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Electrochemical Behavior of Morphology-Controlled Copper (II)
Hydroxide Nitrate Nanostructures
Julien Sarmet, Christine Taviot-Gueho,*Rodolphe Thirouard, Fabrice Leroux, Camille Douard,
Insaf Gaalich, Thierry Brousse, Gwenaëlle Toussaint, and Philippe Stevens
Cite This: https://doi.org/10.1021/acs.cgd.2c01468
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sı Supporting Information
ABSTRACT: Nanostructure control is an important issue when using
electroactive materials in energy conversion and storage devices. In this
study, we report various methods of synthesis of nanostructured copper
(II) hydroxide nitrate (Cu2(OH)3NO3) with a layered hydroxide salt
(LHS) structure using various synthesis methods and investigate the
correlation between nanostructure, morphology, and their pseudocapa-
citive electrochemical behavior. The variations in nanostructure size and
morphology were comprehensively explored by combining X-ray
diraction (XRD) and scanning electron microscopy (SEM), while the
electrochemical activity was characterized using cyclic voltammetry. We
demonstrate that Cu2(OH)3NO3−LHS nanostructured submicron
particles produced by alkaline precipitation with 88% of the copper
cations can cycle with a two-electron redox process. Unfortunately, the
electroactivity decreases rapidly from the first cycle due to the occurrence of structural transformations and subsequent
electrochemical grinding. However, samples obtained by ultrasonication and microwave synthesis, two original synthesis methods for
LHS materials, formed of nanosized crystalline domains agglomerated in micron-sized particles, represent a good compromise
between capacity and cyclability. Moreover, by using pair distribution function analysis on electrode materials after repeated cycling,
we were able to follow the chemical and structural changes occurring in Cu2(OH)3NO3materials during electrochemical cycling
with first a quick transformation to Cu2O and then the appearance of Cu metal and copper acetate Cu(II)2(O2CCH3)4·2H2O.
■INTRODUCTION
The transformation of the energy system into a world without
fossil fuels is astrong driving force to develop eective
electrochemical energy conversion and storage devices based
on batteries, fuel cells, and super capacitors. Eorts must focus
on the development of inexpensive electrode materials with high
redox reversibility and stability, and in this field, layered
hydroxide salts (LHS), also known as layered basic salts, benefit
from a growing interest. Their structure is based on a distorted
Brucite-like structure in which a fraction of the structural
hydroxide groups is replaced by water molecules or anions
leading to the following general chemical formula
M2+(OH)2x−(Am−)x/m·nH2O, where M2+ is a divalent cation,
normally Mg2+, Ni2+, Zn2+, Ca2+, Cd2+, Co2+, or Cu2+, and
(Am−)x/m·nH2O is a counterion, which is hydrated or not.
1
So
far, the main focus has been on nickel- and cobalt-based LHS
materials and promising electrochemical,
2,3
magnetic,
4
and
photophysical properties
5
have been reported. Layered copper
hydroxide salts have been comparatively less studied although
copper (II) hydroxide nitrate Cu2(OH)3NO3is used in vehicle
airbags,
6
as well as a precursor
7
in the preparation of CuO p-type
semiconductor for diverse applications in heterogeneous
catalysis,
8
gas sensors,
9,10
field-emission semiconductors,
lithium ion batteries,
11,12
and high power aqueous bat-
teries.
13−16
These few studies clearly show the potential of
layered copper hydroxide salts but also the need to better
document and understand the synthesis of these materials to
improve their performances.
In this study, by using not only similar synthesis protocol as
reported for layered zinc hydroxide salts (LZH) but also new
synthesis routes based on sonochemistry (US) and microwave
(MW), we report the formation of well-defined nanostructured
Cu2(OH)3NO3with various morphologies and sizes. First,
microcrystalline parameters and morphological structures were
comprehensively explored by combining scanning electron
microscopy (SEM) and X-ray diraction (XRD). In the second
part, the electrochemical properties were investigated by cyclic
voltammetry and pair distribution function (PDF) analyses were
performed to examine phase transformations during cycling.
Received: December 10, 2022
Revised: February 22, 2023
Articlepubs.acs.org/crystal
© XXXX American Chemical Society A
https://doi.org/10.1021/acs.cgd.2c01468
Cryst. Growth Des. XXXX, XXX, XXX−XXX
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■EXPERIMENTAL DETAILS
Synthesis. For the synthesis of Cu2(OH)3(NO3) samples, AR-
grade raw chemicals were used. Copper oxide (CuO, 98%) and copper
nitrate trihydrate (Cu(NO3)2·3H2O, 98%) were purchased from
Sigma-Aldrich, sodium hydroxide (NaOH, 98%) was obtained from
Acros, and urea (CH4N2O, 99%) was purchased from Prolabo. For
synthesis by salt and oxide hydrolysis (SO), 15.9 g of CuO (0.2 mol)
was dispersed into 200 mL of a 1.25 mol·L−1aqueous copper nitrate
solution. The mixture was kept under stirring for 4 weeks under a
nitrogen atmosphere after which the solid was recovered by
centrifugation at 4500 rpm (relative centrifugal force of 3056g),
washed three times with deionized water, and dried at 60 °C in an oven.
For synthesis by alkaline precipitation (AP), 50 mL of a copper nitrate
solution (0.1 or 1 mol·L−1) and 50 mL of NaOH solution (0.1 or 1 mol·
L−1) were added simultaneously for 5 min into a 1 L reactor containing
200 mL of water using peristaltic pumps at a flow rate of 10 mL·min−1.
