Carbon-thin-layer protected WP with no passivation supported on acid-treated expanded graphite as efficient Pt Co-catalysts for methanol oxidation and oxygen reduction reactions

Article abstract of DOI:10.1039/c8ta08285e

Application of direct methanol fuel cells (DMFCs) is hampered by the low activity, poor stability, and low CO/methanol tolerance of Pt-based catalysts used for methanol oxidation reaction (MOR)/oxygen reduction reaction (ORR). In this study, tungsten phosphide-embedded carbon-thin-layer/acid-treated expanded graphite (WP-CL/AEG) composites are synthesized as Pt-supports/co-catalysts for MOR/ORR via a one-step synthesis route. For MOR, Pt-WP-CL/AEG-3 (molar ratio of P to AEG is 3.6 in precursor) shows the highest electrochemical surface area (123.05 m² g_Pt^-1) and mass activity (2217.6 mA mg_Pt^-1), which are respectively 1.92 and 4.44 times higher than those of commercial Pt/C (64.16 m² g_Pt^-1 and 499.2 mA mg_Pt^-1). Pt-WP-CL/AEG catalysts also exhibit higher CO tolerance and stability than Pt/C. Moreover, Pt-WP-CL/AEG-3 also exhibits a higher ORR activity than Pt/C in acidic media, mainly via a 4e^- transfer pathway (OH^- as the main product) for ORR. As expected, AEG greatly enhances the charge transfer capacity of Pt-WP-CL/AEG catalysts; the exposed P active sites induced by W atoms on WP facilitate methanol/O₂ adsorption and activation during MOR/ORR; the CL originated from citric acid can protect WP from being oxidized in air, which contributes to high activity and stability of P active sites; and charges are transferred from AEG (positively charged) to WP via the linked-CL and then from WP to Pt, which contributes to the high MOR/ORR activity of Pt-WP-CL/AEG. It indicates that WP-CL/AEGs can be considered as promising supports/co-catalysts for Pt in MOR/ORR.

Full text of DOI:10.1039/c8ta08285e

Journal of
Materials Chemistry A
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PAPER
Carbon-thin-layer protected WP with no
passivation supported on acid-treated expanded
graphite as efficient Pt Co-catalysts for methanol
oxidation and oxygen reduction reactions†
Cite this: J. Mater. Chem. A, 2018, 6,
2636
2
a
ab
a
a
a
a
Chunyue Zhang, Ying Dai, Hun Chen, Yuanyuan Ma, Baojian Jing, Zhuang Cai,*
a
a
a
Yaqiang Duan, Bo Tang and Jinlong Zou
*
Application of direct methanol fuel cells (DMFCs) is hampered by the low activity, poor stability, and low
CO/methanol tolerance of Pt-based catalysts used for methanol oxidation reaction (MOR)/oxygen
reduction reaction (ORR). In this study, tungsten phosphide-embedded carbon-thin-layer/acid-treated
expanded graphite (WP-CL/AEG) composites are synthesized as Pt-supports/co-catalysts for MOR/ORR
via a one-step synthesis route. For MOR, Pt-WP-CL/AEG-3 (molar ratio of P to AEG is 3.6 in precursor)
2
Ptꢀ1
Ptꢀ1
shows the highest electrochemical surface area (123.05 m
g
) and mass activity (2217.6 mA mg
),
and
). Pt-WP-CL/AEG catalysts also exhibit higher CO tolerance and stability than Pt/C.
Moreover, Pt-WP-CL/AEG-3 also exhibits a higher ORR activity than Pt/C in acidic media, mainly via
2
Ptꢀ1
which are respectively 1.92 and 4.44 times higher than those of commercial Pt/C (64.16 m
g
Ptꢀ
1
4
99.2 mA mg
ꢀ
ꢀ
a 4e transfer pathway (OH as the main product) for ORR. As expected, AEG greatly enhances the
charge transfer capacity of Pt-WP-CL/AEG catalysts; the exposed P active sites induced by W atoms on
2
WP facilitate methanol/O adsorption and activation during MOR/ORR; the CL originated from citric acid
Received 26th August 2018
Accepted 17th October 2018
can protect WP from being oxidized in air, which contributes to high activity and stability of P active
sites; and charges are transferred from AEG (positively charged) to WP via the linked-CL and then from
WP to Pt, which contributes to the high MOR/ORR activity of Pt-WP-CL/AEG. It indicates that WP-CL/
AEGs can be considered as promising supports/co-catalysts for Pt in MOR/ORR.
DOI: 10.1039/c8ta08285e
rsc.li/materials-a
However, Pt active sites readily adsorb the CO intermediates to
cause poisoning of the catalyst, which results in a poor catalyst
1
. Introduction
10
During the past few decades, direct methanol fuel cell (DMFC) durability in the MOR process. In addition, the high cost and
has been considered as a promising energy source due to its scarcity of precious Pt resources are also main obstacles to the
10–13
high efficiency, safe storage, low operating temperature and commercialization of DMFCs.
