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Electrochemical
behaviors of
Dy(III)
and
its
co-reduction
with
Al(III)
inmolten
LiCl-KCl salts
Ling-Ling Su ab Kui Liu b Ya-Lan Liu b Lu Wang b L i-Yong Yuan b Lin Wang b Zi-Jie LibXiu-Liang Zhao a Zhi-Fang Chaibc Wei-Qun Shiba School of Nuclear Science and Technology University of South China HengYang 421000 ChinabKey Laboratory of Nuclear Radiation and Nuclear Energy Technology Institute of High Energy Physics Chinese Academy of Sciences Beijing 100049 Chinac School of Radiological amp Interdisciplinary Sciences Soochow University Suzhou 215123 China
A R T I C L E I N F O
Article history
Received 15 July 2014
Received
in
revised
form
17
September
2014
Accepted 19 September 2014
Available online 22 September 2014
Keywords
molten chlorides
dysprosium
AlCl3intermetallic compounds
co-reduction
A B S T R A C T
In thiswork theelectrochemicalbehaviors ofDy(III) and itsco-reductionwithAl(III) onan inert tungsten
electrode was investigated in LiCl-KCl molten salts at the temperature of 773K by using cyclic
voltammetry (CV) chronopotentiometry (CP) and square wave voltammetry (SWV) techniques The
results showed that the reduction of Dy(III) ions in LiCl-KCl salts is a reversible diffusion controlled
process through a one-step reaction Dy(III) + 3e$ Dy(0) The diffusion coef 1047297cient of Dy(III) ions was
calculated byboth theCV andCPmethods Furthermorethe co-reductionof Al(III) andDy(III) ionson the
inert tungstenelectrodeallowsDy(III) ions tobe reduced at amore positivepotentialthroughforming Al-
Dyalloys Theconcentration ratioofAl(III) cationsto Dy(III) cations hasa large impacton theformationof
Al-Dy alloys In a Dy(III) ion rich system three signals attributed to the formation of Al-Dy intermetallic
compounds were observed in CV and SWV analyses while only two signals corresponding to Al-Dy
intermetallic compoundswere observed in the Dy(III) ion poor system Potentiostatic and galvanostatic
electrolyses performed on an aluminum electrode identi1047297ed the co-reduction by the formation of one
(Al3Dy) and two Al-Dy alloys
(Al3Dy AlDy) respectively Finally the electrolysis products were
characterized by the Scanning ElectronMicroscopy (SEM) coupled with EnergyDispersive Spectroscopy
(EDS) and X-ray diffraction (XRD) analysesatilde
2014 Elsevier Ltd All rights reserved
1 Introduction
Partitioning and Transmutation (PampT) is universally accepted to
be one of the key-steps in any future sustainable nuclear fuel
cycles in which high ef 1047297cient separations of actinides (An) and
lanthanides (Ln) are generally expected [1] Ln could account for as
much as 25 in weight of the whole 1047297ssion products (FP) [2] and
the strong neutron absorption cross sections of Ln would largely
pull down the transmutation ef 1047297ciency [3] However the
physicochemical properties of Ln and An are very similar and
make the separation of An from Ln extremely challenging [4]
The traditional pyrochemical electrore1047297ning process based on
chloride or
1047298uoride molten salts has been regarded to be a
promising alternative for future spent nuclear fuel cycle [5ndash7] The
simple inorganic molten ionic solvents are immune to radiation
damage and transparent to neutrons In a typical electrore1047297ning
process a solid stainless steel cathode and a liquid Cd cathode were
used to achieve the recovery of uranium and transuranium
elements respectively One of drawbacks of using this liquid Cd
cathode is that the content of Ln in deposited products is relatively
high compared to traditional solvent extraction based processes
which restrains the propagation of the process in industrial scale
As for the ef 1047297cient separation of An from Ln active solid
aluminium cathode seems to be a promising alternative It has
been found that the disparity of the deposition potential between
An and Ln is larger on the solid aluminium cathode than that on
other metal electrodes due to the formation of Al-An alloys [78] In
previous works a relatively high separation ef 1047297ciency of An over
Ln [7910] and excellent extraction and recovery of An [91112]
have been identi1047297ed possible on an Al cathode To establish a
reliable Al cathode based electrochemical pyrochemical process it
is still quite necessary to investigate the electrochemical proper-
ties of representative Ln in AlCl3 contaning melts Actually we have
successfully used solid Al cathodes to extract some Ln elements
from melts by forming Al-Ln intermetallic compounds [13ndash16]
Corresponding author Tel +86 010 88233968 fax +86 010 88235294
E-mail addresses shiwqihepaccn (W-Q Shi) 13974753181163com
(X-L Zhao)
httpdxdoiorg101016jelectacta201409095
0013-4686atilde 2014 Elsevier Ltd All rights reserved
Electrochimica Acta 147 (2014) 87ndash95
Contents
lists
available
at
ScienceDirect
Electrochimica
Acta
journa l h omepage wwwe lseviercomloca te e le cta cta
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Actually this method has been con1047297rmed in the LiCl-KCl melt
for the preparation of other Ln and An [1012ndash1623] chloridesIn the electrolytic process anhydrous Dy2O3 and AlCl3 (both AR
grade) powders were directly added to the LiCl-KCl melt In LiCl-
KCl melt with the working temperature of 773 K AlCl3 can be easily
gasi1047297ed and turn into Al2Cl6 part of them reacts with Dy2O3 to
release Dy(III) ions This reaction can be represented as
Dy2O3 (s) + Al2Cl6 (g) == Al2O3 (s) + 2 DyCl3 (l) (2)
From the thermodynamic data [29] the change of Gibbs energy
of this reaction at 773 K is calculated to be -90345 kJ mol1 It
reveals that reaction (2) could proceed forward at our experimen-
tal temperature In this work we 1047297rstly explored the electrochem-
ical behaviors of dysprosium on an inert tungsten electrode hence
after the complete chlorination of Dy2O3 the LiCl-KCl-DyCl3 melt
was puri1047297ed completely out of AlCl3 by bubbling dry argon
continuously until the ICP-MS analysis of the taken melt sample
shows no remnant Al(III) ion The concentration of Dy(III) ions in
the LiCl-KCl-DyCl3 melt was measured at the same time Then the
co-reduction process of Dy(III) and Al(III) were investigated by
increasing the content of AlCl3 in the melts However owing to the
volatility of AlCl3 the concentration of AlCl3 we present below is
the initial fractions when AlCl3 were added into the melts
23 Electrochemical electrodes and characterization of cathodic
deposits
A custom-built quartz structure was used to position all of the
electrodes and the thermocouple in molten salt A silver wire (d
= 1 mm 9999) dipped into the solution of AgCl (1 wt) in LiCl-
KCl melts contained in a Pyrex tube was used as the reference
electrode All potentials were referred to the Ag+Ag couple As for
the counter electrode a 6 mm graphite rod was used The working
electrode consisted of 1 mm tungsten (W)wire with the lower end
polished by SiC paper Before each measurement the working
electrode was cleaned by galvanostatic anodic polarization The
active electrode surface area was calculated after each experiment
by
measuring
the
immersion
depth
of
the
electrode
in
the
moltensalts As to the electrolysis process an aluminum plate (Alfa
99999) with thick to be 2 mm was used as cathode After
electrolysis the aluminum electrode was abraded and polished by
SiC paper followed by ultrasonic cleaning in ethylene glycol and
ethanol (Sinopharm 998) in an ultrasonic bath for 15 min and
stored in the glove box before analysis
3
Results
and
discussion
31 Electrochemical behavior of Dysprosium Ions on the Tungsten
Electrode
311 Cyclic Voltammetry
In
the
present
work
investigations
of
dysprosium
began
withCV measurements to establish the nature of the system and the
reversibility of the observed reactions The typical CV curve of the
pure LiCl-KCl melts is shown in Fig 1 (dotted curve) The
electrochemical window offered by the LiCl-KCl melts have been
reported to be limited between the reduction of lithium ions
(peak L c) and the anodic release of chlorine [1217] The fact that
there is no other additional peak in its electrochemical window
identi1047297es the applicability of the LiCl-KCl melts for our inves-
tigations
Fig 1 also shows the typical CV of LiCl-KCl-DyCl3 (373 105
mol cm3) mixture with the scan rate of 01Vs1 on a W working
electrode at the temperature of 773 K The signals EaEc were
observed in the voltammogram with the reduction peak (Ec) at
-204
V
and
the
corresponding
anodic
peak
(Ea)
at
-184
V
respectively The reduction (Ec) occurs in a single sharp peak
mode with a gradual decay manifesting the deposition of an
insoluble phase [3031] The reverse anodic scan shows an
oxidation peak (Ea) with much higher amplitude than the
reduction peak due to the availability of the deposited metal for
the re-oxidation According to the previous works of Zhang et al
[22] Chang et al [18] and Konishi et al [19] peaks Ea and Ec had
been ascribed to the deposition and dissolution of Dy metal It is
possible that dysprosium metal would be deposited in a single
direct step by direct reduction of Dy(III) ion into Dy(0)
Furthermore the reversibility of the reaction of deposition and
dissolution of Dy(III)Dy(0) was evaluated over a wide scan rate
range from 005 to 03 Vs1 As shown in Fig 2a the peak potential
shifts very slightly with the increasing scan rates Therefore the
reduction of Dy(III) to metal should be considered to be a reversible
process The Nernstian behavior of the reaction at low scan rates
can be further con1047297rmed by plotting the mid-peak potential as a
function of the scan rate As shown in Fig 2c the mid-peak
potential almost remains stable (-196 V) at the scan rates of 005
01 015 and 02 Vs1 In addition the plot of the cathodic peak
current versus the square root of the sweep rates shows a linear
relationship in Fig 2b indicating the process is a diffusion
controlled one Therefore it is plausible to use the Berzin-Delahay
equation [32] in this work for a soluble-insoluble couple accordingto the theory of linear sweep voltammetry
Ip= 0061(nF)32C0D12V12(RT)12S (3)
where n is the number of exchanged electrons F denotes the
Faraday constant (96500C mol1) Co represents the solute con-
centration (mol cm3) D corresponds the diffusion coef 1047297cient
(cm2 s1) v designates the potential scanning rate (V s1) T is the
absolute temperature (K) and S corresponds the electrode area
(cm2)
The measurement of the slope of the curve in Fig 2b yields the
following relation at T = 773 K and C0= 373 105mol cm3
I p
V 1=2 frac14 eth0068 000082THORNAS1=2V 1=2 (4)
Assuming n = 3 through the combination of Eqs (3) and (4) the
diffusion coef 1047297cient (D) of Dy(III) ions under this condition can be
calculated to be 510 106 cm2 s1
Fig 2 (a) CVs for 373 105molcm3DyCl3 in LiCl-KCl melts at various scan rates
Working electrode W (S068 cm2) Scan rates 005 01 015 02 025 and 03 Vs1
(b) Plot of the cathodic peak current as a function of the square root of the scan rate
(c) Mid-peak potential as a function of the scan rate The dashed curve represents
the
average
mid-peak
potential
(-196
V
vs
Ag
+
Ag)
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In addition it can be found that there is always a fuzzy pre-
platform more anodic to the reduction peak Ec in Fig1 and 2which
is very drawn out at all scan rates in Fig 2 suggesting that this
process could not be diffusion controlled However there are still
some discrepancies about the ascription of this pre-platform
[172123] Ref [1723] ascribed it to the reduction of Dy(III) to Dy
(II) whereas Ref [21] held that the similar pre-platform before the
reduction of Dy(III) ions in LiF-CaF2 melt might be an adsorption
effect of Dy(III) ions to the surface of the working electrode
Combining the results of following SWV we prefer to supporting
the pre-platform is attributed to the adsorption effect of Dy(III)
ions to the surface of the W electrode
312 Chronopotentiometry
The electrochemical behavior of the redox couple Dy(III)Dy(0)
was also studied by CP technique Fig 3 shows the evolution of the
CPs of DyCl3 in LiCl-KCl melts with the applied current density from
-16 mA to -24 mA on a W electrode These curves exhibit a single
wave in the same potential range as that observed in the CV curves
and therefore should be associated with the reduction of Dy(III) ions
into metal In the CP technique transition time (t) means the time
necessary to observe the complete depletion of the electroactive
species (here the Dy(III) ion) resulting from the diffusion in the layer
of electrolyte at the electrode surface From Fig 3a it can be found
that t decreases with the increase of the applied current density In
addition the time-current relationship at a constant value in Fig 3b
proves
the
diffusion-controlled
process
of
Dy(III)
to Dy(0)
and
thevalidity of Sands law (Eq (5)) [32]
it 1=2 frac14 05nFSC ethpDTHORN1=2 (5)
where t is the transition time (s) i denotes the applied current (A)
We also assumed the number of exchanged electron n = 3 The
diffusion coef 1047297cient at T = 773 K and C = 346 105mol cm3 was
calculated to be 172 105 cm2 s1
Table 1 gathered the diffusion coef 1047297cients of Dy(III) in LiCl-KCl
eutectic at 773 K Under the same conditions the diffusion
coef 1047297cient calculated by Ref [17] is approximately three times
higher than that measured by Ref [6] The diffusion coef 1047297cient of
Dy(III) ion in LiCl-KCl melts determined in this work was 51 106
cm2 s1 and 172 105 cm2 s1 using CV and CP techniques
respectively The complex chemical behaviors of Dy(III) ions in
LiCl-KCl melts differences in the principles of CV and CP
techniques [33] and imprecise measurement of the wetted length
of the working electrode could account for these discrepancies
Similar
phenomena
were
observed
in
the
case
of
cerium
anduranium in Ref [3435]
In the above calculation of diffusion coef 1047297cient from the results
of CV and CP the number of exchanged electrons was assumed to
be 3 Actually the number of exchanged electrons can be deduced
from combining the results of CV and CP as stated in Ref [3637]
By using Eqs (3) and (5) the following formula was obtained
n frac14
I p
V 1=2 1
ip1=2
05
061
2
D2C 2
D1C 21
p2RT
F (6)
where the concentration (C) and diffusion coef 1047297cients (D)
subscripted with 1 and 2 are related to CV and CP respectively
According to the results obtained from Fig 2b Ip=V1=2 frac14 eth0068
000082THORNAS1=2V1=2 and Fig 3b it 1=2 frac14 25 102 AS 1=2 the
number of exchanged electrons n301 was achieved almostentirely consistent with the theoretical expectation
313 Square wave voltammetry
SWV with a better accuracy to calculate the number of
electrons exchanged in an electrochemical process was then
employed to con1047297rm the number of exchanged electrons in this
experiment
For a single electrochemical reversible process the differential
