(419f) Ir-M Alloy Catalysts for Direct Ethanol Fuel Cells

   Ethanol is
a promising fuel for direct ethanol fuel cells (DEFCs) and can be produced from
a variety of sources, including biomass. A limitation of DEFCs is the anode catalyst
for ethanol oxidation. A common anode catalyst is Pt, which is expensive and
suffers from low efficiency towards complete oxidation. Complete oxidation
produces 12 electrons per ethanol molecule, while partial oxidation may produce
as few as 4 electrons per ethanol molecule. Replacement of Pt with new catalyst
materials is therefore desirable and the goal of this work. During the
oxidation process, a large number of product intermediates may form, which
further complicates mechanistic analysis.

    Through the combined efforts of
theory and experiment, we have synthesized and characterized several Ir-M alloy
catalysts. We previously showed Ir-Sn nanoparticles to be active DEFC catalysts[1],
as well as recently Ir-Ru. We determined the structure and composition of these
alloys using a variety of experimental techniques such as high-resolution
transmission electron microscopy (HRTEM), electron energy loss spectroscopy
(EELS), x-ray diffraction (XRD), and x-ray absorption spectroscopy (XAS). Our
results show that the activity of Ir-Sn exceeds traditional Pt catalysts. Density
functional theory (DFT) simulations were used to determine the stability of
different core-shell particle structures for Ir-Sn. The DFT simulations indicate
that Sn prefers shell, or surface, locations, in agreement with experiment.
Calculations comparing the reactivity (both C-H and C-C bond scission
reactions) of Ir, Ir-Sn, and Pt indicate the Ir-based materials to be more
reactive towards ethanol. Similar theoretical and experimental results were
found for Ir-Ru.

    Based on
our success with Ir-Sn and Ir-Ru, we have also extended our DFT efforts to
determine the surface structures of several other alloys involving Ir and other
late-transition metals, such as late transition metals Pt, Pd, Rh, etc. We
identified whether the alloyed metals prefer surface or bulk locations, in
order to identify possible surface alloy structures.  The rate-determining step of ethanol oxidation
typically involves C-C bond scission, for example of CHCO[2,3]. Using linear-scaling
relationships[3,4] we determined activation energies over the Ir-M catalysts for
C-C bond scission of CHCO. The DFT results provide data to screen a large
number of potential alloy materials and thus guide experimental synthesis
strategies. Our approach thus accelerates the development of new potential DEFC
anode catalysts.

References

[1] W. Du, Q. Wang,
D. Saxner, N. A. Deskins, S. Dong, J. E. Krzanowski, A. I. Frenkel, and X.
Teng. “Highly active iridium/iridium-tin/tin oxide heterogeneous nanoparticles
as alternative electrocatalysts for the ethanol oxidation reaction.” Journal
of the American Chemical Society 133, 2011, 15172-83.

[2] R. Alcala, M.
Mavrikakis, and J. A. Dumesic. “DFT studies for cleavage of C-C and C-O bonds
in surface species derived from ethanol on Pt(111).” Journal of Catalysis
218, 2003, 178-190.

[3] P. Ferrin, D.
Simonetti, S. Kandoi, E. Kunkes, J. A. Dumesic, J. K. Nørskov, and M.
Mavrikakis. “Modeling ethanol decomposition on transition metals: a combined
application of scaling and Brønsted-Evans-Polanyi relations.” Journal of the
American Chemical Society 131, 2009, 5809-15.

[4] S. Wang, T. Burcin,
J. Shen, G. Jones, L. C. Grabow, F. Studt, T. Bligaard, F. Abild-Pedersen, C.
H. Christensen, and J. K. Nørskov. “Universal Brønsted-Evans-Polanyi Relations
for C–C, C–O, C–N, N–O, N–N, and O–O Dissociation Reactions.” Catalysis
Letters 141, 2010, 370-373.