Comparison of electrocatalytic activity of Pt1-: XPdx/C catalysts for ethanol electro-oxidation in acidic and alkaline media

Article abstract of DOI:10.1039/d0ra00483a

In this paper, a comparision of Pt_1-xPd_x/C catalysts for ethanol-oxidation in acidic and alkaline media has been investigated. We prepared Pt_1-xPd_x/C catalysts with different ratios of Pt/Pd (x at% = 0, 27, 53, 77 and 100) by the formic acid reduction method. The obtained Pt_1-xPd_x/C catalysts were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), induced coupled plasma-atomic emission spectroscopy (ICP-AES), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Structural and morphological investigations of the as-prepared catalysts revealed that the metallic particle size increases with increasing Pd content in the catalyst. The electrocatalytic performances and stabilities of Pt_1-xPd_x/C catalysts were tested by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometry (CA) measurements for ethanol oxidation in acidic and alkaline media. The electrochemical measurements demonstrate that Pt_1-xPd_x/C catalysts exhibit much higher electrocatalytic activity for alcohol oxidation in alkaline media than that in acidic media. The composition of Pt/Pd has a significant impact on the ethanol-oxidation in both acidic and alkaline media. The Pt₂₃Pd₇₇/C catalyst shows the highest electrocatalytic performance with a mass specific peak current of 2453.7 mA mg_PtPd^-1 in alkaline media, which is higher than the Pt₇₇Pd₂₃/C with the maximum of peak current of 339.7 mA mg_PtPd^-1 in acidic media. Meanwhile, the effect of electrolyte, CH₃CH₂OH concentrations and scan rates was also studied for ethanol-oxidation in acidic and alkaline media.

Full text of DOI:10.1039/d0ra00483a

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PAPER
Comparison of electrocatalytic activity of Pt Pd /
1
ꢀx
x
C catalysts for ethanol electro-oxidation in acidic
Cite this: RSC Adv., 2020, 10, 10134
and alkaline media†
Qiang Zhang, Ting Chen, Rongyan Jiang and Fengxing Jiang *c
a
a
b
In this paper, a comparision of Pt1ꢀxPd
been investigated. We prepared Pt1ꢀxPd
and 100) by the formic acid reduction method. The obtained Pt1ꢀxPd
x
/C catalysts for ethanol-oxidation in acidic and alkaline media has
/C catalysts with different ratios of Pt/Pd (x at% ¼ 0, 27, 53, 77
/C catalysts were characterized by
x
x
X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), induced coupled plasma-atomic
emission spectroscopy (ICP-AES), X-ray photoelectron spectroscopy (XPS) and transmission electron
microscopy (TEM). Structural and morphological investigations of the as-prepared catalysts revealed that
the metallic particle size increases with increasing Pd content in the catalyst. The electrocatalytic
performances and stabilities of Pt1ꢀxPd
voltammetry (LSV) and chronoamperometry (CA) measurements for ethanol oxidation in acidic and
alkaline media. The electrochemical measurements demonstrate that Pt1ꢀxPd /C catalysts exhibit much
x
/C catalysts were tested by cyclic voltammetry (CV), linear sweep
x
higher electrocatalytic activity for alcohol oxidation in alkaline media than that in acidic media. The
composition of Pt/Pd has a significant impact on the ethanol-oxidation in both acidic and alkaline media.
The Pt23Pd77/C catalyst shows the highest electrocatalytic performance with a mass specific peak
Received 16th January 2020
Accepted 24th February 2020
PtPdꢀ
1
current of 2453.7 mA mg
in alkaline media, which is higher than the Pt77Pd23/C with the maximum
DOI: 10.1039/d0ra00483a
PtPdꢀ1
of peak current of 339.7 mA mg
3 2
in acidic media. Meanwhile, the effect of electrolyte, CH CH OH
rsc.li/rsc-advances
concentrations and scan rates was also studied for ethanol-oxidation in acidic and alkaline media.
Among various metals, Pd is known to be an excellent candidate
to form bimetallic Pt-based catalysts towards ethanol oxida-
1
. Introduction
16
Direct ethanol fuel cells (DEFCs) have received considerable tion, due its ability to remove CO species adsorbed on Pt and
17
interest due to their high energy density, low toxicity and price, then enhance the activity and the anti-poison ability. More-
1,2
and environmentally friendly nature. Currently, Pt is the most over, Pd easily forms alloys with Pt due to the similar atom
3
18
efficient electrocatalyst that has been utilized in DEFCs.
structure, and has a lower price than Pt.
However, it is still a challenge to realise large-scale commer-
Recently, some researchers have been devoted to fabricating
cialization due to their low abundance, high cost, and poor various PtPd nanostructures for ethanol oxidation, including
4
,5
19
20
21
22
tolerance of reaction intermediates (mainly CO). Thus, to alloys,
nanocubes,
nanoowers,
nanorods,
nano-
23
24
25
solve the issue, many researchers have been devoted to dendrites and core–shell structures. For instance, Liu et al.