intensity measured at each step between the successive pulses
exhibits a Gaussian relationship with the potential Mathematical
analysis of the Gaussian peak yields a simple equation associating
with the width of the half peak (W12) with temperature and the
number of electrons exchanged (n) [3839]
W 1=2 frac14 352RT
nF (7)
where R is the universal gas constant T denotes the absolute
temperature in K n represents the number of exchanged electrons
and F designates the Faradays constant
Fig 4 shows a typical SWV of DyCl3 (373 105mol cm3) in the
LiCl-KCl melts on a W electrode at the frequency of 20 Hz A sharp
peak (Ec) associated with the reduction of Dy(III) ion can be clearly
observed in the same potential range as CV and CPHowever peak Ecis not exactly symmetric as predicted by the theory of nucleation
effect Due to which the rise of the current is delayed by the
overpotential caused by the solid phase formation thus the
increasing part of the differential current is sharper than the
decreasing one The disturbance of the signal due to the nucleation
Fig 3 (a) CPs of LiCl-KCl-DyCl3 (346 105molcm3) melts at 773 K Working
electrode W (S068 cm2) Applied current -16 -18 -20 -22 and -24 mA (b)
Relationship
between
the
square
root
of
the
transition
time
and
the
applied
current
Table 1
Diffusion coef 1047297cients of Dy(III) in LiCl-KCl eutectic at 773 K
Reference 105 D cm2 s1 Dy(III) Concentration (104ppm) Technique
[17]
147
201
CP
[6] 046 20 CP
[6] 07 06 CP
this work 051 061 CV
this work 172 057 CP
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However large differences were observed between signals L aL cand AaAc in Fig 5a As for the red curve peak Ec at -204 V and Ea at
about -183 V can be ascribed tothe reduction of Dy(III) tometal and
its subsequent re-oxidation respectively according to the above
results of LiCl-KCl-DyCl3 system and Ref [617] Between peaks EaEcand AaAc two broad anodic peaks at about -130 and -165 V were
observed The CVs measured subsequently in the same system at
different terminal potentials display a broad anodic peak at
approximate -13 V which actually consist of two close peaks Ia and
IIa similar to their cathodic peaks Ic and IIc (Fig 5b) In addition the
SWV in Fig 6 (curve 2) also revealed the existence of the two close
peaks Ic and IIc at approximate -13 V According to the co-reduction
principle redox peaks IaIc IIaIIcand IIIaIIIc should be ascribed tothe
formation
and
dissolution
of
at
least
three
AlxDyy intermetalliccompounds Moreover the closer the deposition potential of the
intermetallic compound to that of Dy metal the higher Dy content
could be formed in the AlxDyy intermetallic compound [1215]
As for the black curve in Fig 5a CV of AlCl3-rich system much
higher current intensities of peaks Aa and Ac were obviously
observed compared to that of AlCl3-poor system Besides other
differences could also be observed in the two curves of Fig 5a For
example the broad anodic peak at -13 V in the red curve separated
into two redox peaks IaIc and IIaIIc in the black curve The CVs
measured at various cathodic terminal potentials in AlCl3-rich
system (Fig 5c) and the SWV (curve 3 in Fig 6) also present two
clearly separated redox peaks IaIc and IIaIIc However the redox
signals IIIaIIIc and EaEc corresponding to the formation and
dissolution of an intermetallic compound with higher Dy content
Fig 5 (a) CVs of LiCl-KCl-AlCl3-Dy2O3 (09 wt) melts with different AlCl3concentrations 08 wt (red curve) and 12 wt (black curve) (b) CVs of the LiCl-
KCl-AlCl3 (08 wt)-Dy2O3 (09 wt) melts (c) CVs of the LiCl-KCl-AlCl3 (12 wt
)-Dy2O3 (09 wt) melts at different inversion potentials Working electrode
W (S068
cm2)
Temperature
773
K
Scan
rate
01
Vs1
Fig 7 CVs of LiCl-KCl-DyCl3 melts (black dotted curve) and LiCl-KCl-AlCl3-DyCl3melts (red solid curve) on an Al electrode Temperature 773 K Scan rate 01
Fig 6 SWVs of LiCl-KCl-DyCl3 (373 105molcm3) melts (curve 1) and LiCl-KCl-
AlCl3-Dy2O3 (09 wt) melts with different AlCl3 concentrations 08 wt (curve 2)
and 12 wt (curve 3) Working electrode W (S068 cm2) Temperature 773 K
Pulse height 10 mV potential step 5 mV frequency 20 Hz Vs1
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and the redox couple of Dy(III)Dy(0) vanished from both the CV
(Fig 5c) and SWV (curve 3 in Fig 6) curves In the meanwhile a
new
couple
of
peaks
marked
as
IVaIVc emerged
with
its
cathodicand anodic potential at approximate -216 and -205 V respectively
which should be ascribed to be the reduction and oxidation of Al-Li
alloy [1217] The reason of the difference between the two curves
in Fig 5 could be as follows With the increase of AlCl3concentration a much thicker layer of Al was deposited on the
W electrode which would facilitate the diffusion of Dy metal
Subsequently the initially generated intermetallic compound
AlxDyy tends to be transformed into another intermetallic
compounds AlxDyy with high Al content Therefore peaks IIIa
IIIc and EaEc which correspond to the formationdissolution of an
intermetallic compound with high Dy content and the redox
couple Dy(III)Dy(0) respectively could not be observed In
addition when the deposited AlxDyy intermetallic compounds
were not fully mantle the Al-covered electrode Al-Li alloys would
have the chance to be formed [15]
Electrochemical behaviors of LiCl-KCl melts containing both
Al(III) and Dy(III) cations were also investigated on an Al electrode
Fig 7 provides a comparison about the CVs of LiCl-KCl-DyCl3 and
LiCl-KCl-AlCl3-DyCl3 melts using Al as the working electrode The
CV of LiCl-KCl-DyCl3 melts without Al(III) cations (black dotted
curve) is consistent with Ref [17] Peaks IcIa are ascribed to the
formation and dissolution of Al-Dy alloys on the Al electrode The
red solid curve in Fig 7 shows a typical co-reduction behavior of
Al(III) and Dy(III) ions on the Al electrode which is very similar to
that obtained in LiCl-KCl-DyCl3 melts although the peaks become
more bulky This could be caused by the formation of different Al-
Dy alloys through the co-reduction of Al(III) and Dy(III) cations at
more cathodic potential [50]
33
Preparation
and
characterization
of
the
Al-Dy
alloys
To
con1047297rm
the
co-reduction
of
Dy(III)
and Al(III) ions andexamine the formation of AlxDyy intermetallic compounds at
various concentration ratio of Al(III) and Dy(III) both potentio-
static and galvanostatic electrolyses were carried out on a
tungsten electrode However only a very small amount of Al-
Dy alloys that adhered to the W electrode could be obtained even
the experiment was repeated for several times This phenomenon
is probably caused by the small cathode current and the high melt
point of the Al-Dy alloys Therefore we further used an Al plate
electrodewith the size of 15 cm 15 cm 02 cm for electrolysis
To prepareAl-Dy alloys at more anodic potential potentiostatic
electrolysis at -14 V -15 V and -16 V each for 3 h respectively
was performed Fig 8 shows the XRD patterns and the cross-
section SEM images coupled with EDS analysis of the cathodic
deposits of potentiostatic electrolysis It turns out that the
electrolysis at -14 V achieved nothing but Al metal while
electrolysis at -15V and -16 V produced a uniform layer covering
on the Al plateelectrode (Fig 8a and c) ByXRD analyses (Fig 8b)
the composition of the deposition layer obtained at -15 V was
con1047297rmed to be Al metal and the intermetallic compound Al3Dy
with crystallographic structure of rhombohedral lattice (R-3 m)
(PDF in XRD data base 18ndash0020) When electrolysis was
performed at more negative potential of -16 V the intermetallic
compound Al3Dy with crystallographic structures of R-3 m and
hexagonal lattice (P63mmc) (PDF in XRD data base 65ndash6363)
could be both obtained
It is well known that potentiostatic electrolysis has the
advantage of controlling the composition of the compound
produced by the cathodic reaction According to the co-reduction
Fig 8 SEM-EDS and XRD results of the potentiostatic electrolysis products of LiCl-KCl-AlCl3 (12 wt)-Dy2O3 (09 wt) melts on the Al electrodes (a) SEM image (deposited
at -16 V) (b) XRD pattern (deposited at -150 V and -160 V) (c) Enlarged SEM image (deposited at -16 V) (d) EDS result (deposited at -16 V)
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behaviors above at least two kinds of AlxDyy intermetallic
compounds
(DyAl3 and
DyAl2)
could
be
formed
by
potentiostaticelectrolysis at -15 and -16V since two very close pairs of
redox peaks were observed in CVs and SWV However only one
kind of intermetallic compound Al3Dy was acquired which was
against with our expectation The main reason might be the low
current density at our experimental concentration Under this
condition even though the intermetallic compound DyAl2 was
formed the formation rate was much slower than its diffusion
rate then the transformation of DyAl2 into the more stable Al-
rich phase (DyAl3) on the Al electrode would take palce Hence
ultimately only one kind of intermetallic compound DyAl3 was
observed in Fig 8 similar phenomena had been observed in the
potentiostatic electrolysis for the preparation of Al-Gd alloys
[15] The fact that intermetallic compound Al3Dy with more
crystallographic structures was obtained by electrolysis at more
negative potential to some extent shows the importance of
nucleation overpotential for the growth of Al-Dy alloys onto the
electrode
To provide a stable current to equably form more AlxDyyintermetallic compounds galvanostatic electrolysis with the
current intensity of -50 mA was also carried out for 25 h in our
experiment During the electrolysis the cathode potential was
controlled within the range of -13 V to -175 V to prevent the
deposition of pure Dy and in the meanwhile cover the two much
more anodic redox peaks (IaIc and IIaIIc) associated with the
formation of AlxDyy intermetallic compounds As shown in the
SEM image in Fig 9a a much thicker layer of approximate 40
mm of
the deposits was obtained than that gained by potentiostatic
electrolysis (Fig 8a) The XRD result in Fig 9b con1047297rms that the
deposits are composed of intermetallic compounds DyAl3 and
DyAl
although
DyAl2 was
still
not
observed
which
proves
onceagain that DyAl2 could not be stable at this temperature and easily
be transformed into DyAl3 The EDS analyses coupled with SEM in
Fig 8d and Fig 9d also con1047297rmed the co-existence of Dy and Al in
the deposits of electrolysis
4
Conclusions
Electrochemical behaviors of Dy(III) cations on an inert W
electrode were studied in molten LiCl-KCl-DyCl3 salts by
combining various electrochemical techniques (ie CV CP and
SWV) The electroreduction of Dy(III) ions on the tungsten
electrode is a single step process with transfer of three electrons
The reduction shows a reversible behavior for polarization rates
range of 50 V 300mV1 which is controlled by the diffusion of
Dy(III) cations in solution Accordingly the diffusion coef 1047297cient of
Dy(III) ion in the LiCl-KCl melts was measured by both CV and CP
techniques The adsorption effect which is surface based was
also observed prior to the reduction of Dy(III) to Dy(0)
The concentration ratio of Dy(III) ions to Al(III) ions has a great
in1047298uence on the co-reduction In a Dy-rich system three signals
corresponding to the formation of three AlxDyy were observed on
the tungsten electrode However when Al(III) cations were
suf 1047297cient only two of which with higher Al content were
observed SEM-EDS and XRD characterizations identi1047297ed inter-
metallic compound DyAl3 was produced by potentiostatic
electrolysis at -15 V and -16 V while two intermetallic com-
pounds DyAl3 and DyAl were obtained through galvanostatic
electrolysis at -50 mA
Fig 9 SEM (ac)-EDS (d) and XRD (b) results of the galvanostatic electrolysis products of LiCl-KCl-AlCl3 (12 wt)- Dy2O3 (09 wt) melts on the Al electrode Current -50 mA
Time 25 h Temperature 773 K
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Actually this method has been con1047297rmed in the LiCl-KCl melt
for the preparation of other Ln and An [1012ndash1623] chloridesIn the electrolytic process anhydrous Dy2O3 and AlCl3 (both AR
grade) powders were directly added to the LiCl-KCl melt In LiCl-
KCl melt with the working temperature of 773 K AlCl3 can be easily
gasi1047297ed and turn into Al2Cl6 part of them reacts with Dy2O3 to
release Dy(III) ions This reaction can be represented as
Dy2O3 (s) + Al2Cl6 (g) == Al2O3 (s) + 2 DyCl3 (l) (2)
From the thermodynamic data [29] the change of Gibbs energy
of this reaction at 773 K is calculated to be -90345 kJ mol1 It
reveals that reaction (2) could proceed forward at our experimen-
tal temperature In this work we 1047297rstly explored the electrochem-
ical behaviors of dysprosium on an inert tungsten electrode hence
after the complete chlorination of Dy2O3 the LiCl-KCl-DyCl3 melt
was puri1047297ed completely out of AlCl3 by bubbling dry argon
continuously until the ICP-MS analysis of the taken melt sample
shows no remnant Al(III) ion The concentration of Dy(III) ions in
the LiCl-KCl-DyCl3 melt was measured at the same time Then the
co-reduction process of Dy(III) and Al(III) were investigated by
increasing the content of AlCl3 in the melts However owing to the
volatility of AlCl3 the concentration of AlCl3 we present below is
the initial fractions when AlCl3 were added into the melts
23 Electrochemical electrodes and characterization of cathodic
deposits
A custom-built quartz structure was used to position all of the
electrodes and the thermocouple in molten salt A silver wire (d
= 1 mm 9999) dipped into the solution of AgCl (1 wt) in LiCl-
KCl melts contained in a Pyrex tube was used as the reference
electrode All potentials were referred to the Ag+Ag couple As for
the counter electrode a 6 mm graphite rod was used The working
electrode consisted of 1 mm tungsten (W)wire with the lower end
polished by SiC paper Before each measurement the working
electrode was cleaned by galvanostatic anodic polarization The
active electrode surface area was calculated after each experiment
by
measuring
the
immersion
depth
of
the
electrode
in
the
moltensalts As to the electrolysis process an aluminum plate (Alfa
99999) with thick to be 2 mm was used as cathode After
electrolysis the aluminum electrode was abraded and polished by
SiC paper followed by ultrasonic cleaning in ethylene glycol and
ethanol (Sinopharm 998) in an ultrasonic bath for 15 min and
stored in the glove box before analysis
3
Results
and
discussion
31 Electrochemical behavior of Dysprosium Ions on the Tungsten
Electrode
311 Cyclic Voltammetry
In
the
present
work
investigations
of
dysprosium
began
withCV measurements to establish the nature of the system and the
reversibility of the observed reactions The typical CV curve of the
pure LiCl-KCl melts is shown in Fig 1 (dotted curve) The
