Defect engineering of nickel hydroxide nanosheets by Ostwald ripening for enhanced selective electrocatalytic alcohol oxidation

Article abstract of DOI:10.1039/c8gc03451f

Selective electrocatalytic oxidation (ECO) of alcohols to aldehydes or acids is an environmentally friendly and economical method in modern industries. Herein, tunable holes and vacancies in nickel hydroxide (h-Ni(OH)₂) by the Ostwald ripening process are successfully fabricated, and the as-prepared electrocatalysts are used for the selective ECO of alcohols into acids or aldehydes with excellent electrocatalytic activity and stability, where a selectivity above 92% to benzoic acid with a benzyl alcohol conversion of 99% on 1.0 h-Ni(OH)₂ was obtained, while a selectivity of >94% to benzaldehyde with a conversion of >90% could also be achieved once the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical was employed. A paired electrolysis system is employed using 1.0 h-Ni(OH)₂ and 1.0 PtO₂/h-Ni(OH)₂ as electrocatalysts at the anode and cathode, respectively, to simultaneously achieve the selective ECO of benzyl alcohol and H₂ production, resulting in a high faradaic efficiency of 192.9%. The excellent ECO activity is mainly due to the abundance of holes and vacancies which facilitate the diffusion rate of the reaction species, adjust the electronic structure and surface properties of h-Ni(OH)₂, and change the charge density around benzyl alcohol, thereby enhancing the adsorption energy of the alcohol. This study introduces a new avenue for the understanding and future design of advanced defect-based electrocatalysts for electrosynthesis and energy conversion.

Full text of DOI:10.1039/c8gc03451f

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Defect engineering of nickel hydroxide nanosheets
by Ostwald ripening for enhanced selective
electrocatalytic alcohol oxidation†
Cite this: DOI: 10.1039/c8gc03451f
Xianlang Chen,
Qiaoqiao Zhang, Guilin Zhuang,
Jian-guo Wang
Xing Zhong, * Bowen Yuan, Suiqin Li, Yongbing Gu,
Xiaonian Li, Shengwei Deng and
*
Selective electrocatalytic oxidation (ECO) of alcohols to aldehydes or acids is an environmentally friendly
and economical method in modern industries. Herein, tunable holes and vacancies in nickel hydroxide
2
(h-Ni(OH) ) by the Ostwald ripening process are successfully fabricated, and the as-prepared electrocata-
lysts are used for the selective ECO of alcohols into acids or aldehydes with excellent electrocatalytic
activity and stability, where a selectivity above 92% to benzoic acid with a benzyl alcohol conversion of
9
9% on 1.0 h-Ni(OH)
90% could also be achieved once the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical was
employed. A paired electrolysis system is employed using 1.0 h-Ni(OH) and 1.0 PtO /h-Ni(OH) as elec-
trocatalysts at the anode and cathode, respectively, to simultaneously achieve the selective ECO of benzyl
alcohol and H production, resulting in a high faradaic efficiency of 192.9%. The excellent ECO activity is
2
was obtained, while a selectivity of >94% to benzaldehyde with a conversion of
>
2
2
2
2
mainly due to the abundance of holes and vacancies which facilitate the diffusion rate of the reaction
Received 5th November 2018,
Accepted 20th December 2018
species, adjust the electronic structure and surface properties of h-Ni(OH) , and change the charge
2
density around benzyl alcohol, thereby enhancing the adsorption energy of the alcohol. This study intro-
DOI: 10.1039/c8gc03451f
duces a new avenue for the understanding and future design of advanced defect-based electrocatalysts
for electrosynthesis and energy conversion.
rsc.li/greenchem
is a very promising green process because of its mild reaction
Introduction
1
7–22
conditions and environmental friendliness.
However, the
Selective oxidation of alcohols into the corresponding carbonyl use of precious metal electrodes (Pt alloy or Pt) or metal-based
1
–11
compounds has attracted significant research attention.
As catalysts entails high costs and metal scarcity during the cata-
important industrial chemicals, benzaldehyde and benzoic lytic process, thereby limiting the development of ECO
2
3–25
acid are widely used not only as intermediates in the pharma- routes.
To overcome these drawbacks, Earth-abundant
ceutical and agrochemical industries but also as dyestuffs and transition-metal compounds, such as (bpy)Cu/2,2,6,6-tetra-
1
2–14
food additives.
Traditional industrial procedures for the methyl-1-piperidine-N-oxyl (TEMPO) co-catalyst systems or
synthesis of benzaldehyde and benzoic acid usually involve TEMPO derivatives, have been used as catalysts for the ECO of
2
2,26,27
oxidation of toluene, heating of phthalate anhydride, or hydro- benzyl alcohol.
Besides, three-dimensional Co O NWs/
3 4
lysis of benzyl chloride, all of which not only utilize hazardous Ti membranes also exhibit outstanding ECO performance for
or corrosive reagents but also release considerable quantities benzyl alcohol, display a high current efficiency toward
1
4–16
of undesired by-products.
Therefore, developing a green benzoic acid, and provide an environmentally friendly protocol
2
8
and sustainable catalytic oxidation mechanism for benzyl for the alcohol oxidation processes. To improve the conver-
alcohol is highly desirable. Electrocatalytic oxidation (ECO) sion and selectivity of the related reactions, investigating and
exploring high-efficiency electrocatalysts with the desired per-
1
5
formance is of great significance.
Institute of Industrial Catalysis, State Key Laboratory Breeding Base of
Green-Chemical Synthesis Technology, College of Chemical Engineering,
Zhejiang University of Technology, Hangzhou 310032, P.R. China.
E-mail: zhongx@zjut.edu.cn, jgw@zjut.edu.cn
Electrocatalytic activity is closely associated with the avail-
ability of active sites. Therefore, maximizing the number of
active sites in a catalyst through defect engineering is highly
desired to improve its performance in the ECO process.
