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Combined photoemission spectroscopy and electrochemical study of a mixture of (Oxy)carbides as potential innovative supports and electrocatalysts

Due to the scarcity and high cost of platinum, the scientific community is putting great effort into finding new catalysts to replace it in energy conversion technologies, such as low temperature fuel cells and electrolyzers. In this context, Group VI transition metal carbides have recently attracted great interest due to their possible use in electrocatalysis, both as catalysts and supports. Among them, tungsten carbides (WC and W2C) are the most promising materials since they have been demonstrated to show electronic and catalytic properties similar to those of the Pt-group metals in some catalytic reactions.In the context of electrochemical production of hydrogen, the Pt-like behavior of tungsten carbide makes it a potential catalyst for the hydrogen evolution reaction (HER). One of the crucial issues for the use of carbides as electrode materials is their stability in electrochemical environments. W2C is not stable under electrochemical conditions, since oxide species are immediately formed on the surface, whereas WC is stable at potentials lower than 0.6 V in acidic solutions.In the present study, in order to get a clear understanding of the chemical changes induced on nonstoichiometric tungsten carbides (WxC) by the electrochemical polarization of the material, we have investigated the behavior of a WxC film deposited on a planar TiOxCy system under electrochemical conditions.
The WxC/TiOxCy system was subjected to an aging treatment to investigate its electrochemical stability and the effect of the influence of the orientation of the Ti substrate grains. Figure 1 shows the SPEM measurements carried out after the electrochemical treatment at the ESCAmicroscopy beamline at Elettra. Images in panels (a) and (b) show a photoemission map of the surface revealing the displacement of the grains on the surface and the corresponding EBSD analysis respectively. In Figure 1(c) and (d), the red regions represent a higher content of oxide species, whereas the blue ones represent a higher content of carbidic species. The Ti 2p and W 4f chemical maps (TiOxCy/TiOx and WxC/WOx ratios, respectively) clearly show the same tendency to undergo oxidation for TiOxCy and WxC on the different grains. The high-resolution XPS spectra, shown in Fig. 1(e) and (f) taken on single grains confirm the different resistance to corrosion of the WxC/TiOxCy film on top of differently oriented substrate grains: the films on grains with an orientation close to [0001] (reddish grains in the EBSD map) are less resistant to oxidation since they present a higher amount of oxide species (TiOx and WOx) after the accelerated ageing treatment. On the contrary, grains with an orientation close to ⟨21̅1̅0⟩ (green) and ⟨101̅0⟩ (blue) are more resistant to oxidation.
 

Figure 1. (a) SPEM micrograph of the studied region recorded at the W 4f (a) core level energy (KE = 717 eV); (b) corresponding EBSD map of the polycrystalline Ti substrate. Calculated TiCxOy /TiO2 (c) and WxC/WOx (d). Chemical maps; and normalized Ti 2p (e) and W 4f (f) photoemission spectra (after Shirley background removal) measured on different grains indicated both in the EBSD map and the SPEM micrograph.

 

Taking into account the results described above about the stability of WxC up to 0.6 V, that indicate only minor oxidation of the film up to this potential, the significant oxidation of WxC supported on TiOxCy on the grains with an orientation close to [0001] can be attributed to the oxidation of the TiOxCy substrate to TiO2. The TiO2 formed under these conditions provides the oxygen for the oxidation of WxC due to the interaction between the two (oxy)carbides, as seen during the preparation of the sample.
In light of these results, we can say that the interaction between WxC and TiOxCy plays an important role both in the composition and the electrochemical stability of WxC. All these observations lead us to conclude that, under the studied electrochemical conditions, TiOxCy growth on grains with orientation close to ⟨21̅1̅0⟩ and ⟨101̅0⟩can stabilize WxC, preventing the oxidation (corrosion) process.


This research was conducted by the following research team:

Laura Calvillo1Carlos Valero-Vidal2, Stefano Agnoli1Hikmet Sezen3Celine Rudiger2Julia Kunze-Liebhauser2and Gaetano Granozzi1


1 Department of Chemical Sciences and INSTM unit, Università di Padova, Padova, Italia
2 Institute of Physical Chemistry, Leopold-Franzens-University Innsbruck, Innsbruck, Austria
3 Elettra - Sincrotrone Trieste, Trieste, Italy



Contact person:

Gaetano Granozzi, email: gaetano.granozzi@unipd.it

 

Reference

L. Calvillo, C. Valero-Vidal,S. Agnoli, H. Sezen, C. Rudiger, J. Kunze-Liebhauser and G. Granozzi, “Combined photoemission spectroscopy and electrochemical study of a mixture of (Oxy)carbides as potential innovative supports and electrocatalysts” ACS Applied Materials & Interfaces 8, 19418 (2016), doi: 10.1021/acsami.6b04414

 

 
Last Updated on Monday, 24 October 2016 14:36