Pt-Ni-P nanocages with surface porosity as efficient bifunctional electrocatalysts for oxygen reduction and methanol oxidation

Article abstract of DOI:10.1039/c9ta00928k

The structure- and composition-controlled synthesis of Pt-based nanocatalysts is highly important for improving their catalytic performance while reducing the overall use of precious Pt. Here, an effective strategy is developed to combine surface porosity

Full text of DOI:10.1039/c9ta00928k

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Materials Chemistry A
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Pt–Ni–P nanocages with surface porosity as
efficient bifunctional electrocatalysts for oxygen
reduction and methanol oxidation†
Cite this: DOI: 10.1039/c9ta00928k
Kai Deng, You Xu, * Dandan Yang, Xiaoqian Qian, Zechuan Dai, Ziqiang Wang,
Xiaonian Li, Liang Wang * and Hongjing Wang
*
The structure- and composition-controlled synthesis of Pt-based nanocatalysts is highly important for
improving their catalytic performance while reducing the overall use of precious Pt. Here, an effective
strategy is developed to combine surface porosity, hollow cavities and metal–nonmetal incorporation
engineering on Pt, resulting in ternary Pt–Ni–P mesoporous nanocages (Pt–Ni–P MNCs). Owing to the
mesoporous surface, hollow structure and synergistic electronic effects derived from Pt, Ni, and P
elements, the as-prepared Pt–Ni–P MNCs exhibit excellent electrocatalytic activity and long-term
stability for both the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR), enabling
them to be promising ORR–MOR bifunctional electrocatalysts for direct methanol fuel cells.
Received 24th January 2019
Accepted 19th March 2019
DOI: 10.1039/c9ta00928k
rsc.li/materials-a
alternative effective strategy is alloying or incorporating Pt
1
. Introduction
with other transition metals to form bi-metallic or multi-
2
2–30
Direct methanol fuel cells (DMFCs) represent a class of prom- metallic Pt-based nanostructures.
The foreign metal
ising clean power sources and are especially important in atoms could modify the electronic properties of Pt by charge
1
–3
powering portable electronic devices. The sluggish kinetics of transfer processes and lowers the adsorption energy of related
the oxygen reduction reaction (ORR) at the cathode and the reaction species onto the Pt surface due to the ligand or the
methanol oxidation reaction (MOR) at the anode is the key electronic effect. Beyond this, one recent research trend in
4,5
obstacle for the commercialization of DMFCs. To date, Pt and this eld is the introduction of phosphorus (P), an inexpen-
Pt-based nanomaterials have been recognized as the most effi- sive metalloid element consisting of abundant valence elec-
6–12
cient electrocatalysts for both the ORR and MOR in DMFCs.
trons, into Pt and Pt-based nanostructures. By using various
However, a number of problems including high cost, limited strategies and P sources, a broad variety of P-containing
utilization and their poisoning by strongly adsorbed species Pt-based nanocatalysts have been developed which demon-
(
e.g., carbon monoxide) have greatly limited their large-scale strated superior electrocatalytic performance and CO toler-
31–35
practical application. Therefore, large efforts are currently ances.
Despite these advances, the synthesis of ternary
being made to signicantly increase the Pt utilization efficiency Pt–non-precious-metal–P hollow nanostructures with a well-
of DMFCs and reduce the extent of catalyst poisoning of Pt while dened mesoporous surface is still challenging yet
maintaining suitable performance.
signicant.
Efforts to boost the catalytic performance of Pt and
Herein, we demonstrate the synthesis of Pt–Ni–P meso-
increase the Pt utilization efficiency have been concentrated porous nanocages (Pt–Ni–P MNCs) by means of a facile strategy.
on the delicate structural design of Pt and Pt-based nano- The current strategy involves the template-assisted deposition
1
3–18
catalysts.