electrochemical window offered by the LiCl-KCl melts have been
reported to be limited between the reduction of lithium ions
(peak L c) and the anodic release of chlorine [1217] The fact that
there is no other additional peak in its electrochemical window
identi1047297es the applicability of the LiCl-KCl melts for our inves-
tigations
Fig 1 also shows the typical CV of LiCl-KCl-DyCl3 (373 105
mol cm3) mixture with the scan rate of 01Vs1 on a W working
electrode at the temperature of 773 K The signals EaEc were
observed in the voltammogram with the reduction peak (Ec) at
-204
V
and
the
corresponding
anodic
peak
(Ea)
at
-184
V
respectively The reduction (Ec) occurs in a single sharp peak
mode with a gradual decay manifesting the deposition of an
insoluble phase [3031] The reverse anodic scan shows an
oxidation peak (Ea) with much higher amplitude than the
reduction peak due to the availability of the deposited metal for
the re-oxidation According to the previous works of Zhang et al
[22] Chang et al [18] and Konishi et al [19] peaks Ea and Ec had
been ascribed to the deposition and dissolution of Dy metal It is
possible that dysprosium metal would be deposited in a single
direct step by direct reduction of Dy(III) ion into Dy(0)
Furthermore the reversibility of the reaction of deposition and
dissolution of Dy(III)Dy(0) was evaluated over a wide scan rate
range from 005 to 03 Vs1 As shown in Fig 2a the peak potential
shifts very slightly with the increasing scan rates Therefore the
reduction of Dy(III) to metal should be considered to be a reversible
process The Nernstian behavior of the reaction at low scan rates
can be further con1047297rmed by plotting the mid-peak potential as a
function of the scan rate As shown in Fig 2c the mid-peak
potential almost remains stable (-196 V) at the scan rates of 005
01 015 and 02 Vs1 In addition the plot of the cathodic peak
current versus the square root of the sweep rates shows a linear
relationship in Fig 2b indicating the process is a diffusion
controlled one Therefore it is plausible to use the Berzin-Delahay
equation [32] in this work for a soluble-insoluble couple accordingto the theory of linear sweep voltammetry
Ip= 0061(nF)32C0D12V12(RT)12S (3)
where n is the number of exchanged electrons F denotes the
Faraday constant (96500C mol1) Co represents the solute con-
centration (mol cm3) D corresponds the diffusion coef 1047297cient
(cm2 s1) v designates the potential scanning rate (V s1) T is the
absolute temperature (K) and S corresponds the electrode area
(cm2)
The measurement of the slope of the curve in Fig 2b yields the
following relation at T = 773 K and C0= 373 105mol cm3
I p
V 1=2 frac14 eth0068 000082THORNAS1=2V 1=2 (4)
Assuming n = 3 through the combination of Eqs (3) and (4) the
diffusion coef 1047297cient (D) of Dy(III) ions under this condition can be
calculated to be 510 106 cm2 s1
Fig 2 (a) CVs for 373 105molcm3DyCl3 in LiCl-KCl melts at various scan rates
Working electrode W (S068 cm2) Scan rates 005 01 015 02 025 and 03 Vs1
(b) Plot of the cathodic peak current as a function of the square root of the scan rate
(c) Mid-peak potential as a function of the scan rate The dashed curve represents
the
average
mid-peak
potential
(-196
V
vs
Ag
+
Ag)
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In addition it can be found that there is always a fuzzy pre-
platform more anodic to the reduction peak Ec in Fig1 and 2which
is very drawn out at all scan rates in Fig 2 suggesting that this
process could not be diffusion controlled However there are still
some discrepancies about the ascription of this pre-platform
[172123] Ref [1723] ascribed it to the reduction of Dy(III) to Dy
(II) whereas Ref [21] held that the similar pre-platform before the
reduction of Dy(III) ions in LiF-CaF2 melt might be an adsorption
effect of Dy(III) ions to the surface of the working electrode
Combining the results of following SWV we prefer to supporting
the pre-platform is attributed to the adsorption effect of Dy(III)
ions to the surface of the W electrode
312 Chronopotentiometry
The electrochemical behavior of the redox couple Dy(III)Dy(0)
was also studied by CP technique Fig 3 shows the evolution of the
CPs of DyCl3 in LiCl-KCl melts with the applied current density from
-16 mA to -24 mA on a W electrode These curves exhibit a single
wave in the same potential range as that observed in the CV curves
and therefore should be associated with the reduction of Dy(III) ions
into metal In the CP technique transition time (t) means the time
necessary to observe the complete depletion of the electroactive
species (here the Dy(III) ion) resulting from the diffusion in the layer
of electrolyte at the electrode surface From Fig 3a it can be found
that t decreases with the increase of the applied current density In
addition the time-current relationship at a constant value in Fig 3b
proves
the
diffusion-controlled
process
of
Dy(III)
to Dy(0)
and
thevalidity of Sands law (Eq (5)) [32]
it 1=2 frac14 05nFSC ethpDTHORN1=2 (5)
where t is the transition time (s) i denotes the applied current (A)
We also assumed the number of exchanged electron n = 3 The
diffusion coef 1047297cient at T = 773 K and C = 346 105mol cm3 was
calculated to be 172 105 cm2 s1
Table 1 gathered the diffusion coef 1047297cients of Dy(III) in LiCl-KCl
eutectic at 773 K Under the same conditions the diffusion
coef 1047297cient calculated by Ref [17] is approximately three times
higher than that measured by Ref [6] The diffusion coef 1047297cient of
Dy(III) ion in LiCl-KCl melts determined in this work was 51 106
cm2 s1 and 172 105 cm2 s1 using CV and CP techniques
respectively The complex chemical behaviors of Dy(III) ions in
LiCl-KCl melts differences in the principles of CV and CP
techniques [33] and imprecise measurement of the wetted length
of the working electrode could account for these discrepancies
Similar
phenomena
were
observed
in
the
case
of
cerium
anduranium in Ref [3435]
In the above calculation of diffusion coef 1047297cient from the results
of CV and CP the number of exchanged electrons was assumed to
be 3 Actually the number of exchanged electrons can be deduced
from combining the results of CV and CP as stated in Ref [3637]
By using Eqs (3) and (5) the following formula was obtained
n frac14
I p
V 1=2 1
ip1=2
05
061
2
D2C 2
D1C 21
p2RT
F (6)
where the concentration (C) and diffusion coef 1047297cients (D)
subscripted with 1 and 2 are related to CV and CP respectively
According to the results obtained from Fig 2b Ip=V1=2 frac14 eth0068
000082THORNAS1=2V1=2 and Fig 3b it 1=2 frac14 25 102 AS 1=2 the
number of exchanged electrons n301 was achieved almostentirely consistent with the theoretical expectation
313 Square wave voltammetry
SWV with a better accuracy to calculate the number of
electrons exchanged in an electrochemical process was then
employed to con1047297rm the number of exchanged electrons in this
experiment
For a single electrochemical reversible process the differential
intensity measured at each step between the successive pulses
exhibits a Gaussian relationship with the potential Mathematical
analysis of the Gaussian peak yields a simple equation associating
with the width of the half peak (W12) with temperature and the
number of electrons exchanged (n) [3839]
W 1=2 frac14 352RT
nF (7)
where R is the universal gas constant T denotes the absolute
temperature in K n represents the number of exchanged electrons
and F designates the Faradays constant
Fig 4 shows a typical SWV of DyCl3 (373 105mol cm3) in the
LiCl-KCl melts on a W electrode at the frequency of 20 Hz A sharp
peak (Ec) associated with the reduction of Dy(III) ion can be clearly
observed in the same potential range as CV and CPHowever peak Ecis not exactly symmetric as predicted by the theory of nucleation
effect Due to which the rise of the current is delayed by the
overpotential caused by the solid phase formation thus the
increasing part of the differential current is sharper than the
decreasing one The disturbance of the signal due to the nucleation
Fig 3 (a) CPs of LiCl-KCl-DyCl3 (346 105molcm3) melts at 773 K Working
electrode W (S068 cm2) Applied current -16 -18 -20 -22 and -24 mA (b)
Relationship
between
the
square
root
of
the
transition
time
and
the
applied
current
Table 1
Diffusion coef 1047297cients of Dy(III) in LiCl-KCl eutectic at 773 K
Reference 105 D cm2 s1 Dy(III) Concentration (104ppm) Technique
[17]
147
201
CP
[6] 046 20 CP
[6] 07 06 CP
this work 051 061 CV
this work 172 057 CP
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However large differences were observed between signals L aL cand AaAc in Fig 5a As for the red curve peak Ec at -204 V and Ea at
about -183 V can be ascribed tothe reduction of Dy(III) tometal and
its subsequent re-oxidation respectively according to the above
results of LiCl-KCl-DyCl3 system and Ref [617] Between peaks EaEcand AaAc two broad anodic peaks at about -130 and -165 V were
observed The CVs measured subsequently in the same system at
different terminal potentials display a broad anodic peak at
approximate -13 V which actually consist of two close peaks Ia and
IIa similar to their cathodic peaks Ic and IIc (Fig 5b) In addition the
SWV in Fig 6 (curve 2) also revealed the existence of the two close
peaks Ic and IIc at approximate -13 V According to the co-reduction
principle redox peaks IaIc IIaIIcand IIIaIIIc should be ascribed tothe
formation
and
dissolution
of
at
least
three
AlxDyy intermetalliccompounds Moreover the closer the deposition potential of the
intermetallic compound to that of Dy metal the higher Dy content
could be formed in the AlxDyy intermetallic compound [1215]
As for the black curve in Fig 5a CV of AlCl3-rich system much
higher current intensities of peaks Aa and Ac were obviously
observed compared to that of AlCl3-poor system Besides other
differences could also be observed in the two curves of Fig 5a For
example the broad anodic peak at -13 V in the red curve separated
into two redox peaks IaIc and IIaIIc in the black curve The CVs
measured at various cathodic terminal potentials in AlCl3-rich
system (Fig 5c) and the SWV (curve 3 in Fig 6) also present two
clearly separated redox peaks IaIc and IIaIIc However the redox
signals IIIaIIIc and EaEc corresponding to the formation and
dissolution of an intermetallic compound with higher Dy content
Fig 5 (a) CVs of LiCl-KCl-AlCl3-Dy2O3 (09 wt) melts with different AlCl3concentrations 08 wt (red curve) and 12 wt (black curve) (b) CVs of the LiCl-
KCl-AlCl3 (08 wt)-Dy2O3 (09 wt) melts (c) CVs of the LiCl-KCl-AlCl3 (12 wt
)-Dy2O3 (09 wt) melts at different inversion potentials Working electrode
W (S068
cm2)
Temperature
773
K
Scan
rate
01
Vs1
Fig 7 CVs of LiCl-KCl-DyCl3 melts (black dotted curve) and LiCl-KCl-AlCl3-DyCl3melts (red solid curve) on an Al electrode Temperature 773 K Scan rate 01
Fig 6 SWVs of LiCl-KCl-DyCl3 (373 105molcm3) melts (curve 1) and LiCl-KCl-
AlCl3-Dy2O3 (09 wt) melts with different AlCl3 concentrations 08 wt (curve 2)
and 12 wt (curve 3) Working electrode W (S068 cm2) Temperature 773 K
Pulse height 10 mV potential step 5 mV frequency 20 Hz Vs1
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and the redox couple of Dy(III)Dy(0) vanished from both the CV
(Fig 5c) and SWV (curve 3 in Fig 6) curves In the meanwhile a
new
couple
of
peaks
marked
as
IVaIVc emerged
with
its
cathodicand anodic potential at approximate -216 and -205 V respectively
which should be ascribed to be the reduction and oxidation of Al-Li
alloy [1217] The reason of the difference between the two curves
in Fig 5 could be as follows With the increase of AlCl3concentration a much thicker layer of Al was deposited on the
W electrode which would facilitate the diffusion of Dy metal
Subsequently the initially generated intermetallic compound
AlxDyy tends to be transformed into another intermetallic
compounds AlxDyy with high Al content Therefore peaks IIIa
IIIc and EaEc which correspond to the formationdissolution of an
intermetallic compound with high Dy content and the redox
couple Dy(III)Dy(0) respectively could not be observed In
addition when the deposited AlxDyy intermetallic compounds
were not fully mantle the Al-covered electrode Al-Li alloys would
have the chance to be formed [15]
Electrochemical behaviors of LiCl-KCl melts containing both
Al(III) and Dy(III) cations were also investigated on an Al electrode
Fig 7 provides a comparison about the CVs of LiCl-KCl-DyCl3 and
LiCl-KCl-AlCl3-DyCl3 melts using Al as the working electrode The
CV of LiCl-KCl-DyCl3 melts without Al(III) cations (black dotted
curve) is consistent with Ref [17] Peaks IcIa are ascribed to the
formation and dissolution of Al-Dy alloys on the Al electrode The
red solid curve in Fig 7 shows a typical co-reduction behavior of
Al(III) and Dy(III) ions on the Al electrode which is very similar to
that obtained in LiCl-KCl-DyCl3 melts although the peaks become
more bulky This could be caused by the formation of different Al-
Dy alloys through the co-reduction of Al(III) and Dy(III) cations at
more cathodic potential [50]
33
Preparation
and
characterization
of
the
Al-Dy
alloys
To
con1047297rm
the
co-reduction
of
Dy(III)
and Al(III) ions andexamine the formation of AlxDyy intermetallic compounds at
various concentration ratio of Al(III) and Dy(III) both potentio-
static and galvanostatic electrolyses were carried out on a
tungsten electrode However only a very small amount of Al-
Dy alloys that adhered to the W electrode could be obtained even
the experiment was repeated for several times This phenomenon
is probably caused by the small cathode current and the high melt
point of the Al-Dy alloys Therefore we further used an Al plate
electrodewith the size of 15 cm 15 cm 02 cm for electrolysis
To prepareAl-Dy alloys at more anodic potential potentiostatic
electrolysis at -14 V -15 V and -16 V each for 3 h respectively
was performed Fig 8 shows the XRD patterns and the cross-
section SEM images coupled with EDS analysis of the cathodic
deposits of potentiostatic electrolysis It turns out that the
electrolysis at -14 V achieved nothing but Al metal while
electrolysis at -15V and -16 V produced a uniform layer covering
on the Al plateelectrode (Fig 8a and c) ByXRD analyses (Fig 8b)
the composition of the deposition layer obtained at -15 V was
con1047297rmed to be Al metal and the intermetallic compound Al3Dy
with crystallographic structure of rhombohedral lattice (R-3 m)
(PDF in XRD data base 18ndash0020) When electrolysis was
performed at more negative potential of -16 V the intermetallic
compound Al3Dy with crystallographic structures of R-3 m and
hexagonal lattice (P63mmc) (PDF in XRD data base 65ndash6363)
could be both obtained
It is well known that potentiostatic electrolysis has the
advantage of controlling the composition of the compound
