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Research Article
Single-Crystalline Mo-Nanowire-Mediated Directional Growth of
High-Index-Faceted MoNi Electrocatalyst for Ultralong-Term
Alkaline Hydrogen Evolution
Zhenpeng Liu, Guoxian Zhang, Jun Bu, Wenxiu Ma, Bin Yang, Hong Zhong,* Shuangming Li,
Tao Wang,* and Jian Zhang*
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ABSTRACT: As appealing alternatives to noble-metal-based electrocatalysts
for catalyzing hydrogen evolution reaction (HER) in alkali electrolyzers,
earth-abundant MoNi-based catalysts have attracted intensive attention.
Unfortunately, the exploration of MoNi-based electrocatalysts with superior
intrinsic activity and ultralong-term stability still remains a grand challenge.
Here, ultralong high-index faceted Mo@MoNi core−shell nanowires were
topochemically synthesized through the thermal reduction of Mo@NiMoO4
core−shell nanowires, where single-crystalline Mo support facilitates the
topological transformation of NiMoO4 into high-index faceted MoNi. When
the as-achieved Mo@MoNi core−shell nanowire film serve as a free-standing
cathode in alkaline solutions, it exhibit a remarkably decreased HER
overpotential of 18 mV at 10 mA cm−2 and a Tafel slope of ∼33 mV dec−1,
which are much lower than those for the state-of-the-art earth-abundant
electrocatalysts and even commercial Pt/C. Experimental and theoretical investigations reveal that the exposed high-index (331)
facets of MoNi can considerably reduce the energy barriers of initial water dissociation and subsequent hydrogen combination steps,
which synergistically accelerates the sluggish alkaline HER kinetics. Significantly, after a 70-day HER operation, the overpotential of
Mo@MoNi electrocatalysts at 10 mA cm−2 decreases by only 4 mV. Therefore, this work sheds a bright light on the rational design
of high-performance HER electrocatalysts and their practical utilization for alkaline electrolyzers.
KEYWORDS: MoNi catalysts, core−shell nanowires, high-index facet, electrocatalytic, hydrogen evolution reaction
mechanisms in the alkaline medium.14−16 Principally, the
alkaline HER kinetics involves two steps: (1) initial water
dissociation process (Volmer step: H2O + e− → H* + OH−)
and subsequent hydrogen combination process (Heyrovský
step: H2O + e− + H* → H2 + OH−, or Tafel step: H* + H* →
H2).15,17 Owing to the strong bonding property of O−H bonds
in water molecules, the prior water dissociation process leads
to a rate-determining energy barrier (ΔG(H2O)).9,18 Meanwhile, the sequent hydrogen combination process also
confronts an energy barrier for the formation and release of
hydrogen gas.19,20 Accordingly, synchronously reducing the
energy barriers of water dissociation and hydrogen combination steps is of paramount significance for speeding up the slow
HER kinetics in alkaline solutions.21 Lately, various NiMo-
1. INTRODUCTION
In terms of its high energy density and environmental
friendliness, hydrogen is an ideal renewable energy carrier
and is thus being pursued as a promising alternative to the
exhausting fossil fuels.1−3 Among various strategies for
hydrogen production, electrochemical hydrogen evolution
reaction (HER) by dissociating water molecules has been
commonly considered as the most economical and effective
pathway for the future hydrogen economy.4−7 Nevertheless,
owing to the sluggish kinetics of HER process, particularly in
alkaline electrolytes, it is of vital necessity to develop highactivity and stable electrocatalysts for intrinsically reducing the
HER overpotential.8,9 Up to now, precious-metal platinum
(Pt)-based materials still remain the benchmark HER catalysts
with high intrinsic activity in basic environments.10 However,
the scarcity, high cost, and poor long-term durability of Ptbased electrocatalysts seriously impede their large-scale
deployment in commercial alkaline water-splitting electrolyzers.11−13
For exploring high-performance HER catalysts, intensive
efforts have been devoted to probing the underlying HER
© 2020 American Chemical Society
Received: June 28, 2020
Accepted: July 15, 2020
Published: July 15, 2020
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based nanomaterials like NiMo nanoparticles,22 MoNi4/MoO2
cuboids,23 NiMoNx/C nanosheets,24 NiMoSx hollow prisms,25
etc. have been developed as alkaline HER electrocatalysts.
