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Electrochemical Deposition of Aluminum from NaCl-AlCl3 Melts
Li, Qingfeng; Hjuler, H. A.; Berg, Rolf W.; Bjerrum, Niels J.
Published in:
Electrochemical Society. Journal
DOI:
10.1149/1.2086512
Publication date:
1990
Document Version
Final published version
Link to publication
Citation (APA):
Li, Q., Hjuler, H. A., Berg, R. W., & Bjerrum, N. (1990). Electrochemical Deposition of Aluminum from NaCl-AlCl3
Melts. Electrochemical Society. Journal, 137(2), 593-598. DOI: 10.1149/1.2086512
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d. Electrochem. Soc., Vol. 137, No. 2, February 1990 9 The Electrochemical Society, Inc.
t r i b u t e s n o t h i n g to < S > ; Eq. [A-5] and [A-7] are unc h a n g e d (so is Eq. [15] in the text). H o w e v e r , < l / S > becomes
<11S>=~-
~
+ -(2-~)
-
electrolyte conductivity, (~ cm) 1
local averaging distance, cm
g e o m e t r i c p a r a m e t e r in Fig. 7, d i m e n s i o n l e s s
constriction factor, d i m e n s i o n l e s s
potentialz V
9c
[A-14]
which on combining with Eq. [A-7]yields Eq. [17].
LIST OF SYMBOLS
a
interfacial area/total pore volume, cm2/cm 3
b
Tafel parameter, V
c(z) local c o n c e n t r a t i o n in micropore, m o l / c m ~
c(z) local c o n c e n t r a t i o n in macropore, m o l / c m ~
C1 integration constant, V/cm
D
diffusivity, cm2/s
F
F a r a d a y constant, C/eq
i(z) interfacial c u r r e n t density, A / c m 2
i0
e x c h a n g e current density, A / c m 2
iavg a v e r a g e interfacial current density, A / c m 2
I'(z) total c u r r e n t along pore axis, A
L
l e n g t h of pore, c m
n
electron n u m b e r of reaction, eq/mol
_n' n u m b e r of spheres per u n i t area in Fig. 5, cm 2
N
n u m b e r of constrictions p e r c h a n n e l in constricted
c u b i c p o r e structure
R
Gas constant, J/tool 9 K
S(z) local cross-sectional area, c m 2
$1 surface area s c a l e i n Eq. [1]-[3]
T
temperature, K
z
d i s t a n c e into p o r e from electrode-electrolyte inter-
face, cm
Greek
transfer coefficient, eq/mol
dimensionless surface roughness size in Eq. [16] and
h
em
K
[17]
d i m e n s i o n l e s s average interfacial current density
m a c r o s c o p i c v o i d v o l u m e fraction, d i m e n s i o n l e s s
integration variable in Eq. [2], cm
overpotential, V
593
REFERENCES
1. J. E u l e r and W. N o n e n m a c h e r , Electrochim. Acta, 2,
268 (1960).
2. J. N e w m a n and C. W. Tobias, This Journal, 109, 1183
(1962).
3. K. Micka and I. Rousar, Electrochim. Acta, 25, 1085
(1980).
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22.
K. Micka, ibid., 27,765 (1982).
G. Wilemski, This Journal, 130, 117 (1983).
J. Van Zee and R. E. White, ibid., 130, 2003 (1983).
E. Royaie and J. Jorne, ibid., 131, 1237 (1984).
K. C. Ho and J. Jorne, ibid., 133, 1394 (1986).
R. E. White, M. A. Nicholson, L. G. Kline, J. Van Zee,
and R. Darby, ibid., 131, 268 (1984).
J. N e w m a n and W. T i e d e m a n n , AIChE J., 21, 25 (1975).
M. K r a m e r and M. Tomkiewicz, This Journal, 131, 1283
(1984).
H. S. Lira, " L o n g Life Nickel E l e c t r o d e s for NickelH y d r o g e n Cells," N A S A CR-174815, p. 106ff (Dec.
1984).
P. Sides and C. W. Tobias, This Journal, 129, 2715
(1982).
O. Lanzi and R. F. Savinell, ibid., 130, 799 (1983).
P. F e d k i w and J. N e w m a n , Chem. Eng. Sci., 33, 1563
(1978).
K. Micka and M. Savata, J. Power Sources, 3, 167 (1978).
T. Kessler and R. Alkire, This Journal, 123, 990 (1976).
O. Lanzi and U. Landau, ibid., 135, 1922 (1988).
O. Lanzi and U. Landau, ibid., 136, 368 (1989).
M. M e n o n and U. Landau, This Journal, 134, 2248
(1987).
G. W. D. Briggs, E. J o n e s , and W. F. K. W y n n e - J o n e s ,
Trans. Faraday Soc., 51, 1433 (1983).
G. C. A k e r l o f and P. Bender, J. Am. Chem. Soc., 70,
2366 (1948).
Electrochemical Deposition of Aluminum from NaCI-AICI3 Melts
Li Qingfeng, H. A. Hjuler,* R .W. Berg, and N. J. Bjerrum
Molten Salts Group, Chemistry Department A, The Technical University of Denmark, DK-2800 Lyngby, Denmark
ABSTRACT
E l e c t r o c h e m i c a l d e p o s i t i o n of a l u m i n u m from NaA1C14 melts saturated w i t h NaC1 onto a glassy carbon e l e c t r o d e at
175~ has b e e n studied by v o l t a m m e t r y , c h r o n o a m p e r o m e t r y , and c o n s t a n t current deposition. The d e p o s i t i o n of alumin u m was f o u n d to p r o c e e d via a n u c l e a t i o n / g r o w t h m e c h a n i s m , and the n u c l e a t i o n process was f o u n d to be progressive.