produced by the cathodic reaction According to the co-reduction
Fig 8 SEM-EDS and XRD results of the potentiostatic electrolysis products of LiCl-KCl-AlCl3 (12 wt)-Dy2O3 (09 wt) melts on the Al electrodes (a) SEM image (deposited
at -16 V) (b) XRD pattern (deposited at -150 V and -160 V) (c) Enlarged SEM image (deposited at -16 V) (d) EDS result (deposited at -16 V)
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behaviors above at least two kinds of AlxDyy intermetallic
compounds
(DyAl3 and
DyAl2)
could
be
formed
by
potentiostaticelectrolysis at -15 and -16V since two very close pairs of
redox peaks were observed in CVs and SWV However only one
kind of intermetallic compound Al3Dy was acquired which was
against with our expectation The main reason might be the low
current density at our experimental concentration Under this
condition even though the intermetallic compound DyAl2 was
formed the formation rate was much slower than its diffusion
rate then the transformation of DyAl2 into the more stable Al-
rich phase (DyAl3) on the Al electrode would take palce Hence
ultimately only one kind of intermetallic compound DyAl3 was
observed in Fig 8 similar phenomena had been observed in the
potentiostatic electrolysis for the preparation of Al-Gd alloys
[15] The fact that intermetallic compound Al3Dy with more
crystallographic structures was obtained by electrolysis at more
negative potential to some extent shows the importance of
nucleation overpotential for the growth of Al-Dy alloys onto the
electrode
To provide a stable current to equably form more AlxDyyintermetallic compounds galvanostatic electrolysis with the
current intensity of -50 mA was also carried out for 25 h in our
experiment During the electrolysis the cathode potential was
controlled within the range of -13 V to -175 V to prevent the
deposition of pure Dy and in the meanwhile cover the two much
more anodic redox peaks (IaIc and IIaIIc) associated with the
formation of AlxDyy intermetallic compounds As shown in the
SEM image in Fig 9a a much thicker layer of approximate 40
mm of
the deposits was obtained than that gained by potentiostatic
electrolysis (Fig 8a) The XRD result in Fig 9b con1047297rms that the
deposits are composed of intermetallic compounds DyAl3 and
DyAl
although
DyAl2 was
still
not
observed
which
proves
onceagain that DyAl2 could not be stable at this temperature and easily
be transformed into DyAl3 The EDS analyses coupled with SEM in
Fig 8d and Fig 9d also con1047297rmed the co-existence of Dy and Al in
the deposits of electrolysis
4
Conclusions
Electrochemical behaviors of Dy(III) cations on an inert W
electrode were studied in molten LiCl-KCl-DyCl3 salts by
combining various electrochemical techniques (ie CV CP and
SWV) The electroreduction of Dy(III) ions on the tungsten
electrode is a single step process with transfer of three electrons
The reduction shows a reversible behavior for polarization rates
range of 50 V 300mV1 which is controlled by the diffusion of
Dy(III) cations in solution Accordingly the diffusion coef 1047297cient of
Dy(III) ion in the LiCl-KCl melts was measured by both CV and CP
techniques The adsorption effect which is surface based was
also observed prior to the reduction of Dy(III) to Dy(0)
The concentration ratio of Dy(III) ions to Al(III) ions has a great
in1047298uence on the co-reduction In a Dy-rich system three signals
corresponding to the formation of three AlxDyy were observed on
the tungsten electrode However when Al(III) cations were
suf 1047297cient only two of which with higher Al content were
observed SEM-EDS and XRD characterizations identi1047297ed inter-
metallic compound DyAl3 was produced by potentiostatic
electrolysis at -15 V and -16 V while two intermetallic com-
pounds DyAl3 and DyAl were obtained through galvanostatic
electrolysis at -50 mA
Fig 9 SEM (ac)-EDS (d) and XRD (b) results of the galvanostatic electrolysis products of LiCl-KCl-AlCl3 (12 wt)- Dy2O3 (09 wt) melts on the Al electrode Current -50 mA
Time 25 h Temperature 773 K
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Actually this method has been con1047297rmed in the LiCl-KCl melt
for the preparation of other Ln and An [1012ndash1623] chloridesIn the electrolytic process anhydrous Dy2O3 and AlCl3 (both AR
grade) powders were directly added to the LiCl-KCl melt In LiCl-
KCl melt with the working temperature of 773 K AlCl3 can be easily
gasi1047297ed and turn into Al2Cl6 part of them reacts with Dy2O3 to
release Dy(III) ions This reaction can be represented as
Dy2O3 (s) + Al2Cl6 (g) == Al2O3 (s) + 2 DyCl3 (l) (2)
From the thermodynamic data [29] the change of Gibbs energy
of this reaction at 773 K is calculated to be -90345 kJ mol1 It
reveals that reaction (2) could proceed forward at our experimen-
tal temperature In this work we 1047297rstly explored the electrochem-
ical behaviors of dysprosium on an inert tungsten electrode hence
after the complete chlorination of Dy2O3 the LiCl-KCl-DyCl3 melt
was puri1047297ed completely out of AlCl3 by bubbling dry argon
continuously until the ICP-MS analysis of the taken melt sample
shows no remnant Al(III) ion The concentration of Dy(III) ions in
the LiCl-KCl-DyCl3 melt was measured at the same time Then the
co-reduction process of Dy(III) and Al(III) were investigated by
increasing the content of AlCl3 in the melts However owing to the
volatility of AlCl3 the concentration of AlCl3 we present below is
the initial fractions when AlCl3 were added into the melts
23 Electrochemical electrodes and characterization of cathodic
deposits
A custom-built quartz structure was used to position all of the
electrodes and the thermocouple in molten salt A silver wire (d
= 1 mm 9999) dipped into the solution of AgCl (1 wt) in LiCl-
KCl melts contained in a Pyrex tube was used as the reference
electrode All potentials were referred to the Ag+Ag couple As for
the counter electrode a 6 mm graphite rod was used The working
electrode consisted of 1 mm tungsten (W)wire with the lower end
polished by SiC paper Before each measurement the working
electrode was cleaned by galvanostatic anodic polarization The
active electrode surface area was calculated after each experiment
by
measuring
the
immersion
depth
of
the
electrode
in
the
moltensalts As to the electrolysis process an aluminum plate (Alfa
99999) with thick to be 2 mm was used as cathode After
electrolysis the aluminum electrode was abraded and polished by
SiC paper followed by ultrasonic cleaning in ethylene glycol and
ethanol (Sinopharm 998) in an ultrasonic bath for 15 min and
stored in the glove box before analysis
3
Results
and
discussion
31 Electrochemical behavior of Dysprosium Ions on the Tungsten
Electrode
311 Cyclic Voltammetry
In
the
present
work
investigations
of
dysprosium
began
withCV measurements to establish the nature of the system and the
reversibility of the observed reactions The typical CV curve of the
pure LiCl-KCl melts is shown in Fig 1 (dotted curve) The
electrochemical window offered by the LiCl-KCl melts have been
reported to be limited between the reduction of lithium ions
(peak L c) and the anodic release of chlorine [1217] The fact that
there is no other additional peak in its electrochemical window
identi1047297es the applicability of the LiCl-KCl melts for our inves-
tigations
Fig 1 also shows the typical CV of LiCl-KCl-DyCl3 (373 105
mol cm3) mixture with the scan rate of 01Vs1 on a W working
electrode at the temperature of 773 K The signals EaEc were
observed in the voltammogram with the reduction peak (Ec) at
-204
V
and
the
corresponding
anodic
peak
(Ea)
at
-184
V
respectively The reduction (Ec) occurs in a single sharp peak
mode with a gradual decay manifesting the deposition of an
insoluble phase [3031] The reverse anodic scan shows an
oxidation peak (Ea) with much higher amplitude than the
reduction peak due to the availability of the deposited metal for
the re-oxidation According to the previous works of Zhang et al
[22] Chang et al [18] and Konishi et al [19] peaks Ea and Ec had
been ascribed to the deposition and dissolution of Dy metal It is
possible that dysprosium metal would be deposited in a single
direct step by direct reduction of Dy(III) ion into Dy(0)
Furthermore the reversibility of the reaction of deposition and
dissolution of Dy(III)Dy(0) was evaluated over a wide scan rate
range from 005 to 03 Vs1 As shown in Fig 2a the peak potential
shifts very slightly with the increasing scan rates Therefore the
reduction of Dy(III) to metal should be considered to be a reversible
process The Nernstian behavior of the reaction at low scan rates
can be further con1047297rmed by plotting the mid-peak potential as a
function of the scan rate As shown in Fig 2c the mid-peak
potential almost remains stable (-196 V) at the scan rates of 005
01 015 and 02 Vs1 In addition the plot of the cathodic peak
current versus the square root of the sweep rates shows a linear
relationship in Fig 2b indicating the process is a diffusion
controlled one Therefore it is plausible to use the Berzin-Delahay
equation [32] in this work for a soluble-insoluble couple accordingto the theory of linear sweep voltammetry
Ip= 0061(nF)32C0D12V12(RT)12S (3)
where n is the number of exchanged electrons F denotes the
Faraday constant (96500C mol1) Co represents the solute con-
centration (mol cm3) D corresponds the diffusion coef 1047297cient
(cm2 s1) v designates the potential scanning rate (V s1) T is the
absolute temperature (K) and S corresponds the electrode area
(cm2)
The measurement of the slope of the curve in Fig 2b yields the
following relation at T = 773 K and C0= 373 105mol cm3
I p
V 1=2 frac14 eth0068 000082THORNAS1=2V 1=2 (4)
Assuming n = 3 through the combination of Eqs (3) and (4) the
diffusion coef 1047297cient (D) of Dy(III) ions under this condition can be
calculated to be 510 106 cm2 s1
Fig 2 (a) CVs for 373 105molcm3DyCl3 in LiCl-KCl melts at various scan rates
Working electrode W (S068 cm2) Scan rates 005 01 015 02 025 and 03 Vs1
(b) Plot of the cathodic peak current as a function of the square root of the scan rate
(c) Mid-peak potential as a function of the scan rate The dashed curve represents
the
average
mid-peak
potential
(-196
V
vs
Ag
+
Ag)
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In addition it can be found that there is always a fuzzy pre-
platform more anodic to the reduction peak Ec in Fig1 and 2which
is very drawn out at all scan rates in Fig 2 suggesting that this
process could not be diffusion controlled However there are still
some discrepancies about the ascription of this pre-platform
[172123] Ref [1723] ascribed it to the reduction of Dy(III) to Dy
(II) whereas Ref [21] held that the similar pre-platform before the
reduction of Dy(III) ions in LiF-CaF2 melt might be an adsorption
effect of Dy(III) ions to the surface of the working electrode
Combining the results of following SWV we prefer to supporting
the pre-platform is attributed to the adsorption effect of Dy(III)
ions to the surface of the W electrode
312 Chronopotentiometry
The electrochemical behavior of the redox couple Dy(III)Dy(0)
was also studied by CP technique Fig 3 shows the evolution of the
CPs of DyCl3 in LiCl-KCl melts with the applied current density from
-16 mA to -24 mA on a W electrode These curves exhibit a single
wave in the same potential range as that observed in the CV curves
and therefore should be associated with the reduction of Dy(III) ions
into metal In the CP technique transition time (t) means the time
necessary to observe the complete depletion of the electroactive
species (here the Dy(III) ion) resulting from the diffusion in the layer
of electrolyte at the electrode surface From Fig 3a it can be found
that t decreases with the increase of the applied current density In
addition the time-current relationship at a constant value in Fig 3b
proves
the
diffusion-controlled
process
of
Dy(III)
to Dy(0)
and
thevalidity of Sands law (Eq (5)) [32]
it 1=2 frac14 05nFSC ethpDTHORN1=2 (5)
where t is the transition time (s) i denotes the applied current (A)
We also assumed the number of exchanged electron n = 3 The
diffusion coef 1047297cient at T = 773 K and C = 346 105mol cm3 was
calculated to be 172 105 cm2 s1
Table 1 gathered the diffusion coef 1047297cients of Dy(III) in LiCl-KCl
eutectic at 773 K Under the same conditions the diffusion
coef 1047297cient calculated by Ref [17] is approximately three times
higher than that measured by Ref [6] The diffusion coef 1047297cient of
Dy(III) ion in LiCl-KCl melts determined in this work was 51 106
cm2 s1 and 172 105 cm2 s1 using CV and CP techniques
respectively The complex chemical behaviors of Dy(III) ions in
LiCl-KCl melts differences in the principles of CV and CP
techniques [33] and imprecise measurement of the wetted length
of the working electrode could account for these discrepancies
Similar
phenomena
were
observed
in
the
case
of
cerium
anduranium in Ref [3435]
In the above calculation of diffusion coef 1047297cient from the results
of CV and CP the number of exchanged electrons was assumed to
be 3 Actually the number of exchanged electrons can be deduced
from combining the results of CV and CP as stated in Ref [3637]
By using Eqs (3) and (5) the following formula was obtained
n frac14
I p
V 1=2 1
ip1=2
05
061
2
D2C 2
D1C 21
p2RT
F (6)
where the concentration (C) and diffusion coef 1047297cients (D)
subscripted with 1 and 2 are related to CV and CP respectively
According to the results obtained from Fig 2b Ip=V1=2 frac14 eth0068
000082THORNAS1=2V1=2 and Fig 3b it 1=2 frac14 25 102 AS 1=2 the
number of exchanged electrons n301 was achieved almostentirely consistent with the theoretical expectation
313 Square wave voltammetry
SWV with a better accuracy to calculate the number of
electrons exchanged in an electrochemical process was then
employed to con1047297rm the number of exchanged electrons in this
experiment
For a single electrochemical reversible process the differential
intensity measured at each step between the successive pulses
exhibits a Gaussian relationship with the potential Mathematical
analysis of the Gaussian peak yields a simple equation associating
with the width of the half peak (W12) with temperature and the
number of electrons exchanged (n) [3839]
W 1=2 frac14 352RT
nF (7)
where R is the universal gas constant T denotes the absolute
temperature in K n represents the number of exchanged electrons
and F designates the Faradays constant
Fig 4 shows a typical SWV of DyCl3 (373 105mol cm3) in the
LiCl-KCl melts on a W electrode at the frequency of 20 Hz A sharp
peak (Ec) associated with the reduction of Dy(III) ion can be clearly
observed in the same potential range as CV and CPHowever peak Ecis not exactly symmetric as predicted by the theory of nucleation
effect Due to which the rise of the current is delayed by the
