Molecular Engineering to Tune the Ligand Environment of Atomically Dispersed Nickel for Efficient Alcohol Electrochemical Oxidation

Article abstract of DOI:10.1002/adfm.202106349

Atomically dispersed metals maximize the number of catalytic sites and enhance their activity. However, their challenging synthesis and characterization strongly complicates their optimization. Here, the aim is to demonstrate that tuning the electronic environment of atomically dispersed metal catalysts through the modification of their edge coordination is an effective strategy to maximize their performance. This article focuses on optimizing nickel-based electrocatalysts toward alcohol electrooxidation in alkaline solution. A new organic framework with atomically dispersed nickel is first developed. The coordination environment of nickel within this framework is modified through the addition of carbonyl (CO) groups. The authors then demonstrate that such nickel-based organic frameworks, combined with carbon nanotubes, exhibit outstanding catalytic activity and durability toward the oxidation of methanol (CH₃OH), ethanol (CH₃CH₂OH), and benzyl alcohol (C₆H₅CH₂OH); the smaller molecule exhibits higher catalytic performance. These outstanding electrocatalytic activities for alcohol electrooxidation are attributed to the presence of the carbonyl group in the ligand chemical environment, which enhances the adsorption for alcohol, as revealed by density functional theory calculations. The work not only introduces a new atomically dispersed Ni-based catalyst, but also demonstrates a new strategy for designing and engineering high-performance catalysts through the tuning of their chemical environment.

Full text of DOI:10.1002/adfm.202106349

ReseaRch aRticle
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Molecular Engineering to Tune the Ligand Environment
of Atomically Dispersed Nickel for Efficient Alcohol
Electrochemical Oxidation
Zhifu Liang, Daochuan Jiang, Xiang Wang, Mohsen Shakouri, Ting Zhang,
Zhongjun Li, Pengyi Tang,* Jordi Llorca, Lijia Liu,* Yupeng Yuan, Marc Heggen,
Rafal E. Dunin-Borkowski, Joan R. Morante, Andreu Cabot,* and Jordi Arbiol*
1
. Introduction
Atomically dispersed metals maximize the number of catalytic sites
and enhance their activity. However, their challenging synthesis and
characterization strongly complicates their optimization. Here, the aim is
to demonstrate that tuning the electronic environment of atomically dis-
persed metal catalysts through the modification of their edge coordination
is an effective strategy to maximize their performance. This article focuses
on optimizing nickel-based electrocatalysts toward alcohol electrooxidation
in alkaline solution. A new organic framework with atomically dispersed
nickel is first developed. The coordination environment of nickel within
this framework is modified through the addition of carbonyl (CO) groups.
The authors then demonstrate that such nickel-based organic frameworks,
combined with carbon nanotubes, exhibit outstanding catalytic activity and
durability toward the oxidation of methanol (CH OH), ethanol (CH CH OH),
The electrochemical oxidation of alcohols
is at the basis of several strategic chem-
ical processes and energy technologies,
including electrochemical organic syn-
thesis, biomass, and organic waste val-
orization, and direct alcohol fuel cells
[1]
(DAFC). The electrooxidation of alcohols
offer numerous advantages over com-
peting valorization strategies and energy
technologies, such as high energy densities
and energy conversion efficiency, opera-
tion/reaction at low working temperature
with high selectivity, and low fuel cost
with easy production, transportation, and
3
3
2
and benzyl alcohol (C H CH OH); the smaller molecule exhibits higher
[1]
6
5
2
storage. However, despite their numerous
catalytic performance. These outstanding electrocatalytic activities for alcohol
electrooxidation are attributed to the presence of the carbonyl group in the
ligand chemical environment, which enhances the adsorption for alcohol, as
revealed by density functional theory calculations. The work not only intro-
duces a new atomically dispersed Ni-based catalyst, but also demonstrates
a new strategy for designing and engineering high-performance catalysts
through the tuning of their chemical environment.
advantages, the deployment of electrocata-
lytic valorization strategies and DAFCs is
hampered by their low cost-effectiveness in
large part associated with the use of mod-
erately efficient catalysts based on high-cost
and low-abundance elements such as plat-
[2,3]
inum and palladium.
