Bimetallic NiCo metal-organic frameworks for efficient non-Pt methanol electrocatalytic oxidation

Article abstract of DOI:10.1016/j.apcata.2021.118159

Bimetallic NiCo-MOF was synthesized via the one-pot synthesis method by using 2,2′-bipyridine-5,5′-dicarboxylic acid as the building block with nickel acetate and cobalt acetate as the mixed precursors. Co and Ni ions were concurrently coordinated with carboxyl groups and pyridyl nitrogen donors. Basic characterizations of the series of MOFs were investigated by diverse techniques. The developed NiCo-MOF consisted of mixed-valence Co(II)/Co(III) and Ni(II)/Ni(III) and rich functional groups. The MOF exhibited a large specific surface area, evenly distributed nanocubes, and high electrocatalytic activity. Compared with the Ni-MOF, Co-MOF, and commercial Pt/C(20 %), the NiCo-MOF catalyst had improved electrocatalytic activity toward methanol oxidation reaction (MOR). High current density of 225 mA cm^?2 at the potential of 1.6 V versus RHE in alkaline medium was achieved. Moreover, remarkable cycling stability in the MOR conditions was observed and only showed a slight decline after 5000 s. The catalytic mechanism of NiCo-MOF toward MOR was provided.

Full text of DOI:10.1016/j.apcata.2021.118159

Applied Catalysis A, General 619 (2021) 118159
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Bimetallic NiCo metal-organic frameworks for efficient non-Pt methanol
electrocatalytic oxidation
Minghua Wang, Changbao Wang, Lei Zhu, Feilong Rong, Linghao He, Yafei Lou,
Zhihong Zhang*
School of Material and Chemical Engineering, Zhengzhou University of Light Industry, No. 136, Science Avenue, Zhengzhou 450001, PR China
A R T I C L E I N F O
A B S T R A C T
′
′
Keywords:
Bimetallic NiCo-MOF was synthesized via the one-pot synthesis method by using 2,2 -bipyridine-5,5 -dicarbox-
ylic acid as the building block with nickel acetate and cobalt acetate as the mixed precursors. Co and Ni ions were
concurrently coordinated with carboxyl groups and pyridyl nitrogen donors. Basic characterizations of the series
of MOFs were investigated by diverse techniques. The developed NiCo-MOF consisted of mixed-valence Co(II)/
Co(III) and Ni(II)/Ni(III) and rich functional groups. The MOF exhibited a large specific surface area, evenly
distributed nanocubes, and high electrocatalytic activity. Compared with the Ni-MOF, Co-MOF, and commercial
Pt/C(20 %), the NiCo-MOF catalyst had improved electrocatalytic activity toward methanol oxidation reaction
Non-Pt electrocatalyst
Bimetallic NiCo-MOF
Methanol oxidation reaction
ꢀ 2
(
MOR). High current density of 225 mA cm at the potential of 1.6 V versus RHE in alkaline medium was
achieved. Moreover, remarkable cycling stability in the MOR conditions was observed and only showed a slight
decline after 5000 s. The catalytic mechanism of NiCo-MOF toward MOR was provided.
1
. Introduction
To address the energy crisis and ameliorate the environmental
their excellent electrocatalytic activities toward the MOR. The addition
of precious metal nanoparticles to some catalysts can significantly
improve the catalytic activity for the MORs. However, in addition to the
high cost and scarcity, the poisoning effect caused by the CO or CHO
species produced during electro-oxidation limit their practical applica-
tion [11]. Therefore, the development of efficient, stable, and inexpen-
sive non-Pt MOR catalysts is crucial. Various catalysts based on
nonnoble transition metals (e.g., Co, Ni, Fe, and Cu) have been exten-
sively developed as potential substitutes because of their good electro-
catalytic activity [12–14]. Most MOR electrocatalysts have been
fabricated by integrating multiple components [15–17] or calcining at
extremely high temperatures [18]. The fussy preparation procedure, low
electrochemical activity, and electrocatalytic performance greatly limit
the applications of these MOR electrocatalysts.
contamination caused by the development of globalization and
increasing living standards, the exploration for new energy resources to
replace the traditional fossil energy has attracted great attention [1,2].
Direct methanol fuel cell (DMFC) is a promising energy conversion de-
vice because of its high energy conversion efficiency, environmental
friendliness, relative ease of handling and storage of liquid fuel [3,4].