overpotential caused by the solid phase formation thus the
increasing part of the differential current is sharper than the
decreasing one The disturbance of the signal due to the nucleation
Fig 3 (a) CPs of LiCl-KCl-DyCl3 (346 105molcm3) melts at 773 K Working
electrode W (S068 cm2) Applied current -16 -18 -20 -22 and -24 mA (b)
Relationship
between
the
square
root
of
the
transition
time
and
the
applied
current
Table 1
Diffusion coef 1047297cients of Dy(III) in LiCl-KCl eutectic at 773 K
Reference 105 D cm2 s1 Dy(III) Concentration (104ppm) Technique
[17]
147
201
CP
[6] 046 20 CP
[6] 07 06 CP
this work 051 061 CV
this work 172 057 CP
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However large differences were observed between signals L aL cand AaAc in Fig 5a As for the red curve peak Ec at -204 V and Ea at
about -183 V can be ascribed tothe reduction of Dy(III) tometal and
its subsequent re-oxidation respectively according to the above
results of LiCl-KCl-DyCl3 system and Ref [617] Between peaks EaEcand AaAc two broad anodic peaks at about -130 and -165 V were
observed The CVs measured subsequently in the same system at
different terminal potentials display a broad anodic peak at
approximate -13 V which actually consist of two close peaks Ia and
IIa similar to their cathodic peaks Ic and IIc (Fig 5b) In addition the
SWV in Fig 6 (curve 2) also revealed the existence of the two close
peaks Ic and IIc at approximate -13 V According to the co-reduction
principle redox peaks IaIc IIaIIcand IIIaIIIc should be ascribed tothe
formation
and
dissolution
of
at
least
three
AlxDyy intermetalliccompounds Moreover the closer the deposition potential of the
intermetallic compound to that of Dy metal the higher Dy content
could be formed in the AlxDyy intermetallic compound [1215]
As for the black curve in Fig 5a CV of AlCl3-rich system much
higher current intensities of peaks Aa and Ac were obviously
observed compared to that of AlCl3-poor system Besides other
differences could also be observed in the two curves of Fig 5a For
example the broad anodic peak at -13 V in the red curve separated
into two redox peaks IaIc and IIaIIc in the black curve The CVs
measured at various cathodic terminal potentials in AlCl3-rich
system (Fig 5c) and the SWV (curve 3 in Fig 6) also present two
clearly separated redox peaks IaIc and IIaIIc However the redox
signals IIIaIIIc and EaEc corresponding to the formation and
dissolution of an intermetallic compound with higher Dy content
Fig 5 (a) CVs of LiCl-KCl-AlCl3-Dy2O3 (09 wt) melts with different AlCl3concentrations 08 wt (red curve) and 12 wt (black curve) (b) CVs of the LiCl-
KCl-AlCl3 (08 wt)-Dy2O3 (09 wt) melts (c) CVs of the LiCl-KCl-AlCl3 (12 wt
)-Dy2O3 (09 wt) melts at different inversion potentials Working electrode
W (S068
cm2)
Temperature
773
K
Scan
rate
01
Vs1
Fig 7 CVs of LiCl-KCl-DyCl3 melts (black dotted curve) and LiCl-KCl-AlCl3-DyCl3melts (red solid curve) on an Al electrode Temperature 773 K Scan rate 01
Fig 6 SWVs of LiCl-KCl-DyCl3 (373 105molcm3) melts (curve 1) and LiCl-KCl-
AlCl3-Dy2O3 (09 wt) melts with different AlCl3 concentrations 08 wt (curve 2)
and 12 wt (curve 3) Working electrode W (S068 cm2) Temperature 773 K
Pulse height 10 mV potential step 5 mV frequency 20 Hz Vs1
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and the redox couple of Dy(III)Dy(0) vanished from both the CV
(Fig 5c) and SWV (curve 3 in Fig 6) curves In the meanwhile a
new
couple
of
peaks
marked
as
IVaIVc emerged
with
its
cathodicand anodic potential at approximate -216 and -205 V respectively
which should be ascribed to be the reduction and oxidation of Al-Li
alloy [1217] The reason of the difference between the two curves
in Fig 5 could be as follows With the increase of AlCl3concentration a much thicker layer of Al was deposited on the
W electrode which would facilitate the diffusion of Dy metal
Subsequently the initially generated intermetallic compound
AlxDyy tends to be transformed into another intermetallic
compounds AlxDyy with high Al content Therefore peaks IIIa
IIIc and EaEc which correspond to the formationdissolution of an
intermetallic compound with high Dy content and the redox
couple Dy(III)Dy(0) respectively could not be observed In
addition when the deposited AlxDyy intermetallic compounds
were not fully mantle the Al-covered electrode Al-Li alloys would
have the chance to be formed [15]
Electrochemical behaviors of LiCl-KCl melts containing both
Al(III) and Dy(III) cations were also investigated on an Al electrode
Fig 7 provides a comparison about the CVs of LiCl-KCl-DyCl3 and
LiCl-KCl-AlCl3-DyCl3 melts using Al as the working electrode The
CV of LiCl-KCl-DyCl3 melts without Al(III) cations (black dotted
curve) is consistent with Ref [17] Peaks IcIa are ascribed to the
formation and dissolution of Al-Dy alloys on the Al electrode The
red solid curve in Fig 7 shows a typical co-reduction behavior of
Al(III) and Dy(III) ions on the Al electrode which is very similar to
that obtained in LiCl-KCl-DyCl3 melts although the peaks become
more bulky This could be caused by the formation of different Al-
Dy alloys through the co-reduction of Al(III) and Dy(III) cations at
more cathodic potential [50]
33
Preparation
and
characterization
of
the
Al-Dy
alloys
To
con1047297rm
the
co-reduction
of
Dy(III)
and Al(III) ions andexamine the formation of AlxDyy intermetallic compounds at
various concentration ratio of Al(III) and Dy(III) both potentio-
static and galvanostatic electrolyses were carried out on a
tungsten electrode However only a very small amount of Al-
Dy alloys that adhered to the W electrode could be obtained even
the experiment was repeated for several times This phenomenon
is probably caused by the small cathode current and the high melt
point of the Al-Dy alloys Therefore we further used an Al plate
electrodewith the size of 15 cm 15 cm 02 cm for electrolysis
To prepareAl-Dy alloys at more anodic potential potentiostatic
electrolysis at -14 V -15 V and -16 V each for 3 h respectively
was performed Fig 8 shows the XRD patterns and the cross-
section SEM images coupled with EDS analysis of the cathodic
deposits of potentiostatic electrolysis It turns out that the
electrolysis at -14 V achieved nothing but Al metal while
electrolysis at -15V and -16 V produced a uniform layer covering
on the Al plateelectrode (Fig 8a and c) ByXRD analyses (Fig 8b)
the composition of the deposition layer obtained at -15 V was
con1047297rmed to be Al metal and the intermetallic compound Al3Dy
with crystallographic structure of rhombohedral lattice (R-3 m)
(PDF in XRD data base 18ndash0020) When electrolysis was
performed at more negative potential of -16 V the intermetallic
compound Al3Dy with crystallographic structures of R-3 m and
hexagonal lattice (P63mmc) (PDF in XRD data base 65ndash6363)
could be both obtained
It is well known that potentiostatic electrolysis has the
advantage of controlling the composition of the compound
produced by the cathodic reaction According to the co-reduction
Fig 8 SEM-EDS and XRD results of the potentiostatic electrolysis products of LiCl-KCl-AlCl3 (12 wt)-Dy2O3 (09 wt) melts on the Al electrodes (a) SEM image (deposited
at -16 V) (b) XRD pattern (deposited at -150 V and -160 V) (c) Enlarged SEM image (deposited at -16 V) (d) EDS result (deposited at -16 V)
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behaviors above at least two kinds of AlxDyy intermetallic
compounds
(DyAl3 and
DyAl2)
could
be
formed
by
potentiostaticelectrolysis at -15 and -16V since two very close pairs of
redox peaks were observed in CVs and SWV However only one
kind of intermetallic compound Al3Dy was acquired which was
against with our expectation The main reason might be the low
current density at our experimental concentration Under this
condition even though the intermetallic compound DyAl2 was
formed the formation rate was much slower than its diffusion
rate then the transformation of DyAl2 into the more stable Al-
rich phase (DyAl3) on the Al electrode would take palce Hence
ultimately only one kind of intermetallic compound DyAl3 was
observed in Fig 8 similar phenomena had been observed in the
potentiostatic electrolysis for the preparation of Al-Gd alloys
[15] The fact that intermetallic compound Al3Dy with more
crystallographic structures was obtained by electrolysis at more
negative potential to some extent shows the importance of
nucleation overpotential for the growth of Al-Dy alloys onto the
electrode
To provide a stable current to equably form more AlxDyyintermetallic compounds galvanostatic electrolysis with the
current intensity of -50 mA was also carried out for 25 h in our
experiment During the electrolysis the cathode potential was
controlled within the range of -13 V to -175 V to prevent the
deposition of pure Dy and in the meanwhile cover the two much
more anodic redox peaks (IaIc and IIaIIc) associated with the
formation of AlxDyy intermetallic compounds As shown in the
SEM image in Fig 9a a much thicker layer of approximate 40
mm of
the deposits was obtained than that gained by potentiostatic
electrolysis (Fig 8a) The XRD result in Fig 9b con1047297rms that the
deposits are composed of intermetallic compounds DyAl3 and
DyAl
although
DyAl2 was
still
not
observed
which
proves
onceagain that DyAl2 could not be stable at this temperature and easily
be transformed into DyAl3 The EDS analyses coupled with SEM in
Fig 8d and Fig 9d also con1047297rmed the co-existence of Dy and Al in
the deposits of electrolysis
4
Conclusions
Electrochemical behaviors of Dy(III) cations on an inert W
electrode were studied in molten LiCl-KCl-DyCl3 salts by
combining various electrochemical techniques (ie CV CP and
SWV) The electroreduction of Dy(III) ions on the tungsten
electrode is a single step process with transfer of three electrons
The reduction shows a reversible behavior for polarization rates
range of 50 V 300mV1 which is controlled by the diffusion of
Dy(III) cations in solution Accordingly the diffusion coef 1047297cient of
Dy(III) ion in the LiCl-KCl melts was measured by both CV and CP
techniques The adsorption effect which is surface based was
also observed prior to the reduction of Dy(III) to Dy(0)
The concentration ratio of Dy(III) ions to Al(III) ions has a great
in1047298uence on the co-reduction In a Dy-rich system three signals
corresponding to the formation of three AlxDyy were observed on
the tungsten electrode However when Al(III) cations were
suf 1047297cient only two of which with higher Al content were
observed SEM-EDS and XRD characterizations identi1047297ed inter-
metallic compound DyAl3 was produced by potentiostatic
electrolysis at -15 V and -16 V while two intermetallic com-
pounds DyAl3 and DyAl were obtained through galvanostatic
electrolysis at -50 mA
Fig 9 SEM (ac)-EDS (d) and XRD (b) results of the galvanostatic electrolysis products of LiCl-KCl-AlCl3 (12 wt)- Dy2O3 (09 wt) melts on the Al electrode Current -50 mA
Time 25 h Temperature 773 K
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In addition it can be found that there is always a fuzzy pre-
platform more anodic to the reduction peak Ec in Fig1 and 2which
is very drawn out at all scan rates in Fig 2 suggesting that this
process could not be diffusion controlled However there are still
some discrepancies about the ascription of this pre-platform
[172123] Ref [1723] ascribed it to the reduction of Dy(III) to Dy
(II) whereas Ref [21] held that the similar pre-platform before the
reduction of Dy(III) ions in LiF-CaF2 melt might be an adsorption
effect of Dy(III) ions to the surface of the working electrode
Combining the results of following SWV we prefer to supporting
the pre-platform is attributed to the adsorption effect of Dy(III)
ions to the surface of the W electrode
312 Chronopotentiometry
The electrochemical behavior of the redox couple Dy(III)Dy(0)
was also studied by CP technique Fig 3 shows the evolution of the
CPs of DyCl3 in LiCl-KCl melts with the applied current density from
-16 mA to -24 mA on a W electrode These curves exhibit a single
wave in the same potential range as that observed in the CV curves
and therefore should be associated with the reduction of Dy(III) ions
into metal In the CP technique transition time (t) means the time
necessary to observe the complete depletion of the electroactive
species (here the Dy(III) ion) resulting from the diffusion in the layer
of electrolyte at the electrode surface From Fig 3a it can be found
that t decreases with the increase of the applied current density In
addition the time-current relationship at a constant value in Fig 3b
proves
the
diffusion-controlled
process
of
Dy(III)
to Dy(0)
and
thevalidity of Sands law (Eq (5)) [32]
it 1=2 frac14 05nFSC ethpDTHORN1=2 (5)
where t is the transition time (s) i denotes the applied current (A)
We also assumed the number of exchanged electron n = 3 The
diffusion coef 1047297cient at T = 773 K and C = 346 105mol cm3 was
calculated to be 172 105 cm2 s1
Table 1 gathered the diffusion coef 1047297cients of Dy(III) in LiCl-KCl
eutectic at 773 K Under the same conditions the diffusion
coef 1047297cient calculated by Ref [17] is approximately three times
higher than that measured by Ref [6] The diffusion coef 1047297cient of
Dy(III) ion in LiCl-KCl melts determined in this work was 51 106
cm2 s1 and 172 105 cm2 s1 using CV and CP techniques
respectively The complex chemical behaviors of Dy(III) ions in
LiCl-KCl melts differences in the principles of CV and CP
techniques [33] and imprecise measurement of the wetted length
of the working electrode could account for these discrepancies
Similar
phenomena
were
observed
in
the
case
of
cerium
anduranium in Ref [3435]
In the above calculation of diffusion coef 1047297cient from the results
of CV and CP the number of exchanged electrons was assumed to
be 3 Actually the number of exchanged electrons can be deduced
from combining the results of CV and CP as stated in Ref [3637]
By using Eqs (3) and (5) the following formula was obtained
n frac14
I p
V 1=2 1
ip1=2
05
061
2
D2C 2
D1C 21
p2RT
F (6)
where the concentration (C) and diffusion coef 1047297cients (D)
subscripted with 1 and 2 are related to CV and CP respectively
According to the results obtained from Fig 2b Ip=V1=2 frac14 eth0068
000082THORNAS1=2V1=2 and Fig 3b it 1=2 frac14 25 102 AS 1=2 the
number of exchanged electrons n301 was achieved almostentirely consistent with the theoretical expectation
313 Square wave voltammetry
SWV with a better accuracy to calculate the number of
electrons exchanged in an electrochemical process was then
employed to con1047297rm the number of exchanged electrons in this
experiment
For a single electrochemical reversible process the differential
intensity measured at each step between the successive pulses
exhibits a Gaussian relationship with the potential Mathematical