Introducing vacancies and holes to the surface of a catalytic
29
†
Electronic supplementary information (ESI) available: Additional characteriz-
ation, electrochemical measurement results. See DOI: 10.1039/c8gc03451f
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material can effectively adjust its electronic structure and is fixed. The amount of loading of Ni(OH)
2
is determined by
surface properties, increase its active sites, and, thus, remark- measuring the mass of CF before and after the hydrothermal
3
0–39
Controlled defect engineering reaction, which is about 0.625 mg cm−2
2 6 2
Preparation of 1.0 PtO /h-Ni(OH) . H PtCl ·6H O (5 mg) was
ably enhance its activity.
enables the adjustment of the adsorption strength of benzyl
alcohol on electrocatalysts, which can affect the conversion dissolved in 40 mL water under vigorous stirring for 30 min.
and selectivity of ECO. Despite extensive studies on this topic, PtCl ·6H O aqueous solution and 1.0 h-Ni(OH) on CF were
.
2
2
H
2
6
2
2
reports elucidating the influence of holes and vacancies for all transferred into a Teflon-lined stainless steel autoclave
the ECO of benzyl alcohol are rare. Nickel hydroxide (Ni(OH)2)- (50 mL), and maintained at 373 K for 4 h. After being cooled
based materials are well-known and promising electrocatalysts down to room temperature, 1.0 PtO
2
/h-Ni(OH)
and washed with deionized water thoroughly before being
can easily facilitate the decomposition of water to form vacuum dried.
2
was taken out
4
0–44
for the oxygen evolution reaction (OER),
since Ni(OH)
2
adsorbed OH− on its surface in alkaline media.
the surface morphology of the catalyst to modulate the elec- The preparation step is similar to that of h-Ni(OH)
tronic environment of Ni(OH) can influence the adsorption that Co, Cu and Fe nitrates are used instead of nickel nitrate.
45–48
Adjusting
on CF.
2 2 3
Preparation of h-Cu(OH) , h-Co(OH) and h-Fe(OH)
2
, except
2
strength of the reactants, and may present a meaningful strat- The loading of the electrocatalysts of h-Fe(OH)
egy for the selective ECO of benzyl alcohol. and h-Cu(OH) is about 0.600, 0.687 and 0.575 mg cm
Based on this concept, herein, we design a facile hydro- respectively.
thermal method combined with Zn etching via the Ostwald Physicochemical characterization studies. XRD data were
3
, h-Co(OH)
2
−
2
2
,
ripening process to fabricate tunable holes and vacancies on obtained using an XPERT-PRO X-ray diffractometer with Cu Kα
Ni(OH) supported on carbon fiber (CF) to construct 3D nano- (λ = 1.54 Å) radiation. SEM images were obtained with a
2
2
hybrids (denoted as h-Ni(OH) ). The defect structure of the Hitachi FE-SEM S-4700 at 15 kV. TEM images were obtained
nanohybrid can be modulated by etching with Zn components, with a Tecnai G2F30S-Twin electron microscope operating at
thereby providing flexibility to achieve favorable ECO beha- an acceleration voltage of 300 kV. EDX mapping and high
2
viors. The obtained h-Ni(OH) nanohybrid is employed as an angle annular dark field-scanning transmission electron
anode to investigate its performance for the selective ECO of microscopy (HAADF-STEM) were performed on a spherical
benzyl alcohols to their corresponding acids or aldehydes. aberration corrected TEM. XPS measurements were performed
Density functional theory (DFT) calculations indicate that the using a Kratos AXIS Ultra DLD XPS system. Nitrogen absorp-
availability of defects plays a crucial role in modulating the tion–desorption isotherms were obtained using
a
adsorption capability of the catalyst toward the intermediates Quantachrome NovaWin instrument to measure BET specific
involved during ECO. Given its unique structural properties, surface areas. Atomic force microscopy (AFM) was performed
defective h-Ni(OH) exhibits significantly enhanced electro- with a Dimension Icon atomic force microscope. CHI 760E
2
catalytic activity for the selective ECO of benzyl alcohols.
and RRDE-3A from ALS were used for CV experiments. A con-
stant current meter was used for ECO. All gas chromatography
(
(
GC) experiments were carried out and recorded by GC-MS
Agilent Technologies, HP-5). High performance liquid chrom-
Experimental section
Synthetic procedures
atography (HPLC) was used to analyze products using a C18
column (4.6 mm × 150 mm) packed with 5 μm diameter par-
Pre-treatment of CF. The commercial carbon fiber (CF) was ticles (Thermo Scientific™ UltiMate™ 3000).
cut into slices (∼2 cm × 4 cm) and ultra-sonicated in HNO
40 wt%), acetone, ethanol and deionized water for 30 min suc-
3
Electrochemical measurements
HER and OER. The HER and OER measurements were per-
(
cessively. Then the CF was dried at 60 °C for further use. The formed with a conventional three-electrode system using a CHI
size (diameter) of the fibers is about 9 µm (Fig. S1†). 760D (CH Instruments, Inc., Shanghai), with 1.0 h-Ni(OH) on
Preparation of h-Ni(OH) on CF. In a typical preparation, CF (1 × 1 cm ) as the working electrode, a graphite rod serving
2
2
2
−
1
nickel(II) nitrate hexahydrate (25 mmol L ), zinc nitrate hexa- as the counter electrode and Ag/AgCl (in 3 M KCl solution)
hydrate, and urea (metal/urea mole ratios of 1 : 5) were dis- serving as the reference electrode. The electrochemical HER
solved in 40 mL of deionized water to form a uniform solution and OER were performed in H
by constant stirring. Then the CF and solution were transferred KOH solution, respectively.
2 2
-saturated and O -saturated 1 M
into a Teflon-lined stainless steel autoclave (50 mL), and kept
The potentials were calibrated to a RHE scale using the fol-
at 120 °C for 12 h. After the autoclave was cooled to room lowing equation:
temperature, the CF was ultra-sonicated with deionized water
to remove the loosely bound products, and dried at 60 °C
under vacuum for 12 h. Finally, the substrate was etched with
EðRHEÞ ¼ EðAg=AgClÞ þ 0:059 pH þ 0:197
ð1Þ
5
M NaOH for 24 h, followed by drying at 60 °C under vacuum.
