Morphology-control and template-free fabrication of bimetallic Cu–Ni alloy rods for ethanol electro-oxidation in alkaline media

Article abstract of DOI:10.1016/j.jallcom.2020.157438

The design of highly efficient and stable non-noble transition metal-based electrocatalysts for the ethanol oxidation reaction (EOR) is imperative for the development of the direct ethanol fuel cells (DEFCs). In this work, we report a simple template-free method for preparing a type of rod-like Cu–Ni alloy particle with the unique porous structure and evaluate it as the electrocatalyst for the EOR in alkaline media. The pH, which was adjusted by the addition of NH₃·H₂O during the liquid-phase coprecipitation process, was found to be a key factor to shape Cu–Ni alloy precursor into a quasi-one-dimensional morphology. After annealing at a reducing atmosphere (H₂/Ar = 5/95, v/v), well-alloyed Cu–Ni rods with the predefined molar ratio (Cu/Ni) of 1:1, a specific surface area of 6.84 m² g^?1, and the average pore size of 30.97 nm were obtained. Cyclic voltammetry (CV) and chronoamperometry (CA) test results show that the prepared Cu–Ni alloy catalyst demonstrated an anodic current peak of 86.10 mA cm^?2 in the presence of 0.2 M ethanol and a 95% retention of current density after 2000 s, indicating its good electrochemical performance in terms of catalytical activity and long-term stability. This bottom-up synthesis strategy would enrich the fabrication methodologies and open up a promising avenue for preparing multiple Ni-based EOR electrocatalysts with the easy-controllable morphologies and porous structure at the industrial scale.

Full text of DOI:10.1016/j.jallcom.2020.157438

Journal of Alloys and Compounds 855 (2021) 157438
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Morphology-control and template-free fabrication of bimetallic CueNi
alloy rods for ethanol electro-oxidation in alkaline media
,
,
, b,
Zelin Miao a 1, Changfan Xu a 1, Jing Zhan a *, Ziwei Xu a
a School of Metallurgy and Environment, Central South University, Changsha, 410083, China
b National Engineering Laboratory for High Efficiency Recovery of Refractory Nonferrous Metals Resources, Changsha, 410083, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The design of highly efficient and stable non-noble transition metal-based electrocatalysts for the
ethanol oxidation reaction (EOR) is imperative for the development of the direct ethanol fuel cells
(DEFCs). In this work, we report a simple template-free method for preparing a type of rod-like CueNi
alloy particle with the unique porous structure and evaluate it as the electrocatalyst for the EOR in
alkaline media. The pH, which was adjusted by the addition of NH3$H2O during the liquid-phase
coprecipitation process, was found to be a key factor to shape CueNi alloy precursor into a quasi-one-
dimensional morphology. After annealing at a reducing atmosphere (H2/Ar ¼ 5/95, v/v), well-alloyed
Received 15 June 2020
Received in revised form
25 September 2020
Accepted 1 October 2020
Available online 6 October 2020
Keywords:
Coordination-coprecipitation
CueNi alloy rods
Porous nanostructure
Ethanol electro-oxidation
CueNi rods with the predefined molar ratio (Cu/Ni) of 1:1, a specific surface area of 6.84 m2 ꢀ1, and
g
the average pore size of 30.97 nm were obtained. Cyclic voltammetry (CV) and chronoamperometry (CA)
test results show that the prepared CueNi alloy catalyst demonstrated an anodic current peak of
86.10 mA cmꢀ2 in the presence of 0.2 M ethanol and a 95% retention of current density after 2000 s,
indicating its good electrochemical performance in terms of catalytical activity and long-term stability.
This bottom-up synthesis strategy would enrich the fabrication methodologies and open up a promising
avenue for preparing multiple Ni-based EOR electrocatalysts with the easy-controllable morphologies
and porous structure at the industrial scale.
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
CueNi alloy, as one of the Ni-based electrocatalysts, exhibits
several advantages including good surface oxidation properties,
Due to the large consumption of fossil fuels and environmental
pollution, direct ethanol fuel cells (DEFCs), an environmentally-
benign and highly-efficient energy conversion device, have drawn
increasing attention with the great potentials to be applied in
portable electronics and electrical vehicles [1e3]. However, the
sluggish kinetics of ethanol oxidation reaction (EOR) has always
been regarded as a critical issue in DEFCs, which needs to be cir-
cumvented by the utilization of high-performance electrocatalysts
[4,5]. While many pieces of researches focused on Pt/Pd and their
alloys or composites [4e8], the deficient global reserves of noble
metals and their vulnerability to CO poisoning have severely
restricted industrial-scale manufacturing and commercial utilities
of these materials [9e11].
excellent electrical conductivity, and superior corrosion tolerance
in alkaline media [12,13], making it a popular choice for numerous
energy conversion processes such as methane decomposition [14],
hydrogen evolution reaction (HER) [15,16], and oxidation reaction
of small organic molecules (SOMs) [17e19]. When catalyzing
electro-oxidation of methanol, CueNi alloy was found to exhibit a
low affinity of CO, which significantly ameliorates the self-
poisoning issue facing by the noble-metal-based materials [18].