analysis of the Gaussian peak yields a simple equation associating
with the width of the half peak (W12) with temperature and the
number of electrons exchanged (n) [3839]
W 1=2 frac14 352RT
nF (7)
where R is the universal gas constant T denotes the absolute
temperature in K n represents the number of exchanged electrons
and F designates the Faradays constant
Fig 4 shows a typical SWV of DyCl3 (373 105mol cm3) in the
LiCl-KCl melts on a W electrode at the frequency of 20 Hz A sharp
peak (Ec) associated with the reduction of Dy(III) ion can be clearly
observed in the same potential range as CV and CPHowever peak Ecis not exactly symmetric as predicted by the theory of nucleation
effect Due to which the rise of the current is delayed by the
overpotential caused by the solid phase formation thus the
increasing part of the differential current is sharper than the
decreasing one The disturbance of the signal due to the nucleation
Fig 3 (a) CPs of LiCl-KCl-DyCl3 (346 105molcm3) melts at 773 K Working
electrode W (S068 cm2) Applied current -16 -18 -20 -22 and -24 mA (b)
Relationship
between
the
square
root
of
the
transition
time
and
the
applied
current
Table 1
Diffusion coef 1047297cients of Dy(III) in LiCl-KCl eutectic at 773 K
Reference 105 D cm2 s1 Dy(III) Concentration (104ppm) Technique
[17]
147
201
CP
[6] 046 20 CP
[6] 07 06 CP
this work 051 061 CV
this work 172 057 CP
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However large differences were observed between signals L aL cand AaAc in Fig 5a As for the red curve peak Ec at -204 V and Ea at
about -183 V can be ascribed tothe reduction of Dy(III) tometal and
its subsequent re-oxidation respectively according to the above
results of LiCl-KCl-DyCl3 system and Ref [617] Between peaks EaEcand AaAc two broad anodic peaks at about -130 and -165 V were
observed The CVs measured subsequently in the same system at
different terminal potentials display a broad anodic peak at
approximate -13 V which actually consist of two close peaks Ia and
IIa similar to their cathodic peaks Ic and IIc (Fig 5b) In addition the
SWV in Fig 6 (curve 2) also revealed the existence of the two close
peaks Ic and IIc at approximate -13 V According to the co-reduction
principle redox peaks IaIc IIaIIcand IIIaIIIc should be ascribed tothe
formation
and
dissolution
of
at
least
three
AlxDyy intermetalliccompounds Moreover the closer the deposition potential of the
intermetallic compound to that of Dy metal the higher Dy content
could be formed in the AlxDyy intermetallic compound [1215]
As for the black curve in Fig 5a CV of AlCl3-rich system much
higher current intensities of peaks Aa and Ac were obviously
observed compared to that of AlCl3-poor system Besides other
differences could also be observed in the two curves of Fig 5a For
example the broad anodic peak at -13 V in the red curve separated
into two redox peaks IaIc and IIaIIc in the black curve The CVs
measured at various cathodic terminal potentials in AlCl3-rich
system (Fig 5c) and the SWV (curve 3 in Fig 6) also present two
clearly separated redox peaks IaIc and IIaIIc However the redox
signals IIIaIIIc and EaEc corresponding to the formation and
dissolution of an intermetallic compound with higher Dy content
Fig 5 (a) CVs of LiCl-KCl-AlCl3-Dy2O3 (09 wt) melts with different AlCl3concentrations 08 wt (red curve) and 12 wt (black curve) (b) CVs of the LiCl-
KCl-AlCl3 (08 wt)-Dy2O3 (09 wt) melts (c) CVs of the LiCl-KCl-AlCl3 (12 wt
)-Dy2O3 (09 wt) melts at different inversion potentials Working electrode
W (S068
cm2)
Temperature
773
K
Scan
rate
01
Vs1
Fig 7 CVs of LiCl-KCl-DyCl3 melts (black dotted curve) and LiCl-KCl-AlCl3-DyCl3melts (red solid curve) on an Al electrode Temperature 773 K Scan rate 01
Fig 6 SWVs of LiCl-KCl-DyCl3 (373 105molcm3) melts (curve 1) and LiCl-KCl-
AlCl3-Dy2O3 (09 wt) melts with different AlCl3 concentrations 08 wt (curve 2)
and 12 wt (curve 3) Working electrode W (S068 cm2) Temperature 773 K
Pulse height 10 mV potential step 5 mV frequency 20 Hz Vs1
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and the redox couple of Dy(III)Dy(0) vanished from both the CV
(Fig 5c) and SWV (curve 3 in Fig 6) curves In the meanwhile a
new
couple
of
peaks
marked
as
IVaIVc emerged
with
its
cathodicand anodic potential at approximate -216 and -205 V respectively
which should be ascribed to be the reduction and oxidation of Al-Li
alloy [1217] The reason of the difference between the two curves
in Fig 5 could be as follows With the increase of AlCl3concentration a much thicker layer of Al was deposited on the
W electrode which would facilitate the diffusion of Dy metal
Subsequently the initially generated intermetallic compound
AlxDyy tends to be transformed into another intermetallic
compounds AlxDyy with high Al content Therefore peaks IIIa
IIIc and EaEc which correspond to the formationdissolution of an
intermetallic compound with high Dy content and the redox
couple Dy(III)Dy(0) respectively could not be observed In
addition when the deposited AlxDyy intermetallic compounds
were not fully mantle the Al-covered electrode Al-Li alloys would
have the chance to be formed [15]
Electrochemical behaviors of LiCl-KCl melts containing both
Al(III) and Dy(III) cations were also investigated on an Al electrode
Fig 7 provides a comparison about the CVs of LiCl-KCl-DyCl3 and
LiCl-KCl-AlCl3-DyCl3 melts using Al as the working electrode The
CV of LiCl-KCl-DyCl3 melts without Al(III) cations (black dotted
curve) is consistent with Ref [17] Peaks IcIa are ascribed to the
formation and dissolution of Al-Dy alloys on the Al electrode The
red solid curve in Fig 7 shows a typical co-reduction behavior of
Al(III) and Dy(III) ions on the Al electrode which is very similar to
that obtained in LiCl-KCl-DyCl3 melts although the peaks become
more bulky This could be caused by the formation of different Al-
Dy alloys through the co-reduction of Al(III) and Dy(III) cations at
more cathodic potential [50]
33
Preparation
and
characterization
of
the
Al-Dy
alloys
To
con1047297rm
the
co-reduction
of
Dy(III)
and Al(III) ions andexamine the formation of AlxDyy intermetallic compounds at
various concentration ratio of Al(III) and Dy(III) both potentio-
static and galvanostatic electrolyses were carried out on a
tungsten electrode However only a very small amount of Al-
Dy alloys that adhered to the W electrode could be obtained even
the experiment was repeated for several times This phenomenon
is probably caused by the small cathode current and the high melt
point of the Al-Dy alloys Therefore we further used an Al plate
electrodewith the size of 15 cm 15 cm 02 cm for electrolysis
To prepareAl-Dy alloys at more anodic potential potentiostatic
electrolysis at -14 V -15 V and -16 V each for 3 h respectively
was performed Fig 8 shows the XRD patterns and the cross-
section SEM images coupled with EDS analysis of the cathodic
deposits of potentiostatic electrolysis It turns out that the
electrolysis at -14 V achieved nothing but Al metal while
electrolysis at -15V and -16 V produced a uniform layer covering
on the Al plateelectrode (Fig 8a and c) ByXRD analyses (Fig 8b)
the composition of the deposition layer obtained at -15 V was
con1047297rmed to be Al metal and the intermetallic compound Al3Dy
with crystallographic structure of rhombohedral lattice (R-3 m)
(PDF in XRD data base 18ndash0020) When electrolysis was
performed at more negative potential of -16 V the intermetallic
compound Al3Dy with crystallographic structures of R-3 m and
hexagonal lattice (P63mmc) (PDF in XRD data base 65ndash6363)
could be both obtained
It is well known that potentiostatic electrolysis has the
advantage of controlling the composition of the compound
produced by the cathodic reaction According to the co-reduction
Fig 8 SEM-EDS and XRD results of the potentiostatic electrolysis products of LiCl-KCl-AlCl3 (12 wt)-Dy2O3 (09 wt) melts on the Al electrodes (a) SEM image (deposited
at -16 V) (b) XRD pattern (deposited at -150 V and -160 V) (c) Enlarged SEM image (deposited at -16 V) (d) EDS result (deposited at -16 V)
L-L Su et al Electrochimica Acta 147 (2014) 87 ndash95 93
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behaviors above at least two kinds of AlxDyy intermetallic
compounds
(DyAl3 and
DyAl2)
could
be
formed
by
potentiostaticelectrolysis at -15 and -16V since two very close pairs of
redox peaks were observed in CVs and SWV However only one
kind of intermetallic compound Al3Dy was acquired which was
against with our expectation The main reason might be the low
current density at our experimental concentration Under this
condition even though the intermetallic compound DyAl2 was
formed the formation rate was much slower than its diffusion
rate then the transformation of DyAl2 into the more stable Al-
rich phase (DyAl3) on the Al electrode would take palce Hence
ultimately only one kind of intermetallic compound DyAl3 was
observed in Fig 8 similar phenomena had been observed in the
potentiostatic electrolysis for the preparation of Al-Gd alloys
[15] The fact that intermetallic compound Al3Dy with more
crystallographic structures was obtained by electrolysis at more
negative potential to some extent shows the importance of
nucleation overpotential for the growth of Al-Dy alloys onto the
electrode
To provide a stable current to equably form more AlxDyyintermetallic compounds galvanostatic electrolysis with the
current intensity of -50 mA was also carried out for 25 h in our
experiment During the electrolysis the cathode potential was
controlled within the range of -13 V to -175 V to prevent the
deposition of pure Dy and in the meanwhile cover the two much
more anodic redox peaks (IaIc and IIaIIc) associated with the
formation of AlxDyy intermetallic compounds As shown in the
SEM image in Fig 9a a much thicker layer of approximate 40
mm of
the deposits was obtained than that gained by potentiostatic
electrolysis (Fig 8a) The XRD result in Fig 9b con1047297rms that the
deposits are composed of intermetallic compounds DyAl3 and
DyAl
although
DyAl2 was
still
not
observed
which
proves
onceagain that DyAl2 could not be stable at this temperature and easily
be transformed into DyAl3 The EDS analyses coupled with SEM in
Fig 8d and Fig 9d also con1047297rmed the co-existence of Dy and Al in
the deposits of electrolysis
4
Conclusions
Electrochemical behaviors of Dy(III) cations on an inert W
electrode were studied in molten LiCl-KCl-DyCl3 salts by
combining various electrochemical techniques (ie CV CP and
SWV) The electroreduction of Dy(III) ions on the tungsten
electrode is a single step process with transfer of three electrons
The reduction shows a reversible behavior for polarization rates
range of 50 V 300mV1 which is controlled by the diffusion of
Dy(III) cations in solution Accordingly the diffusion coef 1047297cient of
Dy(III) ion in the LiCl-KCl melts was measured by both CV and CP
techniques The adsorption effect which is surface based was
also observed prior to the reduction of Dy(III) to Dy(0)
The concentration ratio of Dy(III) ions to Al(III) ions has a great
in1047298uence on the co-reduction In a Dy-rich system three signals
corresponding to the formation of three AlxDyy were observed on
the tungsten electrode However when Al(III) cations were
suf 1047297cient only two of which with higher Al content were
observed SEM-EDS and XRD characterizations identi1047297ed inter-
metallic compound DyAl3 was produced by potentiostatic
electrolysis at -15 V and -16 V while two intermetallic com-
pounds DyAl3 and DyAl were obtained through galvanostatic
electrolysis at -50 mA
Fig 9 SEM (ac)-EDS (d) and XRD (b) results of the galvanostatic electrolysis products of LiCl-KCl-AlCl3 (12 wt)- Dy2O3 (09 wt) melts on the Al electrode Current -50 mA
Time 25 h Temperature 773 K
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However large differences were observed between signals L aL cand AaAc in Fig 5a As for the red curve peak Ec at -204 V and Ea at
about -183 V can be ascribed tothe reduction of Dy(III) tometal and
its subsequent re-oxidation respectively according to the above
results of LiCl-KCl-DyCl3 system and Ref [617] Between peaks EaEcand AaAc two broad anodic peaks at about -130 and -165 V were
observed The CVs measured subsequently in the same system at
different terminal potentials display a broad anodic peak at
approximate -13 V which actually consist of two close peaks Ia and
IIa similar to their cathodic peaks Ic and IIc (Fig 5b) In addition the
SWV in Fig 6 (curve 2) also revealed the existence of the two close
peaks Ic and IIc at approximate -13 V According to the co-reduction
principle redox peaks IaIc IIaIIcand IIIaIIIc should be ascribed tothe
formation
and
dissolution
of
at
least
three
AlxDyy intermetalliccompounds Moreover the closer the deposition potential of the
intermetallic compound to that of Dy metal the higher Dy content
could be formed in the AlxDyy intermetallic compound [1215]
As for the black curve in Fig 5a CV of AlCl3-rich system much
higher current intensities of peaks Aa and Ac were obviously
observed compared to that of AlCl3-poor system Besides other
differences could also be observed in the two curves of Fig 5a For
example the broad anodic peak at -13 V in the red curve separated
into two redox peaks IaIc and IIaIIc in the black curve The CVs
measured at various cathodic terminal potentials in AlCl3-rich
system (Fig 5c) and the SWV (curve 3 in Fig 6) also present two
clearly separated redox peaks IaIc and IIaIIc However the redox
signals IIIaIIIc and EaEc corresponding to the formation and
dissolution of an intermetallic compound with higher Dy content
Fig 5 (a) CVs of LiCl-KCl-AlCl3-Dy2O3 (09 wt) melts with different AlCl3concentrations 08 wt (red curve) and 12 wt (black curve) (b) CVs of the LiCl-
KCl-AlCl3 (08 wt)-Dy2O3 (09 wt) melts (c) CVs of the LiCl-KCl-AlCl3 (12 wt
)-Dy2O3 (09 wt) melts at different inversion potentials Working electrode
W (S068
cm2)
Temperature
773
K
Scan
rate
01
Vs1
Fig 7 CVs of LiCl-KCl-DyCl3 melts (black dotted curve) and LiCl-KCl-AlCl3-DyCl3melts (red solid curve) on an Al electrode Temperature 773 K Scan rate 01
Fig 6 SWVs of LiCl-KCl-DyCl3 (373 105molcm3) melts (curve 1) and LiCl-KCl-
AlCl3-Dy2O3 (09 wt) melts with different AlCl3 concentrations 08 wt (curve 2)
and 12 wt (curve 3) Working electrode W (S068 cm2) Temperature 773 K
Pulse height 10 mV potential step 5 mV frequency 20 Hz Vs1
92 L-L Su et al Electrochimica Acta 147 (2014) 87 ndash95
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and the redox couple of Dy(III)Dy(0) vanished from both the CV
(Fig 5c) and SWV (curve 3 in Fig 6) curves In the meanwhile a
new
couple
of
peaks
marked
as
IVaIVc emerged
with
its
cathodicand anodic potential at approximate -216 and -205 V respectively
