268289 Non-Carbon Electrocatalyst Supports for PEM Fuel Cells
Non-Carbon Electrocatalyst Supports for PEM Fuel Cells
Ying Liu1 and William E. Mustain1
1Department of Chemical, Materials and Biomolecular Engineering; University of Connecticut, Storrs, CT 06268
ISSUES WITH CARBON SUPPORTS FOR Pt IN PEMFCS
Proton exchange membrane fuel cells (PEMFCs) convert the chemical energy of H2 and O2 directly into electrical energy through complementary redox processes that are not limited by Carnot or Rankine heat cycles. Hence, PEMFCs are promising as highly efficient and environmentally friendly energy sources for the 21ST century and have been explored widely for stationary and vehicular applications. The rate limiting reaction in PEMFCs is the oxygen reduction reaction (ORR), which takes place at the PEMFC cathode . Pt and Pt-alloys supported on carbon black are the most popular catalyst for the ORR in PEMFCs. Carbon black has several features that make it nearly ideal as an electrocatalyst support including high electrical conductivity, high surface area, chemical stability and low cost . On the other hand; it is thermodynamically unstable at ORR relevant potentials and bonds to catalyst particles via weak Van der Waals forces. Hence, there have been considerable studies to replace carbon black or enhance its surface and structural properties .
NON-CARBON ELECTROELECTROCATALYST SUPPORTS
This thermodynamic limitation has led several researchers to investigate non-carbon electrocatalyst supports for Pt for use in PEMFCs. One such support that has received considerable attention is tungsten monocarbide (WC). Recently, Pt/WC catalysts were shown to have higher activity than Pt/Vulcan electrocatalysts in a PEMFC  and it was suggested that electron transfer between Pt and WC led to a rearrangement of the Pt d-band, which has a similar effect to alloying Pt with non-noble metals. However, work since then by our group  and Chen's group  has suggested that this enhancement is short lived and WC supports suffer from electrochemical instability at ORR relevant potentials. Despite this, there are several applications where Pt/WC catalysts may be useful, including hydrogen oxidation and hydrogen evolution reactions (HOR/HER). Also, it is important for researchers to understand the stability behavior for Pt/WC catalysts and, by extension, Pt/WO3 electrocatalysts for the ORR and other reactions in aqueous media.
PHYSICAL AND ELECTROCHEMICAL CHARACTERIZATION
In this talk, the electrochemical behavior of Pt/WC and Pt/WO3 electrocatalysts in acid media will be discussed (TEM image for Pt/WC is shown in Figure 1).
Figure 1. TEM image of as-prepared Pt/WC prior to electrochemical treatment.
In particular, this talk will focus on the electrochemical performance of these catalysts for the ORR and HOR/HER. In addition, special attention will be paid to the surface chemistry and structure as a function of potential, probed by TEM, SEM/EDX and XPS, and its influence on electrochemical performance. Most importantly, it was found that the ORR activity for the Pt particles is enhanced on Pt/WC compared with Pt/C, Figure 2; however, during operation, significant performance loss was observed and correlated with the formation of significant Pt detachment and agglomeration, Figure 3.
Figure 2. Linear sweep voltammogram for Pt/WC and Pt/Vulcan (Pt/C) catalysts in O2 saturated HClO4 electrolyte at 25oC.
Figure 3. TEM image of Pt/WC following to electrochemical treatment.
On the other hand, this significant performance loss and agglomeration was not observed by Pt/WC catalysts at lower potentials during the HOR/HER. Differences in the stability of Pt at various potential ranges was found to be a strong function of the oxidation state of the support material, with the formation of WO3 from WC correlating with the onset of performance degradation.
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