Current-voltage relation and charge distribution in mixed ionic electronic solid conductors

Riess, I.

doi:10.1016/0022-3697(86)90121-6

Cited by 103 publications

(57 citation statements)

References 18 publications

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“…For an MIEC electrolyte, we adopt the model developed by Riess et al and Gödickemeier et al [20][21][22] However, our formulation is much simpler in that the gas concentrations at the electrode-electrolyte interfaces can be directly obtained from the solution of the porous electrode model, which had to be evaluated through an assumed transport model for the porous electrodes in these references in order to take the concentration overpotential into account correctly. As a result, our description of the MIEC electrolyte employs Eq.…”

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confidence: 99%

Numerical Modeling of Single-Chamber SOFCs with Hydrocarbon Fuels

Hao

Goodwin

2007

J. Electrochem. Soc.

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A two-dimensional numerical model of a single-chamber solid oxide fuel cell ͑SCFC͒ operating on hydrocarbon fuels is developed. The SCFC concept is a simplification of a conventional solid oxide fuel cell in which the anode and cathode are both exposed to the same premixed fuel-air mixture, and selective catalysts promote electrochemical oxidation of the fuel at the anode and simultaneous electrochemical oxygen reduction at the cathode. Optimization of SCFC stacks requires considering complex, coupled chemical and transport processes. The model accounts for the coupled effects of gas channel fluid flow, heat transfer, porous media transport, catalytic reforming-shifting chemistry, electrochemistry, and mixed ionic-electronic conductivity. It solves for the velocity, temperature, and species distributions in the gas, profiles of gaseous species and coverages of surface species within the porous electrodes, and the current density profile in an SCFC stack for a specified electrical bias. The model is general, and can be used to simulate any electrode processes for which kinetics are known or may be estimated. A detailed elementary mechanism is used to describe the reactions over the anode catalyst surface. Different design alternatives including yttria-stabilized zirconia vs Ce 0.8 Sm 0.2 O 1.9 electrolytes, the effects of mixed conductivity, and the optimal fuel-to-air ratio are explored. A single-chamber fuel cell ͑SCFC͒ is one in which the anode and cathode are both exposed to the same premixed fuel-air stream, and selective electrocatalysts are used to preferentially oxidize the fuel at the anode and reduce oxygen at the cathode. The latest studies of SCFCs demonstrate great improvements in power density and reduction in operating temperature with innovations in material and system design.1-4 While the efficiency is typically lower than that of conventional dual-chamber solid oxide fuel cells ͑SOFCs͒, SCFCs do not require seals, and allow a very simple gas manifold design. For some applications, notably small-scale power generation, the mechanical simplicity of an SCFC design may make it more attractive than a dual-chamber SCFC, even with a lower efficiency.The need for selective electrocatalysts has several implications for the design of an SCFC. First, an SCFC must operate at a temperature low enough that the catalysts maintain some degree of selectivity; this typically limits the temperature to below 700°C, which is significantly lower than that of conventional SCFCs with an yttria-stabilized zirconia ͑YSZ͒ electrolyte. For this reason, SCFCs demonstrated to date have used ceria-based electrolytes, rather than YSZ.Another implication of the need for selective electrocatalysts is that an SCFC is unlikely to run well, if at all, on hydrogen. Any catalyst that promotes electrochemical oxidation of hydrogen, or electrochemical reduction of oxygen, would very likely promote direct catalytic combustion if exposed to a hydrogen-air mixture. This problem can be dealt with by using a hydrocarbon fuel instead of hydr...

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Numerical Modeling of Single-Chamber SOFCs with Hydrocarbon Fuels

Hao

Goodwin

2007

J. Electrochem. Soc.

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“…In Equation 13, the direction of I e_drift must be the same as that of I i throughout the electrolyte. Therefore, I e_drift should be recycled as I e_diffusion throughout the whole electrolyte.…”

Section: E3192mentioning

confidence: 99%

Equilibration Process in Response to a Change in the Anode Gas Using Thick Sm-Doped Ceria Electrolytes in Solid-Oxide Fuel Cells