decreasing the Pt loading by incorporating alternative lower prepared a PtPd alloy catalyst supported on graphene for
6
7
8
9
cost metals into the Pt such as Cu, Pd, Ni, Fe, etc, and ethanol oxidation in alkaline solutions and found that PtPd
enhancing the CO tolerance by introducing some metal oxides with the ratios of 0.46 : 0.54 possessed excellent catalytic activity
10
11
12
13
26
into the catalyst such as CeO
2
,
TiO
2
,
SnO
2
,
RuO
2
,
etc. and durability. Lu and co-workers
fabricated three-
Hence, the catalytic performances and CO resistance of Pt can dimensional platinum–palladium hollow nanospheres with
be boosted by modifying the surface electronic structure of Pt a dendritic shell (PtPd-HNSs) through a facile and economic
1
4,15
and the bifunctional mechanism between the two metals
route, which show remarkable activity and durability toward the
methanol oxidation reaction (MOR) in acidic media, owing to
the coupled merits of bimetallic nanodendrites and a hollow
a
27
School of Science, Shandong Jianzhu University, Jinan 250101, China
interior. And Alcaide et al. revealed that prepared Pd–Pt cata-
b
School of Materials Science and Engineering, Shandong Jianzhu University, Jinan
lysts with a Pd : Pt atomic ratio of 25 : 75 showed the best
performance in direct methanol single fuel cells in acidic
2
50101, China
c
Department of Physics, Jiangxi Science and Technology Normal University, Nanchang
28
media. Shen and co-workers reported that Pd/C has a higher
catalytic activity and better steady-state performance for ethanol
electrooxidation than Pt/C in alkaline media, which probably
3
†
1
30013, P. R. China. E-mail: f.x.jiang@live.cn
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra00483a
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Table 1 Comparison of parameters on as – prepared catalysts
Metal loading (wt%)
Pt : Pd (at/at)
2
ꢀ1
˚
a/A
Catalysts
Pt
Pd
By EDX
By ICP
d/nm by XRD
ECSA/m g
Pt/C
9.14
6.53
5.06
3.02
—
—
—
—
3.921
3.911
3.901
3.891
3.891
4.0
4.3
5.9
9.8
11.5
27.07
25.35
23.12
21.98
16.89
Pt73Pd27/C
Pt47Pd53/C
Pt23Pd77/C
Pd/C
1.33
3.15
5.60
7.79
83 : 17
54 : 46
39 : 61
—
73 : 27
47 : 53
23 : 77
—
due to the fact that OH species formed easily on the Pd surface stirring. Aer that, the above obtained solution was heated to
ꢁ
react with CO-like intermediate species to produce CO
2
and 80 C by an oil bath while stirring and reuxing for 3 h. Finally,
29
30
release the active sites. Carvalho et al. prepared carbon- the Pt1ꢀxPd
x
/C catalysts were washed and dried in a vacuum
ꢁ
supported Pd, Pt, Pt
1
Pd
1
, and Pt
3
Pd
1
electrocatalysts for oven at 60 C. In addition, Pt/C, Pd/C, PtPd (50 : 50)/C and PtPd
ethanol oxidation and compared the electrochemical activities (75 : 25)/C catalysts were also prepared by a similar way. Here,
of catalysts in acid and alkaline media. Although many papers a theoretical amount of PtPd loading on carbon black was
about the best ratio of PtPd for ethanol electro-oxidation in controlled at 10 wt% with a nominal Pt : Pd atomic ratios of
acidic or alkaline medium were investigated, the involving 75 : 25, 50 : 50 and 25 : 75. The EDS spectra showed that the Pt
comparision of the electroactivity toward ethanol oxidation in and Pd elements in the Pt1 Pd /C compositions with the
ꢀx
x
both acid and alkaline solution is not available in the literature atomic ratios of 83 : 17, 54 : 46 and 39 : 61 summarized in
in details.
Table 1, which were close to the feed ratio of the precursors. In
Herein, we report Pt1ꢀxPd
x
/C catalysts with different ratios of addition, the actual loading and the atomic ratios of Pt : Pd
Pt/Pd by formic acidic reduction method for ethanol electro- were 73 : 27, 47 : 53 and 23 : 77 according to the measurement
oxidation in acidic and alkaline media. The obtained samples of inductive coupled plasma emission spectrometer (ICP) and
were characterized by X-ray diffraction (XRD), energy-dispersive the data were summarized in Table 1, which is similar to the
X-ray analysis (EDX), transmission electron microscopy (TEM), results of nominal compositions. Based on the results of the ICP
induced coupled plasma-atomic emission spectroscopy (ICP- data, the as-prepared catalysts were denoted as Pt/C, Pt Pd /C,
7
3
27
AES). The catalytic performance and stability of the composite Pt47Pd53/C, Pt23Pd77/C and Pd/C, respectively.
electrodes for the ethanol oxidation under both in acid and
alkaline conditions were explored by cyclic voltammetry and
chronoamperometry. In addition, the effect of electrolyte and
2.3 Characterizations
A S-4700 energy dispersive X-ray spectrometry (EDX) and 710-ES
ICP were used to obtain the composition of as-prepared PtPd
alloy catalysts. Transmission electron microscope (TEM, JEM-
CH
3 2
CH OH concentrations, scan rates were also studied for
ethanol-oxidation in acidic and alkaline media.
1011, Japanese Electronics Co. Ltd, Japan) was used to charac-
terize the morphology and structure of the as-prepared mate-
rials. Power X-ray diffraction (XRD, PANalytical X'Pert PRO MRD
2
. Experimental
2.1 Materials and reagents
system with Cu K radiation) spectra were used to analyse the
a
H
2
PtCl $6H O, PdCl KOH, HCOOH and
6
2
2
,
H
2
SO
4
were
structure characterisation of PtPd alloy catalysts. X-ray photo-
electron spectroscopy (XPS) experiments were carried on a K-
Alpha instrument (ThermoFisher), and the binding energies
were calibrated by C 1s (284.8 eV).
purchased from Sinopharm Chemical Reagent Co., Ltd. All
chemicals were of analytical grade purity. Carbon black was
obtained from Suzhou Baohua Carbon Black Co., Ltd. A
ꢀ1
deionized water (resistance: 18.25 MU cm ) was used for the
preparation of aqueous solution.
