Unveiling the Axial Hydroxyl Ligand on FeN₄C Electrocatalysts and Its Impact on the pH‐Dependent Oxygen Reduction Activities and Poisoning Kinetics

Abstract

FeNC materials have shown a promising nonprecious oxygen reduction reaction (ORR) electrocatalyst yet their active site structure remains elusive. Several previous works suggest the existence of a mysterious axial ligand on the Fe center, which, however, is still unclarified. In this study, the mysterious axial ligand is identified as a hydroxyl ligand on the Fe centers and selectively promotes the ORR activities depending on different FeN4C configurations, on which the adsorption free energy of the hydroxyl ligand also differs greatly. The selective formation of hydroxyl ligand on specific FeNC configurations can resolve contradictories between previous theoretical and experimental results regarding the ORR activities and associated active configurations of FeNC catalysts. It also explains the pH‐dependent ORR activities and, moreover, a previously unreported pH‐dependent poisoning kinetics of the FeNC catalysts.

MnN4 Oxygen Reduction Electrocatalyst: Operando Investigation of Active Sites and High Performance in Zinc–Air Battery

“…To further confirm the role of MnN 4 sites, the SCN − poison experiment was also carried out to block the MnN sites. [ 43 ] The introduction of SCN − into the ORR system of Mn‐SAS/CN caused an obvious negative shift of the half‐wave potential by 90 mV and notably reduced diffusion limiting current density (≈84% retention), while no obvious change was found in as‐prepared porous CN, indicating that the MnN 4 sites constitute the key catalytically active centers of ORR (Figure 2f). In addition, the stability of Mn‐SAS/CN during ORR was tested by CV (Figure 2e), and the characterizations including XRD, transmission electron microscopy (TEM), and AC HAADF‐STEM images were taken for the Mn‐SAS/CN after 5000 cycles.…”

Transition Metal and Nitrogen Co‐Doped Carbon‐based Electrocatalysts for the Oxygen Reduction Reaction: From Active Site Insights to the Rational Design of Precursors and Structures

“…In particular, unlike in acidic media, the ORR performance in alkaline media finally recovered to the original level of the continuous linear sweep voltammetry (LSV), which was ascribed to the dissociation of SCN − in 0.1 m KOH solution [38] . It is worth noting that recent studies have revealed that the competitive adsorption between the hydroxyl ligand and poisoning anions leads to new pH‐dependent poisoning kinetics of Fe−N−C catalysts [22] . It was observed that the ORR activity declined obviously in acidic solution, but there did not alter significantly in alkaline solution.…”

“…[38] It is worth noting that recent studies have revealed that the competitive adsorption between the hydroxyl ligand and poisoning anions leads to new pHdependent poisoning kinetics of FeÀ NÀ C catalysts. [22] It was observed that the ORR activity declined obviously in acidic solution, but there did not alter significantly in alkaline solution. This interesting result indicates significantly various poisoning mechanisms at high or low pH, which is mainly caused by the competitive adsorption between OH ligand and poisoning anions, especially in alkaline solution, which can be evidenced by the electrochemical experiments and theoretical calculations.…”

“…This work promoted a deeper understanding of the Fe−N 4 site and inspired further exploration of the OH ligands. Recently, detailed research has revealed the pivotal role of the axial OH ligands in the ORR process [22] . Five Fe−N 4 −C structures, including Fe−N 4 −C 10 (generally denoted as D1), Fe−N 4 −C 12 (D2), Fe−N 4 −C 8 (D3), and two other edge Fe−N 4 −C structures (armchair and zigzag, denoted as ZZ‐edge and AM‐edge) were constructed as the theoretical models to investigate the correlation between the structure and activity (Figure 2a–d).…”

“…Recently, detailed research has revealed the pivotal role of the axial OH ligands in the ORR process. [22] Five FeÀ N 4 À C structures, including FeÀ N 4 À C 10 (generally denoted as D1), FeÀ N 4 À C 12 (D2), FeÀ N 4 À C 8 (D3), and two other edge FeÀ N 4 À C structures (armchair and zigzag, denoted as ZZ-edge and AMedge) were constructed as the theoretical models to investigate the correlation between the structure and activity (Figure 2a-d). DFT results indicated that, except for D2, all FeÀ N 4 structures exhibited enhanced ORR performance after introducing OH Figure 1.…”