A Robust PtNi Nanoframe/N-Doped Graphene Aerogel Electrocatalyst with Both High Activity and Stability

Article abstract of DOI:10.1002/anie.202015679

Insufficient catalytic activity and stability and high cost are the barriers for Pt-based electrocatalysts in wide practical applications. Herein, a hierarchically porous PtNi nanoframe/N-doped graphene aerogel (PtNiNF-NGA) electrocatalyst with outstanding performance toward methanol oxidation reaction (MOR) in acid electrolyte has been developed via facile tert-butanol-assisted structure reconfiguration. The ensemble of high-alloying-degree-modulated electronic structure and correspondingly the optimum MOR reaction pathway, the structure superiorities of hierarchical porosity, thin edges, Pt-rich corners, and the anchoring effect of the NGA, endow the PtNiNF-NGA with both prominent electrocatalytic activity and stability. The mass and specific activity (1647 mA mg_Pt^?1, 3.8 mA cm^?2) of the PtNiNF-NGA are 5.8 and 7.8 times higher than those of commercial Pt/C. It exhibits exceptional stability under a 5-hour chronoamperometry test and 2200-cycle cyclic voltammetry scanning.

Full text of DOI:10.1002/anie.202015679

Angewandte
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How to cite: Angew. Chem. Int. Ed. 2021, 60, 9590–9597
International Edition:
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doi.org/10.1002/anie.202015679
doi.org/10.1002/ange.202015679
Electrocatalysis Hot Paper
A Robust PtNi Nanoframe/N-Doped Graphene Aerogel
Electrocatalyst with Both High Activity and Stability
Jing Yang, RenØ Hübner, Jiangwei Zhang, Hao Wan, Yuanyuan Zheng, Honglei Wang,
Haoyuan Qi, Lanqi He, Yi Li, Amare Aregahegn Dubale, Yujing Sun, Yuting Liu, Daoling Peng,
Yuezhong Meng, Zhikun Zheng, Jan Rossmeisl, and Wei Liu*
Abstract: Insufficient catalytic activity and stability and high
cost are the barriers for Pt-based electrocatalysts in wide
practical applications. Herein, a hierarchically porous PtNi
nanoframe/N-doped graphene aerogel (PtNiNF-NGA) elec-
trocatalyst with outstanding performance toward methanol
oxidation reaction (MOR) in acid electrolyte has been
developed via facile tert-butanol-assisted structure reconfigu-
ration. The ensemble of high-alloying-degree-modulated elec-
tronic structure and correspondingly the optimum MOR
reaction pathway, the structure superiorities of hierarchical
porosity, thin edges, Pt-rich corners, and the anchoring effect of
the NGA, endow the PtNiNF-NGA with both prominent
electrocatalytic activity and stability. The mass and specific
electrocatalysis, and correspondingly the large amount of
[2]
precious Pt required. To address these issues, great efforts
have been made on the composition, size, and morphology
control, structure engineering, and optimization of support
[
3]
materials of the electrocatalysts. To date, agreement has
been reached on how to improve activity, with the general
strategies being via increasing the number of active sites and
[
4]
improving the intrinsic activity of each site. Regarding the
electrocatalytic stability, the construction of stabilized Pt
[
1a,5]
alloys, such as alloys with Pt-rich surface,
intermetallic
[6]
[7]
phase, and high alloying degree, etc., has been proved to
be an efficient way. However, these kinds of electrocatalysts
often possess a solid morphology, and the inner Pt atoms are
À1
À2
[8]
activity (1647 mAmgPt , 3.8 mAcm ) of the PtNiNF-NGA
are 5.8 and 7.8 times higher than those of commercial Pt/C. It
exhibits exceptional stability under a 5-hour chronoamperom-
etry test and 2200-cycle cyclic voltammetry scanning.
essentially wasted, resulting in a moderate activity. Recent-
ly, the combination of high-porosity one-dimensional Pt-rich
surface structure and commercial carbon black (C) support
has been demonstrated to be an effective strategy for the
preparation of electrocatalysts with both high activity and
[2d,9]
Introduction
excellent stability.
