Fe azaphthalocyanine unimolecular layers (Fe AzULs) on carbon nanotubes for realizing highly active oxygen reduction reaction (ORR) catalytic electrodes

Hiroya Abe,Susumu Ikeda,Tomokazu Matsue,Jun Nakamura,Yasutaka Matsuo,Yutaro Hirai,Haruyuki Matsuyama,Hiroshi Yabu

doi:10.1038/s41427-019-0154-6

Abstract

A new class of Pt-free catalysts was designed that included molecular iron phthalocyanine (FePc) derivatives, namely, iron azaphthalocyanine (FeAzPc) unimolecular layers (Fe AzULs) adsorbed on oxidized multiwall carbon nanotubes (oxMWCNTs). FeAzPcs were dissolved in organic solvents such as dimethyl sulfoxide (DMSO), and catalytic electrodes modified with molecularly adsorbed FeAzPcs were successfully prepared. The optimized composition of the catalytic electrodes was determined, and the electrodes exhibited superior activity for the oxygen reduction reaction (ORR) and better durability than conventional FePc catalytic electrodes and commercial Pt/C due to the electron-withdrawing properties of the pyridinic nitrogen in FeAzPcs. The catalytic electrodes that were molecularly modified with FeAzPcs have higher activities than those composed of FeAzPc crystals and oxMWCNTs. To the best of our knowledge, among all of the conventional catalysts based on modified MWCNTs and oxMWCNTs, this catalyst exhibits the highest activity. Unlike other Pt-free catalytic electrodes, the Fe AzUL catalytic electrodes can be prepared by low-cost processing without pyrolysis and are therefore promising catalytic electrode materials for applications, such as polymer electrolyte fuel cells and metal–air batteries.

Highlights

Pt-free catalysts with high catalytic activity are in demand as alternative materials to Pt supported on carbon black (Pt/C) to realize efficient, highly stable, and low-cost polyelectrolyte fuel cells[1,2] and metal–air batteries[3,4]
Ultraviolet–visible (UV-Vis) spectra (Fig. S2) for FePc and Fe azaphthalocyanines (FeAzPcs)-4N dissolved in dimethyl sulfoxide (DMSO) showed the Q band, which is a characteristic absorbance peak of phthalocyanine derivatives, at 655 and 627 nm, respectively
DMF dimethyl formamide, DMSO dimethyl sulfoxide, FePc iron phthalocyanine, FeAzPc-4N iron-tetra-2,3-pyridinoporphyrazine, ORR oxygen reduction reaction, oxMWCNT oxidized multiwall carbon nanotube, THF tetrahydrofuran aSurface-adsorbed FeAzPc ratio was measured by X-ray photoelectron spectroscopy (XPS) bCurrent density at 0.5 V

Summary

Introduction

Pt-free catalysts with high catalytic activity are in demand as alternative materials to Pt supported on carbon black (Pt/C) to realize efficient, highly stable, and low-cost polyelectrolyte fuel cells[1,2] and metal–air batteries[3,4]. The molecular size of phthalocyanines is approximately 1.7 nm[28]; the EELS analysis provides evidence for the molecular adsorption of FeAzPc-4N onto oxMWCNTs. ORR activities of the prepared catalysts The differences in the ORR activities of the catalytic electrodes prepared from THF and DMSO solutions of FeAzPc-4N are discussed first.

Results

Conclusion