The development of supported electrocatalysts for the oxidation of fuels in hydroxide exchange membrane fuel cells
Date
2015
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
Hydroxide exchange membrane fuel cells (HEMFCs) offer a number of
advantages over proton exchange membrane fuel cells. One advantage is the higher
stability of catalytic materials in the alkaline environment of a HEMFC, which opens
the possibility for using non-platinum (Pt) group metals to catalyze the oxygen
reduction reaction (ORR) and the hydrogen oxidation reaction (HOR). However, even
on Pt, the activity of the HOR is approximately two orders of magnitude slower in
alkaline environments relative to acidic environments. Pt is an expensive catalyst and
a cheaper catalyst would greatly lower the cost associated with HEMFCs.
In order to develop novel catalysts for the alkaline HOR, catalytic trends must
be established. Previously, trends on monometallic polycrystalline disk electrodes
were established using hydrogen binding energies calculated by density functional
theory as a thermodynamic descriptor. While these trends have helped develop new,
Pt-free catalysts for the alkaline HOR, bulk materials such as disk electrodes are not
suitable for fuel cell devices. Therefore, catalytic trends were also developed for
monometallic, carbon-supported catalysts, which are able to be directly used in a fuel
cell device. Gold, silver, copper, iridium, platinum, palladium, rhodium, nickel, and
cobalt nanoparticles on carbon supports were tested for the alkaline HOR reaction.
While general trends observed for the disk electrodes also hold for the supported
catalysts, the silver, gold, and copper nanoparticles were found to be more active than
the respective disk electrodes. While the trends developed on supported catalysts will help the development
of new electrocatalysts, Pt is the most active metal for the alkaline HOR. To decrease
the amount of Pt needed, gold (Au) substrates were decorated with Pt nanoparticles
and tested for the alkaline HOR. Both Pt-decorated disk Au substrates (Pt/Au) and
supported Au substrates (Pt/Au/C) were synthesized through the galvanic
displacement of an underpotentially-deposited monolayer of copper on the Au
substrate. Characterization of these surfaces through standard electrochemical
techniques indicates that Pt is present on the Au surface as small nanostructures. The
activities of Pt/Au and Pt/Au/C toward the alkaline HOR were shown to be similar to
bulk Pt and far higher than bulk Au, while utilizing much less Pt than state-of-the-art
fuel cell catalysts. Pt/Au/C catalysts therefore may be used to decrease the loading of
Pt required for HEMFC anodes. Though Pt/Au catalysts offer Pt-like activity while
utilizing less Pt, the cost of the catalyst may not decrease significantly since Au is an
expensive substrate. Therefore, transition metal carbide (TMC) powders were also
impregnated with low loadings of Pt and tested for the alkaline HOR, which will
decrease the cost of the catalyst further.
The lack of infrastructure for hydrogen storage and transportation currently
limits the practicality of using hydrogen as a fuel source. Nevertheless, HEMFCs also
have the benefit of utilizing alternative CO2-neutral feuls such as ethylene glycol and
glucose. The kinetic activity of the alkaline oxidation of these fuels was determined
for Pt/Au catalysts. In addition, the stability of each catalyst was compared to bulk Pt.
The Pt/Au catalysts were found to have similar activity as bulk Pt as well as improved
stability for the alkaline oxidation of ethylene glycol. The mechanism of each
oxidation reaction was examined on the Pt/Au surface using in-situ infrared spectroscopy. The increased stability of the Pt/Au surface toward the oxidation of
ethylene glycol is due to the prevention of C-C bond scission, therefore avoiding the
poisoning of Pt active sites.