All the linear sweep voltammetry measurements were iR-
Different Ni/Zn mole ratios of 1.5, 1.25, 1.0, 0.75 and 0.5 for compensated to account for the voltage drop between the refer-
the synthesis of 1.5 h-Ni(OH) , 1.25 h-Ni(OH) , 0.75 h-Ni(OH) ence and working electrodes. Polarization curves were
.0 h-Ni(OH) and 0.5 h-Ni(OH) , which the molar of Ni metal obtained using LSV with a scan rate of 5 mV s
2
2
2
,
−1
1
.
2
2
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Electrochemical impedance spectroscopy (EIS) measure- projector augmented wave (PAW) potentials and the PBE func-
4
9–54
ments were carried out at 1.25 V vs. RHE from 100 kHz to 1 Hz tional for the exchange–correlation energy.
The kinetic
with an amplitude of 5 mV. The values of TOF were calculated energy cutoff was set to 400 eV. The k-point meshes were
according to the following equation:
j ꢀ S
F ꢀ n
obtained using the Monkhorst–Pack scheme with 4 × 4 × 1. In
order to simulate the benzyl alcohol adsorbed on vacancy-free
TOF ¼
ð2Þ Ni(OH)
2
and oxygen defects of Ni(OH)
2
(V
O 2
-Ni(OH) ), the 4 ×
4
4
× 1 supercell was used for V -Ni(OH) and oxygen defects of
O 2
where J (mA cm− ) is the measured current density at the Ni(OH) . During the geometry optimizations, all layers were
2
2
potentials, S represents the surface area of the working elec- fully relaxed. A vacuum of at least 30 Å was considered along
trode (1.0 h-Ni(OH)
2
on CF), the number 4 stands for a four- the z axis. The convergence criteria of the force and energy are
, F is Faraday’s constant 10 meV Å− and 0.01 meV, respectively.
1
electron transfer per mole of O
2
1
(
96 485.3 C mol− ), and n represents the moles of the metal
For the DFT calculation of all structures, the adsorption
atom on the electrode which can be calculated by using the energies (Eads) of benzyl alcohol on vacancy-free Ni(OH) and
2
mass and the molecular weight of the coated catalysts.
Overall water splitting. The overall water splitting (OWP)
measurement was performed in a paired electrolysis system, in
V -Ni(OH) were calculated by using the equation:
O 2
Eads ¼ Etotal ꢁ Eadsorbate ꢁ Esurf
ð6Þ
2 2 2
which the 1.0 h-Ni(OH) and 1.0 PtO /h-Ni(OH) electrocata-
lysts were used as electrodes for the OER and HER, respect-
ively, and linear sweep voltammetry was performed from 1 to
where Etotal, Esurf and Eadsorbate are the total energies of the
adsorption state system, the total energies of the pure surface
and the total energies of benzyl alcohol, respectively.
−
1
2
V with a scan rate of 5 mV s
ECO of benzyl alcohol. The ECO of benzyl alcohol was con-
ducted in a two-electrode H-cell system with a cationic Nafion
17 membrane. The anode compartment contained 10 mL of 1
.
1
Results and discussion
M KOH with 40 mM benzyl alcohol, and 10 mL of 1 M KOH was
employed in the cathode compartment. The ECO electrolysis A schematic of the preparation of the h-Ni(OH)
was carried out at room temperature using a constant current of talyst with holes and vacancies is shown in Scheme 1. First,
0 mA until complete consumption of the substrate. 3 mmol% NiZn(OH) arrays were uniformly grown on CF through a facile
TEMPO was employed in the anode for selective electrocatalytic hydrothermal method in which urea, Zn(NO , and Ni(NO )
3 2
/CF electroca-
2
2
2
)
3
2
alcohol oxidation. To analyze the products, 150 μL of the elec- are dissolved as raw materials in 40 mL of water. The solution
trolyte solution with 150 μL ethyl acetate was periodically col- and a piece of clean CF were transferred to a 50 mL Teflon-
lected during the ECO reaction. The products were analyzed by lined stainless steel autoclave and heated at 120 °C for 12 h.
GC (FuLi GC9790II) to calculate the conversion of benzyl Urea decomposition produces hydroxyl, ammonia, and carbon-
2
+
alcohol and selectivity of benzoic acid and benzaldehyde.
ate, thus providing an alkaline environment in which Ni and
2
+
The benzyl alcohol conversion (%) and the selectivity (%) of Zn are precipitated during the hydrothermal reaction to form
the oxidation products were calculated using eqn (3) and (4):
2 2
NiZn(OH) arrays. Then, the as-obtained NiZn(OH) /CF was
immersed in 5 M NaOH for 24 h, and the Zn component was
selectively removed. Tiny holes and a large number of
moles of benzyl alcohol reacted
Conversion of benzyl alcohol ¼
initial moles of benzyl alcohol
vacancies were expected to form in h-Ni(OH)
sition in the Ni(OH) crystal structure with uniform orientation
via an in situ Ostwald ripening process was observed.
2
/CF, and a tran-
ð3Þ
2
4
8,55
moles of oxidation products
moles of benzyl alcohol reacted
For
Selectivity of oxidation products ¼
ð4Þ
The faradaic efficiency of benzaldehyde formation was
calculated using the equation:
m ꢀ n ꢀ F
Faraday efficiency ¼
ð5Þ
I ꢀ t
Where m is the number of moles of the product, n is the
number of electrons obtained from the reactant to the
−1
product, F is Faraday constant (96 485 C mol ), I is the
current, and t is the time.
Computational section
Density functional theory (DFT) calculations were performed
using the Vienna ab initio simulation package (VASP) code with Scheme 1 Schematic fabrication process of h-Ni(OH) .