2.4 Electrochemical measurements
In order to perform the electrochemical measurements, it was
prepared a dispersion of catalysts, which consists in 10.0 mg as-
2.2 Catalyst preparation
In a typical preparation, the carbon-supported Pt1ꢀxPd
x
(x ¼ 0, prepared catalyst powders dispersing in the mixture of 4.0 mL
0
.27, 0.53, 0.77, and 1) catalysts were prepared by formic acidic deionized water and 50 mL Naon solution for 30 min. Then,
31
reduction method. Briey, 0.61 mL H
.72 mL H PdCl
2
PtCl
6
(7.723 mM) and a 10.0 mL homogeneous catalyst ink was spread on the surface of
ꢁ
0
2
4
solution (22.55 mM) with a corresponding Pt glass carbon electrode (GCE) and dried at 60 C. The metallic
to Pd molar ratio of 25 : 75 were added to 30 mL deionized water loading for Pt/C, Pt73Pd27/C, Pt47Pd53/C, Pt23Pd77/C and Pd/C on
ꢀ
2
ꢀ2
ꢀ2
and mixed with 50 mg carbon black. This mixture solution was GCE were about 0.032 mg cm , 0.028 mg cm , 0.029 mg cm
,
ꢀ2
ꢀ2
stirred for 30 min and ultrasonicated for 30 min at room 0.031 mg cm , and 0.027 mg cm , respectively.
temperature. Then, an aqueous solution of formic acidic (2.0 M, Electrochemical measurements were performed in a clas-
0 mL) was added slowly to the above mixture under rapid sical three-electrodes-cell by a CHI 660B electrochemical
4
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workstation at room temperature. A GCE (3.0 mm in diameter), with value of 0.89, B is the peak width at half maximum
Pt plate and a saturated calomel electrode (SCE) were used as intensity, and q is the peak position. The average particle size
the working, counter and reference electrodes, respectively. for the Pt, Pt Pd , Pt Pd , Pt Pd and Pd on carbon black
7
3
27
47
53
23
77
Unless otherwise stated, all the applied potential were refer- was about 4.0, 4.3, 5.9, 9.8, and 11.5 nm, respectively. Obviously,
enced to SCE. The electrocatalytic activity of ethanol in acid the particle size of carbon supported metals increases with the
medium was carried out 0.5 M H
solution while the alkaline oxidation of ethanol was performed report.
in 1.0 M KOH + 1.0 M CH CH OH by cyclic voltammogram (CV) form a large amount of product with a very tiny nucleus in
2 4 3 2
SO + 1.0 M CH CH OH Pd content increases, which is consist with the previous
36,37
The quick reduction reaction for Pt catalysts could
3
2
ꢀ
1
measurement at the scan rate of 50 mV s . Chronoamper- a short time and result in the well-dispersed nanoparticles,
ometries (CA) tests of samples were recorded at 0.5 V in acid while for Pd catalyst could result in the uneven dispersion of
3
8
solution or ꢀ0.5 V in alkaline solution for 3600 s, respectively. nanoparticles. The increase of particle size for Pt1 Pd alloy
ꢀx
x
The CA curves were performed in the same electrolyte as CV and Pd has an effect on the electrochemical active surface area
measurement.
(ECSA) of catalyst. As seen from the Table 1, the small size of
particles possess larger ECSA compared to the large particles.
The higher ECSA of catalyst is most likely ascribed to the high
dispersions of metal nanoparticles with small size on the
substrates, contributing to its high electrocatalytic performance
3. Results and discussion
3.1 XRD analysis
39,40
for ethanol oxidation.
The crystal structures of as-prepared Pt1ꢀxPd
x
/C catalyst were
determined using XRD and recorded in Fig. 1. All the catalysts
ꢁ
show a broad diffraction peak at about 25 , ascribed to the
reection of C (002) plane. For the pure Pt, the diffraction peaks 3.2 TEM analysis
ꢁ
ꢁ
ꢁ
ꢁ
situated at the 2q value of 39.8 , 46.4 , 67.5 , 81.3 are corre-
To investigate the morphological and structural feature of the Pt/
C, Pt73Pd27/C, Pt47Pd53/C, Pt23Pd77/C and Pd/C catalysts, TEM
analysis was conducted and recorded in Fig. 2. As shown in
Fig. 2A, for Pt/C catalyst, the Pt nanoparticles are uniformly
distributed on the carbon support without obvious agglomera-
tions. However, the Pd nanoparticles with large size on the Pd/C
catalyst appear agglomeration and connect together (Fig. 2E).
Compared with the mono-Pd counterpart, the dispersion of Pt
and PtPd nanoparticles in the mono/bimetallic catalysts exhibits
more uniform. Previous study revealed that Pd tends to form
sponding to (111), (200), (220) and (311) planes of face-centered-
cubic (fcc) crystalline Pt [JCPDS 04-0802], respectively. Mean-
while, the four peaks of the monometallic Pd at about 40.1 ,
6.7 , 68.1 , 82.0 are attributed to the (111), (200), (220) and
32
ꢁ
ꢁ
ꢁ
ꢁ
4
(
311) lattice planes of fcc crystalline Pd [JCPDS no. 46-1043],
14
respectively. For the Pt73Pd27/C, Pt47Pd53/C, Pt23Pd77/C cata-
lysts, the diffraction peaks shi slightly to higher 2q angles with
respect to Pt, with the increment of Pd amount. The lattice
constant in Table 1 and the average particle size were estimated
based on the diffraction peaks of (220). The lattice parameters
/C are smaller than that of Pt/C (3.921 A) and larger
than that of Pd/C (3.891 A), revealing the formation of Pt1ꢀxPd
twinned or multiple twinned seeds while Pt prefers to form single
˚
41,42
of Pt1ꢀxPd
x
crystal seeds,
which may be responsible for the less agglom-
˚
x
eration of Pt-containing Pd1 Pt /C electrocatalysts. Obviously,
ꢀx
x
33,34
alloy.
The average particle size was calculated from the XRD
analysis according to the Debye–Scherrer formula:
the sizes of the metallic nanoparticles increase with the increasing
the content of Pd in all samples, of which Pd/C catalyst shows the
largest particle size. The results of dimensions of metallic nano-
particles in TEM analysis are consist with the XRD patterns.
