Transient Natural Convection near a Plane Vertical Electrode Surface after Reversing the Electrolytic Current

McLarnon

2006

J. Electrochem. Soc.

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Holographic interferometry was applied in situ to measure the ionic mass-transfer rates near upward-facing zinc anodes dissolving at high current densities in aqueous KOH solutions. The time-varying interference fringe patterns were converted to individual concentration profiles of Zn͑OH͒ 4 2− , K + , and OH − ions. The measured ionic concentration profiles agreed well with those predicted by a transient diffusion model for a multicomponent electrolyte. When a zinc anode was electrochemically dissolved in 3 wt % KOH electrolyte, the anode potential abruptly shifted in the noble direction, immediately followed by gas evolution. The onset time for this potential rise and accompanying gas evolution agreed well with the calculated time interval required to drive the OH − ion concentration to zero at the zinc electrode surface. The present model can be applied to not only optimize zinc cell designs but also develop a control method for trouble-free, safe operation of a zinc battery without gas evolution.The increasing international awareness of energy and environmental issues has accelerated the development of rechargeable batteries for applications such as fuel cell electric vehicles ͑FCEVs͒ and hybrid electric vehicles ͑HEVs͒. Batteries for mobile telephones, laptop computers, FCEVs, and HEVs share the common requirement of high specific energy and high volumetric energy density; however, FCEV and HEV batteries also require high power density, very low cost, fail-safe operation in series strings, wide operating and survival temperature ranges, and so on. Furthermore, FCEV and HEV batteries must undergo repeated high-rate, shortduration, shallow charge/discharge cycles near a fixed state of charge ͑SOC͒, rather than the slower, deep charge/discharge cycles characteristic of mobile-telephone or laptop-computer batteries. 1 Thus, the search for electrode materials optimized for FCEV or HEV applications is more challenging than that for mobiletelephone or laptop-computer battery materials. In addition to the obvious need to minimize all battery, cell, and component resistances in order to maximize FCEV and HEV battery power and efficiency, there is a strong need to minimize concentration overpotential. This is a particularly complex and challenging problem because during repeated shallow charge/discharge cycles, the concentration overpotential will change due to the accumulated superposition of transient ionic concentration profiles. This superposition occurs because during typical HEV battery operation there is little or no rest period between charging and discharging cycles to allow the electrodes and electrolyte to equilibrate. An important criterion for the design of a rechargeable battery for HEV or FCEV applications is that the charge/discharge cycle must not result in excessive overpotentials.Among the present and candidate batteries for FCEV and HEV are lead acid, nickel cadmium, nickel zinc, nickel metal hydride, and Li-ion. Each of these batteries has certain advantages and disadvantages, which have been discu...

Section: Analysis and Discussionmentioning

confidence: 99%

Ionic Mass Transfer Accompanying Anodic Dissolution of Zinc in Alkaline Solution

Arise

McLarnon

2006

J. Electrochem. Soc.

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“…Its usefulness in a conventional electrolytic cell with taller electrodes has been confirmed in transient electrolysis. [17,18] According to this theory, the pure diffusion model can be applied to a nondimensional time T(ϵDt/(␦ B ) 2 ) of less than 0.1, while a steady-state natural convection is attained after T ϭ 0.4.…”

Section: Discussionmentioning

confidence: 99%

Electrochemical interfacial phenomena under microgravity: Part I. Anodic dissolution of copper in drop shaft

Tomii

Konishi

et al. 1999

Metall Mater Trans B

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As part of a series of physical modeling studies on the electrochemical etching of a printed wiring board, copper foil was anodically dissolved in a small cavity. Coupling of the ionic mass transfer rate with the undercutting phenomena must be precisely understood in order to finely control high aspect ratio anisotropic etching. Interferometry has been applied to the study of the ionic mass transfer rate accompanying electrochemical reactions, and it may help in the visualization of the concentration profile of the metal ion within a narrow cavity. As an idealistic case, the electrochemical dissolution of copper in a purely diffusional field, especially the Cu 2+ ion transfer rate itself, is now the focus of attention. The copper anode was galvanostatically dissolved in an aqueous solution (0.1M CuSO 4 -1M H 2 SO 4 ) contained in a quasi-two-dimensional electrolytic cell for 8 seconds during free fall in a drop shaft. A disk type of anode, 1 mm in diameter and 100 m in thickness, and a flat shaped ring cathode 20 mm in inner diameter were sandwiched by two sheets of slide glass. The electrolyte layer was 200 m in effective thickness. The interference fringe pattern accompanying the copper dissolution was measured in situ with a common path-type microscopic laser interferometer. Terrestrial experiments were also conducted with a horizontally installed cell under the same electrolytic conditions. No natural convection would be expected in the present electrolytic cell design with such a shallow and horizontal electrolyte layer. The distance between the anode surface and the location at which the outer interference fringe appeared was measured by the duration time. It increased linearly with the square root of time under microgravity, but started to deviate from this linearity a few seconds after starting the electrolysis in the terrestrial experiment. A significant difference in the growing distance between the environments had not been a priori anticipated for such a quasi-twodimensional cell. The local current density distribution along the smaller electrode surface may be influenced by induction of a kind of natural convection.