Notably, NiMo-based alloy catalysts show appealing HER
performance among the current electrocatalysts. Commonly,
there are two strategies for synthesizing NiMo-based alloy
nanomaterials: (1) electrodeposition that is utilized for
preparing an amorphous NiMo alloy layer,26 MoNi4 alloy
hollow nanorods array,27 MoNi4 alloy microsphere,28 etc.; (2)
thermal reduction of NiMo-based precursors for the synthesis
of NiMo nanoparticles,22 MoNi4/MoO2 cuboids,29 MoNi4/
MoO3−x nanowires or nanosheets,30,31 MoNi4 nanosheets,32
etc. However, for NiMo-based alloy electrocatalysts, the
relationship between their crystal structure and electrocatalytic
activity remains elusive. In addition, the water dissociation and
hydrogen combination properties of the current NiMo-based
electrocatalysts are still much inferior to those of the Pt-based
catalysts.33−35 Furthermore, the long-term operation stability
(more than 1 month) of NiMo-based catalysts is far
unsatisfactory (normally <5 days).30,31,36
In this work, we demonstrate a novel strategy for
synthesizing high-index faceted Mo@MoNi core−shell nanowires, where the (110) facets of Mo nanowires (length >50
μm; average diameter ∼186 nm) mediate the topochemical
transformation of surface NiMoO4 into high-index (331) facets
of MoNi shells (thickness 8−25 nm) during the thermal
reduction of Mo@NiMoO4 precursors. When the as-assembled
ultralong Mo@MoNi core−shell nanowires behaved as a freestanding cathode, it exhibited excellent HER activity with an
overpotential of ∼18 mV at a current density of 10 mA cm−2
and a considerably decreased Tafel slope of ∼33 mV dec−1 in a
1 M KOH aqueous solution, which were considerably lower
than those for the currently reported Pt-free electrocatalysts
and even commercial Pt/C. Remarkably, after a 70-day
stability test, the HER overpotential of the Mo@MoNi
cathode at 10 mA cm−2 augmented by only 4 mV. Combined
with experimental investigations and theoretical calculations,
the exposed high-index (311) facets of MoNi in the Mo@
MoNi core−shell nanowires were revealed to effectively reduce
the energy barriers of the initial water dissociation and sequent
hydrogen combination processes, which eventually accelerated
the sluggish HER kinetics.
under ultrasound. The obtained suspension was then assembled onto
a nylon membrane (pore size 0.44 μm; diameter 4 cm) using vacuum
filtration. After drying for 30 min at 60 °C, a free-standing Mo
nanowire film was easily peeled off. The diameter of Mo nanowires
was adjusted by the growth rate (7.2, 25.2 and 180 cm h−1) of NiAl9Mo in directional solidification.
Preparation of the Mo@MoNi Core−Shell Nanowire Film. A
piece of Mo nanowire film (diameter 4 cm, thickness 320 μm) was
immersed in 80 mL of an aqueous solution containing 0.02 M
Ni(NO3)2·6H2O and 0.005 M (NH4)6Mo7O14·4H2O in a 100 mL
Teflon autoclave. The autoclave was then heated for 6 h at 150 °C in
an oven. After washing and drying, the Mo@NiMoO4 core−shell
nanowire film was obtained. Later, the Mo@NiMoO4 core−shell
nanowire film was heated for 3 h at 600 °C under a H2/Ar (4/96, v/
v) atmosphere. Eventually, the Mo@MoNi core−shell nanowire film
was achieved. Higher reduction temperatures (650, 700, 800, 900, and
1000 °C) were carried out to obtain Mo@MoNi core−shell
nanowires with different proportions of high-index (331) facet. The
loading weight (3.07, 5.14, 6.23, and 7.12 mg cm−2) of the MoNi
catalyst on the Mo nanowires was adjusted by changing the
concentration of the Ni(NO3)2·6H2O (0.01, 0.02, 0.03, and 0.04
M) and (NH4)6Mo7O14·4H2O (0.0025, 0.005, 0.0075, and 0.01 M).
Pure MoNi catalyst was prepared through a similar method with the
Mo@MoNi nanowire film except adding the Mo film substrate.
