Electrochemical behaviour of uranium (IV) in DMF at vitreous carbon

Article abstract of DOI:10.1016/j.electacta.2009.07.057

The electrochemical behaviour of UCl₄ (0.01 mol L^-1 up to 0.05 mol L^-1) in 0.1 mol L^-1 TBAPF₆/DMF solution at vitreous carbon was studied, at room temperature, by cyclic voltammetry and potentiostatic

Full text of DOI:10.1016/j.electacta.2009.07.057

Electrochimica Acta 54 (2009) 7318–7323
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Electrochimica Acta
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Electrochemical behaviour of uranium (IV) in DMF at vitreous carbon
M.L. Afonsoa, A. Gomesb,∗,1, A. Carvalhoa, L.C. Alvesc, F. Wastind, A.P. Gonc¸ alvesa
a Instituto Tecnológico e Nuclear/CFMC-UL, P-2686-953 Sacavém, Portugal
b CCMM, Departamento de Química e Bioquímica da Faculdade de Ciências, Universidade de Lisboa, Campo Grande Ed. C8, 4◦Piso,
P-1749-016 Lisboa, Portugal
c Instituto Tecnológico e Nuclear/CFN-UL, P-2686-953 Sacavém, Portugal
d European Commission, JRC, Inst. Transuranium Elements, D-76125 Karlsruhe, Germany
a r t i c l e i n f o
a b s t r a c t
The electrochemical behaviour of UCl4 (0.01 mol L−1 up to 0.05 mol L−1) in 0.1 mol L−1 TBAPF6/DMF solu-
tion at vitreous carbon was studied, at room temperature, by cyclic voltammetry and potentiostatic
techniques. The electrolytic solutions were analyzed by UV spectroscopy (UV), and the electrodeposited
films were characterized by Rutherford Backscattering Spectroscopy (RBS) and X-ray diffraction (XRD).
The cyclic voltammetric results, at low UCl4 concentrations (0.01 mol L−1), point that the reduction of
U(IV) to U(0) occurs in two steps involving mainly U(IV) and U(III) species. The first electron transfer
reaction is quasi-reversible and the second irreversible. The diffusion coefficient of U(IV) in DMF and the
charge rate constant were determined to be 4.78 × 10−7 cm2 s−1 and 1.93 × 10−3 cm s−1 (at 0.02 V s−1),
respectively.
RBS data obtained from samples prepared at constant potential (−3.10 V) during 3 h at room temper-
ature, indicated the presence of uranium particles deposited all over the vitreous carbon surface with
aggregates in some places, confirming that the second reduction step corresponds to uranium electrode-
position. No crystallographic ordering could be seen by XRD, pointing to an amorphous character of the
Article history:
Received 19 February 2009
Received in revised form 16 July 2009
Accepted 18 July 2009
Available online 28 July 2009
Keywords:
Uranium tetrachloride
Dimethylformamide
Cyclic voltammetry
Vitreous carbon electrode
Electrodeposition
uranium films.
© 2009 Elsevier Ltd. All rights reserved.
1. Introduction
plexity of these systems, with the formation of dimerized species
and complexes [2,10].
Electrochemical reduction of heavy elements from solution is
already being used successfully in different fields such as: the sep-
aration of actinides from rare earth elements and spent nuclear
fuels [1], actinides redox flow batteries for the electric power stor-
age [2] and electrodeposition of actinide layers for nuclear targets
[3]. Some of these applications are performed in aqueous media.
