A Distributed Charge Transfer Model for IT-SOFCs Based on Ceria Electrolytes

Morteza Rahmanipour,Alfonsina Pappacena,Alessandro Donazzi,Marta Boaro

doi:10.1149/2.1911712jes

Morteza Rahmanipour, Alfonsina Pappacena + Show 2 more

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https://doi.org/10.1149/2.1911712jes

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Abstract

A distributed charge transfer model for IT-SOFCs with MIEC electrolyte and composite electrodes is developed. A physically-based description of the electronic leakage current in the electrolyte is included, together with mass and charge conservation equations. The model is applied to simulate experimental polarization curves and impedance spectra collected on IT-SOFCs consisting of SDC electrolytes, Cu-Pd-CZ80 infiltrated anodes and LSCF/GDC composite cathodes. Hydrogen electro-oxidation experiments are examined (H2/N2 humidified mixtures, 700°C, 30–100% H2 molar fraction). A significant increase of the ohmic resistance measured in the impedance spectra is revealed at decreasing the H2 partial pressure or increasing the voltage (from 0.71 Ω cm2 at 100% H2 to 0.81 Ω cm2 at 30% H2). Good agreement between the calculated and experimental polarization and EIS curves is achieved by fitting the exchange current density and the capacitance of each electrode. Model and theoretical analyses allow to rationalize the observed shift of the ohmic resistance, highlighting the key-role played by the electronic leakage current. Overall, the model is able to capture significant kinetic features of IT-SOFCs, and allows to gain insight into relevant parameters for the optimal design of the cell (electrochemically active thickness, current and potential distribution, mass diffusion gradients).

Highlights

Samaria doped Ceria (SDC) and Gadolinia doped Ceria (GDC) are reference electrolyte materials for intermediate temperature solid oxide fuel cell (IT-SOFC) applications, thanks to their high ionic conductivity between 500◦C and 700◦C.1–5 Associated to a high ionic conductivity, these materials show mixed ionic and electronic conductive (MIEC) properties, and their electronic conductivity increases when exposed to reducing atmospheres
The originality of the present results stems from the introduction of a scheme for the leakage current in the distributed charge transfer model, and refers to the simulations of electrochemical impedance spectroscopy (EIS) spectra in the presence of the leakage current
A reduction of the OCV from 0.87 V at 100% H2 to 0.86 V at 30% H2 is noted. These results are consistent with other literature reports dedicated to cells based on SDC electrolytes with similar thickness (Table IV): Lu et al.[35,36] find an OCV of 0.85 V when operating an infiltrated Cu-CeO2-SDC/SDC/LSCF cell with humidified H2 at 700◦C; Matsui et al.[37] measure OCV values between 0.85 and 0.90 V with different cells based on the same SDC electrolyte; Fang et al.[38] report an OCV of 0.81 V for Ni-SDC/SDC/Sr-cobaltite cells with 500 μm electrolyte thickness

Summary

Introduction

Samaria doped Ceria (SDC) and Gadolinia doped Ceria (GDC) are reference electrolyte materials for intermediate temperature solid oxide fuel cell (IT-SOFC) applications, thanks to their high ionic conductivity between 500◦C and 700◦C.1–5 Associated to a high ionic conductivity, these materials show mixed ionic and electronic conductive (MIEC) properties, and their electronic conductivity increases when exposed to reducing atmospheres. Starting from continuum rigorous descriptions, a series of models has been derived for the prediction of impedance spectra collected on MIEC electrolytes, the most important examples being those by Jamnik and Maier,[7] by the group of Haile[8,9] and by Atkinson et al.[10] These models propose closed-form, equivalent circuit-like equations, which allow to calculate several relevant parameters of mixed conductors by fitting the spectra, such as the activation energy, the conductivity and the mobility of the ionic and electronic transport processes, the enthalpy and the entropy of reduction, the electrolytic domain boundary, the chemical capacitance, as well as the concentration of the electronic carriers. A general review of the literature seems to indicate that, on the one hand, the available descriptions of MIEC-based cells are steady state, whereas, on the other hand, dynamic, charge-distributed models

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