Electrochemical Measurements. Electrochemical measurements were performed using an electrochemical workstation
(CHI760E) with a three-electrode system in a 1 M KOH aqueous
solution. A standard Hg/HgO electrode and a graphite rod were used
as the reference and counter electrodes, respectively. The Mo@MoNi
nanowires were assembled into a free-standing film and directly used
as the working electrode for electrochemical measurements. Linear
sweep voltammetry (LSV) curves were acquired at a potential sweep
rate of 2 mV s−1. Electrochemical impedance spectroscope (EIS) was
carried out from 105 to 0.01 Hz with an amplitude of 5 mV. Cyclic
voltammetry was applied to measure the electrochemical double layer
at various scan rates. All of the potentials were referred to a reversible
hydrogen electrode (RHE) based on the equation: ERHE = EHg/HgO +
0.099 + 0.059 V × pH. All polarization curves were corrected using iR
compensation. For comparison, commercial 20 wt % Pt/C was dropcast onto the Ni foam as a HER electrocatalyst (with the same loading
weight of MoNi at Mo film: 5.13 mg cm−2). The overall water
splitting was performed in a standard two-electrode system using the
Mo@MoNi core−shell nanowire film and NiFe-LDH on Ni foam as
the cathode and anode in a 1 M KOH aqueous solution, respectively.
Theoretical Calculations. All of the calculations were carried out
with the plane-wave-based density functional theory (DFT) method
as implemented in the Vienna ab initio simulation package
(VASP).38,39 The electron−ion interaction was described with the
projector-augmented wave (PAW) method,40,41 and the electron
exchange and correlation energy was solved with the Perdew−Burke−
Ernzerhof (PBE) exchange-correlation functional within the generalized gradient approximation.42 An energy cutoff of 400 eV was used.
The convergence criteria for energy and force are 10−5 eV and 0.02
eV/Å, respectively. To avoid the errors caused by the interactions
between the periodically repeated slabs, a vacuum layer of 12 Å was
set. To correctly describe the magnetic properties of Ni element, spin
polarizations were included for the computations of Ni and MoNi
systems. The transition state of H2O dissociation is identified with the
climbing image Nudged Elastic Band method.43 The Ni catalyst was
simulated with a (3 × 3) supercell of the FCC Ni(111) surface, and
the Mo catalyst was simulated with a (3 × 3) supercell of the BCC
metal Mo(110) surface, while the high-index facet of MoNi catalyst
was simulated with a (3 × 3) supercell of the FCC MoNi(210)
surface with step and terrace sites, respectively. A (3 × 3 × 1) k-points
mesh grid was used for all three surfaces.
2. EXPERIMENTAL SECTION
Preparation of Mo Nanowire Film. Mo nanowires were
synthesized through selective phase dissolution of directionally
solidified NiAl-9Mo (45.5 atom % Ni-45.5 atom % Al-9 atom %
Mo) eutectic alloys. First, NiAl-9Mo ingots were prepared from pure
Ni plate, Al rod, and Mo plate using vacuum induction melting
furnace. The diameter and length of the formed NiAl-9Mo ingot were
about 80 and 100 mm, respectively. A rod with a diameter of 10 mm
and a length of 100 mm was cut from the NiAl-9Mo ingot using a
wire electrodischarged machine and placed into an alumina crucible
10/11 mm in diameter (inside/outside diameter) and 120 mm in
length for directional solidification in an improved Bridgman vertical
vacuum furnace.37 The rod was heated by a graphite heater to 1900 ±
10 K for 1 h and thermally stabilized for 20 min. Then, it was
directionally solidified with a growth rate of 180 cm h−1 at a thermal
gradient of approximately 300 K cm−1. Second, the directionally
solidified NiAl-9Mo rod (6.58 g) was immersed into the mixture of
HCl (36 wt %), H2O2 (30 wt %), and H2O (10:10:80; v/v/v) for 24 h
at room temperature so that NiAl matrix phase was selectively etched
away. After washing in deionized water and drying at 60 °C, the Mo
nanowires were obtained. Finally, 200 mg of Mo nanowires were
dispersed into 50 mL of deionized (DI) water and treated for 30 min
Research Article
3. RESULTS AND DISCUSSION
Synthesis and Characterization of Mo@MoNi Core−
Shell Nanowire Film. The fabrication procedure of the Mo@
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Figure 1. Schematic illustrations for the fabrication of the Mo@MoNi core−shell nanowire film.