which should be ascribed to be the reduction and oxidation of Al-Li
alloy [1217] The reason of the difference between the two curves
in Fig 5 could be as follows With the increase of AlCl3concentration a much thicker layer of Al was deposited on the
W electrode which would facilitate the diffusion of Dy metal
Subsequently the initially generated intermetallic compound
AlxDyy tends to be transformed into another intermetallic
compounds AlxDyy with high Al content Therefore peaks IIIa
IIIc and EaEc which correspond to the formationdissolution of an
intermetallic compound with high Dy content and the redox
couple Dy(III)Dy(0) respectively could not be observed In
addition when the deposited AlxDyy intermetallic compounds
were not fully mantle the Al-covered electrode Al-Li alloys would
have the chance to be formed [15]
Electrochemical behaviors of LiCl-KCl melts containing both
Al(III) and Dy(III) cations were also investigated on an Al electrode
Fig 7 provides a comparison about the CVs of LiCl-KCl-DyCl3 and
LiCl-KCl-AlCl3-DyCl3 melts using Al as the working electrode The
CV of LiCl-KCl-DyCl3 melts without Al(III) cations (black dotted
curve) is consistent with Ref [17] Peaks IcIa are ascribed to the
formation and dissolution of Al-Dy alloys on the Al electrode The
red solid curve in Fig 7 shows a typical co-reduction behavior of
Al(III) and Dy(III) ions on the Al electrode which is very similar to
that obtained in LiCl-KCl-DyCl3 melts although the peaks become
more bulky This could be caused by the formation of different Al-
Dy alloys through the co-reduction of Al(III) and Dy(III) cations at
more cathodic potential [50]
33
Preparation
and
characterization
of
the
Al-Dy
alloys
To
con1047297rm
the
co-reduction
of
Dy(III)
and Al(III) ions andexamine the formation of AlxDyy intermetallic compounds at
various concentration ratio of Al(III) and Dy(III) both potentio-
static and galvanostatic electrolyses were carried out on a
tungsten electrode However only a very small amount of Al-
Dy alloys that adhered to the W electrode could be obtained even
the experiment was repeated for several times This phenomenon
is probably caused by the small cathode current and the high melt
point of the Al-Dy alloys Therefore we further used an Al plate
electrodewith the size of 15 cm 15 cm 02 cm for electrolysis
To prepareAl-Dy alloys at more anodic potential potentiostatic
electrolysis at -14 V -15 V and -16 V each for 3 h respectively
was performed Fig 8 shows the XRD patterns and the cross-
section SEM images coupled with EDS analysis of the cathodic
deposits of potentiostatic electrolysis It turns out that the
electrolysis at -14 V achieved nothing but Al metal while
electrolysis at -15V and -16 V produced a uniform layer covering
on the Al plateelectrode (Fig 8a and c) ByXRD analyses (Fig 8b)
the composition of the deposition layer obtained at -15 V was
con1047297rmed to be Al metal and the intermetallic compound Al3Dy
with crystallographic structure of rhombohedral lattice (R-3 m)
(PDF in XRD data base 18ndash0020) When electrolysis was
performed at more negative potential of -16 V the intermetallic
compound Al3Dy with crystallographic structures of R-3 m and
hexagonal lattice (P63mmc) (PDF in XRD data base 65ndash6363)
could be both obtained
It is well known that potentiostatic electrolysis has the
advantage of controlling the composition of the compound
produced by the cathodic reaction According to the co-reduction
Fig 8 SEM-EDS and XRD results of the potentiostatic electrolysis products of LiCl-KCl-AlCl3 (12 wt)-Dy2O3 (09 wt) melts on the Al electrodes (a) SEM image (deposited
at -16 V) (b) XRD pattern (deposited at -150 V and -160 V) (c) Enlarged SEM image (deposited at -16 V) (d) EDS result (deposited at -16 V)
L-L Su et al Electrochimica Acta 147 (2014) 87 ndash95 93
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behaviors above at least two kinds of AlxDyy intermetallic
compounds
(DyAl3 and
DyAl2)
could
be
formed
by
potentiostaticelectrolysis at -15 and -16V since two very close pairs of
redox peaks were observed in CVs and SWV However only one
kind of intermetallic compound Al3Dy was acquired which was
against with our expectation The main reason might be the low
current density at our experimental concentration Under this
condition even though the intermetallic compound DyAl2 was
formed the formation rate was much slower than its diffusion
rate then the transformation of DyAl2 into the more stable Al-
rich phase (DyAl3) on the Al electrode would take palce Hence
ultimately only one kind of intermetallic compound DyAl3 was
observed in Fig 8 similar phenomena had been observed in the
potentiostatic electrolysis for the preparation of Al-Gd alloys
[15] The fact that intermetallic compound Al3Dy with more
crystallographic structures was obtained by electrolysis at more
negative potential to some extent shows the importance of
nucleation overpotential for the growth of Al-Dy alloys onto the
electrode
To provide a stable current to equably form more AlxDyyintermetallic compounds galvanostatic electrolysis with the
current intensity of -50 mA was also carried out for 25 h in our
experiment During the electrolysis the cathode potential was
controlled within the range of -13 V to -175 V to prevent the
deposition of pure Dy and in the meanwhile cover the two much
more anodic redox peaks (IaIc and IIaIIc) associated with the
formation of AlxDyy intermetallic compounds As shown in the
SEM image in Fig 9a a much thicker layer of approximate 40
mm of
the deposits was obtained than that gained by potentiostatic
electrolysis (Fig 8a) The XRD result in Fig 9b con1047297rms that the
deposits are composed of intermetallic compounds DyAl3 and
DyAl
although
DyAl2 was
still
not
observed
which
proves
onceagain that DyAl2 could not be stable at this temperature and easily
be transformed into DyAl3 The EDS analyses coupled with SEM in
Fig 8d and Fig 9d also con1047297rmed the co-existence of Dy and Al in
the deposits of electrolysis
4
Conclusions
Electrochemical behaviors of Dy(III) cations on an inert W
electrode were studied in molten LiCl-KCl-DyCl3 salts by
combining various electrochemical techniques (ie CV CP and
SWV) The electroreduction of Dy(III) ions on the tungsten
electrode is a single step process with transfer of three electrons
The reduction shows a reversible behavior for polarization rates
range of 50 V 300mV1 which is controlled by the diffusion of
Dy(III) cations in solution Accordingly the diffusion coef 1047297cient of
Dy(III) ion in the LiCl-KCl melts was measured by both CV and CP
techniques The adsorption effect which is surface based was
also observed prior to the reduction of Dy(III) to Dy(0)
The concentration ratio of Dy(III) ions to Al(III) ions has a great
in1047298uence on the co-reduction In a Dy-rich system three signals
corresponding to the formation of three AlxDyy were observed on
the tungsten electrode However when Al(III) cations were
suf 1047297cient only two of which with higher Al content were
observed SEM-EDS and XRD characterizations identi1047297ed inter-
metallic compound DyAl3 was produced by potentiostatic
electrolysis at -15 V and -16 V while two intermetallic com-
pounds DyAl3 and DyAl were obtained through galvanostatic
electrolysis at -50 mA
Fig 9 SEM (ac)-EDS (d) and XRD (b) results of the galvanostatic electrolysis products of LiCl-KCl-AlCl3 (12 wt)- Dy2O3 (09 wt) melts on the Al electrode Current -50 mA
Time 25 h Temperature 773 K
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However large differences were observed between signals L aL cand AaAc in Fig 5a As for the red curve peak Ec at -204 V and Ea at
about -183 V can be ascribed tothe reduction of Dy(III) tometal and
its subsequent re-oxidation respectively according to the above
results of LiCl-KCl-DyCl3 system and Ref [617] Between peaks EaEcand AaAc two broad anodic peaks at about -130 and -165 V were
observed The CVs measured subsequently in the same system at
different terminal potentials display a broad anodic peak at
approximate -13 V which actually consist of two close peaks Ia and
IIa similar to their cathodic peaks Ic and IIc (Fig 5b) In addition the
SWV in Fig 6 (curve 2) also revealed the existence of the two close
peaks Ic and IIc at approximate -13 V According to the co-reduction
principle redox peaks IaIc IIaIIcand IIIaIIIc should be ascribed tothe
formation
and
dissolution
of
at
least
three
AlxDyy intermetalliccompounds Moreover the closer the deposition potential of the
intermetallic compound to that of Dy metal the higher Dy content
could be formed in the AlxDyy intermetallic compound [1215]
As for the black curve in Fig 5a CV of AlCl3-rich system much
higher current intensities of peaks Aa and Ac were obviously
observed compared to that of AlCl3-poor system Besides other
differences could also be observed in the two curves of Fig 5a For
example the broad anodic peak at -13 V in the red curve separated
into two redox peaks IaIc and IIaIIc in the black curve The CVs
measured at various cathodic terminal potentials in AlCl3-rich
system (Fig 5c) and the SWV (curve 3 in Fig 6) also present two
clearly separated redox peaks IaIc and IIaIIc However the redox
signals IIIaIIIc and EaEc corresponding to the formation and
dissolution of an intermetallic compound with higher Dy content
Fig 5 (a) CVs of LiCl-KCl-AlCl3-Dy2O3 (09 wt) melts with different AlCl3concentrations 08 wt (red curve) and 12 wt (black curve) (b) CVs of the LiCl-
KCl-AlCl3 (08 wt)-Dy2O3 (09 wt) melts (c) CVs of the LiCl-KCl-AlCl3 (12 wt
)-Dy2O3 (09 wt) melts at different inversion potentials Working electrode
W (S068
cm2)
Temperature
773
K
Scan
rate
01
Vs1
Fig 7 CVs of LiCl-KCl-DyCl3 melts (black dotted curve) and LiCl-KCl-AlCl3-DyCl3melts (red solid curve) on an Al electrode Temperature 773 K Scan rate 01
Fig 6 SWVs of LiCl-KCl-DyCl3 (373 105molcm3) melts (curve 1) and LiCl-KCl-
AlCl3-Dy2O3 (09 wt) melts with different AlCl3 concentrations 08 wt (curve 2)
and 12 wt (curve 3) Working electrode W (S068 cm2) Temperature 773 K
Pulse height 10 mV potential step 5 mV frequency 20 Hz Vs1
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and the redox couple of Dy(III)Dy(0) vanished from both the CV
(Fig 5c) and SWV (curve 3 in Fig 6) curves In the meanwhile a
new
couple
of
peaks
marked
as
IVaIVc emerged
with
its
cathodicand anodic potential at approximate -216 and -205 V respectively
which should be ascribed to be the reduction and oxidation of Al-Li
alloy [1217] The reason of the difference between the two curves
in Fig 5 could be as follows With the increase of AlCl3concentration a much thicker layer of Al was deposited on the
W electrode which would facilitate the diffusion of Dy metal
Subsequently the initially generated intermetallic compound
AlxDyy tends to be transformed into another intermetallic
compounds AlxDyy with high Al content Therefore peaks IIIa
IIIc and EaEc which correspond to the formationdissolution of an
intermetallic compound with high Dy content and the redox
couple Dy(III)Dy(0) respectively could not be observed In
addition when the deposited AlxDyy intermetallic compounds
were not fully mantle the Al-covered electrode Al-Li alloys would
have the chance to be formed [15]
Electrochemical behaviors of LiCl-KCl melts containing both
Al(III) and Dy(III) cations were also investigated on an Al electrode
Fig 7 provides a comparison about the CVs of LiCl-KCl-DyCl3 and
LiCl-KCl-AlCl3-DyCl3 melts using Al as the working electrode The
CV of LiCl-KCl-DyCl3 melts without Al(III) cations (black dotted
curve) is consistent with Ref [17] Peaks IcIa are ascribed to the
formation and dissolution of Al-Dy alloys on the Al electrode The
red solid curve in Fig 7 shows a typical co-reduction behavior of
Al(III) and Dy(III) ions on the Al electrode which is very similar to
that obtained in LiCl-KCl-DyCl3 melts although the peaks become
more bulky This could be caused by the formation of different Al-
Dy alloys through the co-reduction of Al(III) and Dy(III) cations at
more cathodic potential [50]
33
Preparation
and
characterization
of
the
Al-Dy
alloys
To
con1047297rm
the
co-reduction
of
Dy(III)
and Al(III) ions andexamine the formation of AlxDyy intermetallic compounds at
various concentration ratio of Al(III) and Dy(III) both potentio-
static and galvanostatic electrolyses were carried out on a
tungsten electrode However only a very small amount of Al-
Dy alloys that adhered to the W electrode could be obtained even
the experiment was repeated for several times This phenomenon
is probably caused by the small cathode current and the high melt
point of the Al-Dy alloys Therefore we further used an Al plate
electrodewith the size of 15 cm 15 cm 02 cm for electrolysis
To prepareAl-Dy alloys at more anodic potential potentiostatic
electrolysis at -14 V -15 V and -16 V each for 3 h respectively
was performed Fig 8 shows the XRD patterns and the cross-
section SEM images coupled with EDS analysis of the cathodic
deposits of potentiostatic electrolysis It turns out that the
electrolysis at -14 V achieved nothing but Al metal while
electrolysis at -15V and -16 V produced a uniform layer covering
on the Al plateelectrode (Fig 8a and c) ByXRD analyses (Fig 8b)
the composition of the deposition layer obtained at -15 V was
con1047297rmed to be Al metal and the intermetallic compound Al3Dy
with crystallographic structure of rhombohedral lattice (R-3 m)
(PDF in XRD data base 18ndash0020) When electrolysis was
performed at more negative potential of -16 V the intermetallic
compound Al3Dy with crystallographic structures of R-3 m and
hexagonal lattice (P63mmc) (PDF in XRD data base 65ndash6363)
could be both obtained
It is well known that potentiostatic electrolysis has the
advantage of controlling the composition of the compound
produced by the cathodic reaction According to the co-reduction
Fig 8 SEM-EDS and XRD results of the potentiostatic electrolysis products of LiCl-KCl-AlCl3 (12 wt)-Dy2O3 (09 wt) melts on the Al electrodes (a) SEM image (deposited
at -16 V) (b) XRD pattern (deposited at -150 V and -160 V) (c) Enlarged SEM image (deposited at -16 V) (d) EDS result (deposited at -16 V)
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behaviors above at least two kinds of AlxDyy intermetallic
compounds
(DyAl3 and
DyAl2)
could
be
formed
by
potentiostaticelectrolysis at -15 and -16V since two very close pairs of
redox peaks were observed in CVs and SWV However only one
kind of intermetallic compound Al3Dy was acquired which was
against with our expectation The main reason might be the low
current density at our experimental concentration Under this
condition even though the intermetallic compound DyAl2 was
formed the formation rate was much slower than its diffusion
rate then the transformation of DyAl2 into the more stable Al-