Besides, current
alcohol electrooxidation catalysts become
rapidly poisoned with carbon monoxide,
Z. F. Liang, T. Zhang, J. Arbiol
Catalan Institute of Nanoscience and Nanotechnology (ICN2)
CSIC and BIST
Campus UAB, Bellaterra, Barcelona, Catalonia 08193, Spain
E-mail: arbiol@icrea.cat
D. Jiang, Y. Yuan
School of Chemistry and Chemical Engineering
Anhui University
Heifei Anhui 230601, China
M. Shakouri
Z. F. Liang, X. Wang, A. Cabot, J.R. Morante
Functional Nanomaterials Group
Canadian Light Source
Saskatoon S7N 0X4, Canada
Catalonia Institute for Energy Research – IREC
Sant Adrià de Besòs, Barcelona Catalonia 08930, Spain
E-mail: acabot@irec.cat
Z. J. Li
School of Physics
Hefei University of Technology
Hefei 230009, P. R. China
P. Y. Tang
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.202106349.
State Key Laboratory of Information Functional Materials, 2020 X-Lab
Shanghai Institute of Microsystem and Information Technology
Chinese Academy of Sciences
©
2021 The Authors. Advanced Functional Materials published by Wiley-VCH
GmbH. This is an open access article under the terms of the Creative Com-
mons Attribution-NonCommercial-NoDerivs License, which permits use
and distribution in any medium, provided the original work is properly cited,
the use is non-commercial and no modifications or adaptations are made.
Shanghai 200050, China
E-mail: py.tang@mail.sim.ac.cn
DOI: 10.1002/adfm.202106349
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which further limits their cost-effectiveness. Thus, the develop-
ment of efficient and stable alcohol oxidation electrocatalysts
based on abundant elements is a topic of major interest.
Several non-precious transition metal catalysts have been
proposed for alcohol electro-oxidation.[4,5] Among them, nickel-
based catalysts are considered the most promising candidates
owing to their high catalytic activity, low price, and notable
2,5-hydroxyterephthalaldehyde (HBC) (Scheme S1, Supporting
Information) and tetramino-benzoquinone (TABQ) (Scheme S2,
Supporting Information) in N-methyl-2-pyrrolidone (NMP)
(see details in Figure 1a; Scheme S3, Supporting Information).
The dark-black color of the obtained solid indicated the for-
mation of a conjugated polymer, which was washed by water
and methanol to remove low molecular mass impurities, and
then dried under vacuum. For comparison, a reference nickel-
based 2D organic framework without carbonyl chemical groups
(Ni–2D–SA) was prepared under the same reaction condition
but using 1,2,4,5-benzenetetraamine tetrahydrochloride (TAB)
[
6]
abundance. Numerous Ni-based catalysts, such as Ni0.75Cu0.25
,
NiSn alloy,[ Ni Bi , NiSe, NiO, and metal–organic frame-
7]
[8]
[9]
[10]
9
3
7
[11]
[12]
works with Ni(OH)4, and Ni-Co hydroxide have been devel-
oped and tested for alcohol electrochemical-oxidation. However,
the reported onset potentials and stabilities are still not fully
satisfactory, which calls for novel strategies to design and engi-
neer nickel-based catalysts with improved performance.
[
18]
instead of TABQ (Scheme S4, Supporting Information).
Scanning electron microscopy (SEM) images (Figure S1a,b,
Supporting Information) show that the Ni–2D–O–SA sample
has a cotton-like morphology. The X-ray diffraction (XRD) pat-
tern (Figure S3, Supporting Information) of the Ni–2D–O–SA
powder was similar to that of graphite, with a diffraction peak
at 26.29° corresponding to the (001) family planes and which
Recently, 2D covalent organic frameworks and metal–organic
frameworks (MOF) with highly tunable pore structure and
functionality, and high density of active sites, have arisen broad
interest as electrocatalysts.[ Optimized electrocatalysts based
on such 2D structures have demonstrated outstanding perfor-
13]
[
19]
indicated π–π layered stacking. In contrast to the clear crys-
talline and layered structure displayed by Ni–2D–O–SA, the
XRD pattern of Ni–D–SA displayed a much broader and less
intense peak at around 26°. This result indicates that the intro-
duction of carbonyl groups enhances the crystallinity of the
nickel-based 2D organic framework used here.
