The heteroatom (N, S, Cl, B, P, Fe, Co, Mn or Ni)-doped carbon materials, such as carbon nanotubes (CNTs)¹,  graphene², graphitic arrays³, and amorphous carbon⁴, have drawn great attention due to their excellent electrocatalytic performance for ORR. However, among all these non-precious carbon-based catalysts, very few are on a competitive level with platinum in acidic media. One family of these most promising alternatives to Pt are mainly the Fe/N co-doped carbon materials. Transition metal was found to be indispensable to catalyze the graphitization of nitrogen−carbon precursor to form the highly graphitized carbon. In addition, Some studies argued that metals could not only help the formation of active sites, but could be an integral active part of the catalytic site⁵. Based on the above conceptions, we studied the effect of Fe content on the catalyst performance by varying initial FeSO₄ 7H₂O/Polyquaternium weight ratio to tune the final Fe content in the catalyst. The obtained optimal catalyst (N/Fe-PAD) showed a comparable ORR activity in acidic media with commercial Pt/C.

The N/Fe-doped porous carbon electrode materials(N/Fe-PAD) were prepared by homogeneously dispersion of polyquaternium (PAD) and ferrous sulfate (FeSO₄ 7H₂O) precursors onto the surface of silica (which was firstly dispersed in hydrochloric acid solution by ultrasonication, the FeSO₄ 7H₂O/Polyquaternium weight ratio is 0.04 and 1.0, respectively). After dried overnight, the resulting solid was ground to a fine powder, and then calcined at 800^oC for 1h in nitrogen atmosphere. The excess amount of sodium hydroxide (NaOH) was added to vacate the silica and the resulting powder was acid-leached using 0.5 M H₂SO₄ at 85^oC to remove the unreacted metallic Fe. Then, the catalytic graphitization of the impregnated carbon was subjected heat treatment at 800^oC for 1h pyrolyzation time again. The catalysts as-prepared are thus designated as N/Fe_X-PAD(X=0.04 and 1.0, respectively).  

The electrocatalytic activity on the N/Fe_X-PAD and Pt/C was investigated using cyclic voltammetry (CV) and linear sweep voltammetry (LSV) employing rotating disk electrode (RDE) technique in 0.1M HClO₄.

Fig. 1 shows CV responses of N/Fe_1.0-PAD in N₂/O₂ saturated 0.1 M HClO₄ solution. In the case of a N₂-saturated solution, the cyclic voltammogram only presents a featureless quasi-rectangular trace. However, when O₂ is introduced into the solution, a distinct ORR response with the peak potential (Ep) at 0.61 V vs. RHE, and the peak current density (Ip) of 2.64 mA cm^-2 can be clearly observed, indicating that this N/Fe_1.0-PAD catalyst has a profound catalytic ORR activity. Figure 2 shows the polarization curves of the N/Fe_1.0-PAD, N/Fe_0.04-PAD and Pt/C catalysts. It can be seen that the high activity of N/Fe_1.0-PAD for oxygen reduction was clearly demonstrated by onset potential of 0.80V, half-wave potential of 0.68V, which are positively shifted more than 25 and 80 mV, respectively, compared to N/Fe_0.04-PAD. In addition, the limiting current densities for N/Fe_1.0-PAD are improved greatly (～7.5 mA cm^-2 at 0.30 V), much higher than Pt/C catalyst.

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