The preparation of NiO/Ni-N/C nanocomposites and its electrocatalytic performance for methanol oxidation reaction

Article abstract of DOI:10.1039/d0nj02045a

Direct methanol fuel cells (DMFCs) have been extensively studied for developing a new non-precious metal catalyst to replace commercial platinum catalysts, due to their expensive cost and susceptibility to CO poisoning. In this work, nanocomposites of nickel oxide/nickel supported by N-doped carbon (NiO/Ni-N/C) were prepared by using a hydrothermal method and further carbonization at different temperatures. The surface morphology and composition of the NiO/Ni-N/C nanocomposites were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). As observed through electrochemical experiments, the NiO/Ni-N/C nanocomposite modified glassy carbon electrode (NiO/Ni-N/C/GCE) exhibits high electrocatalytic activity for the methanol oxidation reaction (MOR) in alkaline solution. Among the nanocomposites, NiO/Ni-N/C carbonized at 500 °C (NiO/Ni-N/C₅₀₀) displays the highest mass activity (1043 mA mg_Ni^?1) at 0.73 V (vs.Ag/AgCl), which is much higher than that of Ni(OH)₂/N-C and NiO/Ni-N/C carbonized at other temperatures (400, 600, and 700 °C). In addition, the catalytic activity of NiO/Ni-N/C₅₀₀is 2.12 times that of the commercial Pt/C. NiO/Ni-N/C₅₀₀/GCE also possesses fast kinetics, excellent stability and CO tolerance. Therefore, the NiO/Ni-N/C₅₀₀nanocomposites have great application potential as MOR catalysts.

Full text of DOI:10.1039/d0nj02045a

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The preparation of NiO/Ni-N/C nanocomposites and its electrocatalytic
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performance for methanol oxidation reaction
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Jingchuang Zhaoa, Yingzhen Zhanga, Xianyu Kanga, Yancai Lia,b,*
a College of Chemistry, Chemical Engineering and Environment, Minnan Normal
University, Zhangzhou 363000, P. R. China
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b Fujian Province Key Laboratory of Modern Analytical Science and Separation
Technology, Minnan Normal University, Zhangzhou 363000, P. R. China
* Corresponding author
Corresponding E-mail: liyancai@mnnu.edu.cn
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Abstract: Direct methanol fuel cells (DMFCs) have been extensively studied in
developing a new non-precious metal catalyst to replace commercial platinum
catalysts, due to its expensive cost and easy to CO poisoning. In this work, nickel
oxide/nickel supported by N-doped carbon nanocomposites (NiO/Ni-N/C) were
prepared through hydrothermal method and further carbonization at different
temperature. The surface morphology and composition of the NiO/Ni-N/C
nanocomposites were characterized by X-ray diffraction (XRD), scanning electron
microscopy (SEM), transmission electron microscopy (TEM) and X-ray
photoelectron spectra (XPS). Through electrochemical experiments, the NiO/Ni-N/C
nanocomposites modified glassy carbon electrode (NiO/Ni-N/C/GCE) exhibits highly
electrocatalytic activity to methanol oxidation reaction (MOR) in alkaline solution.
Among them, the NiO/Ni-N/C carbonization at 500 ℃ (NiO/Ni-N/C500) displays the
highest mass activity (1043 mA·mgNi-1) at 0.73 V (vs Ag/AgCl), which much higher
than that of the Ni(OH)2/N-C and the NiO/Ni-N/C carbonization at other temperature
(400, 600, 700 ℃). In addition, the catalytic activity of the NiO/Ni-N/C500 is 2.12
times that of the commercial Pt/C. The NiO/Ni-N/C500/GCE also possesses fast
kinetics, excellent stability and CO tolerance. Therefore, the NiO/Ni-N/C500
nanocomposites have great application potential as MOR catalyst.
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Keywords: nickel oxide/nickel, methanol oxidation reaction, electrocatalytic
performance, alkaline
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1. Introduction
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The extensive use of fossil energy has brought serious pollution to the
environment and threatened human health, so it's unsustainable energy.[1, 2] To
reduce the continuing pollution of the environment and protect human health, green
and sustainable energy conversion systems are urgently needed.[3, 4] Direct methanol
fuel cells (DMFCs) (a kind of proton membrane fuel cells) have been regarded as a
promising energy conversion system due to its strong advantages in the past
decades.[4-6] Methanol combustion is second only to hydrogen in energy density.