Therefore, there is an urgent
environmental friendliness, and has already been applied in need to explore an efficient and relatively inexpensive co-
1–5
portable electronic devices and electric vehicles. Work effi- catalyst, which can avoid CO poisoning of the Pt active sites
14,15
ciency of the DMFC is mainly dependent on two important and reduce Pt loading for MOR catalysts.
reactions: methanol oxidation reaction (MOR) and oxygen
Numerous efforts have been dedicated to solving these crit-
reduction reaction (ORR). Therefore, it is necessary to develop ical issues. One of the most useful methods is to alloy Pt with
17
6,7
16
11
efficient catalysts that can promote these two reactions. To date, a small amount of precious metal, such as Ru, Au and Pd,
Pt-based catalysts are the most commonly-used catalysts in which tends to rationalize the consumption of expensive Pt
DMFCs because they have good catalytic activity for both MOR metal by increasing its catalytic activity and stability. With the
8
at the anode and ORR at the cathode. Particularly, Pt-based development of many Pt-based bimetallic nanocrystals for
9
catalysts show an irreplaceable catalytic activity for MOR.
MOR, the catalytic activity and stability are generally improved
over commercial Pt/C. In spite of this, it is still impossible to
avoid high cost of Pt-alloy catalysts.
a
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of
Recently, transition-metal compounds have been used as co-
catalysts and/or supports, which have already exhibited high co-
the People's Republic of China, School of Chemistry and Materials Science,
Heilongjiang University, Harbin, 150080, China. E-mail: zoujinlong@aliyun.com
b
18–20
School of Civil Engineering, Heilongjiang Institute of Technology, Harbin, 150050, catalytic activity and Pt utilization for MOR.
Transition-
China
metal phosphides have been used as electrocatalysts in photo-
catalysis, electrocatalysis, hydrogen evolution reactions
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ta08285e
21
22
1
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Journal of Materials Chemistry A
2
3
24
18
19,25
(
C
HER), fuel cells, etc. Pt-MoP/C, Pt-CoP/C
and Pt-Ni
2
P/
20
have shown excellent catalytic activity and stability as DMFC
anode catalyst. WP has been proved as a promising alternative
26
to Pt-based catalysts for HER. It is reported that supported WP
can be used as an active HER electrocatalyst, which exhibits
27
higher activity and stability than other Pt-based catalysts. In
our previous study, WP/porous carbon (WP/C) composite with
porous structure and high special surface area has been used as
28
Pt-support and co-catalyst for MOR. Remarkable CO tolerance
and durability of Pt-WP/C are desirably obtained, which are
attributed to the fast surface transport/elimination of adsorbed
28
CO (intermediates) on different crystalline faces of WP.
Scheme 1 Illustration of the preparation procedures for WP-CL/AEG
Despite better performance than Pt/C, Pt-WP/C still suffers from
the inferior stability of WP/C originating from the weak inter-
actions between porous carbon (relatively low electrical
conductivity) and WP. Thus, enhancing the electrical conduc-
tivity of support (carbon), the stability of WP (in the air or
electrolyte), and the interactions between support and WP
should be of great interest to promote the co-catalytic activity of
WP.
(Step I) and Pt-WP-CL/AEG (Step II).
(
H N O W$H O, A.R.) and 0.015 mol diammonium hydrogen
40 10 41 2
phosphate ((NH ) HPO , A.R.) were dissolved in 30 mL deion-
4
2
4
ized water to form a colorless transparent solution. Meanwhile,
y mmol (y ¼ 0, 0.83, 2.50, 4.16, 5.83 and 7.49) of AEG as support
was also added to the solution with intense stirring (the prep-
aration process for AEG was shown in ESI†). Then, 0.045 mol
citric acid, as a chelating agent, was added to the solution to
The development of new carbon supports to promote the
performance of electrocatalysts has also attracted much atten-
29,30
tions.
Graphitized carbon materials with sequential struc-
ꢁ
promote formation of WP NPs. The solution was kept at 90 C in
ture and high conductivity can signicantly improve the
a water bath for 12 h with intense stirring and then heated at
29–31
performance of Pt-based catalysts.
Commonly-used carbon
ꢁ
1
20 C for 24 h to obtain the WP-CL/AEG precursor. The dried
materials as catalyst supports include carbon nanotubes
ꢁ
sample was carbonized at 900 C for 2 h at a heating rate of
32,33
34,35
36,37
(CNTs),
carbon nanobers (CNFs),
graphene,
etc.
ꢁ
ꢀ1
1
5
C min under owing nitrogen (N
2
) atmosphere (50–60
mL min ). Finally, the sample was naturally cooled to room
temperature under N atmosphere. Obtained samples with AEG
However, the application of low-cost expanded graphite (EG) as
a stable support for electrocatalysis is seldom reported. We
believe that using the excellent electrical conductivity and
layered structure of EG can effectively enhance the co-catalytic
ꢀ
2
additions of 0, 0.83, 2.50, 4.16, 5.83 and 7.49 mmol were marked
as WP-CL/AEG-x (x ¼ 0, 1, 2, 3, 4 and 5, respectively).
38
performance of WP/EG composite as a support.
In this study, nitric acid treated EG (AEG) with abundant
oxygen-containing functional groups is used as the substrate for 2.2. Synthesis of Pt-WP-CL/AEG catalysts
the well-crystallized WP nanoparticles (NPs) to obtain the WP-
Pt-WP-CL/AEG composites with 5 wt% Pt were prepared by the
CL/AEG composites as Pt-supports/co-catalysts for both MOR
and ORR. Citric acid (CiA), used as a chelating agent in the
precursor, is converted to porous carbon-thin-layer (CL) for
protecting WP NPs from oxidation (P) in air (passivation-free)
and linking the WP with AEG. WP NPs are in situ formed in
the carbon skeleton of CL. WP-CL is closely integrated with the
AEG support, which possesses abundant O-groups, excellent
stability and electrical conductivity. This particles-encapsulated
layer-by-layer structure (Dual) of WP-CL/AEG facilitates good
dispersion of Pt NPs, thereby facilitating the exposure of more
Pt active sites and better charge transfer. Therefore, Pt-WP-CL/
AEG catalysts are expected to exhibit higher (or at least,
comparable) activity and stability for both MOR and ORR over
the commercial Pt/C. Pt-WP-CL/AEG catalysts may have the
potential to be used as alternatives for Pt/C for DMFC.