Moreover, the addition of the Cu component could properly modify
the electronic structure of Ni and suppress the formation of
destructive intermediate
g-NiOOH during the electrocatalytic
process, leading to improved activity and stability for electro-
oxidation of SOMs [17]. For example, Habibi et al. [19] fabricated
dendritic-morphology CueNi alloy nanoparticles via an electro-
deposition approach, which showed higher anodic current density
compared with pure Ni towards the oxidation of glycerol.
* Corresponding author. School of Metallurgy and Environment, Central South
University, Changsha, 410083, China..
Apart from the manipulation of elemental composition, the
performance of electrocatalysts can be further improved by
E-mail address: zhanjing@csu.edu.cn (J. Zhan).
The authors have equal contributions.
1
https://doi.org/10.1016/j.jallcom.2020.157438
0925-8388/© 2020 Elsevier B.V. All rights reserved.

Z. Miao, C. Xu, J. Zhan et al.
Journal of Alloys and Compounds 855 (2021) 157438
optimizing the morphology and structure, i.e., to help expose an
increased density of active sites on the surface of the corresponding
materials [4,20]. Resulting from Ostwald ripening and potential
coalescence of particles through migration [21,22], zero-
dimensional (0D) materials/nanoparticles with high surface en-
ergy usually present poor stability, which can be alleviated by
endowing the electrocatalysts with one-dimensional (1D)
morphology. Meanwhile, the simultaneously increased electrical
conductivity originated from the interconnection of the anisotropic
conductor could facilitate electron transportation and further
eliminate the usage of carbon supports [20,23]. Furthermore, the
addition of the porous nanostructure with the increased specific
surface area (SSA) is expected to promote the mass transfer such as
diffusion of ions at the interface of electrode and electrolyte
[10,24,25]. Although many fabrication methods for CueNi alloy
have currently been reported such as mechanical alloying [26],
electrodeposition [27], laser ablation [28], spray pyrolysis [29], etc.,
most of them failed to prepare CueNi alloy with the aforemen-
tioned 1D morphology and porous structure. Template-based
methods may achieve this goal by applying easy-etched tem-
plates, e.g., anodic alumina membranes (AAMs), to precisely control
the morphology and particle size distribution [30]. However, one
should notice that the preparation of the ordered AAMs was a two-
step anodizing process, which would undoubtedly impose
complexity and increased cost to the whole fabrication procedure.
Thus, the exploration of new methods for the fabrication of 1D
porous CueNi alloy with low cost, mild temperature, and facile
control over the morphology and particle size is still indispensable.
Herein, we reported a simple template-free strategy for pre-
paring 1D porous CueNi alloy particles by combining coprecipita-
tion of metal ions (i.e., Cu2þ and Ni2þ) into CueNi alloy precursors
from Cu2þ-Ni2þ-NH3-NHþ4 -C2O42--Cl--H2O system, with an anneal-
ing process. However, in this reaction system, four orders of
magnitude difference of cumulative formation constants of Cu/Ni
complexes with NH3 ligand indicates their thermodynamically
unfavorable simultaneous precipitation from the liquid phase at the
high pH [31], leading to the hard control over elemental distribu-
tion and metallic ratio in CueNi alloy precursors and the corre-
sponding alloy products. In this work, this issue was solved by
applying the ethanol-H2O mixture as the modified solvent instead
of pure H2O, and the efficacy of pH on the morphology of precursors
was systematically investigated. Further, the NH3-induced oriented
growth mechanism for rod-like CueNi oxalate complex salt was
proposed. Following thermal decomposition in a reducing atmo-
sphere, bimetallic CueNi rods with the predefined molar ratio (Cu/
Ni) of 1:1 and a hierarchically porous structure were obtained.
Their application as electrocatalysts for the EOR in NaOH solution
was subsequently evaluated by cyclic voltammetry (CV) and chro-
noamperometry (CA) techniques.
was stirred for an additional 2 h. Precipitated solids were then
filtered, rinsed several times with DI water and ethanol, and dried
in a vacuum oven at 353 K for 12 h. Finally, the precursors were
decomposed in a reducing atmosphere (H2/Ar ¼ 5/95, v/v) at 673 K
for 30 min to obtain the porous bimetallic CueNi rods. Ni powders
with a similar quasi-1D porous fashion were also prepared and the
corresponding physical characterizations were reported in our
previous work [32,33].