Moreover, methanol possesses the advantages of cheaper, more abundant, renewable,
easy to store and transport safely compared to hydrogen. The core reaction of DMFCs
is methanol oxidation reaction (MOR). Methanol is oxidized to CO2, H+ and e− by
anode catalyst in the presence of H2O, according to the following equation (1):
Anode reaction: CH3OH + H2O → CO2 + 6 H+ + 6 e− (1)
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DMFCs still face significant challenges of slow kinetics and CO poisoning in the
MOR.[5] It has been proved that MOR kinetics in alkaline medium is faster than that
in acidic medium.[7] Besides, both challenges can be addressed by developing
efficient anode catalysts. So far, the best MOR catalyst is platinum; however, its
expensive cost and easy to CO poisoning limit its widely application.[8, 9] Therefore,
it is urgent need for developing low-platinum and non-platinum catalysts as an
alternative one.[5]
Notably, nickel-based material is an excellent choice as a non-platinum catalyst
for the MOR, because of its inexpensive cost, its stability to electrochemical process
and its tolerance to poisoning.[10, 11] Positive nickel ions (Ni2+ and Ni3+) will
provide a stable ladder for electron transfer during the MOR.[10, 11] Recently, many
efforts have been made on nickel-based materials.[12-16] Wu et al. used a simple
one-pot method to prepare a unique Cu-Ni alloy electrocatalyst, and the mass activity
-1
(MA) for MOR reached 867.1 mA·mgmetal in 1.0 M KOH electrolyte with 1.0 M
methanol.[12] Pieta et al. embedded the nickel heterostructure in the ultra-thin
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two-dimensional (2D) g-C3N4 by a one-pot hydrothermal method, and obtained a
Ni/CN with better MOR performance under visible light irradiation.[13] Noor et al.
prepared NiO-MOF/rGO composites by hydrothermal method and studied its
electrocatalytic activity for MOR in alkaline medium.[14] Arunachalam et al.
prepared Ni/NiO catalyst and Meso Ni-Co-O-3 catalyst by microwave reaction
method, and showed the methanol oxidation mass activities of 275 mA·mg-1 and 750
mA·mg-1 respectively.[15, 16] No matter how, it is still very necessary to develop
novel nickel-based materials for further studying the MOR kinetics and improving its
electrocatalytic activity.
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Carbon materials have been widely used in the field of catalyst because of their
good conductivity, excellent performance and low cost.[17] Some studies have shown
that the metal active center was aggregated to some extent during electrochemical
experiments.[18, 19] Some researches have demonstrated that carbon materials
(graphene, porous carbon or carbon nanotubes) embrace the active metal or metallic
compound nanoparticles (NPs) [20-26] not only can improve their electrical
conductivity and activity,[23, 26, 27] but also effectively protect the nanoparticles
from the influence of bad environment and weaken the nanoparticle
corrosion/aggregation in the process of electrochemical measurement to enhance its
durability. In addition, recent works have proved that N-doped carbon materials could
avoid the aggregation of composite through electronic effect of the nitrogen, thus
enhancing their stability.[28, 29] Therefore, nitrogen-doped carbon materials are
beneficial to the comprehensive properties of composites. It is hoped that the
electrocatalytic activity, kinetics and CO tolerance of such catalysts will be further
enhanced by doping nitrogen into carbon materials which used to embrace the active
centers of transition metals.
In this work, the nickel hydroxide composites were firstly synthesized by
one-step hydrothermal method, and then were carbonized into nickel-oxide/nickel
based nitrogen-doped carbon nanocomposites (NiO/Ni-N/C). The NiO/Ni-N/C
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nanocomposites were characterized by X-ray diffraction (XRD), scanning electron
microscopy (SEM), transmission electron microscopy (TEM) and X-ray
photoelectron spectra (XPS). The effect of carbonization temperature on the
electrochemical and electrocatalytic properties of NiO/Ni-N/C was also studied. The
results demonstrate that the nanocomposites carbonized at 500 ℃ (NiO/Ni-N/C500)
exhibit the highest electrocatalytic activity for MOR. The NiO/Ni-N/C500/GCE also
displays fast kinetics, excellent stability and CO tolerance.
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2. Experimental section
2.1. Chemicals
Analytical grade nickel (Ⅱ) nitrate hexahydrate (Ni(NO3)2·6H2O), methanol,
ethanol, potassium hydroxide (KOH) and potassium chloride (KCl) were obtained
from Xilong Chemical Co., Ltd. (Shantou, China). 2-methylimidazole (2-MI) and
platinum on carbon (10% Pt/C, Identification of product: P111329) were obtained
from Aladdin Reagent Co., Ltd. (Shanghai, China). Nafion was acquired from
Sigma-Aldrich. All of the chemical reagents are analytical grade and used without
further purification. Ultrapure water was obtained from Milli-Q system Millipore.