39
modied borohydride reduction method (Scheme 1, Step II).
00 mg of WP-CL/AEG was ultrasonically dispersed in aqueous
chloroplatinic acid (H PtCl $6H O, A.R.) solution for 15 min to
1
2
6
2
form a uniform suspension. Then, pH of the suspension was
adjusted to 10 using 1 M NaOH solution. Excess sodium boro-
hydride (NaBH , A.R.) solution was used as a reducing agent
4
and the Pt-species in the suspension was slowly reduced at
room temperature with vigorous stirring. The suspension was
stirred for another 3 h and then allowed to stand for 12 h. Next,
it was washed with deionized water and ethanol several times
+
(
centrifugation at 4000 rpm) to remove residual Na ions and
adsorbed water, respectively. The rinsed precipitate was nally
ꢁ
desiccated at 50 C to obtain the catalyst, which was marked as
Pt-WP-CL/AEG-x (x ¼ 1, 2, 3, 4 and 5).
2
.3 Materials characterization
2
. Experimental
X-ray diffraction (XRD), Raman spectroscopy, scanning electron
microscope (SEM), N adsorption/desorption isotherms, trans-
2.1. Synthesis of WP-CL/AEG composites
2
A simple one-step synthesis method was used to synthesize WP- mission electron microscopy (TEM) and X-ray photoelectron
CL/AEG composites. The detailed process was shown in Scheme spectroscopy (XPS) observations were used to characterize the
1
(Step I). Typically, 0.015 mol ammonium tungstate structure, morphology and composition of the samples. XRD
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patterns were obtained from a Rigaku D/max 2500 diffractom-
QH ¼ S/v
(2)
˚
eter using Cu-Ka radiation (k ¼ 1.5406 A, 40 kV, 20 mA) at step
ꢁ
scan of 0.02 . N adsorption/desorption isotherms were where, Q
H
is the charge for HAD, S is the integral area of
2
measured under 77 K using Micromeritics Tristar II. Specic hydrogen stripping peak (A V) and v is the scanning speed
ꢀ
1
surface area and pore size distribution were calculated using (0.05 V s ); m is the platinum loading on the electrode (mg),
Brunauer–Emmett–Teller (BET) theory and Barrett–Joyner– and 0.21 is the charge required to oxidize the H monolayer on
2
ꢀ
2
7,18,40
Halenda (BJH) method, respectively. Raman spectra were a bright Pt (mC cm ).
captured on an HR 800 micro-Raman spectrometer (Jobin-Yvon)
at 457.9 nm. Analyses of surface composition of the prepared
2.6 ORR measurements
catalyst were conducted using XPS (Kratos-AXIS UL TRA DLD, Al
Ka X-ray source) and data were tted with “XPS peak” soware.
SEM analysis was performed using an S-4800 scanning electron
microscope (Japan) at an accelerating voltage of 5.0 kV. Trans-
mission electron microscopy (TEM) and high-resolution TEM
CV tests were conducted from ꢀ0.2 to 1.0 V in 0.5 M H SO
2
4
ꢀ
1
solution saturated with O at a scan rate of 0.05 V s . LSV tests
2
were conducted in O
electrode rotation speed of 1600 rpm (0.01 V s ). Rotating disk
electrode (RDE) measurements were conducted in O -saturated
solution at a scan rate of 10 mV s and various rotation
2 2 4
-saturated 0.5 M H SO solution at an
ꢀ1
2
(HRTEM) images were obtained using a JEM-2100 electron
ꢀ1
2 4
H SO
microscope (JEOL) with an accelerating voltage of 200 kV.
speeds from 400 to 2500 rpm. To investigate ORR kinetics,
Koutecky–Levich (K–L) equations are used to determine the
2
.4 Electrochemical measurements
number of transferred electrons (n) per O molecule at different
2
rotation rates.
41,42
Electrochemical measurements were conducted at room
temperature (25 C) on an electrochemical workstation
ꢁ
1/2
1
/J ¼ 1/J
L
+ 1/J
K
¼ 1/Bu + 1/J
K
(3)
(4)
(CHI760E, Chenhua, Shanghai) with a typical three-electrode
testing system. The counter electrode was a polished platinum
sheet (1 cm ) electrode, while the reference electrode was
2/3 ꢀ1/6
B ¼ 0.62nFC D
n
0
2
a saturated calomel electrode (SCE, +0.2415 V vs. standard where, J is the apparent current density, J is the kinetic current
K
hydrogen electrode, 3.0 M KCl). All potentials reported in this density, JL is the limiting current density, u is the angular
work were versus SCE. The working electrode was a glassy- velocity of the disk, n is the electron transfer number, F is the
ꢀ
1
carbon electrode (GCE) (diameter of 3 mm and area of Faraday constant (96 485 C mol ), C is the bulk concentration
0
2
ꢀ6
ꢀ3
0
.07065 cm ). The catalyst ink was prepared by ultrasonically of O (1.2 ꢂ 10 mol cm ), D is the diffusion coefficient of O
2
2
ꢀ
5
2
ꢀ1
dispersing a mixture containing 5 mg of catalyst, 100 mL of in electrolytes (1.9 ꢂ 10 cm
s
), and n is the kinematic
18
2
ꢀ1 41,42
ethanol and 50 mL of a 5 wt% Naon (Aldrich) solution. For viscosity (0.01 cm s ).