2
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comparison, the effects of different Ni/Zn precursor molar (Fig. S3†). After Zn removal, the obtained 1.0 h-Ni(OH)
ratios were investigated (Experimental section, ESI†). into porous nanoflakes on CF (Fig. 1b–d). The TEM images of
X-ray diffraction (XRD) was performed to investigate the 1.0 h-Ni(OH) and 1.0 NiZn(OH) are shown in Fig. S4.† In
crystal structure of pristine 1.0 NiZn(OH) and 1.0 h-Ni (OH) Fig. S5–S8,† the SEM images of the as-synthesized h-Ni(OH)
2
2
turned
2
2
2
2
.
In Fig. 1a, the XRD pattern of 1.0 NiZn(OH)2 presents five electrocatalysts with different Ni/Zn precursor molar ratios
obvious different diffraction peaks that could be attributed to before and after Zn removal can be observed. Ostwald ripening
standard α-Ni(OH)
2
(JCPDS Card No. 380715) and occurred during the preferential etching of Zn components,
Zn (CO ) (OH) (JCPDS Card No. 191458); the peaks at 12.80°, and the dissolved Ni species were redeposited onto the surface
5
3 2
6
2
2
4.53°, 33.46°, 36.19°, and 59.98° are consistent with the of Ni(OH) , resulting in the formation of an abundance of
5
6
48,57,58
results of previous studies. Compared with 1.0 NiZn(OH)
2
,
well-distributed holes.
As revealed by the atomic force
the diffraction peaks of 1.0 h-Ni (OH) were observed at 19.25°, microscopy (AFM) image in Fig. 1e, the thickness of 1.0 h-Ni
2
3
2
3.06°, 38.54°, 52.10°, 59.05°, and 62.72°, respectively, corres- (OH) was approximately 12.5 nm. The uniform size distri-
ponding to the (001), (101), (102), (102), (110), and (111) bution of the defects can be observed in Fig. 1f, and the
5
6
planes of hexagonal β-Ni(OH) (JCPDS Card No. 140117). No selected area electron diffraction (SAED) pattern shown in the
2
diffraction peaks for Zn
observed, which demonstrates that Zn
removed from the products and that α-Ni(OH) was unstable obtained for 1.0 h-Ni (OH) (Fig. 1g), and holey defects were
5
(CO
3
)
2
(OH)
6
and α-Ni(OH)
2
were inset in Fig. 1f reveals the crystallinity of h-Ni(OH)
2
with dis-
2
+
was completely crete spots. High-resolution TEM (HRTEM) images were
2
2
during Ostwald ripening because it is easily transformed into found. By contrast, no such defects could be found in 1.0 NiZn
4
7
the β phase under alkaline conditions. The diffraction peaks (OH)
2
(Fig. S9†). Lattice fringes with a spacing of 0.24 nm
of 0.5, 0.75, 1.25, and 1.5 h-Ni(OH) were identical to those of could be assigned to the (101) plane of 1.0 h-Ni (OH) , and
2
2
1
2
.0 h-Ni(OH) (Fig. S2†). To investigate the structural character- uniformly distributed holey defects with sizes of about 3–4 nm
ization and morphology of the samples, scanning electron could be distinctly recognized (Fig. 1g). These findings demon-
microscopy (SEM) and transmission electron microscopy strate that the selectivity of Zn removal in 1.0 NiZn(OH)2
(
(
TEM) were performed. The SEM images show that 1.0 NiZn induces holey defects, which could help accelerate electrolyte
5
9,60
OH)
2
with a network structure was uniformly grown on CF penetration and the release of gas bubbles.
These features
Fig. 1 (a) XRD results of 1.0 h-Ni(OH)
2
and 1.0 NiZn(OH)
. (f, g) HRTEM images of 1.0 h-Ni(OH)
showing the homogeneous distribution of Ni and O without Zn. (i) Dispersions of 0.5, 0.75, 1.0, 1.25, and
(from left to right) in ethanol displaying the Tyndall effect.
2
. (b–d) SEM images of 1.0 h-Ni(OH)
2
. (e) AFM image of 1.0 h-Ni(OH)
2
. Inset: The corres-
ponding line-scan profile of 1.0 h-Ni(OH)
2
2 2
. Inset of (f): The SAED pattern of 1.0 h-Ni(OH) . Scale bar: 5
−1
1
1
nm . (h) EDS images of 1.0 h-Ni(OH)
.5 h-Ni(OH)
2
2
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are beneficial for the electrocatalytic performance of the about 0.3 eV compared with those of 1.0 NiZn(OH)
obtained nanohybrid. In Fig. S10–S12,† 0.5 h-Ni(OH)2, indicating that the Ni content of h-Ni(OH)2 has a modified
.75 h-Ni(OH) , and 1.25 h-Ni(OH) exhibited holey defects local electronic structure, which may be used to approximate
2
similar to those of 1.0 h-Ni(OH) The O 1s peak of 1.0 NiZn(OH) is
2
, thereby
0
2
2
6
2,63
2
. In addition, the TEM the ideal Ni–OH phase.
elemental mappings demonstrated the uniform distribution of positively shifted by about 0.3 eV, which is in agreement with
Ni and O and the insignificant signal of Zn (Fig. 1h), which the results of the Ni 2p spectrum (Fig. 2c). Fig. 2d shows the
means that the Zn component could be effectively removed O 1s XPS of different h-Ni(OH) samples; here, the main peaks of
2
through alkaline etching. The atomic ratio of Ni and O was ver- O1, O2, O3, and O4 at 532.3, 531.5, 530.7, and 529.8 eV corres-
ified by the corresponding energy dispersive X-ray (EDX) spec- pond to surface O–CvO, adsorbed OH, lattice OH, and lattice
6
4,65
trum (Fig. S13†). These results demonstrate that h-Ni(OH)
with abundant holey defects was successfully prepared. The the sum of lattice OH and O is distinctly less than that of
porous structure of this material can increase its surface area, 1.5 h-Ni(OH) , which indicates the presence of more oxygen
thereby providing an abundance of active sites and facilitating vacancies on the surface of 1.0 h-Ni(OH)
mass transport. The Tyndall effect can be observed when an samples also showed that their sum of relative contents of
ethanolic suspension of the sample is irradiated with a laser lattice OH and O was lower than that of 1.5 h-Ni(OH)
beam (Fig. 1i), which demonstrates the excellent dispersibility (Table S1†). These results confirm the presence of oxygen
and colloidal nature of these h-Ni(OH) samples. vacancies on the surface of all holey Ni(OH)2 samples. To
X-ray photoelectron spectroscopy (XPS) characterization determine the specific surface areas of the h-Ni(OH) samples,
studies were conducted to study the valence bond structures of their N adsorption–desorption isotherms were determined. In
Ni and O in the different samples. Fig. 2a shows the XPS Fig. 2e, the plots of the as-synthesized h-Ni(OH) samples
2
O, respectively.