In this regard, hollow nanostructures with well- of Pt mesoporous nanocages (Pt MNCs) and the subsequent Ni,
dened porous surfaces have special advantages over their P-co-incorporation into the pre-made Pt MNCs. The as-
counterparts with solid interiors and compact exteriors as fabricated ternary Pt–Ni–P nanoarchitectures integrate the
they allow for effective use of both the outer and inner structural advantages of hollow nanocages and mesoporous
1
9–21
surfaces of the materials when used as catalysts.
Another surfaces and the compositional properties of metal–nonmetal
multicomponents, allowing for rational engineering of the
density and reactivity of the active sites for electrocatalysis
State Key Laboratory Breeding Base of Green-Chemical Synthesis Technology, College applications. As expected, the as-prepared Pt–Ni–P MNCs
of Chemical Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang
exhibit superior electrocatalytic activities towards both the ORR
3
10014, P. R. China. E-mail: yxu@zjut.edu.cn; wangliang@zjut.edu.cn; hjw@zjut.
and MOR compared with binary Pt–Ni MNCs and commercial
Pt/C, making them promising ORR–MOR bifunctional electro-
catalysts for DMFCs.
edu.cn
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ta00928k
1
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2
.5 Characterization
2
. Experimental
The surface morphology and structure of the catalysts were
characterized using a scanning electron microscope (SEM)
2
.1 Materials and chemicals
All chemicals and solvents were used as received without
further purication. Nickel chloride hexahydrate (NiCl $6H O),
sodium hypophosphite (NaH PO ), sodium borohydride
), ammonium hydroxide solution (NH $H O), hydro-
uoric acid (HF), tetraethyl orthosilicate (TEOS), ethanol (CH3-
(ZEISS SUPRA 55) with an accelerating voltage of 15 kV. The
2
2
hollow structure of the catalysts was investigated using a trans-
mission electron microscope (TEM) (JEOL JEM-2100F) operated
at 200 kV equipped with an energy dispersive X-ray spectrom-
eter (EDX) analyzer. The wide-angle X-ray diffraction (XRD)
analysis was performed on an X-ray diffractometer (PANalytical
X'Pert PRO) with Cu Ka radiation (l ¼ 0.1542 nm) at 40 kV and
2
2
(NaBH
4
3
2

CH OH), methanol (CH OH), perchloric acid (HClO ) and L-
ascorbic acid (AA) were purchased from Aladdin Industrial
Corporation (Shanghai, China). Potassium tetrachloroplatinate
(K PtCl ), 3-aminopropyltrimethoxysilane (APTMS) and Plur-
2 4
onic F127 were provided by Sigma-Aldrich. Commercial Pt/C
catalysts (20 wt%) were ordered from Alfa Aesar.
2
3
4
3
0 mA. The X-ray photoelectron spectroscopy (XPS) analysis was
performed using an ESCALAB MK II spectrometer (VG Scien-
tic, UK) with Al Ka X-ray radiation for excitation. Inductively
coupled plasma mass spectrometry (ICP-MS) measurements
were carried out by using an Elan DRC-e instrument. Nitrogen-
sorption isotherms were collected using a model Autosorb IQ3
instrument (Quantachrome, USA). The surface areas of the
samples were calculated by the Brunauer–Emmett–Teller (BET)
method. The pore-size distribution was derived from the
desorption branch using the Barrett–Joyner–Halenda (BJH)
method.
2
.2 Preparation of SiO
Typically, 200 mL of ethanol was added to a solution containing
5.2 mL of H O and 6.4 mL of NH $H O. Aer stirring vigor-
2
spheres
1
2
3
2
ously for 1 h at room temperature, 12 mL of TEOS was injected
into the above solution. Aer 3 h, the sample was collected by
centrifugation at 10 000 rpm for 5 min, followed by washing
ꢀ
with ethanol and water. The product was nally dried at 50 C in
a vacuum oven for 10 h.
