Unusual Effect of Trace Water on the Structure and Activity of NixCo1?x Electrocatalysts for the Methanol Oxidation Reaction

Article abstract of DOI:10.1002/cssc.201903108

Highly active Ni-based catalysts have attracted much attention but are still facing challenges owing to the immature synthetic method. Herein, polyhedral Ni_xCo_1?x alloy was prepared by a facile modified polyol method in which a trace amount of water could halve the particle size of the alloy. The Ni/Co ratios in Ni_xCo_1?x alloy strictly depended on the used amount of water owing to the different solubilities of the precursors. Among them, the Ni_0.6Co_0.4 nanoparticles obtained with 70 μL of deionized water exhibited the best performance in the methanol oxidation reaction with a peak current density of 116 mA cm^?2 in the presence of 1 m NaOH+0.5 m CH₃OH solution, which is higher than those of Ni_0.7Co_0.3 (80 mA cm^?2) and Ni_0.5Co_0.5 (33 mA cm^?2). The excellent performance of Ni_0.6Co_0.4 is attributed to the unique structure with appropriate Ni/Co ratio, which elongates the C?O bond in methanol and lowers the reaction free energy according to DFT calculations.

Full text of DOI:10.1002/cssc.201903108

Accepted Article
Title: Unusual effect of trace water on structure and activity of NixCo1-
x electrocatalysts for methanol oxidation reaction via the polyol
method
Authors: Ronghui Que, Maolin Li, Hong Yao, Xiuhua Wang, Fan Liao,
and Mingwang Shao
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.201903108
Link to VoR: http://dx.doi.org/10.1002/cssc.201903108
A Journal of
www.chemsuschem.org

ChemSusChem
10.1002/cssc.201903108
Unusual effect of trace water on structure and activity of
Ni Co electrocatalysts for methanol oxidation reaction via
x
1-x
the polyol method
1
1
2
1
3
3
Ronghui Que *, Maolin Li , Hong Yao , Xiuhua Wang *, Fan Liao , Mingwang Shao *
1
. Anhui Key Laboratory of Molecule-Based Materials, The Key Laboratory of
Functional Molecular Solids, Ministry of Education, College of Chemistry and
Materials Science, Anhui Normal University, Wuhu 241000, PR China.
. Library of Anhui Normal University, Wuhu 241000, PR China.
2
3
. Institute of Functional Nano & Soft Materials Laboratory (FUNSOM), Jiangsu Key
Laboratory for Carbon-Based Functional Materials & Devices, Soochow University,
Suzhou, 215123, PR China
ABSTRACT
Highly active Ni-based catalysts have attracted much attention yet still facing
challenges due to the immature synthesis method. Here the polyhedral Ni Co alloy
x
1-x
was prepared by a facile modified polyol method, in which trace amount of water
could halve the particle size of the alloy. The Ni/Co ratios in Ni Co alloy strictly
x
1-x
depended on the used amount of water owing to the different solubility of
precursors. Among them, the Ni Co nanoparticle obtained with 70 μL of
0
.6
0.4
deionized water exhibited the best performance in methanol oxidation reaction with
-2
the peak current density of 116 mA·cm in the presence of 1 M NaOH + 0.5 M
-
2
CH OH solution, larger than those of Ni Co (80 mA·cm ) and Ni Co (33
3
0.7
0.3
0.5
0.5
-
2
mA·cm ). The excellent performance of Ni Co is attributed to the unique
0
.6
0.4
structure with proper ratio of Ni/Co elongates the C-O bond in methanol and lowers
the reaction free energies according to DFT calculation.
Keywords: NiCo alloy; Methanol oxidation reaction (MOR); Electrocatalysis;
Polyhedral nanoparticles.
1
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ChemSusChem
10.1002/cssc.201903108
1. Introduction
Direct methanol fuel cells (DMFC) are a clean and efficient energy conversion
device to directly convert the chemical energy of methanol to electricity and have
1-5
promising potential to be used in portable power and clean energy vehicles
.
However, the performance and power output of DMFCs critically depend on the
kinetics of the methanol oxidation reaction (MOR), which is the most important and
rate limiting step in DMFCs. The challenge for the development of high performance
of DMFCs is thus the enhancing the kinetics and efficiency of MOR via the
development of active electrocatalysts. Substantial scientific efforts have been
devoted to develop Pt-based catalysts with the aim of decreasing Pt usage,
6
-8
increasing poisoning tolerance and improving the catalytic activities and stabilities .
Another important and efficient strategy is to find out low-cost non-noble metals as
alternative catalysts to the precious Pt-based electrocatalysts. Ni-based transition
metals are considered as the most promising candidates owing to their high
poisoning tolerance and low cost. To date, two main ways have been employed to
design and tune the Ni-based catalysts: (a) using catalyst carriers, such as carbon
9
-11
materials (carbon nanofiber and graphene)
morphology, which is related to the catalyst active surface
Various synthesis methods were proposed, such as microemulsion method
and (b) controlling the catalyst
1
2-14
.
