(339e) Metal-Modified Tungsten and Molybdenum Carbide Electrocatalysts for Energy Conversion

At present, the most active electrocatalysts for acidic fuel cells and electrolyzers are based on the expensive and scarce Pt-group metals.  The current Pt loadings in electrocatalysts are too expensive for large scale use in a PEM fuel cell and electrolysis applications.  Additionally, instability of Pt on carbon black, the most common electrochemical support, leads to long-term activity loss in fuel cells. Transition metal carbides have long been studied for their similar catalytic activity to Pt-group metals [1]. Tungsten monocarbide (WC) has been investigated both as electrocatalyst and as a support material for low loadings of Pt [2,3].  The economic motivation for studying WC as a support is clear as tungsten is orders of magnitude less expensive and more abundant than Pt.  One monolayer of Pt on WC shows nearly identical hydrogenation activity to Pt [2], while exhibiting carbon monoxide tolerance, an advantage over Pt which is easily poisoned by carbon monoxide (CO).  Consequently, WC is more active than Pt for methanol oxidation [4].  There is interest in the reduction of carbon dioxide (CO₂) into useful fuels, but the currently known catalysts require high overpotential to reach reasonable rates.  Compared to other metals, Pt only needs relatively low overpotential to reduce CO₂, but due to CO poisoning, produces very little reduction products [5].  In this talk, CO tolerant carbide-based catalysts will be shown to be more active than Pt under CO₂reduction conditions. 

Polycrystalline tungsten foil is carburized in a tube furnace to create a thin film of single-phase WC at the surface. Physical vapor deposition (PVD) of Pt, Cu, and other metals allow for monolayer amounts of metal to modify the surface of the carbide. The surfaces are characterized using X-ray photoelectron spectroscopy (XPS) before and after electrochemical measurements.  The use of relatively smooth carbide thin films for these studies allow for the comparison of the specific activity between samples due to similar surface areas while simultaneously enabling good control over surface carbon content [6].  The electrochemical activities of the surfaces are measured through the use of linear sweep voltammetry and chronopotentiometry in aqueous solutions.  Gas chromatography allows for the quantification of reactant and products, along with the Faradaic efficiency. The experimental activities will be compared to theory for several types of electrocatalytic reactions, including hydrogen evolution, methanol oxidation, and carbon dioxide reduction reactions. The modification of tungsten or molybdenum carbide surface with another metal also allowed for the tuning the electrocatalytic properties [7]. 

[1] H.H. Hwu, J.G. Chen, Chemical Reviews, 105 (2005) 185-212.

[2] D.V. Esposito, S.T. Hunt, A.L. Stottlemyer, K.D. Dobson, B.E. McCandless, R.W. Birkmire, J.G. Chen, Angew. Chem. Int. Ed.49 (2010) 9859.

[3] D.V. Esposito, J.G. Chen, Energy and Environmental Science, 4 (2011) 3900.

[4] E.C. Weigert, A.L. Stottlemyer, M. B. Zellner, J. G. Chen, J. Phys. Chem C., 111 (2007) 14617.

[5] Y. Hori, H. Wakebe, T. Tsukamoto, O. Koga, Electrochim. Acta, 39 (1994) 1833-1839.

[6] Y.C. Kimmel, D.V. Esposito, R.W. Birkmire, J.G. Chen, Int. J. Hydrogen Energy, 37 (2012) 3019.

[7] D.V. Esposito, S.T. Hunt, Y.C. Kimmel, J.G. Chen, J. Am. Chem. Soc., 134 (2012) 3025.