“…The ratio of sum of squares due to regression to the total sum of squares was calculated to be 0.998. concerning the properties of phthalocyanines (2)(3)(4)(5), namely, electrochemical and chemical redox equilibria (6)(7)(8), suggests a reversible reduction of up to four electrons per molecule. The apparent discrepancies gave rise to a more detailed investigation of the electrochemistry of the cell.…”

Section: Appendixmentioning

confidence: 99%

“…The optical methods such as Jamin (3), Mach-Zehnder (4), and holographic interferometry (5,6) are effective for the measurement in the single-component aqueous solutions because of the proportionality between the ionic concentration and the refractive index of the solution.…”

mentioning

confidence: 99%

Measurement of Concentration Profiles of Cu2+ Ion and H+ Ion near a Plane Vertical Cathode by Two‐Wavelength Holographic Interferometry

Denpo

Okumura

et al. 1985

J. Electrochem. Soc.

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A technique of two-wavelength holographic interferometry was applied to simultaneously measure the concentration profiles of Cu 2~ ion and H + ion near a plane vertical cathode placed in an unstirred aqueous solution containing CuSO4 and excess H2SO4. It was revealed that the measured concentration profiles of Cu 2+ ion and H + ion are approximated with the parabolic equations of horizontal distance from cathode surface and that the concentration gradient of Cu 2+ ion at the cathode surface is larger in CuSO4-H2SO4 solution than in CuSO4 solution at the same cathodic current density and vertical distance. The following matters were also clarified: (i) the nondimensional rate of ionic mass transfer is shown as a function of Ray]eigh number based on the boundary layer theory, and (ii) the effect of the addition of excess H2SO4 is expressed by a parameter f which is the ratio of Rayleigh numbers in CuSO4-H2SO4 solution and in CuSO4 solution under the conditions of the uniform distribution of cathodic current density and an excess of H2SO4.The measurement of cathodic concentration profile of ion is important for elucidating not only the ionic mass transfer itself, but also the effect of supporting electrolyte (1, 2). The optical methods such as Jamin (3), Mach-Zehnder (4), and holographic interferometry (5, 6) are effective for the measurement in the single-component aqueous solutions because of the proportionality between the ionic concentration and the refractive index of the solution.In the industrial electrolysis, such as the electrolytic refining of copper, for example, an excess of sulfuric acid is added. In the multicomponent aqueous solutions, the above proportionality no longer holds and the measurement of concentration profiles of ions becomes more difficult.An experimental technique using an image sensor of one-dimensional photodiode array was developed in order to measure the concentration profile of Cu ~+ ion in aqueous CuSO4-H2SO4 solution (7). In this measurement of the light absorbance distribution, however, a noise caused by the light diffraction near the cathode surface was superimposed on the output signal, and the concentration of Cu 24 ion on the cathode surface was estimated by extrapolating the profile toward the cathode surface. The estimation of the concentration of H + ion on the cathode surface was tried by subtracting the light absorbance due to Cu 2+ ion from the refractive index of the solution measured by the conventional holographic interferometry (5). Due to the error of the measurements and the extrapolation, however, the precision and accuracy of the estimated H + ion concentration was relatively poor.It is the intent of the present work to carry out the simultaneous measurement of concentration profiles of Cu 2+ ion and H + ion in the cathodic boundary layer by using a technique of two-wavelength holographic interferometry which is based on the dispersion effect of refractive index of the solution. The problem due to diffraction pattern appearing near the cathode surface can be avo...