Miyashita¹

2017

J. Electrochem. Soc.

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The electronic conductivity of Sm-doped ceria is very low in air but increases substantially in H 2 gas. Conventional models can explain the equilibration processes of yttria-stabilized zirconia electrolytes and thin mixed ionic-electronic conducting electrolytes in response to variations in the fuel composition. However, the equilibration processes of thick samaria-doped ceria electrolytes have not yet been explained. We measured and attempted to explain the equilibration process of a very thick (6.6 mm) samaria-doped ceria electrolyte in response to a change in the anode gas. The measured open-circuit voltage gradually increased to an equilibrium voltage of 0.80 V within 5 min. However, based on the chemical diffusion coefficient equation for the electron diffusion current, the equilibrium time should have been much longer than 5 min. When we assumed a current-independent constant anode voltage loss (0.35 V), the calculations were substantially improved for determining the experimental results. Solid-oxide fuel cells (SOFCs) directly convert the chemical energy of fuel gases, such as hydrogen and methane, into electrical energy. SOFCs use a solid-oxide film as the electrolyte, and oxygen ions serve as the main charge carriers. Typically, yttria-stabilized zirconia (YSZ) is used as the electrolyte material in these cells. The current operating temperature of these cells (873-1273 K) should be reduced to extend their lifespans. Therefore, the use of more advanced ionconducting electrolyte materials, such as samaria-doped ceria (SDC) electrolytes, at lower temperatures is preferred. The open-circuit voltage (OCV) of an SDC cell is approximately 0.8 V, which is lower than the Nernst voltage (V th ) of 1.15 V at 1073 K. The low OCV is attributed to the low value of the ionic transference number (t ion ).The model described by Duncan and Wachsman 1,2 can explain the equilibration processes of thin and thick YSZ electrolytes and thin mixed ionic-electronic conducting (MIEC) electrolytes in response to higher transient voltages or variations in the fuel composition. However, the equilibration process using thick SDC electrolytes in response to a change in the anode gas remains unexplained. In this work, we measured the equilibration process using a very thick (6.6 mm) SDC electrolyte in response to a change in the anode gas and used Wagner's Equations 3, 4 to calculate the oxygen chemical potential profile. Using Rickert's approach, 5 we subsequently analyzed the drift current and electron diffusion current throughout the entire electrolyte under open-circuit conditions. Weppner's method 6,7 was used to calculate the equilibration rate of the electron diffusion current. On the basis of these calculations, we showed that the equilibration time of the electron diffusion current is the limiting factor when using a thick SDC electrolyte. Consequently, the equilibration rates should differ from those obtained using thin and thick YSZ electrolytes and thin MIEC electrolytes. Furthermore, we discussed the problems that occur...

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“…Equation 9 is a classical equation that is still used for modern theoretical calculations [3], which is right within the limits of linear transport theory. T. Miyashita The generalized form of equation 9 allows for Ie to be calculated from the constant field approximation [4].…”

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confidence: 99%

Theoretical Verification Necessity of Leakage Currents Using Sm Doped Ceria Electrolytes in SOFCs~!2009-01-29~!2009-03-03~!2009-03-19~!

Miyashita¹

2010

TOMSJ

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Numerous approaches have been made to solve the basic transport equation that describes a solid oxide fuel cell (SOFC) with mixed conduction. Classically, the open circuit voltage (OCV) is calculated with Wagner's equation, which is right within the limits of linear transport theory. In order to generalize Wagner's equation, many models have been proposed to describe the current-voltage relation in mixed ionic electronic solid conductors (MIEC) using constant field approximation to calculate an electronic current. However, experimental verification necessity of leakage currents in SOFCs using Sm doped Ceria electrolytes has already been pointed out both qualitatively and in quantities. Using the constant field approximation, the limits of linear transport theory can not be clear. In this report, a new model is expressed without using the constant field approximation. This model follows from Wagner's equation and continuity. The calculated electronic current for doped Ceria electrolyte matches the values calculated using conventional models. But the electrical field near the cathode is large enough to cause dielectric breakdown which has never been reported. Continuity is not deniable, so there are limitations in Wagner's equation, coming from the limits of linear transport theory.

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Current-voltage relation and charge distribution in mixed ionic electronic solid conductors

Cited by 103 publications

References 18 publications

Numerical Modeling of Single-Chamber SOFCs with Hydrocarbon Fuels

Numerical Modeling of Single-Chamber SOFCs with Hydrocarbon Fuels

Equilibration Process in Response to a Change in the Anode Gas Using Thick Sm-Doped Ceria Electrolytes in Solid-Oxide Fuel Cells

Theoretical Verification Necessity of Leakage Currents Using Sm Doped Ceria Electrolytes in SOFCs~!2009-01-29~!2009-03-03~!2009-03-19~!

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