Nevertheless, high current densities are needed, which result in
significant hydrogen evolution, compromising consequently the
deposit quality and yield. Furthermore, in the case of uranium elec-
trodeposition, the presence of oxygen promotes the formation of
uranyl ion. This explains the significant amount of studies reported
in the literature concerning uranium electrodeposition in differ-
ent systems [1,3–9]. The knowledge of uranium electrochemical
behaviour in organic media is essential for the development of
technological applications. Until now this behaviour is known only
within a limited extent. The main difficulty arises from the com-
It has been reported by polarographic studies that UCl4 orig-
inates two cathodic waves in dimethylformamide (DMF) and
dimethyl sufoxide (DMSO), corresponding to the reduction steps
U(IV)/U(III) and U(III)/U(0), respectively. However while in DMF the
first reduction is reversible and the second irreversible, in DMSO
all the steps are irreversible [10]. Shirasaki et al. [2] observed
that the behaviour of uranium (IV) dimethylsulfoxide perchlo-
rate in DMSO was strongly dependent on the substrate, giving
rise to more cathodic waves in platinum and gold than in mer-
cury. In dimethylacetamide (DMA), three waves were detected
but not assigned [11]. In tetrahydrofuran (THF), three cathodic
waves were also obtained and all assigned to U(IV)/U(III) reduc-
tion by a mechanism dominated by chloride ion transfer reactions
[12].
From these studies, it was concluded that the electrochemical
behaviour of uranium is strongly influenced not only by the nature
of the working electrode but also by the solvent and electrolyte
composition.
In the present work, the electrochemical behaviour of UCl4 in
dimethylformamide (DMF) with tetrabutylammonium hexafluo-
rophosphate (TBAPF6) as the supporting electrolyte at vitreous
carbon was studied for the first time in order to evaluate the possi-
∗
Corresponding author.
E-mail address: abmg@fc.ul.pt (A. Gomes).
ISE member.
1
0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.07.057

M.L. Afonso et al. / Electrochimica Acta 54 (2009) 7318–7323
7319
bility of uranium electrodeposition by the application of a constant
potential, at room temperature.
2. Experimental
2.1. Chemicals and reagents
The solvent, dimethylformamide (DMF) (Riedel-de Haen p.a.)
and the supporting electrolyte, tetrabutylammonium hexaflu-
orophosphate (TBAPF6) (>99% Fluka) were thoroughly dried,
deoxygenated and stored in a glove box under nitrogen (<2 ppm of
oxygen). UO2(NO3)2·6H2O (E. Merck, p.a.) was converted into anhy-
drous UCl4 in two steps following the procedure described in Ref.
[13] with some adjustments. The first step consisted in the synthe-
sis of UO3 by thermal decomposition of UO2(NO3)2·6H2O (100 ◦C
during 3 h in vacuum followed by 300–350 ◦C during 2 h). The sec-
ond step consisted in adding hexachloropropene to anhydrous UO3
under an argon atmosphere. The mixture was heated slowly (60 ◦C)
under stirring and an exothermal reaction was observed. An ice
bath was used in order to avoid temperatures above 100 ◦C. After
this reaction the formed trichloro acryloyl chloride was refluxed at
158 ◦C during 6 h until the red color of the solution (due to the
dissolved U(V) chloride) disappeared and a green precipitate of
uranium (IV) chloride was formed. Solutions were prepared by dis-
solution of UCl4 in 0.1 mol L−1 TBAPF6/DMF inside the glove box
immediately before each measurement. The electrochemical cells
were filled and sealed inside the glove box.
Fig. 1. Cyclic voltammograms of 0.01 mol L−1 UCl4 in 0.1 mol L−1 TBAPF6/DMF and
0.1 mol L−1 TBAPF6/DMF (inset) on vitreous carbon electrode at ꢀ = 0.10 V s−1, at
room temperature. The potential sweep was started from the open circuit potential
in the cathodic direction.
electrode, is presented in Fig. 1. The inset shows the voltammo-
gram recorded with the solvent and the supporting electrolyte.
The potential scans were started in the cathodic direction from
the open circuit potential, in the potential range between −3.28 V
and 0.20 V. The voltammogram obtained in the solvent and the
supporting electrolyte shows that TBAPF6 is almost inert in
the present experimental conditions, pointing out to its good
characteristics as supporting electrolyte. On the other hand the
voltammogram of the uranium solution exhibits two cathodic
peaks, I and II, at −1.63 V and −2.70 V, respectively, and in the
reverse scan two small waves at −2.21 V and −0.07 V. A direct
comparison among the experimental results and the literature
cannot be done, since the reported work are concerned with UCl4
reduction using other electrolytes, solvents and/or substrates and
reference electrode [2,10,12]. Yet, the comparison of the cyclic
voltammograms attained herein with others obtained for uranium
(IV) suggests that the two well-defined peaks in the cathodic
region might be assigned to processes that involve the reduction
of uranium (IV) to uranium (0) in two steps: U(IV) → U(III) and
U(III) → U(0)[10].
