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

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 FeF20 TPP) 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-FeF20 TPP exhibited the highest ORR activity among the prepared catalysts. When MWCNT-Im-FeF20 TPP 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-FeF20 TPP (OCV=1.30 V, Pmax =100 mW cm-2 ) and MWCNTs-Ox-FeF20 TPP (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.

Design and Preparation of Fe-N5 Catalytic Sites in Single-Atom Catalysts for Enhancing the Oxygen Reduction Reaction in Fuel Cells.

There is an urgent need for developing nonprecious metal catalysts to replace Pt-based electrocatalysts for oxygen reduction reaction (ORR) in fuel cells. Atomically dispersed M-Nx/C catalysts have shown promising ORR activity; however, enhancing their performance through modulating their active site structure is still a challenge. In this study, a simple approach was proposed for preparing atomically dispersed iron catalysts embedded in nitrogen- and fluorine-doped porous carbon materials with five-coordinated Fe-N5 sites. The C@PVI-(DFTPP)Fe-800 catalyst, obtained through pyrolysis of a bio-inspired iron porphyrin precursor coordinated with an axial imidazole from the surface of polyvinylimidazole-grafted carbon black at 800 °C under an Ar atmosphere, exhibited a high electrocatalytic activity with a half-wave potential of 0.88 V versus the reversible hydrogen electrode for ORR through a four-electron reduction pathway in alkaline media. In addition, an anion-exchange membrane electrode assembly (MEA) with C@PVI-(DFTPP)Fe-800 as the cathode electrocatalyst generated a maximum power density of 0.104 W cm-2 and a current density of 0.317 mA cm-2. X-ray absorption spectroscopy demonstrated that a single-atom catalyst (Fe-Nx/C) with an Fe-N5 active site can selectively be obtained; furthermore, the catalyst ORR activity can be tuned using fluorine atom doping through appropriate pre-assembling of the molecular catalyst on a carbon support followed by pyrolysis. This provides an effective strategy to prepare structure-performance-correlated electrocatalysts at the molecular level with a large number of M-Nx active sites for ORR. This method can also be utilized for designing other catalysts.