The resulting dispersions were kept under stirring and a nitrogen
atmosphere at room temperature or 40 °C for dierent aging times (no
aging, 1 day, 4 days) after which the precipitates were recovered by
centrifugation (4500 rpm/3056g), washed three times with water, and
dried at 60 °C in an oven. For synthesis by urea hydrolysis, 3 g of urea
(0.05 mol) was added into 80 mL of a 0.2 mol·L−1copper nitrate
solution. Then, the mixture was heated at 80 °C for 2 h either by reflux
method (urea reflux) or by hydrothermal treatment (urea HT) using a
Teflon-coated stainless steel autoclave. In both cases, the precipitates
were recovered by centrifugation (4500 rpm/3056g), washed three
times with water, and dried at 60 °C in an oven. For the microwave
hydrothermal treatment (MWHT) synthesis, the same mixture as
prepared for urea hydrolysis was poured into 80 mL vessels and
irradiated with microwaves at a power of 300 W using a Discover SP-D
80 microwave for dierent periods of times (30, 60, 240 min) and
dierent temperatures (80, 100 °C); the pressure reached was around
290 psi. For sonochemical synthesis (US), the same mixture as
prepared for urea hydrolysis was also used. After 1 h of stirring at room
temperature under a nitrogen atmosphere, it was subjected to intense
ultrasonic irradiation (20 KHz and 21 W·cm−2) for various times (30,
150, 240 min) using Vibra-Cell 75041 equipment. Solids obtained by
MWHT and US methods were recovered by centrifugation (4500 rpm/
3056g), washed three times with water, and dried at 60 °C in the oven.
The materials were labeled for instance as AP0,1M‑4days‑40°Caccording to
the synthesis method, the concentration of reactants, the reaction or
aging time, and the temperature applied.
Material Characterization. X-ray diraction (XRD) analysis of
the samples was carried out on a Philips X’Pert pro diractometer
equipped with an X’celerator 1D detector (2.122°active length), using
Cu Kα1/Kα2source in a Bragg−Brentano θ−θgeometry from 5 to
90°(2θ) with a scan step of 0.016°. Profile matching and structure
refinement used the FullProf suite package.
17
To properly reproduce
the XRD peak profile and reach good fits, it was necessary to consider
microstructural eects in particular anisotropic size eects, related to
the platelet morphology of the particles, and these were modeled with
linear combinations of spherical harmonics SH as implemented in
FullProf. The SH description also allowed the calculation of the
volume-averaged apparent size of the coherent domains along each
reciprocal lattice vector, and to do so, the LaB6NIST standard was used
to correct for instrumental broadening. The plate shape of the
crystallites also led us to consider preferred orientation PO eects on
the XRD patterns in the [001] direction as the stacking direction of the
hydroxide layers. For that, the exponential function as implemented in
Fullprof was used and G1/G2 parameters were included after the
preliminary refinement with the random orientation of the crystallites
had converged. For Rietveld refinements, the atomic positions reported
by Eenberger for the Rouaite monoclinic polymorph were considered
as the starting structural model.
18
Only the atomic positions within the
hydroxide layer were refined; nitrate anion geometry and N−O bonds
were fixed. The atomic PDFs were obtained from X-ray total scattering
data collected on a PANalytical Empyrean diractometer equipped
with a solid-state GaliPIX3D detector, a focusing X-ray multilayer
mirror, and a Ag anticathode (Kα1= 0.5594 Å, Kα2= 0.5638 Å). The
active material powders or electrode paste mixtures recovered after
various numbers of charge−discharge cycles were placed in glass
Table 1. Synthesis Conditions and Characteristics of Cu2(OH)3NO3Sample Series
synthetic conditions LHS characteristics
method code salt base
conc.
(mol·
L−1) stirring atm
T
(°C)
duration/
aging
yield
a
(wt
%) particle shape TEM
SBET
(m2/
g)
Lz/Lxy
(nm)
SO CuO-
Cu(NO3)2
no 0.2 Y N2RT 4 weeks 92 irregular rodlike. 2 μm long-1 μm broad 1.4 193/132
0.25
AP0,1M‑no aging‑RT Cu(NO3)2NaOH 0.1 Y N2RT 2 min 37 undefined/broken plates. 200 nm-1 μm 17.5 28/35
0.1
AP0,1M‑1day‑RT RT 2 min/1
day
54 undefined plates. 1 μm mixture of small/
broken < 1 μm + large plates 2 μm +
tiny crystal bits < 100 nm
13.3 33/39
AP0,1M‑4days‑RT RT 2 min/4
days
60 10.0
AP0,1M‑4days‑40°C40 2 min/4
days
63 well-dispersed plates, undefined shape. 2
μm
10.0 42/44
AP1M‑4days‑RT 1 RT 2 min/4
days
59 very small platelets < 100 nm 6.7
1
ureareflux Cu(NO3)2urea 0.2 Y N280 2H 83 broken pieces of very wide plates. 20 μm
in-plane/5 μm thick
1.8 185/62
0.5
ureaHT N autoclave 27 radial growth of thick and large plates 1.1 202/52
US30min Cu(NO3)2urea 0.2 N Ar degas. RT 30 min 7 well-defined rectangular/rhombic plates.
5−10 μm in-plane, <1 μm thick
1.5 139/52
0.5
US150min 150 min 42 1.8 138/79
US240min 240 min 54 large and thick disks. 5−15 μm diam., 2
μm thick
<1 178/99
MWHT30 min‑80°CCu(NO3)2urea 0.2 Y air 80 30 min 5 mixture of small/broken < 5 μm + large
rectangular plates 10−20 μm
2.6 64/51
0.5
MWHT 60min‑80°C80 60 min 2 3.1 75/74
MWHT240min‑80°C80 240 min 8 very well-defined rectangular rhombic
platelets 10−20 μm 10-30 μm
<1 157/77
MWHT30min‑100°C100 30 min 5 1.3 218/87
a
Percent yield is the amount of solid obtained with respect to the theoretically expected pure compound amount.