2
In the O 1s XPS spectrum of 1.0 h-Ni(OH) ,
2
6
5
2
.
Other h-Ni(OH)
2
2
2
2
2
2
6
6
2
spectra of the as-synthesized h-Ni(OH) samples; the peaks show a type-IV isotherm with a hysteresis loop, which indi-
obtained indicate the presence of O and Ni. Fig. 2b shows the cates a mesoporous size distribution. The Brunauer–Emmett–
Ni 2p XPS spectra of 1.0 NiZn(OH) ; here, the peaks at 855.7 Teller (BET) specific surface areas of the 0.5, 0.75, 1.0, 1.25, and
2
and 873.5 eV are attributed to the peaks of Ni 2p3/2 and Ni 2p1/2
,
2
1.5 h-Ni(OH) samples are 31.01, 37.94, 63.89, 88.92 and
respectively. Two satellite peaks located at 861.5 and 879.8 eV 108.19 m g−1, respectively (Table S2†). Differences in the
2
6
1,62
are assigned to the same species.
These four peaks are specific surface area of the samples are attributed to differences
negatively shifted in 0.5, 0.75, 1.0, 1.25, and 1.5 h-Ni(OH)
2
by in their Zn content. The Barrett–Joyner–Halenda (BJH) curves of
the pore size distributions of the samples further confirm that
the majority of the holes are about 3–4 nm in size (Fig. 2f),
which fairly satisfactorily matches the HRTEM results. The
2
unique porous features of h-Ni(OH) can provide numerous
channels, promote contact between the catalyst and the electro-
lyte, and lead to rapid charge and mass transport during electro-
lysis, all of which are crucial for improving ECO performance.
The electrocatalytic OER activities of h-Ni(OH)2 were
studied to investigate the effect of defects and vacancies on its
electrocatalytic performance by using a typical three-electrode
device in 1 M KOH aqueous solution. The linear sweep voltam-
metry (LSV) curves of the samples are shown in Fig. 3a. The
polarization curve of all samples shows a peak at around 1.45 V
(
versus that of a reversible hydrogen electrode, RHE), corres-
2
+
3+
ponding to the redox reaction of Ni /Ni ; this result indicates
that the electrocatalytically active β-NiOOH phase,
is vital for the high activity for the OER, is rapidly and sub-
stantially formed. The anodic current density increased dra-
matically under a more positive potential, thus suggesting the
excellent OER activity of the electrocatalyst. As seen in Fig. 3a,
among those of the other samples, the OER performance of
5
6,67,68
which
6
9
1.0 h-Ni(OH)
2
is the best. This phenomenon indicates that
1.0 h-Ni(OH)2 features the optimal number of holes and
oxygen vacancies (Fig. 1f and Table S1†). The electrocatalytic
ability of the holey Ni(OH) is clearly significantly related to
the etched Zn content. The characteristics of Ni-bonding
changed successively during the preferential etching of Zn
components, creating more active sites around holey defects,
causing changes in the local electronic structure, and provid-
2
Fig. 2 (a) XRD spectra of different h-Ni(OH)
spectra of Ni and O in different h-Ni(OH) samples. (d) O 1s XPS spectra
of different h-Ni(OH) samples. (e, f) N adsorption–desorption iso-
therms and the corresponding BJH pore size distributions of different
h-Ni(OH) samples.
2
samples. (b, c) XRD
2
2
2
2
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0.5 h-Ni(OH)
2
,
0.75 h-Ni(OH)
2
,
1.25 h-Ni(OH)
2
,
and
1
.5 h-Ni(OH) , respectively; this result further demonstrates
2
the outstanding activity of the 1.0 h-Ni(OH)
the OER. As can be seen from Fig. S18,† the current density of
.0 h-Ni(OH) exhibits no noticeable degradation after 10 h of
2
electrocatalyst for
1
2
continuous OER operation. The TEM images of 1.0 h-Ni(OH)
2
after the OER demonstrate that the holey defects are main-
tained (Fig. S19†). These results prove that holes and oxygen
vacancies in the catalyst can improve its charge-transfer capa-
bility and electrical conductivity and provide specific active
sites, thereby improving its electrocatalytic activity and stability.
Based on the OER results, the electrocatalytic properties of
the hybrids for the ECO of benzyl alcohol were evaluated. The
LSV of 1.0 h-Ni(OH) was obtained in the presence and
2
absence of 40 × 10− M benzyl alcohol in 1 M KOH solution, as
shown in Fig. 3c. Interestingly, once benzyl alcohol was added
to the reaction system, the current density and anodic oxi-
dation peak significantly increased. The CV results before and
after the addition of 40 × 10− M benzyl alcohol are displayed in
the inset of Fig. 3c. A noticeable positive shift value of the
anodic oxidation peak is 0.05 V and an increase in the peak
current could be observed after the addition of benzyl alcohol.
3
3
2
Fig. 3 (a) OER polarization curves of the different h-Ni(OH) samples.
