Ionic Mass Transfer Rate Research Articles

(Invited) Gravitational Level Effects on Electrochemical Interfacial Phenomena

Electrochemical copper refining technology invented in Niagara area made a great progress over more than 100 years through better understanding of natural convection phenomena. Ionic mass-transfer rate enhanced by natural convection developing along 1m high vertical electrode surfaces has been unconsciously utilized by trial-and-error method. As the local electrolyte composition is partly stratified in an electrolytic cell (roughly 5m long x 1.3m wide x 1.3m depth). The industrial electrolytic tank house is composed of about 1000 cells through which the electrolyte is continuously supplied. Thus, the electrolyte circulation is a key technology in industrial copper refining processes as well as appropriate selection of effective additive and stable sedimentation of anode slime containing precious metals on the electrolytic tank bottom.On the other hand, Cu nanowire arrays were electrodeposited potentiostatically with PC filter template in two kinds of electrolytic cell configurations: cathode over anode (C/A) and anode over cathode (A/C). The effect of gravitational level on the coupling phenomena between the morphological variation and the ionic mass transfer rate in nanosized pores(a mesoscopic scale of a fewμm high and 30nm in diameter) was also found. Hitherto, the coupling phenomena accompanied with the electrochemical interfacial reactions confined in such a mesoscopic space has not been studies in spite of the great technological interest for the production of micro- or meso-scale fabrication techniques encountered in microelectronics and energy conversion & storage technology field. The initial stage including nucleation and growth process during electrochemical phase transformation reactions must be at first focused to profoundly comprehend the present subject in order to reasonably design such an advanced device technology.Fundamental aspects in electrochemical processes in microgravity environments have not been frequently reported. However, Artemis projects may provide the large opportunities on the energy storage and power generation, as well as materials processing. The effects of gravitational level and magnetic field gradient on the electrochemical interfacial phenomena should be understood. The drop tower facility surely offers the appropriate & “unique” experimental opportunities to propose such research program in the future space engineering fields. Electrocrystallization of metals is selected as a good subject for starting direction of research. Its reaction mechanism is relatively simple, the deposition rate can be easily varied by changing the current density or potential, and a smooth copper film is deposited so long as it is not too thick.By the way, the damascene process has been introduced to copper wiring process in ULSI field by IBM almost 25 years ago. It is important to control the nucleation and crystal growth in the initial stages of electrodeposition process. For this purpose, many effects of additive have been intensively examined. However, few quantitative examinations have been reported on the coupling phenomena of ionic mass transfer and nucleation and crystal growth. In the present work, we introduce various gravitational levels as new parameters into the nonequilibrium electrochemical processing in order to create or tailor the unique physical properties in nano space field. It is fruitful in terms of physicochemical hydrodynamics that the experiments are done under various gravitational levels. In fact, it has been confirmed that the gravitational levels as well as the magnetic field introduce the intensified effects from that under 1 G normal environment. Here is a report to analyze the effect of gravitational level on the initial stages of Cu electrodeposition on TaN or TiN substrate.

Ionic Mass Transfer Accompanied with Electrodeposition of Li Metal in Solvate Ionic Liquid

Li metal is a promising negative electrode candidate for next-generation rechargeable batteries. However, the non-uniform electrochemical reaction induces dendrite formation during charging operation and dead lithium during discharging operation, respectively. Many researchers tried to control the morphological variation of electrodeposited Li metal by using additives and artificial interface layer. In general, the electrodeposition of metals are governed by the ionic mass transfer rate in the vicinity of the electrode. In this study, we focused on the Li+ ionic mass transfer phenomenon during the electrodeposition and electrochemical dissolution of Li metal in solvate ionic liquid. The solvate ionic liquid is new electrolyte candidate for the batteries. Especially, it is good candidate for Li-S batteries due to the suppression of dissolution of poly-sulfide. However, the mechanism of electrodeposition of Li metal has not been cleared in solvate ionic liquid system. Here, two in-situ techniques were applied to discuss the mechanism of Li metal electrodeposition from the viewpoint of ionic mass transfer. One is the digital holographic interferometry microscope which can visualize the Li+ concentration profile near the electrode. Another is micro-Raman spectroscopy which can detect the bonding of the chemical species in the electrolyte. We would like to discuss the ionic mass transfer phenomenon in the vicinity of the electrode during the electrodeposition and electrochemical dissolution of Li metal in solvate ionic liquid based on these in-situ measurement results.

