chapter 12 transients may 2010 - zhejiang...
TRANSCRIPT
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Chapter 12
Transients & Periodic Current/Voltage waveforms
a. Galvanostatic polarization b. Potentiostatic polarization c. Non‐DC waveform plating
Pulse Periodic reverse
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Outline:
• Time constants in Electrochemical systems
• Pulse shape
• Transient diffusion equation
Ganlvanodynamic
Potentiodynamic
Semi‐infinite and restricted diffusion
• Waveform Modes
• Why Pulse Plating?
• Controlling the Boundary Layer Thickness
• Analysis of Pulsed Potential vs. Pulsed Current
• Periodic Reverse Plating for Effective Leveling
Introduction:
The fundamental equation controlling electrochemical systems is the Nernst Plank equation:
jj j j j j j
c + v c = F (z u c ) + (D c )
t
[10]
Assuming that no transients are applied on the velocity field (v), we must consider transients in
concentration (C), ohmic potential (), and kinetics (s – not shown explicitly in Eq. [10] but in
its boundary conditions). Since the ohmic potential responds instantaneously ( ~ 10‐13 s), and the activation has a typical time constant of ts = RsCDL ~ s, we are concerned primarily with the
transient response of the concentration field.
Accordingly, once we neglect the potential variation, Eq. [10] can be simplified to:
jj j j
c + v c = (D c )
t
[44]
The boundary conditions are reformulated, recognizing that since the electric field driven
current becomes negligible, the current density now takes the form:
n j j jj
i = - F z D c [45]
On the cell boundaries we have:
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Insulator: i = 0 0RC [46]
Electrode: CV- E= ln ln 1e
b L
RT c RT i
nF c nF i
[47]
In deriving eq. [47] we recognize that S and are negligible compared to C. We furthermore
recognize that the concentration overpotential (in boundary condition [47]) cannot be significant
unless i approaches iL. For this to happen we must have:
ce << cb, or cb ~ 0 [48]
Inspecting eq. [44] and its boundary conditions [46] and [48], we recognize that it is
identical to the convective diffusion equation common in representing transport problems in
non‐electrochemical systems. Accordingly, invoking the mass transport control approximation
causes the problem to lose all its electrochemical characteristics, transforming it to a transient
diffusion problem. Obtaining general analytical solutions to the transient convective diffusion
problem is complex since it requires solving the transient concentration distribution in the cell in
conjunction with the fluid‐flow.
An approximation that applies to systems undergoing transient polarization can be
invoked. Assuming stagnant solution (no velocity) within the region of varying concentration
where the approximation is applied (usually the boundary layer), eq. [44] simplifies to:
2RR R
c = D c
t
[50]
The subscript R in eq. [50] indicates that it is applied to the reactant ion. Also, it is assumed that
the reactant diffusivity is constant, independent of the concentration. Eq. [50] is known as Fick’s
2nd law. Its solution is particularly relevant to problems of transient (periodic) current and
potential applications such as pulse and periodic reverse waveforms when the time constant is
in the range of the concentration profile relaxation time, i.e., about 0.01 seconds or longer (1‐4).
1. J. C. Puippe and F. H. Leaman, Theory and practice of pulse plating, American Electroplaters and Surface Finishers Society, (1986).
2. K. I. Popov and M. D. Maksimovic, in Modern Aspects of Electrochemistry, Vol. 19, B. E. Conway, J. O. M. Bockris, and R. E. White, eds., Plenum Press, New York, p. 193, 1989
3. B. K. Purushothaman, P. W. Morrison and U. Landau, "Reducing Mass Transport Limitations by the Application of Special Pulsed Current Modes”, J. Electrochem. Soc., 152 (4) J33‐J39 (2005)
4. B. K. Purushothaman, and U. Landau, "Rapid Charging of Lithium Ion Batteries Using Pulsed Currents – A Theoretical Analysis”, J. Electrochem. Soc., 153, (3) A533‐542 (2006)
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Transients in Electrochemical Processes
i
t
t
V a
c
a
c
TV E
T a C l
i
0
lna
RT i
F i
ln EC
B
CRT
nF C
10-6 s min10-13 s
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Periodic Reverse Plating for Effective Leveling
U. Landau, Extended Abstract, The ECS Meeting, Hawaii, October 1993
(In the Tafel range)
Wa = = =
L i L
b
i L
RT
F i
a 1
Wa >> 1 Uniform Distribution
Wa << 1 Non-Uniform Distribution
0.5 V
+0 V
+0 V
0 V
+1 V
+1 V
To achieve uniform deposit thickness, apply periodic reverse plating with:
High Wa during plating (for level deposition): Low current density Low C
Low Wa during dissolution (for non-uniformity) High current density High A
PLATE (~uniformly)
DISSOLVE (very non-uniformly)
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