Macrocycle carbon-supported

The possible complete replacement of Pt or Pt alloy catalysts employed in PEFC cathodes by alternatives, which do not require any precious metal, is an appropriate final topic for this section. Some nonprecious metal ORR electrocatalysts, for example, carbon-supported macrocyclics of the type FeTMPP or CoTMPP [92], or even carbon-supported iron complexes derived from iron acetate and ammonia [93], have been examined as alternative cathode catalysts for PEFCs. However, their specific ORR activity in the best cases is significantly lower than that of Pt catalysts in the acidic PFSA medium [93], Their longterm stability also seems to be significantly inferior to that of Pt electrocatalysts in the PFSA electrolyte environment [92], As explained in Sect. 8.3.5.1, the key barrier to compensation of low specific catalytic activity of inexpensive catalysts by a much higher catalyst loading, is the limited mass and/or charge transport rate through composite catalyst layers thicker than 10 pm. 

Based on X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), electron spin resonance (ESR), Mbssbauer, and extended X-ray absorption fine structure spectroscopy (EXAFS) , van Veen and collaborators concluded that the thermal treatment at temperatures where catalytic activity is maximum ( 500-600°C) does not lead to complete destruction of the macrocycles, but rather to a ligand modification which preserves the Me-N4 moiety intact. Furthermore, the stability of this catalytic site is improved because the reactive parts of the ligands are bound to the carbon support and thus are no longer susceptible to an oxidative attack. Thermal treatments at higher temperatures (up to 850°C) led to some decomposition of the Me-N4 moiety, and thus to a decrease of the catalytic activity, and to the reduction of some of the ions to their metallic state. 

EXAFS confirmed that unpyrolyzed ClFeTMPP adsorbed intact on Vulcan and was still intact up to 325°C. After pyrolysis at 700°C (the optimum temperature for catalytic activity) the first shell of atoms around the Fe ion was still Fe-N4, but the interatomic distance contracted and the Fe ion was now in the N4 plane (Fe ion was above that plane for the intact molecule). The Mossbauer spectrum of samples heat treated at 800°C revealed the formation of magnetic iron oxides that could be chemically leached out. The leached material yielded a spectrum similar to that recorded at 700°C and attributed to the Fe-N4 moiety. Van Veen and collaborators proposed a reaction scheme of what they believed happened to the ClFeTMPP chelates upon pyrolysis. This scheme is illustrated in Figure 3.5. However, they warned the reader that, in Figure 3.5, while the overall structure of the iron catalytic site remained close to the original Fe-N4 moiety, there must be substantial local variation in how this unit is attached to the subjacent carbon support. This study is in agreement with an earlier study by McBreen and collaborators", who analyzed 9 wt% CoTMPP and FeTMPP chelates (about 0.7 wt% metal) adsorbed on Vulcan XC-72 and heat treated in inert atmosphere between 600 and 1,000°C. They found that the heat treatment of the macrocycles... 

Figure 3.7. Schematic representation of an active Co-based dimer proposed by Savy and collaborators and obtained after heat treatment at 500°C, in inert atmosphere, of Co naphthophthalocyanine adsorbed on a carbon support. The remaining portions of the macrocycle structures lie in the planes represented by the two lines parallel to the support, (according to Figure 8b in ref. [50] reproduced with permission of Elsevier).

Today, we know that it is possible to produce these Fe- and/or Co-based electrocatalysts by adsorbing related metal-N4 macrocycles on a carbon support and heat-treating this material at about 600°C, the optimum temperature in terms of activity. More stable, but less active catalysts are, however, obtained for heat-treatment temperatures > 800 C. Similar catalysts may also be obtained with cheaper metal and nitrogen precursors (like metal salts and ammonia, for instance). For aU these catalysts, it is generally now believed that two types of catalytic sites are obtained simultaneously, but not in the same proportions. 

Vallejos-Burgos et al. studied the stmcture and activity of a free-base phthalocya-nine, Cu phthalocyanine, and Co phthalocyanine after heat-treating the compounds at different temperatures [39]. The stmctural changes that occurred during the heat treatment are summarized in Fig. 8.4. It is clear from that study that the stmctural evolution of the macrocycle and carbon support associated with a loss of nitrogen or changes in the microporosity of the support depend on the metal center. 

The real structure, or structures, of heat-treated ORR catalysts is yet to be revealed. The catalyst makeup is likely to strongly depend on the precursors used, with the structure of catalysts derived from different macrocycle compounds differing from one another due to such factors as the type of a macrocycle, substituent, carbon support, and the metal center. This is an intriguing subject of future research that promises to produce non-precious metal ORR catalysts with much improved activity and performance durability. 