2.6 Electrochemical measurements
Electrochemical measurements were performed in a conven-
tional three-electrode cell consisting of a working electrode, an
Ag/AgCl (saturated KCl) reference electrode and a Pt wire
counter electrode using an electrochemical workstation (CHI
2
.3 Preparation of Pt MNCs
The obtained SiO spheres were initially modied with an
amino group. 50 mg of SiO particles were dissolved in
2
2
760E). All potentials, if not specied, were recorded relative to
a mixture containing 10 mL of isopropanol and 200 mL of
APTMS. Then the mixed solution was heated to reux at 80 C
ꢀ
the reversible hydrogen electrode (RHE) in this work.
As for ORR measurements, the working electrode was
a glassy-carbon rotating disk electrode (RDE) (3 mm in diam-
eter) modied by the catalyst. 2 mg of the Pt–Ni–P MNCs was
dispersed in 1 mL of deionized water and sonicated for 20 min
to produce an ink. Then, the glassy-carbon RDE was coated with
for 12 h. The functionalized SiO spheres were collected by
2
centrifugation at 6000 rpm for 3 min, followed by washing
ꢀ
with ethanol and water, and dried at 40 C in a vacuum oven
for 10 h. In a typical synthesis of Pt MNCs, the functionalized
SiO
in 2 mL of K
2
spheres (2 mg) and Pluronic F127 (20 mg) were dispersed
PtCl solution (20.0 mM). Aer Pluronic F127
ꢁ2
the catalyst ink to achieve a Pt loading of 28.2 mg cm and
allowed to air dry. Following catalyst addition, 5 mL of Naon
2
4
was completely dissolved, 2 mL of AA solution (0.1 M) was
(0.5 wt%) was coated on the catalyst-modied glassy-carbon
added to the above solution. Then the mixed solution was
ꢀ
RDE surface to ensure catalyst adhesion. The glassy-carbon
RDE modied by other samples was prepared with similar
procedures and the Pt loading on the electrode was kept the
same for different catalysts. The ORR evaluation was performed
by using a RRDE-3A rotation system (ALS Co. Ltd, Japan). Linear
sweep voltammetry (LSV) measurements were carried out in an
sonicated for 1 h at 35 C. The as-formed SiO @Pt product was
2
collected by centrifugation at 6000 rpm for 10 min and
washed for 3 cycles with water. The SiO
2
@Pt was then re-
dispersed in 10 wt% HF (20 mL) for 12 h to erode the SiO
2
templates. The nal Pt MNCs were collected by centrifugation
at 3000 rpm for 10 min and washed with water 5 times.
ꢁ1
4
oxygen saturated 0.1 M HClO solution at a scan rate of 5 mV s
Finally, the Pt MNCs were re-dispersed in H
further use.
2
O (5 mL) for
and rotation rate of 1600 rpm. Chronoamperometric (CA)
investigation for the ORR was conducted in an oxygen saturated
0
1
.1 M HClO solution at 0.9 V (vs. RHE) with a rotation rate of
600 rpm. The kinetic currents were calculated using the Kou-
4
2
.4 Preparation of Pt–Ni–P MNCs
0.5 mL of NiCl $6H O (20.0 mM) and 4.5 mL of NaH PO (20.0
2
2
2 2
tecky–Levich equation:
mM) solutions were added to the as-prepared Pt MNC disper-
sion under stirring, then, a freshly prepared NaBH solution (2
mL, 0.2 M) solution was quickly added. Aer stirring for 10 min
1
1
1
4
¼ j þ j
j
k
d
at room temperature, the slurry was washed with water and where j, j and j represent the measured, kinetic and diffusion-
k
d
collected by centrifugation at 3000 rpm for 5 min. The nal limited current densities, respectively. The electron transfer
ꢀ
product was dried at 50 C in a vacuum oven for 10 h before the number (n) and the peroxide yield (H
2
O
2
%) could be calculated
electrochemical tests.
as follows:
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4I
D
n ¼
ðI
D
þ I =NÞ
R
2
00I
R
H
2
O
2
% ¼
ðI
D
N þ I
R
Þ
where ID and I are the disk current and ring current, respec-
R
tively; and N ¼ 0.4286 is the current collection efficiency of the
Pt ring.