The as-developed bunched Pt-Ni alloy
nanocages/C electrocatalysts exhibited a superior activity and
durability toward oxygen reduction reaction, even in fuel cell
Breakthrough in fuel cell technologies requires electro-
catalysts with both high activity and stability, as well as low
cost. Nevertheless, the state-of-the-art platinum (Pt)-based
electrocatalysts still cannot meet these requirements, due to
the sluggish kinetics, surface CO poisoning, and catalyst lost
caused by aggregation and Ostwald ripening during the
[9]
tests. Moreover, it is believed that the electrocatalytic
stability of the Pt-based electrocatalysts can be further
improved by reasonable design of the support materials, such
as via the construction of a porous support structure to
facilitate mass transfer and accelerate the intermediates
[
1]
[
*] Dr. J. Yang, Y. Y. Zheng, Y. Li, Dr. A. A. Dubale, Dr. Y. J. Sun,
Prof. Y. Z. Meng, Prof. W. Liu
Dr. H. Wan, Prof. J. Rossmeisl
Center for High Entropy Alloy Catalysis (CHEAC), Department of
Chemistry, University of Copenhagen
The Key Laboratory of Low-Carbon Chemistry & Energy Conservation
of Guangdong Province, Key Laboratory for Polymeric Composite and
Functional Materials of Ministry of Education, State Key Laboratory
of Optoelectronic Materials and Technologies, School of Materials
Science and Engineering, Sun Yat-sen University
Guangzhou 510275 (P. R. China)
Universitetsparken 5, 2100 Copenhagen (Denmark)
Dr. H. Y. Qi
Center for Advancing Electronics Dresden (cfaed) and Faculty of
Chemistry and Food Chemistry, Technical University of Dresden
0
1069 Dresden (Germany)
E-mail: liuw226@mail.sysu.edu.cn
and
Dr. H. L. Wang, L. He, Prof. Z. K. Zheng
Central Facility of Electron Microscopy, Electron Microscopy Group of
Materials Science, Universität Ulm
89081 Ulm (Germany)
Key Laboratory for Polymeric Composite and Functional Materials of
Ministry of Education, School of Chemistry, and State Key Laboratory
of Optoelectronic Materials and Technologies, Sun Yat-sen University
Guangzhou 510275 (P. R. China)
Y. T. Liu, Dr. D. L. Peng
School of Chemistry, South China Normal University
Guangzhou 510006 (China)
Dr. R. Hübner
Helmholtz-Zentrum Dresden—Rossendorf, Institute of Ion Beam
Physics and Materials Research
Bautzner Landstrasse 400, 01328 Dresden (Germany)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202015679.
Dr. J. W. Zhang
State Key Laboratory of Catalysis, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences (CAS)
Dalian 116023 (P. R. China)
9
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[
10]
removal, heteroatom doping of the support materials to
reaction (MOR), the PtNiNF-NGA shows both outstanding
[11]
anchor Pt-based electrocatalysts, and increasing the degree
of graphitization of the support to improve corrosion
electrocatalytic activity and prominent durability in acid
environment (0.1 M HClO solution with 1 M CH OH). The
4
3
[
4b]
resistance. Therefore, it is of great promise to simultane-
ously achieve long-term stability, high activity, and low cost by
constructing highly open and stabilized Pt-based alloy elec-
trocatalysts together with the optimization of the support
material. However, this remains a challenge and has been
rarely reported.
Pt alloy nanoframes (NFs) have attracted enormous
attention due to their prominent electrocatalytic performance
endowed by their large surface area per volume, highly open
porous architecture that consists of corners and edges, and
mass activity of the PtNiNF-NGA toward MOR is as high as
À1
1647 mAmgPt , which is 5.8 times higher than that of
À1
commercial Pt/C (285 mAmgPt ). Notably, the PtNiNF-
NGA exhibits a significantly enhanced stability toward
MOR, and the high activity can be well maintained, even
when extending the test time to 5 h or after 2200-cycle cyclic
voltammetry scanning. This work provides a new perspective
for the preparation of high-performance and cost-effective
electrocatalysts.
[2e, 12]
tunable d-band energy level.
However, the most severe
shortcoming of NF electrocatalysts is their insufficient Results and Discussion
[
13]
stability.
On one hand, the NF that consists of thin
interconnected branches often suffers from a poor intrinsic
structural stability, due to a remarkably high surface energy of
corner atoms and obvious fragility of the ultrathin edges.
As shown in Figure 1, the PtNiNF-NGA has a hierarchical
and highly porous 3D structure constructed by interconnected
and crumpled graphene nanosheets with the PtNiNFs uni-
formly deposited on them, and the introduction of PtNiNFs
shows no obvious influence on the highly porous 3D structure
of the NGA (Supporting Information, Figure S1), while the
PtNiNFs without any support tend to aggregate (Figure S2).
The PtNiNFs have an octahedral nanoframe morphology,
with an edge thickness of ca. 2.5 nm, an edge length of ca.
8.8 nm, and a holey inside structure. The PtNiNFs have
a record-small total size with a narrow size distribution (8.8 Æ
[12b,13]
On the other hand, leaching of certain components (especially
the non-precious metal), and agglomeration or detachment in
a harsh electrochemical condition result in an instability
[
12f]
problem during catalysis. Recently, attention has been paid
to solve the instability problem in the field of NF electro-
catalysts, and the introduction of structural support and
construction of corner-fortified structures have been propo-
[
12d,f,14]
sed.
needed, and the corrosion of support materials (usually
However, sophisticated atomic engineering is often
[12a,e,17]
1.0 nm; Figure S3).
Due to the high edge-thickness-to-
[13,15]
commercial carbon black) is urge to be resolved.
Except
length ratio and the very small total size, the superposition of
the octahedral edges observed from different viewing direc-
tions in projection images leads to different appearances of
the PtNiNFs on the GA network, as indicated by the tilted
HAADF-STEM micrographs (Figure S4). The element dis-
for the stability enhancement of the NF corners, that of the
NF edges is rarely reported. A step-forward of Pt-based NFs
to real commercial use requires further great efforts on
enhancing both their activity and especially their intrinsic
structural stability and thus the long-term electrocatalytic
stability.