(Invited) Effect of Gravitational Level on the Initial Stage of Cu Electrodeposition

Material tailoring of nanostructured devices emphasizes the importance to separately control the nucleation rate from the growth process. It is indispensable to uniquely create the heterogeneous interface to result in the required electronic or physical properties. Then, the multi-scale modeling to link the atomic scale process on the heterogeneous interface with the shape evolution phenomena based on the continuous media process is required. It is well known that natural convection is accompanying with physicochemical reactions under ground level (1 G). Microgravity experiments are conducted over 10 s with JAMIC drop tower in order to understand the effect of gravitational level on electrodeposition as well as electrochemical dissolution. Less nucleation rate was observed in μ-G. On the other hand, a centrifugal field has been applied widely in order to create high G environment in electrochemical processing. Various differences in physical and chemical properties were found as well as morphological variations. The main purpose of this study is to clarify the effect of gravitational level from m-G to high G level on the coupling phenomena between the ionic mass transfer rate and the morphological variations of the electrodeposits. The possibility to control the nucleation phenomena, film morphology and nanostructure is explored. In particularly, the effects on the electrodeposited copper are analyzed in a cavity-type electrolytic cell from microgravity field in parabolic flight to high G field in centrifugal field. Cathode over anode configuration simulates the ionic mass transfer rate under quasi-microgravity environment because no macroscopic natural convection is induced in this cell configuration. Copper electrodeposition is conducted in order to examine the nucleation rate of copper on TiN substrate. The effects of cell configuration and gravitational level are focused on these phenomena.

(Invited) Development of Concentration Profile of Li+ Ion Accompanied with Electrodeposition of Li Metal in Solvated Ionic Liquid

Li metal is promising anode candidate for next-generation batteries because Li metal has very large specific capacity and Li/Li+ potential is the lowest. It is indispensable to control the surface morphological variation during cycles in order to utilize Li metal anode in next-generation batteries. In this study, the morphological variation of Li metal electrode in the electrodeposition is discussed from the viewpoint of Li+ ionic mass transfer rate in the vicinity of the electrode surface. In general, the metal electrodeposition and the morphology of electrodeposits are strongly affected by the electrode surface concentration. However, there are only a few researches focused on the relationship between the morphological variations of electrodeposited Li metal and the ionic mass transfer phenomenon. A digital holographic microscope (DHM) was utilized in order to visualize the Li+ concentration profile during the electrodeposition of Li metal. Our previous researches used a holographic interferometry to in-situ measure the surface concentration and concentration boundary layer thickness during the electrodeposition of Li metal. The principle of the technique is same, but the resolution of DHM was drastically improved. Figure 1 shows the DHM image in solvated ionic liquid electrolyte (LiTFSA:tetraglyme(G4) = 1:1). A working electrode was Cu plate, and a counter electrode was Li metal. Applied current density was 3.0 mA cm-2. The image was recorded at 100 seconds after the electrodeposition started. The color variation indicates the Li+ concentration profile near the working electrode, and the image shows that very uniform concentration profile was formed in this case (the precursor of Li dendrite was not detected.). Not only Li+ surface concentration but also concentration boundary layer thickness are focused to discuss the Li+ ionic mass transfer phenomenon. Especially, solvated ionic liquids are new electrolytes for batteries and then it is very important to understand the ionic mass transfer in the solvated ionic liquids. We would like to discuss the relationship between the morphological variations of electrodeposited Li metal and the developing the concentration profile in solvated ionic liquids systems. Figure 1

Gravitational Level Effects on the Coupling Phenomena between Mass Transfer and Morphological Variation Rate

