Boosting Electrochemical Oxygen Reduction Performance of Iron Phthalocyanine through Axial Coordination Sphere Interaction

Abstract

Precise regulation of the electronic states of catalytic sites through molecular engineering is highly desired to boost catalytic performance. Herein, a facile strategy was developed to synthesize efficient oxygen reduction reaction (ORR) catalysts, based on mononuclear iron phthalocyanine supported on commercially available multi‐walled carbon nanotubes that contain electron‐donating functional groups (FePc/CNT‐R, with “R” being −NH2, −OH, or −COOH). These functional groups acted as axial ligands that coordinated to the Fe site, confirmed by X‐ray photoelectron spectroscopy and synchrotron‐radiation‐based X‐ray absorption fine structure. Experimental results showed that FePc/CNT‐NH2, with the most electron‐donating −NH2 axial ligand, exhibited the highest ORR activity with a positive onset potential (Eonset=1.0 V vs. reversible hydrogen electrode) and half‐wave potential (E1/2=0.92 V). This was better than the state‐of‐the‐art Pt/C catalyst (Eonset=1.00 V and E1/2=0.85 V) under the same conditions. Overall, the functionalized FePc/CNT‐R assemblies showed enhanced ORR performance in comparison to the non‐functionalized FePc/CNT assembly. The origin of this behavior was investigated using density functional theory calculations, which demonstrated that the coordination of electron‐donating groups to FePc facilitated the adsorption and activation of oxygen. This study not only demonstrates a series of advanced ORR electrocatalysts, but also introduces a feasible strategy for the rational design of highly active electrocatalysts for other proton‐coupled electron transfer reactions.

Axial Coordination Effect on the Oxygen Reduction Reaction of FeN4 Electrocatalysts Based on Grand Canonical Density Functional Theory

“…24 25−27 In another study, Zhang et al reported molecular iron phthalocyanine supported on functionalized multiwalled carbon nanotubes (CNTs) and obtained an N−Fe−N 4 site with the axial N atom coordinated to the FeN 4 and CNTs, yielding enhanced ORR performance in comparison to the nonfunctionalized FePc/CNT and stateof-the-art Pt/C catalyst. 28 Several theoretical studies have suggested that the activity of Fe−N 4 SACs is restricted by the strong binding of reaction intermediate (e.g., OH*) on the single Fe site, and the introduction of an axial coordination effect can decrease the orbital hybridization of Fe active center with ORR intermediates, leading to accelerated ORR activity. 29−32 It is worth noting that most of the current theoretical understanding of the ORR activity was based on the charge-neutral model (CNM), which keeps constant charge and neglects the surface charge effect.…”

Controlled Modification of Axial Coordination for Transition‐Metal Single‐Atom Electrocatalyst

“…This work explored the axial coordination effect of different electron donating groups on the Fe center of FePc. The −NH 2 axial ligand is the most electron‐donating, which endowed the FePc with the highest ORR activity, including a positive onset potential of 1.0 V versus RHE and a half‐wave potential ( E 1/2 ) of 0.92 V versus RHE [31] . DFT study evidenced that the electron‐donating ligands could facilitate the adsorption and activation of O 2 .…”

“…The À NH 2 axial ligand is the most electron-donating, which endowed the FePc with the highest ORR activity, including a positive onset potential of 1.0 V versus RHE and a half-wave potential (E 1/2 ) of 0.92 V versus RHE. [31] DFT study evidenced that the electrondonating ligands could facilitate the adsorption and activation of O 2 . Inspired by the biosystems, Liu and co-workers designed an ORR electrocatalyst by covalently attaching an axial imidazolecoordinated porphyrin to MWCNTs.…”

“…Yu et al. first modified ligands such as −NH 2 , −OH and −COOH on the surface of commercial MWCNTs, which subsequently axially coordinated with the Fe active center of FePc (Figure 2c) [31] . This work explored the axial coordination effect of different electron donating groups on the Fe center of FePc.…”

“…Yu et al first modified ligands such as À NH 2 , À OH and À COOH on the surface of commercial MWCNTs, which subsequently axially coordinated with the Fe active center of FePc (Figure 2c). [31] This work explored the axial coordination effect of different electron donating groups on the Fe center of FePc. The À NH 2 axial ligand is the most electron-donating, which endowed the FePc with the highest ORR activity, including a positive onset potential of 1.0 V versus RHE and a half-wave potential (E 1/2 ) of 0.92 V versus RHE.…”