(
b) Highest current density of the redox peaks of different h-Ni(OH)
2
2
samples. (c) OER LSV curves of 1.0 h-Ni(OH) in the absence and pres-
−3
ence of 40 × 10 M benzyl alcohol in 1.0 M KOH. Inset: CV curves. (d) These results reveal that the ECO of benzyl alcohol was dramati-
Conversion and selectivity for ECO at a continuous current of 20 mA in cally much easier over 1.0 h-Ni(OH)
than the OER. Oxygen
2
−3
4
0 × 10 M benzyl alcohol with 1.0 M KOH. (e) Conversion, selectivity,
and FE obtained from different electrocatalysts. (f) Conversion, selecti-
vity, and FE obtained after 10 reaction cycles using 1.0 h-Ni(OH)
vacancies as active sites could provide the optimal adsorption
energy to benzyl alcohol and promote the ECO process.
2
.
2
Moreover, Fig. S20a† shows the CV curves of 1.0 h-Ni(OH) from
−1
1
1
.03–1.83 V at various potential sweep rates of 10–50 mV s in
M KOH solution. The anodic and cathodic peak potentials
ing remarkable OER performance. The quasi-reversible oxi- shifted toward more positive and negative directions, respect-
dation and reduction peaks obtained could be attributed to ively, with increasing scan rate. Fig. S20b† shows that the
the conversion from Ni(OH) to active NiOOH, and continuous anodic peak current density (I) in Fig. S20a† has a linear
2
reduction to Ni(OH)
voltammogram (CV) curve (Fig. S14†). Moreover, it is remark- ECO of benzyl alcohol is controlled by adsorption.
able that 1.0 h-Ni(OH) exhibited a much higher intensity of
The concentration profiles of the reactant and products
2
could be observed in a typical cyclic relationship with the scan rate (V), thereby indicating that the
2
this oxidation peak with a greater peak area than the other were changed at different reaction times over 1.0 h-Ni(OH) to
2
samples, which indicates the substantial and fast transform- explore the ECO of benzyl alcohol under a continuous current
ation from Ni(OH) to the more active NiOOH. The highest oxi- (Fig. 3d). The results indicate that benzoic acid, which is
2
dation peak values acquired from the electrocatalysts with stable, is the final product and that the concentration of
different Ni/Zn ratios showed a volcano plot (Fig. 3b). The benzoic acid increases with time. Benzaldehyde is the inter-
electrocatalytic kinetics were supported by the Tafel slopes mediate product, and its concentration initially increases and
2
obtained. The Tafel slope of 1.0 holey Ni(OH) , at 114 mV then decreases after peaking during electrolysis. To compare
−
1
dec (Fig. S15†), is the lowest among those of 0.5 h-Ni(OH)
2
the electrocatalytic activities of samples with different Ni/Zn
255 mV dec− ), 0.75 h-Ni(OH) (160 mV dec ), 1.25 h-Ni ratios, the ECO of benzyl alcohol was performed under similar
OH)
1
−1
(
(
2
(158 mV dec− ), and 1.5 h-Ni(OH)
1
(189 mV dec−1). This optimal conditions using these catalysts (Fig. 3e and S21†).
2
2
result demonstrates the rapid OER kinetics of 1.0 h-Ni(OH)
2
.
2
Among the samples tested, 1.0 h-Ni(OH) achieved the highest
The outstanding performance of this catalyst was proven by conversion rate (99.99%) and selectivity (99.30%) for the ECO
the electrochemical impedance spectroscopy (EIS) data. The of benzoic acid; these values are higher than those of
charge transfer resistance of 1.0 h-Ni(OH) was much lower 0.5 h-Ni(OH) (98.71% and 73.34%), 0.75 h-Ni(OH) (99.55%
2
2 2
than those of the other as-synthesized h-Ni(OH) electrocata- and 83.95%), 1.25 h-Ni(OH)2 (99.42% and 93.73%), and
2
lysts (Fig. S16†) because the abundance of holes and oxygen 1.5 h-Ni(OH) (98.18% and 92.94%). The FE of each catalyst
2
vacancies in the former facilitates charge transfer. for the ECO process was calculated. The 1.0 h-Ni(OH) revealed
2
Furthermore, Fig. S17† shows the turnover frequency (TOF) an FE of 98.62%, which is much higher than those of the
estimation. At the highest anodic oxidation peak of other samples. These results indicate that conversion, selecti-
0.67 mV cm− , the TOF of 1.0 h-Ni(OH)
2
is 127 h , which is vity, and FE are significantly influenced by the Ni/Zn ratio and
0, 4.85, 1.42, and 1.76 times higher than those of the oxygen vacancies can promote ECO. We also increased the
−1
9
2
2
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benzyl alcohol concentration (40 × 10− M) to view the poten-
tial for industrial implementation; benzyl alcohol is still com-
pletely converted at a current density of 200 mA, with 99.5%
conversion and 99.1% selectivity in 130 min (Fig. S22†). In
addition, the ECO performance of the catalysts for benzyl
alcohol is consistent with the redox peak observed in Fig. 3b.
2
When Zn is not removed from 1.0 NiZn(OH)
exhibits lower conversion (95.39%), selectivity (87.97%) and FE
88.62%) compared with those of 1.0 h-Ni(OH) under the
2 2
, 1.0 NiZn(OH)
(
2
same conditions (Fig. S23†). This observation verifies that
holes and oxygen vacancies could promote the electrocatalytic
ECO activity of the catalysts. 1.0 h-Ni(OH) showed high stabi-
2
lity in 1 M KOH and could be recycled for up to 10 consecutive
catalytic runs. Its conversion rate (>99%) remained constant
and its selectivity and FE for benzoic acid were >92% and
>
95%, respectively (Fig. 3f). The TEM and SEM (Fig. S24†)
images of 1.0 h-Ni(OH) showed no obvious change after the
2
recycling experiments, which means the catalyst could be a
stable electrocatalyst for the ECO of benzyl alcohol. A compari-
son of 1.0 h-Ni(OH) and other electrocatalysts for the ECO of
2
benzyl alcohol to benzoic acid is shown in Table S3.† In order Fig. 4 (a) LSV polarization curves of 1.0 h-Ni(OH)
2
in 1.0 M KOH in the
absence and presence of 40 × 10 M benzyl alcohol and TEMPO. (b)
−3
to further explore the applications of 1.0 h-Ni(OH)
formance in the ECO of a series of alcohols was tested
Table 1). Good-to-excellent product yields were obtained for
the oxidation of different kinds of alcohols to their corres-
ponding acids. In most cases, the catalyst could achieve >90% 1.0 h-Cu(OH)
conversion and >90% selectivity for the acid within ≤200 min.