Gravitational Level Effects on the Coupling Phenomena between Mass Transfer and Morphological Variation Rate T. Nishida 1) , K. Nishikawa 1) , H. Matsushima 1) , T. Wakatsuki 1) , T. Homma2) , M. Rosso 3) and Y. Fukunaka4) 1) Dept. of Energy Sci. & Tech., Kyoto University 2) Dept. of Applied Chemistry, Waseda University 3) Ecole Polytecnique, Paris 4) Nanotechnology Research Center, Waseda University e-mail: hirofukunaka@gmail.com Development of advanced battery is the key for future space engineering field. Long life cycling must be maintained as well as the safety issue. Moreover, battery life must be predicted to reduce the number of space debri. Such technology achievements in space engineering field should be fed-back to the terrestrial applications including large scale energy storage devices. Generally speaking, battery reversibility is frequently governed by electrode surface flatness. Metal electrodeposition may generate irregular deposits with various morphologies. Coupling phenomena between the morphological or microstructural variations and ionic mass transfer rate must be fully understood to tailor the interface to create the unique physical properties as well as the ultra-high energy storage capacity. It is also indispensable for us to prolong the charging/discharging operation of secondary battery after longer term utilization under various environments. For liquid electrolyte, a precise study of these morphologies may be complicated by convective motion caused by gravitational force of buoyancy. Now the coupling phenomena are discussed in the case of Li in organic solvent PC or ionic liqid and Ag (or Cu) in AgNO3 (CuSO4) aqueous solution because they have been employed as negative electrode in space application as well as commercial use. The dendrite growth behavior of Li metal galvanostatically electrodeposited on Ni substrate in LiClO4-PC electrolyte was in-situ observed by a laser scanning confocal microscope. A Li dendrite precursor is stochastically evolved on Ni substrate probably through SEI. The initiation period of dendrite precursor becomes shorter with increasing current density and decresing LiClO4 concentration. Once it has been initiated, the ionic mass transfer rate starts to control the growth process of dendrite arm length, exceeding over the surface chemistry determining step. In order to excule the complexities due to SEI formation during the dendrite formation process, more simplified coupling behavior was studied in aqueous solution. 3M AgNO3aqueous solution was contained in a quasi-two dimensional electrolytic cell. Holographic interferometry technique was employed to monitor the ionic mass transfer rate. The thickness of electrolyte layer sandwiched by two slides glasses was measured to be 55 to 70 micrometer. The concentration profiles were formed around dendrite tips of silver electrodeposited at extremely high current densities. A uniform diffusion layer was formed along the cathode surface at the start of electrolysis. Electrochemical nucleation and growth phenomena on a substrate is expected under a considerably high overpotential. Following a certain incubation period, several dendrite tips began to protrude into the diffusion layer. The growth rates of these dendrites were related to concentration gradients around the tips, diffusivity, and transfer number of Ag+ ion. Giant-Macro step flow accompanied with dendritic growth of electrodeposited Ag in AgNO3 solution was dynamically observed in the image of Laser Confocal Scanning Microscope. These experiments must be engaged in microgravity facility which provides an ideal environment to built the physical model on electrolyte/electrode interfacial coupling phenomena without natural convection.

Coupling Phenomena Between Micromorphological Evolution and Ionic Mass Transfer Rate during Ag Electrodeposition in AgNO3 Aqueous Solution

The rechargeable battery concept with maltreat tolerance and safety is essentially indispensable not only in the space application but also in terrestrial renewable energy storage systems. Higher energy density, power capability and longer life requirements make the targets more challenging. A better understanding of the electrolyte/electrode interface phenomena including electrode kinetics is crucial. “Electrochemical Nucleation & Growth” aspect is a fundamental approach how to control the coupling phenomena between nano-structural interface evolution and ionic mass transfer rate enclosed a meso-scale space filled with electrolyte. Such a research has not been frequently challenged in designing the longer life energy storage devices. Metal negative electrode surface shows various morphologies after repetition of electrodeposition and electrochemical dissolution reaction. Dendrite precursor is finally grown after electrochemical nucleation and growth process where the coupling phenomena govern. It is also indispensable for us to prolong the charging/discharging operation of secondary battery after longer term utilization under various environments. For liquid electrolyte, a precise study of these morphologies may be complicated by convective motion caused by gravitational force of buoyancy. Now the coupling phenomena are discussed in the case of silver because it has been employed as negative electrode in space application. 3M AgNO3 aqueous solution was contained in a quasi-2D cell. Holographic interferometry was employed to monitor the ionic mass transfer rate. The concentration profiles were formed around dendrite tips of silver electrodeposited at extremely high current densities. A uniform diffusion layer was formed along the cathode surface at the start of electrolysis. Following a certain incubation period, several dendrite tips began to protrude into the diffusion layer. The growth rates of these dendrites were related to concentration gradients around the tips, diffusivity, and transfer number of Ag+ ion. Giant-Macro step flow accompanied with Ag dendritic growth was dynamically observed with Laser Confocal Scanning Microscope. Transient current variation measurements are now engaged under the double pulse potential which introduces Ag nucleus on the foreign substrate in order to develop a multi-scale physical model of dendritic growth.

