1454
J. EZectrochem. Sot.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y
13. W. S. Ferguson, Ph..D Thesis, U n i v e r s i t y of Illinois,
Urbana, Illinois (1956).
14. Beckwith Carbon Corp., Technical Leaflet.
15. R. O. Johnston, Ph.D. Thesis, U n i v e r s i t y of Illinois,
Urbana, Illinois (1974).
16. " I n t e r n a t i o n a l Critical Table," Vol. 3, p. 301,
S e p t e m b e r I978
McGraw-Hill, New York (1928).
17. R. S. Nicholson, Anal. Chem., 38, 1406 (1966).
18. H. A. L a i t i n e n and B. B. Bhatia, ibid., 30, 1995
(1958).
19. H. A. L a i t i n e n and J. A. Plambeck, J. Am. Chem.
Soc., 87, 1202 (1965).
The Electrochemical Behavior of Selenium and Selenium
Compounds in Sodium Tetrachloroaluminate Melts
J. Robinson and R. A. Osteryoung*
Department o~ Chemistry, Colorado State University, Fort Collins, Colorado 80523
ABSTRACT
The electrochemistry of s e l e n i u m and various selenium compounds in
A1C18:NaC1 melts has been investigated b y a variety of techniques including
pulse and cyclic voltammetry, coulometry, and the r o t a t i n g - d i s k electrode.
It was found that selenium can be reduced, in both acid and basic melts, b y
a single two-electron step to selenide whic'h exists i n the melt as either
A1SeC1 or A1SeC12- (or their analogous solvated species A12SeC15- and
A12SeC162-) depending upon the acidity. The mechanism for the oxidation of
s e l e n i u m to S e ( I V ) was found to be d e p e n d e n t upon the melt acidity. I n basic
melts, selenium was first oxidized by a two-electron step to a n S e ( I I ) species
and then by a f u r t h e r t w o - e l e c t r o n step to S e ( I V ) . I n acid melts the oxidation
was a single quasireversible four-electron step. The reduction of Se(IV) to
s e l e n i u m was a single four-electron step at all melt acidities. The kinetics of
these oxidation and reduction processes were investigated extensively. F r o m
studying SeC14 solutions, it was found that there were two Se(IV) species in
the melt, SeC162- and SeC15-, linked b y the a c i d i t y - d e p e n d e n t e q u i l i b r i u m
SeCI~- -}- C1- ~ S e C I ~ for which the e q u i l i b r i u m constant was calculated to be 6.0 _ 1.0 • 108 on the
mole fraction scale.
I n recent years there has been considerable interest
i n the use of fused-salt media as electrolytes for high
energy density batteries and fuel cells. I n view of this,
there have been several papers dealing with the electrochemical behavior of chalcogens and their compounds in these systems. S u l f u r has been studied i n a
v a r i e t y of these molten salts including most recently
sodium tetrachloroaluminate, both i n this laboratory
(1) and by M a m a n t o v a n d co-workers (2, 3). Studies
of s e l e n i u m are, however, m u c h fewer. Bodewig a n d
P l a m b e c k (4) have investigated its electrochemistry in
fused LiC1-KC1 melts and observed that it undergoes
oxidation and reduction to Se2C12 and Se 2-, respectively, while Shimotake and Cairns (5) have investigated a L i / S e b a t t e r y using a t e r n a r y eutectic of LiF,
LiC1, and LiI as the electrolyte. Recently, while screening cathode materials for use i n a n A1C18:NaC1 b a t tery, Marassi et al. (2) studied the electrochemistry
of Se. They r a n cyclic v o l t a m m o g r a m s on a saturated
solution of Se i n a 68-37 mole percent ( m / o ) A1C13:
NaC1 melt at a p l a t i n u m electrode and observed a
quasireversible oxidation couple close to the anodic
limit of the melt and a reduction process at about 1V
ws. an A1 wire i n the melt. They also observed a second, irreversible, reduction wave that appeared only
on p l a t i n u m electrodes. No coulometric studies were
u n d e r t a k e n i n this study; therefore, it is u n k n o w n
what the reaction products were.
This latter paper appears to be the only published
electrochemical study of Se i n tetrachloroaluminates.
There have, however, been a few studies of stabilization of low oxidation states of Se in these melts. It has
been established that the large anions, A1C14- and
A12C17-, present in acid tetrachloroaluminates are ca* Electrochemical Society Active Member.
Key words: fused salts, voltammetry, electrolysis, kinetics.
pable of stabilizing low oxidation states of metals such
as Cd22+, Sn22+, P b +, and cluster cations of Te (6, 7),
and recently B j e r r u m et al. (8) identified a variety of
low oxidation state species of Se such as Se42+, Ses 2+,
Sel~2+, and Sel62+. These were observed spectrophotometrically i n m i x t u r e s of Se and SeC14 i n acid tetrachloroaluminate melts and the n a t u r e s of the species
were found to be dependent upon the relative a m o u n t s
of Se and Se(IV) present. Corbett et al. (9) have also
studied similar systems, Se-(SeC14 § A1C13) mixtures,
and have identified the species Se42+ and Ses 9'+ as being present.