Crystal Growth & Design pubs.acs.org/crystal Article
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Cryst. Growth Des. XXXX, XXX, XXX−XXX
B
capillaries of 0.7 mm diameter. An empty capillary of the same type or
only filled with electrode additives was measured in the same way for
background subtraction. Data were recorded over the 1−145°2θrange,
which corresponds to an accessible maximum value for the scattering
vector Qmax of 21.4 Å−1. Data merging, background subtraction, and
Kα2stripping were done using HighScore Plus software provided by
PANalytical Corporation. It was also used to generate corrected and
normalized total scattering structure functions S(Q). Finally, the PDF
or G(r) was calculated from the Fourier transforms of S(Q) truncated at
21 Å−1. The program PDFgui was used to extract local structural
information from the measured PDF.
19
As starting models, we
considered the structural data reported by Smura et al.
20
for Cu2O
(COD 1010941), Fortes et al.
21
for Cu (COD 4105040), and Skapski
et al.
22
for Cu(II)2(O2CCH3)4·2H2O. Scanning electron microscopy
(SEM) images were recorded using a JSM-7500F field-emission
scanning electron microscope operating at an acceleration voltage of 3
kV. Samples to be imaged were mounted on conductive carbon
adhesive tabs and coated with a gold thin layer. Infrared spectra were
recorded in transmission mode using the KBr pellet technique (2 wt %)
with a Nicolet 5700 spectrometer from Thermo Scientific over the
wavenumber domain of 400−4000 cm−1with a scan number of 128 at a
resolution of 4 cm−1. Thermogravimetric analysis (TGA) was studied
under airflow in the temperature range of 25−1000 °C with a linear
temperature ramp of 5 °C·min−1. Surface areas were determined from
the BET analysis of nitrogen adsorption isotherms recorded at 77 K
using a Micromeritics ASAP 2020.
Electrochemical Measurements. The electrode pastes were
prepared mixing LHS/SG/PTFE in a 60/30/10 wt. % ratio. The
obtained pastes were dried in the oven at 60 °C and then pressed under
5 tons on a steel grid. Cyclic voltammetry (CV) curves were performed
using a Biologic VMP3 with a three-electrode setup with LSG/SG/
PTFE as a working electrode, Ag/AgCl (KCl sat) as a reference, and
platinum as a counter electrode. The electrolyte was sodium acetate (1
mol·L−1) in a [−1.2;1] V potential range.
■RESULTS AND DISCUSSION
Synthesis and Morphology Analysis. The characteristics
of the samples and the synthesis conditions are summarized in
Table 1. It should be noted that US and MWHT methods have
been used in very few of the LHS material syntheses reported so
far and the results obtained here are therefore highly original.
The formation of Cu2(OH)3NO3phases was confirmed in all
cases based on XRD analysis, supplemented by TGA and IR
analysis given in the Supporting Information (Figures S1 and
S2). The crystallization behavior of layered metal hydroxides is
usually explained by the classical nucleation and growth model,
and these materials tend to develop 2D morphologies. Indeed,
due to the large dierence in the density of atoms between the
stacking and lateral directions, the basal crystallographic planes
are the dominant exposed facets (Bravais’ law) in layered
materials, leading to a growth in the lateral direction to form
platelet-like particles.
23
Here, depending on the synthesis
method applied, particles with dierent shapes, sizes,
aggregation states, and dierent crystallinities were obtained.
SEM images of samples representative of each series are
displayed in Figure 1 together with XRD patterns.
As a first method, we applied the hydrolysis of divalent metal
salts in the presence of metal oxide (SO method), which has
been known for a long time.
24,25
A weak point for this method is
the reaction time that can be very long, thus explaining its
underuse. In the present case, the reaction was conducted over 4
Figure 1. Variation of the morphologies and the XRD patterns of Cu2(OH)3NO3sample series prepared according to five dierent synthesis methods:
(a) hydrolysis of salt and oxide (SO), (b) precipitation with alkaline solution (AP), (c) urea hydrolysis (urea), (d) sonochemistry (US), and (e)
microwave hydrothermal treatment (MWHT).
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Cryst. Growth Des. XXXX, XXX, XXX−XXX
C
weeks and microsized particles were obtained with a coarse
rodlike shape indicating a directed aggregation of primary
platelet crystals in the stacking direction (Figure 1a). Although
not discussed in the former studies, such anisotropic crystal
growth of layered compounds along the stacking direction is
quite unusual and we can attribute it to high supersaturation
conditions, which are maintained constant over a long period
through the controlled release of metal ions from CuO.
26,27
These conditions also explain the very high product yield
achieved by this method, higher than 90 wt %. The high purity
and crystallinity of the sample enabled a Rietveld refinement of
the XRD data (discussed later) and showed a structure similar to
the Rouaite mineral.
18
The second method applied was the
precipitation (AP) with the simple addition of a hydroxide
solution in an acidic aqueous solution containing M2+ cations
and Am−counteranions. This is a widely used method for
preparing LHS materials with many parameters that can be
varied such as the pH, the concentration of the reactants, and the
addition rate of the solution.
11
In the present case, the quick
addition within a few minutes of 0.1 M NaOH led to the
formation of well-dispersed platelets of submicron size with
slightly indented contours (Figure 1b). After 96 h of aging, the
presence of both very small particles and much bigger platelets
suggests the occurrence of the Ostwald ripening process with the
slow disappearance of small platelets in favor of the larger ones.