As for MOR measurements, the working electrode was
a glassy carbon electrode (GCE) modied with the catalyst. The
preparation process of the working electrodes was the same as
that of the ORR measurements. Cyclic voltammograms (CV)
2 2 4
were recorded in a solution of 0.5 M N -saturated H SO solu-
ꢁ
1
tion containing 1 M CH OH at a scan rate of 50 mV s . CA
3
measurements were carried out in a 0.5 M H SO solution
2
4
containing 1 M CH OH at a potential of 0.8 V (vs. RHE). For CO
3
stripping measurements, high-purity CO was pre-adsorbed on
the working electrode by bubbling it in a 0.5 M H
for 30 min while holding the electrode potential at 0.24 V (vs.
RHE). Aer the solution was purged with N for 20 min to
remove the remaining CO, the CO stripping curve was obtained
2 4
SO solution
2
ꢁ
1
by CV at a scan rate of 50 mV s
.
3. Results and discussion
Fig. 2 (a and b) SEM and (c and d) TEM images of Pt–Ni–P MNCs. The
inset in (c) shows the SAED pattern. (e) HRTEM image of the edge part
As illustrated in Fig. 1, the Pt–Ni–P MNCs were fabricated by of a single Pt–Ni–P MNC. (f) The Fourier-filtered lattice fringe image of
a three-step strategy. The rst step involves the synthesis of the marked area in (e). The inset in (f) displays the corresponding FFT
pattern.
2 2
SiO @Pt core–shell mesoporous nanospheres. SiO spheres
(
ꢂ180 nm in diameter), which were obtained by the St ¨o ber
36
method, were used as the hard-templates for the deposition of
Pt nanostructures (Fig. S1†). The presence of F127 as the
structure-directing agent for the growth of Pt contributes to the
formation of a well-dened mesoporous surface on the as-
Pt–Ni–P MNCs is around 200 nm (Fig. S3†). A magnied SEM
image clearly reveals the mesoporous surface of the Pt–Ni–P
MNCs (Fig. 2b). Nitrogen adsorption–desorption isotherms
further reveal the mesoporous features of the Pt–Ni–P MNCs
37,38
deposited Pt nanostructures.
Aer the removal of the SiO
2
templates by HF solution treatment in the second step, Pt MNCs
with uniform morphology can be produced (Fig. S2†). In the
third step, the pre-constructed Pt MNCs served as the precur-
(Fig. S4a†). The hollow structure of these Pt–Ni–P MNCs is
clearly conrmed by TEM observation (Fig. 2c), and an enlarged
TEM image further conrms that the thickness of the shell is
around 20 nm (Fig. 2d). The surfaces of the Pt–Ni–P MNCs
consist of abundant tiny interconnected nanoparticles with
a diameter of around 3 nm, which could serve as catalytically
active sites during electrocatalysis. The polycrystallinity feature
of the as-prepared Pt–Ni–P MNCs is conrmed by the selected
area electron diffraction (SAED) pattern (inset of Fig. 2c). Fig. 2e
shows a high-resolution TEM (HRTEM) image of the edge part
of a single Pt–Ni–P MNC. The lattice fringes with a spacing of
2 2 2
sors for Ni, P-co-incorporation by using NiCl , NaH PO and
NaBH as the Ni source, P source and reduction agent, respec-
4
tively. The Ni, P-co-incorporation step could be achieved at
room temperature without the need for high reaction temper-
ature and dangerous phosphorus sources.
The detailed morphology and structure features of the as-
synthesized Pt–Ni–P MNCs are characterized by SEM and
TEM. The low magnication SEM image (Fig. 2a) reveals that
well-dispersed Pt–Ni–P MNCs with sphere-like morphology are
generated in a high yield. The average diameter of the as-formed
0.23 nm correspond to the (111) planes of the Pt-based face-
centered cubic (fcc) structure, as also revealed by the corre-
sponding fast Fourier transform (FFT) pattern (Fig. 2f).