MOR test, 2.5 mL of the catalyst ink was pipette onto a pre-
Rotating ring disk electrode (RRDE) measurements were
cleaned working electrode. For ORR test, 5 mL of the catalyst conducted at an electrode rotation speed of 1600 rpm with
ꢀ
1
ink was pipetted onto a pre-cleaned working electrode. a scan rate of 10 mV s . A constant ring potential was
Commercial Pt/C (10 wt% Pt on Vulcan carbon black, Hesen controlled at 1.2 V in O -saturated 0.5 M H SO solution. The
2
2
4
Electric Co. Ltd, Shanghai, China) was used as the reference electron transfer number (n) and the hydrogen peroxide yield
catalyst for all tests.
[H O (%)] were obtained from RRDE voltammograms, deter-
2
2
mined by the following eqn (5) and (6):
2
.5 MOR measurements
n ¼ 4I
D
/[I
D
+ (I
R
/N)]
+ (I
(5)
(6)
Electrochemically active surface area (ECSA) of the catalyst layer
H
2
O
2
% ¼ 200(I
R
/N)/[I
D
R
/N)]
was measured in the potential range of ꢀ0.2 to 1.0 V at a scan
ꢀ
1
2 4
rate of 50 mV s in 0.5 M H SO solution. ECSA was calculated
where, ID and IR are the disk current and the ring current,
respectively; N is the H collection efficiency of the Pt ring
2
using the H adsorption–desorption region (as shown in eqn (1)
2 2
O
7,18,40
and (2)).
formed in a N
CH OH solution at room temperature with a scan rate of 50 mV
Electrochemical methanol oxidation was per-
4
2
with a value of 0.4.
2
-saturated solution containing 0.5 M H SO + 1 M
2
4
3
ꢀ
1
s
. Chronoamperometry (CA) tests were performed for 3600 s
3
3
. Results and discussion
.1 Characteristics analyses of (Pt)-WP-CL/AEG composites
to investigate long-term durability of the catalysts in the
selected solution at a constant potential of 0.77 V. Electro-
chemical impedance spectroscopy (EIS) was conducted at 0.77 V
over a frequency range of 10 to 10 Hz with a sinusoidal
perturbation of 10 mV amplitude. CO stripping voltammetry
Fig. 1a shows XRD patterns of WP-CL/AEG-x (x ¼ 0, 1, 2, 3, 4 and
5
ꢀ2
ꢁ ꢁ ꢁ
). The typical diffraction peaks at around 21.1 , 28.7 , 31.0 ,
5
4
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
2.9 , 43.2 , 44.6 , 46.5 , 56.7 , 68.8 , and 73.6 correspond to
ꢀ1
characteristic features of (101), (002), (011), (202), (112), (211),
103), (020), (114) and (222) planes of WP (PDF#29-1364),
was carried out at a scan rate of 10 mV s in 0.5 M H
solution.
2 4
SO
(
2
7,28,43
respectively.
The sharp XRD peaks show high crystal-
ECSA ¼ Q
H
/(m ꢂ 0.21)
(1) linity of the prepared WP particles. Because of strong diffraction
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recognized that more defects should favorably generate more
active sites. Especially, an I /I ratio of 0.42 of Pt-WP-CL/AEG-3
D
G
is higher than that of Pt-WP-CL/AEG-0 (0.37), which indicates
that Pt-WP-CL/AEG-3 composite has more defects than Pt-WP-
CL/AEG-0.
Fig. S2† shows N
size distributions of AEG and WP-CL/AEG-x (x ¼ 0, 1, 2, 3, 4 and
). The specic surface area (SBET) and average pore size are
2
adsorption/desorption isotherms and pore
5
shown in Table 1. All observed curves have type-IV pattern with
obvious hysteresis loops at relative pressures (P/P ) from 0.4 to
0
28
2
ꢀ1
1
.0. WP-CL/AEG-0 has the largest S
of 96.69 m g , while
BET
ꢀ1
2
AEG has the lowest SBET of 12.02 m g . Apparently, as AEG
content increases, specic surface areas of samples gradually
decrease, while the average pore size increases. It implies that
a balance between porosity and electrical conductivity of WP-
CL/AEGs can be achieved for obtaining a high MOR/ORR
activity.
Surface compositions of Pt-WP-CL/AEGs are investigated by
XPS (Fig. 2). Fig. 2a shows XPS survey spectra of Pt-WP-CL/AEG-x
(
x ¼ 0, 1, 2, 3, 4 and 5). These spectra show that Pt, W, P, C and O
elements are all detected in the prepared catalysts. C 1s and O
s peaks can be observed at around 284.6 and 532.2 eV,
1
18
respectively. Meanwhile, W 4f and P 2p peaks are located at
28,43
around 36.3 and 133.5 eV, respectively.
