH) for either 6 or 12 cycles of 30 min milling and 5-min pause at a rotation rate of 400 rpm, and then pyrolyzed at 900 \({}^{\circ}\)C for 1 h under Ar atmosphere, obtaining two samples labelled as FeNC_BM3_P and FeNC_BM6_P. Two further samples were obtained by ball-milling FeNC continuously for 1 h (FeNC_BM1) and then pyrolyzing as described before (FeNC_BM1_P). Figure 1 shows a diagram illustrating the procedure for sample preparation, and Table 1 lists all the samples prepared with the corresponding label. Catalyst ink preparation: The catalyst ink was prepared by dispersing 0.2 mg of catalyst in 270 \(\upmu\)L of DI water and 135 \(\upmu\)L of Isopropanol. This dispersion was treated in an ultrasonic bath (Elmasonic P 30 H, Elma Schmidbauer GmbH) for 45 min. Then, 50 \(\upmu\)L of Nafion solution (5 wt.% in lower aliphatic alcohols and water, Aldrich) were added to the dispersion and treated in the ultrasonic bath for further 15 min. The ink was dropped (7 \(\upmu\)L) onto a working electrode (glassy carbon disk/platinum ring rotating ring disk electrode: RRDE AFE6R2 GCPT, area = 0.2375 cm\({}^{2}\), Pine Research Instrumentation), resulting in a catalyst loading of 0.15 mgcm\({}^{-2}\). Before use, the modified glassy carbon electrode was dried in a convection oven at 40 \({}^{\circ}\)C for 4 min. Fourier Transform Infrared Spectroscopy (FTIR) was carried out by means of a FTIR100 spectrometer (Perkin Elmer) in transmittance mode. Samples were pelleted in 150 mg of KBr using a Specac manual hydraulic press, by applying a pressure of 7 tons for 5 min. The diameter of pellets was 13 mm. X-Ray Diffraction (XRD) was carried out by means of a Philips PW1730 diffractometer with Cu K\(\alpha\) radiation (\(\uplambda\) = 1.5406 A). Cyclic Voltammetry (CV). The catalytic activity toward ORR of the prepared materials was examined by cyclic voltammetry, using a VMP3 Potentiostat (Bio Logic Science Instrument) controlled by computer through EC-Lab V10.18 software. A conventional tree electrode cell was used. The reference electrode was saturated calomel electrode SCE (Amel 303/SCG/12), the auxiliary electrode was a platinum wire (Amel 805/SPG/12) and the glassy carbon disk RRDE modified with the catalyst layer as a working electrode. All potential values in the manuscript were measured vs. SCE and then converted to the reversible hydrogen electrode (RHE) scale, according to Equation (1):  
\[E(RHE)=E(SCE)+E^{0}(SCE)+0.0591 pH \tag{1}\]  
where \(E^{0}(SCE)\) = 0.241 V.  
CV experiment was carried out in 125 mL of neutral phosphate buffer solution (PBS), saturated in either nitrogen or oxygen atmosphere (gas purging for at least 20 min to obtain complete oxygen saturation of the media) at room temperature, with 10 mVs\({}^{-1}\) scan rate and 1.4 V to \(-\)0.4 V vs. RHE potential window.