Figure 2. The SEM images of (a) Mo nanowires and (b) Mo@MoNi core−shell nanowires. (c) TEM, (d−f) HRTEM, and (g) HAADF-STEM
images of the Mo@MoNi core−shell nanowires. The corresponding elemental mapping images of (h) Mo, (i) Ni, and (j) Mo + Ni elements in
Mo@MoNi core−shell nanowires. (k) XRD pattern of the Mo@MoNi core−shell nanowires. The inset in (a) is the photograph of Mo nanowire
film. The insets in (b) and (e) are the SEM image and the related SAED pattern of the Mo@MoNi core−shell nanowires.
MoNi on Mo nanowires was changed from ∼3.07 to 7.12 mg
cm−2 through adjusting the loading amount of NiMoO4
precursors. Unless specially annotated, the loading weight of
MoNi on the ultralong Mo nanowires was 5.14 mg cm−2. By
comparison, Ni nanoparticles were grown on commercial
nickel foam (Figures S14−S16).
To investigate the nanostructure and morphology of the
resultant Mo@MoNi core−shell nanowires, the SEM and
TEM characterizations were carried out. As shown in Figure
2b, dense nanoparticles with sizes of 20−100 nm anchor on
the nanowires. The corresponding elemental mapping images
in Figure S17b,c clearly reveal the overlapped distributions of
Ni and Mo elements over the whole nanowires. In addition,
the morphologies of the Mo@MoNi core−shell nanowires
under higher reduction temperature were investigated using
the SEM. As shown in Figure S18, along with increased
reduction temperature, the nanowire structure was retained.
Unless specially noted, the sample used in this work was
achieved under 600 °C. Afterward, the TEM image in Figure
2c shows the core−shell structure. The diameter of core
nanowires was 140−230 nm, which was consistent with that
for the Mo nanowires. The thickness of the shells was about
MoNi core−shell nanowire film is schematically illustrated in
Figures S1 and 1. First, ultralong single-crystal Mo nanowires
with a diameter of ∼186 nm and the length of >50 μm were
prepared in gram scale through selectively etching the
directionally solidified NiAl-9Mo (DS−NiAl-Mo) eutectic
alloys (Figures S2−S7). Transmission electron microscopy
(TEM), the corresponding selected area electron diffraction
(SAED) pattern, and X-ray diffraction analyses (XRD)
revealed that the Mo nanowires were single crystalline, and
the (110) facets were predominantly exposed (Figures S8 and
S9). Then, above ultralong Mo nanowires were easily
assembled into a free-standing film through the vacuum
filtration and the weight density was 0.53 g cm−3 (inset in
Figures 2a and S10). Second, precursor NiMoO4 nanoparticles
with sizes of 30−200 nm were then grown on the Mo
nanowires via a hydrothermal process at 150 °C in an aqueous
solution containing Ni(NO3)2 and (NH4)6Mo7O14 (Figures
S11−S13a). Eventually, after a 3 h heating treatment at 600 °C
in H2/Ar (4:96, v/v) atmosphere, the NiMoO4 precursors on
the Mo nanowires were topochemically transformed into the
MoNi shells. The ultralong Mo@MoNi core−shell nanowires
were thus achieved (Figure S13b). The loading weight of the
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Figure 3. (a) XRD patterns of the Mo@MoNi core−shell nanowires under different reduction temperatures. (b) Proportions of the (331) facet in
MoNi shells under different reduction temperatures.
Figure 4. (a) Linear sweep voltammetry (LSV) curves and (b) the corresponding Tafel plots of the Mo@MoNi core−shell nanowires, Mo
nanowires, Ni nanoparticles, MoNi catalysts, and commercial Pt/C on a nickel foam in a 1 M KOH aqueous solution. (c) Comparison with the
state-of-the-art HER electrocatalysts under alkaline conditions. (d) Ultralong-term HER stability test of the Mo@MoNi core−shell nanowires at a
current density of 10 mA cm−2.
