Journal of The Electrochemical Society, 158 共3兲 F21-F28 共2011兲
F21
0013-4651/2011/158共3兲/F21/8/$28.00 © The Electrochemical Society
The Structure of Nickel Chloride in the Ionic Liquid
1-Ethyl-3-methyl Imidazolium Chloride/Aluminum Chloride:
X-ray Absorption Spectroscopy
D. F. Roeper,a,* K. I. Pandya,b G. T. Cheek,a,c,* and W. E. O’Gradya,*,z
a
Naval Research Laboratory, Washington, DC 20375, USA
SAIC, Brookhaven National Laboratory, Upton, New York 11973, USA
c
Chemistry Department, United Stated Naval Academy, Annapolis, Maryland 21402, USA
b
The structure of anhydrous nickel chloride in the ionic liquid 1-ethyl-3-methyl imidazolium chloride and aluminum chloride has
been investigated with extended X-ray absorption fine structure 共EXAFS兲 in both Lewis acid and Lewis base solutions. The
EXAFS data of NiCl2·6H2O crystals were also recorded and analyzed to demonstrate the difference file technique. The difference
file technique is used to obtain the structural information for the very closely spaced coordination shells of chloride and oxygen
in NiCl2·6H2O and they are found to agree very closely with the X-ray diffraction data. The difference file technique is then used
to analyze the nickel chloride in the ionic liquid solutions. Even though anhydrous NiCl2 is more soluble in the basic solution than
in the acidic solution, the EXAFS data show a single coordination of four chlorides in a tetrahedron around the nickel atom in the
basic solution. In a weak acid solution, there are six chlorides in a single octahedral coordination shell around the nickel. However,
in a strong acid solution, in addition to the octahedral chloride-coordination shell, there is a second coordination shell of eight
aluminum atoms in the form of a simple cube.
© 2011 The Electrochemical Society. 关DOI: 10.1149/1.3522762兴 All rights reserved.
Manuscript submitted June 29, 2010; revised manuscript received October 27, 2010. Published January 4, 2011.
Nickel is a commercially important element used in a variety of
applications including stainless steels, superalloys, shape memory
alloys,1-3 battery electrodes,4-6 and medical applications.7 It is also
used in applications requiring heat and corrosion protection. Subsequently, there is continued interest in the electrodeposition of metallic nickel and nickel alloys.8,9 The composition of the electrolytes
and the parameters used in the electrodeposition have a strong effect
on the structure and properties of the deposits.10 While nickel can be
deposited from nickel sulfamate baths and other water-based solutions, using nonaqueous solutions can allow for electroplating different alloys or different microstructures than are available from
aqueous solutions. Other metals cannot be deposited from aqueous
solutions because their potentials fall outside the window of stability
for water.
Using ionic liquids 共ILs兲 for the electrodeposition of metals can
avoid some of these difficulties because the region of potential stability is larger than that of water. This would allow for the preparation of some unique alloys. The electrochemical window can be
tailored to fit a particular application by choosing appropriate anions
and cations to form the melt. It is also possible to deposit pure
metals without the oxides and hydrides that can form in aqueous
solutions. The structures formed by the metal ions in the solutions
play an important role in the electrodeposition of the metals and
little is known about these structures in ionic liquids. Extended
X-ray absorption fine structure 共EXAFS兲 is a powerful in situ technique for examining the local structure of metal ions in both aqueous solutions and ionic liquids.11-13
We have investigated the structure of anhydrous nickel chloride
in the ionic liquid 1-ethyl-3-methyl imidazolium chloride 共EMIC兲
and aluminum chloride 共AlCl3兲 as a function of the acid/base character of the IL using EXAFS. AlCl3 is a Lewis acid and the acidity
of the IL can be manipulated by adjusting the ratio of the AlCl3 to
the EMIC. Further, NiCl2 is also a Lewis acid and plays an additional role in the structures formed in the electrolyte. The NiCl2 in
AlCl3/EMIC solution was studied with EXAFS in a series of Lewis
acid and Lewis base melts. This system has been the subject of
several earlier studies with EXAFS14,15 and electrochemistry.16,17
* Electrochemical Society Active Member.
z
Experimental
Reference samples.— Anhydrous NiCl2 共Alfa Aesar 99.9%兲 was
dried on a Schlenk line and transferred to a nitrogen-filled Vacuum
Atmospheres dry box where all the samples were prepared. The
oxygen and water concentrations in the dry box were below 1 ppm.