1
5, 16
,
,
17, 18
19, 20
21-23
microwave radiation synthesis
, polyol reduction
, and electrodeposition
which are effective for the preparation of nickel-based catalysts with various
1
9, 24
15
morphologies and anisotropy such as Ni-Co nanoflowers
, nanoneedles
,
25
26
self-assembled Ni-Co nanoparticles , dendrite-like NiCu alloy , and Ni-Co-Sn alloy
2
7
nanoparticles . Cu/NiCu nanowires synthesized with microemulsion method
showed much larger activities than that of single nickel nanoparticles in oxidizing
2
8
methanol ; The Ni-MnO /C nanoparticles synthesized by R.M. Abdel Hameed using
x
microwave assisted method exhibited several times of catalytic performance than
2
9
that of ordinary Ni/C . A layered Ni-Cu alloy network of Ni0.75Cu0.25 with
microscopically interconnected branches and nanoscale dendritic tentacles showed a
2
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ChemSusChem
10.1002/cssc.201903108
-
2 30
peak current density of 84 mA·cm
. The peak current density of Co-Ni/RGO
-2
synthesized by the traditional polyol method reaches 22.5mA cm , when Co/Ni is 1:4
3
1
.
So far it appears that there are no reports on nickel-based catalysts synthesized by
a modified polyol method for the MOR.
More importantly, these aforementioned reports prepared the Ni-based catalysts
either using a complicated method or with larger sizes (>100 nm), which limit
catalytic activity due to their low specific surface area, and thus hamper the effective
utilization of catalysts. Therefore, the development of a new and simple synthetic
strategy to obtain small Ni/Co bimetallic nanoparticles is highly desired.
Herein, we reported the synthesis of uniform and nano-sized Ni Co bimetallic
x
1-x
nanoparticles using a modified polyol method. When trace amount of water was
added into conventional polyol system, the size of Ni/Co nanoparticles decreased
significantly from 100-150 nm to 50-70 nm. The best results are obtained on the
addition of 70 μL water, resulting in the formation of Ni Co with the best catalytic
0
.6
0.4
-2
activity for MOR and achieving current density of 116 mA·cm for methanol
oxidation at 0.65 V (vs. Ag/AgCl). Density functional theory (DFT) calculations also
proved that Ni Co alloy is energetically more favorable for MOR and thus can
0
.6
0.4
decrease the reaction free energy due to its proper Ni/Co ratio. These results indicate
that Ni Co alloy prepared by the simple and facile method is a promising catalyst
0.6
0.4
for DMFCs applications.
2. Experimental section
2
.1 Chemicals
All chemicals including nickel acetylacetonate (Ni(acac) ), cobalt acetylacetonate
2
(
Co(acac) ), polyvinylpyrrolidone (PVP), ethylene glycol N-N-dimethylformamide
2
(DMF), sodium hydroxide (NaOH) in this work were purchased from Shanghai Aladdin
Chemical Reagent's official website and were used without any further purification.
The used water is deionized water (DIW).
2
.2 Synthesis of Ni-Co alloy catalyst
3
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ChemSusChem
10.1002/cssc.201903108
Ni Co alloy was synthesized by the modified polyol method. First of all, 10 mg PVP,
x
1-x
0
.08 mmol Ni(acac) , and 0.027 mmol Co(acac) were added into a 20 mL autoclave .
2
2
Then 6.4 mL of N, N-dimethylformamide and 4 mL of ethylene glycol were slowly
injected into the autoclave. Finally, 60 μL of deionized water was injected into the
autoclave, then ultrasound for 10 min. The uniformly mixed solution was placed in an
oven and heated to and then maintained at 190℃ for 8 h. The sample is then cooled
to room temperature and washed with absolute ethanol for centrifuge 3-4 times. Dry
the sample in an oven at 70℃ for 10 h to obtain the final products. By changing the
content of deionized water, the synthesized Ni Co alloy has different Ni/Co ratios,
x
1-x
and 60 μL, 70 μL, and 80 μL of deionized water corresponded to Ni Co , Ni Co ,
0.7
0.3
0.6
0.4
Ni Co , respectively.
0
.5
0.5
2
.3 Characterization of Ni Co alloy
x
1-x
The crystal structure of Ni Co alloy was characterized by X-ray powder
x
1-x
diffractometer (XRD) with a Cu Kα radiation (λ = 0.15406 nm). The morphology was
characterized by scanning electron microscopy (SEM) on the Hitachi S8100 electronic
spectrum with energy dispersive spectroscopy (EDX) and transmission electron
microscopy (TEM) testing was carried out at JEM-2100 with an accelerating voltage
of 200 kV. The high resolution transmission electron spectrum (HRTEM) and the
selected area electron diffraction (SAED) were obtained from Tecnai F20 G2 FEG-TEM.
The element distribution was tested by TEM-mapping. X-ray photoelectron
spectroscopy (XPS) was operated by an ESCALAB MKII photoelectron spectrometer
with a laser source Mg target of 1253.6 eV.