3
5
d ¼ kl/B cos q
where d is the crystalline size in nanometers, l is the wavelength
˚
of the radiation (1.54056 A for CuKa radiation), k is a constant
Fig. 1 XRD patterns of Pt/C, Pt73Pd27/C, Pt47Pd53/C, Pt23Pd77/C and Fig. 2 TEM images of Pt/C (A), Pt73Pd27/C (B), Pt47Pd53/C (C), Pt23Pd77
/
Pd/C catalysts. C (D) and Pd/C (E) catalysts.
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.3 XPS analysis
RSC Advances
3
X-ray photoelectron spectroscopy (XPS) was employed to inves-
tigate the surface composition and oxidation states of the as-
prepared samples. Fig. 3A shows the Pt 4f XPS spectra of Pt/C,
Pt73Pd27/C, Pt47Pd53/C and Pt23Pd77/C. Each curve exhibits two
prominent peaks located at 71.10 and 74.41 eV, which can be
43
assigned to the Pt 4f7/2 and Pt 4f5/2, respectively. Compared to
the XPS spectrum of Pt/C, the binding energies of Pt 4f slightly
Fig. 4 CV curves in 0.5 M H
C, Pt73Pd27/C, Pt47Pd53/C, Pt23Pd77/C and Pd/C. The current scale was
normalized by the ESCA based on the oxide reduction peak.
2
SO
4
(A) and 1.0 M KOH (B) solution on Pt/
shi to lower values for the other Pt1 Pd /C catalysts. Mean-
ꢀx
x
while, the shi of the peak increases with the Pd content. The Pt
f peaks of Pt/C can be resolved into two doublets, as shown in
4
Fig. 3C. The two prominent peaks with binding energies at 71.3
and 74.5 eV correspond to the metallic Pt, meanwhile, the other
two small peaks at 72.0 and 77.1 eV are assigned to Pt oxide,
alloys with a shi in binding energy, which is consistent with
the previous literature.
47,48
44
2
such as PtO and PtO. Based on the relative integrated inten-
sity values, metallic Pt is found to be predominant on the Pt/C
catalyst. The Pd 3d spectra of Pd/C, Pt Pd /C, Pt Pd /C and
3
.4 Electrochemical properties of Pt1ꢀxPd
Fig. 4 shows cyclic voltammetry (CV) for Pt/C, Pt73Pd27/C,
Pt47Pd53/C, Pt23Pd77/C and Pd/C catalysts in 0.5 M H SO (A) and
.0 M KOH (B) aqueous electrolytes. It can be seen that both the
x
/C
7
3
27
47
53
Pt Pd /C are given in Fig. 3B. For the Pd/C sample, two peaks
2
3
77
at about 335.45 eV and 340.65 eV are associated with Pd 3d5/2
and Pd 3d3/2, respectively. As shown in Fig. 3D, for Pd/C sample,
the spectrum of Pd 3d can be deconvoluted with two doublets,
the main peaks located at 335.1 eV and 340.4 eV can be assigned
to the Pd 3d5/2 and Pd 3d3/2 spin orbit states of Pd(0), and the
less intense peaks around 336.0 eV and 341.3 eV can be
2
4
1
Pt/C and Pd/C show well dened CV shapes similar to those
reported in previous literatures in acidic and alkaline electro-
lytes.
variation in shape characteristics took to evolve from a typical
for pure Pt/C to one typical for pure Pd/C as the increase of Pd
27,28,49,50
The CV curves for Pt Pd /C catalysts show slight
1ꢀx x
1
assigned to Pd 3d5/2 and Pd 3d3/2 peaks of Pd(II) species.
content. At the high potential range (>0.23 V in H
ꢀ0.55 V in KOH), the peaks in the CV curves are ascribed to the
oxidation and reduction of metal nanoparticles (Pt, Pd, and
Pt1ꢀxPd alloy). The peaks associated with the typical features
2 4
SO and
Notably, by alloying with Pt, the binding energies of Pd 3d for
the binary Pt1ꢀxPd /C catalysts are also found to negatively shi
x
to a lower binding energy compared with pure Pd/C as the Pt
content increases. Together, compared with Pt/C and Pd/C
catalysts, the same phenomenon of shis of the binding
>
x
characteristic of hydrogen adsorption and desorption are
observed in the CV curves for all-prepared catalysts at a low
potential range (<0.06 V in H SO and <ꢀ0.62 V in KOH).
energy for Pt and Pd to a lower peak value in binary Pt1ꢀxPd
x
/C
catalysts, which suggests a change in the electronic density of Pt
2
4
45
However, the hydrogen absorption/desorption peak in the low
potential region on Pd/C catalyst is weaker in comparison to Pt/
and Pd. The above results could be ascribed to the electronic
interactions due to the addition of Pd, leading to electron
46
1ꢀx x
C and Pt Pd /C in alkaline media, indicating that the Pd
nanoparticles are more easily associated with OH at a low
potential region. Relative to Pt/C, the hydrogen absorption/
transfer from Pd to Pt (electro-negativity: Pt, 2.28; Pd, 2.20).
The XPS analysis results further indicate the formation of PtPd
desorption peak of the Pt1ꢀxPd
x
/C sample appeared to be
depressed, which is due to the changes in the crystal lattice
51
parameter and electronic properties. It is worth to note that
the CV curves at a lower potential (<ꢀ0.17 V) presents a high
2 4
peak current density on the Pt23Pd77/C and Pd/C in 0.5 M H SO
solution, which is due to the absorption/desorption of the large
49,52
quantities of hydrogen in bulk Pd.