2.2. Materials characterizations
The morphology and elemental ratio/distribution of the samples
were systematically investigated by transmission electron micro-
scopy (TEM, Tecnai G2 20ST), scanning electron microscopy (SEM,
JSM-6360LV), and the affiliated energy-dispersive X-ray spectros-
copy (EDX, GENESIS 60S). The powder XRD patterns of the pre-
cursors and final products were recorded by an X-ray
diffractometer (Rigaku-TTRIII X, Cu,
l
¼ 0.154056 nm) with the
scanning rate of 10ꢁ minꢀ1. Characterization of the chemical
composition of CueNi alloy precursors was achieved by Fourier
transform infrared spectrometry (FTIR, Nicolet 6700) in the wave-
numbers ranging from 400 to 4000 cmꢀ1. The thermal decompo-
sition profile of the rod-like CueNi alloy precursor (obtained at
pH ¼ 7.6) was recorded by thermogravimetry and differential
thermal analyses (TA, SDTQ). X-ray photoelectron spectroscopy
(XPS, Thermo Scientific K-Alpha) with Al Ka as the excitation source
was utilized to probe the surface chemical composition and valence
states. The Brunauer-Emmett-Teller (BET) surface area and the
distribution of pore size were measured by the surface area and
porosity analyzer (Micromeritics ASAP 2020).
2.3. Electrochemical measurements
The electrocatalyst ink was prepared by ultrasonic dispersion of
25 mg of CueNi alloy powders in 1 mL DI water. Then, 4 mL of this
suspension was deposited onto the surface of the glassy carbon
electrode (GCE, diameter: 3 mm) that had been mechanically pol-
ished by 50 nm
g-Al2O3 slurry and rinsed with DI water and ethanol
for several times. Nafion solution (5 wt%, 2
mL) was added on the
top of the dried electrocatalyst ink to obtain the modified electrode
(CueNi/GCE). Ni/GCE was also prepared using the same drop-
casting method. All electrochemical measurements were per-
formed at room temperature (298 2 K) with an electrochemical
workstation (CHI660B) using a conventional three-electrode setup.
The modified glass carbon electrode (CueNi/GCE and Ni/GCE) with
a mass loading of 1.41 mg cmꢀ2, a saturated calomel electrode
(SCE), and a Pt electrode (1 cm ꢂ 1 cm) served as working, refer-
ence, and counter electrode, respectively.
3. Results and discussions
2. Experimental
3.1. Shape-controlled synthesis of CueNi alloy precursors
2.1. Materials preparation
The CueNi alloy rods were synthesized through a coordination-
coprecipitation procedure coupled with the thermal decomposition
in a reducing atmosphere as illustrated in Fig. 1, where the liquid-
All chemical reagents were of analytical grade and used without
further purification. In a typical process, H2C2O4$2H2O (13.010 g,
0.100 mol) and polyvinylpyrrolidone (PVP, 0.500 g) were dissolved
in a solvent mixture comprising 50 mL ethanol (EtOH) and 50 mL
deionized (DI) water. To this was injected a solution of NiCl2$6H2O
(9.702 g, 0.040 mol), 7.842 g of CuCl2$2H2O (7.842 g, 0.046 mol) and
PVP (0.500 g) in 50 mL ethanol and 50 mL DI water, at the constant
rate of 1.67 mL minꢀ1 in 60 min. The reaction mixture was me-
chanically stirred in a water bath maintained at 323 K, during
which the pH was carefully adjusted to 2.0e8.0 by dropwise
addition of NH3$H2O. After completion of feeding, the suspension
phase precipitation process has
a significant influence on
morphology, composition homogeneity, and elemental distribution
of the final CueNi alloy particles. According to the previous ther-
modynamic analysis of the NieCo system [34], the simultaneous
precipitation of Cu2þ and Ni2þ by C2O42ꢀ with the assistance of NH3
in Cu2þ-Ni2þ-NH3-NH4þ-C2O42--Cl--H2O (298 K) system would also
be a complicated dynamic equilibrium process, during which many
reactions coexist and compete with each other. These include the
coordination of Cu2þ/Ni2þ with NH3 and C2O24ꢀ, hydrolysis of metal
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Z. Miao, C. Xu, J. Zhan et al.
Journal of Alloys and Compounds 855 (2021) 157438
Fig. 1. Schematic illustration of the fabrication process for rod-like porous CueNi alloy.
ions, precipitation of Cu2þ/Ni2þ by C2O24ꢀ, and dissociation re-
actions of weak acids and bases [35]. To determine the theoretical
pH ranges for coprecipitation of Cu2þ and Ni2þ in the water solvent,
a thermodynamic analysis was performed according to the prin-
ciple of matter conservation [34e36] and the calculation process
will be illustrated in another work in detail. It is concluded that due
to the lower dissociation constants of [Cu(NH3)n]2þ(n ¼ 1e5) [35]
compared with [Ni(NH3)m]2þ(m ¼ 1e6) [34], the total concentra-
tion of Cu2þ (free ions and complexes) in solution is higher than
Ni2þ at most time, except for pH ranging from 2 to 6. This makes it
difficult to keep the molar ratio (1:1, for example) of metallic
components in precursors prepared at a higher pH and the corre-
sponding alloy products the same to the ratio of the initial con-
centration of the metal ions added into the system. To expand the
coprecipitation pH range of Cu2þ and Ni2þ, the ethanol-water
mixture was employed as the reaction solvent to replace pure
water since the addition of a low dielectric medium, such as
ethanol, was found to have an obvious effect to adjust the ther-
modynamics of precipitation reaction [37] and to decrease the
solubility of oxalate salts and metal-NH3 complexes [34]. Moreover,
ethanol has lower surface tension compared with water, which is
conducive to reducing the particle size and improving the quality
and dispersion degree of CueNi alloy precursors [38]. These could
also be assisted by adding the proper quantity of PVP, which serves
as the surface stabilizer and dispersion reagent during the nucle-
ation and crystal growth process [39,40]. After the annealing
treatment, CueNi alloy rods with the even distribution of Cu and Ni,
an accurate molar ratio of 1:1, and a unique porous nanostructure
can be finally obtained.