Herein, we present an efficient strategy (tert-butanol-
assisted PtNi alloy NFs/N-doped graphene aerogel formation
strategy) to simultaneously realize exceptional electrocata-
lytic stability and activity of PtNiNFs in acid electrolyte by an
ensemble of several features: a high-alloying-degree-modu-
lated electronic state, Pt-rich corners, and thin edge sizes of
the PtNiNFs, the anchoring effect of the N-doped graphene
aerogel (NGA) to the PtNiNFs, and the highly exposed
hierarchical porous structure of the resulting PtNiNF-NGA.
On one hand, the record-small thin edge size, the modulated
electronic structure with compressive strain and optimized d-
band center, and the highly open structure of the highly
alloyed PtNiNF contribute to the high electrocatalytic activity
and CO anti-poisoning property. Meanwhile, the high alloying
degree of the edges and the Pt-rich corners improve the
intrinsic structure stability of the PtNiNFs, slow down the Ni
leaching during the electrocatalysis process, and protect the
vertices of the NF and significantly fortify the NF structure.
On the other hand, the interconnected and highly porous
NGA, which is a corrosion-resistant support, enhances the
entire mass transfer, anchors the NFs, and avoids the
agglomeration or detachment of the NFs in a harsh electro-
Figure 1. a) and b) SEM images of the PtNiNF-NGA at different
magnifications; c) HAADF-STEM micrograph of the PtNiNF-NGA with
a PtNiNF enlarged in the lower right inset; d) HAADF-STEM image of
the individual PtNiNF on NGA and corresponding EDX-based element
maps (Pt: red; Ni: green); e) TEM image; f and g) HRTEM images of
an individual PtNiNF. The inset of (f) shows the corresponding FFT
pattern.
[16]
catalytic condition, further improving electrocatalytic sta-
bility and activity. When applied for methanol oxidation
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tribution of Pt and Ni in the PtNiNF was analyzed by EDX-
based spectrum imaging analysis in STEM mode (Figures 1d
and S5). The results show the edges of the PtNiNF with
uniformly dispersed Pt and Ni and the corners with a Pt-
enriched surface. The atom ratio of Pt and Ni obtained from
inductively coupled plasma optical emission spectroscopy
observed in the PtNiNF-NGA except for the vanished Ni-rich
facets (Figures S4 and S5). The ICP-OES measurements show
that the atom ratio of Pt and Ni in the PtNiNC-NGA is
44.4:55.6, while that in the PtNiNF-NGA increases to
52.4:47.6. The above results demonstrate the leaching of Ni
from the PtNiNC-NGA in tert-butanol. This was further
confirmed by UV/Vis monitoring of the tert-butanol super-
natant collected after thermal treatment of the PtNiNC-
(
5
ICP-OES) and SEM-EDX (Figure S6) are 52.4:47.6 and
6:44, respectively. A HRTEM image of a PtNiNF and its
[18]
corresponding FFT are given in Figures 1 f and g. The
diffractogram can be described by a [001] zone axis pattern
of a face-centered cubic structure with lattice spacings of
NGA, where oxidized Ni can be detected (Figure S12). The
loading of PtNiNFs on the PtNiNF-NGAs can be adjusted by
controlling the concentration of the metal precursors in the
mixed solution, where the RGO was immersed into. Besides
the PtNiNF-NGA discussed above with 8.0 wt% Pt, PtNiNF-
NGAs with 5.2 wt% and 19.4 wt% Pt loadings can be also
easily obtained (Figure S13). Further systematic investiga-
tions indicate that the initial ratio between the Ni precursor
and Pt precursor has great influence on the final PtNi
structure. The PtNiNF-NGA was prepared using a Ni/Pt
precursor ratio of 1.0:1.3. When the Ni/Pt precursor ratio was
decreased to 1.0:4.0, a Pt Ni -NGA with solid PtNi nano-
0
.187 nm and 0.131 nm, which are ascribed to the (200) and
(
220) planes, respectively. These lattice spacings are a little
smaller than those of pure Pt, indicating the presence of
compressive strain due to the introduction of Ni into the
alloyed structure. ICP-OES characterization shows that the
loading of Pt in the PtNiNF-NGA is 8.0 wt%. The short form
of PtNiNF-NGA used in the following discussions without
specifying the loading amount of Pt means this PtNiNF-NGA
(
8.0 wt% Pt).