2.2. Apparatus and procedure
Cyclic voltammetry and electrodeposition experiments were
performed in a single compartment three-electrode cell specially
designed for air sensitive experiments, and were conducted by
using a potentiostat (BAS Cell power, CV-50W software) system
under a personal computer control. For the cyclic voltammetric
experiments, a cylindrical vitreous carbon electrode (0.16 cm2) was
used as working electrode. It was polished with 0.05 ␮m alumina
on micro-cloth, thoroughly rinsed with distilled water and ace-
tone prior to use in the electrochemical experiments. The counter
electrode was a platinum wire and a pseudo-reference elec-
trode silver–silver ion (0.01 mol L−1 AgNO3, 0.1 mol L−1 TBAPF6)
immersed in dimethylformamide was used. The Ag+ ion elec-
trode was separated from the bulk solution by a VycorTM frit. All
the potentials presented in the present work are referred to this
pseudo-reference electrode. iR compensation was employed for all
the experiments using the software CV-50W Version 2.3. For the
electrodeposition experiments the cathode was a vitreous carbon
removable cylinder, inserted in a Teflon holder, the anode was a
platinum spiral and the reference electrode was the same used in
the cyclic voltammetric measurements. Uranium electrodeposition
was performed at constant potential of −3.10 V during 3 h without
stirring and at room temperature. After the experiment, the elec-
trodes were removed from the Teflon support, washed thoroughly
with acetone and stored under vacuum (10−5 Torr). The solutions
containing 0.01 mol L−1 and 0.05 mol L−1 UCl4 in TBAPF6/DMF were
analyzed by UV spectroscopy using a Varian UV–vis–NIR spec-
trophotometer. The electrodeposited films were characterized by
Rutherford Backscattering Spectroscopy (RBS), using Oxford Micro
beam OM 150 Model 1997, and by X-ray diffraction, with the help
of a Pananytical X’Pert Pro diffractometer (Cu K␣-radiation).
3. Results and discussion
3.1. Cyclic voltammetry
Fig. 2. Cyclic voltammograms of 0.01 mol L−1 UCl4 in 0.1 mol L−1 TBAPF6/DMF
on vitreous carbon electrode at several sweep rates (from ꢀ = 0.020 V s−1 to
ꢀ = 0.250 V s−1) at room temperature. The potential sweeps were started from the
open circuit potential in the cathodic direction.
The cyclic voltammogram obtained in 0.01 mol L−1
UCl4 + 0.1 mol L−1 TBAPF6/DMF solution at vitreous carbon

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M.L. Afonso et al. / Electrochimica Acta 54 (2009) 7318–7323
Fig. 3. Cyclic voltammograms of 0.01 mol L−1 UCl4 in 0.1 mol L−1 TBAPF6/DMF on
vitreous carbon electrode at several sweep rates (from ꢀ = 0.01 V s−1 to ꢀ = 0.20 V s−1
at room temperature. The potential sweeps were started from the open circuit
potential in the cathodic direction.
Fig. 4. Dependence of cathodic peak current with square root of scan rate for the first
reduction process observed in Fig. 2 (0.01 mol L−1 UCl4 in 0.1 mol L−1 TBAPF6/DMF
on a vitreous carbon electrode at room temperature).
)
is the bulk concentration of U(IV), S (cm2) is the geometric surface
area of electrode and D (cm2 s−1) is the diffusion coefficient of U(IV)
[14].