The SEM images of CueNi alloy precursors obtained at different
pH are shown in Fig. 2. It can be observed that the solution pH,
which was initially acidic due to the existence of H2C2O4 and then
adjusted by NH3$H2O, has a prominent effect on the morphology
and particle size of the precursors. At pH ¼ 2.0 (Fig. 2a), these form
as irregular particles intertwined by numerous tiny fibers. At
pH ¼ 5.0 (Fig. 2b), the morphology changes to micro-spheres
agglomerated by nanoparticles. By increasing the pH to 7.6
(Fig. 2c), distinct rod-like CueNi alloy precursors are observed with
an aspect ratio larger than 10. We can conclude that the shape of
precursors could be simply controlled by adjusting the addition of
NH3$H2O as the morphology of precursors experienced from
irregular aggregates to micro-spheres, further to rod-like particles.
However, when the pH reaches to 8.0, the clusters of rods are likely
to be corroded [41], leading to the significant decrease of the uni-
formity of precursors, as shown in Fig. 2d. Meanwhile, some deep
blue crystals were observed after filtration, which are supposed to
be copper oxalate particles derived from the segregated precipita-
tion of Cu2þ at such high pH value. Therefore, to obtain the uniform
rod-like CueNi alloy precursor powders from the modified reaction
system of Cu2þ-Ni2þ-NH3-NH4þ-C2O42--Cl--ethanol-H2O, the pH of
the reaction system should be maintained at 7.6.
obtained at different pH values. CueNi oxalate salts in the absence
of NH3$H2O were also synthesized and measured for comparison. It
can be seen that precursor prepared without NH3$H2O (pH ¼ 0.42
during the reaction) has the same diffraction peaks as those pre-
pared at pH ¼ 2.0 and 5.0. Their peaks overlap with those of
CuC2O4$xH2O and NiC2O4$2H2O, indicating that these precursors
are essentially the mechanical mixture of Cu and Ni oxalate salts.
However, when pH ¼ 7.6 and pH ¼ 8.0, the XRD patterns have a
significant change which implies the formation of new substances
and no existing standard JCPDS cards could be currently referenced
for them. We consider that the addition of excess NH3$H2O would
be the reason for the induction of the rod-like morphology, as the
pH of the solution was the only parameter that we changed.
To confirm this hypothesis, the infrared spectra of the corre-
sponding CueNi alloy precursors were further measured as shown
in Fig. 4. When pH ꢃ 5.0, the infrared spectra are similar. As for the
precursor obtained at pH ¼ 2, the absorption peak at 3419.83 cmꢀ1
and 1628.76 cmꢀ1 can be ascribed to eOH in the water of crystal-
lization [42]. The peaks located at 1358.02 cmꢀ1 and 1319 cmꢀ1 can
be assigned to CeO stretching and OeC]O bending vibrations.
Moreover, the stretching vibration of MeeO contributes to the
absorption peak at 760 cmꢀ1 [43] and the absorption peak located
at 489.26 cmꢀ1 is resulting from the oxalate ring deformation [32].
Unsurprisingly, several peaks related to NH3 appear in the spectra
of precursor samples obtained at high pH. For example, when
pH ¼ 7.6, the original peak located at 3341.83 cmꢀ1 splits into two
peaks at 3443.64 cmꢀ1 and 3351.41 cmꢀ1, corresponding to the
stretching vibration of eOH and NeH, respectively. A new ab-
sorption peak at 1300 cmꢀ1 could be related to the bending vi-
bration of HeNeH and the peak at 691.57 cmꢀ1 is generated by the
rocking vibration of NH3 [44]. The appearance of these new peaks
confirms the existence of NH3 in CueNi alloy precursors obtained at
pH ¼ 7.6 and 8.0.