The similar phenomena
for Pt–Pd alloys have already been observed by Łukaszewski
53,54
50
et al.
and Seweryn et al. In acidic media, it is noted that the
CV curves for all catalysts present an obvious double layer
region (0.06–0.23 V) as well. However, no obvious double layer
region is appeared in CV curves of alkaline media just like that
in acidic media, since the adsorption of OH may even start in
the hydrogen desorption region leading to the formation of
49
metal oxides immediately aer the hydrogen desorption. The
ECSA of Pd and/or Pt-based electrocatalysts was estimated by
the oxide reduction peak according to the following
Fig. 3 XPS spectra of the samples in the binding energy range of Pt 4f
30,55
equation:
(
A) and Pd 3d (B); the fitted XPS spectra of Pt/C for Pt 4f (C) and Pd/C
for Pd 3d (D). The spectra were obtained by calibration based on the C
s peak at 284.5 eV.
1
ECSA ¼ Q
S C
/(Q M)
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where Q
Paper
ꢀ
6
S
is the coulombic charge (in 1 ꢂ 10 C), determined
by integrating the current peak of Pd (or Pt) oxide reduction
divided by the scan rate; Q is the conversion factor, commonly
C
ꢀ
6
ꢀ2
taken as 424 ꢂ 10 C cm , corresponding to PdO (or PtO)
reduction; and M is the metal loading (mg) on the GC surface.
By integrating the coulombic charge associated with oxide
reduction peak from the CV curves, the SECSA of the catalysts
2
ꢀ1
2
follows the sequence: Pt/C (27.07 m g ) > Pt73Pd27/C (25.35 m
ꢀ1
2
ꢀ1
2
ꢀ1
g
) > Pt47Pd53/C (23.12 m g ) > Pt23Pd77/C (21.98 m g ) >
Scheme 1 C1 and C2 pathway of ethanol oxidation on Pt-based
catalysts.
2
ꢀ1
Pd/C (16.89 m
g ). It can be seen that the ECSA value
decreases gradually as the increase of Pd content. This result
shows that the larger ECSA value for pure Pt/C catalyst is
attributed to the smaller particle size for pure Pt nanoparticles
The typical CV curves for ethanol electro-oxidation on
x
than Pt1ꢀxPd alloy and pure Pd, which is consistent with the
Pt1ꢀxPd
featuring three well-dened current peaks,
forward and the other on the reverse potential scan signed as Pf1
x
/C catalysts in acidic media are presented in Fig. 5A,
results of XRD and TEM. To evaluate the catalyst anti-CO
poisoning ability, we further carried out CO stripping experi-
ments. Fig. S1 in the ESI† shows the CO stripping voltammetry
59,60
two on the
(
at ꢃ0.655 V), Pf2 (at ꢃ1.037 V), and P
r
(at ꢃ0.374 V), respec-
on the Pt1ꢀx
Pd /C catalysts. The onset potential (Eonset) and
x
tively. The rst oxidation peak (P ) is ascribed to the dehydro-
f1
peak potential (ECO) of CO oxidation are used to evaluate the
genation of ethanol, whereas the second peak (P ) refers to the
f2
56
anti-CO poisoning ability of catalysts. The Eonset and ECO are
observed at 0.27 and 0.61 V on the Pt73Pd27/C catalyst, which are
both lower than those on the Pt47Pd53/C (0.32 and 0.63 V) and
Pt23Pd77/C (0.39 and 0.67 V) catalysts. The Eonset and ECO of
oxidation process involving with surface-adsorbed oxygen-like
42
species. It can be seen in inset of Fig. 5A, that the catalyst
with the Pd content less than 50% presents slightly higher
electrocatalytic activities (shown as the peak current densities at
x
Pt73Pd27/C are the most negative among the Pt1ꢀxPd /C catalyst,
which indicate that the CO adsorption on Pt73Pd27/C can be
more oxidized easily and removed.
P
f1) than pure Pt/C. Nevertheless, the electrocatalytic perfor-
mance decreases obviously when the Pd content is more than
0% in Pt1ꢀxPd /C, and the electrocatalytic activity is close to
zero on pure Pd/C. However, for the formation of reverse scan
5
x
61
peak (P ) is still unclear. Consequently, the ratio of the two
r
3
.5 Ethanol electro-oxidation
forward peak current density (jf1/jf2) could be used to quantify
61
the capacity of catalysts for the ethanol oxidation. As seen in
Table 2, the jf1/jf2 ratio were calculated to be 0.809 for Pt23Pd77/C
catalyst, which is higher than Pt47Pd53/C (0.798), Pt73Pd27/C
3.5.1 Ethanol electro-oxidation in acidic medium. The
electrocatalytic activity toward ethanol oxidation on as-prepared
catalysts were investigated by cyclic voltammograms (CV), linear
sweep voltammetry (LSV), and chronoamperometry (CA) in both
acidic and alkaline solutions. Initially, Fig. 5 shows the results
of electrochemical measurements for ethanol oxidation in
acidic media. The main steps for the oxidation of ethanol were
x
(0.769) and Pt/C (0.748). The results indicate that the Pt1ꢀxPd /C
catalysts with high Pd loading can enhance the ability to break
the C–C bond in ethanol and thus achieve complete oxidation of
ethanol to CO2.
57,58
Also, it can be found in Fig. 5B that the onset potential (Eop
)
described as follows.
The current for ethanol oxidation on
of ethanol electro-oxidation on as-prepared catalysts is shied
to the positive direction as the increase of Pd content in Pt1ꢀx
Pd /C compared to pure Pt/C, in agreement with the results by
Pt-based catalysts mainly originated from the C1 and C2
pathway (Scheme 1). The nal reaction product is supposed to
the acetic acid in acid medium. Moreover, there are other
possible species that can be formed during the ethanol electro-
oxidation, including reduced species.