2.2. Instruments and measurements
XRD patterns are obtained on Pan-altical X′Pert-pro MPD using Cu Kα radiation.
SEM images are taken on Zeiss supra55 at 5 kV. TEM images are observed by JEM
2100 F at 200 kV. The high angle annular dark field-scanning transmission electron
microscope (HAADF-STEM) and the elemental mapping images are obtained from
FEI Tecnai G2 F30 at 300 kV. The actual loading of Ni in the prepared catalyst has
been determined by inductively coupled plasma-optical emission spectroscopy
(ICP-OES, Agilent 720ES). The Raman spectra were recorded using DXR 2Xi
Raman Microscope using a 532 nm laser excitation. The XPS was measured by
Thermo Fischer Scientific K-Alpha at 15 kV, with C 1s = 284.6 eV binding energy
standard for charge correction.
Electrochemical experiments were investigated by a CHI 650D electrochemical
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workstation (CH Instruments, Inc. Shanghai) with a conventional three-electrode cell.
The modified or unmodified glassy carbon electrode (GCE, Ф = 3 mm) was used as the
working electrode, the Pt wire and Ag/AgCl (3.0 M KCl) electrode were used as the
counter and reference electrodes, respectively.
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2.3. Synthesis of the NiO/Ni-N/C nanocomposites
In a typical procedure, Ni(NO3)2·6H2O (0.5595 g) was dissolved in 15 mL
methanol to form a clear solution, which was subsequently injected into 15 mL of
methanol containing 0.6160 g 2-MI under ultrasonic for 10 min at room temperature.
Then, this fresh mixed solution was heated at 120 ℃ for 4 h in an autoclave (Teflon
lined). The precipitates were centrifuged and washed with ethanol three times and
dried in vacuum at 70 ℃ for overnight. The green powder was ground evenly in agate
mortar and denoted as nickel hydroxide supported by N-doped carbon
nanocomposites (Ni(OH)2/N-C).
The Ni(OH)2/N-C was placed in a tube furnace, then heated to 400 ℃, 500 ℃
600 ℃ and 700 ℃, respectively, for 3 h at the heating rate of 5 ℃/min under flowing
N2 gas, and then naturally cooled to room temperature to obtain the NiO/Ni-N/C
samples. The as-prepared products were directly used without any post-treatment.
The synthesis process of these NiO/Ni-N/C nanocomposites are shown in
Scheme 1. During the carbonization process, the nickel atoms of the Ni(OH)2/N-C
catalyst are partially reduced to pure nickel and gradually reunite or partial oxidize to
form nickel-oxide/nickel particles, while the structure of the composite partially
During the carbonization process, the nickel atoms of the Ni(OH)2/N-C catalyst are
partially reduced to pure nickel and gradually reunite or partial oxidize to form
nickel-oxide/nickel particles, while the structure of the composite partially sintering.
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Scheme 1. Schematic illustration of the synthesis process of the NiO/Ni-N/C nanocomposites
2.4. Preparation of the modified electrode
A catalyst ink was prepared through mixing of the synthesized materials (1 mg) or
10% Pt/C (1 mg), ultrapure water (990 μL) and Nafion (10 μL, 5 wt % solution), and
ultrasound for 30min. Before modification, the GCE was successively polished with
different size of alumina powder (1.0 μm, 0.3 μm and 0.05 μm). The polished GCE
were then alternating ultrasonic cleaning for three times by ultrapure water and ethanol,
and then nitrogen blow-dried for further using. 5 μL as prepared catalyst ink was
deposited on the GCE by micropipette, and dried at room temperature. The obtained
modified electrodes are named as the Ni(OH)2/N-C/GCE and the NiO/Ni-N/C/GCE,
respectively.
3. Results and discussion
The morphology of the different Ni-based nanocomposites was investigated by
SEM. As shown in Fig. 1A and Fig. 1B, the Ni(OH)2/N-C exhibits a unique sheet
structure and interlaces with each other between the different sheets. After
carbonization at 400 ℃, the obtained NiO/Ni-N/C400 (Fig. 1C) can basically maintain
the morphology of the Ni(OH)2/N-C. However, with the increase of carbonization
temperature, the two-dimensional structure gradually appears different degrees of
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sintering, as shown in Fig. 1D of NiO/Ni-N/C500, Fig. 1E of NiO/Ni-N/C600 and Fig.