The electrochemical tests were performed on an electrochemical workstation
(Chenhua Instrument CHI660E, Shanghai, China) with the conventional
2
three-electrode system. The Ni Co electrodes with a geometric area of 0.07 cm
x
1-x
were used as the working electrode; Pt foil and Ag/AgCl (saturated KCl) were used as
the counter and reference electrodes, respectively. All potential values were referred
to the reference electrode. The activity and stability for MOR were investigated by
-
1
cyclic voltammetry (CV) with a sweep rate of 50 mV·s and chronoamperometry (CA)
measurements, respectively. The test solutions used in this study were 1.0 M NaOH
4
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ChemSusChem
10.1002/cssc.201903108
solution with and without addition of methanol (0.5 M). All the experiments were
performed at 298 ± 2 K, and the test solutions were opened to the air.
Before performing a series of electrochemical tests, the glassy carbon electrode
(GCE) was polished with alumina powder. After that, 2 mg of the black sample was
dispersed in 0.4 mL of anhydrous ethanol and 0.6 mL of deionized water
accompanied with 20 μL of 5% Nafion mixed solution. After ultrasonicating for 3 min
to make it uniformly dispersed, 10 μL of ink was vertically dropped onto the glassy
-
2
carbon electrode by a pipette. The obtained Ni Co alloy loading is 0.286 mg·cm . In
x
1-x
this work, the three working electrodes with different samples (Ni Co , Ni Co ,
0
.7
0.3
0.6
0.4
Ni Co ) were prepared with the same operating procedures.
0
.5
0.5
2
.4 Computational Methods
In this work, we utilized Vienna Ab initio Simulation Package (VASP)
perfrom DFT within periodic models. Generalized gradient approximation (GGA)
3
2-35
to
3
6
3
7, 38
with projector augmented wave (PAW)
method was applied in calculations to
better simulate the electronic structures and the core-electron interaction. The
cutoff energy is set to be 500 eV, and the force threshold of optimization
−1
convergence is 0.05 eV Å . The optimized lattice constant of NiCo alloy is 3.48 Å,
which is consistent with experimental value of 3.52 Å with 1.1% error. The NiCo(111)
surface is further built with five-layer 2×2 slab, which is termed as Ni Co (the
0.5
0.5
atomic ratio of Ni:Co is 1:1). We also constructed other two models, i.e. Ni Co
0
.6
0.4
with that atomic ratio of 6:4 and Ni Co with that atomic ratio of 7:3. The Ni Co
0
.7
0.3
0.6
0.4
structure was built by substituting two Co atoms of Ni Co structure with two Ni
0
.5
0.5
atoms. Similarly, the Ni Co structure was built by substituting four Co atoms of
0
.7
0.3
Ni Co structure with four Ni atoms. The vacuum layer of all of the surfaces has
0
.5
0.5
lengths more than 12 Å to avoid spurious interaction. We used a 3 × 3 × 1 k-point grid
39
to sample surface Brillouin zone by using Monkhorst-Pack method . Vibrational
frequencies of all adsorbed and gas-phase species were also calculated.
The electrochemical process is studied by using the computational hydrogen
4
0
electrode (CHE) model . For an elementary step during methanol electro-oxidation,
e.g.
5
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ChemSusChem
10.1002/cssc.201903108
CH OH* -> CH O* + H+ + e-
3
2
The reaction Gibbs free energy is calculated as:
+
∆
G = GCH O* + GH + eU - GCH O*
2
2
+
Where mean Gibbs free energy of adsorbed CH O*, GH mean Gibbs free energy of
2
proton and we assume that it is half of Gibbs free energy of H , U is the electrode
2
potential. When this step is thermodynamically possible under external potentials,
one has the equilibrium as ∆G + eU = 0, and the corresponding U is the limiting
potential of this step. The Gibbs energy of each species is calculated by considering
4
1
42
entropic correction and solvation correction reported before .
3. Results and discussion
Fig. 1a sketches two different paths to obtain different sizes of polyhedral
structures. When the reaction mixture reacted according to the first path, the size of
the product is about 100-150 nm (Fig. S1a and S1b†). With adding a tiny amount of
water into the reaction system, the size of resultant nanoparticles reduced to 50-70
nm (Fig. S1c and S1d†), comparing with the condition of no water adding. A decrease
in particle size indicates the larger specific surface area, and more active sites.
Additionally, the ratios of Ni/Co strictly depend on the water contents. The SEM-EDX
shown in Fig. 1b indicates that the ratio of Ni and Co is 58:42, approximately
corresponding to the yield of Ni Co resulted from 70μL water. Ni Co and
0
.6
0.4
0.7
0.3
Ni Co were respectively produced from the used water of 60 μL and 80 μL with
0
.5
0.5
the actual ratio of Ni and Co of 72:28 and 46:54 (Fig. S2†). The C element in three
samples comes from the conductive tape, and the O element may stem from the
corresponding two metal oxides and the atmosphere.