T h e m o r p h o l o g y of a l u m i n u m deposits was e x a m i n e d with p h o t o m i c r o s c o p y . It was s h o w n that d e p e n d i n g on the current
densities (c.d.) applied, three types of a l u m i n u m deposits c o u l d be obtained, namely, s p o n g y deposits f o r m e d at l o w e r c.d.
(below 0.7 m A / c m 2), s m o o t h layers d e p o s i t e d at i n t e r m e d i a t e c.d. ( b e t w e e n 2 and 10 m A / c m 2), and dendritic or p o r o u s deposits o b t a i n e d at h i g h c.d. (above 15 mA/cm2). H o w e v e r , the s m o o t h a l u m i n u m deposits w e r e a b o u t five t i m e s m o r e volum i n o u s t h a n the theoretical value. The s p o n g y deposits w e r e f o r m e d d u e to difficulties in e l e c t r o n u c l e a t i o n and could be
i n h i b i t e d by application of pulsed currents and/or a d d i t i o n of m a n g a n e s e chloride into the melt.
A l u m i n u m has b e e n used as a n o d e material in n e w l y dev e l o p e d r e c h a r g e a b l e batteries with the NaA1C14 m o l t e n
salt as the electrolyte and transition m e t a l sulfides as cathodes at 175~ (1, 2). D u r i n g t h e c h a r g i n g of t h e batteries, it
was f o u n d (3) that a l u m i n u m dendrites can be f o r m e d
u n d e r certain circumstances. The spacing b e t w e e n the
electrodes in t h e batteries is usually v e r y small; hence, t h e
d e n s i t y of the deposits is of i m p o r t a n c e for the capacity
and life of the batteries. N o n c o m p a c t , dendritic deposits
m a y cause short circuiting, early deterioration, and so on.
B a r t o n and Bockris (4) and Diggle et al. (5) h a v e dev e l o p e d a kinetic m o d e l of d e n d r i t e f o r m a t i o n in w h i c h the
role of a diffusion-controlled process was e m p h a s i z e d to
a c c o u n t for the initiation and g r o w t h of dendrites. A set of
rules g o v e r n i n g the kinetics of d e n d r i t e f o r m a t i o n was es*Electrochemical Society Active Member.
tablished, m a i n l y that (i) a certain critical current density
(overpotential) m u s t be e x c e e d e d in order to p r o v o k e dendrite formation, (ii) d e n d r i t e g r o w t h e x h i b i t s a certain ind u c t i o n period before it b e c o m e s visible, and (iii) the
critical current density or o v e r p o t e n t i a l for d e n d r i t e form a t i o n is directly related to t h e c o n c e n t r a t i o n of the dep o s i t i n g species.
As early as the 1930s, a l u m i n u m d e n d r i t e s w e r e found to
be favored d u r i n g deposition at high current density from
NaC1-A1C13 melts (6, 7). This was later studied by Midorikawa in a series of papers (8-10). Thereafter, several studies h a v e b e e n p e r f o r m e d in this field (11-17). M a n y investigations s u p p o r t e d the a s s u m p t i o n of diffusion-controlled
d e n d r i t e f o r m a t i o n for a l u m i n u m deposition. As e x p e c t e d ,
a critical current density or overpotential is p r e s e n t for alum i n u m d e n d r i t e initiation, and the critical v a l u e is directly
related to the mass transport conditions and the concen-
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594
J. Electrochem. Soc., Vol. 137, No. 2, February 1990 9 The Electrochemical Society, Inc.
tration of species containing a l u m i n u m (12-14). Generally,
in melts w i t h less t h a n 57 m o l e p e r c e n t (m/o) A1C13, the
critical current density is v e r y small and dendrites are easily formed. This finding is unfortunate, since at high concentrations of AIC13 the melts are difficult to h a n d l e because of their high v a p o r pressures.
T h e r e were also reports suggesting that electrode substrates (8, 11, 15), inorganic and organic additives
(10, 13, 16, 17), and s u p e r p o s e d alternating current (8) or
p u l s e d current (15) had significant effects on inhibiting
a l u m i n u m d e n d r i t e formation. These findings cannot, of
course, fit the m e c h a n i s m of diffusion-controlled d e n d r i t e
f o r m a t i o n b u t suggest the g o v e r n i n g role of the f o r m a t i o n
of crystalline nuclei in the deposition (14, 15). In the overvoltage m e a s u r e m e n t in a NaC1-KC1-A1C13 melt, Hayashi
et al. (18) f o u n d that the m a i n part of cathodic polarization
s e e m e d to be due to crystallization overvoltage in the low
polarization potential region. Rolland and M a m a n t o v (19)
f o u n d that the r e d u c t i o n of A12C17- in acidic chloroaluminate m e l t s i n v o l v e d a n u c l e a t i o n process. Apart from this,
little w o r k has been d o n e on the e l e c t r o c h e m i c a l nucleation and g r o w t h of a l u m i n u m from that m e d i u m . Moreover, a l m o s t all p r e v i o u s studies w e r e p e r f o r m e d in acidic
NaCI-A1CI3 melts, and little w o r k has b e e n r e p o r t e d in the
basic NaC1-A1C13 m e l t saturated w i t h NaC1. This s y s t e m is
of m u c h m o r e interest from the technical point of v i e w because of its low v a p o r pressure. It was our i n t e n t i o n to
s t u d y the e l e c t r o c h e m i c a l n u c l e a t i o n process and to e x a m ine h o w and w h e n the a l u m i n u m dendrites form in NaC1saturated melts at 175~
Experimental
P r e p a r a t i o n o f c h e m i c a l s . - - P u r i f i c a t i o n of A1C13 and
NaC1 and other e x p e r i m e n t a l p r o c e d u r e s h a v e b e e n previously d e s c r i b e d (3, 20). The e q u i m o l a r NaC1-A1CI~ melts,
w h i c h w e r e m a d e from distilled A1Cla and analytic-grade
NaC1 dried at 200~ b e c a m e slightly yellow after several
days of e x p e r i m e n t a t i o n and s o m e dark particles w e r e always f o r m e d b o t h on the a l u m i n u m electrode and in the
melt. F u r t h e r purification of the e q u i m o l a r melts was
therefore p e r f o r m e d by zone refining carried out in a glass
furnace consisting of nine stationary ring heaters. A certain a m o u n t of a m i x t u r e of 50.1 m/o distilled A1C13 and
49.9 m/o dried NaC1 was charged into a P y r e x a m p u l of
200 m m length and 25 m m inner diameter. The a m p u l was
t h e n sealed off u n d e r v a c u u m and heated o v e r n i g h t in a
r o c k i n g furnace. The heating zones of 35 m m w i d t h each
w e r e m a i n t a i n e d at 158~ The m o t i o n of the charged
a m p u l i n v o l v e d a d i s p l a c e m e n t range of 75 m m , w h i c h
was r e p e a t e d m a n y times. The travel rate of the m o l t e n
zones was 3 mm/h. The duration of the w h o l e refining proc e d u r e was a b o u t 2 weeks.