Among the different types of clean energy, methanol is a cheap fuel, with
its theoretical energy density higher than that of hydrogen [5]. During
the electro-oxidation of methanol in an acidic system, CO will be pro-
duced and will poison the catalyst, which will negatively affect the
catalytic activity and oxidation reaction. Meanwhile, the catalysts
adsorb OHads from water, thereby blocking the adsorption of methanol
and other reaction intermediates. The high stability of the reaction in-
termediate at the active site is important for the effective methanol
oxidation reaction (MOR) [6,7]. Alkaline DMFC has become another
important source because of its high electrocatalytic activity, and
methanol is oxidized to carbon dioxide in this material [8,9]. In addi-
tion, alkaline DMFC can reduce catalyst loading and the use of precious
metal catalysts [10]. Pt, Ru, Pd, and their alloy catalysts have displayed
Metal-organic frameworks (MOFs) belong to a type of advanced
porous materials and are constructed by organic linking groups and
metal ions through coordination bonds. The MOFs show some intrinsic
advantages, such as high crystallinity, large specific surface area, and
diverse structures and functionalities. Thus, MOFs and their composites
and derivatives have been constructed as diverse electrode materials or
electrocatalysts and applied for energy storage and conversion [19–21],
supercapacitors [22,23], hydrogen evolution reaction [24,25], oxygen
*
Corresponding author.
E-mail address: mainzhh@163.com (Z. Zhang).
https://doi.org/10.1016/j.apcata.2021.118159
Received 4 December 2020; Received in revised form 16 March 2021; Accepted 13 April 2021
Available online 16 April 2021
0
926-860X/© 2021 Elsevier B.V. All rights reserved.

M. Wang et al.
Applied Catalysis A, General 619 (2021) 118159
evolution reaction [26,27], oxygen reduction reaction [28], and MORs
proposed electrocatalyst based on bimetallic MOF provides a new
strategy to efficiently catalyze methanol.
[
29,30]. However, the exploration of MOFs and MOF-based composites
as MOR electrocatalysts is still in its infancy stage. Only limited types of
MOFs have been developed as electrocatalysts for methanol
electro-oxidation in an alkaline medium. Some of these MOFs are
Cu-MOF and graphene oxide composite [29], Ni-doped ZIF-67 [30,31],
2. Experimental
The chemicals and basic characterizations are described in detail in
the S1 part of Supplementary materials.
Bi-MOF doped with Cubane [M
4
(OH)
4
] (M = Ni, Co) clusters [32], and
semiconductive CoNi-MOF [33]. However, most MOFs are characterized
by low conductivity, large size, and low catalytic ability. MOFs nano-
materials are normally integrated with other components with superior
electrocatalytic activity or electrochemical activity or calcined to the
nanohybrids of mesoporous carbon and metal compounds [34]. Few
studies have shown that pristine MOFs can be directly applied as MOR
catalysts [35]. In a previous study, semiconductive CoNi-based MOF
2
.1. Preparation of NiCo-MOF
NiCo-MOF was prepared by a modified method in according to the
′
′
literature [36]. In typically, 2,2 -Bipyridine-5,5 -dicarboxylic acid
0.48 mmol, 117.2 mg), nickel acetate (0.48 mmol, 120 mg), and cobalt
acetate (0.24 mmol, 70 mg) were added to a mixed solution of DMF
8 mL), chlorobenzene (4 mL), and absolute ethanol (4 mL). A homo-
(
[
Co
organic ligand, and showed a current density of 92.8 mA cm
methanol electro-oxidation [33]. Given its limited catalytic ability,
Co Ni3-x(HAB) MOF is inferior to some MOF-based catalysts toward
x 2
Ni3-x(HAB) MOF] was synthesized by using hexaaminobenzene as
(
ꢀ
2
for
geneous dispersion was obtained after the solution was stirred for 1 h at
room temperature. Afterward, the dispersion was removed into a 20 mL
◦
x
2
Teflon-lined stainless steel autoclave for 60 h under 120 C. The pre-
MOR. Therefore, optimizing the preparation conditions to fabricate
MOFs with suitable nanostructure and rich catalytic sites is essential.
In light of the promising catalytic ability of MOFs, bimetallic NiCo-
cipitate was collected by centrifugation (10,000 rpm) after cooling to
room temperature and washed thrice with DMF (12 mL) and absolute
ethanol (12 mL). Finally, the precipitate was dried at 80 ℃ for 24 h. For
comparison, Ni-MOF and Co-MOF were synthesized through a similar
procedure without cobalt acetate or nickel acetate, respectively.