[
14,15]
mances toward oxygen evolution and reduction,
and carbon
dioxide reduction.[
16,17]
However, despite their high potential for
alcohol electrooxidation, these 2D materials and particularly a
nickel-containing 2D organic framework have yet to be devel-
oped and optimized toward this electrocatalytic reaction.
Here, we report the synthesis of an atomically-dispersed
nickel 2D molecular organic framework containing abundant
carbonyl (CO) functional groups within the nickel ion coor-
dination environment. Its atomic structure and how the out-
standing electrocatalytic activities and durabilities obtained for
the electrochemical-oxidation of methanol, ethanol, and benzyl
alcohol were analyzed, are related to the alcohol adsorption
ability of the carbonyl functional groups and the high unsatu-
rated coordination number of the nickel ions within the new
catalyst presented here.
Fourier transform infrared (FT-IR) spectroscopy was used to
evaluate the chemical structure of the samples (Figure S4, Sup-
porting Information). Based on the FT-IR spectra, the signal for
−
1
−1
CN appeared at 1642 cm for Ni–2D–SA, and 1648 cm for
Ni–2D–O–SA. The vibration of the carbonyl bond (CO) in Ni–
−1
2D–O–SA negatively shifted below 1600 cm and overlapped with
other vibrations. For Ni–2D–O–SA, the vibration peak for the
C(CO)C bond, which is a footprint of TABQ, shifted from
−1
1140 to 1010 cm due to the coordination effect and the attraction
between the layers. This result already points out to the successful
[20]
introduction of the carbonyl groups in the organic framework.
The survey X-ray photoelectron spectroscopy (XPS) spectra
revealed the sole presence of C, N, Ni, and O in both sam-
2
. Results and Discussion
[
21]
ples (Figures S5 and S6, Supporting Information). For both
samples, the high-resolution Ni 2p spectrum showed a unique
doublet at binding energies of 856 eV (Ni 2p3/2) and 873.3 eV
The nickel-based 2D organic framework with abundant
carbonyl functional groups (Ni–2D–O–SA) was solvother-
mally synthesized through a Schiff-based reaction between
[
22,23]
(Ni 2p1/2), significantly above those of NiO.
The local environment in Ni–2D–O–SA was further ana-
lyzed and compared with that of NiO using X-ray absorption
fine structure (XAFS) at the Ni K-edge (Figure 1b–d; Figure S7,
Supporting Information). Comparing the spectral features at
the near-edge, that is, the X-ray absorption near-edge structure
P. Y. Tang, M. Heggen, R. E. Dunin-Borkowski
Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and
Peter Grünberg Institute, Forschungszentrum Jülich GmbH
5
2425 Jülich, Germany
(XANES), we observe the absorption onset of Ni–2D–O–SA at
J. Llorca
Institute of Energy Technologies
Department of Chemical Engineering and Barcelona
Research Center in Multiscale Science and Engineering
Universitat Politècnica de Catalunya
EEBE
slightly lower energy than that of NiO (Figure 1b). Differences
are more clearly seen when plotting the first derivative of the
two spectras (Figure S7a, Supporting Information), as shown
in the inset of Figure 2b. Besides, the main absorption peak of
Ni–2D–O–SA has a lower intensity than that of NiO. Both fea-
tures indicate that compared with NiO, where Ni is fully sur-
rounded by O, within Ni–2D–O–SA, Ni atoms are partially
surrounded by a lower electronegativity element, that is, N.