According to the above characterizations and analysis, we pro-
posed that under alkaline conditions, e.g., pH ¼ 7.6 and 8.0, NH3
would coordinate with Cu2þ/Ni2þ and be transferred into precipi-
tated solids, leading to the significant change of XRD patterns and
infrared spectra of CueNi alloy precursors comparing with those
obtained at low pH conditions; the chemical formulas of resulting
precursors at high pH can be preliminarily inferred to CuxNi1-
x(NH3)yC2O4$nH2O (x: 0e1; y: equivalents of coordinated NH3; n:
equivalents of water of crystallization), where the values of x, y, and
n remain to be determined in the further study. The suggested
growth mechanism of rod-like CueNi alloy precursor containing
NH3 is accordingly put forward and schematically illustrated in
Fig. 5. Similar to copper oxalate previously studied [45], CueNi
oxalate complex salts at low pH are constructed by [M(C2O4)]n
(M ¼ Cu/Ni) ribbons where the adjacent two ribbons are perpen-
dicular to each other. Each metal center can form bonds with four
oxygen atoms donated by two C2O24ꢀ ligands in a plane and two
oxygen atoms from C2O24ꢀ of upper and lower ribbons. With the
excessive addition of NH3$H2O, i.e. under alkaline conditions, NH3
Fig. 3 shows the XRD patterns of CueNi alloy precursors
3

Z. Miao, C. Xu, J. Zhan et al.
Journal of Alloys and Compounds 855 (2021) 157438
Fig. 2. SEM images of CueNi alloy precursors prepared at different pH conditions: (a) 2.0, (b) 5.0, (c) 7.6, and (d) 8.0.
molecules are preferentially coordinated with metal centers, lead-
ing to the change of the original surface polarity and further surface
energy of CueNi oxalate crystal [41,45]. Through the ordered ag-
gregation of crystallites in a certain direction under the action of
surface energy disparity [46], the CueNi oxalate complex salts with
a quasi-one-dimensional morphology are finally fabricated. While
at low pH (pH ¼ 5.0), the precursors display as micro-spheres
aggregated by numerous nanocrystals.
3.2. Preparation of porous CueNi alloy rods
The thermal decomposition behaviors of rod-like CueNi alloy
precursors obtained at pH ¼ 7.6 were investigated by the ther-
mogravimetric analyzer and the corresponding TG-DTA curves
were recorded as depicted in Fig. 6a. A continuous mass loss was
observed as the temperature increased up to 370 ꢁC. Four endo-
thermic peaks at 190.35 ꢁC (weight loss: 6.86%), 231.73 ꢁC (weight
loss: 22.46%), 311.43 ꢁC, and 349.37 ꢁC (weight loss:35.86%) could
be assigned to the primary and secondary removal of water of
crystallization, volatilization of coordinated NH3 and decomposi-
tion of residual CueNi complex oxalate salt, respectively. It should
be noted that the thermal decomposition temperature of CuC2O4
and NiC2O4 in an Ar atmosphere was reported to be 295 ꢁC [47] and
370 ꢁC [32]. Therefore, the single endothermic peak at 349.37 ꢁC
displayed in Fig. 6a represents that the as-prepared CueNi alloy
precursor is a type of novel compound rather than a simple mixture
of NH3-coordinated CuC2O4 and NiC2O4 powders. Fig. 6b shows the
XRD pattern of the decomposed sample from rod-like precursor at
673 K in a weak reducing atmosphere (5% H2 in Ar). The three
Fig. 3. XRD patterns of CueNi alloy precursors prepared at different pH conditions.
diffraction peaks located at 2
q
of 43.9ꢁ, 51.2ꢁ, and 75.3ꢁ are
unambiguously assigned to the (111), (200), and (220) facets of a
face-centered cubic (fcc) CueNi alloy (JCPDS no.65e9047). No other
diffraction peaks that would be suggestive of impurities were
detected, such as metallic Cu (JCPDS no.70e3039) and Ni (JCPDS
no.70e0989), indicating the formation of CueNi solid solution
during this process. The sharp diffraction peak is also indicative of
Fig. 4. Infrared spectra of CueNi alloy precursors prepared at different pH conditions.
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Z. Miao, C. Xu, J. Zhan et al.
Journal of Alloys and Compounds 855 (2021) 157438
Fig. 5. NH3-induced oriented-growth mechanism of rod-like CueNi alloy precursor.
Fig. 6. (a) TG-DTA curves of the rod-like CueNi alloy precursor (obtained at pH ¼ 7.6) in the Ar atmosphere. (b) XRD pattern CueNi alloy rods and Cu (red dash line) and Ni (blue
dash line) references.
the high crystallinity of this material. The mean crystallite size of
alloy particles was further calculated to be 20.7 nm through the
Electron microscopy techniques were applied to observe the
morphology and microstructure of the as-prepared CueNi alloy
particles. The SEM image shown in Fig. 7a indicates the CueNi alloy
perfectly inherits the quasi-one-dimensional morphology of the
Scherrer equation [48,49]: D ¼ k
l/Bcos
q
, where k is Scherrer con-
stant (taken as 0.89), other parameters including
l
, B, represent
q
the wavelength of X-ray source (0.154056 nm), full width at half the
maximum intensity (FWHM) and the peak position, respectively.
precursor obtained at pH ¼ 7.6 with diameters of 0.4e0.6
mm and
lengths of tens of micrometers. The porous structure we observe
Fig. 7. (a) SEM, (b) TEM, (c) HRTEM images and (d) elemental mapping of CueNi alloy rods.