-
x
50
62
Seweryn et al. and Lopes et al. The result indicates that, as in
the case of ethanol oxidation in acid media on Pt, the cleavage
63
of the C–C bond is rather difficult on the Pd catalyst. Likewise,
the oxidation of ethanol on the Pt1 Pd /C (x > 0.5) is more
ꢀx
x
Table 2 Comparison of parameters of measurements on catalysts in
acidic medium
In acidic media
ꢀ1
ꢀ1
Catalysts
Pt/C
E
op (V)
E
p
(V)
j
f1 (mA mg
)
j
f2 (mA mg
)
jf1/jf2
0.332
0.655
0.678
0.674
0.674
—
403.3
482.1
463.1
349.6
—
538.9
627.1
580.3
432.1
0.748
0.769
0.798
0.809
—
Pt73Pd27/C 0.358
Fig. 5 CV curves (A) and LSV curves (B) of ethanol oxidation on Pt47Pd53/C 0.385
different electrodes in 0.5 M H SO + 1.0 M CH CH OH. Inset image in Pt23Pd77/C 0.421
panel A is the histograms of calculated mass activity of first forward Pd/C
scan peaks (Pf1) on different electrocatalysts for ethanol oxidation.
2
4
3
2
—
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difficult than on pure Pt/C, which is probably attributed to that the process of ethanol electrooxidation on all samples is under
the dissociative chemisorption of ethanol requires the existence diffusion controlled process. This implies that the reaction
of several adjacent Pt ensembles and the presence of atoms of between catalysts and ethanol relies on the rate of diffusion of
the second metal around the Pt active sites could block ethanol ethanol in order to reach the catalysts.
6
2,64
adsorption on Pt sites due to the dilution effect.
The effect of the concentration of electrolyte (H
CH
electrodes was investigated. As shown in Fig. 6, the oxidation CH
3.5.2 Ethanol electro-oxidation in alkaline medium. Fig. 7
shows the relevant electrochemical measurements on as-
2
SO
4
, CH
3
-
2
OH) and the scan rates on the peak current density at the prepared catalysts for ethanol oxidation in 1.0 M KOH + 1.0 M
CH OH solution. As shown in Fig. 7A, the CV curves only
3
2
peak current density is closely related to the concentration of present two well-dened oxidation peaks (Pf1 in the forward
electrolyte solution and scan rates. For all catalysts, it can be scan and P in reverse scan). The forward scan peak (P ) is
r
f1
observed that the peak current density increases with the related to the oxidation of freshly chemisorbed species issued
concentration of H SO at the initial stage and then declines from alcohol adsorption, whereas the other peak (P ) in the
2
4
r
gradually to a stable value (Fig. 6A). In acid media, the current reverse scan is ascribed to the removal of the incompletely
density on the Pt73Pd27/C is always higher than that on the other oxidized carbonaceous species formed during the forward
32
catalyst. And the forward peak current density reaches the scan. The electrocatalytic activities for EOR of pure Pd and
ꢀ
1
maximum at the concentration of 0.1 mol L of H
2
SO
4
x
. Electro- Pt1ꢀxPd /C alloy catalysts are remarkably higher than that of
activity of the all the samples for the ethanol oxidation was pure Pt in alkaline media. This result is quite in agreement with
63
further examined at different ethanol concentrations, as shown the previous reports. As we known, both Pt-based and Pd-
in Fig. 6B. For the Pt/C and Pt Pd /C catalysts, the peak based catalyst exhibits much higher electrocatalytic activity
7
3
27
current density increases as the ethanol concentration for alcohol oxidation in alkaline media than that in acidic
33
increases. The increase in the current density with ethanol can media. Furthermore, the activity of Pd for ethanol electro-
be claried by the adsorption reaction of ethanol and its further oxidation in alkaline media is higher than that of Pt, which is
2
0,65
oxidation.
x
For the Pt1ꢀxPd /C (x > 0.5) catalyst, when the mainly attributed to the higher oxyphilic characteristics of Pd/C
ꢀ1
29,49
ethanol centration reaches 3.0 mol L , the forward peak and the relatively inert nature of Pd/C on C–C bond cleavage.
current density reaches the maximum and a saturation of the Simultaneously, during the whole process, the Pt23Pd77/C
current response is observed at the ethanol concentration of exhibits the highest oxidation peak current density of 2453.7
ꢀ
1
ꢀ1
3
.0 mol L . This indicates that oxidation rate of the electrode mA mg , which is 3.15, 1.73, 5.41 and 1.46 times as large as
ꢀ1
surface is related to the concentration of ethanol, and the active those of Pt Pd /C (778.4 mA mg ), Pt Pd /C (1421.6 mA
sites of the electrode reach the saturation at the ethanol mg ), Pt/C (453.2 mA mg ), and Pd/C (1676.4 mA mg ),
concentration of 3.0 mol L . Subsequently, the peak current respectively. Compared to various Pt or Pd-based catalysts re-
shows a decrease gradually with higher ethanol concentrations, ported in previously literatures previously literatures (summa-
which may be due to the accumulation of absorbed species on rized in Table S1 in the ESI†), it is noteworthy that the mass
the surface of catalyst. The oxidation peak current densities activity of the Pt23Pd77/C catalyst is also obviously higher.
7
3
27
47
53
ꢀ
1
ꢀ1
ꢀ1
ꢀ
1
increase as the scan rate increased in acid media. As depicted in
The onset potential of ethanol electro-oxidation (Table 3) is
Fig. 6C, the peak current densities are proportional to the obtained on as-prepared catalysts in alkaline media based on
1
/2
square root of the scan rate (v ). The above results suggest that the LSV curves in Fig. 7B. The pure Pd/C and Pt1 Pd /C show
ꢀx
x
a lower onset potential compared to pure Pt/C. It can be found
that the onset potential for ethanol electro-oxidation on
Pt23Pd77/C and Pt47Pd53/C shi negatively 125 mV and 118 mV
in comparison with that on pure Pt/C, respectively. More than
100 mV reduction on the onset potential for anodic reaction
indicates the signicant enhancement in the kinetics of the
29,66
ethanol oxidation reaction (EOR).