The similar XRD characteristic peaks of those three bimetals (Fig. 1c) at about
4
4.23, 51.54 and 76.18 can be indexed as fcc crystal structures’ crystal planes (111),
(200) and (220), respectively (Ni PDF # 700989, Co PDF# 897093). When the water
content gradually decreases in the reaction system, the diffraction peak
corresponding to the (111) crystal plane shifts toward a high angle (Fig. 1d),
4
3-45
indicating that the crystal lattice gradually shrinks
. The reason why the lattice
6
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ChemSusChem
10.1002/cssc.201903108
constant (3.524 Å, 3.527 Å ,3.530 Å, respectively, Table S1†) becomes larger when
the nickel content increases is that the small radius nickel atom partially replaces the
cobalt atom with a large radius, which indicates that three bimetals initially reach the
4
6
degree of alloying .
Small and uniform nanoparticles prepared by the adding of 70 μL deionized water
can be seen in SEM and TEM images (Figs. 1h, S1 and S3†). A large number of edges
and corners are clearly observed on the particles (Fig. 1h), forming an irregular
polyhedral structure. There are observed aggregation of small nanoparticles after
adding 10 μL of water (Fig. S4a and S4e†). The morphologies of those nanoparticles
obtained with 60 and 80 μL of water (Fig. S5 and S6†) are almost the same as
Ni Co alloy. When the water content increases to 100 μL, the particle size (125 nm)
0
.6
0.4
of polyhedral becomes larger than that of Ni Co (Fig. S4d and S4h†). In short, we
0
.6
0.4
can see from Fig.S4 and S7† that the particle size becomes larger as the water
content increases, and the cobalt content in Ni Co1-x also increases as the water
x
content increases.
It is easy to see from the Fig. 1g,h that the crystal faces (111) of Ni Co alloy
0
.6
0.4
corresponds to a lattice spacing of 0.204 nm. What we should note is that the
interplanar spacing corresponding to the (111) crystal plane of Ni Co is larger
0
.6
0.4
푎
than that of Ni element by 0.005Å (according to the equation d = √ 2
, d is
2
2
ℎ +푙 +푘
the interplanar spacing, a is a lattice constant, and h, k, l are crystal plane indices)
which may be in accordance with the slightly shift in XRD pattern. The other two
alloys (Ni Co and Ni Co ) have similar results with the interplanar spacing
0
.7
0.3
0.5
0.5
decreasing by 0.01 Å and increasing by 0.02 Å, respectively (also according to the
푎
equation d = √ 2
2). The calculated lattice plane spacing is close to the actual
2
ℎ +푙 +푘
measurement result, which indirectly proves that the material is an alloy. As can be
seen in the TEM-mapping in Fig. 1e, the Ni element and the Co element are
uniformly dispersed within the region of the entire nanoparticle.
The XPS full spectra (Fig. S8 and S9†) of Ni Co , Ni Co and Ni Co alloys
0
.6
0.4
0.7
0.3
0.5
0.5
show the existence of Ni, Co, C and O elements, of which C is from conductive tape,
7
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ChemSusChem
10.1002/cssc.201903108
O element comes from the oxides or air. The molar ratios of the Ni/Co among three
alloys calculated by XPS are 2.3:1, 1.5:1, and 1:1, which are very close to the EDX
results. The binding energies at 852.6 eV and 869.7 eV can be respectively assigned
0
to Ni 2p3/2 and Ni 2p , which represent the chemical valence of Ni at these two
1/2
2
+
positions (Fig. 1h). The Ni peaks and satellite peaks are located at 855.7 eV, 873.5
eV and 861.5 eV, 879.8 eV, respectively, in good agreement with the reported
2
+
30
positions of the Ni binding energies . The observed peaks at 777.8 eV and 793.3
eV correspond to Co 2p3/2 and Co 2p , reflecting that the chemical valence of Co is
1/2
2
+
zero. The binding energies at 780.9 eV and 796.5 eV are assigned to Co and his
4
7
10
2+
2+
satellite peaks are at 785.5 eV
and 802.3 eV . These peaks of Ni and Co
indicate the existence of some oxides on the surface.
It should be especially noted that some additional information can be observed
from the XPS spectra and Table S2. It can be seen from the table that the oxide of
cobalt is much larger than that of nickel. Due to the difference in electronegativity
between nickel (1.91) and cobalt (1.88), cobalt will more easily lose electrons to form
CoO, and NiO is intend to get electrons to form elemental nickel. This phenomenon
of gain and loss of electrons causes the Co 2p peak to shift to a high binding energy,
2
8, 48
while the binding energy of Ni 2p shifts to a low field.
consistent with the XRD result.
