DMFC Catalyst Development
One of the factors that limit the performance of DMFC is
the poisoning of the anodic catalyst. During the oxidation
of methanol, the reaction intermediate carbon monoxide
strongly binds to the catalyst surface and blocks the
active sites. Pure Pt catalysts are particularly vulnerable
to this poisoning effect. Among several different methods
available, the use of the Pt-Ru alloy catalyst has been
most successful to solve this problem. Addition of Ru
substantially improves the CO tolerance of the catalyst,
and there has been a great deal of research on the effect
of this bimetallic catalyst composition and structure as
well. The incorporation of a metal element to another metal
can be achieved in several ways and the catalytic
reactivity can be greatly improved depending on the
preparation method.
Our group's current efforts include the development of
highly reactive catalysts which allow simple surface
structure control and scale-up synthesis. One of the main
strategies is to utilize the spontaneous deposition
phenomena. When clean noble metal surfaces are used as
substrates, the deposition from a noble metal ion solution
may result in the formation of nano-sized islands. This
simple method can be applied to nanoparticle catalyst
modification to provide active heterogeneous surfaces.
Spontaneous deposition on nanoparticle catalysts can be
achieved by 3 steps: electrochemical cleaning, immersion in
the metal precursor solution and electrochemical reduction
of the adsorbed metal species. The packing density of
ad-metal can be easily adjusted in a sub-monolayer
deposition regime. The Pt/Ru catalyst prepared in this
manner was proven to have superior reactivity toward
methanol oxidation compared to the commercial Pt-Ru alloy
catalyst. Encouraged by this success, we are currently
expanding the method to various ad-metals and core metals
in search of highly active catalysts for methanol oxidation
and other fuels such as formic acid.
Electrocatalysis of Admetal-Modified Platinum in Alkaline
Media
Most research on platinum based electrocatalysts for fuel
cell electrodes has focused on catalysis in acidic media.
However, the use of alkaline media may be a promising
avenue toward increased catalytic activity. Platinum based
anodes in acidic media are plagued by CO poisoning, an
effect that can be mitigated, but not eliminated, through
incorporation of other metals (e.g. ruthenium). The
function of these metals is to preferentially adsorb OH,
which then oxidizes the CO on adjacent Pt sites to CO2.
Similarly, in alkaline media, the abundance of OH adsorbed
to the Pt surface leads to faster CO oxidation, even
without the incorporation of Ru.
A new avenue of research in our group investigates the
effects of adding Ru and other metals to Pt electrodes in
alkaline media. Although the switch from acidic to alkaline
media greatly improves CO oxidation kinetics, it is
expected that the addition of other metals will yield
further kinetic improvements. This investigation is being
carried out both on single crystal surfaces and
nanoparticles, using cyclic voltammetry to study oxidation
of methanol, formate ions, and CO. The performance of these
catalysts will be further studied using laminar flow fuel
cell technology, in collaboration with Dr. Kenis in the
department of chemical and biomolecular engineering.