1F of NiO/Ni-N/C700. Among them, the NiO/Ni-N/C500 nanocomposites only occur
slight structural sintering. In addition, the content of Ni was determined by ICP-OES
analysis, and the results demonstrate that the Ni content of each catalyst (from Fig. 1B
to Fig. 1F) was 53.55 wt. %, 82.73 wt. %, 89.43 wt. %, 98.66 wt. % and 98.64 wt. %,
respectively.
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Fig. 1. SEM images of the Ni(OH)2/N-C (A and B), the NiO/Ni-N/C400 (C), the NiO/Ni-N/C500
(D), the NiO/Ni-N/C600 (E) and the NiO/Ni-N/C700 (F) nanocomposites
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Fig. 2. XRD patterns of the Ni(OH)2/N-C nanocomposites (A); XRD patterns of (a) the
NiO/Ni-N/C400, (b) the NiO/Ni-N/C500, (c) the NiO/Ni-N/C600 and (d) the NiO/Ni-N/C700
nanocomposites at different 2θ ranges in (B) and (C); XRD patterns of the NiO/Ni-N/C500
nanocomposites (D)
The structure and composition of the different Ni-based nanocomposites were
investigated by XRD. The XRD pattern shows that the Ni(OH)2/N-C has a crystal
structure similar to that of the Ni(OH)2 (JCPDS No.38-0715), as shown in Fig. 2A.[30]
The sharp diffraction peaks at 11.16° can be ascribed to the addition of the like-GO
carbon (001) and the Ni(OH)2.[31] This demonstrates the formation of the
Ni(OH)2/N-C. The XRD patterns of the NiO/Ni-N/C carbonization at different
temperature are shown in Fig. 2B and 2C. The diffraction peaks of Fig. 2B at 44.46°,
50.80° and 76.34° can be indexed to (111), (200) and (220) planes of pure Ni with
fcc-structure (JCPDS No.01-087-0712).[18] As shown in Fig. 2C, the broad peak at
25.7° can be attributed to graphene-like carbon (d(002), JCPDS No.41-1487).[32, 33]
Combined with the diffraction peak of the Ni(OH)2/N-C and the NiO/Ni-N/C
nanocomposites, it can be concluded that like-GO carbon is converted into
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graphene-like carbon during the carbonization process. Fig. 2D provides the
magnified XRD patterns of the NiO/Ni-N/C500, the diffraction peaks at 37.25°, 43.30°
and 62.88° can be indexed to (111), (200) and (220) planes of NiO (JCPDS
No.47-1049), which indicates the existence of a small amount of nickel oxide in the
nanocomposites.[34] It has been proved that partially oxidized nickel is beneficial to
enhance its catalytic activity, so the NiO/Ni-N/C500 nanocomposites are expected to
have good catalytic properties.[35, 36]
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Fig. 3. Raman spectra of (A) the Ni(OH)2/N-C and (B) the NiO/Ni-N/C400 (a), the NiO/Ni-N/C500
(b), the NiO/Ni-N/C600 (c) and the NiO/Ni-N/C700 (d) nanocomposites
The Raman spectrum of the Ni(OH)2/N-C is shown in Fig. 3A. The peak at 440
cm-1 should be assigned to the Ni-OH stretching mode.[37] The peak at 1038 cm-1 is
due to the adsorbed nitrate anions, which indicates that the Ni(OH)2/N-C contains the
intercalated nitrate anions.[38] The peak at 3664 cm-1 is related to the stretching
vibration of the hydroxyl groups.[37, 39] The peaks at 698, 1157, 1451 and 2905 cm-1
can be assigned to the v(CCH3), ρ(CH3), δ(CH3) and v(CH3) modes of
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-1
2-methylimidazole groups, respectively.[40] The peaks at 976, 1277 and 2797 cm
can be assigned to the δ(R), δ(NH) and 2v(R) modes of imidazole ring (R = ring),
respectively.[40] Fig. 3B displays the Raman spectrum of the NiO/Ni-N/C carbonized
at different temperature. The four peaks correspond to one-phonon transverse optical
(TO) and longitudinal optical (LO) modes (about 350~640 cm-1), two-phonon 2TO
(about 660~740 cm-1), TO + LO (about 913 cm-1) and 2LO (about 955~1240 cm-1)
modes, respectively.[41, 42] Studies have shown that NiO defect-rich will
significantly increase the scattering intensity of one-phonon.[41] In this work, it is
obviously observed the strongest scattering strength of one-phonon LO modes of the
NiO/Ni-N/C500 (curve b of Fig. 3B). This result indicates that the NiO/Ni-N/C500 has
more NiO defects, which will be beneficial for a catalyst.
