Diffusional Processes in Solid Electrolyte Fuel Cell Electrodes

Abstract

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-...

Overpotential Behavior of Stabilized Zirconia Solid Electrolyte Fuel Cells

“…8 and 11 at 1000~ is 0.25, 1.1, ,~3, and ~0.6 mA/cm 2 for 0.016%, 0.77%, 50~/d, and 98.9% CO, respectively. In partial support of these data, minimum overpotential was recently observed at 50-70% CO (21), while Zahradnik (22) has shown that earlier results ( 14) exhibited a trend towards maximum exchange current densities at 50% CO. However, Eq.…”

“…12 will apply. Equation [19] simply becomes /o = kdol-a(1 --Oco) a [22] In CO-COs mixtures, 0co >> eo and, therefore, the fraction of free sites ,~1 --0co.…”

“…In the case of CO-CO2 mixtures, diffusion overpotential (6,13,20,21,23), transition overpotential (20,21), and reaction overpotential (20) at 800~176 have all been reported. Tafel behavior is often followed (2,6,14,20,21) although, except in two studies (20,22), mechanisms have not been given to account for the results. There is serious disagreement concerning the value of the transition factor (2,14,20) and the influence of gas composition on overpotential (13,14,20,21).…”

“…The overpotential behavior of such cells is of interest from both theoretical and practical viewpoints. Nevertheless, only a limited amount of research has been done in this area and there is considerable disagreement among the available results on the overpotential at metal or oxide electrodes in contact with ZrO2-based electrolytes and either O2-inert gas (2)(3)(4)(5)(6)(7)(8)(9)(10)(11), H2-H20 (12)(13)(14)(15)(16)(17)(18)(19), or CO-COs (2,6,13,14,17,(20)(21)(22)(23) mixtures.…”

Electrochemical Oxidation of CO on Pt and Ni Point Electrodes in Contact with an Yttria-Stabilized Zirconia Electrolyte

“…According to Cheboshin et al, 20 diffusion of CO and CO 2 molecules in the solid electrolyte plays a role in the reaction mechanism by increasing the width of the reaction zone along the three-phase boundary ͑TPB͒ between electrode, electrolyte, and gas. In a later paper, the authors, however, found charge transfer between CO and oxygen at the TPB was rate limiting and resulted in anodic limiting currents, 21 Zahradnik 22 also suggested a reaction mechanism in which the transport of gaseous CO and oxygen ions to reaction sites on the surface of a porous electrode was accompanied by a slow electrochemical reaction. Etsell and Flengas 6 confirmed this, concluding that charge transfer involving gaseous or adsorbed CO and gaseous CO 2 was the rate-determining step.…”