The overall performance of fuel cells is limited by the sluggish kinetics of the oxygen reduction reaction (ORR). That´s why large amounts of platinum are used at large-scale fuel cells as it shows superior ORR activity. However, because of its high cost and limited availability a great deal of effort has been made to develop ORR catalysts free of platinum group metals (PGM). Among the most active PGM-free ORR electrocatalysts are metal-nitrogen-carbon (M-N-C), such as Fe-N-C. These are a type of PGM-free catalysts in which single atomic iron (or other TM atoms such as Co or Ni) coordinated to several N atoms are dispersed in a C-C sp2 network. They are usually synthesized following the same procedure: firstly, a mixture of C, N and Fe precursors are mixed and subjected to one or more thermal treatments under flow of an inert or reactive gas (NH₃ at high temperature (between 600ºC and 1100ºC). Then, acid leaching of the pyrolized catalyst is carried out to remove metallic Fe, iron oxides, Fe₃C and Zn particles. Lastly, another thermal treatment is further applied. These FeCxNy ensembles contains different Fe-N_x moieties, which can participate in the ORR to a different extent. It is generally accepted that the Fe(II)N₄ sites are the most active ones for the ORR in acid. However, in order to deep into the understanding of the catalytic activity it is essential to analyse the N environment on the FeNxCy architecture as N atoms in different configurations has been identified, namely FeNx-pyrrolic (FeN₄C₁₂) and FeNx-pyridinic (FeN₄C₁₀). Furthermore, they have a different role in the activity and stability shown by the catalyst in the ORR. Previous studies indicated that whereas the initial activity of the fresh Fe-N-C catalyst might be ascribed to FeN_x-pyrrolic, the FeNx-pyridinic sites are actually the most active and stable. Besides, pyrrolic environments are prone to oxidize resulting in demetallation by formation of Fe₂O₃ species. However, other studies regarding N-doped C materials concluded that pyridinic N and pyrrolic N yield higher activity compared to the other N species, but pyridinic N turns into pyrrolic N over the ORR. The flow of inert gas used during the thermal treatments of the Fe-N-C catalysts have been reported to affect the content of the different N-containing groups and, therefore it can be optimize to increase the ORR activity. It is also important to highlight that catalysts obtained from CxNy precursors with high molecular weight displaying a high N/C atomic ratio and high porosity, typically MOFs or POP, render the most active ORR catalysts.

In this work, we have synthesized a Fe/N/C catalyst using a CxNy precursor obtained via the wet-polymerization of an imine-based framework. Melamine and terephthalaldehyde were dissolved in dimethylsufoxide, after that, zinc nitrate and iron nitrate were added. The mixture was stirred and heated to 180 ⁰C for 16 hours. The resulting material was subjected to a thermal treatment at 900 ⁰C under different flows of an inert gas (N₂). The material obtained (ImFeZn 1HT) displayed high ORR activity (see Figure 1). Both catalysts have been thoroughly characterized by XRD, TEM, XAS, CO cryo-chemisorption to evaluate its site density, stripping of NO by exposure to aqueous nitrite and TOF. Characterization data indicate the formation of FeNx ensembles, with pyridinic-N and pyrrolic-N moieties, and a site density of 4.4·10¹⁹ sites·g^-1 and 1.5·10¹⁹ sites·g^-1 estimated by electrochemical stripping of NO and CO cryo-chemisorption respectively. The ORR activity has been evaluated in a RRDE configuration in 0.1 M HClO₄. After acid leaching and a further thermal treatment (ImFeZn AL 2HT) the ORR activity increased significantly (see Figure 1) reaching a mass-specific activity of 3.4 A/g at 0.8 V with E_onset and E_1/2 of 0.95 V and 0.76 V, respectively. These values are among the highest activities reported in acid electrolyte.