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Fig. 4. (A-F) TEM images of the NiO/Ni-N/C500 nanocomposites; (D) the enlarged TEM image of
the partially marked area in C; (G) the corresponding FFT pattern of the partially marked area in F;
(H) HAADF-STEM and the corresponding elemental mapping images of (I) C, (J) N, (K) O and
(L) Ni
The surface morphology and composition of the NiO/Ni-N/C500 was further
studied using TEM and elemental mapping, as shown in Fig. 4 and Fig. S1. It can be
clearly observed the cross-connected structure of the NiO/Ni-N/C500 from TEM
images in Fig. 4A-B. The enlarged TEM images of the NiO/Ni-N/C500 are shown in
Fig. 4C-F, and the lattice spacing of the NiO/Ni-N/C500 is also presented by scientific
measurement and calculation. In the Fig. 4D and Fig. 4E, the lattice spacing is about
0.206 nm and 0.205 nm, which is very close to the standard value of pure Ni (111)
plane with fcc-structure (JCPDS No.01-087-0712).[18] In the Fig. 4E and Fig. 4F, the
lattice spacing is about 0.242 nm, which is very close to the standard value of NiO
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(111) plane (JCPDS No.47-1049).[34] In addition, the lattice spacing about 0.210 nm
in Fig. 4F should be attributed to (200) plane of NiO (JCPDS No.47-1049).[34] The
corresponding fast Fourier transform (FFT) pattern viewed from the [101] direction of
the partially marked area in Fig. 4F is shown in Fig. 4G, which also present the NiO
(111) and NiO (200) planes.[43] The lattice spacing and FFT pattern of the
NiO/Ni-N/C500 nanocomposites are consistent well with the XRD pattern, which can
further prove the formation of nickel (partially oxidized) nanoparticles. Fig. 4H-L
display the HAADF-STEM and the elemental mapping images of the NiO/Ni-N/C500
nanocomposites, in which reveals the homogeneous distribution of the four elements
in the NiO/Ni-N/C500 nanocomposites. Notably, such microscopic pore structure can
provide a large contact area between the catalyst and electrolyte solution, which will
be beneficial for electrocatalytic reactions.[44, 45] In addition, the special structure of
the nickel nanoparticles embraced by the carbon is shown in Fig. S1. This structure
can not only improve the conductivity of the catalyst, but also protect the metal active
center from the aggregation of nanoparticles, so as to improve the stability of the
MOR process.[27, 46]
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Fig. 5. XPS spectra of C 1s (A), N 1s (B), O 1s (C) and Ni 2p (D) of the NiO/Ni-N/C500
nanocomposites
The surface chemical composition and the oxidation states of the as-prepared
materials were characterized by XPS. As shown in the XPS survey spectra of the
NiO/Ni-N/C500 in Fig. S2, it can be clearly observed C, O and Ni signals, and also the
weak N characteristic peak. Meanwhile, the contents of C, N, O and Ni elements were
calculated as 41.52, 2.4, 34.5 and 21.58 %, respectively. As shown in Fig. 5A, the
high-resolution C 1s XPS spectrum of the NiO/Ni-N/C500 appears three small peaks,
which can be classified as C=C sp2 (284.7 eV), C-C sp3/C=N sp2 (285.6 eV) and
C=O/C-N (288.6 eV). From the N 1s XPS spectrum in Fig. 5B, it can be found the
presence of pyridinic N (399.3 eV) and pyrrolic N (400.2 eV), but haven’t found
Graphitic N and Oxidized N. This result strongly proves a certain number of N atoms
doped into the carbon framework. The high-resolution O 1s XPS spectrum of the
NiO/Ni-N/C500 nanocomposites exhibit a clear peak (529.2 eV) for the metal-oxygen
bond (Fig. 5C), indicating the existence of NiO. The other two characteristic peaks in
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the O1s spectrum can be attributed to O-C=O/C=O (531.1 eV) and C-O (532.5 eV). In
addition, a weak peak at 529.0 eV may correspond to the Ni-O bond in nickel
hydroxide (Fig. S3C).[47] As to the Ni 2p XPS, the two typical peaks of Ni are
located at 855.6 eV and 873.5 eV, belonging to the spin-orbit splitting of 2p3/2 and
2p1/2, respectively (Fig. 5D). In addition, the two peaks centered on 861.0 eV and
879.4 eV belong to the Ni 2p3/2 and 2p1/2 vibration peaks, respectively. Furthermore,
two fitting peaks appear in the XPS spectrum of Ni 2p, located at the binding energy
of 853.5 eV and 871.5 eV (Fig. 5D), which should be attributed to Ni(0).[19, 48]
However, there is no peak corresponding to Ni(0) in the high-resolution Ni 2p XPS
spectrum of Ni(OH)2/N-C nanocomposites (Fig. S3E). The Ni 2p XPS results suggest
that nickel atoms should be in the form of Ni0 and Ni2+ in the NiO/Ni-N/C500
nanocomposites. The XPS results of C1s and O1s also exhibit a large number of
oxygen-containing functional groups which expose to the surface of the
NiO/Ni-N/C500 nanocomposites.[49] Thus, the XPS results prove nickel atoms exist in
the metallic phase of the NiO/Ni-N/C500 nanocomposites as the form of Ni(0) and
NiO, which is consistent with the XRD results.