2.1
1
The preparation process of the PtNiNF-NGA is illustrated
crystals was obtained. When the Ni/Pt precursor ratio was
increased to 4.0:1.0, a Pt Ni -NGA with highly open
in Scheme 1, and the evolution of the PtNiNF-NGA was
monitored by various techniques. The RGO hydrogel was
prepared by using ascorbic acid to chemically reduce the GO
without any disturbing. Then, the solvent exchange process
was introduced to get the RGO gel filled with DMF. After the
solvothermal treatment of the precursor-containing DMF
solution in the presence of the RGO gel, nitrogen was doped
onto the graphene network, and solid octahedral PtNiNCs
were in situ deposited on the N-doped graphene network, as
seen from the TEM, HAADF-STEM, and tilted HAADF-
STEM images (Figures S7 and S8). Then, the obtained
composite gel was washed thoroughly by water and ethanol
for several times. The PtNiNCs within the composite gel kept
their solid morphology and showed little change during the
washing process (Figure S9). From EDX-based spectrum
imaging analysis of an individual PtNiNC, it can be seen that
the Pt and Ni elements are uniformly distributed along the
edges of the octahedron, and the corners are Pt-rich, while the
facets and interior of the octahedron are Ni-rich (Figures S10
and S11). After further thoroughly washed by tert-butanol,
the composite gel was kept in tert-butanol at 708C for 24 h,
and the PtNiNF-NGA was finally obtained followed by
washing and freeze-drying. Edges and corners with similar
elemental distribution to that for the PtNiNC-NGA are
1
2.6
dendrite-like PtNi nanoframes was prepared (Figure S14).
The specific surface areas and porosities of the NGA,
PtNiNC-NGA, and PtNiNF-NGA were determined from N2
physisorption isotherms. The pore size distributions of the
NGA and PtNiNC-NGA, analyzed by density functional
theory (DFT), are nearly the same, indicating that there are
no pore contributions from the solid PtNiNCs (Figures 2a and
S15). Meanwhile, the specific surface area estimated from
a Brunauer-Emmett-Teller (BET) plot decreases from ca.
2
À1
2
À1
336.1 m g for the NGA to ca. 300.8 m g for the PtNiNC-
NGA after the introduction of the PtNiNCs (Table S1). As for
the PtNiNF-NGA, it shows an increased number of meso-
pores (ca. 3.8 nm) and much larger BET surface area (ca.
2
À1
352.0 m g ) (Figure 2a), which further proves the holey
structure of the PtNiNFs and is consistent with the HADDF-
Figure 2. a) Pore size distribution of GA, PtNiNC-NGA, and PtNiNF-
NGA; b) XRD patterns of PtNiNC-NGA and PtNiNF-NGA; c) Pt 4f XPS
spectra of commercial Pt/C, PtNiNC-NGA, and PtNiNF-NGA; d) Ni 2p
XPS spectra of Ni foam, PtNiNC-NGA, and PtNiNF-NGA.
Scheme 1. Illustration of the preparation process of the PtNiNF-NGA.
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STEM and ICP-OES results. In addition, the content of NiO
calculated from the valence band measured by XPS. The d-
and Ni(OH) on the surface of the PtNiNF-NGA analyzed by
XPS (Figure 2d) are significantly lower than those on the
PtNiNC-NGA, demonstrating the diminished NiO and Ni-
band center is closely correlated to surface adsorption of
intermediates. As seen in Figure S18, the d-band center of the
PtNiNF-NGA is down-shifted, which is attributed to the
electron transfer from Ni to Pt, as indicated by the XPS
results, and can weaken the CO bonding on the Pt surface by
donating more electrons to the PtÀCO antibonding orbital,
2
(
OH) surface in the Ni leaching process. Based on the above
2
experimental results, and the fact that Ni is easy to be
oxidized by dissolved O2 under ambient condition, as
[
12a]
[23]
confirmed by the XPS results and literatures,
and tert-
thereby increasing the catalytic performance. As for the
2
+ [19]
butanol can form soluble complexes with Ni , we propose
that in the tert-butanol-assisted structure reconfiguration
strategy, the synergy of the oxidation effect from the dissolved
PtNiNC-NGA, the d-band center is further down-shifted,
which can be ascribed to the Ni-rich surface in the facets and
interior of the PtNiNCs (Figures S10 and S11).
O to Ni and the complexation effect from the tert-butanol to
oxidized Ni drives the leaching of Ni from the solid PtNiNC-
NGA to form the holey PtNiNF-NGA.
Furthermore, the X-ray absorption fine structure (XAFS)
analyses at the Ni K-edge and the Pt L3-edge for the PtNiNC-
NGA and PtNiNF-NGA, and the corresponding Ni foil, NiO
2
As shown from the XRD characterizations in Figure 2b,
both PtNiNCs and PtNiNFs exhibit a face-centered cubic
powder, Pt foil, and PtO powder references were conducted
2
to examine the valence state and coordination environment of
the Ni and Pt atoms. The oxidation state of the metal in
a statistical nature can be analyzed from the XANES region
(
fcc) crystal structure. The four specific peaks corresponding
to the (111), (200), (220), and (311) lattice planes (2q > 358)
are located between those for Pt (PDF#04-0802) and Ni
[24]
based on the edge position and white line intensity. The
results demonstrate that the platinum in both PtNiNC-NGA
and PtNiNF-NGA bimetallic alloys is statistically metallic
platinum, while the nickel is obviously in oxidation state
(Figures 3a and b). The oxidation state of Ni can be ascribed
to the synergy effect of electron transfer from Ni to Pt and
surface oxidation of Ni. EXAFS was utilized to investigate the
local atomic structure and to acquire structural parameters,
such as the coordination numbers (CNs), bond lengths, and
alloying degree in a bimetallic nanoparticle. As shown from
the radial distance space spectra in Figures 3c and S19, the
approaching scattering path of PtÀNi (NiÀPt) bonds signal in
(
PDF#04-0850), indicating lattice contraction of Pt and the
[12a,20]
formation of a PtNi alloy in the PtNiNF-NGA.