From the slope of the linear dependence of the cathodic peak
current on square root scan rate it is possible to determine the
diffusion coefficient D, according to Eq. (1). The value ˛n˛ can be
estimated from Eq. (2):
With the purpose of improving the characterization of the two
cathodic processes, sets of cyclic voltammograms were performed
at different sweep rates, on distinct potential regions. For the
study of the first process, the measurements were performed in
the potential region up to −2.00 V, using sweep rates between
0.02 V s−1 and 0.25 V s−1 (Fig. 2). For the second cathodic process a
more negative potential region was analyzed, applying a different
range of sweep rates (0.01–0.20 V s−1), Fig. 3.
ꢀ
ꢀ
ꢀ
ꢀ
0.0477
˛n˛
Ep − Ep/2
=
(2)
In Fig. 2, the shape of the peaks I and Iꢀ point out for a reversible
process, but the treatment of the experimental data revealed
that the process is more complex than expected. The separations
between the peak and half-peak potentials in the voltammograms
were measured and the values |Ep − Ep/2| obtained are slightly
larger (≈90 mV at 298 K) than the expected value for a reversible
process (59/n mV at 298 K, where n is the number of electrons
involved in the reduction reaction which is assumed to be 1), indi-
cating that the reduction of U(IV) at vitreous carbon electrode is
not only controlled by diffusion, but also by charge transfer kinet-
ics (Table 1) [14]. The graphical representation of the cathodic
peak current, Jp, against the square root of sweep rate (Fig. 4)
reveals a linear behaviour with an intercept slightly higher than
zero which shows that the reaction is not totally reversible [15].
The relation between the cathodic peak current (Jp, A) and the
scan rate (v, V s−1) for an irreversible reduction reaction (which can
be used for quasi-reversible process) for solubles species is given
by
where Ep/2 is the half-peak potential, in V, at 298 K[14]. The value
of ˛n˛ was found to be 0.53 0.06, using the average value for
|Ep − Ep/2|, 90 10 mV, for sweep rate up to 0.250 V s−1. Substituting
this value on Eq. (1), the diffusion coefficient of U(IV) was deter-
mined to be 4.78 × 10−7 cm2 s−1 at 298 K. This value is lower than
that obtained for the [U(dmso)9]4+ complex, 1.1 × 10−6 cm2 s−1
[16] and similar to that determined for the solvato complex
U(dmso)8(ClO4)4, 2.3 × 10−7 cm2 s−1 [2].
The magnitudes of cathodic and anodic peak potentials at vari-
ous sweep rates are shown in Table 1. It is observed that the values
are practically constant and varies slightly with the increase of
sweep rate. The values of ꢁEp vary between 91 mV and 106 mV
for the 0.02 V s−1 and 0.250 V s−1, respectively. These values are
higher than expected for the reversible process. This result also sug-
gests that the reduction of U(IV) is not only controlled by diffusion
but also charge transfer kinetics. The rate constant for the charge
transfer reaction, ks (cm s−1), can be estimated by the Nicholson’s
method [17]. According to Nicholson’s theory, the function ꢂ is
related to the standard rate constant, ks, by Eq. (3) and related to
the peak potential difference, ꢁEp:
Jp = 299n(˛n˛)1/2SCD1/2v1/2
(1)
where n˛ is the number of electrons transferred in the rate-
determining step, ˛ is the charge transfer coefficient, C (mol cm−3
)
ks
ꢂ =
(3)
[ꢃDnvF/(RT)]1/2
Table 1
Effect of sweep rate on peak potentials and current densities of the first cathodic
The values of ꢂ at various ꢁEp are reported in the Nichol-
son working curve at 298 K [17], and in this case, the parameter
ꢂ has the values: 0.770 and 0.485 for ꢁEp = 91 mV and 106 mV,
respectively. From Eq. (3), ks was evaluated as 1.9 × 10−3 cm s−1
and 9.3 × 10−4 cm s−1, using n = 1 and D = 4.78 × 10−7 cm2 s−1. This
standard constant rate values fall in the range 0.3v1/2 ≥ ks ≥ 2 ×
10−5v1/2 cm s−1 confirming that the reduction of U(IV) in DMF solu-
tion is quasi-reversible at 298 K [18].
process in 0.01 mol L−1 UCl4 at room temperature.