′
′
MOF was explored by using 2,2 -bipyridine-5,5 -dicarboxylic acid
bpydc) as the organic ligand with nickel acetate and cobalt acetate as
(H
2
the mixed precursors (Scheme 1). The specific objectives of this research
are as follows: (i) to explore and analyze the synthesis of bimetallic MOF
nanostructure; (ii) to probe the catalytic ability toward the MOR of the
bimetallic NiCo-MOF; and (iii) to discuss the mechanisms of the meth-
anol catalysis in an alkaline medium of NiCo-MOF. The results verify
that the NiCo-MOF consisted of diversely mixed metal ions, even particle
size, large specific surface area, and enhanced electrochemical activity.
2
.2. Electrochemical measurements
Electrochemical measurements were recorded on CHI760D electro-
chemical workstation (Shanghai Chenhua Instrument Co., China). The
MOR performance was measured in alkaline medium (1.0 M KOH so-
lution containing 0.5 M methanol) with three-electrode system,
including glassy carbon electrode (GCE) working electrode, platinum
wire counter electrode, and Ag/AgCl (saturated KCl) reference elec-
trode. For working electrode modification, 1.0 mg of catalyst was
dispersed in 1.0 mL deionized water and stirred ultrasonically to ob-
2
+
With the integration of various coordination modes of metal ions (Co
2
+
and Ni ) with carboxylate groups and pyridyl nitrogen donors, the
obtained NiCo-MOF demonstrated an efficient MOR catalyst comparable
to the sole Ni-MOF and Co-MOF. When electrocatalytically oxidizing
methanol in an alkaline medium, the obtained NiCo-MOF catalyst dis-
played an extremely high current density and good durability. The
tained a homogeneous dispersion. Then, 20
μ
L of the dispersion was
spread on the GCE surface and dried naturally. Accordingly, the mass
Scheme 1. Schematic diagram of the preparation of NiCo-MOF and its application as an efficient electrocatalyst for MOR.
2

M. Wang et al.
Applied Catalysis A, General 619 (2021) 118159
loading of active materials is 0.02 mg. For the MOR measurements,
cyclic voltammograms (CVs) were determined in alkaline medium using
a rotating disk electrode (RDE, 3 mm) at different scanning rates within
the potential of -0.2 to 0.6 V. Electrochemical impedance spectroscopy
3. Results and discussion
3.1. Surface morphologies of the synthesized MOFs
(
EIS) was recorded at the potential of 1.6 V in the frequency ranging
The surface morphologies and microstructures of Ni-MOF, Co-MOF,
and NiCo-MOF were investigated by scanning electron microscopy
(SEM) and transmission electron microscopy (TEM). As shown in Fig. 1a
and b, Ni-MOF exhibits a flower-like nanostructure with sheet thickness
of approximately 30 nm. The TEM images of Ni-MOF (Fig. 2a and b)
show that partial nanosheets are stacked together, while no clear sub-
stantial lattice is observed in the HR-TEM (Fig. 2c). These results suggest
the low crystallinity of Ni-MOF. The SEM images of Co-MOF (Fig. 1c and
d) demonstrate that the MOF is composed of a cube with irregular
5
ꢀ 1
from 10 to 10 Hz with an amplitude of 5 mV. The potentials were
corrected to reversible hydrogen electrode (RHE) by the following
equation: E (versus RHE) = E (versus Ag/AgCl) + (0.197 + 0.0591 ×
pH) V.
Fig. 1. Low- and high-magnification SEM images of (i) Ni-MOF, (ii) Co-MOF, (ⅲ) NiCo-MOF.
3

M. Wang et al.
Applied Catalysis A, General 619 (2021) 118159
Fig. 2. TEM and HR-TEM images of (a, b, c) Ni-MOF, (d, e, f) Co-MOF, and (g, h, i) NiCo-MOF composites. Elemental mapping images of homogenously distributed
Ni (blue), Co (green), C (red), N (brown), and O (yellow) containing in (j) Ni-COF, (k) Co-MOF, and (l) NiCo-MOF. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article).
4

M. Wang et al.
Applied Catalysis A, General 619 (2021) 118159
shapes and uneven size distribution. This result is further confirmed by
the TEM images (Fig. 2d and e). No lattice is obtained in the HR-TEM
because of the low crystallinity of Co-MOF (Fig. 2f). Meanwhile, small
nanocubes are found in the SEM (Fig. 1e and f) and TEM images of NiCo-
MOF (Fig. 2g–i), and these nanocubes are smaller than that of Co-MOF
and have even particle distribution. These characteristics are beneficial
to improve the number of active sites, further boosting the catalytic
activity. Fig. 2j–l shows the TEM-energy dispersive spectrometer (EDS)
elemental mapping of synthesized MOFs. The C, O, and N elements are
homogeneously distributed over the entire architecture. Besides, Ni and
Co elements are uniformly contained in the Ni-MOF and Co-MOF,
respectively, while the two elements coexist in NiCo-MOF sample.
on crystal structure of the as-synthesized MOFs [40–42]. Fourier trans-
form infrared (FT-IR) spectra of the MOFs (Fig. 4b) verify that metal ions
are joined by organic ligand. The asymmetric and symmetric stretching
vibration bands of the carboxyl group (
–
COOH) are expected to be
ꢀ 1
ꢀ 1
1500–1630 cm and 1350–1460 cm , respectively [43], whereas the
2
ꢀ
are observed at 1613 and
stretching vibration bands of bpydc
ꢀ 1
1380 cm [44].