After zone refining, the m o l t e n salts w e r e colorless and
allowed a l u m i n u m electrodes in the s u b s e q u e n t experim e n t s to r e m a i n shiny and bright. H o w e v e r , a tiny a m o u n t
of dark particles still a p p e a r e d in the m e l t after s o m e days
of use.
Cell a s s e m b l y a n d o v e n . - - T o facilitate the visual observ a t i o n of t h e a l u m i n u m deposits, the cells w e r e m a d e of
s q u a r e P y r e x tubes. The w o r k i n g electrode was m a d e of a
glassy carbon rod (Type V10 from Le Carbone Lorraine) of
3 m m d i a m e t e r (area 0.07 cm2), as described earlier (3). T w o
a l u m i n u m rods of 99.999% purity j o i n e d to t u n g s t e n leads
w e r e u s e d as the c o u n t e r and r e f e r e n c e electrode, respectively. T h e r e f e r e n c e electrode was placed in t h e cell in
such a w a y that it was s u r r o u n d e d mainly by NaA1CI4 saturated w i t h NaC1 (i.e., the c o m p o s i t i o n of the m e l t before an
e x p e r i m e n t started). The test cell was placed into a transp a r e n t o v e n of our o w n construction, heated with circulated air. The a l u m i n u m deposits could be o b s e r v e d and
p h o t o g r a p h e d directly.
I n s t r u m e n t s a n d m e a s u r e m e n t s . ~ V o l t a m m o g r a m s and
potentiostatie current transients w e r e obtained with an
e l e c t r o c h e m i c a l system built in this laboratory (21) and rec o r d e d on a H e w l e t t - P a e k a r d X-Y r e c o r d e r (Type 7004B).
After each set of m e a s u r e m e n t s , the w o r k i n g electrode
was k e p t at a positive potential of 100 m V vs. a l u m i n u m for
at least half an h o u r and t h e n left for another half h o u r to
eliminate the c o n c e n t r a t i o n polarizations.
A l u m i n u m deposits were a c h i e v e d by electrolysis at
c o n s t a n t current, w h i c h was delivered by a c h r o n o a m perostat built in this laboratory. To e n s u r e that sufficient
m e t a l was d e p o s i t e d to p r o v i d e a visible layer of a l u m i n u m
deposits, a total charge of ca. 30 C/cm 2 was e m p l o y e d in
each run, c o r r e s p o n d i n g theoretically to a 0.01 m m thickness of u n i f o r m l y plated a l u m i n u m . The duration of each
d e p o s i t i o n e x p e r i m e n t varied f r o m several m i n u t e s to
m o r e t h a n 20h, d e p e n d i n g on the current density applied.
After deposition, the a l u m i n u m deposits w e r e o b s e r v e d by
u s i n g a Zeiss J e n a T e c h n i v a l 2 Stereo M i c r o s c o p e
e q u i p p e d w i t h an O l y m p u s OM-2 S L R c a m e r a and a
S e h o t t K L 1500 cold light source. The m o r p h o l o g y of the
a l u m i n u m deposits was e x a m i n e d by m i e r o p h o t o g r a p h y
and the specific v o l u m e of the s m o o t h deposits was measu r e d as averages from the pictures.
Results and Discussion
V o l t a m m o g r a m s . F i g u r e 1 s h o w s v o l t a m m o g r a m s obtained in b o t h slightly acidic (Fig. la) and slightly basic
(saturated w i t h NaC1, Fig. lb) NaC1-A1CI~ melts at 175~
The small current peak A in Fig. la m a y be attributed to
the A12C1C r e d u c t i o n w h i c h starts at ca. - 5 0 m V vs. the
a l u m i n u m reference. Rolland and M a m a n t o v (19) f o u n d
the s a m e overpotential v a l u e in their e l e c t r o c h e m i c a l
study. The peak was followed by the A1C14 reduction. I n
t h e v o l t a m r n o g r a m f r o m the basic melts (Fig. lb), the current p e a k disappeared. In this case only AIC1C r e d u c t i o n
o c c u r r e d starting at ca. - 6 5 m V vs. a l u m i n u m . In the cat h o d i c b r a n c h e s of b o t h v o l t a m m o g r a m s , cross-over loops
a p p e a r e d on reversing sweeps characterizing the p r e s e n c e
of a n u c l e a t i o n / g r o w t h process. Integration of both the cat h o d i c and the anodic currents of the v o l t a m m o g r a m s obt a i n e d f r o m the basic melts gave a charge ratio, QJQa, bet w e e n 1.01-1.05, indicating that the r e d u c t i o n of A1C1C
b e h a v e s reversibly in a c h e m i c a l sense.