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Some studies suggest that the electrocatalytic activity of nickel-based materials
to methanol oxidation is due to unpaired d-electrons or d-vacant orbitals in
NiOOH.[50, 51] These unpaired d-electrons and empty d-orbitals could form
chemical bonds with the absorbed material.[50, 51] The electrooxidation mechanism
of methanol over NiO/Ni-N/C catalyst could be assumed as following:[34, 52]
Ni + 2OH− → Ni(OH)2 + 2e−
NiO + OH− → NiOOH + e−
(2)
(3)
(4)
Ni(OH)2 + OH− → NiOOH + H2O + e−
NiOOH + CH3OH → Ni(OH)2 + products (5)
As shown in equations (3) and (4), the formation of NiOOH is an important step
in the electrooxidation process of methanol based on nickel-based materials.[18] Thus,
before the electrocatalytic activity tests, Ni(OH)2/N-C/GCE and the NiO/Ni-N/C/GCE
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were pretreated by cyclic voltammetry (CV) over 50 cycles at a scan rate of 200
mV·s-1 and 20 cycles at a scan rate of 50 mV·s-1 in 1.0 M KOH in order to enrich
hydroxide, which will increase the thickness of the electrocatalytic layers.[18, 34]
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Fig. 6. The ECSA of (a) the Ni(OH)2/N-C/GCE, (b) the NiO/Ni-N/C400/GCE, (c) the
NiO/Ni-N/C500/GCE, (d) the NiO/Ni-N/C600/GCE and (e) the NiO/Ni-N/C700 /GCE in 1.0 M KOH
The electrochemical activity specific surface area (ECSA) is an important
parameter to measure the catalytic active of materials.[30, 53, 54] Herein, the ECSA
of the Ni-based materials were calculated by CV in non-Faradaic potential region. As
shown in Fig. S4A-E, CV curves were taken in potential range from -0.15 V to -0.05
V (vs. Ag/AgCl) at various scan rates of 5, 10, 15, 20 and 25 mV·s-1 in 1.0 M KOH
solution. Fig. S4A'-E' are the capacitive current ((Ia-Ic)/2) at -0.1 V vs Ag/AgCl as a
function of various scan rates, in which the double capacitance (Cdl) are equal to the
slopes. The Cdl of the Ni(OH)2/N-C/GCE, NiO/Ni-N/C400/GCE, NiO/Ni-N/C500/GCE,
NiO/Ni-N/C600/GCE and NiO/Ni-N/C700/GCE are 7.01×10-3, 12.19×10-3, 10.05×10-3,
9.27×10-3 and 7.32×10-3 mF, respectively. According to formula (6):
ECSA = Cdl / (Cs×l)
(6)
(Cs : the specific capacitance of the sample, Cs = 0.04 mF·cm-2 in KOH solution; ‘l’ :
the catalyst loading in ‘mg’ of working electrode)[55]
The ECSA of (a) Ni(OH)2/N-C/GCE, (b) NiO/Ni-N/C400/GCE, (c)
NiO/Ni-N/C500/GCE, (d) NiO/Ni-N/C600/GCE and (e) NiO/Ni-N/C700/GCE were
calculated as 35.05, 60.95, 50.25, 46.35 and 36.60 cm2·mg-1, respectively (Fig. 6).