This is in
accordance with the HRTEM results. When compared with
the PtNiNC-NGA, the PtNiNF-NGA shows positively shifted
peaks, indicating a higher compressive strain and alloying
degree in the PtNiNF-NGA, which may be due to an atom
rearrangement during the tert-butanol-assisted Ni leaching
[
7c]
process.
The composition and the valence state of the
elements on the surface of the PtNiNF-NGA, the PtNiNC-
NGA, and commercial Pt/C were characterized by XPS
(
4
(
Figures 2c and d). As for the PtNiNF-NGA, the peaks of Pt
f5/2 and Pt 4f7/2 at 74.5 and 71.2 eVare assigned to metallic Pt
54.4%), and the peaks at 75.9 eV and 72.5 eV correspond to
the Pt L3-edge and Ni K-edge in both PtNiNC-NGA and
PtNiNF-NGA indicate their bimetallic alloying structure, and
the PtNiNF-NGA exhibits more merged scattering path of
PtÀPt (NiÀNi) and PtÀNi (NiÀPt) bonds, indicating a higher
2
+
Pt (45.6%). Notably, the Pt 4f binding energies of both the
PtNiNC-NGA and the PtNiNF-NGA are down-shifted com-
pared to those of commercial Pt/C, which indicates the change
in the electron density of states of the d-band, and may
weaken the adsorption of reaction intermediates on the
electrocatalyst surface and in turn enhance their electro-
alloying degree. Moreover, the wavelet transformation of c(k)
was conducted to directly demonstrate the alloying degrees in
the PtNiNC-NGA and PtNiNF-NGA as compared with the
corresponding Ni and Pt foil references (Figures 3d–g). For
the Pt L3-edge wavelet transform of the c(k) spectra of
PtNiNC-NGA and PtNiNF-NGA (Figures 3d and e), only
[21]
catalytic properties. Besides, the ratio of Pt to Ni on the
surface of the PtNiNF-NGA (70:30) obtained from XPS
analysis is higher than that in PtNiNC-NGA (51:49), further
demonstrating the Ni leaching during the structure evolution
from PtNiNC-NGA to PtNiNF-NGA. The ratios of Pt to Ni in
both the PtNiNC-NGA and PtNiNF-NGA obtained from
XPS are higher than those from ICP-OES, which can be
ascribed to the Pt-rich corners in the PtNiNC and PtNiNF, as
shown from the EDX line scan profiles. (Figures S5 and S11).
The Ni 2p XPS spectra of the PtNiNC-NGA and PtNiNF-
NGA clearly show an increased binding energy when
compared with Ni foam, further proving the electron transfer
from Ni to Pt (Figure 2d).
Moreover, as shown in Figures S16 and S17, and Table S2,
there exist nitrogen sites (ca. 1.0 at% for N) and residual
oxygen-containing groups (ca. 8.0 at% for O) on the NGA,
which can help binding the PtNiNFs well to prevent the
aggregation of them on the graphene network and endow an
enhanced catalytic performance during the electrochemical
Figure 3. Normalized X-ray absorption near-edge structure (XANES)
c(E) spectra of PtNiNC-NGA and PtNiNF-NGA: a) Ni K-edge with the
Ni foil and NiO as references; b) Pt L3-edge with the Pt foil and PtO2
as references; c) radial distance c(R) space spectra of PtNiNC-NGA
and PtNiNF-NGA; 3D contour wavelet transform extended X-ray
absorption fine structure (WTEXAFS) map with 2D projection of
[
22]
process. The d-band center shifts of the PtNiNC-NGA and
the PtNiNF-NGA vs. Pt in commercial Pt/C were further
PtNiNC-NGA (d), PtNiNF-NGA (e), Pt foil (f), and PtO (g).
2
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one merged scattering path signal of PtÀPt and PtÀNi bonds
located at [c(k), c(R)] of [6.71, 2.38] and [6.50, 2.28] is
observed, respectively. Compared with that of the PtÀPt bond
in the Pt foil ([7.4, 2.82]) (Figure 3 f), both the c(k) and c(R)
of PtNiNC-NGA and PtNiNF-NGA move toward lower
values, which can be attributed to the formation of hetero-
geneous metallic bond (Pt-Ni), and the PtNiNF-NGA shows
the lowest values, implying a larger number of PtÀNi bonds
and thus a high alloying degree, as well as large compressive
strain. No detectable characteristic scattering path signal of
the PtÀO bond at [5.6, 1.94] was observed in both PtNiNC-
NGA and PtNiNF-NGA, again clearly indicating the statisti-
cally metallic platinum state in them (Figure 3g).
