Influence of Molybdenum Carbide Additive on the Oxygen Reduction Reaction Kinetics at Molybdenum Carbide Derived Carbon Electrode

Transition metal carbides can be used to synthesise various micro-mesoporous carbons with different physical and electrochemical properties. By varying the metal carbide and synthesis temperature, carbons with different crystallographic structure and hierarchical porosity can be synthesised, which makes carbide derived carbons (CDC) promising materials for supercapacitors [1] and catalyst carriers for proton exchange membrane fuel cells (PEM‑FC). [2]

The main limiting process in a PEM-FC is the oxygen reduction reaction (ORR), which is highly dependent on the catalyst as well as the catalyst support used. The ORR has been studied on different molybdenum carbide derived carbons as well as on Pt nanoclusters deposited onto Mo₂C derived carbons. [2] Mo₂C is also used in heterogeneous catalysis. [3] The aim of this study was to analyse the effect of Mo₂C additive on the oxygen reduction reaction at molybdenum carbide derived carbons.

In this study molybdenum carbide powder was chlorinated at 750°C to obtain carbons with different amounts of Mo₂C nanoparticles present in the product by varying the synthesis temperature. These carbide derived carbons were analysed using various physical (XRD, XRF, N₂ sorption/desorption, Raman spectroscopy) and electrochemical (rotating disc electrode, cyclic voltammetry and impedance spectroscopy) methods.

XRD patterns of the different materials show traces of γ‑MoC, β′‑Mo₂C and MoO₂. The Raman spectra in Fig. 1. taken from different places in samples, synthesised applying different reaction times, show that the chlorination reaction has not fully terminated and samples contain traces of Mo compounds (MoO₃, Mo₂C) in addition to the pure amorphous carbon particles. [1,4,5] XRF measurements show that after two hours of hydrogen treatment at 800°C there is no residual chlorine from the synthesis present in the studied materials. N₂ sorption/desorption measurements show a decrease in surface area with the increase of Mo₂C content in partially chlorinated carbon powders. The prepared materials exhibit micro-mesoporous structure.

Surprysingly, the carbon materials with small additives of Mo₂C performed electrochemically similarly to very pure molybdenum carbide derived carbons – the over potentials of ORR on C-Mo₂C systems were slightly higher, the number of electrons transferred per one molecule of O₂ was between three and four, and kinetic and electric double layer charging/discharging currents were of similar magnitude. With the increase of Mo₂C additive, over potential increases and currents decrease. The electrochemical stability of the C-Mo₂C materials was also studied and based on preliminary results was reasonably good.  

Acknowledgements

This work was supported by Estonian target research project SF0180002s08, Estonian Centre of Excellence in Research Project TK117T "High-technology Materials for Sustainable Development", Estonian Energy Technology Programme project SLOKT 10209T and ESF Grant ETF8267.

Authors thank J. Aruväli for XRD measurements and analysis of data.

References

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[2] K. Vaarmets, J. Nerut, E. Härk, E. Lust, Electrochimica Acta 104 (2013) 216.

[3] Y. Liu, T.G. Kelly, J.G. Chen, W.E. Mustain, ACS Catalysis 3 (2013) 1184.

[4] M. Dieterle, G. Mestl, Phys. Chem. Chem. Phys. 4 (2002) 822.

[5] T. Xiao, A.P.E. York, V.C. Williams, H. Al-Megren, A. Hanif, X. Zhou, M.L.H. Green, Chem. Mater. 12 (2000) 3896.