Me-N4 macrocyclic complexes (with N4-macrocycles being, for instance, tetraazaannulene [TAA], tetramethoxypheny 1-porphyrin [TMPP], phthalocyanine [Pc]), and Me = Co or Fe were the first to be used as precursors of the catalytic sites. In particular, unsupported C0-N4 chelates (CoTAA, CoTMPP, and CoPc) or C0-N4 chelates adsorbed on a carbon support were used in the 1970s as ORR electrocatalysts [11]. However, both unsupported and carbon-supported C0-N4 chelate electrocatalysts underwent a rapid decline in the activity. An important discovery in these early years was that catalyst stability as well as activity toward ORR could be improved by subjecting C0-N4 chelate/carbon samples to thermal treatment in an inert gas like N2 or Ar [12]. 

Non-precious metal catalyst research covers a broad range of materials. The most promising catalysts investigated thus far are carbon-supported M-N /C materials (M = Co, Fe, Ni, Mn, etc.) formed by pyrolysis of a variety of metal, nitrogen, and carbon precursor materials [106]. Other non-precious metal electrocatalyst materials investigated include non-pyrolyzed transition metal macrocycles [107-122], coti-ductive polymer-based complexes (pyrolyzed and non-pyrolyzed) [123-140], transition metal chalcogenides [141-148], metal oxide/carbide/nitride materials [149-166], as well as carbon-based materials [167-179]. The advances of these types of materials can be found in Chaps. 7-10 and 12-15 of this book. 

Since extensive research has been carried out in the last five decades to understand ORR mechanisms on various N4-macrocyclic metal complexes with and without heat treatment in both acid and alkaline solutions, here we will not repeat what has been reviewed by other experts in the field but only focus on the application aspects of carbon-supported N4-macrocyclic metal complexes (M-N4/C) in alkaline conditions and for AEMFCs. Here, the ORR activity on M-N4/C catalysts... 

Chapters 7-12 focus on the electrocatalysis of carbon-based non-precious metal catalysts. The unique properties and fuel cell applications of various carbon based catalysts are intensively discussed in these chapters. Chapter 7 summarizes the fundamental studies on the electrocatalytic properties of metallomacrocyclic and other non-macrocyclic complexes. Chapter 8 and 9 review the progress made in the past 5 years of pyrolyzed carbon-supported nitrogen-coordinated transition metal complexes. Chapter 10 gives a comprehensive discussion on the role of transitional metals in the ORR electrocatalysts in acidic medium. Chapter 11 introduces modeling tools such as density functional theory (DPT) and ah initio molecular dynamics (AIMD) simulation for chemical reaction studies. It also presents a theoretical point of view of the ORR mechanisms on Pt-based catalysts, non-Pt metal catalysts, and non-precious metal catalysts. Chapter 12 presents an overview on recent progresses in the development of carbon-based metal-free ORR electrocatalysts, as well as the correlation between catalyst structure and their activities. 

In 1964, Jasinski found out that different metallophthalocyanines were able to reduce oxygen in alkaline media [18]. Soon after, this finding was expanded to a variety of MeN4 macrocycles that were applied for the oxygen reduction at low and high pH values. In 1976, Jahnke et al. published an improvement of the ORR activity and stability when carbon-supported complexes were heat treated in inert atmosphere at temperatures ranging from 400 °C to 1,000 °C [19]. 

Fmthermore, it has been foimd that heat treatment processing to the mixmre of metal macrocyclic molecules and carbon materials can improve both the activity and durability of the attained M-N4/C electrocatalysts. Jahnke et al. [27] showed that heat treatment had a significant effect on the catalytic activity of carbon-supported CoTAA (colbalt-dihydrodibenotetraazaannulene) for ORR. They found that all the heat-treated CoTAA samples had better catalytic performance than those untreated... 

ORRs on Carbon-Supported Heat-Treated M-N4 Macrocyclic Molecules... 

Jahnke et al. reported that heat treatment processing could not only improve the catalytic activity of CoTAA for ORR but also enhance the stability of the catalysts in an electrochemical environment [27]. Ever since, the heat treatment of M-N4 macrocyclic complexes on carbon support in inert gas has been employed as an effective method to promote the ORR catalytic activity of the M-N4 macrocycles... 

Figure 6 shows the typical morphology of the attained carbon-supported heat-treated (pyrolyzed) M-N4 macrocyclic molecules. We summarize processing procedures and parameters on the synthesis of heat-treated (pyrolyzed) M-N4 macrocyclic complexes as ORR electrocatalysts in Table 2. It has been recognized that the heat treatment temperature, atmosphere, and duration are important parameters determining the final catalytic activity of the attained pyrolyzed M-N4 macrocyclic complexes. 

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