47.3°, respectively. Noticeably, in comparison with the very
weak signals of the (240) and (233) facets of MoNi, its highindex (331) facet gave a particularly intensified diffraction
peak, which was well in agreement with the HRTEM results.36
In principle, the formation and exposure of high-index facets
are closely related to the calcination temperature. For
comparison, the crystal information of the Mo@MoNi core−
shell nanowires under different reduction temperatures (500−
1000 °C) were examined (Figures 3a and S20). As revealed in
Figure S20, under 500 °C reduction, the sample consists of
Mo, MoO2 and MoNi phases, indicating that the NiMoO4 was
partially transformed into MoNi. Markedly, at 600 °C, the
proportion of the high-index (331) facet in MoNi reached
78.3%. However, along with increased temperature, the
intensity and proportion of the (331) facet of MoNi sharply
decreased, as shown in Figure 3b. Later, X-ray photoelectron
spectroscopy (XPS) analyses were managed to probe the
chemical compositions and bonding information of the Mo@
MoNi core−shell nanowires. The XPS survey curve in Figure
S21a confirms the existence of Mo and Ni elements in the
Mo@MoNi core−shell nanowires. As depicted in Figure S21b,
Mo 3d peaks appeared at 228.6 and 232.2 eV, which were
assigned to metallic Mo° (3d5/2) and metallic Mo° (3d3/2),
respectively.45 The high-resolution Ni 2p spectrum in Figure
8−25 nm. The related SAED analysis exposed that the
diffraction signals of the (331), (240), (352), and (265) facets
of MoNi and the (110) facet of Mo were well distinguished
(the inset in Figure 2d), indicating that the core−shell
nanowires consisted of Mo and MoNi. Subsequently, in highresolution TEM (HRTEM) images in Figure 2e,f, lattice
spaces of 0.220 and 0.209 nm were exclusively distinguished in
the core and shell, which were attributed to the (110) facet of
Mo and the high-index (331) facet of MoNi, respectively
(Figure S19). Obviously, similar lattice distances facilitate the
topochemical growth of the high-index (331) facet of the
MoNi on the (110) facet of Mo. The high-angle annular darkfield scanning transmission electron microscopy (HAADFSTEM) and the corresponding elemental mapping images
indicated that the distributions of Mo and Ni elements were
well overlapped over the core−shell nanowires, but the Mo
element mainly existed in the cores (Figure 2g−j).
To survey the crystal information of the Mo@MoNi core−
shell nanowires, the XRD analyses were conducted. As shown
in Figure 2k, the sharp diffraction peaks at 40.5, 58.6, 73.7, and
87.6° originated from the (110), (200), (211), and (220)
facets of Mo (JCPDS no. 42-1120), respectively.44 The
characteristic peaks of the (331), (240), and (233) facets of
the MoNi (JCPDS no. 48-1745) appeared at 43.2, 44.3, and
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Figure 5. (a) Polarization curves of the Mo@MoNi core−shell nanowires/NiFe-LDH couple and the Pt/C−Ir/C couple for overall water splitting.
(b) Long-term stability tests for the Mo@MoNi core−shell nanowires/NiFe-LDH couple and the Pt/C−Ir/C couple.
KOH aqueous solution at −0.1 V vs RHE with 5 mV
amplitude of AC potential from 10 kHz to 0.01 Hz (Figure
S27). The electron transfer resistance (Rct) of the Mo@MoNi
core−shell nanowires was ∼2.8 Ω, which was much lower than
those for Mo nanowires (126.7 Ω), Ni nanoparticles on Ni
foam (>800 Ω), and Mo@NiMoO4 (>800 Ω), indicating fast
electron transport during the HER. To clarify the influence of
the active surface area on the electrocatalytic HER activity, the
electrochemical double-layer capacitances (Cdl) of the Mo@
MoNi core−shell nanowires, Mo nanowires, and Ni nanoparticles were estimated based on cyclic voltammetry (CV)
scans at different scan rates.50 As demonstrated in Figure S28,
the Mo@MoNi core−shell nanowires had a considerably
higher Cdl (0.67 F) than those of Mo nanowires (0.075 F) and
Ni nanoparticles on Ni foam (0.007 F). The turnover
frequency (TOF) was derived for further understanding the
intrinsic catalytic activity of the Mo@MoNi core−shell
nanowires. As illustrated in Figure S29, the TOF value of the
Mo@MoNi core−shell nanowires was about 0.65 s−1 at the
HER overpotential of 100 mV, which was far higher than 0.047
s−1 for Mo nanowires and 0.006 s−1 for Ni nanoparticles on Ni
foam. Then, the recorded cathodic current density of the Mo@
MoNi core−shell nanowires was further normalized versus
their Brunauer−Emmett−Teller specific surface area (18.1 m2
g−1) (Figure S30). Clearly, the HER overpotential of the Mo@
MoNi core−shell nanowires at 0.1 mA cm−2 was 120 mV,
which was much lower than 165 mV for the Pt/C (Figure
S31). Besides, the HER performance in acid and neutral
solutions were also evaluated. As shown in Figure S32, in an
Ar-saturated 0.5 M H2SO4 and 1.0 M PBS aqueous solutions,
the Mo@MoNi core−shell nanowire cathode showed overpotentials of ∼108 mV and ∼43 mV at a current density of 10
mA cm−2, respectively.