The Pt 4f peaks at
around 71.6 eV are also observed, indicating that Pt species has
44
been loaded on the surface of WP-CL/AEG supports.
High resolution XPS spectra of Pt 4f for Pt-WP-CL/AEG-x (x ¼
0
, 1, 2, 3, 4 and 5) are shown in Fig. 2b and S3† and can be
divided into three pairs of doublets, which are attributed to
Fig. 1 (a) XRD patterns of WP-CL/AEG-x (x ¼ 0, 1, 2, 3, 4 and 5). (b)
28,44,47
different valence states of Pt.
The most intense double
Raman spectra of Pt-WP-CL/AEG-0 and Pt-WP-CL/AEG-3.
peaks (at around 71.5 and 74.8 eV) correspond to metallic Pt
2
8
(
Pt(0)). The weaker doublet states (at around 72.4 and 75.9 eV)
44
can be assigned to Pt oxidation state in PtO. The weakest
doublet (at around 73.5 and 77.4 eV) should correspond to the
PtO2. Compared with Pt 4f peak of Pt/C, the binding energies
ꢁ
peaks of WP, the characteristic peak of EG at around 26.5 is
hard to observe. As shown in Fig. S1a,† the peaks at around
47
ꢁ ꢁ ꢁ
3
9.8 , 46.2 , and 67.5 correspond to respective characteristic
of Pt(0) in Pt-WP-CL/AEG-3 catalyst reveal a negative shi of
features of (111), (200), and (220) planes of Pt with face-centred
cubic (FCC) structure (PDF no. 65-2868, JCPDS, 2004) observed
in both commercial Pt/C and Pt-WP-CL/AEG-x (x ¼ 0, 1, 2, 3, 4
0
.6 eV, indicating that electron transfer should occur from the
48
WP moiety to Pt. This charge transfer may change the elec-
tronic structure and density of states of metal catalysts, which
correspondingly weakens the binding energy of strongly-
44,45
and 5).
Broad peaks of Pt are observed for all prepared Pt-
WP-CL/AEG-x catalysts. Due to the strong characteristic peaks of
WP and relatively low loading of Pt (5 wt%), the characteristic
peaks of Pt are not obvious in XRD patterns. For WP-CL/AEG-
20,25
adsorbed and poisonous intermediates.
Thus, the charge
transferred from WP to Pt in the Pt-WP-CL/AEG catalysts should
lead to a signicant increase in the electron density around Pt
sites, which helps to eliminate chemically adsorbed toxic
0
and Pt-WP-CL/AEG-0, no graphite peak can be detected at
ꢁ
around 26.5 , which indicates an amorphous nature of the citric
28
acid-derived carbon in this study. The characteristic peaks of
WP in WP-CL/AEG-x and Pt-WP-CL/AEG-x (x ¼ 1, 2, 3, 4 and 5)
are very sharp, implying that the addition of AEG does not affect
the formation of WP NPs. Due to the superior stability and
electrical conductivity of AEG, its addition may enhance overall
Table 1 Textural properties of AEG and WP-CL/AEG-x (x ¼ 0, 1, 2, 3, 4,
5) composites
BET surface
area (m g
Pore volume
Average pore
size (nm)
2
ꢀ1
3
ꢀ1
performance of the co-catalyst. Fig. S1b† shows the character- Samples
)
(cm g
)
ꢁ
istic peaks of graphite at around 26.5 , and peak intensity is
AEG
Pt-WP-CL/AEG-0
Pt-WP-CL/AEG-1
12.02
96.69
72.22
70.33
68.82
41.75
32.97
0.026
0.084
0.067
0.08
0.078
0.059
0.057
8.62
3.46
3.66
4.55
4.55
5.55
6.96
seen to increase with increasing AEG content. Typical D
ꢀ
1
ꢀ1
(
1350 cm ) and G (1587 cm ) bands are observed for both Pt-
WP-CL/AEG-0 and Pt-WP-CL/AEG-3 (Fig. 1b). The intensity ratio Pt-WP-CL/AEG-2
3
2
Pt-WP-CL/AEG-3
Pt-WP-CL/AEG-4
Pt-WP-CL/AEG-5
D G
of D and G bands (I /I ) is the ratio of sp /sp carbons, which is
7,46
also an indicator of the degree of disorder.
It is generally
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while the WP particles are coated by a porous carbon layer,
derived from the carbonization of citric acid (Fig. 3b and h). As
shown in Fig. 3c–g, the WP-CL particles with spherical shape are
tightly bound to AEG. High electrical conductivity of AEG is
expected to improve the charge transfer and enhance MOR/ORR
catalytic activity of the catalysts. As shown in Fig. 3h, a large
number of NPs (Pt and WP) with similar sizes are dispersed on
the carbon skeleton. The measured data for particle sizes (WP
or Pt) are presented inset in Fig. 3h, in accordance with the
Gaussian function. As shown in Fig. 3i, WP and Pt NPs are well
crystallized in the carbon skeleton. The lattice spacings of 0.288,
0.203 and 0.196 nm correspond to (001) and (211) planes of WP
and (200) plane of Pt, respectively, and are consistent with XRD
25,50
results.
Pt and WP(-CL) NPs are in close contact with each
other, implying that the charge transfer pathways should be
smooth among Pt, WP-CL and AEG, and more available Pt active
sites of Pt-WP-CL/AEG-3 should be exposed.
