Electrochemical study of methanol tolerant oxygen reduction reaction PdxCoy/C-catalysts from organometallic molecular precursors

Article abstract of DOI:10.1016/j.ica.2017.06.034

In this work, we study the mechanism and kinetics of the oxygen reduction reaction (ORR) over Pd/C, Pd₂Co/C and PdCo₂/C electrocatalysts synthesized by thermal reduction of organometallic precursors, such as [Pd₃(μ-3-Ph-pz)₆], [Et₄N]₂[Pd₂Co(μ-4-I-3,5-Me₂-pz)₄Cl₄], and [Et₄N]₂[PdCo₂(μ-4-I-3,5-Me₂-pz)₄Cl₄]. The methanol tolerance and formation of peroxide under oxygen reduction reaction conditions were determined using a rotating disk electrode (RDE) and a rotating ring disk electrode (RRDE). The Pd₂Co/C catalysts prepared from [Et₄N]₂[PdCo₂(4-I-3,5-Me₂-pz)₄Cl₄] showed the best ORR activity and higher methanol tolerance, compared with commercial platinum catalysts. The ORR electron transfer number for Pd₂Co/C was 3.7, similar to the Pd/C commercial catalysts. Compared to the commercial catalysts, Pd₂Co/C and PdCo₂/C showed the highest hydrogen peroxide formation (%H₂O₂), 16 and 24%, respectively. Electrogeneration of hydrogen peroxide has potential application in advanced oxidation processes.

Full text of DOI:10.1016/j.ica.2017.06.034

Accepted Manuscript
Research paper
Electrochemical Study of Methanol Tolerant Oxygen Reduction Reaction
Pd Co /C-Catalysts from Organometallic Molecular Precursors
x
y
Lisandra Arroyo-Ramírez, Raphael G. Raptis, Carlos R. Cabrera
PII:
S0020-1693(17)30030-0
http://dx.doi.org/10.1016/j.ica.2017.06.034
ICA 17683
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
6 January 2017
5 June 2017
8 June 2017
Please cite this article as: L. Arroyo-Ramírez, R.G. Raptis, C.R. Cabrera, Electrochemical Study of Methanol
Tolerant Oxygen Reduction Reaction Pd Co /C-Catalysts from Organometallic Molecular Precursors, Inorganica
x
y
Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.06.034
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Electrochemical Study of Methanol Tolerant Oxygen Reduction Reaction Pd
Catalysts from Organometallic Molecular Precursors
x y
Co /C-
1
Lisandra Arroyo-Ramírez , Raphael G. Raptis , and Carlos R. Cabrera *
2
1
1
Department of Chemistry, Molecular Sciences Research Center, University of Puerto Rico, Río
Piedras Campus, San Juan, Puerto Rico 00931-3346
2
Department of Chemistry and Biochemistry, Florida International University, Miami, FL
3199-0001
3
*
Corresponding author E-mail: carlos.cabrera2@upr.edu
Abstract
In this work, we study the mechanism and kinetics of the oxygen reduction reaction (ORR) over
Pd/C, Pd Co/C and PdCo /C electrocatalysts synthesized by thermal reduction of organometallic
precursors, such as [Pd (µ-3-Ph-pz) [Et N] [Pd Co(µ-4-I-3,5-Me -pz) Cl ], and
Et N] [PdCo (µ-4-I-3,5-Me -pz) Cl ]. The methanol tolerance and formation of peroxide under
oxygen reduction reaction conditions were determined using a rotating disk electrode (RDE) and
a rotating ring disk electrode (RRDE). The Pd Co/C catalysts prepared from [Et N] [PdCo (4-I-
,5-Me -pz) Cl ] showed the best ORR activity and higher methanol tolerance, compared with
commercial platinum catalysts. The ORR electron transfer number for Pd Co/C was 3.7, similar
to the Pd/C commercial catalysts. Compared to the commercial catalysts, Pd Co/C and PdCo /C
showed the highest hydrogen peroxide formation (%H ), 16 and 24%, respectively.
2
2
3
6
]
,
4
2
2
2
4
4
[
4
2
2
2
4
4
2
4
2
2
3
2
4
4
2
2
2
2
O
2
Electrogeneration of hydrogen peroxide has potential application in advanced oxidation
processes.
1