The NiCl2 powder was ground with a mortar and pestle, then spread
on Kapton tape and sealed in a polyethylene bag for transport to the
synchrotron. The NiCl2·6H2O 共Alfa Aesar 99.95%兲 powder sample
was not prepared in the dry box where it could lose water content
but was made by grinding it with a mortar and pestle under atmospheric conditions. It was then spread on Kapton tape and sealed in
a Kapton bag. A 7 ␮m nickel foil was used as a reference sample to
calibrate the energy of the beamline.
Ionic liquid sample preparation.— EMIC and AlCl3 were prepared and purified as described previously.18 All solution preparations were performed in the dry box, where the oxygen and water
concentrations were below 1 ppm. The NiCl2 solutions were prepared by adding the dried anhydrous NiCl2 to the basic AlCl3/EMIC
melt where the mole fraction of AlCl3 was N = 0.43. A melt with a
value of N ⬍ 0.5 is basic by definition and it is acidic with N
⬎ 0.5. To increase the concentration of NiCl2 in the acidic melts, the
solutions were prepared by adding dry anhydrous NiCl2 to the basic
AlCl3/EMIC melt, then adding AlCl3 to adjust the mole fraction of
AlCl3 to N ⬎ 0.5, giving the acidic solution. Concentrations of
AlCl3 were determined using the density calculations reported by
Fannin et al.19 The concentration of NiCl2 in the N = 0.43 melt is
102 mM and it is 30 mM for both the N = 0.54 and the N
= 0.60 melt. The ionic liquid solutions were sealed in either glass
ampoules or Kapton tubing 共polyimide tubing from Small Parts,
Inc.兲 and placed in a nitrogen-filled vacuum desiccator for transportation to the synchrotron.
X-ray Absorption Experiments.— The EXAFS experiments were
conducted on beamline X-11A at the National Synchrotron Light
Source at Brookhaven National Laboratory. The nickel K-edge
共8333 eV兲 X-ray absorption data were recorded in transmission
mode for the reference materials, 7 ␮m Ni foil, anhydrous NiCl2,
and Ni共OH兲2. A Lytle fluorescence detector was used to record the
spectra of the ionic liquid samples. All the EXAFS data were recorded at room temperature and the monochromator was detuned
20% at 400 eV above the edge to remove any higher harmonics
present in the beam. The storage ring operated at 2.8 MeV with
E-mail: [email protected]
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Journal of The Electrochemical Society, 158 共3兲 F21-F28 共2011兲
兺 A 共k兲sin关2kR + ␾ 共k兲兴
beam currents in the range of 100 to 260 mA. The experimental
setup on X-11A includes a Si共111兲 double-crystal monochromator
and three gas-filled ionization detectors to monitor the X-ray intensities of the incident and transmitted beams and a third chamber to
record the absorption of a reference foil to maintain the monochromator calibration. A Lytle detector using argon gas was used to
obtain the fluorescence EXAFS data for the very dilute NiCl2/ionic
liquid samples. No filters were used in the collection of this data.
Three scans were recorded for each sample and the analysis was
carried out on the averaged data.