X-ray photoelectron spectra (XPS) measurements were employed to
further probe the chemical structure and environment of the MOFs. As
demonstrated in Fig. 4c, the survey scan spectra show peak signals
centered at 284.6, 399.2, and 531.0 eV corresponding to C 1s, N 1s, and
O 1s, respectively, which originate from the used building organic block.
Co 2p signal (781 eV) is obtained in the Co-MOF, while the Ni 2p signal
(855 eV) appears in Ni-MOF. Co 2p and Ni 2p are concurrently observed
in NiCo-MOF. To further investigate the chemical valence states and
chemical environments of each element in the MOFs, the Co 2p and Ni 2p
core-level XPS spectra for the three samples were analyzed (Fig. 5). The
Co 2p spectrum of Co-MOF (Fig. 5a) can be separated into two couples of
peaks at the binding energy (BE) centers of 796.6 eV and 780.1 eV,
which correspond to Co 2p1/2 and Co 2p3/2. The deconvoluted peaks at
3
.2. The porous structures of the synthesized MOFs
The porous structures of the prepared MOFs were investigated by N
2
adsorption-desorption isotherms (Fig. 3a). Typical type I isotherms can
be seen from the samples, indicating the microporous structures of Ni-
MOF, Co-MOF, and NiCo-MOF materials. The steep uptakes below P/
0
P < 0.01 indicates the existence of numerous micropores [37,38].
2
+
Fig. 3b displays the density functional theory (DFT) pore-size distribu-
tions, which further confirm the microporous features of the prepared
MOFs. Careful comparison of the pore size distributions of three samples
reveals their similar pore size distribution, for which two main narrow
distributions centered at 0.9 and 1.1 nm. It suggests that the pores of the
developed MOFs are in the microporous region, which is helpful to boost
the electrocatalytic performance. It indicates that the type of metal ion
has little effect on the channel size of MOFs, but the doping of multiple
metals will improve the performance of other aspects of MOFs, such as
electron transport and electrocatalytic activity [39]. Given the syner-
gistic effect that occurred in the combination of the different metal
atoms of MOFs, the catalytic performances of the obtained catalysts
were superior to those of sole metal-based MOF catalysts [33]. The loose
the BEs near 782.3 eV and 797.3 eV are assigned with Co , whereas
those at 780.4 eV and 796.2 eV are ascribed to Co3 . Four additional
+
peaks are also observed at 785.2, 787.3, 803.9, and 804.1 eV, which can
be assigned to satellite subpeaks of the Co3 and Co species. According
+
2+
to the XPS analysis (Fig. 5b), the BEs for Ni 2p3/2 are centered at
855.1 eV and 856.1 eV, which correspond to the Ni2 and Ni species,
+
3+
respectively, while the BEs of 860.0 eV and 862.7 eV are due to the
satellite peaks of Ni2 and Ni species. The peaks located at 872.5 eV
+
3+
2
+
3+
and 873.3 eV are confirmed to Ni and Ni of the Ni 2p1/2, and their
satellite peaks of 877.3 eV and 880.6 eV are also observed. As demon-
strated in Fig. 5c and d, the Co 2p and Ni 2p core-level spectra show
similar deconvoluted peaks with Co or Ni in Co-MOF and Ni-MOF. Thus,
these MOFs are composed of mixed-valence of Co2 /Co or Ni /Ni
3+
,
+
3+
2+
nanosheet structure endows Ni-MOF
a
markedly higher Bru-
which could facilitate the electron transfer. In addition, the high-
resolution C 1s, N 1s, and O 1s XPS spectra were analyzed, and the re-
sults are displayed in Fig. S1. Similar deconvoluted components are
observed for the three MOFs. From C 1s spectra of the three MOFs, four
parts at the BEs of 284.5, 285.1, 285.9, and 288.2 eV are observed,
2
ꢀ 1
nauer ꢀ Emmett ꢀ Teller (BET) specific surface area (382.05 m g
)
2
ꢀ 1
2
ꢀ 1
than Co-MOF (273.59 m g ) and NiCo-MOF (314.47 m g ). The
3
ꢀ 1
pore volumes of the samples are 0.1232 cm g
(Ni-MOF),
3
ꢀ 1
3
ꢀ 1
0
.0809 cm g (Co-MOF), and 0.0960 cm g (NiCo-MOF) (Table S1).