Keywords: PdCo; Oxygen Reduction Reaction; Organometallic Molecular Precursors; Rotating
2 2
Ring Disk Electrode; H O
Introduction
Direct methanol fuel cells (DMFC) are promising power sources for portable devices due to
their high power density and low operating temperature [1, 2]. However, DMFCs face multiple
obstacles that hinder their commercialization. Two of these obstacles are: (1) the crossover of
methanol from the anode to the cathode through the membrane and (2) the sluggish kinetics of
the oxygen reduction reaction (ORR) at the cathode. Methanol crossover reduces the DMFC
efficiency and performance (ca. 200 mV), caused by the mixed potential generated at the cathode
[
3]. A competition between oxygen reduction and methanol oxidation occurs in Pt
electrocatalysts when the methanol crosses over from the anode to the cathode through the
membrane [4]. Platinum alloy and non-Pt based catalysts have been studied to reduce the high
cost, and improve the ORR activity and methanol tolerance [5-12].
Besides their lower cost, Pd-based electrocatalysts have been widely studied because of their
selectivity for ORR in acid media [10, 13-16]. For example, carbon-supported Pd-Co alloy
catalysts have shown high methanol tolerance as cathode material in acidic direct methanol fuel
cells [4, 17-20]. Moreover, such catalysts have demonstrated good ORR performance [2].
The ORR can proceed by a 4-electron transfer direct mechanism with water as the final
2 2
product, or an indirect 2-electron transfer mechanism with hydrogen peroxide (H O ) as an
–
intermediate [21, 22]. Equation 1 presents the direct 4e transfer pathway where oxygen is
reduced directly to water.
ꢁ
ꢂ
O + 4H + 4e → 2H O
[1]
ꢀ
ꢀ
2

–
Equations 2 to 4 present the 2e transfer pathway for oxygen reduction with hydrogen peroxide
as an intermediate.
ꢁ
ꢂ
O + 2H + 2e → H O
ꢀ
[2]
[3]
[4]
ꢀ
ꢀ
ꢁ
ꢂ
H O + 2H + 2e → 2H O
ꢀ
ꢀ
ꢀ
2
H O → 2H O + O
ꢀ ꢀ ꢀ ꢀ
–
In the 2e transfer pathway, the peroxide formed is electrochemically further reduced to water, or
it disproportionates to water and dioxygen [23]. The peroxide formation causes degradation of
the membrane, leading to decrease of cell durability [24, 25]. For ORR, the direct 4e- pathway is
desired, but it has been difficult to find catalysts to achieve it. Several theoretical studies of the
mechanism and kinetics of ORR in alloy catalysts have been reported [26-28]. Zhang et al. have
presented an overview of the proposals put forward towards the improvement of ORR in alloys
[
29]. The incorporation of Co in Pd electrocatalysts can improve the kinetics and mechanism of
ORR.
In this work we studied Pd
precursors [Pd (µ-3-Ph-pz)6,], [Et
,5-Me -pz) Cl ]. The ORR electrocatalytic activity and methanol tolerance of the palladium and
x
Co
y
/C catalysts derived from the organometallic molecular
3
4
N]
2
[Pd Co(µ-4-I-3,5-Me -pz) Cl ], and [Et N] [PdCo (µ-4-I-
2
2
4
4
4
2
2
3
2
4
4
palladium-cobalt catalysts synthesized by chemical and thermal reduction of the organometallic
precursors was studied. The kinetics and mechanism of the ORR was determined using the
rotating ring disk electrode (RRDE) technique. The number of electrons transferred and the
2 2
amount of peroxide (H O ) formed during ORR was also determined. Electrogeneration of
hydrogen peroxide has potential application in advanced oxidation processes [30].
3

