Fe-NX/C catalysts

Research group Starting precursors of Fe-Nx-C catalysts developed, and pyrolyzation conditions Max power density (mW cm ) Specific power density (mW mg iUi) PEMFC conditions used... 

Fig. 13 Fe-Nx-C catalysts EESEM images of Fe-Nx-C SBA15 (a) and Fe-Nx-C meso (b) BET adsorbed volumes, included also the non-templated Fe—Nx-C catalyst (c), and XPS analysis of the Fe-Nx-C meso catalyst (d). Data from [71]...

The Fe-Nx C meso catalyst resulted more stable compared to the other Fe-Nx/ C catalysts home-made previously mentioned (Fig. 11). 

We have shown several examples in this chapter where residual strategic metals contents <0.2 wt% (Fe or Co) in N-doped catalysts, considered negligible by some researchers, can in fact result in non-negligible catalytic activity for ORR in acid medium, given that this activity increases quickly with the Fe or Co content in N-doped catalysts (see Figs. 10.3 and 10.16b). We also believe that the existence of Fe-Nx sites in Fe/N/C catalysts have now been proven with little or no doubt by ToF-SIMS, Mossbauer spectroscopy, STEM imaging and EELS analysis, and by some carefully performed poisoning experiments. These four types of proofs are now discussed. 

The second proof of the presence of Fe-Nx catalytic sites and their involvement in ORR in Fe/N/C catalysts comes from recent experiments performed by Mossbauer... 

Thus, to summarize the results obtained by Mossbauer spectroscopy, it is clear from the similarity between the changes in ORR activity of these Fe/N/C catalysts illustrated in Fig. 10.21a and the changes in the amount of N-FeN2 + 2- NH C highly active sites illustrated in Fig. 10.21d in the same catalysts that Fe-Nx catalytic sites are indeed active toward ORR in these Fe/N/C catalysts. 

The most conclusive evidence to demonstrate the presence of Fe-Nx active sites for ORR in Fe/N/C catalysts would be to show the possibility of poisoning these sites with a strong ligand to the iron ion of Fe-Nx in contact with an O2 saturated electrolytic solution. In such a case, the poisoning hgand would prevent access of the site to O2 and hence suppress its subsequent reduction. CO would seem to be a good poison candidate since it is well known to poison hemoglobin in neutral pH. However, it has been shown that CO does not poison Fe-based catalysts in acid... 

Fig. 7 Fe-Nx/MPC catalyst FESEM (a, b), pore diameter distribution of the two different MPC used as precursors (c), and XPS (d) analyses. Data from [65]...

Thus, we tried to synthesize a rGO-based catalyst by preserving the characteristic planar structure of the graphene sheets, to maintain the same electric properties of the starting support, and by incorporating surface active ensembles between the sheets, to favor the catalytic activity, as in the ideal scheme of Fig. 5a. The RDE measured ORR activity of the Fe-Nx/rGO catalyst was not as good as expected with an half-wave potential of 0.58 V versus RHE, its performance was 60 mV lower compared to the Fe-Nx/MPC catalyst previously developed (Fig. 6, Table 2). Interestingly, the Nx/iGO, produced without iron, showed a very limited activity toward the ORR, 0.27 V versus RHE, much lower compared to the Fe-Nx/rGO catalyst, sign that probably also N/C ensembles show activity toward ORR. 

As a solution to provide a long-term solution to Pt cost and scarcity, a variety of non-noble metal-based catalysts has been explored as promising cathode catalysts for fuel cells. These ORR catalysts include heat-treated metal-nitrogen-carbon complexes (M-Nx/C, M = Fe or Co), carbon-supported chalcogen-ides, and carbon-supported metal oxides. These catalysts have been synthesized and showed considerable ORR activity and stability when compared to those of Pt/C catalyst. In the exploration, RDE/RRDE techniques are the most commonly employed tools in evaluating the catalysts activity and stability toward ORR and its associated mechanism. 

Fig. 10 Fe-Nx/MPC PPY catalyst TEM (a) BJH pore distribution for the catalyst pyrolyzed one or two times (b), and XPS analysis (c). Data from [72]...

C for Pt. The latter result, which has been measured by Kocha, is shown as a black circle in Figure 3.27. It should also be mentioned that the Tafel slopes at low current densities for the Fe-based and Pt catalysts are practically identical 70 mV/decade. These results indicate that the mass activity of these non-noble metal catalysts is quite good and practically at the level of the mass activity of Pt. Better activity for non-noble metal catalysts would be expected if it were possible to increase the density ofC-Nx functionalities on the carbon support. 

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