The micropores structures in the MOFs could promote the diffusion and
adsorption capacity of methanol molecular, which is beneficial to
enhancement of catalytic activity for MOR [40,41].
correspond to the C
additional peak centered at 291.9 eV is also found, and this peak cor-
responds to the * bond. The N 1s spectra of the three MOFs only
demonstrate one main group at the BE of 399.4 eV, which is originated
C, C N, C O, and COO species, respectively. An
– – –
πꢀ π
3
.3. Crystal and chemical structure of the synthesized MOFs
from the tertiary N bonds to the carbon atoms of N–(C)
3
or H–N–(C)
2
.
Three components are separated from the O 1s spectra at the BEs of
530.6, 531.3, and 532.0 eV, which are assigned with O vacancies, C–
O,
As displayed in the X-ray diffraction (XRD) patterns (Fig. 4a), all the
–
̊
̊
̊
̊
–
and C O groups, respectively. The presence of O vacancies can
three MOFs show main characteristic peaks at 7.0 , 11.8 , 12.4 , 19.3 ,
̊
remarkably boost the electron transfer of the catalyst.
and 23.9 , suggesting that the Ni-MOF, Co-MOF and NiCo-MOF have
similar crystal structure. It indicates that the metal ions have little effect
Fig. 3. (a) N
2
adsorption-desorption isotherms and (b) DFT-pore size distributions of Ni-MOF, Co-MOF, and NiCo-MOF.
5

M. Wang et al.
Applied Catalysis A, General 619 (2021) 118159
Fig. 4. (a) XRD, (b) FI-IR patterns, and (c) XPS survey scan spectra of (i) Ni-MOF, (ii) Co-MOF, and (ⅲ) NiCo-MOF.
Fig. 5. High-resolution XPS of (a) Co 2p containing in Co-MOF, (b) Ni 2p containing in Ni-MOF, and (c) Co 2p and (d) Ni 2p containing in NiCo-MOF.
3
.4. MOR performances of the synthesized MOFs
determined by CV measurements in alkaline solution in the absence and
presence of methanol (Fig. 6). The CV curves of the catalysts in 1 M KOH
without methanol (Fig. 6a) illustrate well-defined peaks with different
locations, indicating a reversible redox reaction. Previous studies have
shown that Co ions with different valances can occur charge transfer
Considering the promising catalytic ability caused by the mixed
metal ions, the non-Pt MOR performances of the synthesized MOFs were
measured by electrochemical technique. For comparison, the MOR ac-
tivities of Ni-MOF and Co-MOF, commercial Pt/C (20 %), were
◦
2+
3+
4+
reaction of Co ↔ Co ↔ Co ↔ Co in alkaline medium [45].
6

M. Wang et al.
Applied Catalysis A, General 619 (2021) 118159
Fig. 6. (a) CV curves of Ni-MOF, Co-MOF, NiCo-MOF, and Pt/C(20 %) recorded in 1.0 M KOH solution and (b) 1.0 M KOH + 0.5 M CH
3
OH at the scan rate of 50 mV
ꢀ 1
s
. (c) Comparison of the current density of the NiCo-MOF catalyst with the reported catalysts.
◦
2+
3+
ꢀ 2
electrode reaches 225 mA cm at the potential of 1.6 V versus RHE,
Meanwhile, the redox process of Ni /Ni /Ni may also occur [46].
The Ni-MOF catalyst presents a pair of definite redox peaks, namely an
anodic peak at 0.40 V versus Ag/AgCl and a cathodic peak at 0.30 V
versus Ag/AgCl, which are originated from the electrochemical oxida-
ꢀ 2
which is outstandingly higher than those of Ni-MOF (12 mA cm ), Co-
ꢀ
2
ꢀ 2
MOF (22 mA cm ), and commercial Pt/C(20 %) (16 mA cm ).
Moreover, the onset potential is a key parameter for assessing the cat-
alytic activity of MOR [50]. The results reveal that the onset potentials
3
+
2+
tion and reduction reactions of the Ni /Ni
couple. The
ꢀ 2
Co-MOF-modified electrode shows a distinct anodic peak at 0.11 V and
three weak cathodic peaks at 0.03, 0.23 and 0.43 V versus Ag/AgCl.