A more quantitative picture was obtained by analyzing the
extended XAFS (EXAFS) features. The fitting of the Fourier trans-
formed EXAFS spectrum suggests that Ni is coordinated with
two N atoms and two O atoms, at a bond length 1.87 and 2.05 Å,
Barcelona Catalonia 08019, Spain
L. Liu
Department of Chemistry
Western University
1
151 Richmond Street, London ON N6A5B7, Canada
E-mail: lijia.liu@uwo.ca
A. Cabot, J. Arbiol
ICREA
Pg. Lluís Companys 23, Barcelona Catalonia 08010, Spain
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Figure 1. a) Scheme of the synthesis procedure used to produce Ni–2D–O–SA. b) Ni K-edge XANES spectrum of Ni–2D–O–SA compared with a
commercial NiO powder. c) Fourier transformed Ni K-edge EXAFS spectra of Ni–2D–O–SA and NiO plotted in R-space. d) Fitting of the Fourier trans-
formed EXAFS spectrum in R-space of Ni–2D–O–SA.
respectively (Figure 1c,d).[24] The contribution at longer radial
distances comes from C atoms with a coordination number of
electrocatalysts, they were loaded on multi-wall carbon nano-
tubes (CNT) through the π–π packing interaction. The XRD
patterns of the obtained Ni–2D–O–SA–CNT and Ni–2D–SA–
CNT composites, showed a unique peak at ≈26° (Figure S10,
Supporting Information), which was assigned to the overlap
between the CNT and the organic framework structures. SEM
(Figure S1c,d, Supporting Information), STEM images, and
EDX compositional maps (Figure S11, Supporting Information)
showed the composites morphology and confirmed the homo-
geneous loading of Ni–2D–O–SA on the CNTs.
6
and a bond length of 2.65 Å. The detailed fitting parameters
can be found in Table S1, Supporting Information. Overall, XAFS
analysis provided strong evidence of the formation of the NiN O2
2
salophen structure unit in the Ni–2D–O–SA framework.
High angle annular dark-field (HAADF) aberration-corrected
scanning transmission electron microscopy (STEM) was used
to analyse the nickel dispersion. As shown in Figure 2a–c;
Figure S8a–c, Supporting Information, Ni–2D–O–SA contained
a high density of isolated and homogeneously distributed nickel
atoms. The uniform distribution of Ni, C, N, and O was fur-
ther confirmed by energy-dispersed X-ray spectroscopy (EDS)
elemental mapping (Figure 2d).
Figure 3a exhibits the cyclic voltammograms (CV) of the
two samples in the range from 0 to 0.65 V in 1.0 ꢀ KOH
−
1
electrolyte with a scan rate of 50 mV s . The anodic peak at
0.50 V versus Hg/HgO measured in the forward scan with
Ni–2D–O–SA–CNT is attributed to Ni2 oxidation, and the
cathodic peak in the backward scan at 0.38 V corresponds to the
reduction of Ni3 species. For Ni–2D–SA–CNT, the anodic peak
shifted to 0.54 V and the cathodic peak to 0.42 V. The current
density at the Ni2 oxidation peak in the Ni–2D–O–SA–CNT
+
The surface area of Ni–2D–O–SA and Ni–2D–SA, evaluated
from N adsorption–desorption isotherms at 77 K (Figure S9,
2
+
Supporting Information) using the Brunauer–Emmett–Teller
2
−1
equation was 21.1 and 83 m g , respectively. Thus, the intro-
duction of carbonyl functional groups significantly decreased
the surface area of the material, which we associate with an
enhancement of the layer–layer interaction that results in
slightly more densely packed frameworks.
+
−
2
electrocatalyst was 19 mA cm , which is ≈9.5 times higher
−
2
than that of Ni–2D–SA–CNT, 2 mA cm . These results indicate
a much higher electrochemical activity of the Ni ions within
3+
To improve the electrical conductivity of these nickel-
based organic frameworks, as required to apply them as
Ni–2D–O–SA–CNT and their easier oxidation to Ni . Besides,
the ratio between the anodic and the cathodic peak current
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Figure 2. a–c) HAADF–STEM images of Ni–2D–O–SA displaying the presence of atomically dispersed nickel atoms. d) Low magnification
HAADF–STEM image and EDS elemental mapping.
densities was close to 1 for both catalysts, indicating a reversible
slightly improved compared to that of Ni–2D–SA–CNT. How-
ever, the Ni–2D–O–SA–CNT catalysts showed much higher
activity toward methanol oxidation reaction (MOR), with cur-
2
+
3+
[25]
Ni /Ni redox reaction in both materials.