Selenide anions are k n o w n to exhibit interesting
chemistry in A1C18:NaC1 melts. Metal oxides and chalcogenides have been observed to react with A1Cla in
closed tubes at temperatures a r o u n d 800 ~ to yield the
species A1OC1 (10), A1SC1 (10), A1SeC1 (11), and
A1TeC1 (12) as the products of reactions such as
A1C13 -t- ZnSe ~ A1SeC1 -t- ZnCle
[1]
In a recent study in this l a b o r a t o r y (13), it was shown
that similar reactions occur i n acidic A1CI~:NaC1 melts
where the oxide and chalcogenide ions behave as tribases reacting with the melt to yield the same species.
In basic melts these anions were shown to be dibases
and the product of reaction with the melt was then a
species such as A1SeC12-. The relative strengths of the
tribases were found to be
Te 2-, Se 2- ~ S 2- ~ 0 2The papers discussed above are the only published
studies of s e l e n i u m and its compounds in tetrachloroa l u m i n a t e melts. This paper is concerned with the
electrochemistry, principally at 175~ of these species
with p a r t i c u l a r attention being given to its acidity de-
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Vol. 125, No. 9
SELENIUM
pendence. I n order to e x a m i n e this the acid-base properties of the solvent must be f u l l y understood. The
three principal equilibria l i n k i n g the m a j o r species
present are (14)
2A1CI~(1) ~,~--A12C16(1) Ko = 2.86 X 107
A1C14- 4- A1C13 ~---A12C172A1C14- ~ A 1 2 C 1 7 - + C1-
[2]
K2 = 2.4 X 104
[3]
K m = 1.06 • 10 -7
[4]
where Ko, /<2, Km are the mole fraction e q u i l i b r i u m
constants at 175~C. The d o m i n a n t acid-base equilibr i u m is that described by Eq. [4] with C1- as the Lewis
base a n d A12C17- the Lewis acid. The pC1- of the
n e u t r a l melt, 50 m / o A1Cla, is 3.5 on the mole fraction
scale; however, a melt of a n y desired acidity can be
made (15) by anodizing a n A1 wire into the melt u n t i l
the desired value, as indicated b y an a l u m i n u m wire
electrode, is reached.
Experimental
Melt preparation and purification.--The p r e p a r a t i o n
of pure N a C l - s a t u r a t e d NaA1C14 melts by the preelectrolysis of a m o l t e n m i x t u r e of A1C13 (A. G. iron-free
F l u k a ) a n d a n excess of NaCI(A. C. S. Fisher) between two a l u m i n u m electrodes has been described
elsewhere (15). After purification, a sample of the
melt was filtered t h r o u g h a m e d i u m porosity frit to
remove a n y excess NaC1 or suspended a l u m i n u m p a r ticles and was t h e n t r a n s f e r r e d to the small P y r e x
glass e x p e r i m e n t a l cell.
The melt p r e p a r a t i o n and all subsequent electrochemical e x p e r i m e n t s were performed i n a dry box
( V a c u u m Atmospheres Company) u n d e r a n argon
atmosphere. This dry box was fitted with a purification
system (Vacuum Atmospheres d r i - t r a i n HE 193-2) that
recycled the atmosphere through activated copper and
molecular sieves to remove oxygen and water, respectively. The level of oxygen in the atmosphere was deterrnined by b u r n i n g a 25W t u n g s t e n lamp filament
(General Electric Company) exposed to the atmosphere (16). A b u l b lifetime of several days, indicating
a n atmosphere containing a r o u n d 1 p p m of oxygen,
was considered satisfactory.
Experimental cells and electrodes.--The cells were
made of P y r e x glass, with a volume of a p p r o x i m a t e l y
75 cm 3, and had a fitted Teflon cap. This cap was
drilled to facilitate the m o u n t i n g of the secondary- and
reference-electrode compartments, which consisted of
fine porosity frits m o u n t e d on P y r e x glass tubes,
the thermocouple well, a n d t'he w o r k i n g electrode.
The working electrode c o m p a r t m e n t coulct readily be
stirred using a glass-coated magnetic stirrer bar. The
assembly was m o u n t e d in a tube furnace and the t e m p e r a t u r e was m a i n t a i n e d w i t h i n _ 1~C by a controller
(Thermo-Electric Model 32422) in conjunction with a
C h r o m e l - A l u m e l thermocouple.
For all experiments, the reference electrode was a
coiled a l u m i n u m wire (Alfa Inorganics m5N) immersed in a n N a C l - s a t u r a t e d melt at 175~ T h r o u g h out this paper, all potentials are referred to this electrode. The secondary electrode was also a coiled a l u m i n u m wire.
The electrodes i n the v o l t a m m e t r i c studies were
either t u n g s t e n (% in. diam Alfa Inorganics m3N8) or
vitreous carbon (3 m m diam Atomergic Chemetals V25
or 2,.5 m m d i a m Tokai GC 30), m o u n t e d i n l~yrex glass.
These were ground flat on a n e m e r y wheel and then
polished to a m i r r o r - l i k e finish using a l u m i n a (Type
B Fisher). R o t a t i n g - d i s k studies were carried out on
a vitreous carbon disk electrode (5 m m diam Tokai
GC 30) m o u n t e d and prepared in the same w a y as the
above electrodes. This electrode was rotated b y a
servocontrolled m o t o r - t a c h o m e t e r (Pine I n s t r u m e n t
C o m p a n y Type ASR 2), the rotation rate of which
could be controlled b e t w e e n 50 and 10,000 r p m with an
accuracy of greater t h a n 1%.