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The results demonstrated that the carbonization process can increase the ECSA of the
Ni-based materials, which could be mainly attributed to the formation of nickel
nanoparticles. In addition, the ECSA decreases continuously with the increase of
carbonization temperature, possibly due to the sintering of materials.
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The Nyquist plots of the electrochemical impedance spectroscopy (EIS) were
recorded at an applied potential of 0.6 V (vs Ag/AgCl) over a frequency range of 1 to
100000 Hz to preliminary understand the MOR kinetics, as shown in Fig. S5.[56, 57]
Obviously, the Nyquist diagram shows a semicircle shape, indicating that the
electrode reaction is controlled by dynamic charge transfer process. The Nyquist plots
are fitted by the equivalent circuit model (Fig. S5), which includes a solution
resistance (Rs), constant phase element (CPE) and a charge transfer resistance (Rct) for
the electrochemical reaction. Due to the possible defects (such as kinks and charge
inhomogeneities) of the catalyst surface, CPE was used instead of Cdl between the
electrode modification material and the electrolyte interface.[12, 57, 58] The Rct of the
(Ni(OH)2/N-C, NiO/Ni-N/C400, NiO/Ni-N/C500, NiO/Ni-N/C600 and NiO/Ni-N/C700)
are calculates as 771, 101, 68, 143 and 177 Ω, respectively. The Rct of the carbonized
NiO/Ni-N/C is all much lower than that of the Ni(OH)2/N-C, suggesting that
carbonization process is helpful to reduce resistance and increase conductivity.
Among them, the NiO/Ni-N/C500 nanocomposites have the best conductivity and the
MOR kinetics. In addition, compared with the reported NiO-MOF/rGO 5 wt% (Rct =
252.5 Ω) methanol oxidation catalyst, the NiO/Ni-N/C500 nanocomposites possess
better electrical conductivity.[14] Therefore, it can be predicted that the
NiO/Ni-N/C500 would exhibit good catalytic performance based on its higher ECSA
and smaller resistance.
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Fig. 7. (A) CV curves (scan rate: 50 mV s − 1) of (a) the Ni(OH)2/N-C/GCE, (b) the
NiO/Ni-N/C400/GCE, (c) the NiO/Ni-N/C500/GCE, (d) the NiO/Ni-N/C600/GCE and (e) the
NiO/Ni-N/C700/GCE at 1.0 M KOH containing 3.0 M CH3OH, inset is the enlarged onset potential
part of these modified electrodes; (B) Mass activity (normalized by mass of catalyst) and specific
activity (normalized by ECSA of catalyst) of the above a, b, c, d and e
The electrocatalytic properties of the Ni(OH)2/N-C/GCE and the
NiO/Ni-N/C/GCE were tested by CV at 50 mV·s-1 in 1 M KOH with 3 M CH3OH. As
shown in the CV curves of Fig. 7A, it can be found that the NiO/Ni-N/C500/GCE
displays the best electrocatalytic activity to MOR at 0.73 V than the other modified
electrodes. The lower catalytic activity of the NiO/Ni-N/C carbonized at 600 or
700 ℃ may be related to more severe sintering. MA (normalized by mass of catalyst)
and specific activity (SA, normalized by ECSA of catalyst) of five catalysts are
calculated and the histogram are shown in Fig. 7B, which we can observed the
NiO/Ni-N/C500/GCE displays the highest MA (1043 mA·mgNi-1) and SA (18.57
mA·cm-2). The well catalytic performance of the NiO/Ni-N/C500/GCE may be resulted
in the coefficient of the partially oxidized nickel atoms and the formation of nickel
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nanoparticles.[35, 36] Although the ECSA of the NiO/Ni-N/C500/GCE is not the
largest one, it has the best conductivity, which may be the decisive reason for its
highest catalytic activity. At the same time, an obvious anodic peak appears in the
reverse scanning because of the oxidation of the adsorbed intermediate formed in the
forward scanning, indicating that these catalysts possess good anti-poisoning
performance.[59-61] We also compared the performance of the Ni-based
nanocomposites with some reported catalysts, as shown in Table S1. It can be found
that the NiO/Ni-N/C500 nanocomposites exhibits better electrocatalytic activity.