Experimental Section
The synthesis and characterization of the carbon supported electrocatalysts was described
in detail elsewhere [31]. Briefly, the chemical reduction (CRht) was carried out by mixing the
tailored organometallic precursor such as [Pd (µ-3-Ph-pz) ] [32-35], [Et N] [Pd Co(µ-4-I-3,5-
Me -pz) Cl ], and [Et N] [PdCo (µ-4-I-3,5-Me -pz) Cl ] [36] (See Table 1) with the Vulcan XC-
2R in THF solvent under sonication. After uniform dispersion was achieved, excess of reducing
agent (NaBH ) was slowly added dropwise with stirring, the solution was filtered, washed and
dried. Then, the resulting solid was heat treated at 600 °C under a N atmosphere. The thermal
reduction (TR), of the organometallic complexes, was done by mixing the precursor with Vulcan
XC-72R in CH Cl under constant stirring followed by sonication until solvent evaporation. The
reduction process was performed in a closed tube furnace by increasing the temperature over 620
C under H atmosphere. The samples were labeled, according to the synthesis method and
precursor used as Pd/C (CRht), Pd/C (TR), Pd Co/C (TR) and PdCo /C (TR). The commercial
3
6
4
2
2
2
4
4
4
2
2
2
4
4
7
4
2
2
2
°
2
2
2
catalysts on carbon support used were 20% Pd/C (BASF), 20% Pt/C (Alfa Aesar) and 20%
PtCo/C (BASF).
The electrochemical measurements were performed using an AutoLab potentiostat
(
PGSTAT 30) and PINE bipotentiostat (AFCBP1) with a MSRX rotator (PINE Instruments) for
the RDE and RRDE experiments, respectively. A three-electrode cell was employed in all
experiments with a graphite rod as auxiliary electrode and Hg|Hg SO (K SO , sat.; 0.64 V vs.
2
4
2
4
NHE) as reference electrode. For RDE technique, a glassy carbon electrode (PINE Instruments,
E2M) modified with the catalysts ink was employed as working electrode. For RRDE technique,
glassy carbon (GC) disk and platinum (Pt) ring from PINE Instruments was used. Working
2 4
electrodes were polished mechanically in alumina and electrochemically in 0.5 M H SO . The
4

catalyst ink was prepared by mixing 1 mg of catalyst, 250 µL of 2-propanol (Aldrich) and 8 µL
®
Nafion 5 wt% (Aldrich), followed by sonication for one-half hour. Finally, the GC surface was
modified with 8 µL (RDE) and 10 µL (RRDE) of the catalysts ink mixture and was allowed to
completely dry before the voltammetric experiments were carried out.
The cyclic voltammetry (CV) was carried out in a potential window between -0.65 V and 0.9
V vs. Hg|Hg
between -0.64 V to 0.66 V and -0.65 V to .076 V vs. Hg|Hg
electrocatalysts and Pt ring, respectively. ORR activity was determined by linear sweep
voltammetry (LSV) at potentials between 0.6 V to -0.5 V vs. Hg|Hg SO in oxygen saturated 0.5
M H SO solution with a scan rate of 5 mV/s. The study of the electrocatalysts tolerance toward
methanol was carried out in 1.0 M MeOH/0.5 M H SO solution purged with oxygen for 15
2
SO
4
in 0.5 M H
2
SO
4
at a scan rate of 100 mV/s under argon. A potential window
2
SO was used for the Pt
4
2
4
2
4
2
4
minutes (saturated) at a scan rate of 5 mV/s. The RDE and RRDE measurements were done at
rotating speed of 1,600 rpm. Also, a study with varying rotation speed between 500 rpm to 3000
2 4
rpm was performed. The ring potential was held constant at 0.56 V vs. Hg|Hg SO (ca. 1.2 V vs.
RHE) for the peroxide formation detection. The deionized water used for the experiments was
previously distilled and pumped through a Nanopure system (Barnstead) to give 18 MΩ-cm
resistivity. All electrochemical experiments were carried out at room temperature.
2
The geometrical area of the glassy carbon (GC) electrode was 0.196 cm . The RRDE
2
have surface areas of 0.2475 cm for the GC disk and 0.1866 cm for the Pt ring. The
2
electroactive surface area (ESA) was determined by measuring the charge of the Pd oxide
2 4
reduction region, from the CV in 0.5 M H SO , divided by the theoretical charge for a palladium
2
oxide monolayer reduction (424 µC/cm ) [37-39]. For platinum, ESA was determined by
measuring the charge of the Pt hydrogen adsorption/desorption region, from the cyclic
5