1455
Exhaustive controlled potential coulometry was carried out in either a t u n g s t e n crucible (Research Org a n i c / I n o r g a n i c Chemical Corporation Wl20) or a vitreous carbon crucible (Atomergic Chemetals C o m p a n y
V25-16) used both as the cell and w o r k i n g electrode.
Adequate mass transport was achieved by stirring with
a glass-coated magnetic stirring bar and the total
charge passed i n these e x p e r i m e n t s was measured b y a
coulometer (Acromag Incorporated).
Electrochemical instrumentation.--All cyclic v o l t a m m e t r y and r o t a t i n g - d i s k studies were made with a
m u l t i p u r p o s e i n s t r u m e n t (17) constructed in this laboratory and the data were recorded on a n X - Y r e corder (Hewlett Packard Model 7046A). Pulse v o l t a m metric studies were performed on a pulse polarograph
(PARC Model 174), modified (18) to permit p a r a metric studies. The melt acidity was adjusted b y the
anodization of a n a l u m i n u m wire (Alfa Inorganics
m5N) into the melt using a constant c u r r e n t source
(Sargent Model IV coulometric c u r r e n t source) as has
been discussed previously (13).
Chemicals.--NafSe
(Cerac Pure, Incorporated),
NafSeO4, NafSeO3, SeCL~, and Se2C12 (Alfa Inorganics)
were all used w i t h o u t f u r t h e r purification. The most
readily soluble type of selenium was found to be the
red amorphous form, p r e s u m a b l y due to its structure,
long chains, as compared to the ring structure of the
vitreous form. This was prepared by reducing a solution of Na2SeO8 in 9M HC1 with SO~ (Matheson Gas
Products) (25). The red precipitate was then filtered
and washed, dried with diethyl ether, and finally dried
u n d e r a Vacuum.
Results and Discussion
A cyclic v o l t a m m o g r a m of 1.18 X 10-3M selenium
in a n NaC1 saturated melt at 175~ is shown in Fig. 1.
It can be seen that there is a reduction peak at 0.880V
on the cathodic going sweep from the electrode rest
potential. On the r e t u r n sweep there is a very slight
shoulder at about 0.950V followed by a peak at 1.140V;
there are then two oxidation peaks close to the anodic
limit of the melt, at 1.88 a n d 2.02V. Finally, o n the r e t u r n sweep there is a peak at 1.80V. Successive sweeps
are identical to the first. Figure 2 is a cyclic v o l t a m m o g r a m of a solution of sodium selenide, also i n a n
NaC1 saturated melt, showing the same features as Fig.
1. As the acidity is increased, Fig. 2b and 2c, the behavior changes significantly. The peaks previously seen
at 0.880 and 1.140V in an N a C l - s a t u r a t e d melt move
together and also in a n anodic direction while the features close to the anodic limit of the melt change considerably. The cyclic v o l t a m m o g r a m i n the most acid
~2
2.0
1.5
1.0
0.5
0.0
E/VvsAt
Fig. 1. Cyclic voltammogram of Se in NaCI-saturated melt (pCI
1.9) at o glassy carbon electrode. Temperature ~ 175~
sweep rate ~ 100 mV sec-1; concentration of Se (as monomer)
1.18 mM.
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1456
J. Electrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y
1.0mA.cm2
S e p t e m b e r 1978
1.48
a
't
1.48
cI
1,48
~
~
(c)
b
/
/
b e/
1A
I
/9
9..~/
j.J
j.J
j9
9
1.4~
~ 1.44
1.40
9
:>
~o
9
9
.~J
o ~
1AI
1.40
~-2
b
25
2.0
15
1.0
E/VvsAt
0.5
O.0
Fig. 2. Cyclic voltammogram of 3.6 mM Na2Se at a glassy carbon electrode. Temperature ---- 175~
sweep rate ---- 200 mV
sec-L Melt pCI. (a) 1.9; (b) 2.9; and (c) 5.2.
melt, Fig. 2c, closely resembles that observed by Marassi et al. (2) for selenium on a p l a t i n u m electrode in
an acid melt, except that the second selenium r e d u c tion peak seen on p l a t i n u m is not seen here, eithei: for
a vitreous carbon or tungsten electrode. The v o l t a m m o g r a m s shown here w e r e all made w i t h a vitreous
carbon electrode. The b e h a v i o r of a tungsten electrode
was, however, v e r y similar except that selenium, as
e v i d en ced by the shape of the cyclic v o l t a m m e t r i c
peak, appeared to adsorb strongly. F o r this reason all
subsequent w o r k was carried out at a vitreous carbon
electrode.
To facilitate this discussion the electrochemistry of
selenium is considered in two parts. The first deals
with the selenium reduction, observed at 0.880V in an
N a C l - s a t u r a t e d melt, and the subsequent oxidation of
the product, which, as is shown later, is the selenide
ion. The second part considers the oxidation of selen i u m to selenium cations and t h e ir reduction back to
selenium.
The selenium-selenide couple.--Exhaustive constant
potential c o u l o m e t r y was p e r f o r m e d on a ~olution of
selenium in both basic and acidic melts at a potential
corresponding to the diffusion plateau of the reduction
peak. It was found that for basic melts (pC1 _-- 1.9) 2.03
_ 0.05 e l e c t r o n s / s e l e n i u m atom w e r e passed, while the
corresponding v a lu e in an acidic melt (pC1 ---- 6.0) was
1.99 _ 0.05. A cyclic v o l t a m m o g r a m of the resulting
solution was identical to that of a solution of sodium
selenide. In the same way, sodium selenide was oxidized by a t w o - e l e c t r o n step to give a solution b e h a v ing like a solution of selenium. It can th e r e fo r e be
concluded that selenium is reduced by a t w o - e l e c t r o n
step to selenide w h ic h itself can be oxidized back to
selenium, again by a single t w o - e l e c t r o n transfer.
