(600ab) 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 21^ST
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 [1].  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 [2]. 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 [3].

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
[4] 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 [5] and Chen's group [6] 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 25^oC.    

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. 

REFERENCES

1. Haile, S. M. Acta Mater. 2003, 51, p. 5981-6000.

2. Kinoshita, K. Carbon:
Electrochemical and Physicochemical Properties, John Wiley & Sons: New
York, 1988.

3. Shrestha, S.; Liu, Y.; Mustain, W.
E. Catal. Rev. Sci. Eng. 2011, 53, p. 256-336.

4. Nie, M.; Shen,
P.K.; Wu, M.; Wei, Z.; Meng, H. J. Power Sources 2006, 162, 173-176.

5. Liu, Y. and W.E.
Mustain, ACS Catalysis, 2011. 1(3):
p. 212-220.

6. Esposito, D.V. and
J.G. Chen, Energy & Environmental
Science, 2011. 4(10): p.
3900-3912.