2
voltammetry in 0.5 M H
currents were normalized to the ESA of the electrode. The mass activity (MA) was determined
by dividing the current by the catalyst mass (mg , where M is Pd or Pt).
The number of electrons transferred per oxygen (n) and the molar fraction of peroxide
2 4
SO , divided by the theoretical charge of 210 µC/cm [40-42]. All
M
formation (ꢃ
) were calculated using the following equations [43, 44]:
ꢅ
ꢄ ꢆ
ꢅ
ꢈꢉꢊ
ꢇ =
[5]
[6]
ꢉ ꢁꢋꢉ /ꢍꢎ
ꢊ
ꢌ
ꢀ
ꢉ /ꢍ
ꢌ
ꢃ
=
ꢉ ꢁꢋꢉ /ꢍꢎ
ꢊ
ꢏ ꢐ
ꢅ
ꢅ
ꢌ
where i
The collection efficiency (N) of 37% was provided by the manufacturer of the ring-disk electrode
8]. The currents (i and i ) at 0.06 V vs. Hg|Hg SO (ca. 0.7 V vs. RHE) corresponding to
D R
is the disk current, i is the ring current and N is the collection efficiency of the RRDE.
[
D
R
2
4
typical fuel cell operating potential was used for determination of number of electrons
transferred and peroxide formation.
2 2
The morphology of the Pd Co and PdCo nanoparticles was determined by electron microscopy
techniques. A Carl Zeiss LEO-922 TEM microscope with Scanning TEM (STEM) capabilities
was used to obtain transmission electron microscopy images of the carbon-supported
nanoparticles. Also, a FEI Tecnai F20 with acceleration voltage of 200kV was used. The FEI
T12 Spirit TEM STEM with accelerating voltage of 120 kV was used to obtain the scanning
transmission electron microscopy images. High resolution scanning electron microscopy (SEM)
images and energy-dispersive X-ray fluorescence spectroscopy (EDS) mapping was performed
in a FEG Hitachi S-5500 ultra high resolution electron microscope at 30 kV.
Results and Discussions
6

The morphology and composition of the Pd-based catalytic nanoparticles supported on
carbon was reported previously [31]. Figure 1 shows the cyclic voltammograms for Pd/C,
Pd
adsorption/desorption peaks was observed for the Pd/C (CRht) and Pd/C (TR) catalysts (see
Figure 1a). However, the hydrogen adsorption/desorption peaks were not defined for Pd Co/C
TR) and PdCo /C (TR) (see Figure 1b and 1c). The voltammetry for the Pd Co/C (CRht) and
PdCo /C (CRht) catalysts are not presented, because previous studies showed incomplete
2 2 2 4
Co/C and PdCo /C electrocatalysts in 0.5 M H SO . From the voltammograms, the hydrogen
2
(
2
2
2
reduction of their precursors [31]. X. Li et al. have explained that Co incorporation within the
Pd-Co catalysts prevents the dissolution of hydrogen into Pd [45]. L. Zhang et al. proposed that
the differences in CV might be due to the formation of Pd-Co alloy [46]. Table 2 presents the
electrochemical surface area (ESA) obtained from the CV. The Pd/C (CRht) has larger ESA than
Pd/C (TR) catalysts, attributed to the smaller particle size. The Pd
have practically the same ESA, probably because the % of metal in the PdCo
than Pd Co/C (TR) [21].
2
Co/C (TR) and PdCo
2
/C (TR)
2
/C (TR) is greater
2
Linear sweep voltammetry (LSV) in acid solution saturated with oxygen at rotation speed of
,600 rpm was used to determine the catalysts’ activity toward oxygen reduction reaction (ORR)
1
and their methanol tolerance. Figure 2 shows linear sweep voltammograms of the (a) Pd/C, (b)
Pd Co/C and (c) PdCo /C catalysts without and with 1 M MeOH in 0.5 M H SO . Figure 2a
2
2
2
4
shows that Pd/C (CRht) has less ORR activity than the Pd/C (TR) catalyst. Table 2 summarizes
the cathodic current density and mass activity for the oxygen reduction reaction at 0.1 V vs.
Hg|Hg
2
SO
4
in 0.5 M H
2
SO
4
solution for the carbon supported catalytic nanoparticles using RDE
Co/C (TR).
techniques. The Pd/C (TR) has a somewhat higher current density than Pd
2
Nevertheless, Pd
2
Co/C (TR) has higher mass activity than Pd/C (TR).
7

