Non-Noble Metal Oxygen Reduction Reaction Catalysts Based on Tungsten Carbide Supports and Fe-N Structures: Rrde Studies in Acid and Alkaline Media
Authors: Rego Ulisses A. , Lopes Thiago , Bott-Neto José Luiz , Tanaka Auro A , Ticianelli Edson Antonio 
Published: February 2020

Tungsten carbide (WC) has been receiving considerable attention in recent years due to its interesting properties, such as, a Pt-like catalytic behavior, [1] high electrical conductivity, chemical stability in acidic electrolytes, and much lower cost compared to platinum. [2] Non-noble catalysts of the type Fe-N/C (iron atoms coordinated to nitrogen atoms in a graphene packed structure) have also been considerably studied given their high catalytic activity toward the oxygen reduction reaction in both, alkaline and acid electrolytes. In this work, Fe-N structures were incorporated into various tungsten carbide nanostructures and their catalytic activities toward the oxygen reduction reaction are assessed in both, acid and alkaline media. Nanostructured tungsten carbides supported on carbon (WC(20, 30, 40%)/C) were prepared by depositing suitable amounts of WO3 on the Vulcan XC72R carbon black, followed by a thermal treatment at 850 0C for 4 hours in a controlled CH4/H2 atmosphere with a subsequent passivation step in a Ar/O2 (99%/1%) atmosphere for 12 hours at 25 oC. The Fe-N structure precursor was obtained by complexing an iron salt ((NH4)2Fe(SO4)2 x 6 H2O) with a nitrogen ligand (2,4,6-Tris(2-pyridyl)–1,3,5–Triazina), followed by deposition on the various WC/C supports with a subsequent pyrolysis step at 700 0C or 800 0C for 2 hours in a controlled N2 atmosphere. The electrocatalysts were physically characterized by X-ray Diffraction, X-ray Photoelectron Spectroscopy and Energy-dispersive X-ray spectroscopy. XRD spectra revel that the WC(30%)/C presents the highest WC proportion among the different supports, while the deconvoluted XPS spectra (W4f peak) evidence the presence of W+6, W+4 and W+2 at the surface of the material, with a much higher proportion of W+2 compared to insignificant amounts of W+6 and W+4. In contrast, when Fe-N structures are prepared on the WC(30)/C support, a higher proportion of W+6 is observed. The catalytic activity of these composites were assessed through cyclic voltammetry on a thin catalyst layer rotating ring-disk electrode, where the durability of the catalysts were conducted, followed with EDX analyses of the thin layer. Considering the ORR electrocatalytic activity, as a trend, it was seen that the Fe-N/WC/C catalyst presents higher activity compared to the support itself (WC/C) in both acidic and alkaline media with a prevalence of the direct 4e- reaction pathway. Further results for the dissolution of Fe upon accelerate durability test revels loss of metal along the cycling, and this information was important for the understanding of the ORR characteristics in these hybrid nanomaterials.

Acknowledgements: Authors acknowledge the São Paulo Research Foundation (FAPESP - Procs. 2013/16930-7, 2016/07848-3), Brazil, for financial supports. The authors also wish to thank Brazilian Nanotechnology National Laboratory (LNNano) for assisting with the XPS measurements.

References

[1]. R. Levy, M. Boudart. Science. 1973, 181, 547.

[2] S. T. Oyama. Catal. Today. 1992, 15, 179.