Axial Ligand Coordination Tuning of the Electrocatalytic Activity of Iron Porphyrin Electrografted onto Carbon Nanotubes for the Oxygen Reduction Reaction

Abstract

The oxygen reduction reaction (ORR) is essential in many life processes and energy conversion systems. It is desirable to design transition metal molecular catalysts inspired by enzymatic oxygen activation/reduction processes as an alternative to noble‐metal‐Pt‐based ORR electrocatalysts, especially in view point of fuel cell commercialization. We have fabricated bio‐inspired molecular catalysts electrografted onto multiwalled carbon nanotubes (MWCNTs) in which 5,10,15,20‐tetra(pentafluorophenyl) iron porphyrin (iron porphyrin FeF20TPP) is coordinated with covalently electrografted axial ligands varying from thiophene to imidazole on the MWCNTs’ surface. The catalysts’ electrocatalytic activity varied with the axial coordination environment (i. e., S‐thiophene, N‐imidazole, and O‐carboxylate); the imidazole‐coordinated catalyst MWCNTs‐Im‐FeF20TPP exhibited the highest ORR activity among the prepared catalysts. When MWCNT‐Im‐FeF20TPP was loaded onto the cathode of a zinc−air battery, an open‐cell voltage (OCV) of 1.35 V and a maximum power density (Pmax) of 110 mW cm−2 were achieved; this was higher than those of MWCNTs‐Thi‐FeF20TPP (OCV=1.30 V, Pmax=100 mW cm−2) and MWCNTs‐Ox‐FeF20TPP (OCV=1.28 V, Pmax=86 mW cm−2) and comparable with a commercial Pt/C catalyst (OCV=1.45 V, Pmax=120 mW cm−2) under similar experimental conditions. This study provides a time‐saving method to prepare covalently immobilized molecular electrocatalysts on carbon‐based materials with structure–performance correlation that is also applicable to the design of other electrografted catalysts for energy conversion.

In Situ Imaging and Computational Modeling Reveal That Thiophene Complexation with Co(II)porphyrin/Graphite Is Highly Cooperative

“…Electron paramagnetic resonance spectroscopy confirmed a 5-coordinate thiophene CoTTP complex at low temperature in toluene solution . Fe­(II)­tetra­(pentafluorophenyl) porphyrin electrografted onto carbon nanotubes and axially coordinated with thiophene produced an increased rate of oxygen reduction compared with the porphyrin on nanotubes alone …”

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

“…Liu et al constructed a molecular catalyst by modifying iron porphyrin on the surface of multiwall carbon nanotubes (MWCNTs) through axial coordination. [28] They studied the effect of various ligands including S-thiophene, N-imidazole, and Ocarboxylate, of which N-imidazole proved to be the best axial ligand for the composite catalyst with efficient ORR activity. Figure 2a shows the configuration of the N-imidazole-coordinated catalyst.…”

Engineering the Electronic Structure of Active Centers in Metalloporphyrins to Boost Oxygen Reduction Reaction Activity

“…With this feature taken into consideration, people have paid more attentions to engineering the coordination configuration of the M-N 4 active sites in metallomacrocycles for achieving higher activity. [30][31][32][33][34][35][36][37] For instance, a combined collaborative effect has been achieved between the pyridine axial ligand on carbon substrate surface and the peripheral chlorine substituents of iron phthalocyanine in a 16(Cl)FePc-py-SWCNTs composite, which in turn leaded to an enhanced ORR activity. [38] Moreover, the axial coordination of 2-methylimidazole ligands toward the hemin Fe (III) sites in a hemin-modified UiO-66 MOF film was found to greatly accelerate the charge transfer rate and boost the intrinsic ORR performance.…”

“…Since these ligands are tethered directly with the metal active centers, they play an important role in regulating the overall reactivity. With this feature taken into consideration, people have paid more attentions to engineering the coordination configuration of the M‐N 4 active sites in metallomacrocycles for achieving higher activity [30–37] . For instance, a combined collaborative effect has been achieved between the pyridine axial ligand on carbon substrate surface and the peripheral chlorine substituents of iron phthalocyanine in a 16(Cl)FePc‐py‐SWCNTs composite, which in turn leaded to an enhanced ORR activity [38] .…”