. This outcome is mutually
Based on the above results, the reaction mechanism and the role of water can be
proposed as shown in Fig. 2. In the classical synthesis, the precursors of Ni(acac) and
2
Co(acac) are respectively reduced into metal elements by ethylene glycol, then the
2
bimetal rapidly nucleates. Since there are a lot of oxygen-containing functional group
(-O and –OH) in ethylene glycol and its produced aldehydes, the nucleus maybe
wrapped around and the existence of PVP (a surfactant) can reduce the surface
49
energy of the crystal face to a certain extent . Using ethylene glycol alone as a
2
+
2 +
solvent is difficult to precipitate Ni and Co , because it is not easy for these ions to
o
diffuse in the high viscosity of ethylene glycol (20 C/ η reaches 19.9). DMF
containing aldehyde groups can be used as a reducing agent and also decrease so
that the corresponding reaction may take place easily. When water was added, nickel
8
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ChemSusChem
10.1002/cssc.201903108
acetylacetonate and cobalt acetylacetonate quickly became nickel ions and cobalt
ions. These ions would be reduced to metal clusters quickly and served as nuclei,
leading to small particles. It is because metal ions are more easily to be reduced than
their acetylacetonate compounds. As the increase of the amount of water, the
5
0
concentrations of nickel ions and cobalt ions decrease, leading to large particles .
Ni(acac) and Co(acac) are slightly soluble in water, but they will preferentially
2
2
ionize when water is added into the system. Then the ions are reduced into atoms by
reducing agents and assemble into nucleus. The existence of ethylene glycol and PVP
can similarly control the particle size. Because nickel acetylacetonate and cobalt
acetylacetonate only can partly dissolve into the water at first for their low solubility,
this fact results in lower ions concentration. This will directly and effectively lead to a
controlled growth rate of nanoparticles and further reducing the particle size. So
when the water content is very low (say 10 μL-40 μL), the two precursors are reduced
to atoms, and the low concentration of ions can only reach the conditions for
forming homogeneous nucleation, so the final particle size is small. When
homogeneous nucleation occurs, more nickel-cobalt atoms (monomers) are
generated, and it is easy to reach the conditions of heterogeneous nucleation.
5
1
Heterogeneous nucleation is easier than homogeneous nucleation . The energy
barrier required for heterogeneous nucleation is half that of homogeneous
nucleation, the excessive precursors will successively undergo the second process
under modest water content until they are completely consumed, so the particles
formed by 100 μL of water are also larger. Therefore, modest water content (60 μL-80
μL) is a relatively suitable ratio for the ionization speed and growth rate to get an
-
1
ideal size. Since the solubility of Co(acac) (5.1 g·L ) is a little larger than that of
2
-
1
Ni(acac) (4.8 g·L ), the Co percentage in the alloy increases with the water content
2
in the reaction system, resulting in the different ratio of Ni/Co. There is a trace
amount of water in DMF and ethylene glycol itself. As the water content increases,
the ability of ethylene glycol to act as a reducing agent increases, resulting in an
increase in the amount of Co in the final product, which may serve as another reason
for producing different proportions of Ni-Co alloy.
9
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ChemSusChem
10.1002/cssc.201903108
Electrocatalytic properties of methanol oxidation were recorded by
electrochemical measurements under basic condition. We firstly explored the effect
of different water contents on the catalytic performance of nickel-cobalt alloy for
MOR. When 70 μL deionized water was added into the reaction system, the
-
2
electrode made with Ni Co achieves as many as 116 mA·cm of anodic current
0
.6
0.4
density (Fig. 2a), whereas the electrode obtained by 10 μL or 100 μL of deionized
water participating in the reaction only exhibits less than one sixth of Ni Co
0
.6
0.4
-
2
electrode, which is the lowest oxidation current density (20 mA·cm (vs. Ag/AgCl)).
The scatter plot of the anodic current density for different water contents in Figure
2
b also clearly indicates that Ni Co alloy formed by adding 70 μL deionized water
0
.6
0.4
has the highest current density.
The electrocatalytic activation performance of Ni Co , Ni Co and Ni Co
0.6
0.4
0.7
0.3
0.5
0.5
in 1 M NaOH solution without methanol is shown in Figs. 2c and S10†. The entire
-
1
activating process was carried out at a sweep rate of 50 mV·s with the voltage
2
+
3+
window of 0 to 0.6 V. Obviously, a pair of Co /Co redox peaks appears
2
+
preferentially due to more Co content on the alloy surface (Fig. 2c), which has been
testified by XPS results. The anodic peak potential at +0.18 V in the forward scan and
the cathodic peak potential at +0.11 V in the backward scan may originate from the
2
+
52
mutual conversion between Co and CoOOH . In the first four cycles, the nickel
atoms in the alloy begin to ionize resulting in the appearance of the second position
2
+
in the forward sweep. With the accumulation of a large amount of Ni on the
2
+
surface of the electrode and the forward sweep going on, Ni on the surface of the
2
+
electrode is more easily oxidized into NiOOH. The oxidation peak of Ni/Ni gradually
disappeared after four turns (blue curves in Fig. 2c), then a second pair of redox
2
+
3+
peaks Ni /Ni forms at +0.36 V in the forward scan and the cathodic peak at +0.26 V
5
3
in the return scan .The activation reaction is as follows:
-
-
CoO + OH - e ↔ CoOOH
-
-
Ni - 2e + 2OH → Ni(OH)
2
-
-
Ni(OH) + OH - e ↔ NiOOH + H O
2
2
The activity testing of three alloys in MOR are carried out in 1M NaOH solution
10
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ChemSusChem
10.1002/cssc.201903108
with 0.5 M CH OH solution (Fig. 2d) with the voltage window of 0-1.0 V at a scan rate
3
-1
of 50 mV·s . The oxidizing current density of Ni Co electrode is obviously higher
0.6
0.4
than that of Ni Co and Ni Co electrode, indicating that Ni Co is more likely
0
.7
0.3
0.5
0.5
0.6
0.4
to catalyze methanol oxidation. We further explain the activation mechanism and
5
4
introduce the electrochemically active surface area (ECSA) .The ECSA of these three
2
2
2
electrode materials is calculated to be 0.228 cm , 0.245 cm , and 0.254 cm
according to Fig. S12. Thus, the oxidation current density of methanol oxidation with
-
2
-2
-2
ECSA correction are 22.86 mA cm , 33.14 mA cm , 9.09 mA cm , respectively.