The onset potential is an important criterion to evaluate the performance of
catalysts, the lower onset potential means easier oxidation process on catalysts
surface.[62, 63] As shown in Fig. 7A, the onset potentials for methanol
electrooxidation of Ni(OH)2/N-C (a), NiO/Ni-N/C400 (b), NiO/Ni-N/C500 (c),
NiO/Ni-N/C600 (d) and NiO/Ni-N/C700 (e) are at 0.376 V, 0.385V, 0.379 V, 0.380 V
and 0.394 V, respectively. The lower onset potential of the NiO/Ni-N/C500 also can
demonstrate its well electrocatalytic activity for MOR.
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Fig. 8. (A) Tafel Plots and (B) I-t curves (for 10000s at 0.6 V vs Ag/AgCl) of (a) the
Ni(OH)2/N-C/GCE, (b) the NiO/Ni-N/C400/GCE, (c) the NiO/Ni-N/C500/GCE, (d) the
NiO/Ni-N/C600/GCE and (e) the NiO/Ni-N/C700/GCE. All electrochemical tests were performed at
1.0 M KOH containing 3.0 M CH3OH
Electrochemical reaction kinetics can be described by Tafel plots. Generally, a
smaller Tafel plot slope implies faster electrochemical kinetics and higher catalytic
activity.[56, 64, 65] The Tafel slopes of the five samples (Ni(OH)2/N-C,
NiO/Ni-N/C400, NiO/Ni-N/C500, NiO/Ni-N/C600 and NiO/Ni-N/C700) are 53.58, 29.70,
27.68, 32.07 and 34.40 mV·dec−1, respectively (Fig. 8A). The NiO/Ni-N/C500 displays
the lowest Tafel slope, which demonstrate the fastest MOR electrochemical reaction
kinetics and the highest catalytic activity.[56, 64, 65] The Tafel slope of the
NiO/Ni-N/C500 is much smaller than that of the reported Ni-Co-2 material (159
mV·dec−1),[16] which indicates that the NiO/Ni-N/C500 possesses fast kinetics
characteristics. The stability of the modified electrodes was studied under
chronoamperometric conditions by recording the current at 0.6 V (vs Ag/AgCl) in a
1.0 M KOH containing 3.0 M methanol. Chronoamperometry curves show that all the
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catalysts exhibit relatively stable electrocatalytic activity within 10,000 seconds (Fig.
8B). Among them, the NiO/Ni-N/C500/GCE maintains the highest current density,
indicating that the NiO/Ni-N/C500 has the strongest resistance to carbon monoxide
poisoning. Finally, the catalytic activity and stability of the NiO/Ni-N/C500 and the
commercial Pt/C were compared. As shown in curve c of Fig. 7A and Fig. S6A, the
catalytic activity of the NiO/Ni-N/C500 is about 2.12 times that of the commercial Pt/C;
additionally, its stability is much higher than that of the commercial Pt/C (Fig. S6B).
4. Conclusion
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In conclusion, we synthesized the NiO/Ni-N/C nanocomposites by means of
hydrothermal and high-temperature annealing method. The NiO/Ni-N/C
nanocomposites were characterized by XRD, SEM, TEM and XPS. Through
electrochemical experiments, the NiO/Ni-N/C nanocomposites exhibit highly
electrocatalytic activity to MOR under alkaline conditions, in which the
NiO/Ni-N/C500 possesses the best electrocatalytic performance with highly mass
activity up to 1043 mA·mgNi-1. The NiO/Ni-N/C500/GCE also displays fast kinetics,
excellent stability and good anti-toxicity. Therefore, this NiO/Ni-N/C500
nanocomposites have great potential applications as MOR catalyst and should be of
great significance in sustainable energy and environmental applications.
Acknowledgments
The work was supported by the National Natural Science Foundation of China
(21905125) and the Innovation Base Foundation for Graduate Students Education of
Fujian Province.
Conflict of Interest
The authors declare that they have no conflict of interest.
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Graphical Abstract
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The preparation of NiO/Ni-N/C nanocomposites and its electrocatalytic
performance for methanol oxidation reaction
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Jingchuang Zhao, Yingzhen Zhang, Xianyu Kang, Yancai Li*
College of Chemistry, Chemical Engineering and Environment, Minnan Normal
University, Zhangzhou 363000, P. R. China
The nickel oxide/nickel supported by N-doped carbon nanocomposites (NiO/Ni-N/C)
were prepared through hydrothermal method and further carbonization at different
-1
temperature. The NiO/Ni-N/C500 displays the highest mass activity (1043 mA·mgNi
and specific activity (18.57 mA·cm-2) for methanol oxidation reaction.
)
25