Graphene-Supported ' Core-Shell '� Carbon Nitride Fe- and Sn-Based Electrocatalysts for the Oxygen Reduction Reaction (ORR)

One of the major obstacles for the development of feasible low-temperature fuel cells (e.g., proton-exchange membrane fuel cells, PEMFCs, and anion-exchange membrane fuel cells, AEMFCs) is the sluggishness of the oxygen reduction reaction (ORR) kinetics. As of today, carbon-supported Pt-based nanocrystals are the most efficient electrocatalysts (ECs) for the ORR. However, the low abundance of platinum and the insufficient durability of these ECs, which results from the degradation of the carbon support, constitute some of the major obstacles for large-scale commercialization of PEMFC and AEMFC technology [1, 2].

In this study, new "Pt-free" electrocatalysts are prepared and studied for the ORR process. These ECs consist of alloyed Fe-Sn nanoparticles embedded in a carbon nitride “shell” a few nanometers thick, supported on conducting micrometric graphene sheets that act as the “core”. The electrocatalytic precursors are prepared by a sol-gel/gel-plastic process following a protocol previously developed in our laboratory [3]; afterwards, they undergo suitable pyrolysis and activation processes. The proposed electrocatalysts, both pristine (e.g., FeSn_0.5-CN_l 900/Gr) and activated (e.g., FeSn_0.5-CN_l 900/Gr_a) are extensively characterized in order to gain a full understanding of their structural features, proprieties and electrocatalytic performance [4]. The chemical composition is determined by inductively-coupled plasma atomic emission spectroscopy (ICP-AES). The structure is elucidated by powder X-Ray diffraction (XRD); the morphology is inspected by high-resolution scanning electron microscopy (HR-SEM) and high-resolution transmission electron microscopy (HR-TEM) (Fig. 1). Finally, “ex situ” cyclic voltammetry with the rotating ring-disk electrode (CV-TF-RRDE) and “in situ” single fuel cell measurements are carried out to evaluate the electrocatalytic performance as well as to study the ORR mechanism. The preliminary CV-TF-RRDE investigations in an alkaline medium exhibit promising results. Indeed, the catalysts exhibit an overpotential ca. 70 mV higher with respect to a 20 wt.% Pt/C reference (Fig. 2).

Acknowledgements

The authors would like to thank the European Union for the financial support provided by the GRAPHENE Flagship, project “GRAFUS - Graphene and related materials as supports for innovative metal carbon nitride electrocatalysts for anion-exchange membrane fuel cells”.

REFERENCES

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[4] V. Di Noto, E. Negro, S. Polizzi, F. Agresti, G.A. Giffin, ChemSusChem 5, 2451 (2012).