28
The measured product yields confirm this mechanism with the
lowest value obtained for AP0.1M‑noaging‑RT but which increases
noticeably upon aging. An increase in temperature, even as low
as 40 °C, produced an acceleration of this process, leading to
microsized homogeneous particles with better crystallinity as
shown by the comparison of XRD data. On the contrary, the use
of 1 mol·L−1of NaOH led to the formation of many small nuclei
due to a rapid increase in pH but which grew smaller and are
much less crystalline, as evidenced by the broad character of
XRD peaks. The XRD data also indicate the presence of
impurities that do not disappear upon aging and thermal
treatment. In the end, the maximum yield achieved for these two
NaOH concentrations is practically the same, in the order of 60
wt %, but with net dierences in terms of crystallinity and purity.
As a third method, urea hydrolysis was used, which is less
frequently used than the previous two for LHS materials. It is
based on the slow hydrolysis of urea providing hydroxide groups
for the reaction with the salt solution.
29
CO2is also produced,
which can lead to the formation of carbonate anions, and unless
it is the desired anions, this can appear as a contamination in the
final product. We applied thermal urea hydrolysis at 80 °C either
under reflux in an oil bath (ureareflux) or combined with a
hydrothermal treatment in aTeflon-lined stainless steel
autoclave (ureaHT). As explained in the case of the LDH
materials, the progressive hydrolysis of urea upon heating
enables nucleation to occur at a slow rate (low supersaturation
conditions), which is favorable to the growth of very large
platelets.
29
In the present case, platelets of several dozens of
micron sizes with a sharp contour were easily attained under
reflux at 80 °C within just 2 h (Figure 1c). Interestingly,
assemblies of platelets, arranged in an apparently cylindrical
manner (radial morphology) with ca. 15−20 μm in diameter,
were obtained under hydrothermal treatment, which are likely to
emanate from a common nucleus. The highly anisotropic
character of the particles is visible on the XRD patterns with very
intense 00lbasal reflections relative to nonbasal hk reflections,
for instance, 120. This preferential orientation is prominent for
the sample obtained by reflux heating that also shows better
crystallinity with very narrow XRD peaks and a good product
yield, higher than 80 wt %. The yield is considerably decreased
upon hydrothermal heating, lower than 30 wt %, and XRD
analysis indicates the presence of CuO. The absence of stirring
in the autoclave vessel used and a too short heating time are
likely explanatory factors.
For the last two methods applied, urea hydrolysis was coupled
with an ultrasonic treatment (US method) and a microwave
hydrothermal treatment (MWHT). Since the reactants and the
concentrations were the same in both synthesis, a direct
comparison can be done between these two methods and urea
hydrolysis described above. Sonochemistry has been applied
very little for the synthesis of LHS so far. Using a high-intensity
ultrasonic horn (20 kHz, 100/50 W·cm−2), the generation of
OH°radicals by the sonolysis of water was reported to lead first
to the formation of Cu(0)/Cu2O particles, which then react with
NO3
−to form LHS copper hydroxide nitrate.
30,31
In the present
case, a lower power intensity was applied (20 kHz, 21 W·cm−2)
and we did not observe the formation of any reduced Cu species
but it clearly accelerated the formation of Cu2(OH)3NO3in only
3 min. Hot spots generated by acoustic cavitation induced
homogeneous nucleation and crystallization of very well-defined
rectangular platelets of a few microns as can be seen in Figure 1d
with some intergrowth sites visible. Interestingly, by increasing
the sonication time, the rectangular platelets stack and align side
by side, then fused together, as in a sintering process, leading to
very large disk-shaped particles with a diameter of about 10
microns and a thickness a few microns (US240min), never
reported so far. One can attribute this disk shape outline to high-
velocity impacts between particles driven by ultrasounds.
32
The
synthesis yield also increases significantly with the sonication
time, reaching values comparable to the AP method after a
sonication time of 150 min but with a much higher crystallinity.
The last method applied was microwave hydrothermal treat-
ment (MWHT). As already reported in the literature, both
nucleation and crystal growth can be considerably accelerated by
combining microwave dielectric heating and hydrothermal
conditions due to uniform bulk heating and an increased
solubility and reactivity of reactants.
33
This method has never
been applied as such to LHS synthesis so far; the only studies
available concern microwave-assisted postsynthesis modifica-
tions of LHS.
34
The reasons are probably some drawbacks
similar to those reported by using MW for LDH materials, i.e.,
ZnO phase formation/segregation when Zn2+ cations are
involved, an accelerated oxidation process of Co2+ or Mn2+
cations, etc.
35
In the present case, we did not observe any
contamination in the final product, even for long microwave
irradiation times up to 240 min. While at 60 °C no product was
formed, very large platelets of submicron-thick and in-plane
dimensions of several microns were obtained at 80 °C for an
irradiation time of only 30 min (Figure 1e). The intergrowth of
more uniform rectangular platelets of tens of micron wide was
observed at 100 °C after an irradiation time of 30 min, also
displaying abetter crystallinity. However, the significant
reduction in synthesis time is counterbalanced by very low
product yields, lower than 10 wt %. Such a low synthesis yield is
of course an important issue to address by optimizing the MW
field homogeneity, MW power, volume/stirring of reaction
system, heating rate/temperature/time, etc., but this was
beyond the scope of this study.
In addition to SEM images, BET analysis (Figure S3) was
used to further characterize the textural properties of LHS
materials. Nitrogen adsorption/desorption isotherms indicate
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the formation of nonporous materials with a very small surface
area (Table 1); no hysteresis loops attributable to mesoporous
interparticle pores were detected either. The slight dierences
observed here only arise from dierences in particle size and
dispersion/aggregation state. Hence, the surface area of AP
samples, composed of small and well-dispersed particles, is
larger (∼10−20 m2·g−1) than that measured for the other
sample series displaying much bigger particles (<3 m2·g−1).
Highly crystalline samples such as US240min, MWHT240min‑80°C
even have no measurable specific surface area.