Fig. 6 Effect of the concentrations of H
the scan rates (C) on the peak current density in acid media, respec- different electrodes in 1.0 M KOH + 1.0 M CH
tively. Inset of (C) j vs. the square root of scan rates. Catalysts: Pt/C, panel A is the histograms of calculated mass activity of different
SO
2 4
(A), CH CH
3 2
OH (B) and Fig. 7 CV curves (A) and LSV curves (B) of ethanol oxidation on
3
CH OH. Inset image in
2
p
Pt73Pd27/C, Pt47Pd53/C, Pt23Pd77/C and Pd/C.
electrocatalysts for ethanol oxidation.
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Table 3 Comparison of parameters of measurements on catalysts in As a result, the imbalance in the coverage of hydroxyl and ethoxi
alkaline medium
will lead to the decrease in the peak current density of ethanol
oxidation. To investigate the effect of CH CH OH on the cata-
3
2
In alkaline media
op (V) (V)
ꢀ0.576 ꢀ0.294
lytic performance, different concentration of CH CH OH
3
2
ꢀ1
ꢀ1
ꢀ1
Catalysts
Pt/C
Pt73Pd27/C ꢀ0.582 ꢀ0.281
E
E
p
j
f
(mA mg
)
j
b
(mA mg
)
j
f
/j
b
ranging from 0.01 to 5.0 mol L was applied (Fig. 8B). In
alkaline media, for Pt/C and Pt1ꢀxPd /C catalysts, the peak
current reaches the maximum value at the ethanol centration of
x
453.2
778.4
516.8
0.877
0.705
0.758
1104.1
1875.5
2615.9
2157.5
ꢀ
1
1
.0 mol L . The increase in the peak current density with the
Pt47Pd53/C ꢀ0.694 ꢀ0.276 1421.6
Pt23Pd77/C ꢀ0.701 ꢀ0.254 2453.7
0.938 concentration of ethanol between 0.1 M and 1 M can be
0.777 explained by the adsorption reaction of ethanol and its further
Pd/C
ꢀ0.608 ꢀ0.273 1676.4
oxidation. The increase of ethanol concentration can promote
the formation of the adsorbed ethanol, thereby yielding an
69
increase in the oxidation peak density of ethanol. While,
ethanol concentrations higher than 1.0 M, adsorption of
ethanol on the catalyst reaches to the saturated state, leading to
the reduction of the ethanol oxidation current. However, for Pd/
C electrode, the peak current is enhanced with increasing the
ethanol concentration. And it is worth mentioning that the peak
current can not reach a maximum value when the concentration
The effect of the concentration of electrolyte (KOH, CH
3
-
CH OH) and the scan rates on the peak current density at the
2
electrodes was investigated in alkaline media. Fig. 8A shows the
peak current density on the different electrodes (Pt/C, Pd/C and
Pt1 Pd /C) in a 1.0 M ethanol solution containing various
ꢀx
x
concentrations of KOH solutions ranging from 0.010 to 3.0 M.
For all catalysts, it can be observed that the peak current density
increases with the concentration of KOH at the initial stage and
then declines gradually to a stable value in alkaline media,
which is consist with the acid media (Fig. 7A). The above results
show that the kinetics of the ethanol oxidation were improved
ꢀ1
of ethanol reaches to 5.0 mol L , which is attributed to the
intrinsic property of Pd in alkaline media, in agreement with
68
the results by Zhao et al. The result indicates that the increase
in the coverage of adsorbed ethoxi as the ethanol concentration
increase yields an increase in the oxidation current of the EOR.
The peak current densities of the catalysts via varying the scan
rates were investigated 1.0 M KOH + 1.0 M CH CH OH (Fig. 8C).
ꢀ
by the greater availability of OH ions in solution and/or
ꢀ
67
a higher OH coverage of the electrode surface. For Pt/C,
Pt Pd /C and Pt Pd /C catalysts, the forward peak current
3
2
7
3
27
47
53
The oxidation peak current densities increase as the scan rate
increased in alkaline media. The peak current densities are
density reaches the maximum at the concentration of
ꢀ
1
0
.2 mol L , while the other catalyst appears at the concentra-
1/2
proportional to the square root of the scan rate (v ) during the
ꢀ
1
tion of 1.0 mol L in alkaline media. At the initial stage, the
increase of the KOH concentration not only accelerate the
dehydrogenation process but also lead to a higher coverage of
the reactive Pd–OHads, which facilitates the ethanol oxidation
by the removal of the adsorbed ethoxi in alkaline medium.
However, at higher KOH concentrations, excessive hydroxyl
coverage on the catalysts will be dominant, thereby conning
whole process of electrooxidation of ethanol in alkaline media.
The correlation coefficient of Pt/C, Pt73Pd27/C, Pt47Pd53/C,
Pt23Pd77/C and Pd/C is 0.9905, 0.9993, 0.9925, 0.9778, 0.9902,
respectively. Accordingly, the ethanol electrooxidation is
dominated by the diffusion-controlled process in alkaline
70
media.
3.5.3 Stability of catalyst. As the long-term stability of
68
the adsorption of ethanol with inadequate coverage of ethoxi.
catalyst has been recognized as another important factor for
71,72
fuel cells,
we also evaluated the long-term stability of as-
prepared catalysts through repeated CV scan for 100 cycles
and CA for 3600 s in both acid and alkaline solution. Fig. 9A and
C exhibits the forward oxidation peak (Pf1) current density for
ethanol electro-oxidation as a function of cyclic scan number in
acid and alkaline media, respectively. In acid media, the peak
current density on Pt Pd /C and Pt Pd /C is about 19.4%
7
3
27
47
53
and 13.2% higher than pure Pt/C at the 10th cycle, respectively.