C h r o n o a m p e r o g r a m s . - - F i g u r e 2 shows a set of potentiostatic current-time transients obtained at different overpotentials in NaA1C14 saturated w i t h NaC1. For all overpotentials applied, there appears to be a p e a k current
t r a n s i e n t c o r r e s p o n d i n g to the charging of the electric
d o u b l e layer. For overpotentials b e l o w 65 m V vs. t h e A1
reference, there is no r e d u c t i o n current. A n u c l e a t i o n overpotential of at least 65 m V is r e q u i r e d for any significant
e l e c t r o d e p o s i t i o n to take place. This is in a g r e e m e n t w i t h
the overpotential o b s e r v e d in v o l t a m m e t r i c m e a s u r e m e n t
for the s a m e electrode and the s a m e NaCl-saturated melt.
F o r overpotentials larger t h a n 70 mV, the capacitive charging c u r r e n t was followed by a rising current due to the form a t i o n of n e w nuclei. As the nuclei grew, the overlap of
nuclei or n e i g h b o r i n g diffusion zones gave rise to a current
m a x i m u m followed by a falling portion of current. The
t i m e to reach the m a x i m u m current d e c r e a s e d as the applied overpotentials increased (Fig. 2a).
~176176
0.02
~e
O.
0
- 0.04
0.4
-o2
0
02
-o;~
;
o;~
potentiat (V)
Fig. ]. Voltammograms
on glassy carbon electrodes st | 75~
sweep
rates were 10 mY s-1. (a) 49.7 m/o NaCl-50.3 m/o AICI3 melt. (b)
NaAICI4 saturated with NaCl.
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J. Electrochem. Soc., Vol. 137, No. 2, February 1990 9 The Electrochemical Society, Inc.
0.8
595
2
i = - zF~r ( 2 D c ) ~312)(Mtp)(1/2)Nt (3/2)
[4]
3
0.6
H e r e z F is t h e m o l a r c h a r g e of t h e d e p o s i t e d species, D is
its d i f f u s i o n coefficient, c is its c o n c e n t r a t i o n i n tool c m -3,
M is its m o l e c u l a r weight, a n d p is t h e d e n s i t y of t h e electrolyte. T h i s is in a c c o r d a n c e w i t h t h e c o n c l u s i o n o b t a i n e d
f r o m t h e d i m e n s i o n l e s s analysis. T h e s t r a i g h t l i n e s of i 2/3
v s . t i n t e r c e p t t h e t i m e axis at treal = t - to = 0, i.e., t = to.
T h e r e f o r e , t h e d e l a y time, to, n e e d e d for t h e t i m e correct i o n c a n b e o b t a i n e d . A f t e r c o r r e c t i o n of t h e t i m e scale
w i t h to o b t a i n e d t h i s way, t h e d i m e n s i o n l e s s plot of t h e exp e r i m e n t a l d a t a is well s u p e r i m p o s e d o n t h e t h e o r e t i c a l
curve, c o r r e s p o n d i n g to a p r o g r e s s i v e n u c l e a t i o n p r o c e s s
(Eq. [2]), as s h o w n in Fig. 4.
(a)
0.4
(12
2
3.0
4
6
8
10
Aluminum
dendrite
formation.--Aluminum
deposits
o b t a i n e d b y e l e c t r o c h e m i c a l r e d u c t i o n f r o m NaA1C14 m e l t
s a t u r a t e d w i t h NaC1 c a n v a r y g r e a t l y i n t h e i r o u t e r a p p e a r ance depending on the depositing conditions. They can be
s m o o t h , r o u g h , d e n d r i t i c , or p o r o u s as t h e c u r r e n t densities increase. S e v e r a l t y p e s of s u r f a c e m o r p h o l o g y of alu-
17o
"~
"E 2 I
.
150
(b)
C
~
0.7
140
130
1.C)
o 75 mY
17
16~-12 - -
9 80mV
o 85 mV
0.6
T40~
130--
0
2
/+
time
6
(s)
8
10
Fig. 2. Potentiostatic current transients for deposition of aluminum
onto a glassy carbon electrode (3 mm diam) from a NaCI-saturated
melt (a) at low overpotentials (mV), and (b) at high overpotentlals
(mV). Applied overpotentials (mV) vs. aluminum reference are indicated in the figure. For clarity not all measured current transients are
shown. Electrode area, 0.07 cm2, temperature, 175~
W h e n l a r g e o v e r p o t e n t i a l s are applied, t h e t i m e r e q u i r e d
to r e a c h t h e m a x i m u m c u r r e n t b e c o m e s so s h o r t t h a t t h e
c a p a c i t i v e c u r r e n t p e a k is n o t d i s t i n g u i s h a b l e f r o m t h e risi n g p o r t i o n of t h e c u r r e n t t r a n s i e n t s d u e to t h e f a r a d a i c
p r o c e s s . A f t e r t h e first m a x i m u m in t h e t r a n s i e n t s , t h e curr e n t r i s e s again, a n d a s e c o n d c u r r e n t m a x i m u m a p p e a r s
t h e r e a f t e r w h e n o v e r p o t e n t i a l s l a r g e r t h a n 120 m V are app l i e d (Fig. 2b). As n u c l e a t i o n p r o c e e d s c o n t i n u o u s l y , n e w
n u c l e i are f o r m e d o n t o p of t h e first d e p o s i t e d l a y e r l o n g
b e f o r e it h a s c o v e r e d t h e w h o l e area of t h e e l e c t r o d e surface, i.e., m u l t i n u c l e a r m u l t i l a y e r g r o w t h o c c u r s (22).
