Structure of Active Sites of Fe-N-C Nano-Catalysts for Alkaline Exchange Membrane Fuel Cells.

Hirofumi Kishi,Daiju Matsumura,Hirohisa Tanaka,Plamen Atanassov,Takeshi Kato,Barr Zulevi,Kateryna Artyushkova,Tomokazu Sakamoto,Susumu Yamaguchi,Kazuhisa Tamura,Alexey Serov,Koichiro Asazawa,Yasuo Nishihata

doi:10.3390/nano8120965

Hirofumi Kishi, Daiju Matsumura + Show 11 more

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https://doi.org/10.3390/nano8120965

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Abstract

Platinum group metal-free (PGM-free) catalysts based on transition metal-nitrogen-carbon nanomaterials have been studied by a combination of ex situ and in situ synchrotron X-ray spectroscopy techniques; high-resolution Transmission Electron Microscope (TEM); Mößbauer spectroscopy combined with electrochemical methods and Density Functional Theory (DFT) modeling/theoretical approaches. The main objective of this study was to correlate the HO2− generation with the chemical nature and surface availability of active sites in iron-nitrogen-carbon (Fe-N-C) catalysts derived by sacrificial support method (SSM). These nanomaterials present a carbonaceous matrix with nitrogen-doped sites and atomically dispersed and; in some cases; iron and nanoparticles embedded in the carbonaceous matrix. Fe-N-C oxygen reduction reaction electrocatalysts were synthesized by varying several synthetic parameters to obtain nanomaterials with different composition and morphology. Combining spectroscopy, microscopy and electrochemical reactivity allowed the building of structure-to-properties correlations which demonstrate the contributions of these moieties to the catalyst activity, and mechanistically assign the active sites to individual reaction steps. Associated with Fe-Nx motive and the presence of Fe metallic particles in the electrocatalysts showed the clear differences in the variation of composition; processing and treatment conditions of SSM. From the results of material characterization; catalytic activity and theoretical studies; Fe metallic particles (coated with carbon) are main contributors into the HO2− generation.

Highlights

Platinum has a limited availability on Earth, and its substitution with a platinum group metal-free (PGM-free) electrocatalyst is needed in order for the fuel cell to become feasible and widely spread technology [1,2,3].Among several classes of PGM-free catalysts, transition metal-nitrogen-carbon (M-N-C) nanomaterials, where the transition metal (M) is usually Fe, Ni, Co are most often studied and discussed
The second peak around 2.2 Å is assigned to Fe-Fe originating from Fe metallic particles
NCB represents the peak of Fe-Fe higher than NCB-N and PPM-N, indicating that acid treatment with HNO3 facilitates removal of Fe metallic particles

Summary

Introduction

Among several classes of PGM-free catalysts, transition metal-nitrogen-carbon (M-N-C) nanomaterials, where the transition metal (M) is usually Fe, Ni, Co (or few others) are most often studied and discussed. M-N-C nanomaterials are highly active electrocatalysts for oxygen reduction reaction (ORR). An iron-nitrogen-carbon electrocatalyst (Fe-N-C) has attracted attention due to its highest ORR activity among the other M-N-C electrocatalysts and demonstrated multiple synthesis protocols to yield desired nanomaterial. Fe-N-C have been employed in experimental proton exchange membrane fuel cells (PEMFC) and have demonstrated promise, while still being much inferior to PGM catalysts [4]. In alkaline media M-N-C demonstrate performance at par with PGM, as such find applications as cathode materials in Anion Exchange Membrane Fuel Cells (AEMFCs) [5]

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