Table 3 presents the results for the ORR onset potentials at specific current density and mass
activity for the Pd/C, Pd Co/C and PdCo /C catalysts with and without methanol, using the RDE
2
2
technique. The Pd/C (TR) has a positive onset potential of 28 mV when compared with Pd/C
2
CRht) at a current density of -20 µA/cm . This result was unexpected, because the Pd/C (CRht)
(
has the smallest particle size and largest superficial area and should thus have greater ORR
catalytic activity. The oxygen reduction reaction in Pd
of 30 mV when compared with PdCo
mass activity, the Pd
compared with PdCo
2
Co/C (TR) has a positive onset potential
2
2
/C (TR) at a current density of -20 µA/cm . In term of
2
Co/C (TR) had a positive shift of the ORR onset potential of 26 mV when
/C (TR). The Pd Co/C (TR) catalyst has a similar ORR onset potential
2
2
when compared with Pd/C (TR) catalysts. These results confirm that the incorporation of the
non-noble metal (cobalt) into the catalyst does not affect the electrocatalytic activity towards
ORR and might even be improving it.
The methanol tolerance of the Pd catalysts was determined. The Pd
2 2
Co/C (TR) and PdCo /C
(
TR) have higher current densities and mass activity in the presence of methanol (see Figures 2b
and 2c). For all Pd catalysts, slightly positive shifts in the ORR onset potentials in presence of
methanol were observed. Tarasevich et al. has explained that methanol increases the solubility of
oxygen, which may be the reason for this increased activity [47]. These results confirm the
activity toward ORR with high methanol tolerance of the Pd-based carbon supported catalysts.
The voltammetric measurements of the commercial carbon-supported catalysts were carried
out to compare their ORR activity and MeOH tolerance with those of our catalysts. The Pt/C
catalyst has the higher electrochemical surface area (see Table 2). LSV in presence of methanol
was carried out to determine the selectivity of the catalysts for oxygen reduction. Figure 3 shows
the oxygen reduction electrocatalytic activity without and with methanol for Pd/C, Pt/C and
8

PtCo/C commercial catalysts. In the LSV, can be observed the well-defined kinetics, mixed and
diffusion regions of commercial Pt/C and PtCo/C electrocatalysts. The ORR activity trend is
PtCo/C > Pt/C > Pd/C. The presence of cobalt in the Pt catalyst improves the ORR
electrocatalytic activity. The Pd/C commercial catalyst has overpotential of 67 mV and 128 mV
than Pt/C and PtCo/C commercial catalysts, respectively (see Figure 3).
The PtCo/C commercial catalyst presents the higher mass activity towards the oxygen
reduction reaction in acid solution. In presence of MeOH, the observed ORR electrocatalytic
activity was from best to worst: Pd/C >> Pt/C >>> PtCo/C; the latter two catalysts have poor
electrocatalytic activity towards ORR in presence of methanol. In the Pt/C catalyst, with MeOH,
the methanol oxidation and oxygen reduction reaction occur simultaneously, causing a mixed
potential [3]. It is well known that Pt carbon-supported catalysts have better ORR activity and
worse MeOH tolerance than Pd carbon-supported catalysts [48, 49]. The incorporation of cobalt
in the platinum catalyst improves the methanol tolerance, but was not selective to ORR, as
shown Figure 3. The advantage of the Pd catalysts is its selectivity to oxygen reduction reaction
in acid media.
Figure 4 shows a comparison between our Pd-based and the commercial catalysts in ORR
activity and methanol tolerance shows that the former have higher current density (Fig. 4a). The
2
ORR onset potential decreases in the following order: PtCo/C > Pt/C > Pd Co/C (TR) > Pd/C
(
TR) > Pd/C > PdCo /C (TR) at -2 mA/mg . As discussed above, the Pt catalysts have better
2 M
ORR activity and poor MeOH tolerance. The better ORR activity of Pt/C catalyst compared with
Pd/C and Pd-Co/C catalysts has also been reported by others [4, 46, 47]. On the other hand, Pd-
based carbon-supported catalysts have a high tolerance towards methanol. Also, a slight positive
shift of the onset potential for our catalysts in presence of methanol is noted (see Figure 4). This
9