Structural and Microstructural Analyses. Copper
hydroxide nitrate in its anhydrous form is known to occur in
two polymorphic forms, orthorhombic (P212121) and mono-
clinic (P21/m) forms that refer to Gerhardtite and Rouaite
crystal minerals, respectively.
18,36
Both structures are derived
from Cu(OH)2with strongly edge-bonded Jahn−Teller
distorted Cu2+ octahedra coordinated to six hydroxide anions;
one-quarter of the hydroxides are replaced by nitrate anions,
which point alternately outside the layers, connected through
N−O bonds (Figure 2). The only dierence between the two
structures is in the orientation of the NO3with a constrained
orientation for the monoclinic polymorph and a free orientation
in the case of orthorhombic polymorph. The monoclinic
polymorph is considered as a metastable phase whose synthesis
is kinetically favored.
31
These two polymorphs are rather
dicult to distinguish based on XRD patterns, but one
noticeable dierence, however, concerns the second most
intense peak at ca. 25.77°(2θ), which in the case of the
monoclinic polymorph is the contribution of two hkl reflections,
i.e., 002 and 111, leading to an XRD peak with a right shoulder,
while in the case of the orthorhombic polymorph, a single peak is
expected attributed to 040 (Figure 3). In the present case, the
formation of the monoclinic polymorph was confirmed by XRD
data for all of the methods of synthesis applied.
Most of the samples appear to consist of pure LHS single
phases, and it was possible to perform Rietveld refinements of
the powder XRD data for many of them. Details are given in the
Supporting Information with refinement results in Table S1 and
Figure S4, refined atomic parameters in Table S2, and main
distances within the hydroxide layers and in the interlayer space
in Table S3. As an example, the graphical output of the Rietveld
refinement for the US150min sample is displayed in Figure 3. A few
remarks can be made for some samples, first for AP1M samples
for which no refinement could be conducted, nor even a profile
refinement, due to important peak profile anisotropy and the
presence of an amorphous component that could not be
identified but assumed to be a copper oxyhydroxide. On the
other hand, for urea samples, CO2produced by urea hydrolysis
led to the formation of Malachit Cu2(OH)2CO3in small
amounts of <2 wt % and the application of the hydrothermal
treatment resulted in the formation of CuO up to 6 wt %.
The dierences observed between samples in terms of
crystallinity can be further characterized by X-ray diraction
line profile analysis, which enables the size of individual perfect
crystalline domains within particles to be determined. Using the
anisotropic spherical harmonic size approach, the coherence
lengths along all crystallographic directions can be calculated. In
the present case, we were particularly interested in the [00l] and
[120] directions, reflecting the extent of the structural coherence
length along the stacking direction (Lz) and in the plane of the
hydroxide layers (Lxy), respectively. The values are reported in
Table 1 and indicate the initial formation of nanosized primary
particles ranging from ca. 25 to 200 nm. Therefore, the
(sub)microsized particles observed by SEM must be considered
as polycrystalline secondary particles resulting from the growth
by orientated aggregation of the above primary nanoparticles. It
can also be noted that the large particles seen by SEM are made
up of large crystalline domains.
Interesting dierences are observed concerning the shape
anisotropy of the crystalline domains measured through the
crystal aspect ratio Lz/Lxy of the thickness to lateral size. At first
sight, it might be expected that particle morphologies would
originate from crystalline domains of similar shapes, i.e., Lz/Lxy <
1 for platelets and Lz/Lxy > 1 for rodlike particles. For SO and AP
particles, it appears that growth follows the shape of the primary
crystallites with an Lz/Lxy value of 1.6 for SO rod-shaped
particles and 0.8−1.0 for AP platelets. On the other hand, for the
other sample series displaying Lz/Lxy values from 1.6 to 3.5, the
aggregative growth occurs in the opposite direction (lateral
direction) than expected from crystalline domains (stacking
direction). It is thus inferred that the dominant exposed facets
must remain in the basal crystallographic planes (ab-planes) and
crystal growth occurs in the lateral direction to form plate-like
particles.
Figure 2. Polyhedral representation of the copper hydroxide nitrate
monoclinic structure obtained from Rietveld refinement parameters for
the US150min sample.
Figure 3. Graphical results of the Rietveld refinement for the US150min
sample. Experimental X-ray diraction pattern (red cross), calculated
pattern (solid black line), Bragg reflections (green ticks), and dierence
profile (solid blue line). The quality of the fit is more visible in the
insets. The fitting of the peak at 25.77°(2θ) with the contributions of
both 002 and 111 reflections is consistent with the monoclinic
polymorph. Low Bragg angle range (2θ< 10°) was excluded from the
refinement because it contains the strongest peak, which is most
aected by experimental errors and may strongly influence the
refinement.
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Electrochemical Behavior. The electrochemical behavior
of all samples was investigated using a three-electrode setup
through cyclic voltammetry (CV) in a 1 M NaCH3COOH
aqueous solution as an electrolyte and a wide potential window
from −1.2 to 1 V vs Ag/AgCl to follow all of the transformations
undergone by the material. This wide potential range induces a
water electrolysis reaction. One hundred CVs were recorded for
each sample with significant peak shifts and dierences in the
peak area indicating important changes upon cycling. Even if we
note some dierences depending on the synthesis method, CV
plots are rather similar among all samples (Figures 4 and S5).
Copper redox peaks are clearly observed in the first scan
attributed to Cu(II)/Cu(I) and Cu(I)/Cu(0) couples. As
shown in the case of LDH and also applies to the case of other
layered materials, the reversible oxidation/reduction of the
cations within the hydroxide layers induces changes in the
interlayer space with a release of the interlayer anions and the
intercalation of the anion electrolyte.
37
A delamination of the
layers may also occur.
The discussion first focuses on the CV curves of the first cycle
since it refers to the starting composition i.e., Cu2(OH)3(NO3).
