Molecular and Electronic Structures of Transition-Metal Macrocyclic Complexes as Related to Catalyzing Oxygen Reduction Reactions: A Density Functional Theory Study

Abstract

Transition-metal (TM) macrocyclic complexes have potential applications as nonprecious electrocatalysts in polymer electrolyte membrane fuel cells. In this study, we employed density functional theory calculation methods to predict the molecular and electronic structures of O2, OH, and H2O2 molecules adsorbed on TM porphyrins, TM tetraphenylporphyrins, TM phthalocyanines, TM fluorinated phthalocyanines, and TM chlorinated phthalocyanines (here TM = Fe or Co). Relevant to their performance on catalyzing oxygen reduction reaction (ORR), we found for the studied TM macrocyclic complexes: (1) The type of the central TM is the most determinant factor in influencing the adsorption energies of O2, OH, and H2O2 (chemical species involved in ORR) molecules on these macrocyclic complexes. Specifically, the calculated adsorption energies of O2, OH, and H2O2 on the Fe macrocyclic complexes are always distinguishably lower than those on the Co macrocyclic complexes. (2) The peripheral ligands are capable of modulating the binding strength among the adsorbed O2, OH, and H2O2, and the TM macrocyclic complexes. (3) A N–TM–N cluster structure (like N–Fe–N) with a proper distance between the two ending N atoms and a strong electronic interaction among the three atoms is required to break the O–O bond and thus promote the efficient four-electron pathway of the ORR on the TM macrocyclic complexes.

Systematic study of transition-metal (Fe, Co, Ni, Cu) phthalocyanines as electrocatalysts for oxygen reduction and their evaluation by DFT

“…The adsorption energy between catalyst and adsorbate molecule is one of the methods to evaluate whether the adsorbate is energetically favourable to be adducted to the surface of the molecular catalyst. 16 Negative adsorption energy indicates that the adsorption reaction is energetically favourable to happen. Besides, lower adsorption energy means stronger bonding energy between catalyst and adsorbate.…”

“…14,19 It is widely believed that the catalytic site is the central transition-metal atom. 16 Since TMPc molecules (Fig. 6a) only owe one catalytic site, the competing adsorption of TMPc with H 2 O (Fig.…”

“…13,14 Particularly, the type of the central transition-metal (TM) atom is the most determinant factor in influencing catalytic activity. [15][16][17][18] It has been reported that iron or cobalt incorporated macrocycle exhibits high measurable levels of catalytic activity for ORR via an approximated 4-electron transfer or 2-electron pathway and the positive ORR onset potential by the experimental and theoretical methods. [19][20][21][22] Considering the ORR performance of metal macrocycle compounds is related with the highest occupied molecular orbital (HOMO) of metal macrocycle compounds.…”

Structure-activity relationship in high-performance iron-based electrocatalysts for oxygen reduction reaction

“…Interestingly, as shown in Table 1, the calculation shows, on pristine C surface, the distance between adsorbed O 2 and pristine C was about 2.8 Å and the D OeO was 1.238 Å, with the Wiberg Bond Index (WBI, which reflects the strength of covalent OeO bond) value of 1.856 and adsorption energy (E ad ) 1.485 eV, all indicating very weak interaction between adsorbed O 2 and pristine C surface or weak electrocatalytic activity of pristine C for ORR [33,34]. While the single doping of Fe 4 or N on carbon can enhance the ORR activity indicated by the decreased D OeC , WBI and E ad or the increased D OeO as shown in Table 1, confirming the observation shown in Fig.…”

The role of trace Fe in Fe–N-doped amorphous carbon with excellent electrocatalytic performance for oxygen reduction reaction

“…Therefore, we believe that by adding FeAc(II) into d N -C-700 enriched with pyridinic N (9.4 at.%) and then annealing at According to the reports that d N -C enriched with the pyridinic N can be used to easily synthesize d Fe-N -C catalyst with a high electrocatalytic performance [22], and that the adding of some Fe salts into the d N -C will offer efficient coordination between pyridinic N and Fe to form high active FeN 4 site [23]. This means that only the formation of FeN 4 site between Fe and pyridinic N atoms can exhibit a high electrocatalytic performance [24][25][26]. Although d Fe-N -C annealed at 600 o C has a high pyridinic N content of 9.0 at.%, its catalytic activity is not high (Table S1), which may be due to that at 600 o C as the pyrrolic N exists, it is not so Table S1.…”