These results are due to the probable Faradic redox reactions [47]. The
CV curve of the Pt/C modified electrode presents a pair of obvious redox
peaks at 0.30 and 0.16 V (versus Ag/AgCl). Notably, the electrode
modified with NiCo-MOF has redox peaks of 0.19 V and 0.33 V versus
Ag/AgCl at the anode and 0.11 V and 0.26 V versus Ag/AgCl at cathode,
which are similar to those of sole metallic MOF catalysts. These peaks
(@10 mA cm ) were 1.47, 1.55, 1.51 and 1.33 V for Pt/C, Ni-MOF,
Co-MOF, and NiCo-MOF, respectively. The catalytic activity of the
NiCo-MOF electrode apparently outperforms those of the sole metallic
MOFs.
According to the used dosages of nickel acetate and cobalt acetate
added in the synthesis of the NiCo-MOF, the atomic ratio of Ni/Co is 2:1.
The ratio of the two components in the bimetallic MOFs remarkably
affect the properties of the final product. Thereby, the used dosages of
nickel acetate and cobalt acetate were optimized. Besides the NiCo-MOF
with the atomic ratio of Ni/Co of 2:1, the two NiCo-MOFs with the
atomic ratio of Ni/Co of 1:1 and 3:1 were also synthesized, and their
MOR performances were investigated, too. As shown in Fig. S2a and b,
2
+
3+
may be due to the simultaneous electrochemical reactions of Co ||Co
2
+
3+
and Ni ||Ni , leading to the overlapping redox peaks. As compared
with Ni-MOF and Co-MOF, the NiCo-MOF catalyst displays the highest
peak current density and largest enclosed area. This result suggests that
the NiCo-MOF exhibits the best electrochemical activity among the
three MOFs, which may be attributed to the synergistic effect of the Ni
and Co species. This characteristic could be ascribed to the following
main reasons: (i) the integration of two types of active components (Co
and Ni) can increase their high dispersion [48]; (ii) doping with another
metal species changes the microstructure of the MOF catalyst; (iii) the
synergistic effect is mainly attributed to the electronic interaction
among the different components. These factors lead to enhanced cata-
lytic performance of the material [49].
ꢀ 2
the onset potentials (@10 mA cm ) of NiCo-MOF(1:1) and NiCo-MOF
(3:1) were 1.34 and 1.37 V, respectively. Both are higher than that of
NiCo-MOF(2:1). Meanwhile, the current density of the NiCo-MOF(1:1)
ꢀ 2
ꢀ 2
(161 mA cm ) and NiCo-MOF(3:1) (128 mA cm ) are lower than
that of NiCo-MOF(2:1). Therefore, the atomic ratio of Ni/Co is 1:1 the
optimized atomic ratio for excellent MOR performance.
Compared with the previously reported Co- or Ni-based electro-
catalysts, the catalytic MOR performance of NiCo-MOF are superior
(Fig. 6c and Table 1). The nanocube architecture enables the full
exposure of active sites to the electrolyte, which offers a short diffusion
path for electrons-ions transport. Thus, faster kinetics, lower over-
potential, and higher electro-catalytic reactivity are realized [11]. The
desirable electrocatalytic activity is mainly due to the richer surface
electroactive sites of Co and Ni species, leading to the enhanced elec-
trocatalytic activity [51]. Further, Co and Ni ions show synergistic effect
MOR performances of the synthesized MOFs were investigated in the
3
presence of 0.5 M CH OH in alkaline medium (Fig. 6b and Table S2).
The CV curves reveal that the anode currents of the Co-MOF- and Ni-
MOF-modified electrodes only have small current densities toward
MOR, whereas the NiCo-MOF catalyst shows a significantly high current
density toward methanol. The current density of the NiCo-MOF
7

M. Wang et al.
Applied Catalysis A, General 619 (2021) 118159
Table 1
Comparison of the MOR performances of the NiCo-MOF catalyst with other reported MOR electrocatalysts in alkaline media.