␹共k兲 is the summation of the backscattering interference from the jth
shell of atoms around the absorber atom. The sine argument has two
contributions; one is from the electron traveling twice the absorber–
scatterer distance, Rj, and the other a phase shift term due to the
electron experiencing the potential of the absorber atom twice and
the potential of the backscattering atom once. Aj共k兲 is the amplitude
function and is given by
XAFS Data Analysis
Aj共k兲 = 共Nj /kRj2兲S20共k兲Fj共k兲exp关− 2共Rj兲/␭ − 2k2␴j2兴
When a beam of X-rays is transmitted through a material, the
intensity of the incoming X-rays is reduced by photoelectric absorption that is characteristic of the particular material. The decrease in
intensity dI due to absorption by a thickness dx of the material is
given by
where Nj is the number of atoms at Rj, the mean distance between
the absorbing atom and the backscattering atom in the jth coordination shell. S20共k兲 is an amplitude reduction factor to take into account
the shake up and shake off processes that occur. Fj共k兲 is the backscattering amplitude of the atoms in the jth coordination shell. Here,
␭ is the mean free path of the photoelectron which accounts for the
finite lifetime of the core hole and interactions with the valence
electrons. ␴2j accounts for deviations in Rj due to thermal vibrations
and static disorder.
The analysis of the X-ray absorption data is carried out using the
XDAP data analysis code.22 The EXAFS portion of the spectrum,
␹共k兲, is initially extracted from the absorption background using a
second-order polynomial removal, extrapolated from the pre-edge
data. This is followed by the removal of the postedge background
using a cubic spline removal technique. The EXAFS data are then
normalized to a per atom basis by dividing through by the step
height of the absorption edge.
The contribution of an individual shell in the EXAFS is isolated
by carrying out a kn共n = 1–3兲 weighted Fourier transform 共FT兲23 on
the EXAFS data which transforms it from k- to R-space, producing
a radial structure function 共RSF兲. The peaks in the RSF correspond
to the individual coordination shells in the sample. These peaks are
shifted from their true values to lower values of R due to the phase
shifts, discussed above. An individual shell can be isolated by carrying out an inverse FT from R-space back to k-space over a range
of ⌬R determined from the nodes in the imaginary portion of the FT
on either side of the shell of interest. Using this technique on data
for reference compounds with well-defined structural parameters,
the phase ␾j共k兲 and amplitude Aj共k兲 functions can be determined.
This reference phase and amplitude information are used to determine the structural parameters of an unknown sample by carrying
out a nonlinear least-squares fit in k-space following the isolation of
the ␹共k兲 from the experimental data. Methods for determining the
error bars for the calculated parameters have been previously discussed by Sayers20 and Koningsberger.22 FEFF 5 is the software program that was used to generate the theoretical spectra that were used
in the analysis.24,25 The model image was generated using CRYSTALMAKER: a crystal and molecular structures program from CrystalMaker Software Ltd, Oxford, England.
dI = − ␮共E兲I dx
where ␮共E兲 is the linear absorption coefficient and is a function of
photon energy. The Beer–Lambert law is obtained when Eq. 1 is
integrated over the total thickness of the material
It = I0e−␮共E兲x
where It is the intensity of the transmitted beam and I0 is the intensity of the incident beam. The linear absorption coefficient is proportional to the probability that an event will occur and is a function
of the initial state and final state wave functions. The X-ray spectrum is generally a smooth decreasing function of the photon energy.
However, when the energy of the X-rays is high enough to excite a
core level electron to an unoccupied level or to the continuum, there
is a sharp rise in the absorption intensity and this is the absorption
edge. At energies above the edge, the electron kinetic energy, Ek, is
given by
Ek = h␯–Eb
where hv is the energy of the incoming radiation, h is Planck’s
constant, v is the frequency of the X-ray, and Eb is the binding
energy of the electron, which corresponds to the energy of the absorption edge. Quantum mechanically, the ejected photoelectron can
be represented by an outgoing spherical wave with a de Broglie
wavelength ␭ given by
␭ = 2␲/k
and k is the photoelectron wave vector given by
k = 关8m␲2 /h2共h␯–Eb兲兴1/2
where m is the mass of the electron. In a single unbound atom such
as krypton, the photoelectron wave can travel without interference.