Quantitative c(R) space spectra fitting was performed to
investigate the local atomic structure and obtain the CNs of
Ni or Pt in the PtNiNC-NGA and PtNiNF-NGA bimetallic
alloys (Figure S20 and Table S3). Both the first and the second
coordination shell were taken into consideration in Ni K-edge
and Pt L3-edge c(R) space spectra fitting for the PtNiNC-
NGA and PtNiNF-NGA. The combination of c(R) and c(k)
space spectra fitting is the intuitive way to distinguish such
merged scattering path signal contributions from each
Figure 4. a) CV curves of the commercial Pt/C, PtNiNC-NGA, and
PtNiNF-NGA in 0.1 M HClO and 1.0 M methanol solution. Scan rate:
4
À1
50 mVs . b) Mass activities and specific activities of these electro-
catalysts. c) Chronoamperometric curves of the commercial Pt/C,
PtNiNC-NGA, and PtNiNF-NGA at 0.56 V (vs. Ag/AgCl) for 4000 s.
d) Long-term chronoamperometric curve of PtNiNF-NGA at 0.56 V (vs.
Ag/AgCl) for 5 h.
[
25]
other. In the Pt L3-edge c(R) space spectra, the PtNiNC-
NGA displays PtÀPt bonding with a CN approaching 7.0 and
PtÀNi bonding with a CN approaching 2.5, while the PtNiNF-
NGA displays PtÀPt bonding with a decreased CN approach-
ing 5.0 and PtÀNi bonding with an increased CN approaching
The electrocatalytic performances of the PtNiNF-NGA,
the PtNiNC-NGA, and commercial Pt/C toward MOR were
comparatively measured in N -saturated 0.1 M HClO con-
2
4
taining 1 M methanol (Figure 4). The mass activity of the
À1
5
.0. The good fitting results of c(R) and c(k) space spectra
PtNiNF-NGA toward MOR is as high as 1647 mAmgPt
,
with reasonable R-factor and the obtained fitting parameters
indicate quantitatively that the PtNiNC-NGA possesses more
phase separation and preserves more characteristic homoge-
neous metallic bonding with higher corresponding NiÀNi and
PtÀPt CNs (Table S3), while the PtNiNF-NGA exhibits more
which is 1.7 and 5.8 times higher than that of PtNiNC-NGA
À1
À1
(949 mAmgPt ) and commercial Pt/C (285 mAmgPt ), re-
spectively (Figure 4b and Table S4). However, there is no
detectable signal for the NGA toward MOR, indicating that
pure NGA is inactive toward MOR and the activity of the
PtNiNF-NGA is mainly contributed from the PtNiNFs on the
3D NGA network (Figure S21). The specific activity based on
the electrochemically active surface area (ECSA) of the
completely fused state with a higher CN of heterogeneous
metallic PtÀNi (NiÀPt) bonding. Furthermore, the alloying
degree (J) can be calculated quantitatively as in Equa-
[26]
À2
tion (1):
PtNiNF-NGA (3.8 mAcm ) is 1.4 and 7.8 times that of the
À2
PtNiNC-NGA (2.8 mAcm ) and commercial Pt/C
À2
(
0.49 mAcm ), respectively (Figures 4b and S22, and Ta-
JA ¼ Pobserved=Prandom 100 %
ð1Þ
ble S5). To the best of our knowledge, the mass and specific
activity of the PtNiNF-NGA are among the highest reported
values for the Pt-based electrocatalysts, especially the Pt-
nonprecious-metal bimetallic electrocatalysts, toward MOR
in acid environment (Table S4).
Then, chronoamperometric measurements of all the
electrocatalysts were conducted at 0.56 V (vs. Ag/AgCl) in
N -saturated 0.1 M HClO4 containing 1.0 M methanol to
2
where J stands for the alloying degree of atom A, Pobserved is
A
the parameter which can be calculated from the CN of AÀB
bonding and the total CN of atom A (Pobserved = N /ÆN ),
A-B
A-i
and Prandom is the parameter of atom-A-based perfect
bimetallic alloy under the ideal condition, which can be
calculated from the atom number (n) [Eq. (2)].
evaluate their stability (Figure 4c). During the entire time
course, the PtNiNF-NGA exhibits a much higher current
density than the PtNiNC-NGA and commercial Pt/C. After
4000 s, the current density of the PtNiNF-NGA still retains as
Prandom ¼ n =ðn þ n Þ 100 %
ð2Þ
B
A
B
The PtNiNC-NGA shows a lower JPt (48%) and JNi
À1
(
52%), which are both much lower than 100%, implying
high as 381.2 mAmgPt , which is 53 and 37 times higher than
À1
the inferior distribution of both Ni and Pt atoms in PtNiNCs.
