Experimental and Theoretical Study of Concentration Distributions in a Model Pore Electrode: I . Measurement of Two‐Dimensional Concentration Gradients in a Zinc Model Pore

McLarnon

2006

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: Methodsmentioning

confidence: 99%

mentioning

confidence: 99%

Ionic Mass Transfer Accompanying Anodic Dissolution of Zinc in Alkaline Solution

Arise

McLarnon

2006

“…An electrochemical cell with a horizontal upward-facing Zn anode nearby a plan-parallel Ni(OH) 2 /NiOOH electrode has been used frequently in order to suppress the natural convection induced by buoyancy forces. [22][23][24][25] Eisenberg pointed out the importance of natural convection and diffusion under gravity field effects. 26 We also recognized natural convection was very important, especially considering the use of a secondary battery for large-scale storage in an electrical energy network system.…”

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confidence: 99%

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

Arise

Kawai

et al. 2012

A horizontal upward-facing Zn anode was electrochemically dissolved in aqueous alkaline electrolyte. A plane-parallel upwardfacing 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.Rechargeable batteries for electric vehicle (EV) and hybrid electric vehicle (HEV) applications have been a subject of intense focus for the last decade or so. Although Li-based systems have received the most attention, there has been increasing interest in Zn-based systems because of their lower costs, material abundance, and compatibility with nonflammable aqueous electrolytes. When a Zn anode is used in a secondary battery, its morphological variations introduce some serious problems of short life, poor reliability, and safety issues. The coupled phenomena between ionic mass transfer rates and electrode surface morphological variations must be well understood, in order to develop a fully optimized rechargeable Zn battery.The ultimate goal of our studies is to incorporate the fundamental knowledge of Zn electrochemical processes into a numerical model for a hybrid electric vehicle (HEV) battery as well as large-scale battery operation. The model may predict optimal conditions for operations such as high-rate, short-duration, shallow charge/discharge cycles in alkaline solution. In addition, it can provide a new insight to control HEV batteries for the depletion of hydroxide (OH − ) ion at rates greater than 10 mA/cm 2 .Many researchers have studied the surface morphological variations of Zn anodes in alkaline solution over the past few decades. Powers and Breiter first studied the surface morphology of ZnO precipitates by optical microscopy. 1 They classified ZnO into two categories as type I and type II films. Type I films are white, loose, and flocculent, and they appear to homogeneously precipitate in a supersaturated solution layer of zincate (Zn(OH) 4 2− ) ion near the electrode surface in the absence of convection. Type II films are light gray to black, more compact, and appear to form directly on the surface rather than by homogeneous precipitation in the absence of convection.Szpak and Gabriel carri...

“…Many experimental and numerical studies have been carried out with both planar and porous Zn electrodes. [1][2][3][4][5][6][7][8][9][10] However, only a few papers have been reported to compare the numerically calculated Zn concentration profiles with the measured profiles. [5][6][7][8] During the discharge of an alkaline cell, Zn is oxidized to form a highly soluble Zn͑OH͒ 4 2− ion with a concomitant OH − ion consumption, as described in the electrochemical reaction ͑Eq.…”

mentioning

confidence: 99%

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

Arise¹,

Kawai

et al. 2010

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 Zn(OH)42− and OH− 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.