In a system where the absorbing atom is bound to other atoms, the
photoelectron wave will undergo scattering from the neighboring
atoms and produce a backscattered wave. The outgoing wave and
the backscattered wave can interfere constructively or destructively
depending on the distance between the absorbing atom and backscattering atom. The final state wave function is modulated by the
interference and hence the absorption coefficient displays a fine
structure due to this modulation. The EXAFS function characterizing the scattering of the outgoing wave by the neighboring atoms
forms the oscillatory part of the total absorption and is defined as
␹共E兲 = 关␮共E兲–␮0共E兲兴/␮0共E兲
where ␮共E兲 is the absorption coefficient of the sample and ␮0共E兲 is
the absorption coefficient of an isolated absorber atom and gives rise
to the atomic background. Using the plane wave approximation and
assuming a single electron, a single scattering event ␹共k兲 as a function of the wave vector is given by20,21
␹共k兲 =
j
j
j
j
Results and Discussion
Reference samples.— Anhydrous NiCl2, Ni共OH兲2, and Ni foil
were used as reference compounds. A preliminary analysis of this
data can be found in Ref. 15. A detailed analysis of the NiCl2·6H2O
data will demonstrate the use of the difference file technique,22
which we have used extensively in the later analysis of the NiCl2 in
the ILs. The analysis of EXAFS data requires the phase and amplitude functions for specific absorber–backscatterer pairs and these
functions are obtained from EXAFS data of reference compounds
with well-defined crystal structures containing the specific scattering
pairs of interest.
The normalized k1 weighted EXAFS spectra for NiCl2·6H2O and
anhydrous NiCl2 are compared in Figs. 1a and 1b and significant
differences are seen. X-ray diffraction data26,27 indicate that in
NiCl2·6H2O, the Ni–O and Ni–Cl bond distances differ by less than
0.5 Å, leading to a significant overlap in the O and Cl shells. This is
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Journal of The Electrochemical Society, 158 共3兲 F21-F28 共2011兲
F23
Figure 1. 共a兲 EXAFS spectrum for NiCl2·6H2O, and 共b兲 EXAFS spectrum
for anhydrous NiCl2.
Figure 3. Phase-corrected fits for the individual shells in NiCl2 · 6H2O for
共a兲 the oxygen shell and 共b兲 the chlorine shell.
clearly seen in the FT shown in Fig. 2b. An initial analysis over the
range 1 ⬍ R ⬍ 3 is carried out by isolating this area of interest with
an inverse FT. Then, this isolated portion of the data is analyzed
utilizing a two-shell fit in both k-space and R-space and the results
are shown in Figs. 2a and 2b. The fits are extremely good. A theoretical spectrum is calculated for the Ni–Cl contribution using the
data from the two-shell fit. The resulting spectrum is subtracted from
the experimental data, resulting in a spectrum which contains primarily the contribution of the Ni–O shell. This Ni–O difference
spectrum is then analyzed and the fit results are shown in Fig. 3a.
This procedure is repeated, a theoretical Ni–O is calculated and
subtracted from the experimental data, yielding a Ni–Cl difference
spectrum which is analyzed and the fit results are shown in Fig. 3b.
The final step in the difference file technique is to calculate a final
spectrum by adding together the spectra obtained for the individual
Ni–Cl and Ni–O contributions and taking the FT of the sum and
comparing it to the FT of the experimental data to determine the
quality of the fit achieved. In Figs. 4a and 4b, the fits in both k-space
and R-space are shown for the experimental data and the theoretical
fits obtained from the difference file technique, respectively. The ⌬k
and ⌬R parameters used in the analysis of NiCl2·6H2O are given in
Table I and the results of the fits are summarized in Table II.
Figure 2. Two-shell fit for NiCl2·6H2O in 共a兲 k-space and 共b兲 R-space.
