Metal/N/C catalysts

The objective of most research in the area of pyrolyzed metal/N/C materials has centered around understanding the nature of the active site for the ORR. Similar to heat-treated macrocycles, there has been a parallel controversy over the nature of the active sites and the role of Fe or Co in these metal-nitrogen-carbon catalysts. Based on the activity attainable from a wide-range of precursors, it seems safe to assume that above a certain temperature, the active site formed is the same regardless of the metal-nitrogen-carbon starting material (macrocycle or otherwise). Initially, some researchers believed that the metal clusters protected by a layer of carbon (which prevented leaching of the metal in the acidic electrolyte) were the source of catalytic... 

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 controversy was further igniting when it became clear that instead of the macrocycles, much simpler molecules might be used as precursors [23, 24]. Nowadays, we understand that for the preparation of a Me-N-C catalyst, one might mix nearly any kind of metal, nitrogen, and carbon source after the heat treatment at appropriate temperatures, the material will have some activity for the ORR. Some metal, nitrogen, and carbon sources that were successfully applied for the preparation of highly active Me-N-C are summarized in Fig. 1. 

Fuel Cells, Non-Precious Metal Catalysts for Oxygen Reduction Reaction, Fig. 4 (a) Polarization curves of several Me-N-C catalysts all examined under the same experimental conditions (1.5 bar H2/O2, 1 mg cm see Ref. [12] et al. for details), (b) today s most active NPMC catalysts (p-Fe-N-C) as prepared by the pore-filling method (PFM) described in Lefevre et al. [44] and by the method described by Proietti et al. [10]. The values... 

Another interesting approach taken to stmcturaUy control M-N-C catalysts was first pioneered by Ma et al. [69] and involves metal-organic framework (MOF)-derived catalysts. In this work they heat-treated an in-house prepared cobalt imi-dazolate MOP to prepare a catalyst that showed promising half-cell electrochemical activity toward oxygen reduction. Proietti et al. [ 14] advanced on this work, instead... 

As an outlook to further improvements of catalyst kinetics and durability in low-and high-temperature polymer electrolyte fuel cells, several possibilities are currently under investigation [73] (1) extended large-scale Pt and Pt-alloy surfaces [70] (2) extended nanostructured Pt and Pt-aUoy films [74] (3) de-alloyed Pt-alloy nanoparticles [75] (4) precious metal free catalyst as described by Lefevre et al. [76], e.g., Fe/N/C catalysts and (5) additives to the electrolyte which modify both adsorption properties of anions and spectator species and also the solubility of oxygen [77]. The latter approach is specific to fuel cells using phosphoric acid as electrolyte. 

Number of active sites per volume of catalyst. For the calculation of site densities of Me-N-C catalysts, often two assumptions have to be made (1) the density is similar to other carbon-based catalysts (0.4 g/cm ) and (2) each metal atom is related to an active site. In some cases, authors determine the exact mass density and/or number of active sites, so that the value becomes more accurate. In all other cases, it is usually overestimated as not all metal atoms are associated with active sites. 

In 1989, Gupta and coauthors showed that highly active catalysts can be prepared from less complex molecules [63]. In their work, they impregnated a carbon black with polyacrylonitril (PAN) and an iron or cobalt acetate. The precursors were heat-treated at different temperatures and the ORR activity was measured. It was found that one can generalize the preparation of Me-N-C catalysts Whenever a metal precursor is heat-treated with nitrogen and carbon sources at temperatures of >600°C (Co) or >700°C (Fe), an active catalyst can be obtained. A scheme of their preparation route and the achieved ORR activities (as a function of pyrolysis temperature) are given in Fig. 16.18. 

It is beheved that the search for alternative supports for Fe-N-C catalysts will become dominant in the future. Especially, different transition metal oxynitrides (TiOxNy, TaOxNy, ZrO Ny), and Nb- or Sb-doped Ti02 or Sn02 or PANI appear to be promising in this context [55, 224—227]. It will remain a major challenge to enable the implementation of molecular centers onto the surface of the new support material. It has already been pointed out that PANI possesses a considerable potential with this respect [189]. Since none of the other promising support materials contain carbon, it is questionable how catalytically active Fe-N-C catalysts could be implemented. 

---