To obtain m o r e i n f o r m a t i o n about the n a t u r e of the
selenium and selenide species in the melt, the dependence o2 the e q u i l i b r i u m potential of a vitreous carbon
indicator electrode on the concentrations of selenium
and selenide ion present was investigated. A solution
of k n o w n concentration of sodium selenide was p r e pared in a vitreous carbon crucible. A small a m o u n t of
the selenide ion was then oxidized to selenium using
the crucible as a w o r k i n g electrode and the rest potential of the indicator electrode was measured. The concentrations of selenium and selenide ion were r ead i l y
d e t e r m i n e d from the n u m b e r of coulombs passed. This
pr o ced u re was repeated m a n y times until all the selenide had been oxidized to selenium. The potential of
the crucible was then changed and the selenium was
reduced, in a stepwise fashion, back to selenide w i t h
e q u i l i b r i u m potential m e a s u r e m e n t s again being made.
Figure 3 shows th r e e attempts at fitting this data to
possible r ed o x reactions. C u r v e (a) corresponds to the
-1
1
~0
0
2
22
1
3
24
2
4
26
5
28
30
Fig. 3. Nernst plots for various Se/Se 2 - redox couples at a pCI
of 5.7 and at 175~ using a glassy carbon electrode. The units
of the abscissa are (a) Ioglo[Se]/[Se2-]; (b) Iogzo[Se2]/
[Se~-]2; and (c)Ioglo[Ses]/[Se2-] s.
plot for the e q u i l i b r i u m
Se + 2 e - ~ Se 2-
[5]
Se2 + 4 e - ~ 2Se 2-
[6]
Se8 + 16e- ~ 8Se 2-
[7]
c u r v e (b) to
and cu r v e (c) to
It can i m m e d i a t e l y be seen that w h i l e
(c) are curved, plot (b) is a straight
thermore, its slope is 22 m V w h i ch is
w i t h the v al u e predicted, 22.3 mV at
Nernst equation for this process
E -- E ~ + 2.303
RT
3F
log
plots (a) and
line and f u r in a g r e e m e n t
175~
by the
[Sef]
[Sef-] 2
[8]
This implies that the potential d e t e r m i n i n g selenium
species is the d i m er Se2, as was o b s e r v e d previously
for the sulfur-sulfide couple (1). The above data w e r e
obtained in an acidic m e l t (pC1 ---- 5.7); however,
similar results, indicating the Se2 d i m er to be the
potential d e t e r m i n i n g species, w e r e obtained for all
the melt acidities investigated (pC1 ---- 1.9-6.0). This
is p er h ap s surprising in v i e w of the fact that Se2 is
not n o r m a l l y stable except as a v ap o r at t e m p e r a t u r e s
g r eat er than 900~
The t h e r m o d y n a m i c a l l y stable
f o r m of selenium at 175~ is the g r a y form, which
consists of long chains (15) b e t w e e n 1'03-104 units
long. This is not, however, k n o w n to be soluble in
any solvents. At room t e m p e r a t u r e t h er e are two
unstable allotropes, both of w h i c h consist of p u c k e r e d
Ses rings. These forms, which are soluble in several
solvents such as CSf, rapidly convert to the gray form
on heating. Since the t e m p e r a t u r e of the melt is
above this transition temperature, any Se added
will r a p i d l y convert to the gray form; the m e l t must
then have the effect of splitting off Se2 groups which
it stabilizes. This p r o b a b l y explains the v e r y slow
solubilization of Se observed in this present work.
These potentiometric e x p e r i m e n t s w e r e carried out
at a range of m e l t acidities and it was found that the
standard potential for the couple showed a v e r y strong
dependence on the pC1. This observation has been
discussed in an ear l i er p ap er (13), and, as outlined
in the introduction to this study, is due to the basic
properties of the selenide ion. In melts m o r e acidic
than a pC1 of 3.5, on the mole fraction scale, the
selenide ion behaves as a tribase and reacts with the
acid species in the melt, A12C17-, to form the species
A1SeC1, or, m o r e probably, the solvated species
A12SeCI~-. Similarly, in basic melts the selenide ion
behaves as a dibase and is present either as A1SeCI2or A12SeC162-. The r ed o x reaction should t h er ef o r e be
w r i t t e n as
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VoL
125, N o . 9
1457
SELENIUM
Se2 + 4 e - + 2A1C14- ~ 2A1SeC1 + 6C1-
[9]
i n acid melts, a n d
Se2 + 4 e - + 2A1C14- ~_- 2A1SeC12- + 4C1-
[10]
i n basic melts.
It has been shown (1) that the basic properties of
the sulfide ion result i n very high solubilities of
metal sulfides, for example CuS, i n acidic melts.
Very similar behavior was observed for selenides; in
a 60 m / o AICI~:NaCI melt the solubility of CuSe was
found to be 0.32M. Oxides and tellurides were also
found to be solubilized to the same degree, which
suggests that acidic t e t r a c h l o r o a l u m i n a t e melts m a y
have some application in the processing of metal ores.
