(339c) Electronic Effect On Oxidation of Formic Acid On Supported Pd-Cu Bimetallic Surface

Electronic Effect on
Oxidation of Formic Acid on Supported
Pd-Cu Bimetallic Surface

Shuozhen Hu^a, Louis Scudiero^b, Su Ha^a

^a The
Gene and Linda Voiland School of Chemical Engineering and Bioengineering,
Washington State University, Pullman, WA 99164

^b Chemistry
Department and Materials Science and Engineering Program, Washington State University, Pullman, WA 99164

Formic acid is
the energy carrier in fuel cells that produces carbon-neutral electric power
when it is used with regenerative direct formic acid fuel cells (DFAFCs). In
regenerative DFAFCs, formic acid is oxidized at the anode to produce
electricity and carbon dioxide.  Carbon dioxide then reacts with water in
another reactor to form formic acid through an electrochemical reduction
process using renewable energy sources, such as solar energy, wind energy, or
tidal energy. Therefore, CO₂-derived fuels hold interesting
prospects for future energy systems based on non-fossil energy sources.

Direct formic
acid fuel cells (DFAFCs) have many advantages over H₂ proton
exchange membrane (PEM) fuel cells and direct methanol fuel cells due to: 1)
non-toxicity, 2) easy to store and transport, and 3) low crossover flux through
membrane. Among the anode catalysts for DFAFC, palladium (Pd) offers the best
electrochemical activity towards the formic acid oxidation reaction. However,
it is expensive and shows a poor long-term stability due to surface poisoning caused by intermediate species, such
as CH₂(OH)₂,that form during the reaction.
To minimize poisoning of the anode and lower the cost, bimetallic surfaces of
Pd using inexpensive transition metals as supporting materials are
investigated. In this study, the layered configuration of thin film samples was
chosen versus nanoparticles to minimize contributions from uneven particle
size, polycrystallinity of particles, and surface component effects that are
typically associated with bimetallic nanoparticles. Furthermore, to avoid
possible contaminations from precursors commonly used in wet chemistry, thermal
vapor deposition method was used to grow Pd films on Cu or other transition
metal (M) thick films over glassy carbon surfaces to facilitate X-ray
photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and
chronoamperometry (CA) measurements. A positive binding
energy shift of the Pd peaks and an opposite shift for M = Cu peaks were
measured. These opposite binding energy shift indicate that there is charge
transfer from Pd to Cu. Similarly, a change of the valence band shape and shift
of the d-band center away from the Fermi level as Pd layer decreased were
observed and measured by XPS. This electronic perturbation of the Pd-Cu systems
results in a lower bond energy between Pd and adsorbate. As a result, a drastic
increase in current density for a thin layered Pd-Cu bimetallic sample compare
to bulk Pd, and an improved stability for formic acid electro-oxidation were measured
by both CV and CA. These findings indicate that Pd-Cu bimetallic surfaces are
better electrocatalytic materials than bulk Pd and could be use to enhance
direct formic acid fuel cell (DFAFC) performance.