ꢀ
2
ꢀ 1
Current density/Weight (A g )
Catalysts
Current density (mA cm
)
Applied potential
Electrolyte
Refs
NiCu-220@C
41.12
92.8
98
1028
649
60
0.6 V @ Ag/AgCl
0.6 V @ Ag/AgCl
0.6 V @ Ag/AgCl
0.6 V @ Ag/AgCl
0.6 V @ Ag/AgCl
0.6 V @ Ag/AgCl
0.6 V @ Ag/AgCl
1.0 M KOH + 1.0 M methanol
1.0 M KOH + 0.5 M methanol
1.0 M KOH + 0.5 M methanol
1.0 M KOH + 0.5 M methanol
1.0 M KOH + 0.5 M methanol
1.0 M KOH + 0.5 M methanol
1.0 M KOH þ 0.5 M methanol
[53]
Co
x
Ni3-x(HAB)
2
MOF-2
[33]
NiCo
NiCo
NiCo
O
2 4
O
2 4
O
2 4
/carbon xerogel
[54]
/Ni(OH)
/rGO
2
132
78
92.3
273
61
[55]
[56]
NiO NS@NW/NF
89
[57]
NiCo-MOF
225
788
This work
to activate the reactants and reduce the reaction energy barrier in
methanol electrocatalytic-oxidation [52].
Determining whether high-activity catalysts can be reused is crucial.
The stability of the catalysts was investigated in a final set of experi-
ments, hinting its good stability. The CV curve of NiCo-MOF in MOR
remains stable after 3000 cycles (Fig. S3). The chronoamperogram (CA)
curves were recorded in the presence of 0.5 M methanol in alkaline
solution for 5000 s (Figs. 7c and S2c). The NiCo-MOF displays higher
catalytic activity than the sole MOFs, NiCo-MOF(1:1) and NiCo-MOF
(3:1), and Pt/C(20 %) over the whole CA measurements. The current
densities of the three catalysts only slight decrease during the mea-
surement, indicating that the as-prepared samples are highly active and
stable electrocatalysts for MORs [60]. After the stability test, the SEM
and XRD measurements of the NiCo-MOF catalyst were investigated
(Fig. S4). From Figs. S4a and b, no substantial change in surface
morphology or crystal composition was observed after long-term sta-
bility testing. The above results imply the good stability of NiCo-MOF
To study the effect of methanol concentration on the catalytic ac-
tivity of NiCo-MOF in different concentrations, the CV curves were
performed (Fig. 7a). The anode peak current density increases with in-
crease of methanol concentration range from 0.2 M to 0.7 M, and the
maximum current density is proportional to methanol concentration
(
Fig. 7a inset). The electrocatalytic activity of the NiCo-MOF at different
ꢀ 1
scan rates from 20 to 300 mV s was also measured by CV. The results
Fig. 7b) demonstrate an increase of the enclosed area along with the
(
scan rate, and this result is due to the rapid electron transfer between the
electrode surface and the electrolyte [58]. The obvious linear relation-
ship between the peak current and the scan rate is observed, manifesting
the surface controlled kinetic process in the methanol electro-oxidation
reaction [59].
Fig. 7. (a) CV curves of Ni-MOF, Co-MOF, and NiCo-MOF in 1.0 M KOH containing CH
oxidation current at 1.6 V versus RHE and concentration of CH OH for NiCo-MOF). (b) CV curves of NiCo-MOF electrode in 1.0 M KOH containing 0.5 M CH
different scan rates (inset: the relationship of the anodic peak currents versus scan rates of NiCo-MOF electrode). (c) Chronoamperometric curves of Ni-MOF, Co-
3
OH with different concentrations (inset: the relationship between the
3
3
OH at
MOF, NiCo-MOF and Pt/C(20 %) recorded in 1.0 M KOH + 0.5 M CH
3
OH solution at 1.6 V versus RHE. (d) EIS plots of Ni-MOF, Co-MOF, NiCo-MOF, and Pt/C(20 %)
in 1.0 M KOH solution.
8

M. Wang et al.
Applied Catalysis A, General 619 (2021) 118159
during the MOR reaction.
4. Conclusion
To further explicate the electrode dynamics of the catalysts for MOR,
the charge transfer resistance (Rct) of all catalysts was evaluated. Fig. 7d
and S2d indicate the EIS Nyquist diagrams of the three catalysts. The EIS
plots were fitted by a modified Randles equivalent circuit (inset of
Fig. 7d) to probe the catalytic mechanism of NiCo-MOF toward MOR.