The CV curves (first cycle) at a scan rate of 10 mV·s−1for
representative samples of each synthesis method are displayed in
Figure 4. Direct capacities were determined from these CV
curves and are given in Figure S6. These experimental values can
be compared to the theoretical capacity of Cu2(OH)3(NO3)
calculated according to this equation:
=
◊
QxF
M
mAh
g 3.6
Ä
Ç
Å
Å
Å
Å
Å
Å
Å
É
Ö
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
where
Δxis the number of electrons exchanged, Fis the Faraday
constant in As/mol, and Mis the molar mass of the compound in
g.mol−1. Thus, assuming a two-electron transfer, owing to the
ability of copper (II) to be reduced to the Cu metal, a theoretical
capacity of 223 mAh.g−1should be obtained. The best capacity
at the first cycle is observed for AP samples with a value of 197
mAh.g−1for AP0.1M‑4days‑40°C, which corresponds to 88% of the
maximum capacity expected for Cu2(OH)3(NO3) if we assume
that there is no contribution from water electrolysis. Such a high
capacity close to the theoretical one supports the presence of
bulk redox reactions not only limited to the surface of the
material at the beginning of the cycling. Thus, the percentage of
copper atoms aected by the electrochemical process ranges
from 88% for AP0.1M‑4days‑40°Cto 25% for SO. Interestingly, the
capacities measured on the first cycle can be related to the size of
the crystalline domains determined above as shown in Figures 5
and S7. Based on the Lz/Lxy ratio, the materials can be divided
into three groups: those very well crystallized both in the in-
plane and stacking directions leading to an Lz/Lxy ratio between
1 and 2 (group 1), those also well crystallized but more in the
stacking direction leading to a ratio of Lz/Lxy > 2 (group 2), and
finally those much less crystallized with small coherent domains
in particular in the stacking direction leading to a ratio of Lz/Lxy<
1 (group 3).
Clearly, the capacities are much higher for this latter group,
indicating an influence of nanostructure size and morphology on
the LHS electrochemical properties. The beneficial eect of
nanostructuration on active surface sites giving rise to superior
electrochemical performances has been documented in many
studies.
38
Additionally, the 2D nanostructure morphology of AP
particles is expected to have the highest surface to volume ratio
providing even more active surface sites. This study of the Lz/Lxy
ratio also suggests that stacking order could have an unfavorable
eect on the electrochemical properties. Indeed, long stacking
coherence lengths are synonymous of stacking stability and we
believe this results in poor diusion properties in the interlayer
space, thus preventing the anion electrolyte intercalation and
deintercalation reactions during the electrochemical process.
39
Cycling stability measurements for up to 100 cycles show the
influence of the average apparent size of the crystalline domains
on the capacity stability and the occurrence of phase
transformation (Figure S8). Thus, small particles such as AP
particles show a rapid loss of capacity in the initial cycles (80%
capacity loss after 100 cycles for AP), while larger particles show
a more gradual capacity decrease. Yet, after 100 cycles, all of the
samples tend to the same capacity of ca. 40 mAh.g−1. It is
believed that the small particles with a large interfacial area
provide more active sites, which in return accelerate phase
transformation. Additionally, an electrochemical grinding from
the edges of the particles to the core is observed. The same phase
transformation occurs in all samples during electrochemical
cycling but is delayed depending on the particle size. US and
MWHT samples belonging to the group 1 Lz/Lxy ratio between
1 and 2 and displaying large particle sizes can represent a good
compromise between capacity and cycling stability retaining up
to 30 and 40%, respectively, of their original capacity at 100
cycles.
Figure 4. First cyclic voltammetry for Cu2(OH)3(NO3) sample series,
measured in 1 mol·L−1of sodium acetate at 10 mV·s−1.
Figure 5. Correlation between the capacities obtained from the CV of
the first cycle and structural coherence lengths Lzand Lxy obtained from
XRD analysis for Cu2(OH)3(NO3) sample series.
Crystal Growth & Design pubs.acs.org/crystal Article
https://doi.org/10.1021/acs.cgd.2c01468
Cryst. Growth Des. XXXX, XXX, XXX−XXX
F
By varying the scan rate in cyclic voltammetry and by plotting
the evolution of the Q/Qmax ratio with Qmax, the capacity
obtained at 1 mV.s−1(Figures 6 and S9), it is possible to
distinguish the impact of morphology on capacity. These data
clearly indicate that the electrochemical process is not purely
controlled by the surface and that the electrochemical reaction is
diusion-limited because at high scan rates (50 mV.s−1), less
than 10% of Qmax was reached for all syntheses. However, AP
sample provides 93% of Qmax at 10 mV.s−1in comparison with
the SO sample, which gives 62%. This is a direct consequence of
morphology and nanostructuration.
Phase Transformations during Cycling. Structural
changes that occur during cycling were investigated through
X-ray pair distribution function analysis carried out on the SO
sample recovered after 1, 10, 100, and 500 complete cycles and
at 0 V vs Ag/AgCl (Figure 7). PDF, as a total scattering
technique, probes both the crystalline and amorphous phases
and is an interesting tool in the structural evolution study of
electrode materials.
40
Using PDFgui software, we were able to
extract quantitative structural information. It was thus shown
that Cu2(OH)3(NO3) rapidly undergoes transformation into
cuprous oxide Cu2O (cycle 10). Then, the disproportionation
reaction of Cu(I)2O in acetate electrolyte can easily explain the
formation of the dimeric copper (II) acetate monohydrate
Cu(II)2(O2CCH3)4·2H2O as observed at cycle 100 and also the
formation of Cu(0) clearly visible at cycle 500. The
compositions of the electrode materials in weight % deduced
from PDF modeling are the following: 100% Cu2O (cycle 10),
36% Cu2O−64% Cu acetate (cycle 100), and 51% Cu2O−15%
Cu−34% Cu acetate (cycle 500).