Electrolyte Circulation in Copper Refinery

The importance of natural convection accompanied with electrochemical reactions along vertical electrodes has been recognized in the field of industrial electrochemical engineering as well as electrodeposition. Brenner applied a freezing method to analyze the concentration distribution[1]. Wagner demonstrated that the limiting current density distribution along the vertical cathode was governed by natural convection, based on the boundary layer theory [2, 3]. Wilke, Eisenberg and Tobias described a similarity between the natural convection caused by electrodeposition and by heat transfer [4]. Ibl and Muller examined the ionic mass transfer rate by the interferometry technique [5]. Since then, numerous studies have been carried out on the ionic mass transfer rate associated with natural convection caused by the electrochemical deposition and dissolution of copper in unstirred CuSO4-H2SO4aqueous electrolyte solutions [6-15]. However, most studies have focused on the steady state phenomena along a relatively short electrode at most 15 cm high confined in semi-infinite electrolyte media and have neglected the effects caused by the existence of the counter electrode. In industrial scale of copper refinery, vertical impure casted anode and starting sheet of electrolytic copper cathode are installed in the tank. Significant electrolyte stratification may frequently cause various troubles to induce poor cathode quality. Each refinery designs their own electrolyte circulation system with their confidence. Additive control technique is a key technology. Sometime a slightly different circulation system is introduced even in one refinery in the same company. It is simply because the principle of electrolyte circulation has been never discussed from the fundamental electrochemical engineering point. It may be a starting point for this problem in such a modernized computational technology.

Gravitational Effects on the Initial Stage of Cu Electrodeposition

The electrochemical processing has been favorably reviewed in the microelectronics industry. It is essential to control the nucleation and crystal growth in the initial stages of electrodeposition process. For this purpose many effects of additive have been examined. However, few quantitative examinations has been reported on the coupling phenomena of ionic mass transfer rate and microstructural/morphological variations through nucleation and crystal growth.This work discusses various gravitational levels as new parameters into the nonequilibrium electrochemical processing in order to create or tailor the unique physical properties in micrometer space field. It is fruitful in terms of not only physical chemistry but also industrial applications to comprehensively understand the coupling phenomena. In fact, it has been confirmed that the gravitational levels as well as the magnetic field introduce the intensive effects to the coupling behavior different from that under 1 G normal environment.Here is a report to analyze the effect of gravitational level on the initial stages of Cu electrodeposition on TaN or TiN substrate installed in a cavity-type electrolytic cell. In case of Cu on TaN substrate, cathode, anode and reference electrode are TaN substrate, copper sheet and copper wire, respectively. Electrolyte solution is 0.05 M CuSO4 – 0.05 M H2SO4. Two kinds of electrolytic cell configuration are used; one is a cathode facing downward over anode horizontally (C/A), and another is an anode over a cathode (A/C). A centrifugal field is applied to introduce a high gravitational field.Time dependence of nucleus number density shows that the nucleation of Cu on TaN is “progressive” nucleation. Under 1 G, saturated nucleus number density (N0) increases and mean diameter (dmean) becomes smaller as the electrode potential decreases. At any potential, N0 in A/C configuration is higher and dmean is smaller than C/A. The current transient shows the difference caused by two cell configurations and macroscopic natural convection due to the concentration difference should be inevitably induced with time progressed in a cavity in the A/C configuration. The image analysis indicates the nucleus distribution is considerably deviate from the Poisson distribution as long as the nearest-neighbor distance concerns. The existence of “exclusion zone” around the growing nuclei in nucleation phenomena has been found. Under high G level field, N0 becomes slightly increased and is saturated as G level increases. The effect of G level on the current transient is quite large. It is almost saturated under G level over 100 G. It is moreover found that the nucleation is saturated under high G level earlier than under low G level. The effect of ionic mass transfer rate due to the concentration difference becomes significant to enhance the nucleation rate knN0.

