No abstract

Although the planar square Fe-N 4 is considered to be the basic unit of the active Fe-N 4 -based moieties, the exact local structure of such moieties has not yet been determined due to that the axial ligands and the second coordination sphere (i.e., the surrounding carbon matrix) of Fe-N 4 are unclear. [8] Based on the computational hydrogen electrode (CHE) model, the theoretical ORR onset potentials of Fe-N 4 in different models were as low as 0.25-0.43 V versus RHE (V RHE ), [9,10] much lower than the experimentally measured ones on Fe-N-C (0.82-0.95 V RHE ). Besides, some new active moieties in Fe-N-C (e.g., FeON 4 , Fe(OH)N 4 , and FeN 4 Cl) were proposed by using in situ characterizations or CHE modeling, [11][12][13][14] revealing that the axialligand coordination played a critical role in high-activity Fe-N 4 -based moieties. However, these studies were carried out on Fe-N-C that contained not only the Fe-N 4 sites but also a large number of other ORR active sites (such as the N-doped carbon and intrinsic defects of carbon), in which the implemented modifications would affect the ORR activities of different active sites and sometimes even alter the structure of the carbon matrix. [15] That is, the observed ORR activity improvement of the Fe-N-C after such modifications could not be solely attributed to the enhancement of the intrinsic ORR activity of Fe-N 4 and thus the mechanism associated with the improvement is still ambiguous. To this end, developing a model catalyst as a studying platform is highly demanded to reveal the regulation mechanism of axial-ligand coordination on the catalytic activity of Fe-N 4 sites.Iron porphyrin (FePr) and iron phthalocyanine (FePc) are widely used molecular model catalysts with Fe-N 4 as the only kind of ORR active sites. [16] Although some modified FePc and FePr with axially coordinated Fe-N 4 moieties have been reported in recent papers, [17][18][19] only a few strong ligands (e.g., CN − and pyridine) could perform such axial coordination due to that the FePc and FePr are more inclined to exist as the D4h symmetry structure with highly concentrated local electrons. [17] In order to systematically study the correlation between the axial ligands and the catalytic activity of Fe-N 4 sites, it is necessary to explore a Fe-N 4 -based molecular model catalyst with a stronger axial coordination ability. Poly(phthalocyanine iron) (PFePc) is an alkali-soluble Identifying the actual structure and tuning the catalytic activity of Fe-N 4based moieties, well-recognized high-activity sites in the oxygen reduction reaction (ORR) are challenging problems. Herein, by using poly(iron phthalocyanine) (PFePc) as an Fe-N 4 -based model electrocatalyst, a mechanistic insight into the effect of axial ligands on the ORR catalytic activity of Fe-N 4 is provided and it is revealed that the ORR activity of Fe-N 4 sites with OH desorption as a rate-determining step is related to the energy level gap between the OH p x p y and Fe 3d z 2 , which can be tuned by regulating the field strength of the...

The recent progress on the fabrication of two-dimensional metal–organic frameworks and their derivatives as well as their applications in electrochemical energy storage and electrocatalysis are reviewed.

Atomically dispersed iron doped-MOF-derived carbon with high iron loading and nitrogen content for the oxygen reduction reaction via a cage-confinement strategy shows excellent catalytic performance.