5

Z. Miao, C. Xu, J. Zhan et al.
Journal of Alloys and Compounds 855 (2021) 157438
likely results from the release of volatiles such as NH3, H2O, and CO2
during the annealing process [50]. It can also be seen from the TEM
image (Fig. 7b) that the rod is constructed by numerous inter-
connected nanoscale particles and nanopores, which could endow
the CueNi alloy with the prominently enlarged SSA. The HR-TEM
image of Fig. 7c displays the distinct lattice fringes with an inter-
planar distance of 0.206 nm (inset image) and this can be indexed
to the (111) crystal plane of the CueNi alloy. EDX analysis results of
two selected areas shown in Fig. S1 demonstrates that the atomic
ratio for Cu and Ni is 1:1, and the elemental mapping result in-
dicates their uniform distribution over CueNi alloy rods (Fig. 7d).
The elemental valence states of CueNi alloy rods in the near-
surface region were examined by XPS and the corresponding
spectra and the deconvoluted peaks are displayed in Fig. S2. The
survey spectrum (Fig. S2a) shows the appearance of the peaks of Cu,
Ni, and O elements, implying the existence of oxidized states on the
surface of alloy powders. The high-resolution XPS spectrum of Ni
2p (Fig. S2b) exhibits the fitted peaks corresponding to metallic Ni0
at around 870.3 eV and 852.8 eV and Ni2þ located at 873.5 eV and
855.8 eV for Ni 2p1/2 and Ni 2p3/2, respectively [12]. Besides, the
peaks with the binding energy of 879.5 eV and 861.4 eV are
ascribed to shakeup satellites of Ni2þ. The presence of nickel oxides
enables the potential promotion of the formation of the electro-
active intermediates, which is conducive to facilitate the EOR pro-
cess. Likewise, The XPS spectrum of Cu 2p (Fig. S2c) also shows the
fitting peaks of metallic Cu0 and Cu2þ at 952.4 eV and 954.7 eV for
Cu 2p1/2 and 932.6 eV and 934.6 eV for Cu 2p3/2 [51], respectively,
whereas the other peaks fitted in the spectrum are assigned to the
satellites of Cu0 (962.3 eV and 941.9 eV) and Cu2þ (962.8 eV and
944.0 eV). The partial oxidation of Ni and Cu in the surface region
resulting from their exposure to the air could prevent the further
oxidation of the inner metallic phase. Moreover, the surface atomic
ratio of Cu and Ni obtained by XPS analysis was near 1:1, which is
consistent with the above EDX results and the predetermined
CueNi ratio in the raw materials, further indicating the uniform
distribution of Cu and Ni in the alloy rods.
CueNi alloy rods was calculated to be 30.97 nm and the Brunauer-
Emmett-Teller (BET) surface area is 6.84 m2 ꢀ1, which is close to
g
that of the reported quasi-1D porous nickel powders (7.9 m2 gꢀ1
)
[33]. It is expected that the hierarchical mesoporous structure and
the large SSA of the prepared CueNi alloy rods could provide suf-
ficient active sites and shorten the electronic transportation and
ionic diffusion pathways, significantly improving electrocatalytic
activity comparing with nonporous materials when being used on
the interface of electrolyte and electrodes.
3.3. Electrocatalytic test for CueNi alloy rods
Cyclic voltammetry (CV) tests of CueNi/GCE in 1 M NaOH so-
lution in the absence and presence of ethanol (scan rate: 20 mV sꢀ1
)
were both conducted to investigate the electrocatalytic property of
CueNi alloy rods for EOR at the room temperature. In Fig. 9a, when
using blank NaOH as the electrolyte, a pair of the redox peaks at the
potential around 0.35 V and 0.50 V (vs. SCE) is observed, which
could be ascribed to the diffusion-controlled transformation be-
tween Ni(OH)2 and NiOOH [12,48,53]. After the addition of 0.1 M
ethanol in solution, due to the surface-active substance of Ni (III),
ethanol molecules tend to be oxidized and a peak with the current
density of ~50 mA cm-2 is observed at 0.65 V, which is about two
times higher than that of porous Ni fibers (~22 mA cm-2 shown in
Fig. S3). The following descending current response might be
associated with the diminished reaction rate arising from the
depletion of ethanol molecules near the surface of the electrode
and their slow diffusion from the bulk solution. Owing to the
refreshed surface of electrocatalyst by reactivation of Ni (III) from Ni
(II) at high potentials, ethanol or intermediates absorbed can be still
electro-oxidized [54,55], leading to the slower decrease of current
density at 0.65e0.7 V during the reverse scan. Since the electro-
catalytic reaction mainly takes place at the surface of active ma-
terials, only portions of NiOOH are consumed during the EOR
process [48], making the cathodic peak still recognizable. However,
unlike the decreased cathodic current density on Ni/GCE (Fig. S3),
the slightly increased current density observed in Fig. 9a on CueNi/
GCE might be related to the continuous activation of electrocatalyst
during the CV test, i.e., thicker layer of NiOOH was formed with the
increased numbers of sweeping cycles [53].