In contrast, the PtNiNF-NGA shows significantly increased
J and J , which are 104% and 106%, respectively, indicating
that of the PtNiNC-NGA (7.2 mAmg ) and commercial Pt/
Pt
À1
C (10.3 mAmgPt ), respectively. When the stability test was
further extended to 5 h, the PtNiNF-NGA still shows a con-
Pt
Ni
À1
the improved atomic distribution of both Pt and Ni atoms, and
more heteroatomic interactions than homoatomic interac-
tions in the entire PtNiNF-NGA.
consistent with the XRD results.
siderable current density, which is as high as 155.8 mAmgPt
(Figure 4d), the PtNiNFs remain well dispersed on the NGA
network, and most of the PtNiNFs maintain their morphology
well (Figure S23). Furthermore, the activity of the PtNiNF-
[26]
This result is highly
9
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NGA retained as high as 86% after 2200 CV cycles, while that
of commercial Pt/C was only 38% under the same conditions
Figure S24). This long-term stability of the PtNiNF-NGA is
(
superior to Pt-based electrocatalysts reported recently under
acidic conditions (Table S4).
To dig out the reason for the greatly enhanced durability
of the PtNiNF-NGA as compared to that of the PtNiNC-
NGA and the Pt/C, we further monitored their morphology
and composition change after the durability test. It was found
that the PtNiNFs in the PtNiNF-NGA maintained their
morphology and dispersion well during the 4000-s stability
test (Figure S25). Notably, even after the 5 hꢀs durability test
(
Figure S23), the atom ratio of Pt to Ni for the PtNiNF-NGA
was retained at 69:31, and the morphology and dispersion
were still maintained well. Moreover, the PtNiNF-NGA kept
its crystalline structure well and showed negligible change in
size during the long-term stability test (Figure S26). In
contrast, for the PtNiNC-NGA, the morphology and size
changed, and more serious Ni leaching happened even during
the 4000-s stability test (Pt:Ni = 76:24); however, the dis-
persion of the PtNiNCs on the NGA was maintained. In the
case of the commercial Pt/C, there was an obvious aggrega-
tion and ripening of Pt nanoparticles (Figure S27), thus
demonstrating the important role of the NGA in anchoring
the PtNi catalyst to avoid particle mobility and aggregation.
XPS spectra were also used to monitor the evolution of the
element composition and electronic state of the PtNiNF-
NGA during the long-term stability test. The results indicate
that the surface leaching of Ni happened during this 5-hour
stability test, while the modulated electronic state was well
maintained (Figure S28). What is more important, in combi-
nation with the XAFS analyses, the results show that the
higher alloying degree of Pt and Ni in the PtNiNF-NGA can
greatly slow down the leaching of Ni from the PtNiNF-NGA,
which is most probably the main origin for the higher
durability.
Figure 5. Free energy diagram for the MOR on Pt (111) (purple line),
Pt Ni (111) (red line), and PtNi (111) (indigo line) with H (g), CO
7
2
2
2
(
g), and H O (g) as a reference.
2
[30]
in Figure 5 and in the literature, Pt is poisoned by strongly
adsorbed *CO. When it comes to bimetallic PtNi alloys with
different compositions, it is obvious that the presence of Ni,
on one hand, increases the barrier for forming the adsorbed
*CH OH intermediate (shown in Figure 5), on the other
2
hand, benefits MOR toward CO2 by weakening *CO
adsorption on the Pt atoms and providing adsorbed *OH as
[29]
the oxidant at a lower potential on the Ni atoms. On PtNi
(111) with lower alloying degree, the higher content of Ni
requires much higher potential for CH OH molecule de-
3
protonation to form *CH OH, which becomes the PDS
2
(0.64 V). In the case of Pt Ni (111) with higher alloying
7
2
degree, the potential to form *CH OH (0.52 eV) is better
2
than for PtNi (111), and notably, Pt Ni just requires around
7
2
The removal of CO from the surface is vital to increase the
0.61 V to conduct the CO oxidation. Both experiments
(similar CO stripping potential in Figure S29) and computa-
tional results (in Figure 5) illustrate that the potential for *CO
oxidation is similar on Pt Ni (111) and PtNi (111). However,
[
27]
stability of Pt-based electrocatalysts toward MOR. There-
fore, CO stripping voltammetry was carried out for commer-
cial Pt/C, PtNiNC-NGA, and PtNiNF-NGA (Figure S29).
Compared with commercial Pt/C (0.51 V vs. Ag/AgCl), the
PtNiNF-NGA and PtNiNC-NGA show surprisingly similar
and much better anti-CO poisoning capability, as indicated by
the significantly decreased oxidation peak potential toward
7
2
Pt Ni (111) with higher alloying degree can balance the
7
2
barriers for *CH OH formation and *CO oxidation in a better
2
way. Furthermore, despite a slightly stronger *OH adsorption,
a weaker *CO adsorption on Pt Ni (111) also chemically
7
2
[28]
CO at 0.45 V vs. Ag/AgCl. Since both the PtNiNCs and
PtNiNFs are anchored on the NGA, the contribution of the
NGA support anchoring effect to the stability is believed to
be similar, too.
promotes a higher driving force toward CO2 formation,
leading to a higher current density shown in Figure 4a.
The electrocatalytic performances of the unsupported
PtNiNFs, the Pt Ni -NGA, Pt Ni -NGA, PtNiNF-NGA
2.1
1
1
2.6
Density functional theory (DFT) predictions of possible
mechanistic pathways for the MOR were further conducted
to understand the high MOR performance of the PtNiNF-
NGA and the difference between the PtNiNC-NGA and the
PtNiNF-NGA (Figures 5, S30 and S31). The relative free
(5.2 wt% Pt), and PtNiNF-NGA (19.4 wt% Pt) toward
MOR were further investigated (Figure S21, S32, S33).