NiCl2 in ionic liquids.— The experimental EXAFS data
共k1-weighted兲 of anhydrous NiCl2 dissolved in the N = 0.43 basic
ionic liquid and the N = 0.54 and N = 0.60 acidic ionic liquids are
shown in Figs. 5a-5c, respectively. The radial structure functions
resulting from the k-weighted Fourier transforms of these EXAFS
data are shown in Figs. 6a-6c, respectively.
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Journal of The Electrochemical Society, 158 共3兲 F21-F28 共2011兲
The FT for the N = 0.43 sample appears fairly symmetrical
without any shoulders, suggesting that there is only a single
absorber–backscatterer pair contributing to the shell. Using a Ni–Cl
phase-corrected Fourier transform will verify this and it is shown in
Fig. 7b. Here a symmetric envelope function for the magnitude
共solid line兲 is observed together with a simple symmetric imaginary
part 共dotted line兲 which peaks at the magnitude of the radial structure function, all indicating that there is only one type of scatterer
and that there are no other backscatterers present. This single shell is
isolated using an inverse FT 共k1, ⌬k = 1.8–12.2 Å−1, 0 ⬍ R ⬍ 3.0兲.
The single-shell analysis is carried out using a tetrahedral NiCl4
model and the resulting phase-corrected data in both k-space and
R-space are shown in Figs. 7a and 7b. The analysis indicates that
there are four chlorine atoms in the coordination shell. This is in
good agreement with previous EXAFS work on this ionic liquid
system.14,28 Gale et al.29 also found nickel chloride to be tetrahein a basic 1-butylpyridinium
drally coordinated as NiCl2−
4
chloride/AlCl3 ionic liquid. Aqueous solutions of NiCl2 in the presence of excess chloride,30 which makes them Lewis basic, also exhibit tetrahedrally coordinated nickel as NiCl2−
4 . The ⌬k and ⌬R
parameters used in the analysis are given in Table I and the results of
the fits are shown in Table II.
For NiCl2 dissolved in the acidic IL, N = 0.54, the data are
shown in Figs. 5b and 6b. The radial structure function resulting
from the FT, shown in Fig. 6b, shows one symmetric peak similar to
that found in the basic melt. This suggests that there is only one kind
of backscattering atom contributing to the coordination shell in this
system as well. A single-shell analysis was performed which yielded
a good fit, again indicating that there is only one kind of backscattering atom. However, in the acidic solution Ni is found to have a
coordination of six chlorine atoms. Although the nickel chloride is
more soluble in the basic ionic liquid which contains excess chloride
Figure 4. Comparison of the NiCl2 · 6H2O experimental data with the sum
of the calculated oxygen and chlorine shells in 共a兲 k-space and 共b兲 R-space.
Figure 5. EXAFS spectra for NiCl2 in the ILs. 共a兲 N = 0.43 basic solution;
共b兲 N = 0.54 acidic solution; 共c兲 N = 0.60 acidic solution.
Figure 6. Corresponding Fourier transforms for NiCl2 in the ILs. 共a兲 N
= 0.43 basic solution; 共b兲 N = 0.54 acidic solution; 共c兲 N = 0.60 acidic
solution.
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Journal of The Electrochemical Society, 158 共3兲 F21-F28 共2011兲
ions,31 the nickel coordinates with more chlorides in the acidic ionic
liquid. McMath and Hardacre also studied Ni ions with EXAFS. In
the acidic electrolyte, N = 0.55 EMIC/AlCl3 system using
NiCl2共PiPr3兲2 and 关Ni共MeCN兲6兴关BF4兴2, they found that the Ni ions
were coordinated by four chlorides, NiCl2−
4 , with no additional aluminum coordination at larger distances. This is the same coordination they found in the basic solution. McMath28,32 found a coordination of four chlorides while we found a coordination of six
chlorides. They used different nickel compounds and this could easily play a role in determining the coordination number. Most importantly, both research groups found the nickel to be coordinated with
only chloride ions in a low acid melt. The additional chloride coordination must come from either the AlCl−4 or the Al2Cl−7 ions in the
solutions. The single-shell analysis is carried out using an octahedral
NiCl6 model and the fits in k-space and the phase-corrected FT in
R-space are shown in Figs. 8a and 8b, respectively. The ⌬k and ⌬R
parameters used in the analysis are given in Table I and the results of
the fits are summarized in Table II.