CV of the reduction/oxidation process allowed to estimate the
2+
3+
−2
surface coverage (Γ) of Ni /Ni within Ni–2D–O–SA–CNT to
rent densities increasing from 31 to 106 mA cm in the range
−
7
−2
be 1.32 × 10 mol cm , two orders of magnitude above that of
0.5–0.6 V versus Hg/HgO, that is, six-fold higher current den-
sities compared with Ni–2D–SA–CNT. Thus, the introduction
of the carbonyl group (CO) at the ligand environment of the
dispersed Ni-atoms active sites clearly results in much higher
MOR catalytic activities.
−9
−2
Ni–2D–SA–CNT, 5.17 × 10 mol cm (Figure 3a; Equation (S1),
Supporting information).[ Figure 3b; Figure S12, Supporting
Information, display CV curves at different scan rates, from
26]
−
1
1
0 to 100 mV s , obtained with these two catalysts in 1.0 ꢀ
KOH electrolyte. The variation of the anodic and cathodic peak
current densities with the square root of the voltage scan rate
The charge transfer resistances of these two catalysts were
evaluated through electrochemical impedance spectroscopy
(EIS). The EIS Nyquist plot of Ni–2D–O–SA–CNT (Figure 3e)
2
was linearly fitted with R > 0.99 for both samples (Figure S12,
2+
3+
Supporting Information), demonstrating the Ni /Ni redox
reaction to be diffusion limited.[ From this dependence, an
apparent diffusion coefficient D′ was evaluated (see Equation S3,
Supporting Information). The D′ obtained with Ni–2D–O–SA–CNT
presents a smaller semicircle than Ni–2D–SA–CNT at
25]
0.56 V in 1 ꢀ KOH + 1 ꢀ methanol electrolyte, denoting a
lower charge transfer resistance (R ) and thus faster MOR
3
[
27]
kinetics. On the other hand, the charge transfer resistance
for OER of Ni–2D–O–SA–CNT is similar to that of Ni–2D–
SA–CNT at 0.67 V (Figure S13, Supporting Information),
which is consistent with the similar OER activities of these
two catalysts.
−
6
−1 −1
is 2.3 × 10 g cm s , which is close to an order of magnitude
−7
−1 −1
higher than with Ni–2D–SA–CNT, 3.3 × 10 g cm s .
The alcohol electrooxidation performance of Ni–2D–O–SA–
CNT and Ni–2D–SA–CNT electrocatalysts was analyzed by
linear sweep voltammetry (LSV) in 1 ꢀ KOH and 1 ꢀ KOH + 1 ꢀ
methanol solutions using a conventional three-electrode setup
As shown in the chronopotentiometry (CP) curve (Figure 3f),
the Ni–2D–O–SA–CNT electrocatalyst maintained constant
−
2
(Figure 3c,d). The OER activity of Ni–2D–O–SA–CNT was just
current densities of 10 and 30 mA cm for 12 h without a
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Figure 3. a) CV of Ni–2D–O–SA–CNT and Ni–2D–SA–CNT electrocatalysts in 1 ꢀ KOH at 50 mV s−1 potential sweep rate. b) CVs of Ni–2D–O–SA–CNT
−
1
electrocatalysts in 1 ꢀ KOH obtained at an increasing potential sweep rate, from 10 to 100 mV s . c,d) LSV curves obtained with Ni–2D–SA–CNT,
Ni–2D–O–SA–CNT, and reference carbon nanotubes (CNTs) catalysts in 1.0 ꢀ KOH (c) and 1 ꢀ KOH + 1 ꢀ methanol in the potential range 0 to
−
1
0
.9 V versus Hg/HgO at a scan rate of 5 mV s (d). e) EIS Nyquist plots at 0.56 V in 1 ꢀ KOH + 1 ꢀ methanol solution. f) CP long-term stability of
2
−
Ni–2D–O–SA–CNT at 10, 30, and 60 mA cm . No iR compensation was applied to the displayed data.
significant increase in the required potential. When increasing
Ni–2D–O–SA. What is more, upon adsorption, the HOCH
3
−
2
the current density to 60 mA cm , the potential raised ≈4.8%
during 12 h, which is attributed to the methanol consumption.