F o r a t h r e e - d i m e n s i o n a l n u c l e a t i o n , e x p r e s s i o n s for t h e
theoretical current transients describing both the rising
a n d t h e falling p a r t s h a v e b e e n o b t a i n e d for b o t h i n s t a n t a n e o u s a n d p r o g r e s s i v e n u c l e a t i o n s (23, 24). A c o n v e n i e n t
c r i t e r i o n for d i s t i n g u i s h i n g b e t w e e n t h e t w o m e c h a n i s m s
is to r e p r e s e n t t h e e x p e r i m e n t a l d a t a i n t h e d i m e n s i o n l e s s
plot, (i/im) 2 VS. t/tm, w h e r e im a n d t~ are t h e c o o r d i n a t e s of
t h e c u r r e n t m a x i m u m o n t h e c u r r e n t t r a n s i e n t s . T h i s analysis c a n b e m a d e b y c o m p a r i n g t h e e x p e r i m e n t a l d a t a w i t h
t h e o r e t i c a l p l o t s r e s u l t i n g f r o m Eq. [1] a n d [2] for i n s t a n t a n e o u s a n d p r o g r e s s i v e n u c l e a t i o n s , r e s p e c t i v e l y (24)
(ilim) 2 = 1.9542 (tlt~) (-l~ {1 -- e x p [-1.2564 (tltm)]} 2
[1]
(ilim) 2 = 1.2254 (tlt=) (-1) {1 -- e x p [-2.3367 (tit,,)2]} 2
[2]
It is f o u n d t h a t t h e d i m e n s i o n l e s s plot o f o u r e x p e r i m e n t a l
d a t a fits to t h e t h e o r e t i c a l c u r v e c o r r e s p o n d i n g to p r o g r e s sive n u c l e a t i o n e x c e p t for l o w v a l u e s of t/tm. AS d e m o n s t r a t e d (25), t h e e x p e r i m e n t a l c u r r e n t t r a n s i e n t s are aff e c t e d b y a c e r t a i n t i m e delay, to. T h e e x a c t c o m p a r i s o n is
p o s s i b l e o n l y a f t e r c o r r e c t i o n of t h e t i m e scale b y d e f i n i n g
treal = t - to
[3]
F i g u r e 3 s h o w s t h e g r a p h i c a l a n a l y s i s of t h e r i s i n g port i o n o f c u r r e n t t r a n s i e n t s . A l i n e a r d e p e n d e n c e of i 2/3 o n
t i m e is f o u n d , i n d i c a t i n g t h e p r o g r e s s i v e n a t u r e of t h e n u c l e a t i o n p r o c e s s as r e q u i r e d b y t h e f o l l o w i n g e q u a t i o n (25)
/Do/
~
o~
<
0.4
~_
0.3
o/
/
o "~176
oO
02
o., / / /
0
0
/2 ~
I
0.5
I
1.0
t(S)
I
1.5
I
2.0
Fig. 3. Plots of i2/3 vs. time for the rising portions of some current
transients. Overpotentials indicated in the figure.
121
o 75mY
9 8 0 mV
o 85 mV
10
#/
0
ll0
2r0
310
t real / t m
Fig. 4. Dimensionless plots for overpotentials of 75, 80, and 85 mV
with time correction. Theoretical curves (A) for instantaneous and (B)
for progressive nucleation.
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596
J. Electrochem. Soc., Vol. 137, No. 2, February 1990 9 The Electrochemical Society, Inc.
Fig. 6. Aluminum deposits on a glassy carbon electrode of 3 mm diam
at 175~ (a) Constant current, 0.285 mA/cm2, 3.5h, NaAICI4 saturated with NaCI. (b) Constant current, 1.14 mAJcm2, 6.95h, NaAICI4
saturated with NaCI. (c) Pulsed current, 1.14 mA/cm2 (id/A) (A = elec~
trode area), 6.95h, NaAICI4 saturated with NaCI. (d) Constant current,
1.14 mA/cm2, 6.95h, NoAICI4 saturated with both NaCI and MnCI 2.
Fig. 5. Different types of aluminum deposits obtained from constant
current deposition on a glassy carbon electrode (3 mm diam) in
NaAICI+ saturated with NaCI at 175~ (a) Spongy deposits, 0.43
mA/cm2, 18.5h. (b) Spongy deposits, 0.57 mA/cm2, 14h. (c) Smooth deposits, 4.3 mA/cm2, 1.85h. (d) Needle-like deposits, 21 mA/cm 2, 22
rain. (e) Fir-tree-like dendrites, 28 mA/cm 2, t 7 min. (f) Porous deposits,
43 mA/cm2, 8 min.
m i n u m d e p o s i t s are s h o w n in Fig. 5. I n t h e NaA1C14 m e l t
s a t u r a t e d w i t h NaC1 at 175~ it s e e m s t h a t s p o n g y dep o s i t s are always f o r m e d at low c u r r e n t d e n s i t i e s (< c a .
0.7 m A / c m 2, see Fig. 5a a n d b); at i n t e r m e d i a t e r a n g e s of
c u r r e n t d e n s i t i e s b e t w e e n 2 a n d 10 m A / c m 2, a l u m i n u m dep o s i t s a p p e a r s m o o t h a n d g r a n u l a r (Fig. 5c), w h i l e at v e r y
h i g h c u r r e n t d e n s i t i e s (> c a . 15 m A / c m ~) n e e d l e - l i k e or firtree-like d e n d r i t e s or v e r y p o r o u s d e p o s i t s are o f t e n obt a i n e d (Fig. 5d, e, a n d f).