For NiCl6 dissolved in the acidic IL, N = 0.60, the data are
shown in Figs. 5c and 6c. The RSF, Fig. 6c, shows a broad structure
with a large overlapping lobe on the high R side that is not seen in
the N = 0.54 acidic melt. An inverse FT is carried out over the
range 0 ⬍ R ⬍ 3.5 Å and a two-shell fit is carried out on the resulting data. There appear to be only two shells with a large overlap and
the two peaks are less than 1.0 Å apart. This makes it virtually
impossible to separate the two shells by the standard Fourier filtering technique and so the difference file technique will be
applied.22,33 The two possible coordinating species are Cl and Al as
there are no other species in the solution. A k1-weighted chloride
phase-corrected FT was carried out on the data isolated in the initial
inverse FT which contains the data for the two peaks, shown in Fig.
9. Here we see the magnitude of the peaks but they are not sufficiently separated to be helpful. However, the imaginary part of the
FT is peaked at the same position as the peak of the magnitude,
which tells us the first peak is due to Cl.22 The peak of the imaginary
part of the second peak has its maximum pointing in a position that
is offset from the maximum amplitude of the second peak, which
means that it is out of phase with that of Cl and therefore cannot be
Cl. The only other possibility is Al and, according to the calculations
of Teo and Lee,34 Al and Cl are an average of ⬃40° out of phase as
found experimentally.
Using the results from the two-shell fit, an individual ␹共k兲 shell
for aluminum was calculated and subtracted from the experimental
data. The resulting ␹共k兲 then contains only the contribution from the
chlorine shell. The single-shell analysis of these chlorine data resulted in the phase-corrected FT as shown in Fig. 10a. These chlorine results were used to calculate a chlorine ␹共k兲, which was then
subtracted from the experimental data to produce a spectrum with
the contribution due only to the aluminum scattering. A single-shell
Figure 7. 共a兲 Fit in k1- weighted k-space for NiCl2 in the N = 0.43 basic
solution. 共b兲 Phase-corrected Fourier transform of the Cl shell fit in the basic
solution.
Table I. Fourier transform parameters used for isolating the
Ni–O, Ni–Cl, and Ni–Al contributions from the experimental
compounds.
Sample
Shell
⌬k 共Å−1兲
⌬R 共Å兲
NiCl2·6H2O
Ni–O
Ni–Cl
Ni–Cl
2.1–13.3
2.2–11.3
1.8–12.2
0.9–2.3
0.8–2.4
0–3.0
Ni–Cl
2.0–10.4
0.2–3.0
Ni–Cl
Ni–Al
1.9–10.3
1.8–9.7
0–3.3
1.5–3.3
NiCl2 in basic
melt, N = 0.43
NiCl2 in acidic
melt, N = 0.54
NiCl2 in acidic
melt, N = 0.60
F25
Table II. EXAFS parameters determined for NiCl2·H2O and NiCl2 in the acidic and basic ionic liquids. N: coordination number, R: bond
distance, ⌬␴2: mean square relative displacement, E0: inner potential correction, SSR: sum of the square of residuals between the experimental
and the calculated spectra, indicating goodness of fit.