rich phase (DyAl3) on the Al electrode would take palce Hence
ultimately only one kind of intermetallic compound DyAl3 was
observed in Fig 8 similar phenomena had been observed in the
potentiostatic electrolysis for the preparation of Al-Gd alloys
[15] The fact that intermetallic compound Al3Dy with more
crystallographic structures was obtained by electrolysis at more
negative potential to some extent shows the importance of
nucleation overpotential for the growth of Al-Dy alloys onto the
electrode
To provide a stable current to equably form more AlxDyyintermetallic compounds galvanostatic electrolysis with the
current intensity of -50 mA was also carried out for 25 h in our
experiment During the electrolysis the cathode potential was
controlled within the range of -13 V to -175 V to prevent the
deposition of pure Dy and in the meanwhile cover the two much
more anodic redox peaks (IaIc and IIaIIc) associated with the
formation of AlxDyy intermetallic compounds As shown in the
SEM image in Fig 9a a much thicker layer of approximate 40
mm of
the deposits was obtained than that gained by potentiostatic
electrolysis (Fig 8a) The XRD result in Fig 9b con1047297rms that the
deposits are composed of intermetallic compounds DyAl3 and
DyAl
although
DyAl2 was
still
not
observed
which
proves
onceagain that DyAl2 could not be stable at this temperature and easily
be transformed into DyAl3 The EDS analyses coupled with SEM in
Fig 8d and Fig 9d also con1047297rmed the co-existence of Dy and Al in
the deposits of electrolysis
4
Conclusions
Electrochemical behaviors of Dy(III) cations on an inert W
electrode were studied in molten LiCl-KCl-DyCl3 salts by
combining various electrochemical techniques (ie CV CP and
SWV) The electroreduction of Dy(III) ions on the tungsten
electrode is a single step process with transfer of three electrons
The reduction shows a reversible behavior for polarization rates
range of 50 V 300mV1 which is controlled by the diffusion of
Dy(III) cations in solution Accordingly the diffusion coef 1047297cient of
Dy(III) ion in the LiCl-KCl melts was measured by both CV and CP
techniques The adsorption effect which is surface based was
also observed prior to the reduction of Dy(III) to Dy(0)
The concentration ratio of Dy(III) ions to Al(III) ions has a great
in1047298uence on the co-reduction In a Dy-rich system three signals
corresponding to the formation of three AlxDyy were observed on
the tungsten electrode However when Al(III) cations were
suf 1047297cient only two of which with higher Al content were
observed SEM-EDS and XRD characterizations identi1047297ed inter-
metallic compound DyAl3 was produced by potentiostatic
electrolysis at -15 V and -16 V while two intermetallic com-
pounds DyAl3 and DyAl were obtained through galvanostatic
electrolysis at -50 mA
Fig 9 SEM (ac)-EDS (d) and XRD (b) results of the galvanostatic electrolysis products of LiCl-KCl-AlCl3 (12 wt)- Dy2O3 (09 wt) melts on the Al electrode Current -50 mA
Time 25 h Temperature 773 K
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and the redox couple of Dy(III)Dy(0) vanished from both the CV
(Fig 5c) and SWV (curve 3 in Fig 6) curves In the meanwhile a
new
couple
of
peaks
marked
as
IVaIVc emerged
with
its
cathodicand anodic potential at approximate -216 and -205 V respectively
which should be ascribed to be the reduction and oxidation of Al-Li
alloy [1217] The reason of the difference between the two curves
in Fig 5 could be as follows With the increase of AlCl3concentration a much thicker layer of Al was deposited on the
W electrode which would facilitate the diffusion of Dy metal
Subsequently the initially generated intermetallic compound
AlxDyy tends to be transformed into another intermetallic
compounds AlxDyy with high Al content Therefore peaks IIIa
IIIc and EaEc which correspond to the formationdissolution of an
intermetallic compound with high Dy content and the redox
couple Dy(III)Dy(0) respectively could not be observed In
addition when the deposited AlxDyy intermetallic compounds
were not fully mantle the Al-covered electrode Al-Li alloys would
have the chance to be formed [15]
Electrochemical behaviors of LiCl-KCl melts containing both
Al(III) and Dy(III) cations were also investigated on an Al electrode
Fig 7 provides a comparison about the CVs of LiCl-KCl-DyCl3 and
LiCl-KCl-AlCl3-DyCl3 melts using Al as the working electrode The
CV of LiCl-KCl-DyCl3 melts without Al(III) cations (black dotted
curve) is consistent with Ref [17] Peaks IcIa are ascribed to the
formation and dissolution of Al-Dy alloys on the Al electrode The
red solid curve in Fig 7 shows a typical co-reduction behavior of
Al(III) and Dy(III) ions on the Al electrode which is very similar to
that obtained in LiCl-KCl-DyCl3 melts although the peaks become
more bulky This could be caused by the formation of different Al-
Dy alloys through the co-reduction of Al(III) and Dy(III) cations at
more cathodic potential [50]
33
Preparation
and
characterization
of
the
Al-Dy
alloys
To
con1047297rm
the
co-reduction
of
Dy(III)
and Al(III) ions andexamine the formation of AlxDyy intermetallic compounds at
various concentration ratio of Al(III) and Dy(III) both potentio-
static and galvanostatic electrolyses were carried out on a
tungsten electrode However only a very small amount of Al-
Dy alloys that adhered to the W electrode could be obtained even
the experiment was repeated for several times This phenomenon
is probably caused by the small cathode current and the high melt
point of the Al-Dy alloys Therefore we further used an Al plate
electrodewith the size of 15 cm 15 cm 02 cm for electrolysis
To prepareAl-Dy alloys at more anodic potential potentiostatic
electrolysis at -14 V -15 V and -16 V each for 3 h respectively
was performed Fig 8 shows the XRD patterns and the cross-
section SEM images coupled with EDS analysis of the cathodic
deposits of potentiostatic electrolysis It turns out that the
electrolysis at -14 V achieved nothing but Al metal while
electrolysis at -15V and -16 V produced a uniform layer covering
on the Al plateelectrode (Fig 8a and c) ByXRD analyses (Fig 8b)
the composition of the deposition layer obtained at -15 V was
con1047297rmed to be Al metal and the intermetallic compound Al3Dy
with crystallographic structure of rhombohedral lattice (R-3 m)
(PDF in XRD data base 18ndash0020) When electrolysis was
performed at more negative potential of -16 V the intermetallic
compound Al3Dy with crystallographic structures of R-3 m and
hexagonal lattice (P63mmc) (PDF in XRD data base 65ndash6363)
could be both obtained
It is well known that potentiostatic electrolysis has the
advantage of controlling the composition of the compound
produced by the cathodic reaction According to the co-reduction
Fig 8 SEM-EDS and XRD results of the potentiostatic electrolysis products of LiCl-KCl-AlCl3 (12 wt)-Dy2O3 (09 wt) melts on the Al electrodes (a) SEM image (deposited
at -16 V) (b) XRD pattern (deposited at -150 V and -160 V) (c) Enlarged SEM image (deposited at -16 V) (d) EDS result (deposited at -16 V)
L-L Su et al Electrochimica Acta 147 (2014) 87 ndash95 93
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behaviors above at least two kinds of AlxDyy intermetallic
compounds
(DyAl3 and
DyAl2)
could
be
formed
by
potentiostaticelectrolysis at -15 and -16V since two very close pairs of
redox peaks were observed in CVs and SWV However only one
kind of intermetallic compound Al3Dy was acquired which was
against with our expectation The main reason might be the low
current density at our experimental concentration Under this
condition even though the intermetallic compound DyAl2 was
formed the formation rate was much slower than its diffusion
rate then the transformation of DyAl2 into the more stable Al-
rich phase (DyAl3) on the Al electrode would take palce Hence
ultimately only one kind of intermetallic compound DyAl3 was
observed in Fig 8 similar phenomena had been observed in the
potentiostatic electrolysis for the preparation of Al-Gd alloys
[15] The fact that intermetallic compound Al3Dy with more
crystallographic structures was obtained by electrolysis at more
negative potential to some extent shows the importance of
nucleation overpotential for the growth of Al-Dy alloys onto the
electrode
To provide a stable current to equably form more AlxDyyintermetallic compounds galvanostatic electrolysis with the
current intensity of -50 mA was also carried out for 25 h in our
experiment During the electrolysis the cathode potential was
controlled within the range of -13 V to -175 V to prevent the
deposition of pure Dy and in the meanwhile cover the two much
more anodic redox peaks (IaIc and IIaIIc) associated with the
formation of AlxDyy intermetallic compounds As shown in the
SEM image in Fig 9a a much thicker layer of approximate 40
mm of
the deposits was obtained than that gained by potentiostatic
electrolysis (Fig 8a) The XRD result in Fig 9b con1047297rms that the
deposits are composed of intermetallic compounds DyAl3 and
DyAl
although
DyAl2 was
still
not
observed
which
proves
onceagain that DyAl2 could not be stable at this temperature and easily
be transformed into DyAl3 The EDS analyses coupled with SEM in
Fig 8d and Fig 9d also con1047297rmed the co-existence of Dy and Al in
the deposits of electrolysis
4
Conclusions
Electrochemical behaviors of Dy(III) cations on an inert W
electrode were studied in molten LiCl-KCl-DyCl3 salts by
combining various electrochemical techniques (ie CV CP and
SWV) The electroreduction of Dy(III) ions on the tungsten
electrode is a single step process with transfer of three electrons
The reduction shows a reversible behavior for polarization rates
range of 50 V 300mV1 which is controlled by the diffusion of
Dy(III) cations in solution Accordingly the diffusion coef 1047297cient of
Dy(III) ion in the LiCl-KCl melts was measured by both CV and CP
techniques The adsorption effect which is surface based was
also observed prior to the reduction of Dy(III) to Dy(0)
The concentration ratio of Dy(III) ions to Al(III) ions has a great
in1047298uence on the co-reduction In a Dy-rich system three signals
corresponding to the formation of three AlxDyy were observed on
the tungsten electrode However when Al(III) cations were
suf 1047297cient only two of which with higher Al content were
observed SEM-EDS and XRD characterizations identi1047297ed inter-
metallic compound DyAl3 was produced by potentiostatic
electrolysis at -15 V and -16 V while two intermetallic com-
pounds DyAl3 and DyAl were obtained through galvanostatic
electrolysis at -50 mA
Fig 9 SEM (ac)-EDS (d) and XRD (b) results of the galvanostatic electrolysis products of LiCl-KCl-AlCl3 (12 wt)- Dy2O3 (09 wt) melts on the Al electrode Current -50 mA
Time 25 h Temperature 773 K
94 L-L Su et al Electrochimica Acta 147 (2014) 87 ndash95
7252019 1-s20-S0013468614019264
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7252019 1-s20-S0013468614019264
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behaviors above at least two kinds of AlxDyy intermetallic
compounds
(DyAl3 and
DyAl2)
could
be
formed
by
potentiostaticelectrolysis at -15 and -16V since two very close pairs of
redox peaks were observed in CVs and SWV However only one
kind of intermetallic compound Al3Dy was acquired which was
against with our expectation The main reason might be the low
current density at our experimental concentration Under this
condition even though the intermetallic compound DyAl2 was
formed the formation rate was much slower than its diffusion
rate then the transformation of DyAl2 into the more stable Al-
rich phase (DyAl3) on the Al electrode would take palce Hence
ultimately only one kind of intermetallic compound DyAl3 was
observed in Fig 8 similar phenomena had been observed in the
potentiostatic electrolysis for the preparation of Al-Gd alloys
[15] The fact that intermetallic compound Al3Dy with more
crystallographic structures was obtained by electrolysis at more
negative potential to some extent shows the importance of
nucleation overpotential for the growth of Al-Dy alloys onto the
electrode
To provide a stable current to equably form more AlxDyyintermetallic compounds galvanostatic electrolysis with the
current intensity of -50 mA was also carried out for 25 h in our
experiment During the electrolysis the cathode potential was
controlled within the range of -13 V to -175 V to prevent the
deposition of pure Dy and in the meanwhile cover the two much
more anodic redox peaks (IaIc and IIaIIc) associated with the
formation of AlxDyy intermetallic compounds As shown in the
SEM image in Fig 9a a much thicker layer of approximate 40
mm of
the deposits was obtained than that gained by potentiostatic
electrolysis (Fig 8a) The XRD result in Fig 9b con1047297rms that the
deposits are composed of intermetallic compounds DyAl3 and
DyAl
although
DyAl2 was
still
not
observed
which
proves
onceagain that DyAl2 could not be stable at this temperature and easily
be transformed into DyAl3 The EDS analyses coupled with SEM in
Fig 8d and Fig 9d also con1047297rmed the co-existence of Dy and Al in
the deposits of electrolysis
4
Conclusions
Electrochemical behaviors of Dy(III) cations on an inert W
electrode were studied in molten LiCl-KCl-DyCl3 salts by
combining various electrochemical techniques (ie CV CP and
SWV) The electroreduction of Dy(III) ions on the tungsten
electrode is a single step process with transfer of three electrons
The reduction shows a reversible behavior for polarization rates
range of 50 V 300mV1 which is controlled by the diffusion of
Dy(III) cations in solution Accordingly the diffusion coef 1047297cient of
Dy(III) ion in the LiCl-KCl melts was measured by both CV and CP
techniques The adsorption effect which is surface based was
also observed prior to the reduction of Dy(III) to Dy(0)
The concentration ratio of Dy(III) ions to Al(III) ions has a great
in1047298uence on the co-reduction In a Dy-rich system three signals
corresponding to the formation of three AlxDyy were observed on
the tungsten electrode However when Al(III) cations were
suf 1047297cient only two of which with higher Al content were
observed SEM-EDS and XRD characterizations identi1047297ed inter-
metallic compound DyAl3 was produced by potentiostatic
electrolysis at -15 V and -16 V while two intermetallic com-
pounds DyAl3 and DyAl were obtained through galvanostatic
electrolysis at -50 mA
Fig 9 SEM (ac)-EDS (d) and XRD (b) results of the galvanostatic electrolysis products of LiCl-KCl-AlCl3 (12 wt)- Dy2O3 (09 wt) melts on the Al electrode Current -50 mA
Time 25 h Temperature 773 K
94 L-L Su et al Electrochimica Acta 147 (2014) 87 ndash95
7252019 1-s20-S0013468614019264
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7252019 1-s20-S0013468614019264
httpslidepdfcomreaderfull1-s20-s0013468614019264 99