ꢀ
ꢀ
ꢀ
ꢀ
I
ꢀ (V s−1
)
−EpI (V)
−EpIꢀ (V)
ꢁE(EpI − EpIꢀ) (V)
Ep − EI
(mV)
jpIꢀ/jpI
p/2
0.02
0.05
0.10
0.15
0.20
0.25
1.652
1.652
1.636
1.636
1.644
1.644
1.560
1.561
1.530
1.530
1.530
1.538
0.092
0.091
0.106
0.106
0.106
0.106
91
91
90
83
76
1.1
1.1
1.1
1.1
1.0
1.1
106
As referred before, for the study of the second reduction pro-
cess, assigned to U(III)/U(0), cyclic voltammograms up to −3.28 V
Values taken from the plots presented in Fig. 2.

M.L. Afonso et al. / Electrochimica Acta 54 (2009) 7318–7323
7321
Fig. 5. Cyclic voltammograms of 0.01 mol L−1 UCl4 in 0.1 mol L−1 TBAPF6/DMF on
vitreous carbon electrode at various cathodic switching potentials, at room tem-
perature. The potential sweeps were started from the open circuit potential in the
cathodic direction.
Fig. 7. Absorption spectra of UCl4 (−) 0.01 mol L−1 and (−) 0.05 mol L−1 in 0.1 mol L−1
TBAPF6/DMF solutions, at room temperature.
were performed (Fig. 3). The absence of the anodic counterpart of
cathodic peak II at potential scans higher than 0.01 V s−1 and the
shift to more negative potentials indicates that the second reduc-
tion step, U(III)/U(0), is irreversible in DMF. An interesting feature is
the detection of a slightly cathodic wave (a) in the potential region
situated between the two well-defined peaks at higher sweep rates.
This strongly suggests the presence of more electroactive species
suffering a fast reduction process.
The study of the influence of the switching potential (Eꢄ) (Fig. 5)
reveals that the first cathodic peak (I) has a corresponding anodic
peak (Iꢀ) for Eꢄ lesser than −2.60 V. Additionally, at this switching
potential the current intensity of peak Iꢀ decreases pointing that
U(0) irreversible species are already being formed at the cathode
surface. Conversely, the current of the anodic wave observed at
−0.066 V increases as the cathodic switching potential is reach-
ing more negative values. The origin of this wave is not yet clear.
Suryanarayanan et al. [19] reported that TBAPF6 in organic solvents
(acetonitrile, propylene carbonate, dimethylformamide, tetrahy-
drofuran, etc.) at vitreous carbon electrode is not completely inert
for positive potentials. In this sense one could assign the observed
behaviour to processes due to the supporting electrolyte. However,
the cyclic voltammogram of the electrolyte, shown in Fig. 1, points
to the absence of any significant electron transfer in the potential
range studied. Therefore, this wave should be related with ura-
nium species and not to processes occurring from the supporting
electrolyte.
The effect of UCl4 concentration in DMF solution on the electro-
chemical behaviour of U(IV) was studied over a concentration range
of 0.01–0.05 mol L−1. Representative voltammograms obtained
from solutions containing different UCl4 concentrations on vitreous
carbon electrode are shown in Fig. 6. As the concentration increases,
both cathodic peaks (I and II) are shifted negatively being the poten-
tial shift more pronounced for the cathodic peak II. The cathodic
wave (a) observed before for the highest scan rate performed in
the solution containing UCl4 0.01 mol L−1 (see Fig. 3) is clearly
seen at concentrations higher than 0.02 mol L−1. The potential shift
of peak II is about 0.50 V when comparing the cyclic voltammo-
grams obtained in the lower and higher concentrations studied.
This potential shift with increasing concentration is expected from
a Nernstian behaviour. Nevertheless, it cannot be excluded the
hypothesis that the species responsible for wave (a) inhibit the
U(III)/U(0) reduction. An increase in the anodic wave detected at
positive potentials is also observed. This was already expected since
this wave is probably related to the oxidation of uranium species, as
referred before. These results strongly suggest the presence of addi-
Fig. 6. Cyclic voltammograms of UCl4 in 0.1 mol L−1 TBAPF6/DMF on vitreous car-
bon electrode obtained for different UCl4 concentrations (0.01 mol L−1, 0.02 mol L−1
,
0.03 mol L−1 and 0.05 mol L−1) at room temperature. The potential sweeps were
started from the open circuit potential in the cathodic direction.
