1D Pd-Based Nanomaterials as Efficient Electrocatalysts for Fuel Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 336

HCOOH ad ! COOH ad þ H þ þ e ! CO 2 þ 2H þ þ 2e

ð 9 Þ

HCOOH ad ! HCOO ad þ H þ þ e ! CO 2 þ 2H þ þ 2e ð 10 Þ

HCOOH ad ! CO ad þ H 2 O ! CO 2 þ 2H þ þ 2e ð 11 Þ

In the first and second paths, through dehydrogenation with forming different
intermediates formic acid can be oxidized to CO 2 directly. The Eq. ( 11 ) represents
the indirect pathway, in which the produced CO intermediate can adsorb strongly
on the surface of catalyst, leading to the poisoning of catalyst. To overcome the
heavy CO-poisoning and high-cost of Pt-based catalysts, recent extensive research
efforts have been devoted to the development of non-platinum anode electrocat-
alysts. Recent studies, including in situ spectroelectrochemical studies showed that
Pd-based catalysts can catalyze the oxidation of formic acid at the anode of
PEMFCs with greater resistance to CO than Pt catalysts [ 42 , 111 ].
Recently, we successfully synthesized nanoneedle-covered 1D palladium-silver
nanotubes through a galvanic displacement reaction with Ag nanorods at 100 C
(PdAg-100) and room temperature (PdAg-25) [ 76 ]. TEM and SEM measurements
displayed that the synthesized PdAg nanotubes exhibit a hollow structure with a
nanoneedle-covered surface, which provide the perfect large surface area for
catalytic applications. From the HRTEM images of PdAg-25 shown in Fig. 18 a, b,
the surface of the PdAg-25 nanotubes is decorated with crystalline Pd nanopar-
ticles with Pd(111) planes, and meanwhile, Ag and AgCl particles are dispersed in
the inner space of the nanotubes. From the elemental mapping and cross-sectional
line profiles shown in Fig. 18 d-g, silver is dispersed in the core and Pd mainly
distributes on the shell of the nanotubes. From the CVs in 0.1 M HClO 4 solution in
Fig. 19 a, more charge for hydrogen desorption was obtained with PdAg-25
nanotubes compared with that of PdAg-100 with the same loading on electrode
surface, indicating rougher surface and thus larger ECSA of PdAg-25 nanotubes.
By comparing the CV curves of the electrodes in 0.1 M HClO 4 + 0.5 M HCOOH
solution (Fig. 19 b), one can see that the PdAg-100 nanotubes exhibit more neg-
ative anodic peak potential (+0.285 vs. +0.316 V) and much larger anodic peak
current density (3.82 vs. 1.97 mA cm -2 ) of formic acid oxidation than those of
PdAg-25 under same conditions. The higher electrocatalytic activity of PdAg-100
nanotubes possibly as the consequences of the higher ratio of Pd to Ag (36:64) in
PdAg-100 compared with that in PdAg-25 (25:75) and the annealing process of the
nanotube surface structures at 100 C. Chronoamperometric analyses (Fig. 19 c-e)
were carried out to evaluate the activity and stability of the PdAg nanotubes for
formic acid electrooxidation. It can be seen that at the three studied potentials, the
maximum initial and steady-state oxidation current densities obtained from both of
the PdAg nanotubes are much larger than that from the bulk Pd electrode over the
entire time period. On the other hand, the initial current density on the bulk Pd
electrode decays much more rapidly than those of the as-synthesized PdAg
nanotubes. These results indicate that the synthesized PdAg nanotubes exhibit