After 36 h chronopotentiometry test, LSV curves recorded
with the used electrocatalysts in a fresh 1 ꢀ KOH + 1 ꢀ
methanol electrolyte (Figure 3d) showed almost no variation
with respect to those obtained with the fresh electrocatalysts,
what demonstrates the excellent stability of Ni–2D–O–SA–CNT
during MOR. After the stability test, the pH value of the solu-
tion is 14.12, which is not a significant change compared to the
initial solution (pH = 14).
angle in Ni–2D–O–SA increases from 108.25° to 109.04°, while
the HOCH angle in Ni–2D–SA remains almost unchanged
3
after optimization. This result indicates that Ni–2D–O–SA can
facilitate the activation of the methanol molecule. We also iden-
tify the O of the CO group as a methanol adsorption site. DFT
calculations show that the methanol molecule tends to adsorb
at such O site with the OH group in methanol parallel to the
surface. The shortest distance between the methanol molecule
and the surface is 1.87 Å, which is much smaller than in the
Ni–2D–SA surface (2.5 Å). Thus, based on the DFT calcula-
tions, it is reasonable to conclude that the addition of carbonyl
(CO) groups to the organic framework results in more effec-
tive adsorption as well as activation of the methanol molecule,
which explains the higher activity of Ni–2D–O–SA toward MOR
when compared with Ni–2D–SA.
The XRD pattern of Ni–2D–O–SA–CNT after MOR
(
Figure S14, Supporting Information) displayed no significant
variation with respect to the sample before the electrochem-
ical tests, which confirmed the excellent structural stability of
the 2D material and the growth of no additional crystalline
phase of the nickel or nickel oxide nanoparticles during the
reaction, indicating there is no agglomeration of nickel atom
active sites.
Methanol adsorption is the initial and a limiting step in
MOR. To understand the contribution of CO groups to meth-
anol adsorption, density functional theory (DFT) calculations
were carried out. Selected fragments were shown in Figure S15,
Supporting Information. Figure 4 shows the optimal methanol
adsorption configurations on top of a Ni site in both Ni–2D–
SA and Ni–2D–O–SA. The adsorption energy is −0.248 eV and
Beyond MOR, we also examined the electrocatalytic activity
of the produced catalysts toward the oxidation of other alcohols.
Compared with Ni–2D–SA–CNT, Ni–2D–O–SA–CNT presented
significantly higher electrocatalytic performance toward the
electrochemical oxidation of both ethanol (EOR) (Figure 5a)
and benzyl alcohol (BOR) (Figure 5b). Especially for the BOR,
the current density at 0.6 V obtained with Ni–2D–O–SA–CNT
−
2
was ≈77 mA cm , which is ≈eight times higher than that of
−
2
Ni–2D–SA–CNT (9.7 mA cm ).
CP measurements were used to test the stability of
Ni–2D–O–SA–CNTforEOR. As shown inFigureS16, Supporting
Information, Ni–2D–SA–CNT exhibited excellent long-term
stability for EOR, similar to that obtained for MOR. For BOR,
chronoamperometric measurements (Figure S17, Supporting
Information) show the current density decay associated with the
consumption of benzyl alcohol. Notice in this respect that the
initial concentration of benzyl alcohol was just 0.1 ꢀ, since
the steric hindrance of benzene may prevent an efficient
−
0.225 eV in Ni−2D−SA and Ni−2D−O−SA, respectively. The
negative adsorption energies suggest that methanol adsorption
is thermodynamically favored in both cases. Interestingly, the
adsorption distance (d) between the O atom of methanol and
Ni in Ni–2D–O–SA is 3.11 Å, which is shorter than in Ni–2D–
SA (3.27 Å). Compared with Ni–2D–SA, the 10% lower adsorp-
tion energy and 5% shorter adsorption distance indicate a more
effective adsorption of the methanol molecule on the surface of
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Figure 4. a,b) Optimal methanol adsorption configurations on top of the Ni site in both Ni–2D–SA (a) and Ni–2D–O–SA (b). c,d) Optimal methanol
adsorption configurations on top of the carbonyl site in both Ni–2D–SA (c) and Ni–2D–O–SA (d) (red is oxygen, light blue is nitrogen, grey is nickel,
brown is carbon).