3.2 MOR activity of Pt-WP-CL/AEG catalysts
CV curves (0.5 M H SO ) and calculated results of ECSAs are
2
4
shown in Fig. 4a and Table S1,† respectively. Table S1† shows
that Pt-WP-CL/AEG-3 exhibits the highest ECSA among all
prepared catalysts. Especially, ECSA of Pt-WP-CL/AEG-3 is
2
ꢀ1
1
23.05 m gPt , which is approximately 1.3 times that of Pt-WP-
Fig. 2 XPS survey spectra for Pt-WP-CL/AEG-x (x ¼ 0, 1, 2, 3, 4 and 5)
2
ꢀ1
CL/AEG-0 (94.66 m gPt ), and 1.92 times that of commercial
Pt/C (64.16 m
means better catalytic activity for MOR. Therefore, MOR
(
a) and high resolution XPS of Pt 4f (b), W 4f (c), P 2p (d), C 1s (e) and O
2
ꢀ1
gPt ). In general, higher ESCA of a catalyst
1
s (f) for Pt-WP-CL/AEG-3.
51
activity of Pt-WP-CL/AEG-x (x ¼ 0, 2 and 3) catalysts may be
intermediates (COads) on Pt active sites and promote methanol better than that of Pt/C. The mass activity and specic activity
48
oxidation.
for these catalysts are shown in Fig. 4b and S8,† respectively. It
High resolution XPS spectra of W 4f (Fig. 2c and S4†) can be can be seen from Fig. 4b that Pt/C shows an oxidation peak at

tted to two peaks. The peaks at around 33.6 and 31.5 eV are
assigned to W 4f5/2 and W 4f7/2 of WP, respectively, which
correspond to the +6 oxidation state of surface W due to slight
49
air oxidation. No metallic W(0) peak at around 31.4 eV is found
for Pt-WP-CL/AEG, which implies that besides formation of
a small amount of WO , W atoms are mainly combined with P
3
to form WP in the carbon skeleton. As shown in P 2p spectrum
(
Fig. 2d and S5†), the two peaks at around 129.6 and 133.6 eV
27,49
correspond to P 2p3/2 and P–O, respectively.
It indicates that
although WP NPs are closely wrapped by CL, a small amount of
phosphorus oxides and WO
WP-CL/AEG composites.
3
inevitably exist on the surface of
As shown in Fig. 2e and S6,† four peaks at around 284.6, 286.9,
2
87.5, and 289.1 eV are designated as C]C, C–C, C–O, and C]O,
7
respectively. Oxygen-containing groups (C–O and C]O) can
increase hydrophilicity of the Pt-WP-CL/AEG catalysts, thereby
facilitating the mass transfer and absorption that promotes the
MOR (ORR) activity in an acidic electrolyte. As shown in Fig. 2f
and S7,† two components can be observed by decomposing the O
1
s peak. The peak at around 531.2 eV corresponds to W-bonded
2ꢀ
oxygen (O ), while the peak at around 532.8 eV is related to
chemisorbed oxygen ions in the oxygen vacancy zone.
44
To explore surface morphologies and internal features of the
prepared samples, SEM and TEM tests are conducted (Fig. 3).
AEG exhibits a relatively irregular lamellar structure in Fig. 3a,
Fig. 3 SEM images of AEG (a), and WP-CL/AEG-x (x ¼ 0, 1, 2, 3, 4 and
) (b, c, d, e, f and g, respectively); TEM image of Pt-WP-CL/AEG-3 (h)
and HRTEM images of Pt-WP-CL/AEG-3 (i).
5
22640 | J. Mater. Chem. A, 2018, 6, 22636–22644
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Journal of Materials Chemistry A
(Fig. 4c). Results show that the highest and most durable
current density is achieved by Pt-WP-CL/AEG-3. In the rst
00 s, current densities of all catalysts decrease sharply with an
increase in testing time, and then reach a stable state. Aer
4
3
2
600 s, current densities of Pt/C and Pt-WP-CL/AEG-x (x ¼ 0, 1,
, 3, 4 and 5) exhibit 31.68 (decline of 92.88%), 325.68 (decline
of 67.35%), 159.03 (decline of 64.77%), 279.97 (decline of
2.59%), 751.89 (decline of 62.31%), 274.58 (decline of 63.29%)
6
ꢀ
1
and 147.32 (decline of 61.23%) mA mgPt , respectively. Obvi-
ously, the stabilities of Pt-WP-CL/AEG-x (x ¼ 0, 1, 2, 3, 4 and 5)
catalysts are far higher than that of Pt/C. Fig. S9† shows the CA
curves of Pt-WP-CL/AEG-3 and Pt/C at a potential of 0.6 V (vs.
SCE). The decline in current densities of Pt/C and Pt-WP-CL/AEG-
3
are 94.99 and 60.05%, respectively. Good stability of Pt-WP-CL/
AEGs could be mainly attributed to high co-catalytic activity of
WP, high utilization efficiency of Pt (active sites), protective roles
of CL and excellent electrical conductivity of AEG. Fig. 4d shows
Fig. 4 CV curves of Pt/C and Pt-WP-CL/AEG-x (x ¼ 0, 1, 2, 3, 4 and 5)
2 4 2 4 3
in (a) 0.5 M H SO and (b) 0.5 M H SO + 1 M CH OH at a scan rate of the Nyquist plots and the equivalent circuit diagram (inset) for all
ꢀ1
5
0 mV s ; (c) CA curves and (d) Nyquist plots measured in 0.5 M
catalysts. As shown in Table S1,† the charge transfer resistances
R ) of commercial Pt/C and Pt-WP-CL/AEG-x (x ¼ 0, 1, 2, 3, 4 and
H
2
SO + 1 M CH OH.