Fig. 8. Potentiostatic transient of 0.01 mol L−1 UCl4 in 0.1 mol L−1 TBAPF6/DMF on
vitreous carbon electrode at −3.10 V potential step at room temperature.

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M.L. Afonso et al. / Electrochimica Acta 54 (2009) 7318–7323
Fig. 9. RBS spectra obtained from three different points in the electrodeposited uranium film in a vitreous carbon surface, obtained by applying a constant potential
(E = −3.10 V) during 3 h to a solution containing 0.01 mol L−1 UCl4 in 0.1 mol L−1 TBAPF6/DMF. (a) Point with a thicker U layer; (b) point with a thinner U layer and (c) random
“bulk” point.
tional electroactive species promoted by the UCl4 concentration
increase.
graphic peaks. The inexistence of any peak points to an amorphous
nature of the electrodeposited uranium films.
In order to confirm the presence of additional species, UV anal-
ysis was performed on the initial electrolytic solutions. The UV
spectra obtained for lower and higher limits of concentrations stud-
ied, is shown in Fig. 7. In a first approach, the spectra of both
solutions are similar, and may be assigned to uranium (IV) com-
plexes with intense peaks detected at 654 nm, 681 nm and 564 nm
and several peaks with low intensity observed at higher wave-
lengths (Laporte-allowed transitions 5f3–5f26d1, attributed to a
inner sphere halide complexes) [20,21]. Apparently, the differ-
ence between the two spectra is not significant: one slight band
is observed in the spectral region of 404 nm and shoulders around
600 nm are observed for the more concentrated medium.
These results suggest that uranium IV complexes are formed
even at low concentrations of UCl4, and its formation is favoured
with increasing concentration of the reagent.
Efforts will be pursued to optimize the electrolysis conditions
in order to improve the quality of the films.
4. Conclusions
The electrochemical behaviour of uranium in DMF at a carbon
vitreous electrode proceeds mainly by two reduction steps; in the
first step U(IV) is reduced to U(III) and in the second step U(III)
is reduced to U(0) which is deposited on the electrode surface.
The electrochemical system used in the present work (dimethylfor-
mamide, tetrabutylammonium hexafluorophosphate and vitreous
carbon substrate) is suitable for uranium electrodeposition. How-
ever, the electrodeposition conditions must be optimized in order
to obtain metallic uranium coatings, suitable for technological
applications.
3.2. Electrodeposition
Acknowledgements
The cyclic voltammetric measurements, for the lower UCl4 con-
centration, indicate that the reduction of U(III) to U(0) takes place at
about −2.70 V. In order to prove this hypothesis a constant poten-
tial of −3.10 V was applied during 3 h at room temperature to
guarantee uranium electrodeposition. Fig. 8 shows the potentio-
static transient obtained during the first deposition hour. The slight
increase in the current observed after approximately 50 s of the
beginning of the transient, should be assigned to the first stages
of uranium nucleation process. After electrodeposition the sample
was examined by RBS and XRD. The RBS analysis (Fig. 9) indicates
the presence of U particles deposited all over the surface of vit-
reous carbon substrate with aggregates in some places. Since the
potential required for deposition is very high (−3.1 V), the elec-
trodeposit may be considered as being uranium in metallic form.
These results confirm that uranium electrodeposition occurs, which
strongly suggest that the voltammetric cathodic peak II is due to
U(III)/U(0) reduction. Therefore it is possible to conclude that the
main reduction steps involve U(IV) and U(III) species. The XRD char-
acterization of the films shows only the broad bands characteristic
of vitreous carbon electrode, with no evidence of any crystallo-
Part of this work has been carried out as a joint project (JP 07-
09) supported by the European Commission within the frame of
the ACTINET-6 network of excellence.
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