The oxidation of selenide ions, or more precisely
the species AISeC1 and A1SeC12-, to selenium was
investigated initially by pulse v o l t a m m e t r y . Figure 4
shows such a pulse v o l t a m m o g r a m i n a n N a C l - s a t u rated melt. The first wave is due to the oxidation
of selenide to s e l e n i u m and there is also a slight
indication of waves due to the oxidation of selenium,
as observed i n cyclic v o l t a m m e t r y (Fig. 1 and 2a).
These latter waves, however, are v e r y small. The first
wave, w h e n analyzed b y the method of Oldham a n d
P a r r y (20), yielded a l i n e a r plot from which an~
was calculated to be 0.59. The limiting c u r r e n t for
the wave also showed a linear dependence on selenide
ion concentration, and the diffusion coefficient of the
diffusing species, p r o b a b l y the solvated form of
A1SeC12-, was calculated to be 2.3 __ 0.2 X 1O-6 cm 2
sec -1. While the position of the wave was strongly
dependent u p o n the pC1 of the melt, the values of
ana a n d D w e r e , despite the change i n the n a t u r e
of the selenide species, i n d e p e n d e n t of the acidity.
Pulse v o l t a m m e t r y proved to be a fairly successful
technique for investigating the kinetics of the oxidation process; however, the method of choice is the
use of the r o t a t i n g - d i s k electrode (RDE). Very large
currents are obtained for low concentrations of electroactive m a t e r i a l and, since the m e a s u r e m e n t s are
made at a steady state, there is no charging c u r r e n t
adding to the background.
Figure 5 shows some results of a series of RDE
experiments carried out on a solution of sodium selenide. These plots were obtained by sweeping the
electrode potential at a rate of 2 mV sec - t at various
fixed rotation rates. A plot of the diffusion current,
measured at a steady state, as a function of the
square root of the rotation rate, ~'/~, is a straight line
at all melt acidities, indicating that the oxidation of
selenide is diffusion controlled. The diffusion c u r r e n t
also varies l i n e a r l y with the concentration of sodium
selenide. F r o m the i vs. ~'/2 plots, the diffusion co8
7
1 . 0
mA.cm-2
I
I
2.0
I
I
1.6
E/VvsAt
I
1.2
Fig. 5. RDE experiment for 2.06 mM Na2Se at a glassy carbon
electrode. Melt pCI ..~ 4.5; rotation rates are (a) 400; (b) 900;
(c) 1600; (d) 2500; (e) 3600; (f) 4900; (g) 6400; and (h) 8100
rpm; temperature -- 175~
efficient of the selenide species is readily calculated
(21) and values at a range of melt acidities are
shown i n Table I. These are i n close agreement with
the values obtained earlier from the pulse v o l t a m m e t r y
and are i n d e p e n d e n t of the melt acidity.
The rising portions of the plots i n Fig. 5 are fairly
d r a w n out; this behavior is typical of a process for
which the rate is being controlled by a slow electron
transfer, and the rate constant for this step can be
d e t e r m i n e d (21). Figure 6 shows a kinetic plot
( 1 / i vs. I/~ I/2) for a sodium selenide solution u n d e r
the same conditions as Fig. 5, the steady-state currents at various potentials within the rising portion
of the waves being d e t e r m i n e d for a range of rotation rates. The plots are a series of parallel lines as
would be expected for a slow e l e c t r o n - t r a n s f e r step.
If these lines are extrapolated to Where ~-I/2 equals
zero, that is to an infinite rotation rate, the intercept
on the y-axis then corresponds to the pure kinetically
controlled c u r r e n t since the rate of mass transfer is
infinite. F r o m these intercepts the forward rate constant can be calculated, and Fig. 7 shows a plot of
logl0-k vs. potential. This plot is a straight line and
the value of ana can be calculated from the slope.
K n o w i n g the value of the s t a n d a r d potential, Eo, from
potentiometry, the standard rate constant, koe, c a n
be determined. The values of ko~ and an~ obtained
6
E 4
Table I. Kinetic parameters and diffusion coefficients, obtained
from RDE experiments, for oxidation of Na2Se at 175~
3
2
pC1a
koo x 105b
~n~
1.9
2.7
3.5
4.5
6.2
4.6
6.3
8.9
7.1
17.8
0.53
0.53
0.55
0.58
0.55
D
x 10er
1
0
2.0
1.5
E / VvsAt
1.0
f15
Fig. 4. Normal pulse of 5.2 mM Na2Se at a glassy carbon electrode. Melt pCI ~_1.9; pulse width 48 msec; sweep rate 5 mV
sec-1; temperature ~ 175~
-
1.99
1.95
2.03
2.01
2.04
Mole fraction scale.
b crfl sec -I.
e CEB2 sec-l.
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1458
2.0
1.5
r
J. Electrochem. Soc.: E L E C T R O C H E M I C A L
j jxjXJx '
fxJ
E
u
1.0
r
'0
x
x
0.5
1.0
{5
21o
2's
~1/2x10 2 /(rpm~ 1/2
Fig. 6. Kinetic plot for 2.06 mM Na2Se at 175~ in a melt with
4.5. Potentials (a) 1.5V; (b) 1.55V; (c) 1.6V; (d) 1.65V;
(e) 1.7V.
pCI ~
~x~
T-~o-2
x
x~
o
•
116
1.~5
E / VvsA[
S e p t e m b e r 1978
lion could be obtained as selenium was found to
adsorb slightly at the vitreous carbon electrode. This
adsorption, which is greatest in basic solutions, can
also be seen on cyclic v o l t a m m o g r a m s (Fig. 1) w h e r e
the reduction p e a k for selenium is h i g h e r and s h a r p e r
t h a n the corresponding o x i d a t i o n peak.