Sample
Shell
N
共 ⫾ 10%兲
R
共Å ⫾ 0.02 Å兲
⌬␴2
共Å−2 ⫾ 5%兲
E0
共eV ⫾ 10%兲
SSR
NiCl2·6H2O
Ni–O
Ni–Cl
Ni–Cl
3.7
2.0
3.9
2.22
2.43
2.28
−0.0017
−0.0049
−0.0027
−9.30
6.89
−1.84
0.019
0.002
0.134
Ni–Cl
6.3
2.37
0.0046
0.89
0.080
Ni–Cl
6.2
2.41
0.0016
2.03
0.033
Ni–Al
7.9
3.25
0.0090
−0.40
0.017
NiCl2 in
basic melt
N = 0.43
NiCl2 in
acidic melt
N = 0.54
NiCl2 in
acidic melt
N = 0.60
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F26
Journal of The Electrochemical Society, 158 共3兲 F21-F28 共2011兲
Figure 8. 共a兲 Fit in k-space for NiCl2 in the N = 0.54 acidic solution. 共b兲
Phase-corrected Fourier transform of the Cl shell fit in the acidic solution.
Figure 10. 共a兲 Phase-corrected Fourier transform of the Cl shell fit from the
NiCl2 in N = 0.60 acidic melt. 共b兲 Phase-corrected Fourier transform of the
Al shell fit from the same solution.
analysis of this aluminum shell resulted in the aluminum phasecorrected FT shown in Fig. 10b. If the second shell is phase corrected with the chloride reference, the peaks of the absolute envelope and the imaginary portion do not align themselves as they do in
Fig. 10b. This indicates that the second shell cannot be a farther out
shell of chloride ions. The separate calculated shells for Cl and Al
were then summed together and compared to the original data and
this is shown in Fig. 11. There is a good correlation between the sum
of the shells and the original data and this fit further confirms the
results and validates the analysis technique. The ⌬k and ⌬R parameters used in the analysis are given in Table I and the results of the
fits for both shells are shown in Table II. In this more acidic solution, the nickel is again coordinated by six chlorides, which are
further coordinated by eight aluminum atoms. The model for the
Figure 9. k1-weighted phase-corrected Fourier transform of the anhydrous
NiCl2 in N = 0.60 acidic melt.
Figure 11. The sum of the two calculated shells, Cl and Al, is compared to
the original data from NiCl2 in N = 0.60 acidic melt.
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Journal of The Electrochemical Society, 158 共3兲 F21-F28 共2011兲
Figure 12. A model of the nickel ion structure in the N = 0.60 acidic melt.
Ni is the central atom surrounded by the octahedron of Cl with an Al above
each face of the octahedron.
nickel ion structure in the N = 0.60 acidic IL is shown in Fig. 12.
Ni is in the central position with the first octahedral shell of Cl− and
a further shell of Al positioned above each face of the octahedron.
Acidic solutions have fewer free chloride ions available due to
the lower concentration of the Cl− contributing EMIC. The chloride
ions that are initially there will react with the AlCl3 to form AlCl−4
and EMI+ 共Refs. 31 and 35兲 as shown in Eq. 1. However, we have
found that the nickel coordinates with more chloride in the acidic
solution than in the basic solution. There are several further reactions, shown in Eq. 2 and 3, that can occur in these solutions.31 A
metal chloride can give up a chloride ion to react with Al2Cl−7 in Eq.
2, or as shown in Eq. 3, as the acidity of the solution increases, less
Cl− is available and, depending on the concentration of AlCl3, the
equilibrium of Reaction 3 will shift to the right, forming more
Al2Cl−7
AlCl3 + EMIC = AlCl−4 + EMI+
关1兴
AlCl−4 + AlCl−4 = Al2Cl−7 + Cl−
关2兴
AlCl−4
+ AlCl3 =
Al2Cl−7
Al2Cl−7
关3兴
can withdraw chloride from the
In strong acidic solutions,
dissolved metal chloride salt if the dissolved metal chloride is a
weaker Lewis acid than Al2Cl−7 共Ref. 35兲 and shift the equilibrium of
Reaction 2 to the left. If a metal chloride is dissolved in an acidic
AlCl3/EMIC solution and does not lose its chlorides, this suggests
that the added metal chloride is comparable in Lewis acid strength
to Al2Cl−7 . In a weaker acidic solution, e.g., N = 0.54, the anion
fraction is ⬃80% AlCl−4 with only ⬃20% Al2Cl−7 being formed.36
NiCl2 is a stronger Lewis acid than AlCl3 under these conditions and
there is a surplus of AlCl−4 that can contribute a Cl− to the Ni to
achieve the octahedral coordination that is observed in the EXAFS
data.