It is well known that the onset oxidation potential and the peak current density
are two essential parameters that determine the catalytic activity for MOR. In this
work, the potential reaching a 20% of the current density of oxidation may be used
5
5
as a measurement of the onset potential of methanol oxidation. The onset
oxidation potential of Ni Co was calculated to be +0.42 V, which is the same as
0
.6
0.4
Ni Co (+0.42V) but lower than that of Ni Co (+0.43V), suggesting that MOR is
0
.7
0.3
0.5
0.5
much easier to be triggered on Ni Co and Ni Co surface. Fig. 2d compares the
0
.6
0.4
0.7
0.3
current densities of Ni Co , Ni Co and Ni Co from two different aspects,
0.7
0.3
0.6
0.4
0.5
0.5
-
2
showing that the highest specific current density of Ni Co is 116 mA·cm , which is
0
.6
0.4
-
2
1
.47 times and 3.52 times larger than that of Ni Co (80 mA·cm ) and Ni Co
0
.7
0.3
0.5
0.5
-
2
electrodes(33 mA·cm ), respectively. Furthermore, the mass current density of
-1
Ni Co (406 mA·mg ) is the highest one among those three catalysts in Figure 2e.
0
.6
0.4
After ensuring that methanol participates in the reaction, we have compared the
voltages at the same current density. It is easy to observe through Figure S12 that for
the Ni Co electrode, only 0.45V voltage (vs.Ag/AgCl) is needed to achieve a
0
.6
0.4
-
2
current density of 10mA cm For Ni Co and Ni Co electrodes, the voltages of
0
.7
0.3
0.5
0.5
5
0mV and 260mV respectively need to be increased to reach the same current
density value.
The tolerance of the catalyst to CO poisoning is measured by the ratio of the peak
current density of the forward scan (I ) to the backward scan (I ) in Fig. 2d. In this
f
b
work, the value of I /I in Ni Co is 1.13, which is 1.04 times of Ni Co (1.11) and
f
b
0.6
0.4
0.7
0.3
1
.27 times of Ni Co (0.89). Therefore, the Ni Co electrode has the best CO
0.5
0.5
0.6
0.4
11
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ChemSusChem
10.1002/cssc.201903108
anti-poisoning ability. More importantly, higher I /I means that methanol and
f
b
intermediate products (including CO) adsorbed on the catalyst surface can be more
7, 56, 57
easily converted into CO during the anodic sweep
. Although the polyhedral
2
Ni-Co structure with a size of 100-150 nm synthesized from the classical method also
has a catalytic effect on methanol oxidation (Fig. S11†), its oxidation peak current
-
2
density is only about 20 mA·cm , proving that small particle size with rough surface
can efficiently increase the extensive specific surface area and the resulted
electrocatalytic activity for MOR.
The long-term stability and durability of the catalyst for methanol oxidation
needs to be measured by chronoamperometry. As shown in Fig. 2f, the current
density versus time is recorded after 1000 s at +0.6 V. It is clear that the current
density of Ni Co decreases to 67% within 76 s and gradually approaches a balance
0
.6
0.4
after 1000 s; while the current densities of Ni Co and Ni Co drop rapidly to 65%
0.7
0.3
0.5
0.5
and 52% during the same period, respectively. These decays mean that the remained
average anodic current density of Ni Co alloy is much higher than that of the
0
.6
0.4
other two, suggesting its better stability and durability.
It is reported that methanol is first oxidized to CH O*, CO, and/or other
3
5
8-60
carbonaceous intermediates during the forward anodic scan
. The by-products of
carbonaceous intermediates accumulated on Ni Co electrodes surface and are
x
1-x
oxidized to CO during the back anodic scan, corresponding to the anodic current
2
-
2
density of 100 mA·cm (vs. Ag/AgCl). The whole reaction mechanism is as follows:
2
2
NiOOH + 2CH OH + 2.5O → 2Ni(OH) + 2H O+3CO
3
2
2
2
2
CO + O → 2CO
2
2
Studies have shown that methanol molecules are more likely to bind to nickel,
52
and intermediate products are more likely to bind to cobalt . It is also reported that
61, 62
β-NiOOH acts as the oxidizing agent in the whole reaction process
. So the
procedure of MOR is shown in Fig. 3a. CH OH adsorbed on Ni is firstly oxidized to be
3
CH O*, which can be quickly trapped by Co and further dehydrogenate to become
3
CH O*. Then the intermediates are changed into CO and the final product of CO in
2
2
the presence of β-NiOOH. The key reaction pathway of this complex reaction process
12
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ChemSusChem
10.1002/cssc.201903108
is as follows: CH OH→ CH O*→ CH O*→CO → CO + H O.