A l u m i n u m g r a i n s f o r m e d d u r i n g d e p o s i t i o n c a n easily
b e o b s e r v e d w i t h a m i c r o s c o p e b y u s i n g grazing i l l u m i n a tion. It is f o u n d t h a t t h e g r a i n size of t h e a l u m i n u m dep o s i t s d e c r e a s e s as t h e c u r r e n t d e n s i t y increases. A t a v e r y
l o w c u r r e n t d e n s i t y (0.285 mA/cm2), s e p a r a t e d a l u m i n u m
p a r t i c l e s c a n b e o b t a i n e d o n t h e e l e c t r o d e s u r f a c e as
s h o w n i n Fig. 6a, a n d i n t h i s case t h e s p o n g y a l u m i n u m dep o s i t s will f o r m after p r o l o n g e d d e p o s i t i o n . A l u m i n u m
p a r t i c l e s s h o w a g r e a t v a r i e t y of sizes, w h i c h c o n f i r m s t h e
p r o g r e s s i v e c h a r a c t e r of t h e n u c l e a t i o n process.
I t is k n o w n f r o m t h e e l e c t r o c r y s t a l l i z a t i o n t h e o r y (26)
t h a t t h e g r a i n size is d e t e r m i n e d p r i m a r i l y b y t h e n u m b e r
of n u c l e i f o r m e d d u r i n g t h e d e p o s i t i o n process. T h e form a t i o n o f t h e s p o n g y d e p o s i t s at l o w e r c u r r e n t d e n s i t y c a n
t h u s b e a t t r i b u t e d to t h e lack of n u c l e i originally f o r m e d
s e r v i n g as g r o w t h c e n t e r s . It w a s k n o w n t h a t a l u m i n u m
d e p o s i t i o n f r o m NaC1-A1C13 m o l t e n salts p r o c e e d s via a n u c l e a t i o n / g r o w t h m e c h a n i s m . T h e difficulty in e l e c t r o n u c l e a t i o n of a l u m i n u m d e p o s i t s is t h u s a s s u m e d to b e t h e
c a u s e of f o r m a t i o n of s p o n g y d e p o s i t s at low c u r r e n t
density.
A critical t h i c k n e s s of a l u m i n u m d e p o s i t s , as well as t h e
v a r i a t i o n of a l u m i n u m g r a i n sizes w i t h c u r r e n t d e n s i t y of
d e p o s i t i o n , w a s also o b s e r v e d in a n a l u m i n u m e l e c t r o p l a t ing s t u d y (14). T h e a s s u m p t i o n of t h e p r e d o m i n a n t role of
t h e e l e c t r o n u c l e a t i o n of a l u m i n u m in t h e d e p o s i t i o n pro-
cess w a s also s u p p o r t e d b y t h e h i g h effect o f o r g a n i c surf a c t a n t s (13, 14, 16, 17) s u c h as t e t r a m e t h y l a m m o n i u m
c h l o r i d e (TMA) a n d urea. T h e o r g a n i c s u r f a c t a n t s w e r e
s u p p o s e d to b l o c k t h e a c t i v e sites o n t h e c a t h o d e s u r f a c e
a n d r e s u l t in a n e n e r g e t i c h o m o g e n i z a t i o n of t h e s u r f a c e
a n d c o n s e q u e n t l y l e a d to a n i n c r e a s e d n u m b e r o f g r o w i n g
nuclei.
W i t h a n i n c r e a s e in c u r r e n t d e n s i t y (overpotential), t h e
e l e c t r o n u c l e a t i o n p r o c e s s b e c o m e s f a s t e r a n d m o r e prec u r s o r y n u c l e i c a n b e f o r m e d d u r i n g t h e d e p o s i t i o n process. T h e f o r m a t i o n o f t h e s p o n g y d e p o s i t s will t h u s b e
suppressed during the subsequent growth. Smooth alumin u m d e p o s i t s c a n t h u s b e f o r m e d w h e n d e p o s i t i o n is d o n e
at i n t e r m e d i a t e c u r r e n t densities.
A t h i g h c u r r e n t d e n s i t y , t h e w h o l e d e p o s i t i o n p r o c e s s is
b r o u g h t u n d e r t h e c o n t r o l o f slow t r a n s p o r t o f d e p o s i t i n g
i o n s f r o m t h e b u l k to t h e e l e c t r o d e surface. I n t h i s case,
a l u m i n u m d e n d r i t e s as p r e v i o u s l y d i s c u s s e d (4, 5) are
f o r m e d . T h e m u c h f a s t e r g r o w t h of a d e n d r i t i c a l u m i n u m
p r o t r u s i o n r e l a t i v e to a l u m i n u m o n t h e r e s t of t h e elect r o d e s u r f a c e lies m a i n l y in t h e d i f f e r e n t d i f f u s i o n c o n d i tions, i.e., t h a t s p h e r i c a l d i f f u s i o n o c c u r s a r o u n d t h e d e n dritic tips w h e r e a s l i n e a r d i f f u s i o n p r e v a i l s e l s e w h e r e . T h e
c o n c e n t r a t i o n g r a d i e n t of species c o n t a i n i n g a l u m i n u m res u l t i n g f r o m electrolysis m a y also d r i v e t h e e x c e s s i v e
g r o w t h of a l u m i n u m d e n d r i t e s as p r o p o s e d i n o u r previo u s s t u d y (3).
