Macrocyclic M-N4 complexes

Table 1 Summary of experimental measurements on the activity of M-N4 macrocyclic complexes for ORR... 

The type of the central metal atom is a dominant factor influencing the catalytic activity of the M-N4 macrocyclic complexes for ORR. At early time, it had been known that the ORR activity in acid electrolyte would decrease in the order of Fe > Co > Ni > Cu > Mn for the MPc, while in the order of Co > Mn > Fe > Ni > Cu for the MTAA macrocycle complexes [27, 47]. Figure 2 shows an example electrochemical measurement result systematically examining the ORR activity on various metal tetra-sulfonated-phthalocyanines (MTsPc) in alkaline electrolytes. It can be observed in Fig. 2 that the ORR activity decreases in the order of Fe > Mn > Co > Cr > Ni Zn > Cu for the MTsPc macrocycles. Importantly, the results in Fig. 2b revealed a correlation between the observed ORR activity and the number of the d electrons in the central transition metal of the M-N4 macrocycles. It appears that the MTsPc macrocycles could possess superior ORR catalytic activity when their central transition metal atoms, such as Fe, Mn, and Co, have nearly half-filled d orbitals. Such a correlation had also been confirmed for the ORR on the MPc macrocycles in alkaline medium, namely the ORR activity of the MPc were found to decrease following the sequence of Fe > Co > Mn, Pd, Pt > Zn [79]. It notes that Mn-N4 macrocycle complexes exhibit low stability in both acid... 

More importandy, extensive studies have shown that the central transition metals would dictate the oxygen reduction pathways on these M-N4 macrocyclic complexes. It has been found that most C0-N4 macrocyclic complexes promote only 2e reduction of oxygen to produce hydrogen peroxide as final products [18, 50, 81, 92, 93]. In contrast, Fe-N4 macrocyclic complexes are found to likely promote 4e reduction of oxygen to produce water with cleavage of 0-0 bond [18, 50, 81, 92, 93]. 

It has been reported that polymerization of M-N4 macrocyclic complexes could enhance the catalytic activity for ORR [87, 92, 94]. It is believed that the... 

High specific area carbon is usually used as support materials for M-N4 macrocyclic complexes due to its large surface area, high conductivity, and beneficial surface groups [81, 94]. Pretreatment on the carhon support materials affects the catalytic activity of the M-N4 macrocycles on the supports. It was found that the carbon substrates covered with basic surface groups helped the M-N4 macrocycles to have better ORR catclytic performance than the substrates covered with neutral or acidic surface groups [92]. 

On M-N4 macrocyclic complexes, oxygen could be completely reduced to water through a 4-electron pathway or be incompletely reduced through a 2-electron pathway, producing hydrogen peroxide. 

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. 

Table 2 Summary of processing procedures on the synthesis of heat-treated (pyrolyzed) M-N4 macrocyclic complexes as ORR electrocatalysts... 

Two fashions of stable O2 adsorption on M-N4 macrocyclic complexes (as shown in Fig. 10) have been predicted from DFT calculations. Figure 10a, b show the end-on adsorption configurations in which one oxygen atom in the O2 molecule is adsorbed on the top of central metal atom while the other one tilted away. Figure 12c, d show the side-on adsorption configurations in which the axis of the O2 molecule is parallel to the molecule plane of the M-N4 macrocyclic complexes and the distances between the central metal atom and the two oxygen atoms in the O2 molecule are equal. 

Analysis of the molecular orbitals obtained by the DFT calculations reveals that these two types of O2 adsorption correspond to different bonding formed between the adsorbed O2 molecule and the central metal atom of the M-N4 macrocyclic complexes. 

Fig. 10 Four possible configurations of O2 adsorption on M-N4 macrocyclic complexes, a and b are end-on configurations, c and d are side-on configurations...

Furthermore, the DFT study revealed that the adsorption energy of O2 molecule on M-N4 macrocycUc complexes were related to the energy level of the nonbonded 3(i-orbitals and their occupation numbers of the central metal atom. In Table 3, we listed the calculated adsorption energy and structural properties of O2 adsorption on MnPc, FePc, and CoPc [169]. Liu et al. have established a linear correlation between the O2 adsorption energy and the nonbonding /-orbital center of metal-loporphyrins and phthalocyanines macrocyclic complexes [169]. It was found that for the M-N4 macrocyclic complexes with central metal atom varying from Cr to Ni, the interaction between the M-N4 macrocyclic complexes and the adsorbed O2 molecule became weaker as the number of electrons on the d-orbitals of the metal atom increases. 

The first-principles DFT calculations of ORR on various M-N4 macrocyclic complexes have been carried out by several research groups [93, 168-173]. Through comparative study of O2 dissociation on different metalloporphyrins (MnP, FeP, CoP, NiP), the trends of the activation barriers for the O2 dissociation with respect to LUMO-HOMO characters of these metalloporphyrins have been discussed by Tsuda et al. [172]. FeP is demonstrated to be the best one due to the large d electrons contribution to the LUMO-HOMO level of the FeP and the stable Fe-O bond [172]. Shi and Zhang performed the DFT calculation on the O2 adsorption on various iron and cobalt porphyrins and phthalocyanines [171]. The catalytic activities of the transition metal macrocyclic complexes were positively related with... 

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