Abstract
In exploration of low-cost electrocatalysts for direct methanol fuel cells (DMFCs), Pt modified tungsten carbide (WC) materials are found to be great potential candidates for decreasing Pt usage whilst exhibiting satisfactory reactivity. In this work, the mechanisms, onset potentials and activity for electrooxidation of methanol were studied on a series of Pt-modified WC catalysts where the bare W-terminated WC(0001) substrate was employed. In the surface energy calculations of a series of Pt-modified WC models, we found that the feasible structures are mono- and bi-layer Pt-modified WCs. The tri-layer Pt-modified WC model is not thermodynamically stable where the top layer Pt atoms tend to accumulate and form particles or clusters rather than being dispersed as a layer. We further calculated the mechanisms of methanol oxidation on the feasible models via methanol dehydrogenation to CO involving C-H and O-H bonds dissociating subsequently, and further CO oxidation with the C-O bond association. The onset potentials for the oxidation reactions over the Pt-modified WC catalysts were determined thermodynamically by water dissociation to surface OH* species. The activities of these Pt-modified WC catalysts were estimated from the calculated kinetic data. It has been found that the bi-layer Pt-modified WC catalysts may provide a good reactivity and an onset oxidation potential comparable to pure Pt and serve as promising electrocatalysts for DMFCs with a significant decrease in Pt usage.
Highlights
Cell devices have to operate in strongly basic or acidic electrolyte media, the corrosion of the electrode materials is problematic, resulting in the inevitable usage of novel metals such as Pt and Pd.[12]
In order to understand the catalytic performance of the Pt-modified WC catalysts in a direct methanol fuel cells (DMFCs), two key issues have been considered: (i) What are the reasonable effective theoretical models to describe the Pt-modified WC structures in reality? (ii) What are the possible pathways of methanol dehydrogenation and oxidation on the Pt-modified WC surfaces and what are the onset potentials and activities for the surface reactions? With the above questions in mind, we illustrated the surface energies of a series of Pt-modified WC models to determine the thermodynamic stability with an increase in the number of surface Pt atoms
The electrooxidation of methanol in a DMFC has been investigated on a series of Pt-modified WC(0001) model surfaces
Summary
Cell devices have to operate in strongly basic or acidic electrolyte media, the corrosion of the electrode materials is problematic, resulting in the inevitable usage of novel metals such as Pt and Pd.[12]. Density functional theory (DFT) calculations have been widely used to understand electrochemical catalytic reactions at the atomic level.[35] Theoretical studies of the electrooxidation of methanol have been performed extensively.[36,37,38,39,40,41,42,43,44,45,46,47,48,49,50] Zhang et al showed that the decomposition of methanol could occur via both C–H bond and O–H bond dissociations on the closed-pack flat (111) surface.[36] Greeley et al carried out DFT calculations by investigating the reaction energy and activation barriers of the elementary steps for methanol decomposition to CO on Pt(111).[38,39] Ferrin et al showed the surface structure sensitivity of methanol electrooxidation on transition metals.[40] Cao et al studied the methanol decomposition on three well-defined low index platinum single crystal planes by combining the experimental and theoretical methods.[41] The decomposition pathways in methanol oxidation were calculated over some bimetallic surfaces such as PtAu, PtRu and PdIn.[42,43,44] Kramer et al presented a model of the surface kinetics of methanol dehydrogenation on transition metals.[45] Stottlemyer et al calculated the methanol activation on the Pt-modified WC(0001) surface via C–H and O–H bonds.[48]. The calculated results, including the mechanisms of methanol oxidation and C–H bond and O–H bond dissociations, are discussed in detail
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