EDX analysis validates the presence of Pt, Ni and P elements
in the Pt–Ni–P MNCs (Fig. S5†). Quantitative analysis results
from the EDX spectrum show the composition of Pt–Ni–P MNCs
to be 61.39 at% Pt, 11.87 at% Ni, and 26.74 at% P, which is
almost in accordance with the ICP-MS data (62.73 at% Pt, 10.56
at% Ni, and 26.71 at% P). Fig. 3a shows the high-angle annular
dark eld-scanning TEM (HAADF-STEM) image of a single
Fig. 1 Schematic illustration of the formation process of Pt–Ni–P
MNCs.
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crystal structure. No other impurity diffraction peaks were
observed from the XRD pattern of Pt–Ni–P MNCs, indicating the
formation of a ternary alloyed structure. When compared with
standard diffraction patterns of pure Pt (JCPDS no. 65-2868), the
diffraction peaks of Pt–Ni MNCs clearly shi to a higher 2q
angle due to the partial substitution of larger Pt atoms by
smaller-sized Ni atoms. It can also be seen that the diffraction
peaks of Pt–Ni–P MNCs exhibit a slightly negative shi relative
to Pt–Ni MNCs, suggesting the expansion of the lattice due to
39
the introduction of P atoms.
The surface composition and electronic structure of the Pt–
Ni–P MNCs and Pt–Ni MNCs are further investigated by XPS.
The XPS survey spectrum further conrms the presence of Pt, Ni
and P elements on the surface of Pt–Ni–P MNCs (Fig. S7†). In
the Pt 4f XPS spectrum of Pt–Ni–P MNCs (Fig. 4b), two peaks
0
centered at 74.5 eV and 71.3 eV could be assigned to Pt 4f5/2
0
II
and Pt 4f7/2, respectively. A small percentage of Pt exists as Pt
in Pt–Ni–P MNCs. Meanwhile, a slight negative shi (0.31 eV) of
the Pt 4f binding energy for the Pt–Ni–P MNCs compared with
the Pt–Ni MNCs is also observed. As seen from the Ni 2p spec-
Fig. 3 (a) HAADF-STEM image and (b–d) elemental mapping images
of a single Pt–Ni–P MNC.
II
trum, the majority of the surface Ni was in the form of Ni .
Positive shis are observed for the binding energy of Ni 2p in
Pt–Ni–P MNCs relative to the Pt–Ni MNCs (Fig. 4c). In the P 2p
spectrum, the two peaks of P 2p at 131.4 and 133.3 eV can be
Pt–Ni–P MNC, which also demonstrates that the inner cavities
and well-dened surface porous structure. The elemental
mapping images reveal the Pt, Ni and P elements are homoge-
neously distributed throughout the whole Pt–Ni–P MNC
0
ascribed to elemental P and oxidized P species, respectively
0
(
Fig. 4d). The binding energy position of the P peak exhibits
(Fig. 3b–d). These results indicate the Ni and P elements have
40,41
a positive shi compared with that of pure P (130.4 eV).
presence of oxidized species (Pt , Ni , and P ) arises from the
The
been successfully incorporated into the Pt structure, forming
ternary Pt–Ni–P nanostructures.
II
II
I
supercial oxidation of the Pt–Ni–P MNCs when exposed to air,
The crystallinity and phase purity of the Pt–Ni–P MNCs are
conrmed by wide-angle XRD analysis. The Pt–Ni MNC coun-
terparts without the P component were also synthesized for
comparison (Fig. S6†). As shown in Fig. 4a, all diffraction peaks
observed for the samples can be assigned to the corresponding
31–34,42
as also reported in other previous studies.
