Current Distribution at a Gas-Electrode-Electrolyte Interface

An analysis is made of the roles of convection and concentration polarization during oxidation of hydrogen in acidic supermeniscus films. In this preliminary analysis activation overvoltage is assumed constant throughout the reaction zone. It is concluded that both convection and concentration polarization are important, and that they are in part responsible for formation of films above the normal meniscus. It is postulated that concentration polarization induces water recireulation, up in the surrounding vapor phase, and down as a gravity-driven falling film on the anode surface. Calculated film heights and thicknesses are in agreement with published observations. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.6.218.72 Downloaded on 2015-06-01 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.6.218.72 Downloaded on 2015-06-01 to IP Vol. 113, No. 6 ELECTROCHEMICAL PROCESSES IN THIN FILMS ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.6.218.72 Downloaded on 2015-06-01 to IP

“…The purity of the oxygen used has been reported previously (3). rod was obtained from the Carborundum Metals Climax Company.…”

Section: Methodsmentioning

confidence: 99%

Section: Electrochemical Processes In Thin Filmsmentioning

confidence: 99%

See 1 more Smart Citation

Electrochemical Processes in Thin Films

Lightfoot¹

1966

“…5). However, as Dm increases, the relationship is no longer linear, but distinctly curved, as shown in tributions are found along the meniscus of a liquid electrolyte fuel cell electrode (8). However, in the case of solid electrolyte fuel cell anodes, the high current densities that are developed in the vicinity of the electrode-electrolyte interface will produce large heat effects, due to resistance heating in the electrode, which may in turn lead to rupture of the electrodeelectrolyte contact.…”

Section: Discussionmentioning

confidence: 99%

Diffusional Processes in Solid Electrolyte Fuel Cell Electrodes

Zahradnik

1970

A mechanism is proposed to account for the polarization voltage loss encountered in the electrochemical oxidation of CO at porous, high-temperature, solid-electrolyte fuel cell anodes. The mechanism involves the transport of gaseous CO and oxygen ions to reaction sites, followed by slow electrochemical reaction. The mechanism results in an involved polarization expression, which under certain conditions simplifies to the familiar Tafel form.High-temperature fuel cells employing either calcia or yttria stabilized zirconia electrolyte have received considerable attention as potential power generators using gaseous fuels derived from coal (1, 2). At the 1000~ operating temperature of solid electrolyte fuel cells, such coal-derived gases as well as prospective fuels such as reformed hydrocarbons will contain large amounts of CO. Oxidation of CO at metallic and mixed oxide fuel cell anodes, however, is accompanied by large polarization losses (3). The presence of hydrogen reduces this loss somewhat, but it is not clear that either the high current densities which are expected from such cells or the complete combustion of CO to CO2 can be achieved without incurring severe voltage losses. For this reason, it is important to understand in a fundamental way the irreversibilities which occur at solid-electrolyte fuel cell anodes. The purpose of this paper is to propose a possible scheme by which this polarization occurs and to derive the equations which quantitatively relate polarization voltage loss to electrode current density.Many such analyses for liquid electrolyte fuel cell electrodes have appeared in the literature, and a complete review of the work in this area is not presented here. The pioneering work of Will should be mentioned, however, since it was the first to treat partially immersed electrodes and proposed a method for treating the electrolyte meniscus which considerably clarified the polarization phenomena (4). Will's work has been reviewed and generalized by Lightfoot, who included effects of convection as well as concentration polarization in his analysis (5).Not all liquid electrolyte fuel cell electrodes follow the thin-meniscus model, however, and analyses of flooded porous fuel cell electrodes have been made, e.g. Brown and Rockett (6).With a liquid electrolyte, it is possible for the gaseous species participating in the electrode reaction to reach the electrode reaction site either through a meniscus film or bulk electrolyte phase, and the reported polarization studies analyze the various ways this transport can occur. In the case of a solid electrolyte, however, such transport is impossible and alternate mechanisms must be responsible for electrode Key words: fuel cell, porous electrode, polarization (and/or) overvoltage, diffusion, limiting current, Damkohler number. polarization. One possibility is for the reactants (CO and oxygen ions) to counterdiffuse to a mutually acceptable location somewhere on the electrode, and there react. In an earlier paper, Zahradnik analyzed this situation for a one-...

“…ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.192.114 19. Downloaded on 2015-05-24 to IP…”

mentioning

confidence: 99%

A LiAl / Cl2 Battery with a Four‐Component Alkali‐Metal Chloride Electrolyte

Thomas¹,

Bennion²

1989

Self Cite

9ABSTRACT A LiA1/C12 cell operating at 280~ was investigated. The electrolyte is a mixture of LiC1, KC1, RbC1, and CsC1 with a eutectic melting point of 258~ The positive electrode is a gas-diffusion electrode formed by coating one side of a porous carbon electrode (Union Carbide PC-60) with PTFE. The limiting discharge current of the cell was controlled by solidstate diffusion of Li in the LiA1 alloy. Polarization of the C12 electrode was caused by the low cross-sectional area of the electrolyte film compared with the pore cross-sectional area. Deactivation of the positive electrode was caused by impurities, such as Cu § in the electrolyte. Mathematical models of the negative and positive electrodes in a LiA1/C12 cell with a gas diffusion C12 electrode have been formulated. A thin film gas diffusion electrode model was used for the positive electrode, while solid-state diffusion of Li in ~-LiA1 was assumed to limit the negative electrode and the cell current. The thin film is liquid salt on the porous electrode walls through which chlorine diffuses. Comparison of the experimental results with the model indicate that the diffusivity of Li in ~-LiA1 is of the order of 10 -12 cm2/s. Lithium and chlorine are attractive materials for the electrodes in high specific energy batteries because of their high reactivities, high cell potential, and low equivalent weights. An early demonstration of these reactants in an electrochemical system was the Li/LiC1/C12 cell constructed and tested at General Motors by Swinkels and coworkers (1-6). This cell used pure molten LiC1 and operated at 650~ Molten lithium was wicked into the cell from an external reservoir by stainless steel felt-metal (2). Chlorine was charged and discharged at a two-layer annular gas diffusion valve electrode (3).Discharge currents as high as 10 A/cm 2, which are possibly the highest steady-state battery discharge current densities ever reported, were measured for this cell (4). However, the following problems were encountered. (i) Alkali metals are soluble in their own molten salts, hence the electrolyte became electronically conducting. Chemical shorting of the lithium occurred, and the corrosiveness of the electrolyte increased. (ii) Graphite is not completely inert to C12, particularly at this temperature. It is especially corroded when used as an anode on charge. (iii) Sealing the cell was difficult.Li/C12 cells operating at lower temperatures were tested using 58 mole percent (m/o) LiC1-42 m/o KC1, which has a eutectic melting point of 352~ (7, 8). Cells similar to Swinkel's cell were operated for up to 323 cycles at coulombic efficiencies of 94-98% using the mixed electrolyte. It was observed that precipitation of KC1 in the electrolyte did not reduce the current. The following difficulties were encountered. (i) The graphite C12 electrode flooded because molten KC1 wets graphite. Thus C12 had to diffuse through a liquid layer which drastically reduces the mass transfer. (ii) Lithium metal reacted with potassium ions to form lithium ions and potassi...