In contrast, the lower peak current density was observed on
Pt23Pd77/C compared to Pt/C. A similar phenomenon has been
73
74
observed by Wang et al. and Zhang et al. on Pd@Pt for
ethanol electro-oxidation. For the enhancement of peak current
density, it is directly related to the electronic structure of PtPd
75
alloy. Pd as an oxyphilic metal in Pt1 Pd alloy accelerates the
ꢀx
x
oxidative removal of COads species by the addition of a large
number of active oxygen-containing species (such as PdO/
x
PdO ), but too low a Pt atom content decreases the probability
of nding the neighboring Pt atoms for ethanol chemisorption
3 2
Fig. 8 Effect of the concentrations of KOH (A), CH CH OH (B) and the
scan rates (C) on the peak current density in alkaline media, respec-
tively. Catalysts: Pt/C, Pt73Pd27/C, Pt47Pd53/C, Pt23Pd77/C and Pd/C.
74,75
and hinders ethanol oxidation.
Also, it can be seen that the
electrocatalytic activity suffers from the loss in the order of
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in Fig. 9D. The polarization current density for EOR shows
a rapid decay on Pt1 Pd /C catalysts in the rst 300 s. The
ꢀx
x
decline of the peak density is attributed to the charging current
or the catalyst poisoning during the process of the EOR. Also,
the nal current density indicates a decreasing electrocatalytic
stability for EOR on as-prepared catalysts following in the order
of Pt23Pd77/C > Pd/C > Pt47Pd53/C > Pt73Pd27/C > Pt/C, in
consonance with the results of repeated CV scan. Nonetheless,
it also means that the electrocatalytic activity of catalyst not only
concerns the ECSA, but also more concerns the surface
composition. Moreover, these results demonstrate that the Pt/
Pd ratio in Pt1 Pd /C alloy signicantly affects the electro-
ꢀx
x
catalytic activity and stability of Pt1ꢀxPd
media, which coincides well with the previous work.
x
/C for EOR in alkaline
77
Fig. 9 Long-term electrocatalytic stability referring to j
on Pt/C, Pt73Pd27/C, Pt47Pd53/C, Pt23Pd77/C and Pd/C catalysts in acid (A)
and alkaline (C) solutions, respectively, scan rate: 50 mV s ; chro-
noamperometric curves of different catalysts at the potential of 0.5 V or
f
over 100 cycles
4
. Conclusions
ꢀ1
In summary, the Pt1 Pd /C catalysts with different Pt/Pd ratios
ꢀx
x
prepared by formic acidic as reducing agent have been investi-
gated to compare the electrochemical performance of ethanol
oxidation in acidic and alkaline solutions. The electrocatalytic
activity toward ethanol oxidation on as-prepared catalysts were
investigated by CV, LSV, and CA in both acidic and alkaline
ꢀ
0.5 V in acid (B) and alkaline media (D). Solution: 0.5 M H
2 4
SO + 1.0 M
CH CH OH (A and B); 1.0 M KOH + 1.0 M CH CH OH (C and D).
3
2
3
2
Pt Pd /C (18.6%) < Pt Pd /C (21.4%) < Pt/C (28.9%) <
Pt Pd /C (34.8%). While in alkaline solution, it should be
7
3
27
47
53
x
solutions. Compared with acidic media, Pt1ꢀxPd /C catalysts
2
3
77
exhibit much higher electrocatalytic activity for alcohol oxida-
tion in alkaline media, which is mainly attributed to the higher
oxyphilic characteristics of Pd/C and the relatively inert nature
of Pd/C on C–C bond cleavage. The ratio of Pt/Pd has a signi-
cant impact on the ethanol-oxidation in acidic and alkaline
noted that the oxidation peak current density for the rst cycle
on Pt73Pd27/C is about ve and eight times higher than that on
pure Pt/C and Pd/C catalysts, respectively (Fig. 9C). During the

rst 10 cycles, the oxidation peak current density drastically
decreases on pure Pt/C and Pt1ꢀxPd /C catalysts due to the
x
46
73 27 23 77
media. The Pt Pd /C and Pt Pd /C catalysts shows the
accumulations of poisonous carbonaceous species. Contrast-
ingly, the peak current density of pure Pd/C catalyst shows
a rapid increase in the rst 20 cycles and then decreases grad-
ually with successive scans. The Pt23Pd77/C catalyst exhibits the
highest electrocatalytic performance with a mass specic peak
ꢀ
1
current of 2453.7 and 339.7 mA mgPtPd , respectively, and long-
term stability in acidic and alkaline media. Moreover, the
electrochemical performance of ethanol oxidation in both
acidic and alkaline solutions is easily inuenced by electrolyte
f b
highest oxidation peak current density with a higher j /j value
of 0.938 during the whole process. Aer 100 cycles, the oxida-
tion peak current density of Pt23Pd77/C catalyst retains ca. 40.7%
3 2
and CH CH OH concentrations, scan rates.
ꢀ
1
relative to the maximum value of 5238.94 mA mg . However,
the values for the current density of the Pt47Pd53/C, Pt73Pd27/C
and Pt/C catalysts are 26.85%, 22.06%, and 31.52%,
respectively.
Conflicts of interest
The authors declare that they have no known competing
Fig. 9B shows chronoamperometric experiments at a xed nancial interests or personal relationships that could have
potential of 0.5 V to further determine the stability of as- appeared to inuence the work reported in this paper.
prepared catalysts. A decay of current density normalized by
the mass of Pt1ꢀxPd
catalysts, indicating that the catalyst surface is prone to
poisoning by an accumulation of ethanolic residues. At 3600 s,
the electrocatalytic activity is the highest on Pt Pd /C, fol-
x
can be observed at the CA curves for all the
Acknowledgements
76
This work was supported by the National Natural Science
Foundation of China (Grant No. 51762018) and National
Natural Science Foundation of China grant (No. 21603122).
7
3
27
lowed by Pt Pd /C, Pt/C, Pt Pd /C and Pd/C catalysts. The
4
7
53
23
77
result is consistent with the repeated CV scan mentioned above,
further conrming that the Pt73Pd27/C catalyst has a better long-
term stability than others in acidic media. From the zoomed Notes and references
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