4
3
(
ct
5
) catalysts are 73.54, 21.61, 72.29, 31.04, 17.41, 40.75 and 80.42
.77 V during the forward scan. Mass activity and specic U, respectively. Pt-WP-CL/AEG-3 catalyst exhibits the lowest Rct
activity calculated from current densities at peak potential or which indicates high catalytic activity toward MOR.
.77 V (vs. SCE) for all catalysts are listed in Table S2.† Obvi- CO stripping voltammograms are carried out to evaluate the
ously, Pt-WP-CL/AEG-3 exhibits the best mass activity among all CO tolerance of these catalysts (Fig. 5 and Table S4†). Onset
0
,
0
ꢀ
1
studied catalysts of 2217.6 mA mgPt at 0.77 V, which is 4.44 potentials and peak potentials of Pt-WP-CL/AEG catalysts for CO
ꢀ
1
times higher than that of Pt/C (499.2 mA mgPt ).
oxidation have obvious negative shis compared with Pt/C,
Meanwhile, the mass activities of Pt-WP-CL/AEG-x (x ¼ 0, 2 meaning that the Pt-WP-CL/AEG catalysts are more effective
ꢀ1
52
and 4) catalysts (1034.4, 787.2 and 775.2 mA mg
, respec- than Pt/C in the oxidation (removal) of COads on Pt active sites.
Pt
tively) at 0.77 V are also much higher than that of Pt/C. Table These results indicate that Pt-WP-CL/AEG catalysts have better
S3† summarizes the mass activities of Pt-WP-CL/AEG-3, Pt-MoP/ CO tolerance capacity than the commercial Pt/C. Better CO
C18, Pt-Ni
2
P/C20 and Pt-CoP/C25 catalysts. The mass activity of poisoning resistance of Pt-WP-CL/AEGs can be partly attributed
Pt-WP-CL/AEG-3 is much higher than those of other reported to the fast mass transport of COads on different crystalline faces
15
catalysts, attributed to both conductive AEG and connected (such as (001) and (211)) of WP to Pt active sites. Moreover, the
bridge of CL contributing to efficient electron transfer from WP structure of WP closely-linked with AEG by CL has excellent
to Pt surface to obtain high MOR activity. The inset of Fig. 4b electrical conductivity, which can effectively improve charge
shows the onset potential–current curves for all catalysts. The transfer during the COads oxidation process. Therefore, anti-
onset potentials of commercial Pt/C and Pt-WP-CL/AEG-x (x ¼ 0, COads poisoning property of Pt-WP-CL/AEGs are mainly due to
1
0
0
, 2, 3, 4 and 5) catalysts are 0.36, 0.30, 0.27, 0.28, 0.27, 0.28 and the synergistic effects among Pt, WP-CL and AEG.
.29 V, respectively. All onset potentials of Pt-WP-CL/AEG-x (x ¼
, 1, 2, 3, 4 and 5) are lower than that of Pt/C, which implies that for MOR can be attributed to the following aspects: (1) small
The high catalytic activity and stability of Pt-WP-CL/AEG-3
Pt-WP-CL/AEG-xs can exhibit excellent methanol electro- sized Pt is uniformly distributed on the surface of WP-CL/AEG
40
chemical oxidation activity. High performance (high mass to enable full utilization of the Pt active sites; (2) highly
activity and low onset potential) of Pt-WP-CL/AEG-x (x ¼ 0, 2, 3 conductive AEG with abundant O-groups can promote electron
and 4) catalysts are attributed to the unique structure of WP-CL/ transfer from WP to Pt surface to obtain low impedance, fast
AEGs and more available Pt active sites. This excellent activity of MOR kinetics and large ECSA; (3) CL-protected WP facilitates
Pt-WP-CL/AEG-x (x ¼ 0, 2, 3 and 4) may also be related to the the exposure and stability of P active sites; (4) synergistic effects
higher active surface area and faster electron transfer from WP between WP-CL and AEG greatly enhance CO tolerance; and (5)
(
-CL/AEG) to Pt. Note that the number of P active sites plays the unique nanostructure of WP-CL/AEG not only avoids the
a major role in affecting co-catalytic activity of WP-CL/AEGs. As aggregation of Pt NPs, but also ensures strong cycling stability
reported, the exposed P active sites are capable of activating H
of the catalyst during long-term operation.
molecules to generate –OH species and eliminate the COads
2
O
28
species on Pt active sites. Moreover, the lamellar structure of
AEG can effectively facilitate the dispersion of WP-CL and Pt NPs
to improve the MOR performance of Pt-WP-CL/AEG catalysts.