The results
reveal that there is a strong interaction between the Pt, Ni and P
atoms, which should also be due to the formation of the Pt–Ni–
P alloy. Based on the above characterization and analysis
results, it can be concluded that ternary Pt–Ni–P nanostructures
with a well-dened mesoporous surface and hollow cavities
were successfully synthesized. Such a unique nanoarchitecture
could integrate the structural advantages of hollow nanocages
and the mesoporous structure and the compositional properties
of metal–nonmetal multicomponents, allowing for rational
engineering of the density and reactivity of the catalytically
active sites.
(111), (200), (220), (311) and (222) planes of the fcc Pt-based
We rst evaluate the electrocatalytic performance of our as-
made Pt–Ni–P MNCs towards the ORR in acidic media, bench-
marked against the Pt–Ni MNCs and commercial Pt/C catalyst.
The BET surface areas of these three catalysts are measured to
2
ꢁ1
2
ꢁ1
be 21.47 m g for the Pt–Ni–P MNCs, 20.44 m g for the Pt–
2
ꢁ1
Ni MNCs, and 85.36 m g for the Pt/C (Fig. S4†). The elec-
trochemically active surface areas (ECSAs) of the electrocatalysts
can be calculated from the charges associated with the areas of
both the desorption and adsorption hydrogen region aer
double-layer correction in CV curves recorded in N -purged
2
ꢁ1
0
.5 M H SO solution with a scan rate of 50 mV s (Fig. S8†).
Accordingly, the ECSAs are calculated to be 44.3 m
5.3 m gPt and 57.6 m gPt for Pt–Ni–P MNCs, Pt–Ni MNCs
2 4
2
ꢁ1
g
Pt
,
2
ꢁ1
2
ꢁ1
3
Fig. 4 (a) XRD patterns of the Pt–Ni–P MNCs and Pt–Ni MNCs. The
ꢀ
and Pt/C, respectively. We further performed electrochemical
double-layer capacitance (Cdl) measurements. The correspond-
ing Cdl values of the catalyst-modied electrodes can be
inset image shows the slow scan XRD profile at 2q ¼ 35–45 . XPS
spectra of the Pt 4f (b), Ni 2p (c) and P 2p (d) for the Pt–Ni–P MNCs and
Pt–Ni MNCs.
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calculated using the slope extracted from the linear relationship indicating an efficient four-electron reaction route is the main
of the current density with the increasing scan rates. The C reaction on Pt–Ni–P MNCs for the ORR process. Rotating ring-
values are evaluated to be 0.27 mF cm , 0.24 mF cm , and disk electrode (RRDE) investigations reveal that the yield of
dl
ꢁ2
ꢁ2
ꢁ
2
0.32 mF cm
for Pt–Ni–P MNCs, Pt–Ni MNCs and Pt/C, H O for Pt–Ni–P MNCs is lower relative to that of the Pt/C,
2 2
respectively (Fig. S9†).
2 2
implying that the production of H O is hindered in the
Fig. 5a shows the ORR polarization curves of the electro- process of the ORR catalyzed by Pt–Ni–P MNCs (Fig. S11c†). The
catalysts. The Pt–Ni–P MNCs exhibit more positive onset electron transfer number obtained from RRDE experiments is
potential (Eonset, 0.99 V) and half-wave potential (E1/2, 0.94 V) determined to be around 4 (Fig. S11d†), which further testies
compared to the Pt–Ni MNCs (Eonset ¼ 0.97, E1/2 ¼ 0.91 V) and the four-electron reaction pathway for the ORR.