A c h a r a c t e r i s t i c of d i f f u s i o n - c o n t r o l l e d d e n d r i t e f o r m a t i o n is t h e critical c u r r e n t d e n s i t y or o v e r p o t e n t i a l for t h e
initial d e n d r i t e f o r m a t i o n (12-14). T h e v a l u e s of critical curr e n t d e n s i t i e s are d i r e c t l y r e l a t e d to t h e c o m p o s i t i o n o f t h e
melt. F o r t h e NaA1C14 m e l t s a t u r a t e d w i t h NaC1, t h e irregular d e n d r i t e s or p o r o u s d e p o s i t s w e r e o f t e n o b t a i n e d w h e n
c u r r e n t d e n s i t i e s e x c e e d i n g 15 m A / c m 2 w e r e a p p l i e d to a
glassy c a r b o n electrode. G r j o t h e i m a n d Matia~ovsk~r (13)
f o u n d t h a t at c u r r e n t d e n s i t i e s u p to 50 m A / c m 2 a l u m i n u m
w a s d e p o s i t e d in t h e f o r m of a fine c r y s t a l l i n e l a y e r o n a
steel s u b s t r a t e f r o m a n e l e c t r o l y t e c o n t a i n i n g 63.6 m/o
A1C13. F e l l n e r e t a l . (14) o b s e r v e d a s i m i l a r critical v a l u e o n
i r o n c a t h o d e s i n a t e r n a r y NaC1-KC1-A1CI3 s y s t e m c o n t a i n i n g 56.8-63.6 m/o A1Cla. S m i t h e t at. (27, 28) f o u n d t h a t t h e
critical c u r r e n t d e n s i t y d e c r e a s e d d r a m a t i c a l l y to c a .
1 m A / c m 2 in t h e t e r n a r y A1C13-based m e l t s c o n t a i n i n g c a .
50 m/o A1C13.
All o f t h e r e s u l t s cited w e r e o b t a i n e d b y u s i n g i r o n or
steel s u b s t r a t e s . T h e r e f o r e t h e d i f f e r e n c e in critical c u r r e n t
d e n s i t i e s b e t w e e n t h e cited l i t e r a t u r e s a n d o u r s m a y b e
p a r t l y a t t r i b u t e d to t h e d i f f e r e n t c o m p o s i t i o n of t h e m e l t
a n d to t h e d i f f e r e n t e l e c t r o d e s u b s t r a t e s used. M o r e o v e r ,
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J. Electrochem. Soc., Vol. 137, No. 2, February 1990 9 The Electrochemical Society, Inc.
Nayak et al. (11) found that electrodeposition of aluminum
on brass from 36.4 m/o NAC1-63.6 m/o A1Cl~ led to nondendritic crystal deposits at current densities of up to
53 mA/cm 2. Pretreatment of a mild steel substrate by electropolishing in the electrolyte itself was also found to be
very helpful in eliminating dendrite formation (12). The
fact that the substrates have such a pronounced effect on
aluminum deposition also suggests the importance of the
nucleation process. It seems correct to say that even in the
case of diffusion-controlled dendrite formation, the electronucleation process may play a certain role in dendrite
formation.
The smooth aluminum deposits obtained at intermediate current densities between ca. 2 and 10 mA/cm 2 are
not compact. Figure 7 shows the results of the average volu m e of the aluminum deposits per coulomb vs. the current
density that was applied. The volumes were all measured
on photographs of the deposits. The deposits obtained
from NaA1C14 saturated with NaC1 by constant current
deposition look porous. The values for the volume of alum i n u m are about five times or more larger than the
theoretical value of 0.0345 mm3/C for a compact layer of
aluminum. The larger values obtained at low and high current densities during constant current deposition reflect
the formation of two types of deposits, viz., the electronucleation-controlled formation of spongy deposits at low
current densities and the diffusion-controlled formation of
dendrites at high current densities.
It seems of importance to distinguish the two mechanisms from one another. Consequently, different techniques may be used to inhibit the formation of individual
kinds of dendrites. For instance, to avoid dendritic formation caused by the slow nucleation process at low current
density, a pulsed current shown in Fig. 8 was used. A single pulse of current (ip) ten times as large as the deposition
current (ia) and with a duration of 10s was applied at the
beginning of the experiment in order to accelerate the nucleation process. A small deposition current (id) was then
used to avoid the dendrite formation caused by the diffusion-controlled process afterward. In this way, the spongy
deposits can be completely eliminated. The volume of the
a l u m i n u m deposits obtained by the pulsed current deposition is also shown in Fig. 7. It can be seen that the
compactness of the aluminum deposits was much improved by this pulse technique, especially at low current
densities. Apparent differences between aluminum deposits obtained at the same deposition current density
(1.14 mA/cm 2) with and without the initial current pulse
are shown in Fig. 6b and c. Furthermore, the addition of
manganese chloride into the melt can also significantly improve the quality of deposits, as seen in Fig. 6d. Detailed
studies are now under way to explore the effect of manganese chloride addition on aluminum deposition.
(.9
O5
+
,constont current
,pulsed current
E
E
0.4
E
=
-5
>
Q3
~
(12
v
4,
>
O
9?-
4-
03
c~
i/1
sohd alurnlnum----
0
0
i
I
l
I
2
4
current density
~
r
6
( m A / c m 2)
i
I
8
Fig. 7. Specific average volume of smooth layer of aluminum deposits, in mm 3 per coulomb, on a glassy carbon electrode in NaAICI4
saturated with NaCI as function of current density.
tp
09
t..
t..
O
597
-<
T
~p
~d
I time
Fig. 8. Pulsed current diagram: ip, initial current pulse, tp, duration of
current pulse, and id, deposition current, tp is always 10s, ip is always
ten times as large as id, and id continued until a total charge of ca. 30
C/cm 2 has been transferred.
