posted on 2016-07-18, 00:00authored byBart Geboes, Jon Ustarroz, Kadir Sentosun, Hans Vanrompay, Annick Hubin, Sara Bals, Tom Breugelmans
Nanoporous Pt based
nanoparticles (NP’s) are promising fuel
cell catalysts due to their high surface area and increased electrocatalytic
activity toward the ORR. In this work a direct double-pulse electrodeposition
procedure at room temperature is applied to obtain dendritic Pt structures
(89 nm diameter) with a high level of porosity (ca. 25%) and nanopores
of 2 nm protruding until the center of the NP’s. The particle
morphology is characterized using aberration-corrected high angle
annular dark field scanning transmission electron microscopy (HAADF-STEM)
and electron tomography (ET) combined with field emission scanning
electron microscopy (FESEM) and macroscopic electrochemical measurements
to assess their activity and stability toward the ORR. Macroscopic
determination of the active surface area through hydrogen UPD measurements
in combination with FESEM and ET showed that a considerable amount
of the active sites inside the pores of the low overpotential NP’s
were accessible to oxygen species. As a result of this accessibility, up to a 9-fold enhancement of
the Pt mass corrected ORR activity at 0.85 V vs RHE was observed at
the highly porous structures. After successive potential cycling upward
to 1.5 V vs RHE in a deaerated HClO4 solution a negative
shift of 71 mV in half-wave potential occurred. This decrease in ORR
activity could be correlated to the partial collapse of the nanopores,
visible in both the EASA values and 3D ET reconstructions.