reaction at higher concentrations. The EIS spectra for EOR and
BOR were similar to those obtained for MOR, confirming that
Ni–2D–O–SA–CNT is also characterized by a highly efficient
charge transfer and a highly favorable EOR and BOR kinetics
(Figure S20, Supporting Information) of the products from
benzyl alcohol oxidation showed a chemical shift in the range
of ≈7–8 ppm, which is assigned to the characteristic chemical
shift of the proton in the benzene ring of benzoate. The char-
acteristic chemical shift belonging to the aldehyde (CHO)
of benzaldehyde was not observed at the range of ≈9–10 ppm,
demonstrating that the final product of the benzyl alcohol
(Figure S18, Supporting Information).
Finally, in order to confirm the product of the electrochem-
ical oxidation of alcohol, nuclear magnetic resonance (NMR)
tests were conducted to analyze the associated products of
electrochemical oxidation of alcohol. As shown in Figure S19,
Supporting Information, the NMR spectrum after the MOR
stability test displays the characteristic chemical shift of
−
oxidation was benzoate (C H COO ). The NMR spectra of
6
5
the EOR electrolyte did also not display the characteristic
chemical shift of the aldehyde (CHO). The characteristic
−
chemical shift of CH COO appears at 1.83 ppm (Figure S21,
3
−
HCOO at 8.33 ppm, while the characteristic chemical shift
Supporting Information).
of HCHO in the range of ≈9–10 ppm was not observed,
Taking into account the above NMR results, we propose the
following electrochemical reaction for the alcohol electrochem-
ical oxidation (Figure 6):
demonstrating that the final product of methanol oxida-
−
tion is formate (HCOO ). Furthermore, the NMR spectrum
−
1
Figure 5. a) LSV of Ni–2D–SA–CNT, Ni–2D–O–SA–CNT, and CNT in 1 ꢀ KOH+1 ꢀ methanol from 0 to 0.9 V versus Hg/HgO at a scan rate of 5 mV s .
−
1
b) LSV of Ni–2D–SA–CNT, Ni–2D–O–SA–CNT, and the CNT in 1 ꢀ KOH + 0.1 ꢀ benzyl alcohol from 0 to 0.9 V versus Hg/HgO at a scan rate of 5 mV s .
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Conflict of interest
The authors declare no conflict of interest.
Figure 6. Scheme of the electrochemical oxidation reaction of alcohol.
Data Availability Statement
Research data are not shared.
3
. Conclusion
In conclusion, we detailed the synthesis of a new nickel-based
D organic framework and proposed a facile molecular engi-
Keywords
2
neering strategy to tune the ligand coordination environments
of nickel ion centers. Through this strategy, carbonyl (CO)
groups were introduced and proved to decisively contribute
to an efficient alcohol electro-oxidation in alkaline solution.
EXAFS and XANES analysis confirmed the Ni salophen unit
structure, NiN O , within the 2D organic frameworks. The
alcohol oxidation, atomically dispersed metals, electrocatalytic oxidation,
nickel, 2D organic frameworks
Received: July 2, 2021
Revised: August 12, 2021
Published online:
2
2
atomically dispersed nickel within a framework containing the
carbonyl groups displayed outstanding MOR, EOR, and BOR
performance. DFT calculations suggested that the introduc-
tion of the CO functional groups favor the absorption and
activation of methanol molecules, which is at the origin of the
improved MOR observed. The present work not only offers
novel materials and a promising strategy to design more effi-
cient atomically dispersed nickel-based catalysts for alcohol
oxidation, but it could also benefit other research fields related
to electrochemical-organic synthesis, energy conversion and
storage.
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