Optical observation of Li dendrite growth in ionic liquid

The electrodeposition process of Li metal in 1.0M LiTFSI–ionic liquid (N-methoxymethyl-N-methylpyrrolidinium) bis(trifluoromethane-sulfonyl) imide was observed in situ by optical microscope. Morphological variations of electrodeposited Li dendrite and its growth rate were examined and the ionic mass transfer rate in the vicinity of the electrode surface was discussed. Once the dendrite starts to grow, its length is proportional to the square root of time. This indicates that the Li+ mass transfer rate affects its growth. Dendrite growth process can be classified into two regions depending on its growth rate: the initiation period and the growing period with swinging behavior probably caused by residual stress.

Coupling Phenomena between Zinc Surface Morphological Variations and Ionic Mass Transfer Rate in Alkaline Solution

A horizontal upward-facing Zn anode was electrochemically dissolved in aqueous alkaline electrolyte. A plane-parallel upward-facing Ni(OH)2/NiOOH cathode was combined with the Zn anode in a single cell, divided by a polymer separator. This configuration can be regarded as a single pore model inside the porous space of a Zn-NiOOH battery cell. It also insures that the ionic mass transfer rate by diffusion and migration mechanism is dominant over that by natural convection. The morphological variations of the Zn anode surface were examined during the discharge operation of the Zn-NiOOH model cell. Two-dimensional transient concentration profiles of each ion accompanying the electrochemical dissolution of Zn anode in alkaline electrolyte were numerically calculated. The surface morphological variations of the Zn anode were discussed with the aid of numerical simulation. ZnO precipitation on the Zn anode surface was confirmed to be dependent on the horizontal distance from the separator. The present numerical model provides insight to the analysis of observed morphological variations along the Zn anode surface.

The effect of hydrodynamic conditions on the rate of copper cathodic reduction from aqueous ethanol environments

The effect of hydrodynamic conditions on copper electroplating from aqueous ethanol CuSO4 solutions is studied. Adding ethanol is found to result in a decrease in the ionic mass transfer rate in the solution. Experimental data are analyzed on the basis of the convective diffusion equation for a rotating disc electrode. Diffusivities of copper cations and kinetic current densities are estimated.

Morphological Variation of Electrodeposited Li in Ionic Liquid

Li metal negative electrode is expected for the next-generation Li battery electrode material, and the control of morphological variation of the Li metal electrode surface is very important issue. In-situ observation of the Li dendrite growth in an ionic liquid by an optical microscope was done in order to discuss the relationship between the Li dendrite growth and the Li+ ionic mass transfer rate. Li dendrite growth in the ionic liquid starts before the surface Li+ ion concentration which is calculated by a simple model become zero. The development of the dendrite length shows the linearity with the square root of time, and this slope has a bending point just after the Li+ ion is depleted. These phenomena mean the Li+ ionic mass transfer rate is also an important parameter for the Li dendrite growth, as well as the surface state of the Li metal electrode.

(Invited) Zinc Surface Morphological Variations and Ionic Mass Transfer Rates in Alkaline Solution

The present simplified electrochemical model of a horizontal upward-facing Zn anode combined with a Ni(OH)2/NiOOH cathode thus reasonably simulates the time-dependent coupling phenomena between the electrode surface morphological variations and the ionic mass transfer rate inside the model-pore mesoporous microstructure that extends across the width of a Zn battery cell.

Li dendrite growth and Li+ ionic mass transfer phenomenon

Li metal dendrite growth in LiPF6–propylene carbonate (PC) electrolyte was observed in situ by a laser scanning confocal microscope (LSCM) with a metallographic microscope. The development of the electrodeposited Li dendrite length was analyzed, and compared with dendrites grown in LiClO4–PC electrolyte solution. Different anions make different Solid Electrolyte Interphase (SEI) layers on the electrodeposited Li metal. This difference is one factor which causes the morphological variation and the different dendrite growth behavior of Li metal in these electrolytes. The relationship between the Li dendrite growth and Li+ ionic mass transfer rate associated with the electrodeposition of Li metal is examined, and dendrite initiation and growth rate are discussed. The authors find that ionic mass transfer plays an important role in dendrite growth behavior.