I n t e r e s t i n g q u a l i t a t i v e i n f o r m a t i o n was, however,
obtained f r o m the t e m p e r a t u r e dependence of the
cyclic v o l t a m m e t r y of selenium solutions. F i g u r e s 8a,
8b, and 8c show cyclic v o l t a m m o g r a m s at 175 ~ 225 ~
and 275~ respectively, of a selenium solution with
all the potentials a d j u s t e d to the same reference
electrode; an A1 w i r e in an N a C l - s a t u r a t e d m e l t at
175~ The m e l t used was of fixed composition, that
of an N a C l - s a t u r a t e d m e l t at 175~
The increase
in t e m p e r a t u r e , therefore, caused a reduction in the
m e l t a c i d i t y due to the increased dissociation of
A1C14-. These pC1 changes were, however, m u c h too
small to account for the gross changes observed on
the cyclic v o l t a m m o g r a m s . The selenium reduction
potential changes from a p p r o x i m a t e l y 0.840V at 175~
to 0.985V at 225~ and to 1.090V at 275~ The corresponding selenide o x i d a t i o n potentials a r e 1.16, 1.13,
and 1.15V. Thus, it can be seen that, w i t h i n e x p e r i m e n t a l error, the position of the selenide oxidation
p e a k h a r d l y changes w i t h t e m p e r a t u r e , w h i l e the
selenium reduction p o t e n t i a l moves a n o d i c a l l y 250
mV for a t e m p e r a t u r e increase of 100~
the p e a k
s e p a r a t i o n decreasing f r o m 320 to 60 mV, which is
close to the value for a t w o - e l e c t r o n r e v e r s i b l e process
at 7 5 ~
(54 m V ) . The most p r o b a b l e e x p l a n a t i o n
for this b e h a v i o r is t h a t there is a chemical step
involved in the reduction, the r a t e of which is g r e a t l y
affected b y t e m p e r a t u r e . One such possible chemical
step could be the m o n o m e r i z a t i o n of the selenium
dimer. The a p p a r e n t increase in r e v e r s i b i l i t y as the
pC1 is increased, Fig. 2a, 2b, and 2c, m a y also be
e x p l a i n e d b y the increased r a t e of the same chemical
step.
A t this point, it should be m e n t i o n e d t h a t the n a t u r e
of the slight s h o u l d e r seen at about 0.950V on an
anodic going sweep of a s e l e n i u m solution, Fig. 1
and 8a, is uncertain, t h o u g h it m a y be associated
with the selenium a d s o r p t i o n discussed earlier. It
only a p p e a r s on cyclic v o l t a m m o g r a m s in the most
basic melts and d i s a p p e a r s as the t e m p e r a t u r e is increased, Fig. 8b and 8c.
The electrochemical oxidation of selenium and the
behavior o] selenium cations.--From Fig. 1 and 2 it
can be seen t h a t the oxidation of selenium and the
r e d u c t i o n of the p r o d u c t s of this o x i d a t i o n a r e f a i r l y
J
117
SCIENCE AND TECHNOLOGY
14
113
Fig. 7. Plot of the potential dependence of log "~ for the oxidation of Na2Se at 175 ~ in a melt with pCI = 4.5.
at various pCl's are given in Table I. The values of
aria a r e i n d e p e n d e n t of pC1 and a r e in good a g r e e m e n t
w i t h those calculated from pulse v o l t a m m e t r y . The
values of koe show a slight i n c r e a s e w i t h m e l t acidity.
Tt was observed e a r l i e r on the cyclic v o l t a m m o g r a m s
of sodium selenide (Fig. 2) t h a t the peaks corresponding to the reduction of selenium and the s u b sequent r e o x i d a t i o n of selenide m o v e d closer with
increasing pC1; t h a t is, the reaction a p p e a r e d to b e come m o r e reversible. The RDE experiments, h o w ever, show t h a t changing the pC1 has only a slight
effect on the rate of o x i d a t i o n of selenide, certainly
not enough to account for the a p p a r e n t increase in
r e v e r s i b i l i t y . The o b s e r v e d decrease in p e a k s e p a r a tion must, therefore, be due to the increased acidity
affecting the reduction of selenium. Both pulse v o l t a m m e t r y and RDE e x p e r i m e n t s w e r e a t t e m p t e d as a
m e a n s of investigating this, but no kinetic i n f o r m a -
Ilom
15
10
E/V vsA[
05
Fig. 8. Cyclic voltammogram of 1.18 mM Se (as monomer) in a
basic melt (NaCI saturated at 175~
at a glassy carbon electrode. Sweep rate ~- 200 mV sac -1. Temperature (a) 175~
(b) 225~ (c) 275~
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"Col. 125, No. 9
complex i n these melts. Considering first the behavior
i n the most basic solutions, there are two rather
poorly defined oxidation peaks for s e l e n i u m at 1.88
and 2.02V with a corresponding reduction wave at
1.79V. If the potential scan is reversed after the first
oxidation peak, there is still a single reduction wave
at 1.79V. I n order to determine the o v e r - a l l stoichiometry, exhaustive coulometry was carried out on a sel e n i u m solution i n a vitreous carbon crucible. If the
coulometry was performed at about 1.9V in an NaC1saturated melt (pC1 --_ 1.9) a total of 2.03 _ 0.05
e l e c t r o n s / s e l e n i u m atom were passed to yield an
S e ( I I ) species. Continued coulometry at about 2.05V
gave a f u r t h e r two electrons, after correction for
the b a c k g r o u n d c u r r e n t which was r a t h e r large due
to the p r o x i m i t y of the anodic limit of the melt. The
final oxidation product, therefore, appears to be an
Se(IV) species. These coulometry experiments could
be carried out fairly r a p i d l y (about 15 m i n each),
and therefore no problem of product volatility was
encountered. W h e n potentiometric m e a s u r e m e n t s were
made on these couples, it was found that both the
Se(IV) and S e ( I I ) species were slowly lost from
the melt, the latter being lost more rapidly. It was,
therefore, not possible to make a n y Nernst plots.