The electrocatalytic durability is another crucial parameter
for assessing a HER catalyst. The electrochemical stability of
the Mo@MoNi core−shell nanowires was first evaluated using
the CV scans. After 2000 CV cycles between 0.075 and −0.625
V vs RHE at 50 mV s−1, the HER overpotential of the Mo@
MoNi core−shell nanowires increased by only 1 mV (Figure
S33). Significantly, the ultralong-term stability measurement of
the Mo@MoNi core−shell nanowires was operated in a 1 M
KOH aqueous solution at a current density of 10 mA cm−2.
Unprecedentedly, after a 70-day stability test, the HER
overpotential of the Mo@MoNi core−shell nanowires
augmented by only 4 mV, which unambiguously proved its
excellent durability and promising implementation for
commercial water-splitting electrolyzers (Figure 4d). Meanwhile, the experimentally detected hydrogen amount using gas
S21c shows two peaks at 852.6 and 870.3 eV, which originate
from metallic Ni° (2p2/3) and Ni° (2p1/3), respectively.30
HER Performance. The electrocatalytic HER performance
of all electrocatalysts was evaluated using a typical threeelectrode system in an Ar-saturated 1 M KOH aqueous
solution with a Hg/HgO electrode and a graphite rod as the
reference and counter electrode, respectively (Figure S22). All
potentials were referred to the reversible hydrogen electrode
(RHE) and the ohmic potential loss from the electrolyte had
been compensated (Figure S23). As disclosed in Figures 4a,
S24, and S25, the Ni nanoparticles on a nickel foam and the
Mo nanowires and pure MoNi on a nickel foam exhibited HER
overpotentials of ∼278, ∼157, and ∼62 mV, respectively, at a
current density of 10 mA cm−2. By contrast, the Mo@MoNi
core−shell nanowire cathode performed a near-zero onset
HER overpotential. Remarkably, the HER overpotential of the
Mo@MoNi core−shell nanowire cathode at 10 mA cm−2
drastically decreased to ∼18 mV, which was extremely lower
than ∼36 mV for commercial Pt/C and the values for
preciously reported alkaline HER electrocatalysts, e.g., MoNi
porous nanosheets (∼72 mV at 10 mA cm−2),35 MoNi
nanowires on nickel foam (∼30 mV at 10 mA cm−2),36 Ni/
NiO heterostructures (∼30 mV at 10 mA cm−2),46 Ni-doped
MoS2 nanosheets (∼98 mV at 10 mA cm−2),47 Co@Co3O4
nanosheets (∼95 mV at 10 mA cm−2),48 and Ni0.89Co0.11Se2
mesoporous nanosheets (∼85 mV at 10 mA cm−2)49 (Table
S1). Moreover, for delivering a large current density of 500 mA
cm−2, the Mo@MoNi nanowires required an overpotential of
as low as 268 mV.
Figure 4b displays the Tafel plots of various electrocatalysts,
which provide deep insights into their HER kinetics.
Apparently, the Mo@MoNi core−shell nanowires showed a
substantially declined Tafel slope of 33 mV dec−1, which was
much lower than 98 mV dec−1 for the Ni nanoparticles on a
nickel foam, 152 mV dec−1 for the Mo nanowires, 104 mV
dec−1 for pure MoNi, and even 34 mV dec−1 for commercial
Pt/C. Accordingly, the electrocatalytic HER kinetics on the
Mo@MoNi core−shell nanowires proceeded following a
Volmer−Tafel mechanism and the rate-deciding step was the
Tafel process, suggesting that the energy barrier of the initial
water dissociation process had been drastically reduced.21 In
addition, the exchange current density of the Mo@MoNi
core−shell nanowires was calculated to be ∼4.5 mA cm−2,
which was larger than those for Ni nanoparticles on Ni foam
(∼0.02 mA cm−2), Mo nanowires (∼1.2 mA cm−2), pure
MoNi (∼2.2 mA cm−2), and commercial Pt/C (∼3.3 mA
cm−2) (Figure S26). The electrochemical impedance spectroscopy (EIS) of the catalysts was measured in Ar-saturated 1 M
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Figure 6. (a) HER polarization curves of the Mo@MoNi core−shell nanowires with different proportions of the high-index (331) facet. (b)
Correlation between the proportion of the high-index (331) facet and the HER overpotential at 10 mA cm−2.