Compared with these samples, the PtNiNF-NGA (8 wt%
Pt) shows greatly enhanced electrocatalytic activity and
especially prominent durability. These results further confirm
the significant influences of catalyst-support interaction,
alloyed state, composition, and the PtNiNF loading amount
on the electrocatalytic performances.
energies of the most stable surface intermediates on Pt Ni2
7
(
111) and PtNi (111) (following the XPS results for the
PtNiNF-NGA and PtNiNC-NGA, respectively) and Pt (111)
were calculated. As for Pt (111), its potential-determining
step (PDS) is the oxidation of water to form adsorbed *OH
Based on the above results, the prominent activity and
greatly enhanced durability of the PtNiNF-NGA are pro-
posed to be ascribed to the following reasons: First, the newly
+
À
[29]
(
H O!*OH + H + e ), which requires 0.76 V. As shown
2
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9595

Angewandte
Research Articles
Chemie
developed tert-butanol-assisted NF formation and structure
reconfiguration strategy endows the PtNiNFs supported on
the NGA with a high alloying degree and Pt-rich corners. The
high alloying degree of Pt and Ni not only leads to an
optimized d-band center, compressive strain effect, and
further promoting the remarkable durability enhancement of
the PtNiNF-NGA. This work opens up new avenues for
designing electrocatalysts with both high activity and dura-
bility.
correspondingly lowered barrier for forming *CH OH and
2
promoted *CO oxidation, which greatly improves the cata- Acknowledgements
lystꢀs intrinsic catalytic activity and durability, but also slows
down the Ni-leaching in acidic condition, further significantly
enhancing the durability. The Pt-rich corners protect the
vertices of the NF and significantly fortify the NF structure.
Second, the appropriate ratio of Pt and Ni in the nanoframe
and a suitable PtNiNF loading amount on the NGA endow
the PtNiNF-NGA with superior electrocatalytic activity (Fig-
ure S32 and S33). Third, the hierarchically porous structure of
the PtNiNF-NGA consisting of an interconnected macro-
porous graphene network and highly exposed mesoporous
PtNi nanoframes facilitates the mass and electron transfer
and the exposure of the active sites. The faster electron
transfer in the PtNiNF-NGA is clearly demonstrated by the
lowest semi-circle and highest slope of the PtNiNF-NGA in
the electrochemical impedance spectroscopy (EIS) results in
Figure S34. Fourth, the NGA serves as a corrosion-resistive
support for anchoring the PtNiNFs, ensures the uniform
dispersion of the PtNiNFs, avoids serious aggregation,
prevents Ostwald ripening, and stabilizes the PtNiNFs during
the catalytic process, thus increasing the activity and stability
Financial supports from the National Natural Science Foun-
dation of China (51801238, 51803241, 51873236, 51803239,
and 21701168), the Natural Science Foundation of Guang-
dong Province (2018A030313458), the National Key Basic
Research Program of China (2020YFA0406101), the 100 Top
Talents Program-Sun Yat-sen University, Fundamental Re-
search Funds for the Central Universities (19lgpy09) and
Dalian high level talent innovation project (2019RQ063) are
gratefully acknowledged. Furthermore, the use of the HZDR
Ion Beam Center TEM facilities and the funding of TEM
Talos by the German Federal Ministry of Education of
Research (BMBF; grant No. 03SF0451) in the framework of
HEMCP are acknowledged. Besides, BL14W1 beamline of
Shanghai Synchrotron Radiation Facility (SSRF) and 1W1B
beamline of Beijing Synchrotron Radiation Facility (BSRF)
are gratefully acknowledged for providing beamtime. The
Center for High Entropy Alloys Catalysis is sponsored by the
Danish National Research Foundation centers of excellence,
Project DNRF149. The authors thank the XPS group from the
surface and structure analysis platform of the Instrumental
Analysis & Research Center, Sun Yat-sen University.
(
Figures S23, S25, and S27).
Conclusion
Conflict of interest
We have developed a facile tert-butanol-assisted structure
reconfiguration strategy to synthesize hierarchically porous
high-alloying-degree PtNi nanoframes supported on N-doped
graphene aerogel (PtNiNF-NGA) electrocatalysts with both
excellent activity and prominent durability toward methanol
oxidation in acidic condition. Systematic structural and
compositional characterizations unveil that the surfactant-
free solvothermal reaction and the mild in situ tert-butanol
leaching during the pretreatment before freeze drying play
the key roles in the evolution of the PtNi nanocrystals
supported on the N-doped graphene aerogel (PtNiNC-NGA)
into the PtNiNF-NGA with high alloying degree. The
PtNiNF-NGA shows a hierarchically porous structure con-
sisting of an interconnected porous graphene network and
highly exposed mesoporous PtNiNFs with record-small size
The authors declare no conflict of interest.
Keywords:
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