As it was found the Se (IV) was the highest oxidation
state of selenium as formed SeC14 was added to the
melt. A cyclic v o l t a m m o g r a m of a solution of this
showed all the same features as one of selenium or
sodium selenide, and coulometry at about 1.7V showed
that 4.1 +_ 0.05 electrons were passed for each molecule
of SeC14 added. Sodium selenite behaved in exactly
the same way as SeC14, showing the high affinity of
the melt for oxide ions (13). Sodium selenate, when
added to the melt, first evolved chlorine and then
behaved as SeC14 while Se2C12 appeared to decompose
when added to the melt.
As the pCl is increased, Fig. 2b, the situation becomes more complex. The oxidation peaks formerly
at 1.88 and 2.02V and the reduction .peak at 1.79V
are all still present b u t have shifted in the anodic
direction and there is now a n e w oxidation wave at
2.19V with its corresponding reduction wave at 2.14V.
As the acidity is increased further, Fig. 2c, the picture
simplifies, there being a single oxidation peak and
corresponding reduction peak. Coulometry at such
an acidity shows that both the oxidation and reduction are four electron steps. It is therefore clear that,
as i n basic melts, the highest oxidation state formed
is S e ( I V ) , b u t the oxidation of s e l e n i u m to Se(IV)
now goes by a different route.
The cyclic v o l t a m m e t r y of a n SeCI~ solution of
i n t e r m e d i a t e pC1, such as the pC1 of Fig. 2b (pC1
_-- 2.9) shows two reduction waves. I-lowever, if
coulometry is carried out on the first wave at about
2.1V four electrons/SeCl~ molecule are passed with
the formation of a solution of selenium. It therefore
appears that there are two Se (IV) species in the melt
linked by a p C l - d e p e n d e n t equilibrium. This is supported by the fact that the more anodic wave grows
at the expense of the other as the melt is made more
acidic. The e q u i l i b r i u m constant for this e q u i l i b r i u m
is calculated later from RDE results. The oxidation
of selenium in v e r y basic melts can therefore be
s u m m a r i z e d as the oxidation to S e ( I I ) and then to
a n Se(IV) species followed by the subsequent fourelectron reduction back to selenium. This changes
i n acid melts to a single quasireversible four-electron
oxidation to a different Se(IV) species while at
i n t e r m e d i a t e acidities both mechanisms are seen to
operate.
Attempts to investigate more closely the oxidation
of selenium i n basic melts by either pulse v o l t a m m e t r y
or RDE's proved fruitless. This was due i n part to
the p r o x i m i t y of the waves to the anodic limit of
the melt b u t p r i n c i p a l l y to the fact that the currents
observed for these processes were very much less
SELENIUM
1459
t h a n would be predicted for a diffusion-controlled
process. It can be concluded from this that either a
very slow e l e c t r o n - t r a n s f e r step is occurring or more
p r o b a b l y that there are chemical steps involved in
these processes. F r o m the cyclic v o l t a m m e t r y it can
be seen that the peaks for the S e - t o - S e (II) and Se ( I I ) t o - S e ( I V ) oxidations both move in a n anodic direction as the acidity increases, indicating that chloride
ions are involved in both these steps. It is a reasonable assumption, therefore, that both the S e ( I I ) and
Se(IV) species are chloro-complexed. It was observed
that the cyclic v o l t a m m e t r y peak potentials for the
reduction of solutions of Se (IV) and Se (II) (prepared
by potentiostatic oxidation of a Se solution) were
identical (1.79V at a pC1 of 1.9). This suggests that
the Se(II) species undergoes a chemical step prior
to reduction, either a disproportionation into Se(IV)
and Se or, a l t e r n a t i v e l y it is a polymeric species that
undergoes decomposition to Se(IV) and Se. Such a
chemical step is supported b y the observation that
the acidification of S e ( I I ) solution with A1C13 results
i n the formation of Se and Se(I.V) b y either a disproportionation or decomposition mechanism. Similar
instability of the Se(II) ion to disproportionation is
observed in fluoroacetic acid solutions (22).
I n contrast to the behavior of basic melts, the
oxidation of selenium in acid melts is readily studied,
and Fig. 9 shows a n o r m a l pulse v o l t a m m o g r a m at a
pC1 of 5.23 at three pulse widths. Plots of the diffusion c u r r e n t vs. 1/t 1/2 and concentration are both
linear, indicating the process to be diffusion controlled,
and the diffusion coefficient of selenium was calculated
to be 3.0 _.+ 0.3 • 10 -6 cm 2 sec -1.