3
3
2
2
2
Another unnegligible factor is the sensitivity of the binding of methanol
molecules to the crystal faces. The oxidation of methanol molecules and
intermediates is more likely to take place on (111) plane than on other crystal planes
6
3
.
Therefore, DFT calculations were carried out to calculate methanol
42
electro-oxidation reactions according to previous procedures
planes of the three alloys. The optimal reaction pathway of the three structures (Fig.
b) shows that CH O*→CH O* is a potential limiting step. It is possible that the
on (111) crystal
3
3
2
adsorption of CH O* is relatively weak on these surfaces and hence its formation is
2
difficult. The free energy (∆G, 1.51 eV) of Ni Co is smaller than that of Ni Co
0
.6
0.4
0.7
0.3
(
1.54 eV) and Ni Co (1.59 eV), which means that the methanol oxidation may
0
.5
0.5
have higher activity on Ni Co structure than the other two counterparts.
0
.6
0.4
-
2
The red curve in Fig. 4a shows that the catalytic activity (116 mA·cm ) of
Ni Co alloy is obviously better than single nickel and cobalt metals (black and blue
0
.6
0.4
curves), indicating a good synergy effect between Ni and Co with proper ratio. The Ni
atoms tend to lengthen the C-O bond length in the methanol molecule, which also
can be easily absorbed by NiOOH mainly because of its empty d-orbital or unpaired
5
7
electrons . Although cobalt does not have the effect of oxidizing methanol (blue
curve in Fig. 4a), it still plays an important role in Ni Co . First, the conductivity of
x
1-x
Co(OH) in solution is better than that of Ni(OH) , which is beneficial to electron
2
2
64
transport ; Additionally, CO and the intermediate products absorbed on cobalt can
3
+
greatly facilitate their further oxidation by Ni and decrease the CO poisoning to Ni
6
3
.
Fig. 4b displays the cyclic voltammograms in 0 V-0.60 V voltage range and at
various scan rates of Ni Co electrode under 0.50 M NaOH media. Both the anodic
0.6
0.4
and cathodic current density peaks gradually rise with the increasing of scan rate
-
1
from 10 to 100 mV·s . Moreover, the position of the anodic peak shifts positively to
a higher potential while the corresponding cathodic potential shifts to the negative
direction with the increase of scan rate. These shifts could be attributed to
electrochemical polarization and the limitation of reaction kinetics, resulting in the
13
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ChemSusChem
10.1002/cssc.201903108
2
7, 65
formation of NiOOH species that would be insufficient at higher scan rates
It can be seen from Fig. 4c that the anode peak and cathode peak current
density have a good linear relationship with the sweep speed at low sweep speed,
.
2
+
3+
indicating that this is a surface control process, and the formation of Ni /Ni is
2
2, 66
dominant
. The second important point to explain the synergy effect can be
2
+
3+
illustrated by the different surface coverage of redox species Ni /Ni , which can be
calculated by the following formula:
푛ꢀ퐹ꢀ
퐼푝 =
푣퐴Γ푐
4푅푇
Where 퐼 is the peak current, 푛 is the electron transfer number, 퐹 is the Faraday
푝
constant, 푅 is the gas constant, 푇 is the thermodynamic temperature, 휈 is the
sweep speed, 퐴 is the electrode surface area, and 푇 is the surface coverage of the
푐
redox species.
When the Co content is low (Ni Co ), the calculated surface coverage of
0
.7
0.3
2
+
3+
-8
-2
Ni /Ni redox species is 0.64×10 mol·cm . As the Co content increases (Ni Co ),
0
.6
0.4
-8
-2
푇 grows up to the summit of 1.48×10 mol·cm . With the further increase of Co
푐
-8
-2
content (Ni Co ), the value of 푇 (1.21×10 mol·cm ) starts to decrease,
0
.5
0.5
푐
indicating that the surface coverage of the most abundant redox species will be
obtained only at proper content ratio of Ni and Co. This is also the reason why the
Ni Co alloy has the largest anodic peak methanol oxidation.
0
.6
0.4
-
2
To the best of our knowledge, this current density of 116 mA·cm in MOR for
non-precious metals is almost the highest of among Ni-based catalysts reported to
date (Fig. 4d). For example, NiCu nanoparticles supported on titanium nitride
-2
(
Ni-Cu(0.5:1)/TiN) showed an anodic current density of 106.8 mA·cm (vs. Ag/AgCl)
6
7
.