3
.3 ORR activity of Pt-WP-CL/AEG catalysts
To verify the possibility of Pt-WP-CL/AEGs as an ORR catalyst,
CA measurements are conducted to evaluate the durability of ORR-related tests, including CV, LSV, RDE and Tafel plots, are
Pt/C and Pt-WP-CL/AEG-x (x ¼ 0, 1, 2, 3, 4 and 5) catalysts conducted for Pt-WP-CL/AEG-3 and Pt/C. Both Pt/C and Pt-WP-
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Fig. 6 CV curves of Pt-WP-CL/AEG-3 and Pt/C (a); LSV curves of Pt-
WP-CL/AEG-3 and Pt/C in O
1
2
-saturated 0.5 M H
600 rpm (10 mV s ) (b); LSV curves of Pt-WP-CL/AEG-3 (c) and Pt/C
d) in O -saturated 0.5 M H SO solution at different rotation rates
10 mV s ); the calculated Koutecky–Levich plots of Pt-WP-CL/AEG-
and Pt/C at 0.3 V (e); and Tafel plots of Pt-WP-CL/AEG-3 and Pt/C (f).
2 4
SO solution at
ꢀ1
Fig. 5 CO stripping voltammograms of Pt/C and Pt-WP-CL/AEG-x (x
(
(
2
ꢀ
2
4
¼
0, 1, 2, 3, 4 and 5) catalysts.
1
3
CL/AEG-3 show well-dened cathodic ORR current peaks
(
Fig. 6a). Pt-WP-CL/AEG-3 exhibits a more positive peak poten- 3.93, respectively. This result shows that Pt-WP-CL/AEG-3 and
ꢀ
tial at around 0.49 V (vs. SCE) than that of Pt/C (0.38 V vs. SCE). Pt/C conduct electrocatalysis via the 4e pathways in ORR.
Moreover, Pt-WP-CL/AEG-3 shows a much higher peak current RRDE measurements are conducted to further investigate the
density than that of Pt/C. To further investigate the ORR ORR pathways by monitoring the formation of intermediate
performance, LSV measurements on a rotating disk electrode peroxide species (Fig. S10†). Remarkably, ORR on Pt-WP-CL/
are carried out (RDE, Fig. 6b). Pt-WP-CL/AEG-3 and Pt/C have AEG-3 yields 2.03 to 6.7% H over a wide potential range
42
2 2
O
similar half-wave and onset potentials. The half-wave potentials from 0.0 to 0.6 V, with n ranging from 3.87 to 3.96 in 0.5 M
of Pt-WP-CL/AEG-3 and Pt/C are 0.61 and 0.58 V, meanwhile the H SO solution, suggesting that the ORR process of Pt-WP-CL/
2
4
ꢀ
onset potentials are 0.72 and 0.69 V, respectively. The onset AEG-3 has high selectivity for generating OH as the main
potentials, half-wave potentials and limiting current densities product. These results are in accordance with the RDE
42
of Pt-WP-CL/AEG-3 and Pt/C are listed in Table S5.† Comparable measurements, indicating that the Pt-WP-CL/AEG-3 performs
ꢀ
42
ORR activity of Pt-WP-CL/AEG-3 (Pt, 5 wt%) and Pt/C (10 wt%) via the efficient 4e ORR pathway in acid media. In addition,
may be attributed to the increased number of exposed Pt active- similar Tafel slope values are obtained for Pt-WP-CL/AEG-3
ꢀ
1
ꢀ1
sites and improved conductivity by offering easy charge transfer (139.3 mV dec ) and Pt/C (136.6 mV dec ), conrming that
44
pathways. In addition, the more exposed P active sites induced ORR activity of Pt-WP-CL/AEG-3 is slightly better than that of Pt/
by W on WP can contribute to faster adsorption and activation C (Fig. 6f).
of oxygen molecules during ORR. Charges are expected to be
Scheme 2 shows a diagram of MOR and ORR processes on Pt-
transferred from AEG (positively charged) to WP through the WP-CL/AEG catalysts. During MOR, some methanol molecules
linked-CL and then from WP to Pt, which contributes to the can be easily adsorbed on the WP-CL/AEG surface with its
promising ORR activity of Pt-WP-CL/AEG.
abundant oxygen-containing functional groups (OH/COOH/
As shown in Fig. 6c and d, current densities for Pt-WP-CL/ COC), which are then oxidized on the adjacent Pt active sites.
AEG-3 are higher than those for Pt/C as the rotation rate This process is more effective and durable than the direct
changes from 400 to 2500 rpm. In general, ORR can occur via adsorption (oxidation) of methanol on Pt surface. The well-
ꢀ
ꢀ
53
the 4e or 2e pathways. The preferred electrocatalysts usually dispersed WP NPs with permissible W and P sites are able to
ꢀ
53
conduct the ORR via the 4e pathway. Fig. 6e shows the K–L activate H O molecules to provide sufficient –OH reactive
curves for Pt-WP-CL/AEG-3 and commercial Pt/C at 0.3 V. The species. A large amount of produced –OH species can promote
calculated n values of Pt-WP-CL/AEG-3 and Pt/C are 3.95 and rapid oxidation of COads species around the periphery of Pt NPs
2
54
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Acknowledgements
We acknowledge the support by National Natural Science
Foundation of China (21806031, 51578218 and 51108162),
Natural Science Foundation of Heilongjiang Province
(QC2015009), Postdoctoral Science Foundation of Heilongjiang
Province (LBH-Q14137), Scientic and technological innovation
talents of Harbin (2016RQQXJ119), and Excellent Young
Teachers Fund of Heilongjiang University and Hundred Young
Talents in Heilongjiang University.
Notes and references
Scheme 2 Schematic diagram of methanol oxidation reaction and
oxygen reduction reaction processes on Pt-WP-CL/AEG catalysts.
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WP-CL/AEGs as MOR/ORR catalyst supports with high co-
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