The reduction of Se(IV) was studied using a n
RDE, and Fig. 10 shows a series of slow voltage scans,
at-fixed rotation rate, for three pC1 values. In the
most basic melt there is a very small wave followed
b y a m u c h larger one, b u t as the acidity is increased
the first wave grows at the expense of the other u n t i l
i n the most acid melts there is only one wave.
This type of behavior is identical to that seen in
the cyclic v o l t a m m e t r y and is a t t r i b u t e d to there
being two Se(IV) species i n a n a c i d - d e p e n d e n t equilibrium. At all pCl's the relative heights of the two
waves are i n d e p e n d e n t of the rotation rate and of
the Se(IV) concentration, which indicates that the
interconversion of the two species is slow and that
there is no difference in the degree of polymerization
between them. The diffusion c u r r e n t for the wave
in the most acid melt varies l i n e a r l y with ~1/2 a n d
Se(IV) concentration, showing the reduction to be
diffusion-controlled and the diffusion coefficient of
the S e ( I V ) species was calculated to be 6.9 • 10 -6
O
,Ns 4
q
<
E 3
b
C
i
2.4
2.2
2:0
i;8
E / V vs A!
Fig. 9. Normal pulse voltammograms for the oxidation of !.78
mM Se (as monamer) at a glassy carbon electrode in an acid melt
(pCI ~ 5.25); temperature ~ 175~ Pulse widths (a) 20 msec;
(b) 50 msec; (e) 100 reset.
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1460
J. Electrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y
c
S e p t e m b e r 1978
oxidation of s e l e n i u m in acidic melts where they have
been observed spectrophotometrically (7). This m a y
be due to the m u c h lower concentrations used i n the
present work; i n this present study the solubility of
s e l e n i u m (as the m o n o m e r ) was found to be about
3 • 10-3M, whereas in B j e r r u m ' s study (8) the
combined Se and SeC14 concentration was 10-100
times greater t h a n this. It is, however, possible that
the species of formal oxidation state 4-2 formed d u r ing oxidation of Se in basic melts is polymeric, though
electrochemical studies of the reduction of T e ( I V )
i n similar melts (24) suggest that T e C l s - a n d TeC12
are the p r e d o m i n a n t T e ( I I ) species present in this
case and S e ( I I ) might be expected to behave i n a
similar m a n n e r .
b
Acknowledgment
This work was supported b y the Air Force Office
of Scientific Research u n d e r G r a n t No. AFOSR-76-2978.
2.5
.
1.5
E/V
1.0
vsAt
Manuscript submitted Feb. 6, 1978; revised m a n u script received April 7, 1978.
Fig. 10. RDE experiments for the reduction of Se (IV) at a
glassy carbon electrode. Rotation rate = 4900 rpm; temperature
---- 175~ pCI = (a) 1.9; (b) 3.5; (c) 5.8.
A n y discussion of this paper will appear in a Discussion Section to be published in the J u n e 1979 JOURNAL.
All discussions for the J u n e 1979 Discussion Section
should be submitted b y Feb. 1, 1979.
cm 2 sec -1. The reduction wave was analyzed i n the
same w a y as the oxidation of Na2Se and ~1, was
calculated to be 0.70. The s t a n d a r d potential for the
process is not known, so koe could not be calculated.
The rate constant at the rest potential, 2.32V, was,
however, d e t e r m i n e d and was found to be 4.8 •
10 -4 cm sec -1 at a pC1 of 5.8. The reduction process
in a basic melt was analyzed in the same way. The
process was found to be diffusion controlled and by
assuming that the value of D calculated for the acidfavored Se(IV) species did not v a r y with pC1. The
diffusion coefficient of the species favored i n basic
melts was found to be 1.94 • 10 -6 cm 2 sec -1, avle
was calculated to be 0.36, and the rate constant at
the rest potential (2.04V) in an N a C l - s a t u r a t e d melt
(pC1 ___=1.9) was d e t e r m i n e d to be 3.8 • 10 -4 cm sec -1.
K n o w i n g the diffusion coefficients for both Se(IV)
species and the heights of the two waves at various
pCl's it was possible to fit the data to various equilibria and the one that appears to operate is shown
below
Publication costs of this article were assisted by
Colorado State University.
S e ( I V ) (C1)z C4-z)+ + C1- ~ Se(IV) (C1)z+lr 3-~)+
[11]
with an e q u i l i b r i u m constant of 6.0 ___ 1.0 • 108 on
the mole-fraction scale. F r o m other studies in chloriderich media (23) it was found Se(IV) is u s u a l l y present as the hexachloro complex. The probable equil i b r i u m in the present case is therefore
SeC15- + C1- ~__ SeC162-
[12]
though comparable equilibria b e t w e e n SeC15- and
SeC14 or SeC15 and SeC13 + cannot be ruled out.
The stoichiometry of the electrochemical oxidation
of selenium and reduction of s e l e n i u m cations in
A1C13:NaC1 melts can be summarized as follows
(a) I n acid melts
Se~ + 10C1- ~ 2SeC15- + 8 e -
[13]
(b) I n basic melts
nSe2 + 2 y C I - --> 2SenCly(~-2~)- + 4neSe,Cly C~-en)- + (6n -- y) C1- --> nSeC162- 4- 2ne25eC162- + Be- --> Se2 + 12C1-
[14]
El5]
[16]
where the values of n and y are either zero or integral.
It is interesting that no evidence is found for the
formation of polymeric selenium cations d u r i n g the
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