commercial Pt/C catalyst (Eonset ¼ 0.92 V, E ¼ 0.85 V), sug-
The durability test of the Pt–Ni–P MNCs and commercial Pt/
gesting the easier reduction of oxygen on the surface of Pt–Ni–P C catalysts toward the ORR is investigated through an acceler-
1/2
ꢁ
1
MNCs. The Tafel slope of the Pt–Ni–P MNCs (62.4 mV dec ) is ated durability test (Fig. S12a and b†). The degradation of E1/2 of
ꢁ
1
lower than those of the Pt–Ni MNCs (76.3 mV dec ) and Pt/C the Pt–Ni–P MNs (15 mV) is much lower than that of the Pt/C (43
ꢁ1
(
63.5 mV dec ), indicating that Pt–Ni–P MNCs are more mV) aer 10 000 cycles. In addition, the mass activity of the Pt–
prone to catalyzing oxygen reduction (Fig. 5b). The kinetic Ni–P MNCs drops by only 22.3% aer 10 000 cycles, while the
currents of the catalysts are derived from the ORR polarization commercial Pt/C catalyst decreases by 41.5% (Fig. S12c†).
curves using the Koutecky–Levich equation. Fig. 5c and Chronoamperometric investigations are further carried out to
d compare the kinetic currents of these catalysts at 0.9 V, compare the stability of the Pt–Ni–P MNCs and Pt/C. Aer
normalized to the ECSA (specic mass) and the Pt loading mass testing for 5 h, the Pt–Ni–P MNCs retain 77.98% of their initial
(
mass activity), respectively. The Pt–Ni–P MNCs show a specic current density, which is higher than that of the Pt/C (48.76%)
ꢁ2
activity of 2.35 mA cm , which is 3.5 and 9.0-times higher than (Fig. S12d†). XPS was further conducted on Pt–Ni–P MNCs aer
that of the Pt–Ni MNCs (0.67 mA cm ) and commercial Pt/C the ORR durability test (Fig. S13†). In the Pt 4f spectrum, both
(
obtained Pt–Ni–P MNCs (1.21 mA mgPt ) exhibited 2.1 and 7.6- species can be detected (Fig. S13a†). In the Ni 2p spectrum, the
fold enhancement as compared to that of the Pt–Ni MNCs (0.58 surface Ni component is present in the form of Ni owing to the
mA mgPt ) and commercial Pt/C (0.16 mA mgPt ), respectively. exposure to air (Fig. S13b†). The decrease in Ni content
In order to study the inuence of composition on the catalytic compared with that before the ORR test could be ascribed to the
performance, we further compared various Pt–Ni–P catalysts chemical etching of Ni from the outer surface of Pt–Ni–P MNCs
with different Ni contents and demonstrated the superior ORR during testing.
activity of our typical Pt–Ni–P MNCs (Fig. S10†). the ORR (Fig. 4d), the P 2p peaks disappear aer a long-term
ꢁ
2
ꢁ
2
0.26 mA cm ) catalysts, respectively. The mass activity of the the metallic state of Pt and a small amount of oxidized Pt
ꢁ
1
II
ꢁ1
ꢁ1
42,43
Differing from those of the samples before
The polarization curves of the Pt–Ni–P MNCs at different durability test (Fig. S13c†), indicating that the P species at the
rotation rates are recorded to further examine the ORR kinetics. surface have been etched or dissolved into the electrolyte, as
42,44,45
Fig. S11a and b† show the rotation-rate-dependent current– reported in previous studies.
EDX analysis conrms that
potential curves of the Pt–Ni–P MNCs and the corresponding Pt, Ni and P elements are still present in the Pt–Ni–P MNCs aer
K–L plots at different potentials, respectively. Based on the K–L the durability test (Fig. S14†), while the Ni content (8.54 at%)
plots, the electron transfer number is calculated to be around 4, and P content (19.21 at%) are slightly lower than those before
the ORR test (11.87 at% Ni and 26.74 at% P) due to the partial
dissolution of P and Ni at the surface.