Cyclic voltammograms of CueNi/GCE in NaOH with various
concentrations of ethanol are also recorded in Fig. 9b. From 0 M to
0.2 M, the onset potentials negatively shift with the increasing
current density of the anodic peak during the forward scan.
Impressively, the anodic peak current density reaches as high as
86.10 mA cmꢀ2 when adding 0.2 M of ethanol in the electrolyte,
which is competitive to most of those reported Ni-based EOR
electrocatalysts listed in Table S1. It should be admitted that the
onset potential for ethanol oxidation on CueNi/GCE is not com-
parable to that of the commercial Pt/C (0.009 V vs. SCE) or Pd/C
(0.159 V vs. SCE) [56]. However, the recently reported onset po-
tential as low as ꢀ0.086 V (vs. SCE) on CuNiPt ternary alloy nano-
particles supported on graphite pencil lead electrode (GPLE) [57] in
an acidic environment might give us the hint that this issue could
probably be solved by integrating the prepared CueNi alloy rods
with Pt/Pd nanoparticles or fabricating the novel rod-like Pt/Pd-
involved multi-component alloys.
To further elucidate and clarify their pore structure, the SSA and
the pore size distribution of the as-prepared CueNi alloy rods were
measured through the N2 adsorption/desorption technique. The
isotherms are presented in Fig. 8, which displays a typical Ⅳ type
Langmuir curve with a hysteresis loop, indicating the mesoporous
structure of CueNi alloy rods [52]. As displayed in the inset image
of Fig. 8, the corresponding Barrett-Joyner-Halenda (BJH) pore size
distribution shows a hierarchical distribution of pores where a
relatively narrow peak and a broad peak are located at around
2.47 nm and 29.30 nm, respectively. The average pore diameter of
The electrocatalytic mechanism of Ni-based alloy/composites
has been well studied in the few past decades, which illustrated
that the component of Ni in these materials mainly serves as the
active substances for the oxidation of SOMs [30,58]. Similarly, the
whole process for EOR on the fabricated CueNi/GCE could be
described as follows [12,13,59,60]:
Fig. 8. N2 adsorption and desorption isotherms for CueNi alloy rods (inset image: pore
size distribution).
Ni þ 2OHꢀ / Ni(OH)2 þ 2eꢀ
(1)
6

Z. Miao, C. Xu, J. Zhan et al.
Journal of Alloys and Compounds 855 (2021) 157438
Fig. 9. Cyclic voltammograms of CueNi/GCE (a) in 1 M NaOH solution in the presence and absence of 0.1 M ethanol (scan rate: 20 mV sꢀ1); (b) in 1 M NaOH solution containing
various concentrations of ethanol (scan rate: 20 mV sꢀ1); (c) in 1 M NaOH solution with 0.1 M ethanol at different scan rates (5 mV sꢀ1e100 m V sꢀ1); and (d) relationship between
the anodic current densities and the square root of the scan rates; inset image: background current densities of CueNi/GCE measured in 1 M NaOH solution at the scan rates from
5 mV sꢀ1 to 100 mV sꢀ1
.
voltammetry (LSV) measured at the scan rate of 1 mV sꢀ1 with
NiðOHÞ2 þ OHꢀ#NiOOH þ H2O þ eꢀ
NiOOH þ ethanol / Ni(OH)2 þ products
(2)
(3)
0.1 M ethanol are displayed in Fig. S4, where the CueNi/GCE and Ni/
GCE demonstrate the slopes of 236 mV decꢀ1 and 352 mV decꢀ1
,
respectively, indicating the faster charge transfer and more active
electro-oxidation of ethanol molecules on CueNi/GCE than Ni/GCE.
In order to evaluate the electrocatalytic stability of the CueNi
alloy rods, chronoamperometry (CA) testing was performed in an
electrolyte of 1 M NaOH solution with 0.1 M ethanol at the polar-
ization potential of 0.6 V (vs. SCE) for 2000 s, as displayed in Fig. 10.
The current density decreased rapidly due to the double-layer
charging and the adsorption of intermediates on the active sites
of electrocatalyst [39]. After that, it gradually reached a quasi-
equilibrium steady-state and maintained 95% of the original cur-
rent density measured at 100 s after 2000 s, indicating their good
After immersing the modified electrode in the alkaline media,
Ni(OH)2 thin layer immediately forms on the surface of the elec-
trocatalyst (Eq. (1)) and is then oxidized to NiOOH during the for-
ward sweeping, corresponding to the current peak at 0.5 V in the
blank NaOH solution (Eq. (2)) [48]. With the addition of ethanol in
the electrolyte, the ethanol will be oxidized to intermediates or
final products, i.e., acetaldehyde or acetic acid, parallel to the
reduction of NiOOH to Ni(OH)2 (Eq. (3)). The repeat of processes
described in Eq. (2) and Eq. (3) leads to the continuous electro-
oxidation of ethanol molecules on the electrode. Cu atoms are
thought to contribute significantly to finely tune the electronic
structure of Ni and preventing the transformation of NiOOH from
b-
phase to -phase, where the latter one is a passivating electro-
g
chemical species and usually shows the inferior activity to
in alkaline media [12,61].
