1D Pd-Based Nanomaterials as Efficient Electrocatalysts for Fuel Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 318

On the basis of the mechanism of ORR, O 2 molecules are firstly adsorbed on
the surface of catalysts, and then electrochemically reduced either directly to water
or indirectly to intermediate of H 2 O 2 . According to the previous studies on the Pd
and Pt-catalyzed ORR, the adsorption energy (AE) of O adsorption can serve as a
good descriptor for the catalytic activity of the catalyst surface toward the ORR
[ 56 , 96 ]. Therefore, it would be of great interest to develop a robust and practical
Pd or Pd-based catalysts with low AE toward O species. Recently, Abruna et al.
[ 44 ] succeeded in tailoring the morphology of the deposited Pd from nanoparticles
to nanorods by simply adjusting the precursor concentration in the electrochemical
deposition of Pd. They found that the surface-specific activity of Pd nanorods (Pd
NRs) toward ORR is not only higher than that of Pd nanoparticles (Pd NPs), but
also becomes comparable to that of bulk Pt catalysts under fuel cell operating
potentials. It can be seen from Fig. 1 a, b that, under the experimental conditions of
1 9 10 -5 M PdCl 2 precursor and 3000 s electrochemical deposition time, only Pd
nanoparticles were formed with the size range from 5 to 10 nm. However, when
the PdCl 2 concentration increased to 3 9 10 -4 M and deposition time decreased
to 100 s, uniform Pd nanorods can be produced with an average diameter of 5 nm
and an aspect ratio of *8. By comparing the electrocatalytic activities of the
formed Pd NPs, Pd NRs, and bulk Pt for ORR as shown in Fig. 1 c, the half-wave
potential (E 1/2 ) obtained from Pd NRs shifts positively by 85 mV compared to that
of Pd NPs and approaches that of bulk Pt catalyst. Figure 1 d demonstrates that the
kinetic current density (j K ) of the Pd NRs approaches that of bulk Pt and is 10-fold
higher than that of the Pd NPs at +0.85 V (a practical operating potential of a
PEMFC cathode). The superior ORR activity of Pd NRs was attributed to the
exceptionally weak interaction between the exposed Pd(110) facet of Pd NRs and
the adsorbed O atoms, which was confirmed by the CO stripping experiments and
density functional theory (DFT) calculations. This study indicated that 1D Pd
nanostructures could be an efficient and cost-effective cathodic electrocatalysts for
PEMFCs.
By taking advantage of the higher activity offered by certain platinum alloys,
Yan and coworkers [ 75 ] reported that unsupported PtPd nanotubes (PtPdNTs)
exhibited much enhanced mass activity toward ORR as fuel cell cathode elect-
rocatalysts. In the study, Pt and PtPd nanotubes were prepared through a galvanic
replacement reaction between the pre-synthesized silver nanorods and Pt or Pd
precursors. Figure 2 shows the SEM and TEM images of the as-synthesized PtPd
NTs. It can be clearly seen that the PtPd NTs display uniform diameter (45 nm),
wall thickness (7 nm), and length (10 lm). The ORR activity and durability test
are shown in Fig. 3 . From Fig. 3 b, it can be seen that the half-wave potential of the
PtPdNTs (0.851 V) is higher than that of the synthesized Pt nanotubes (PtNTs)
(0.837 V), platinum black (0.817 V), and Pt/C (0.828 V). The shift to more
positive potentials suggests clearly that the overpotential of ORR can be effec-
tively reduced by using the bimetallic system. Moreover, the calculated mass- and
specific-activity of the PtPd nanotubes are 1.4 and 1.8 times higher than those of
the platinum black, respectively. Figure 3 a shows the ECSA change with number
of cyclic voltammetry (CV) cycles at three electrocatalysts. After 1000 cycles,