In the stronger acidic melt N = 0.60, a significant amount of
Al2Cl−7 is expected to form.31,36 Earlier research on this ionic liquid
and AlCl3 with other cations31,37,38 indicates that at N = 0.60 and
greater, additional species such as Al3Cl−10 can form as well. The
second shell of aluminum atoms in the N = 0.60 melt suggests that
at this level of acidity, the nickel chloride and Al2Cl−7 are of com-
F27
parable Lewis acidity and hence share the chlorides. A Raman spectroscopy study by Gale et al.39 showed a progressive change in the
spectra as the acidity increased from neutral at N = 0.50 to N
= 0.67. Certain peaks attributed to the Al2Cl−7 are not observed until
N reaches a value of 0.60 and peaks attributed to AlCl−4 are lost.
Additional changes occur at higher acidities between N = 0.60 and
N = 0.67. Similar results have been observed with infrared
spectroscopy.40 Although they did not record any data between N
= 0.50 and N = 0.60, it is reasonable to expect progressive changes
in the species in that region as well.
The detailed study of the nickel chloride coordination in this
work is in good agreement with the results in the paper published by
Dent et al.,14 which showed none of the data analysis. However, the
work of Dent et al. did not observe the more complex coordination
of the Ni as the acidity of the IL increased. Whereas our data indicate a complete second shell of eight aluminum atoms in the N
= 0.60 melt, they offered an estimated result of three aluminum
atoms. Their paper does not provide the details of their analysis.
Therefore it is difficult to compare the results for aluminum. Further,
our data extend to higher energies, which provide more details. They
had a stronger concentration of nickel chloride, 50 vs 30 mM, and
this may further account for some of the difference seen in the
results. As shown above, NiCl2 is also a strong Lewis acid and the
higher concentration could have changed the speciation in their solution to create another intermediate coordination with only three
aluminum atoms in the second shell. There is a transition of coordination geometry as the solution changes from basic 共4 Cl alone兲 to
mildly acidic 共6 Cl alone兲 to strongly acidic 共6 Cl followed by 8 Al兲,
and there may be another transition to 3 Al second shell coordination in different acidic solutions.
Conclusions
The difference in the coordination environment for NiCl2 in the
basic and acidic ionic liquids has been revealed using EXAFS analysis. Although anhydrous NiCl2 is more soluble in the basic solution
than in the acidic solution, the Ni ion has tetrahedral coordination
with four chlorides in the N = 0.43 basic solution. In the weakly
acidic solution, N = 0.54, the Ni has octahedral coordination with
six chlorides. The lack of a second shell of aluminum coordination
indicates that Al2Cl−7 has not formed to a great extent and that NiCl2
is a stronger Lewis acid than AlCl3 because it can withdraw chlorides from the main component, AlCl−4 . In the stronger acidic solution of N = 0.60, a second shell of eight aluminum atoms is found,
indicating that the shell of chlorides in the stronger acidic melt is
due to coordination with the aluminum chloride in the melt. This
indicates that NiCl2 is comparable to Al2Cl−7 in Lewis acidity.
Acknowledgments
The authors thank Professor Hardacre for helpful discussions.
The authors acknowledge the financial support of the Office of Naval Research and the American Society for Engineering Education
共ASEE兲. This research was carried out at the National Synchrotron
Light Source, Brookhaven National Laboratory, which is supported
by the U.S. Department of Energy, Division of Materials Sciences
and Division of Chemical Sciences, under contract no. DE-AC0298CH10886.
Naval Research Laboratory assisted in meeting the publication costs of
this article.
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