b-NiOOH
The CVs of CueNi/GCE measured at different scan rates ranging
from 5 mV sꢀ1 to 100 mV sꢀ1 in the presence of 0.1 M ethanol were
plotted in Fig. 9c. Faster potential scans could usually decrease the
thickness of the diffusion layer, leading to the higher obtained
current densities of anodic peaks relating to ethanol oxidation on
the electrode surface [62]. Besides, the corresponding potentials are
observed shifting positively when increasing the scan rates. The
anodic peak current densities subtracting the background current
(measured without ethanol) are plotted with a linear response
against the square root of scan rates (Fig. 9d), which implies that
the electrocatalytic oxidation reaction of ethanol on the CueNi/GCE
might be a diffusion-controlled process that is consistent with the
previous reports [10,39,63]. Tafel plots derived from linear sweep
Fig. 10. CA curve of CueNi/GCE and Ni/GCE at 0.6 V (vs. SCE) for 2000 s (electrolyte:
1 M NaOH solution in the presence of 0.1 M ethanol).
7

Z. Miao, C. Xu, J. Zhan et al.
Journal of Alloys and Compounds 855 (2021) 157438
j.electacta.2017.05.022.
long-term stability. Moreover, the steady current of CueNi/GCE
during the whole process is observed constantly higher than that of
Ni/GCE with the same mass loading and test conditions as shown in
Fig. 10, which further confirms the relative superior electrocatalytic
performance of CueNi alloy rods for the EOR process.
[2] Y. Qin, Z. Xiong, J. Ma, L. Yang, Z. Wu, W. Feng, T. Wang, W. Wang, C. Wang,
Enhanced electrocatalytic activity and stability of Pd nanoparticles supported
on TiO2-modified nitrogen-doped carbon for ethanol oxidation in alkaline
media, Int. J. Hydrogen Energy 42 (2017) 1103e1112, https://doi.org/10.1016/
j.ijhydene.2016.09.060.
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N. Han, F. Zhao, J. Zhou, W. Cai, Y. Li, Promoting effect of Ni(OH)2 on palladium
nanocrystals leads to greatly improved operation durability for electro-
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4. Conclusions
In summary, highly dispersed CueNi alloy particles with a rod-
like morphology and hierarchical porous structure were success-
fully prepared via a simple template-free approach, combining a
coordination-precipitation process and the post thermal treatment.
To achieve the compositional homogeneity and uniform distribu-
tion of Cu and Ni in alloy products, water-ethanol mixture as the
solvent was used to prepare CueNi alloy precursors. The addition of
NH3 not only adjusted the pH of the solution, but also shaped the
CueNi alloy precursors into the quasi-one-dimensional
morphology when a pH of 7.6 was maintained, and the corre-
sponding NH3-induced oriented growth mechanism was proposed.
After thermal decomposition, well-alloyed CueNi rods were ob-
tained in good crystallinity, with a high BET surface area of
6.84 m2 gꢀ1, an average pore diameter of 30.97 nm, and the desired
atomic ratio of Cu/Ni ¼ 1:1. A large anodic current density of
86.10 mA cmꢀ2 in presence of 0.2 M ethanol and retention of 95%
original current density after 2000 s indicated the excellent elec-
trocatalytic activity of this material, and its high stability for the
EOR in alkaline media. This work may offer a promising strategy for
large-scale synthesis of Ni-based alloys or compounds that could
serve as high-performance anodic electrocatalysts for DEFCs.
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CRediT authorship contribution statement
Zelin Miao: Investigation, Writing - original draft. Changfan Xu:
Investigation, Writing - review & editing. Jing Zhan: Supervision,
Conceptualization, Writing - review & editing, Funding acquisition.
Ziwei Xu: Validation.
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in graphitic carbon shells as electrocatalysts for hydrogen evolution reaction,
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Adv. Energy Mater.
8
(2018) 1701759, https://doi.org/10.1002/
aenm.201701759.
€
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Acknowledgments
This work was financially supported by the Natural Science
Foundation of Hunan Province, China (2020JJ4735); Science and
Technology Department of Hunan Province Tackling Key Scientific
and Technological Problems and Transformation of Major Scientific
and Technological Achievements (2018GK4001); and the Hunan
Key Laboratory for Rare Earth Functional Materials (2017TP1031).
The authors acknowledge Dr. Michael S. Inkpen and Mr. Joseph M.
Parr in the Department of Chemistry, University of Southern Cali-
fornia for their insightful suggestions and language modification.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jallcom.2020.157438.
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