Chelate electrocatalysts

Some Ligands Used in Preparation of Chelate Electrocatalysts... 

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]. 

Beck 23,37,38) carried out voltammetric investigations of chelates in solution (phthalocyanine and tetra-aza-annulene in 85—90% H2SO4). The particularly active electrocatalysts (FePc and CoTAA) showed very positive redox potentials and a pronounced positivization of the polarographic oxygen step in 85 % sulfuric acid. On the basis of these results, Beck proposed the following mechanism... 

All the different N4 chelates, however, cannot be used as electrocatalysts in fuel cells as they are oxidized and decomposed under operation conditions in a couple of hours or days. 

Pyrolyzed N4 chelates and nitrogen-containing polymers on porous carbons and soots might become an option as electrocatalysts for 02... 

Re diimine complexes act as photocatalysts and/or electrocatalysts for CO2 reduction to CO. Examples include the tricarbonyl complexes yac-[Re(Q -diimine)(CO)3L]" [n = 0, L = halide n = 1, L = NCMe, P(OR)3 a-diimine = 1,4-disubstituted 1,4-diazabuta-l,3-dienes or bpy and related chelating N-heterocycles], for example, fac-[Re(dmb)(CO)3(NCMe)]+, 5 [Re(dmb)(CO)3]2" and fac-[Re(bpy)(CO)3 P(OPfl)3 ]+. Electron-transfer from an amine electron donor (e g. triethanolamine or triethylamine) to the excited state complex is usually considered as the initiation of the photocatalysis, and metallocarboxylates and metallo-carboxyUc acids have been proposed as intermediates in the formation of CO. The electrocatalytic process is triggered by a 1-electron or a 2-electron cathodically induced chloride (X) or L ligand dissociation to form the catalytic species. ... 

Systematic studies of the role of such factors as the nature of the metal center and the detailed structure of the chelating ring, particularly its peripheral functionalization, can afford valuable information toward unveiling structure-activity relationships for macrocycles as electrocatalysts for oxygen reduction. The following sub-sections describe some of the most salient aspects of a selected number of transition metal phthalocyanines and porphyrins, including the effects of redox and non-redox active substituents on the properties of Co porphyrins. 

As far as the kinetics and mechanistic aspects of oxygen reduction on these non-noble metal electrocatalysts are concerned, it has been shown that these catalysts may reduce O2 to water with an apparent number of electrons transferred, n, that may reach values very close to 4. This is especially true for Fe-based electrocatalysts made either from Fe-N4 chelates or from cheaper Fe salt precursors. It seems also that the Fe-N2/C catalytic site, which is the most active site in catalysts obtained after a pyrolysis temperature > 800°C, is characterized by a low release of peroxide. Co-based catalysts release, on average, more peroxide than the corresponding Fe-based materials. Studies that were undertaken to decouple the direct 4-electron reduction of oxygen to water from the successive 2 X 2-electron reduction indicate that the direct 4-electron reduction path may be important for these catalysts. This result is in agreement with the quantum theoretical approach of Anderson and Sidik about a model of the pyrolyzed... 

Wiesener, K. N4-chelates as electrocatalysts for cathodic oxygen reduction (1986). Electrochim. Acta 31, 1073-1078. 

Wiesener, K. (1982). Electrocatalysts based on N4 organic chelates for the cathodic reduction of oxygen in acidic solutions. Translated from lektrokhimiyia 18, 758-765. 

It has been demonstrated that a pyrolysis step is necessary and critical in improving both the activity and the stability of Fe- and Co-Nx ORR electrocatalysts, van Veen et al. discussed four models in an effort to explain this pyrolysis effect (1) improving the dispersion of the supported chelate (2) catalyzing the formation of a special type of carbon, which is actually the active phase (3) generating the M-Nx species and (4) promoting a reaction between chelate and subjacent carbon in such a way as to modify the electronic structure of the central metal ion with retention of its N4 coordinated environment. Actually, the active sites should be M—N4 or M—N2 units, depending on the heat treatment temperature (see Figure 3.8). 

Wiesener K (1986) N4-chelates as electrocatalyst for cathodic oxygen reduction. Electrochim Acta 31 1073-1078... 

Within the various NNM electrocatalysts, the most promising ones seem to be heat-treated Fe(II) and/or Co(ll) chelates and macrocycles supported on carbon particles [12, 40-56]. The formation of metal-nitrogen (M-Nx/C) and metal-carbon (M/C) active ensembles after the heat treatment is necessary for ORR (Fig. 4), as emphasized by the groups of Yeager [48-50], Zelenay [12, 52, 53] and Dodelet [44, 51, 52, 57]. 

Ait H, Binder H, Lindner W, Sandstede G (1971) Metal chelates as electrocatalysts for oxygen reduction in acid electrolytes. J Electroanal Chem 3LA19-A22... 

A similar class of alternative electrocatalysts for fuel cell applications is based on iron as core atom of the chelate centers. The pyrolysis of ClFeTMPP... 

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