The Rct of the NiCo-MOF is 34.3 Ω, which is remarkably smaller than
those of Ni-MOF (138.6 Ω), Co-MOF (58.9 Ω), NiCo-MOF(1:1) (42.3 Ω),
NiCo-MOF(3:1) (45.5Ω), and Pt/C (55.2 Ω). Apparently, the NiCo-MOF
Bimetal NiCo-MOF with bipyridine and carboxyl groups was pre-
pared and applied as an efficient electrocatalyst to catalyze methanol
oxidization in an alkaline solution. The NiCo-MOF network consisted of
nanoparticles, mixed-valence of Co2 /Co and Ni /Ni , rich func-
tional C-related groups and O vacancies, microporous structure and
large specific surface area. These favorable characteristics endowed the
NiCo-MOF with high electron transfer rate and enhanced methanol
adsorption, which further improved the electrocatalytic ability. Given
the synergistic effect between Co and Ni ions, the integration of different
coordination modes of metal ions and functional groups, and fast elec-
tron transfer kinetics, the constructed NiCo-MOF catalyst exhibited
significantly improved electrocatalytic performance toward the MOR
compared with the sole metallic Ni-MOF and Co-MOF. In addition, the
+
3+
2+
3+
(
2:1) modified electrode exhibits fast and efficient electron transport
32]. Furthermore, Cdl was performed to assess ECSA through CV
[
ꢀ 1
measurements at scan rates of 10–100 mV s in a non-faradaic region
of 0.1~0.2 V versus RHE (Fig. S5). The ECSA can be calculated from the
following equation [61]:
C
C
DL
ꢀ 2
ECSA =
NiCo-MOF displayed a large current density of 225 mA cm at 1.6 V
versus RHE in alkaline medium containing 0.5 M methanol. NiCo-MOF
catalyst also showed superior long-term stability. This study laid the
foundation for the development of new energy catalysts with high-
efficiency DMFC and high-performance clean energy.
s
2
s
where C is the specific capacitance of 1 cm atomically smooth standard
ꢀ
2
2
electrode (here C
s
= 0.04 m F cm ), CDL = Cdl * 0.07 cm . As calculated
from CVs (Fig. S5), the Cdl and ECSA of NiCo-MOF(2:1) are 20.1 m F
ꢀ
2
2
cm and 35.18 cm , respectively, which are much higher than those of
ꢀ 2
2
CRediT authorship contribution statement
Ni-MOF (Cdl = 0.40.7 m F cm ; ECSA = 0.7 cm ), Co-MOF (Cdl = 2 m F
ꢀ
2
2
ꢀ 2
9 m F cm ;
cm
;
ECSA = 3.5 cm ), NiCo-MOF(1:1) (Cdl
=
2
ꢀ 2
Minghua Wang: Investigation, Validation, Writing - original draft.
Changbao Wang: Methodology, Validation, Formal analysis. Lei Zhu:
Validation, Formal analysis. Feilong Rong: Formal analysis, Investiga-
tion. Linghao He: Methodology, Formal analysis. Yafei Lou: Formal
analysis, Investigation. Zhihong Zhang: Writing - review & editing,
Supervision.
ECSA = 15.75 cm ), and NiCo-MOF(3:1) (Cdl = 8 m F cm ; ECSA = 14
2
cm ). It hints that the developed NiCo-MOF(2:1) exposes more active
sites, resulting in a higher electrochemical activity for MOR.
On the basis of the above results, superior electrocatalytic activity
and high stability of the NiCo-MOF catalyst can be ascribed to several
factors. First, the microporous structure is beneficial to methanol
diffusion [62]. Second, the abundant Ni2 and Co active sites can
effectively enhance the reaction kinetics, which lead to significant
enhancement of the electrocatalytic MOR performance [55,63]. Third,
the activity and stability of the catalyst is due to the synergies of
different metal ions [64,65]. Furthermore, metal ions enter the nodes of
the frame to prepare the bimetal MOF in the hydrothermal reaction. The
inorganic nodes or secondary-building units in the framework facilitate
the synergistic effects of the bimetallic system [66–68]. The combina-
tion of aforementioned factors can significantly improve the elc-
trocatalytic activity and durability of the NiCo-MOF toward MOR.
In addition, the entire process of MOR undergoes some necessary
basic steps, including adsorption of methanol, methanol dehydrogena-
tion to intermediates, and oxidation of these intermediates to carbon
dioxide [69]. The mechanism of the NiCo-MOF catalyst toward MORs in
alkaline medium may obey the following reactions:
+
2+
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
This work was supported by the Distinguished Youth Science Foun-
dation of Henan Province (CN) (No. 202300410492), the Excellent
Youth Science Foundation of Henan Province (CN) (No.
202300410494), and the Young Backbone Teacher Training Program in
Universities of Henan Province (CN) (No. 2018GGJS089).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2021.118159.
NiCo-MOF + CH
NiCo-MOF-CH OHads → NiCo-MOF-(CH
NiCo-MOF-(CH
NiCo-MOF-(CHOH)ads →NiCo-MOF-(COH)ads + H+ + eꢀ
NiCo-MOF-(COH)ads →NiCo-MOF-(CO)ads + H+ + eꢀ
3 3
OH → NiCo-MOF-CH OHads
OH)ads + H+ + eꢀ
3
2
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