The excellent conversion and selectivity of 1.0 h-Ni(OH) may
be due to the presence of abundant holes and oxygen
vacancies, which promote the O–H cleavage of alcohols and profiles of benzaldehyde (97.45%), and the FE (75.86%)
accelerate the ECO process.
obtained under a constant current of 20 mA are presented in
2
, its per-
Conversion and selectivity during ECO at a constant current of 20 mA in
1
.0 M KOH with TEMPO and 20 × 10− M benzyl alcohol. (c) ECO con-
3
(
version, selectivity, and FE of 1.0 h-Ni(OH)
with TEMPO. (d) OER LSV curves of 1.0 h-Ni(OH)
, and 1.0 h-Fe(OH) . (e) ECO conversion and selectivity of
different electrocatalysts. (f) ECO FE of different electrocatalysts.
2
after 10 successive cycles
2
, 1.0 h-Co(OH) ,
2
2
2
2
Selective ECO of benzyl alcohols to benzaldehydes is a very Fig. 4b and S25.† The results demonstrate that TEMPO can
important reaction in the pharmaceutical and fine chemicals facilitate the selective ECO of benzyl alcohol to produce benz-
industries. Herein, we employed TEMPO in the anode to deter- aldehyde within a specified time. As depicted in Fig. 4c,
mine its effect on the selective ECO of benzyl alcohol over 1.0 h-Ni(OH)
.0 h-Ni(OH)
(Fig. 4a). A high anodic oxidation peak and an tive catalytic cycles without significant loss, which illustrates
early initial potential of 1.0 h-Ni(OH)
could be observed its robustness for the selective ECO of benzyl alcohol. We com-
during ECO after the introduction of TEMPO and benzyl pared the electrocatalytic performance of different metal
2
displays notably high activity after 10 consecu-
1
2
2
alcohol. The conversion of benzyl alcohol (92.0%), the product hydroxides (Fig. 4d) and found that 1.0 h-Ni(OH)
2
possesses
Table 1 ECO of various alcohols by using 1.0 h-Ni(OH)
2
Entry
1
Substrate
Product
Time (min)
135
Conv. (%)
99.4
Sel. (%)
98.6
FE (%)
94.1
2
3
135
150
92.3
92.7
95.1
92.1
85.8
76.4
4
5
200
135
98.8
96.5
98.5
93.2
94.9
88.8
6
130
99.9
99.3
98.6
Reaction conditions: Alcohol (40 mmol L− ), 1.0 h-Ni(OH)
1
(0.625 mg cm−2), current density of 20 mA, 1 M KOH. The products were determined
2
by GC/MS and HPLC analysis.
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3 2
much better OER performance than Fe(OH) , Co(OH) , or
Cu(OH) ; it also showed the best ECO performance for benzyl
2
alcohol among these hydroxides (Fig. 4e and f).
We studied the in situ formation of ultrafine PtO
particles (NPs) on 1.0 h-Ni(OH)2 through a simple hydro-
thermal treatment (1.0 PtO /h-Ni(OH) ) to investigate the
2
nano-
2
2
electrocatalytic performance of the system in the hydrogen
evolution reaction (HER). In Fig. S26,† the TEM image of 1.0
PtO
on h-Ni(OH)
2
/h-Ni(OH)
. Fig. S27a† shows the excellent HER activity of
.0 PtO /h-Ni(OH) , which only required an overpotential of
2 2
proves that PtO NPs were successfully loaded
2
1
6
2
2
−
2
1 mV to reach 10 mA cm . The CF and 1.0 holey Ni(OH)
2
delivered the required current densities of 604 and 511 mV
Fig. S27b†), respectively. The outstanding HER performance
of the 1.0 PtO /h-Ni(OH) electrode was supported by the Tafel
plots obtained from the related LSVs (Fig. S27c†). The Tafel
(
2
2
−
1
slope of 1.0 PtO /h-Ni(OH) (67 mV dec ) was much
2
2
−1
lower than those of CF (425 mV dec ) and 1.0 h-Ni(OH)
2
1
(
246 mV dec− ). The FE of 1.0 PtO
was determined through chronoamperometry over a period of
120 s. In Fig. S27d,† the measured amount of H obtained is
very consistent with the theoretical value calculated based on Fig. 5 (a) Schematic of the ECO of benzyl alcohol and HER equipped
the transferred charge, resulting in a high FE of 97.8%. The with 1.0 h-Ni(OH) and 1.0 PtO /h-Ni(OH) , respectively. (b) LSV curves
and 1.0 PtO /h-Ni(OH) electrocatalysts in the absence
2 2
/h-Ni(OH) during the HER
2
2
2
2
2
of 1.0 h-Ni(OH)
2
2
2
stability of 1.0 PtO
2 2
/h-Ni(OH) for the HER was investigated in
Fig. S28,† and only slight degradation could be observed.
These results reveal that 1.0 PtO /h-Ni(OH) , which features an
and presence of 40 × 10− M benzyl alcohol and TEMPO. (c) Schematic
of the paired electrolysis system. (d) Photograph of the electrolyzer for
the ECO of benzyl alcohol and HER. Here, 1.0 h-Ni(OH)2 (right) and 1.0
3
2
2
interfacial synergy between that of PtO
2
and h-Ni(OH)
2
, pos- PtO /h-Ni(OH)2 (left) are used as electrocatalysts. (e) Photograph of the
2
sesses excellent HER activity.
product for the ECO of benzyl alcohol.