We also assess the catalytic performance our catalysts
2 4 3
towards the MOR in 0.5 M H SO containing 1.0 M CH OH. To
evaluate the electrocatalytic activity, the MOR current was
normalized to the ECSA (specic activity) and the mass loading
of Pt (mass activity), respectively. Fig. 6a shows the specic
activities of the catalysts. The forward peak current density on
ꢁ2
the Pt–Ni–P MNCs is 2.28 mA cm , which is 1.4 and 4.1 times
ꢁ
2
higher than that of the Pt–Ni MNCs (1.62 mA cm ) and
ꢁ
2
commercial Pt/C (0.56 mA cm ), respectively. As for the mass
activities (Fig. 6b), the Pt–Ni–P MNCs yield a forward peak
ꢁ
1
current density of 1.22 mA mgPt , about 1.4 and 2.8 times
ꢁ
1
greater than those of the Pt–Ni MNCs (0.87 mA mg
(
) and Pt/C
Pt
ꢁ1
0.44 mA mgPt ), respectively. Moreover, our Pt–Ni–P MNCs also
exhibit a higher mass activity compared with some previously
reported Pt-based catalysts (Table S1†). Chronoamperometric
curves were recorded to probe the stability of the catalysts
(Fig. 6c). The Pt–Ni–P MNCs show a higher specic current
density and slower current decay than the Pt–Ni MNCs and
commercial Pt/C during the whole test process. The CO
Fig. 5 (a) ORR polarization curves of the catalysts in an O
2
-saturated
solution with a scan rate of 5 mV s and a rotation rate of
600 rpm. (b) The corresponding Tafel plots. (c) Specific activities and
d) mass activities of the electrocatalysts at 0.9 V.
ꢁ1
0
.1 M HClO
4
1
(
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addition, the structural stability of the as-formed mesoporous
nanocages could effectively prevent particle aggregation and
Ostwald ripening under electrocatalysis operation conditions;,
endowing them with superior catalytic stability and durability.
4. Conclusions
In summary, we have developed an effective strategy to achieve
surface porosity, hollow cavities and metal–nonmetal incorpo-
ration engineering on Pt to boost the electrocatalytic perfor-
mance. The as-fabricated ternary Pt–Ni–P nanoarchitectures
integrate the structural advantages of hollow nanocages and
mesoporous surfaces and the compositional properties of
metal–nonmetal multicomponents. As a result, structural and
electronic effects remarkably enhance the activity and durability
in electrocatalysis, making Pt–Ni–P MNCs an efficient ORR–
Fig. 6 (a) Specific activities and (b) mass activities of the MOR for the MOR bifunctional electrocatalyst for DMFCs. This study
2 4 3
catalysts recorded in a mixture of 0.5 M H SO and 1 M CH OH at
a scan rate of 50 mV s . (c) Chronoamperometric curves for the MOR
at 0.8 V. (d) CO stripping voltammograms.
provides an alternative avenue in designing highly efficient Pt-
based nanocatalysts by the combination of geometrically and
compositionally favorable factors.
ꢁ1
Conflicts of interest
tolerance of catalysts is conrmed by CO stripping curves. As
shown in Fig. 6d, we can see that the onset potential of CO
anodic oxidation on the Pt–Ni–P MNCs (0.89 V) is obviously
more negative than that on the Pt–Ni MNCs (0.90 V) and
commercial Pt/C (0.92 V). The results demonstrate that the
Pt–Ni–P MNCs have an enhanced catalytic activity and CO-
tolerant performance in the MOR due to the incorporation of
P element.
There are no conicts to declare.
Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (No. 21601154, 21776255, and
21701141).
As for the enhancement in electrocatalytic performance
towards both the ORR and MOR, the unique electronic and
surface structure effects derived from ternary Pt–Ni–P MNCs are
believed to make a major contribution (Fig. 7). On the one hand,
the mesoporous surface and hollow structure not only increase
the catalytically active area accessible to the reactant molecules
but also improve electron mobility in the skeletons of the
nanocages and provide effective mass transfer in the electro-
catalysis reactions. On the other hand, the synergistic electronic
effects derived from Pt, Ni, and P elements could modify the
electronic properties of the Pt by charge transfer processes and
lowers the adsorption energy of related reaction species onto
the Pt surface, and thus facilitate ORR and MOR catalysis. In
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