2
comparison shows that Pd Co/C (TR) is the best among the Pd catalysts studied here, in
agreement with literature reports [4]. The incorporation of cobalt to palladium catalysts improves
the electrocatalytic oxygen reduction activity without affecting the methanol tolerance.
Figure 5 shows the ORR activity for the Pd
speed from 500 rpm to 3000 rpm. Similar voltammetry was observed for the Pd/C (CRht), Pd/C
TR) and PdCo /C (TR) catalysts (not shown). In the voltammograms, the kinetic regions are the
2
Co/C (TR) catalysts upon variation of rotation
(
2
same with changes in the rotation speeds. The mixed kinetic-diffusion and diffusion-controlled
regions show an increase of current densities with increasing rotation speeds. The ORR
voltammograms are highly affected by the rotation speed of the electrode, as higher rotation
speed facilitates the mass transport of oxygen to the electrode surface [50]. Similar results were
observed for the commercial catalysts (not shown). The ORR mechanism can be inferred from
1
/2
the Koutechy-Levich plot (1/i vs. 1/ω ), according the following equation [42]:
ꢑ
ꢉ
ꢑ
ꢑ
ꢑ
ꢑ
= ꢉ + = +
[7]
ꢉ
ꢉ
ꢒ
ꢔ.ꢕꢀꢖꢗꢘꢙꢅ/ꢚꢛꢜꢝꢞ/ꢟꢠꢞ/ꢅ
ꢒ
ꢓ
where i
K
L
is the kinetic current density, i the mass transport limiting current, n the number of
–
electrons (e ) transferred in the reaction, F Faraday’s constant, A the electrode area, D the
diffusion coefficient, C the concentration of oxygen in the solution, v the kinematic viscosity and
ω is the rotation speed. Figure 6 shows the nearly straight line Koutechy-Levich (K-L) plots for
2 2
our (a) Pd/C (CRht) and Pd/C (TR), (b) Pd Co/C (TR) and PdCo /C (TR) catalysts, and (c) Pd/C,
Pt/C and PtCo/C commercial catalysts, indicating a first-order oxygen reduction reaction [4]. The
results reveal that the ORR principally follows a 4e- transfer pathway, with lesser contributions
by 4e- and 2e- pathways with peroxide as intermediate. The RRDE data presented below
corroborate the conclusions derived from the K-L plots.
10

Rotating ring disk electrode technique was applied to the study of the ORR kinetics and to
–
distinguish between the 4e and 2e transfer mechanisms at carbon supported palladium and
–
palladium-cobalt nanocatalysts by determining the amount of peroxide formed. In the RRDE
experiment, the potential of ring was held at 0.56 V vs. Hg|Hg
formed on the disk electrode during ORR is limited by diffusion [51].
Figure 7 presents the RRDE voltammograms of our Pd/C (CRht), Pd/C (TR), Pd
and PdCo /C (TR) catalysts for the oxygen reduction reaction and the peroxide formation. It is
observed that above +0.1 V the reaction is kinetically controlled, but diffusion controlled below -
.1 V. In the -0.1 to +0.1 V range, the ORR is controlled by a mix of kinetic and diffusion
2 4
SO , where the oxidation of the
2 2
H O
2
Co/C (TR)
2
0
process. Figure 8 shows the oxygen reduction reaction and the peroxide formation for the Pd/C,
Pt/C and PtCo/C commercial catalysts at rotation speed of 1,600 rpm. The ring current is not
significant above 0.1 V and starts to increase from the mixed region to the diffusion process
2 2
region. The calculated percentage of peroxide formation (%H O ) and number of electrons
transferred (n) during the oxygen reduction are listed in the Table 4. Because the formation of
peroxide is dependent of the potential, the values were calculated at a typical fuel cell operating
potential of 0.70 V vs. RHE. The ring currents are negligible with respect to disk current for the
Pt/C commercial, PtCo/C commercial and our Pd/C (TR) electrocatalysts. The negligible
peroxide formation (<10%) indicates a four-electron process with water as principal product
[
10]. The number of electrons transferred in the oxygen reduction reaction is 3.9 for the Pt/C,
PtCo/C and Pd/C (TR). For the Pd/C commercial, Pd/C (CRht) and Pd Co/C (TR), similar
formation of peroxide was found, indicating a mix of 4e and 2e transfer pathways. The value of
n is the same for the Pd/C commercial and Pd Co/C (TR) catalyst (n = 3.7). Typically, the
overall n < 4, because of the oxygen reduction mixed pathways (2e and 4e ). Adzic and co-
2
–
–
2
–
–
11