Figure 7. (a) Adsorption free energy diagrams of the initial Volmer process and (b) sequent hydrogen combination process on the MoNi surface
(red), Ni surface (green), and Mo surface (black). The initial state, the transition state (TS), and the final state are indicated in the diagrams with
the corresponding adsorption free energies on the catalyst surfaces. The red, white, blue, and gray spheres represent O, H, Ni, and Mo atoms,
respectively.
atmosphere (Mo@NiMoO4-Ar, Figure S41) and Mo nanowires presented large overpotentials of 204 and 157 mV at 10
mA cm−2, respectively (Figures S42 and 4a). Therefore, the
high HER activity of the Mo@MoNi core−shell nanowires
originated from outer MoNi shells rather than inner Mo cores.
The inner Mo nanowires mainly served as conductive
substrates. To profoundly demonstrate the correlation between
the facets and HER activity of MoNi shells, the Mo@MoNi
core−shell nanowires with different proportions of the MoNi
(331) facets were evaluated for the alkaline HER. As shown in
Figure 6a,b, when the proportion of the high-index (331) facet
in MoNi shells decreased from 78.3 to 20.5%, the HER
overpotential of the Mo@MoNi core−shell nanowires at 10
mA cm−2 drastically increased from 18 to 222 mV.
Unambiguously, these results evidence that the high-index
(331) facet of MoNi shells primarily contributes to the
outstanding HER performance of the Mo@MoNi core−shell
nanowires.
DFT Computations. To better understand the outperformance of MoNi catalyst in HER than its parental Mo
and Ni metals, the energy barriers of the initial Volmer step
(water dissociation) and the Tafel step (hydrogen combination) on three catalysts were calculated with the density
functional theory (DFT) method. The potential energy
diagram in Figure 7a shows that MoNi has a substantially
reduced energy barrier of 0.56 eV for the water dissociation
step, which is much lower than 0.91 eV for Ni and 0.65 eV for
Mo. Meanwhile, the energy barrier of the hydrogen
combination step on MoNi dramatically decreased to 0.49
eV compared with 0.57 and 0.73 eV on the Ni and Mo
catalysts, respectively (Figure 7b). Eventually, the reduced
chromatography was well equal to the theoretical hydrogen
production, suggesting a Faradic efficiency of ∼100% (Figure
S34). After the long-term stability measurement, the XRD,
SEM, and TEM characterizations (Figures S35−S37) were
conducted for investigating the structural durability of the
Mo@MoNi core−shell nanowires. Decidedly, the structural
morphology and chemical composition of the Mo@MoNi
core−shell nanowires showed no apparent variations.
Encouraged by its excellent HER performance, an alkaline
electrolyzer (Figure S38) was assembled using the Mo@MoNi
core−shell nanowire film as the cathode and a reported NiFelayered double hydroxide (NiFe-LDH) on Ni foam as the
anode51 (Figures S39 and S40) for catalyzing the overall water
splitting in a 1 M KOH aqueous solution. As revealed in Figure
5a, the Mo@MoNi core−shell nanowires/NiFe-LDH couple
needed a very small cell voltage of only ∼1.48 V to deliver a
current density of 10 mA cm−2, which was greatly lower than
that of a noble-metal-based Pt/C−Ir/C couple (1.60 V at 10
mA cm−2). The Mo@MoNi core−shell nanowires/NiFe-LDH
couple exhibited superior stability with a negligible overpotential augment over 20 hours at 10 mA cm−2. However, the
cell voltage of the Pt/C−Ir/C couple sharply increased with
increasing operation time (Figure 5b). Additionally, the
current density of the water-splitting electrolyzer using the
Mo@MoNi core−shell nanowires/NiFe-LDH couple reached
100 mA cm−2 at a low voltage of ∼1.73 V (Video S1).
