(589b) Design of Highly Active and Stable Pt-Cu Fuel Cell Catalysts with Controlled Morphology towards Oxygen Reduction Reaction

     A
bimetallic Pt-Cu catalyst on mesoporous carbon has been designed to improve
activities and stabilities for the oxygen reduction reaction, which is of great
interest in polymer electrolyte membrane fuel cells (PEMFC). Recently, Koh and
Strasser reported Pt-Cu bimetallic catalysts, which showed an activity
enhancement of ~4x over commercial catalysts after the copper was
electrochemically dealloyed from the nanoparticle surface.¹

An emerging
concept in catalyst design is to pre-synthesize metal nanocrystals coated with
stabilizing ligands to control their morphology, and then to infuse the
particles onto high surface area supports. The decoupling of nanocrystal
synthesis and infusion provides exquisite control of the nanocrystal size, composition,
morphology, and dispersibility within the porous support. We have designed PtCu
catalysts by first synthesizing PtCu nanoparticles with controlled size (<
3nm) and composition via stabilizing, short-chain, hydrocarbon ligands (oleic
acid and oleylamine), followed by infusion onto the high surface area
mesoporous carbon supports.  XRD
spectra shows that the catalyst is predominantly a single phase Cu rich alloy
with <5% of a Pt rich phase. The catalysts were electrochemically dealloyed
to preferentially leach out copper from the nanoparticle surface. These
catalysts show significantly improved specific mass and area activities for ORR
at 0.9V (vs. NHE) as measured using rotating disk electrodes.
      

The stability of
fuel cell catalysts for ORR was investigated by potential cycling up to 1.2V. For
automotive applications, catalyst support stability at 1.2V is required for an
accumulated life time of the vehicle, with a maximum allowable performance loss
of < 30 mV. Typical amorphous carbon supports, for example Vulcan carbon,
easily oxidize at potentials of 1.2V, which leads to significant activity
losses. Herein we use corrosion resistant graphitic supports to enhance the
stability of the catalysts. The strong metal interaction with the ¹ electron support sites was used to enhance
the catalyst stability. We have studied the stability of the PtCu catalysts by
accelerated durability test (ADT) by potential cycling between 0.5V and 1.2V at
50 mV/s for 1000 cycles. Mesoporous carbon supports with different degree of
graphitization have investigated, and high stabilities have been achieved.

(1) Koh, S.; Strasser, P. Electrocatalysis on bimetallic surfaces:
modifying catalytic reactivity for oxygen reduction by voltammetric surface
dealloying. J Am Chem Soc.  2007, 129,
12624-12625.