The changes in pH were measured during cycling (Figure
S10), and it is quite interesting to note that the pH values
measured at potential values −1.2, 0, and 1 V vs Ag/AgCl on
successive CV cycles for the SO sample and placed on the
Pourbaix diagram of Cu−H2O−acetate (Figure S11) allow a
better understanding of material transformations and PDF
results. This allows us to propose the following sequence of
reactions (eqs 1−3). The present system is, however, a very
heterogeneous solid system, composed of domains more or less
exposed to the electrochemical process, which departs from the
Figure 6. Evolution of Q/Qmax depending on the scan rate for
Cu2(OH)3(NO3) sample series, measured in 1 mol·L−1of sodium
acetate at the first cycle.
Figure 7. (a) PDF data electrode materials for the SO sample after 1, 10, 100, and 500 complete CV cycles. (b) Fitting of the very local structure 1.5 < r
< 10 Å: experimental data are red stars, fits are black curves, and the dierence is below in blue.
Crystal Growth & Design pubs.acs.org/crystal Article
https://doi.org/10.1021/acs.cgd.2c01468
Cryst. Growth Des. XXXX, XXX, XXX−XXX
G
ideality of Pourbaix’s diagram. First, it is worth noting that the
stability of Cu2(OH)3(NO3) in 1 M sodium acetate at pH 8.2
was checked over a period of 24 h (Figure S12). After 10 cycles,
all of the LHS is reduced according to eq 1. This first hypothesis
is supported by the increase of pH following the release of the
hydroxyl group in the solution. Then, the Cu2O is reduced to the
Cu metal at negative potentials based on eq 2. At around −0.8 V
vs Ag/AgCl, water reduction occurs following eq 3, also
accompanied by a release of OH-species. In addition, Figure 8
clearly shows hydrogen evolution from 0.8 V vs Ag/AgCl after
10 cycles. This hydrogen evolution decreases with more cycles,
probably due to passivation of the copper or there is no more
copper formation
+
+ + +
Cu (OH) (NO ) 2e
Cu O OH NO H O
2 3 3
2 3 2
(1)
+ + +Cu O H O 2e 2Cu 2OH
2 2
(2)
+ +2H O 2e H 2OH
2 2
(3)
On the return scan, as the applied potential becomes positive,
the transformation of copper metal to cuprous oxide is observed
first at −0.2 V vs Ag/AgCl. We did not observe that the
transformation of cuprous oxide to cupric (expected according
to Pourbaix’s diagram) in the potential range between 0.15 and 1
V oxide takes place. After a few dozen cycles, in particular, at the
50th cycle, a new phase appears, which is Cu(II) acetate
Cu(II)2(O2CCH3)4·2H2O. We can draw a parallel between our
results and the study reported by Karantonis et al.
41
on the
anodic oxidation of copper. The formation of cuprous oxide on
the electrode surface was followed by extensive dissolution
mainly in the potential region where the Cu(II) monoxide layer
is likely to be formed. It was also established that the role of
acetate ions is not limited to the adjustment of the pH but is also
involved in the formation of basic copper acetate Cu-
(II)2(O2CCH3)4·2H2O.
If we return to the CV curves (Figure 8), at the 100th cycle,
three reductions peaks (−0.25, 0.5, and −0.88 V) and two
oxidation peaks (−0.62 and −0.44 V) are observed that can be
attributed to Cu(0)/Cu(I) and Cu(I)/Cu(II) transitions. At the
250th cycle, we observe a net increase in the cathodic peak
current densities and two anodic at −0.23 and −0.55 V,
indicating a change in the proportion of the phases. At 500th, a
single reversible redox peak is measured at −0.25 and −0.52 V
attributed to the Cu(II)/Cu(I) transition. The capacity loss is
important during the first 50 cycles and then stabilizes around 40
mAh.g−1(Figure S6). This decrease of capacity is a direct
consequence of phase transformation together with electro-
chemical grinding. We also assume that part of the material
transformed into the Cu metal is no longer subject to reversible
redox processes. Finally, regardless of the method of synthesis of
Cu2(OH)3(NO3), the similar capacity values observed at the
100th cycle all around 40 mAh.g−1strongly suggest a similar
phase mixture, i.e., [(Cu2(OAc)4·2H2O)1−x−y(Cu)x(Cu2O)y].
After a high number of cycles, the electrochemical grinding is
likely to annihilate the initial microstructural and morphological
dierences.
■CONCLUSIONS
In this study, we first explored dierent methods of syntheses for
the preparation of Cu2(OH)3(NO3)LHS, of which two
methods, the sonication and microwave methods, are original.
Significant eects were observed on the nanostructure size and
particle morphology and comprehensively investigated includ-
ing the determination of the structural coherence lengths along
the stacking direction (Lz) and in the plane of the hydroxide
layers (Lxy). The electrochemical activity of Cu2(OH)3(NO3)
was examined by cyclic voltammetry and clearly shows a
beneficial eect of nanostructuration on the capacities measured
on the first cycles. Those recorded for nanostructured
submicron AP particles displayed the smallest crystalline
domains and reached nearly 90% of the theoretical capacity
for a two-electron redox process with a capacity of 197 mAh.g−1
(assuming there is no water electrolysis). Unfortunately, the
electrochemical stability of this compound is poor and the
capacity rapidly drops, which makes it unsuitable for any storage
application at this stage.