According to the above analysis, a two compartment H cell
was employed to be utilized for the ECO reaction of benzyl
alcohol and HER, equipped with 1.0 h-Ni(OH)
h-Ni(OH)2 for the anode and cathode, respectively (Fig. 5a). −0.33 eV, thus implying that benzyl alcohol forms a weak
Electrolysis of 1.0 PtO /h-Ni(OH) and 1.0 h-Ni(OH)
yielded interaction. When V -Ni(OH) is employed, the adsorption
an overall water-splitting (OWP) current density of 10 mA cm
energies of benzyl alcohol increase to −0.78 eV, which is much
at 1.68 V. The electrocatalytic performance was improved when stronger than that on vacancy-free Ni(OH) , consistent with
Meanwhile, the oxygen atom in benzyl
obtain a current density of 10 mA cm at 1.48 V (Fig. 5b). alcohol obtains more charges on V -Ni(OH) than on vacancy-
More importantly, the FE could reach as high as 192.9%. In free Ni(OH) (Table S4†). These results indicate that oxygen
2
and 1.0 PtO
2
/
2
adsorption of benzyl alcohol on vacancy-free Ni(OH) is only
2
2
2
O
2
−
2
2
4
0 × 10− M benzyl alcohol was added to the reaction system to previous studies.
3
70,71
−2
O
2
2
addition, once TEMPO was added to the reaction system, the vacancies enhance the adsorption energy of benzyl alcohol.
1
1
0 mA cm− current density point shifted further toward The analysis results of charge density differences agree well
2
.41 V. This result indicates that the addition of TEMPO is an with the Bader charge (Fig. 6c and d). The electron localization
effective strategy to save energy when replacing the OER with functions (ELFs) of benzyl alcohol on V
the selective ECO of benzyl alcohol and that 1.0 h-Ni(OH)
may free Ni(OH) were analyzed in Fig. 6e and f, and a much stron-
be an ideal ECO electrocatalyst. To obtain a potential approach ger interaction between benzyl alcohol and V
for practical application, a paired electrolysis system equipped that between benzyl alcohol and vacancy-free Ni(OH)
with 1.0 h-Ni(OH) and 1.0 PtO /h-Ni(OH)
cathode, respectively, was employed for the simultaneous ECO Ni(OH)
of benzyl alcohol and HER (Fig. 5c and S29†). A representative by the density of states (DOS) in Fig. 6g, compared to the
photograph of the electrolyzer (Fig. 5d) and the electrolytes vacancy-free Ni(OH) , the Fermi surface of V -Ni(OH) appar-
containing the benzoic acid or benzaldehyde products are ently transfers to the conduction band edge, which confirms
shown in Fig. 5e, indicating its great potential for imple- that the conduction band minimum of V -Ni(OH) can obtain
more carriers. This result manifested that the DOS on the
To reveal the role of oxygen vacancies in h-Ni(OH) in the Fermi surface of vacancy-free Ni(OH) increases significantly,
possesses higher conductivity
and carrier density to facilitate electron transfer during the
O
-Ni(OH)
2
and vacancy-
2
2
-Ni(OH)
than
was
O
2
2
for the anode and observed. Furthermore, the electronic structure of vacancy-free
and Vo-Ni(OH) has been further studied. As shown
2
2
2
2
2
2
O
2
O
2
7
2
mentation at a scale with industrial practicability.
2
2
ECO of benzyl alcohol, DFT calculations were used to investi- demonstrating that V
gate the adsorption of benzyl alcohol on vacancy-free Ni(OH)
and vacancy-rich Ni(OH) (V -Ni(OH) ). In Fig. 6a and b, the ECO process. In general, the DFT calculations revealed that
O 2
-Ni(OH)
2
7
3
2
O
2
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1.0 h-Ni(OH)
2
, and the prepared electrode exhibited remark-
able electrocatalytic performance with superior activity and
prominent durability for OER; the catalyst was also success-
fully applied to the selective ECO of benzyl alcohol in an alka-
line medium under optimal reaction conditions, as well as to
the simultaneous selective ECO of benzyl alcohol and H pro-
2
duction with a high FE in a paired electrolysis system. By
virtue of the high uniform orientation of nanosheets, the
2
abundant holes and oxygen vacancies in h-Ni(OH) provide a
highly exposed surface area and an abundance of active sites,
which are beneficial for mass transfer and improve the electro-
catalytic reaction. DFT calculations confirmed that the for-
mation of oxygen vacancies increases the charge transfer
between V -Ni(OH) and benzyl alcohol, thus facilitating
O
2
increases in adsorption capability toward benzyl alcohol and
promoting ECO. The proposed strategy offers new opportu-
nities to develop defective electrocatalysts for the green chemi-
Fig. 6 DFT-calculated adsorption energies of benzyl alcohol on (a) cal industry and energy conversion.
vacancy-free Ni(OH) and (b) oxygen vacancies (V -Ni(OH) ); the charge
density difference of benzyl alcohol on (c) vacancy-free Ni(OH) and (d)
oxygen vacancies (V -Ni(OH) ), negative and positive charges are pre-
sented in green and yellow, respectively; the ELF of benzyl alcohol on (e)
vacancy-free Ni(OH) and (f) oxygen vacancies (V -Ni(OH) ). The ELF
2
O
2
2
O
2
Conflicts of interest
2
O
2
There are no conflicts to declare.
value is set in the range of 0–1, where 1, 0.5, and 0 respectively rep-
resent covalent, metallic, and no-bonding characteristics; and (g) the
density of states (DOS) of vacancy-free Ni(OH)
-Ni(OH) ). (h) Reaction pathways for the ECO of benzyl alcohol with
.0 h-Ni(OH) as the electrocatalyst. Navy, red and gray colors stand for
2
and oxygen vacancies
(
1
V
O
2
Acknowledgements
2
Ni, O, and C atoms respectively, and white and yellow colors stand for H
atoms.
The authors acknowledge the financial support from the
National
Natural
Science
Foundation
of
China
(
NSFC-21625604, 21776251, 21878272 and 21671172).
the unique defective structure of V -Ni(OH) plays a vital role
O
2
in influencing the adsorption energy of benzyl alcohol, thus
facilitating the entire ECO process and enhancing the electro-
catalytic activity of the nanohybrids.
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