workers calculated the number of electrons involved in the ORR to be approximately 3.9 for a
Pd Co/C alloy electrocatalyst [10]. The higher H formation was found in the PdCo /C (TR)
2
2
O
2
2
catalysts with a value of 24%. RRDE voltammograms in presence of MeOH of the Pd/C and Pd-
Co/C bimetallic catalysts are similar to those obtained in the absence of methanol (not shown).
–
The oxygen reduction by 4e transfer pathway is the principal one for all catalysts studied here,
–
although 2e transfer pathways are present.
Electron Microscopy of Pd2Co/C Catalyst
2 2
STEM image and elemental maps line scan of Pd Co/C and PdCo /C catalysts particle was done.
The X-ray fluorescence emission line scan shows that the nanoparticle is composed of the
elements palladium and cobalt (see Figure 9). The nanoparticle is bimetallic and not an oxide of
the metals. Figure 10 and 11 show FE-SEM and X-ray fluorescence mapping images for the Pd
(
2 2
red), Co (cyan) and C (green) elements for the Pd Co/C (TR) and PdCo /C catalysts,
respectively. In the X-ray fluorescence mapping image, it can be observed the carbon signals
from the support (Vulcan) throughout the image and the Pd and Co signals at only one area. The
bimetallic particle was formed like a palladium core and cobalt shell. The literature explains that
homogeneous alloys on the surface of a metal with high negative fee energy for oxide formation
(
Co) and metal with high standard reduction potential (Pd) can increase the ORR catalytic
activity [19, 52].
Conclusions
The RDE technique was used successfully to study the ORR activity and MeOH
tolerance of the Pd-based carbon supported electrocatalysts. The most positive onset potential
was determined for the Pd Co/C (TR) electrocatalyst, which exhibits the highest mass activity
2
12

compared to the other Pd-based catalysts. A positive onset potential shift was found for the Pd-
based/C catalyst in presence of MeOH. The palladium catalysts have better methanol tolerance
than Pt catalysts. The Pd-Co/C bimetallic catalysts are selective to the oxygen reduction reaction.
The percentage of H
2
2
O formation for the Pd-based catalyst was between 7% and 24%. The
–
oxygen reduction by 4e transfer pathway with water as product was the principal pathway for all
–
catalysts, although 2e transfer pathway with peroxide as intermediate also occurred, but to a
lesser extent. The number of electrons transferred in the oxygen reduction reaction was found for
the Pd
was found to have the same kinetics and mechanism as the Pd/C commercial catalysts. The
Pd Co/C can be an excellent cathode electrocatalyst for direct methanol fuel cells. Compared to
the commercial catalysts, Pd Co/C and PdCo /C showed the highest hydrogen peroxide
formation (%H ), 16 and 24%, respectively. Electrogeneration of hydrogen peroxide has
2 2
Co/C (TR) electrocatalyst to be 3.7. From the RRDE measurements, our Pd Co/C catalyst
2
2
2
2
O
2
potential application in advanced oxidation processes.
Acknowledgments
The authors would like to acknowledge the financial support of NASA-MIRO Center for
Advanced Nanoscale Materials (CANM) Grant numbers NNX08BA48A and NNX10AQ17A
and the NASA Puerto Rico Space Grant Consortium (NASA Training Grant No.
NNG05GG78H).
13

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17

FIGURES
2
Figure 1. Cyclic voltammograms of (a) Pd/C (CRht) and Pd/C (TR), (b) Pd Co/C (TR) and (c)
2 2 4
PdCo /C (TR) catalysts in 0.5 M H SO at a scan rate of 100 mV/s.
18