It was also shown that the stacking order would have an
adverse eect on the electrochemical properties by limiting the
access of the electrolyte to the interlayer space. On the other
hand, cycling stability is more dependent on particle size, and for
this reason, US and MWHT samples formed of nanosized
crystalline domains, but agglomerated in micron-sized particles,
may represent a good compromise between capacity and
cyclability. The loss of capacity is mainly attributed to successive
phase transformations into Cu2O, Cu metal, and copper (II)
acetate as evidenced by PDF analysis performed on electrode
materials after repeated cycling. This probably contributes to an
electrochemical grinding, which is why all of the samples tend to
the same capacity of ca. 40 mAh.g−1after the 100th cycle.
■ASSOCIATED CONTENT
*
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.cgd.2c01468.
FTIR spectra, TGA curves, BET analysis, Rietveld
refinement results of PXRD data, cyclic voltammetry
curves, capacity evolution curves, measurement of pH
changes, and Cu−H2O−acetate Pourbaix diagram are
available (PDF)
■AUTHOR INFORMATION
Corresponding Author
Christine Taviot-Gueho −Clermont Auvergne INP, CNRS,
Institut de Chimie de Clermont Ferrand, UniversitéClermont
AuvergneInstitut de Chimie de Clermont Ferrand, F-63000
Figure 8. Cyclic voltammetry of 10, 50, 100, 250, and 500th cycles of
the Cu2(OH)3(NO3) SO sample at 10 mV.s−1in 1 mol·L−1of sodium
acetate.
Crystal Growth & Design pubs.acs.org/crystal Article
https://doi.org/10.1021/acs.cgd.2c01468
Cryst. Growth Des. XXXX, XXX, XXX−XXX
H
Clermont−Ferrand, France; orcid.org/0000-0002-9468-
2684; Email: christine.taviot-gueho@uca.fr
Authors
Julien Sarmet −Clermont Auvergne INP, CNRS, Institut de
Chimie de Clermont Ferrand, UniversitéClermont
AuvergneInstitut de Chimie de Clermont Ferrand, F-63000
Clermont−Ferrand, France; orcid.org/0000-0001-5827-
2031
Rodolphe Thirouard −Clermont Auvergne INP, CNRS,
Institut de Chimie de Clermont Ferrand, UniversitéClermont
AuvergneInstitut de Chimie de Clermont Ferrand, F-63000
Clermont−Ferrand, France
Fabrice Leroux −Clermont Auvergne INP, CNRS, Institut de
Chimie de Clermont Ferrand, UniversitéClermont
AuvergneInstitut de Chimie de Clermont Ferrand, F-63000
Clermont−Ferrand, France
Camille Douard −Clermont Auvergne INP, CNRS, Institut de
Chimie de Clermont Ferrand, UniversitéClermont
AuvergneInstitut de Chimie de Clermont Ferrand, F-63000
Clermont−Ferrand, France; Present Address: Nantes
Universite, CNRS, Institut des Materiaux de Nantes Jean
Rouxel, IMN, 2 rue de la Houssiniere BP32229, 44322
Nantes, Cedex 3, France. Reseau sur le Stockage
Electrochimique de l’Energie (RS2E), CNRS FR 3459, 33
rue Saint Leu, 80039 Amiens, Cedex, France
Insaf Gaalich −Clermont Auvergne INP, CNRS, Institut de
Chimie de Clermont Ferrand, UniversitéClermont
AuvergneInstitut de Chimie de Clermont Ferrand, F-63000
Clermont−Ferrand, France; Present Address: Nantes
Universite, CNRS, Institut des Materiaux de Nantes Jean
Rouxel, IMN, 2 rue de la Houssiniere BP32229, 44322
Nantes, Cedex 3, France. Reseau sur le Stockage
Electrochimique de l’Energie (RS2E), CNRS FR 3459, 33
rue Saint Leu, 80039 Amiens, Cedex, France
Thierry Brousse −Clermont Auvergne INP, CNRS, Institut de
Chimie de Clermont Ferrand, UniversitéClermont
AuvergneInstitut de Chimie de Clermont Ferrand, F-63000
Clermont−Ferrand, France; Present Address: Nantes
Universite, CNRS, Institut des Materiaux de Nantes Jean
Rouxel, IMN, 2 rue de la Houssiniere BP32229, 44322
Nantes, Cedex 3, France. Reseau sur le Stockage
Electrochimique de l’Energie (RS2E), CNRS FR 3459, 33
rue Saint Leu, 80039 Amiens, Cedex, France;orcid.org/
0000-0002-1715-0377
Gwenaëlle Toussaint −Clermont Auvergne INP, CNRS,
Institut de Chimie de Clermont Ferrand, UniversitéClermont
AuvergneInstitut de Chimie de Clermont Ferrand, F-63000
Clermont−Ferrand, France; Present Address: EDF R&D,
Department LME, Avenue des Renardieres, 77818 Moret-
sur-Loing, Cedex, France. Reseau sur le Stockage
Electrochimique de l’Energie (RS2E), CNRS FR 3459, 33
rue Saint Leu, 80039 Amiens, Cedex, France.
Philippe Stevens −Clermont Auvergne INP, CNRS, Institut de
Chimie de Clermont Ferrand, UniversitéClermont
AuvergneInstitut de Chimie de Clermont Ferrand, F-63000
Clermont−Ferrand, France; Present Address: EDF R&D,
Department LME, Avenue des Renardieres, 77818 Moret-
sur-Loing, Cedex, France. Reseau sur le Stockage
Electrochimique de l’Energie (RS2E), CNRS FR 3459, 33
rue Saint Leu, 80039 Amiens, Cedex, France.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.cgd.2c01468
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
The authors would like to thank the French National Research
Agency (ANR-20-CE05-0024, LaDHy project) and Labex
STORE-EX (ANR-10-LABX-76-01) for financial support.
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Crystal Growth & Design pubs.acs.org/crystal Article
https://doi.org/10.1021/acs.cgd.2c01468
Cryst. Growth Des. XXXX, XXX, XXX−XXX
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