Zinc Surface Morphological Variations and Ionic Mass Transfer Rates in Alkaline Solution

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In Situ Observation of Dendrite Growth of Electrodeposited Li Metal

The dendrite growth behavior of Li metal galvanostatically electrodeposited on Ni substrate in a –propylene carbonate electrolyte solution was in situ observed by a laser scanning confocal microscope with a metallographic microscope. A Li dendrite precursor is stochastically evolved on Ni substrate probably through a solid electrolyte interphase layer produced by the surface chemical reaction between a reduced Li metal and an organic electrolyte. The measured length of randomly growing Li dendrite arms was statistically analyzed. The initiation period of the dendrite precursor becomes shorter with increasing current density and decreasing concentration. Once it has been initiated, the ionic mass transfer rate starts to govern the growth process of the dendrite arm length, exceeding over the surface chemistry controlling step. The dendrite arm length averaged over the substrate surface grows linearly proportional to the square root of time. The lower the concentration of , the steeper the inclination of the line at , whereas the concentration dependence of inclination is not evident at .

Ionic Mass Transfer Rate Accompanying Pulsed Current Electrodeposition of Silver

Ionic Mass Transfer Rate Accompanying Pulsed Current Electrodeposition of Silver

Numerical Calculation of Ionic Mass-Transfer Rates Accompanying Anodic Zinc Dissolution in Alkaline Solution

Transient ionic mass-transfer rates during the anodic dissolution of an upward-facing horizontal zinc electrode in a 10 wt % (1.95 M) KOH aqueous electrolyte were monitored by holographic interferometry. The anode potential during zinc dissolution was also measured against a HgO/Hg reference electrode. A two-dimensional mathematical model, wherein a supporting electrolyte also takes part in the electrochemical reactions, was developed to characterize the electrolyte transport phenomena. Transient mass-transfer rates of both and ions were numerically analyzed, and calculated transient refractive index profiles and anode overpotentials were compared to measured values. The electrochemical dissolution and passivation phenomena of the zinc anode in an alkaline electrolyte solution were discussed.

Numerical Simulation of Ionic Mass-Transfer Rates with Natural Convection in CuSO4 – H2SO4 Solution: II. Comparisons Between Numerical Calculations and Optical Measurements

A mathematical model is developed in Part I of this study for the ionic mass-transfer rates associated with natural convection developing along both electrodes immersed in a aqueous electrolyte. The additional effect of an excess amount of is discussed through the comparisons with the optical measurements. The concentration profiles of both and ions and the natural convective velocity profile have been measured by a two-wavelength holographic interferometer and the tracer method. The present calculation quantitatively agrees with the measured ionic mass-transfer rates toward the electrode surface except for copper electrolysis in an unsupported electrolyte above one-half of the limiting current density. The optical observation suggests that a substantially steady state is attained within 180 s after starting the electrolysis in every case. The numerical calculation predicts a further development of ionic mass-transfer phenomena over 600 s. It is closely related to both secondary flow and electrolyte stratification phenomena.

Numerical Simulation of Ionic Mass-Transfer Rates with Natural Convection in CuSO4 – H2SO4 Solution: I. Numerical Study on the Developments of Secondary Flow and Electrolyte Stratification Phenomena

The ionic mass-transfer rates accompanying natural convective electrolyte flow in a aqueous electrolyte solution acidified with an excess amount of are numerically analyzed. The effects of a supporting electrolyte and an interaction behavior between both cathodic upward and anodic downward natural convections are examined. Both anodic and cathodic current density distributions along the vertical height are also calculated. A mathematical model is extended by incorporating an additional boundary condition at the limiting current. A measure of ionic migration effect ε, a ratio of limiting current to limiting diffusion current, involving the transference number of a discharging metallic ion is introduced for this purpose. The present calculation predicts the oscillation behaviors in transient variations in both electrode surface concentration and maximal natural convective velocity, which are deeply related to the periodic fluctuating electrolyte flow patterns distorted by secondary flow. The addition of maintains an ε value around 1 and prevents the further development to the transition or turbulent natural convection.