Figure 2. Rotating disk electrode (RDE) linear sweep voltammograms for (a) Pd/C, (b) Pd
2
Co/C
and (c) PdCo /C catalysts without (solid line) and with 1 M MeOH in 0.5 M H SO (dashed line)
2
2
4
purged with oxygen for 15 min before each experiment, scan rate of 5 mV/s and rotation speed
of 1,600 rpm.
19

Figure 3. Rotating disk electrode (RDE) linear sweep voltammetry for the Pd/C, Pt/C and
PtCo/C commercial catalysts without (solid line) and with 1 M MeOH in 0.5 M H SO (dashed
2
4
line) purged with oxygen for 15 min at 10 mL/min before each experiment, scan rate of 5 mV/s
and rotation speed of 1,600 rpm.
20

Figure 4. Rotating disk electrode (RDE) linear sweep voltammetry for (a) oxygen reduction
reaction activity and (b) methanol tolerance for Pt/C (solid line), PtCo/C (dashed line), Pd/C
(
PdCo /C (short dashed line) catalysts. Scan rate of 5 mV/s and a rotation speed of 1,600 rpm.
dotted line) commercial and our Pd/C (dash-dotted line), Pd
2
Co/C (dash-dot-dotted line) and
2
21

Figure 5. Rotating disk electrode (RDE) linear sweep voltammetry of the Pd
0
2
Co/C catalyst in
.5 M H SO saturated with oxygen at varying rotation speeds and scan rate of 5 mV/s.
2
4
22

Figure 6. Koutechy-Levich plots of the RDE study of ORR in 0.5 M H
CRht) and Pd (TR), (b) Pd Co/C (TR) and PdCo /C (TR), and (c) commercial Pd/C, Pt/C and
PtCo/C catalysts.
2 4
SO of the (a) Pd/C
(
2
2
23

Figure 7. Rotating ring disk electrode (RRDE) voltammograms of the (upper) disk (i
D
) and
Co/C (TR)
SO saturated
(
(
lower) ring (i
dash-dotted line) and PdCo
R
) currents for Pd/C (CRht) (solid line), Pd/C (TR) (dashed line), Pd
/C (TR) (dash-dot-dotted line) catalysts in 0.5 M H
4
2
2
2
with oxygen at scan rate of 5 mV/s and speed rotation of 1,600 rpm. The ring potential was held
at 0.56 V vs. Hg|Hg SO (ca. 1.2 V vs. RHE) for the peroxide formation detection.
2
4
24

Figure 8. Rotating ring disk electrode (RRDE) voltammograms for the commercial Pd/C (solid
line), Pt/C (dashed line) and PtCo/C (dash-dotted line) catalysts in 0.5 M H SO saturated with
oxygen at scan rate of 5mV/s and speed rotation of 1,600 rpm; (upper) i and (lower) i are disk
SO (ca. 1.2 V vs.
2
4
R
D
and ring measured currents. The ring potential was held at 0.56 V vs. Hg|Hg
RHE) for the peroxide formation detection.
2
4
25

Figure 9. STEM images and elemental line scan in particle after thermal reduction process of
Pd Co precursor on carbon Vulcan.
2
26

Figure 10. Scanning electron microscopy image and energy dispersive X-ray analysis (SEM-
EDS) elemental mapping showing the Pd, Co, and C X-ray fluorescence maps for the Pd
catalysts.
2
Co/C
27

Figure 11. High resolution SEM with energy dispersive X-ray analysis (SEM/EDS) elemental mapping
images for the Pd, Co, and C elements in the PdCo /C catalyst material.
2
28

TABLES
Table 1. Chemical and molecular structures of the organometallic palladium
and palladium-cobalt precursors.
Chemical formula
Molecular structure
3 6
*[Pd (µ-3-Ph-pz) ]
[
4 2 2 2 4 4
Et N] [Pd Co(µ-4-I-3,5-Me -pz) Cl ]
I2
N4
Cl2
N3
Co1
Cl1
Pd1
*
4 2 2 2 4 4
*[Et N] [PdCo (µ-4-I-3,5-Me -pz) Cl ]
N1
N2
I1
Note: *From Ref. 33-36. Copyright © 2004, American Chemical Society. **From Ref. 37. Copyright
Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.
29