HER Active Centers. For unveiling the high-performance
HER active centers in the Mo@MoNi core−shell nanowires, a
series of reference electrocatalysts were evaluated. First, in
contrast to ∼18 mV for the Mo@MoNi core−shell nanowires,
the Mo@NiMoO4 that was annealed for 3 h at 600 °C in an Ar
36264
https://dx.doi.org/10.1021/acsami.0c11716
ACS Appl. Mater. Interfaces 2020, 12, 36259−36267
ACS Applied Materials & Interfaces
www.acsami.org
energy barriers for both Volmer and Tafel processes on MoNi
synergistically accelerate its sluggish alkaline HER kinetics for
rapid hydrogen evolution.
Engineering, Northwestern Polytechnical University, Xi’ an
710129, P. R. China
Jun Bu − Key Laboratory of Special Functional and Smart
Polymer Materials of Ministry of Industry and Information
Technology, School of Chemistry and Chemical Engineering,
Northwestern Polytechnical University, Xi’ an 710129, P. R.
China
Wenxiu Ma − Key Laboratory of Special Functional and Smart
Polymer Materials of Ministry of Industry and Information
Technology, School of Chemistry and Chemical Engineering,
Northwestern Polytechnical University, Xi’ an 710129, P. R.
China
Bin Yang − State Key Laboratory of Solidification Processing,
Northwestern Polytechnical University, Xi’ an 710129, P. R.
China
Shuangming Li − State Key Laboratory of Solidification
Processing, Northwestern Polytechnical University, Xi’ an
710129, P. R. China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsami.0c11716
■
CONCLUSIONS
In summary, we have highlighted a unique strategy for
synthesizing an ultralong Mo@MoNi core−shell nanowire
featuring high-index (331) facets, which simultaneously reduce
the energy barriers of the water dissociation and the hydrogen
combination processes for accelerating alkaline HER kinetics.
When the achieved Mo@MoNi core−shell nanowires behaved
as a free-standing cathode in alkaline electrolytes, they showed
an excellent HER activity, which exceeds those of the
previously reported Pt-free electrocatalysts and even Pt/C
catalysts. Furthermore, the Mo@MoNi core−shell nanowires
exhibit an unprecedented 70-day HER stability. Therefore, this
work not only provides a high-activity and stable HER
electrocatalyst for replacing Pt/C for commercial alkali
water-splitting electrolyzers but also opens up a window for
designing high-performance electrocatalysts for other energy
conversion processes, e.g., water oxidation, oxygen reduction
reaction, and CO2 reduction.
■
Notes
The authors declare no competing financial interest.
■
ASSOCIATED CONTENT
sı Supporting Information
*
ACKNOWLEDGMENTS
This work was financially supported by the Fundamental
Research Funds for the Central Universities (Grant nos.
G2019KY05318 and 3102017jc01001) and the National
Natural Science Foundation of China (Nos. 51705407 and
51674201). This work is partially sponsored by the Specialized
Research Fund for the Doctoral Program of Higher Education
(2016M600785, 2016BSHEDZZ126, and 2018T111048). We
would like to thank Prof. Xuetao Gan and the Analytical &
Testing Center of Northwestern Polytechnical University for
the morphologic and spectroscopic characterizations. We
acknowledge Prof. Kaijie Chen and Jianwei Cao for the XRD
and BET measurements. We also appreciate Yumin Wang and
Long Yang for helpful discussions.
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsami.0c11716.
Materials, characterizations, XRD results, SEM images,
TEM images, XPS results, polarization curves, CV
curves, EIS, Faradic efficiency, BET results, and
comparison of HER performance of the state-of-the-art
MoNi-based and other electrocatalysts (PDF)
Video S1: Water splitting at a current density of 100 mA
cm−2 (MP4)
■
Research Article
AUTHOR INFORMATION
Corresponding Authors
Hong Zhong − State Key Laboratory of Solidification Processing,
Northwestern Polytechnical University, Xi’ an 710129, P. R.
China; Email: zhonghong123@nwpu.edu.cn
Tao Wang − SUNCAT Center for Interface Science and
Catalysis, Department of Chemical Engineering, Stanford
University, Stanford, California 94305, United States;
orcid.org/0000-0003-4451-2721; Email: twang86@
stanford.edu
Jian Zhang − Key Laboratory of Special Functional and Smart
Polymer Materials of Ministry of Industry and Information
Technology, School of Chemistry and Chemical Engineering,
Northwestern Polytechnical University, Xi’ an 710129, P. R.
China; orcid.org/0000-0002-0912-1197;
Email: jianzhang@nwpu.edu.cn
■
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