Conclusions
Electrochemical deposition of aluminum onto glassy
carbon electrodes from NaA1C14 saturated with NaC1 at
175~ has been investigated and found to proceed via a nucleatiorggrowth mechanism. The nucleation process was
found to be progressive. Several types of aluminum deposits can be electrodeposited from the melt, mainly depending on the current densities applied. It is observed
that only at intermediate current densities, between 2 and
10 mA/cm 2, can a smooth layer of aluminum be formed.
The smooth deposits are still about five times more voluminous than the theoretical value. At high current densities, above ca. 15 mA/cm 2, needle-like or fir-tree-like dendrites or very porous deposits were often obtained. The
deposits were attributable to the slow diffusion process of
species containing aluminum. At lower current densities,
below ca. 0.7 mA/cm 2, aluminum was deposited in the
form of spongy deposits, apparently due to the slow nucleation process. The spongy deposits can be inhibited by
using pulsed current deposition or by adding manganese
chloride to the melt.
Acknowledgments
We gratefully acknowledge the Danish Ministry of
Energy, the Danish Technical Research Council, and
Myhrwolds Fond for financial si~pport.
Manuscript submitted April 3, 1989; revised manuscript
received July 13, 1989.
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J. Electrochem. Soc., Vol. 137, No. 2, February 1990 9 The Electrochemical Society, Inc.
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Analysis of the Response of Ion Sensing FETs with a Chemically
Modified Gate Insulator
Hubert Perrot, Nicole Jaffrezic-Renault, and Paul Clechet*
Laboratoire de Physico-chimie des Interfaces, URA CNRS 404, Ecole Centrale de Lyon-BP 163,
69131 Ecully Cedex, France
ABSTRACT
This paper is devoted to the analysis of the physicochemical parameters influencing the response of ion sensing FETs
obtained by chemical grafting of the silica gate insulator. The site binding theory has been applied to the modified silica/
electrolyte interface; for the first time the equilibria involving H § and M + ions have been taken into account. The established theoretical expression ~o-pM shows that the sensitivity of the ion sensing FETs is better when the n u m b e r of sensitive sites and the complexation constant of these sites are higher. The experimental response of the Ag § sensing I S F E T
was fitted with this model. The percentage of cyanografted sites is determined to be 8%; their complexation constant
is -0.9.
ISFET-type chemical sensors consist of an insulated
gate field effect transistor (IGFET) where the metallic gate
is replaced by a sensitive membrane, an electrolyte, and a
reference electrode. At the interface between the insulator
and the solution, the electric potential difference depends
on the composition of the solution (pH or other ions) and
on the nature of the insulator surface. Several dielectrics
were used for pH detection: SiO~ (1), Si3N4 (2), A1203 and
Ta205 (3). For ion detection the dielectric surface was modified by adding several types of specific membrane to the
insulator surface: special glass for Na + and K + detection
(4), insoluble salts for halogenide detection, polymeric
membranes with inserted ionophore groups for detection
of K +, Na § Ca 2+ (5), NO3- (6), C1- (7) ions. Ion specificity
was also conferred onto the directly modified insulator
surface by ion implantation (8) for Na + detection or by
chemical grafting. The feasibility of an Ag + sensing I S F E T
through chemical grafting of the silica surface with cyanopropylsilane has been shown (9, 10). The devices obtained have a long lifetime (11) and a short response time
(9). This paper is devoted to the analysis of the parameters
influencing the ion response of this type of device.
The potential/pH response of the oxide membrane
I S F E T is based on the acid/base behavior of hydroxyl
groups on the surface. The site binding theory which was
first applied to pH sensitive I S F E T by Siu and Cobbold
(]2) allows the fit of the response of the ISFETs by introducing a sensitivity constant ~ (13, 14). Very recently, the
site binding theory was extended to a dielectric surface
with two types of sites: the Si3N4 membrane with amphoteric hydroxyl groups and the basic amine groups (15).
For both sites the potential determining ion is H +. In this
paper, we present the site binding theory for a modified insulator surface: a silica I S F E T grafted with ionophore
groups which presents a specific response for ions different from H § the remaining amphoteric hydroxyl groups
being pH sensitive. This model is applied to the cyanografted silica membrane; the potential determining ions
are Ag + and H + (9-11).
* Electrochemical Society Active Member.
Site Binding Theory Applied to Grafted Silica Surface
Surface reactions.--The two types of sites on the grafted
silica surface are amphoteric hydroxyl sites, called SOH,
and the ionophore grafted sites called S-X.
It is assumed that no surface complexation o f M + occurs
with hydroxyl groups, as has been observed in the case of
Ag § ions, the flatband potential of St/bare SiO2/electrolyte
only being slightly shifted in the presence of Ag § ions (10).
The S-X sites are assumed to present no amphoteric properties as has been shown through the electrokinetic properties of CN-grafted silica in colloidal aqueous suspensions (16). Thus, each type of site reacts with one specific
ion: H § with the amphoteric SOH sites and M § with the
S-X sites. The formation of surface complexes between
charged surface sites (SOH2 § and SO-) and counterions in
the solution will be neglected. Such surface complexes
have been shown to have a negligible effect on the surface
potential (14).
The equations describing the surface reactions of the
two sites are:
Amphoteric reactions of SOH.-SOH2 + <-->SOH + Hs +
k'oH -
SOH ~ SO- + H~+
k"oH--
(H~+)(SOH)
(SOH~ +)
(Hs+)(SO -)
(SOH)
[1]
[2]
Surface complexation of M +with S-X sites.-S-X + M~+ ~ S-X --- M §
K•
(S-X --- M §
(S-X) (Ms+)
[3]
In these equations, the quantities in parentheses are
ionic activities, in the case of bulk concentrations, or numbers of sites per unit surface area in the case of surface concentrations. The activity coefficients for the surface con-
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