Bimetallic Ni Co synthesized by electrodeposition method presented the current
2
2
-
2
52
density of 58 mA·cm (vs. Ag/AgCl) , which is the half of our work although we
used the same metal elements. More importantly, the mass activity of Ni Co is far
0
.6
0.4
more lager than that of commercial Pt/C (Table S3†). These results suggest that the
non-precious metal catalyst of Ni Co exhibited excellent methanol
electrooxidation performance. The size of Ni Co becomes larger than before due
0
.6
0.4
.6
0
0.4
14
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ChemSusChem
10.1002/cssc.201903108
to the existence of large amount of Nafion on the surface after cycling 300 times (Fig.
S13a†). But the skeleton of polyhedral morphology still remains clear (Fig. S13b†),
indicating its excellent stability.
4. Conclusion
In summary, a simple modified polyol method was used to regulate the size of the
Ni Co alloy nanoparticles by a tiny amount of water. When the water content is
x
1-x
different in the reaction system (60μL, 70μL and 80μL), three alloys with different
Ni/Co ratios (Ni Co , Ni Co and Ni Co ) are obtained, which are characterized
0
.7
0.3
0.6
0.4
0.5
0.5
by XRD, TEM and XPS. The anodic current density of Ni Co exhibits a maximum
0
.6
0.4
-
2
value of 116 mA·cm (vs. Ag/AgCl) in MOR, which is several times larger than that of
the other two alloys. The synergistic effect between nickel and cobalt and the peak
2
+
3+
value of calculated surface coverage of the redox species Ni /Ni in Ni Co
0
.6
0.4
-
8
-2
(
1.48×10 mol·cm ) can partly account for the high performance. DFT calculation
clearly shows that the potential-limiting steps for all of the three structures are all
the dehydrogenation reaction of CH O* to CH O* and follow the energetic trend:
3
2
Ni Co > Ni Co > Ni C . More importantly, the activity and stability of
0
.6
0.4
0.7
0.3
0.5 0.5
Ni Co is the highest among the previous non-precious metals reports in MOR.
0
.6
0.4
This preparation is simple and novel and will be of great importance in the future.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the online version, at
DOI:
SEM and TEM images, XPS and cyclic voltammogram of Ni Co and Ni Co alloys,
0
.7
0.3
0.5
0.5
Comparison of the specific current density with precious reports.
AUTHOR INFORMATION
Corresponding Author
*
qrhui123@mail.ahnu.edu.cn
wangxh@mail.ahnu.edu.cn
*
15
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ChemSusChem
10.1002/cssc.201903108
*mwshao@suda.edu.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was partially supported by the National Natural Science Foundation of
China (Nos. 21301006 and 21301007) and Collaborative Innovation Center of Suzhou
Nano Science & Technology.
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ChemSusChem
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Fig. 1 (a) The schematic preparation illustration of two different sizes of polyhedral Ni Co
x
1-x
bimetal; (b) EDX of Ni Co bimetal prepared with 70 μL water; (c) From top to bottom, the XRD
x
1-x
patterns of Ni Co , Ni Co , and Ni Co bimetals are respectively obtained; (d) the enlarged
0.7
0.3
0.6
0.4
0.5
0.5
XRD pattern of the (111) crystal plane; (e) TEM-mapping for Ni and Co element distribution
(
green for Ni, yellow for Co); (f) HRTEM for three regions in (e); (g) TEM image of Ni Co alloy;
0.6 0.4
And (h) and (i) XPS spectra of Ni 2p, Co 2p and their fitted curves.
19
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ChemSusChem
10.1002/cssc.201903108
Fig. 2 The reaction mechanism and role of water on the polyhedral.
20
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ChemSusChem
10.1002/cssc.201903108
Fig. 3 (a) Cyclic voltammograms were recorded by adding 10 uL, 40 uL, 50, 60, 70, 80, 100
uL of deionized water into the reaction solvent in a 1 M NaOH + 0.5 M CH OH solution at a
3
sweep rate of 50 mV/s; (b) the scatter plot of the anodic current density for different water
contents; (c) Cyclic voltammogram of Ni Co in the first 8 cycles of activation process; (d)
0.6
0.4
The CV curves of Ni Co in 1 M NaOH with and without CH OH; (e) Specific activity and
0.6
0.4
3
mass activity of Ni Co , Ni Co and Ni Co ; And (f) Chronoamperograms of three
0.7
0.3
0.6
0.4
0.5
0.5
alloys in the presence of 1 M NaOH + 0.5 M CH OH solution at +0.6V.
3
21
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ChemSusChem
10.1002/cssc.201903108
Fig. 4 (a) Proposed mechanisms of methanol electro-oxidation reactions to CO ; and (b) Energy
2
profile of methanol electro-oxidation process.
22
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ChemSusChem
10.1002/cssc.201903108
Fig. 5 (a) Cyclic voltammogram of Ni, Co and Ni Co ; (b) Cyclic voltammograms of Ni Co
0.4
0.6
0.4
0.6
recorded at a sweep rate of 10-100 mV/s in a 0.5 M NaOH solution; (c) The relationship between
the corresponding oxidation peak current density and the reduction peak current density of (b)
and the sweep speed at different sweep speeds; And (d) a bar graph comparing of Ni Co alloy
0.6
0.4
with the maximum reported oxidized methanol current density in the literature.
23
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