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

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 (H 2 /N 2 humidified mixtures, 700 • C, 30-100% H 2 molar fraction). A significant increase of the ohmic resistance measured in the impedance spectra is revealed at decreasing the H 2 partial pressure or increasing the voltage (from 0.71 cm 2 at 100% H 2 to 0.81 cm 2 at 30% H 2 ). 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). 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, however, these materials show mixed ionic and electronic conductive (MIEC) properties, and their electronic conductivity increases when exposed to reducing atmospheres. The MIEC character of the electrolyte, in turn, results in the onset of electronic leakage currents (or short circuit currents) and low open-circuit voltage values. The leakage current has a chemical origin and is due to the partial reduction of Ce 4+ ions to Ce 3+ ions, prompted by the different chemical potential at the electrodes: this current is active also in the absence of an electric field applied on the cell, since it stems from the spontaneous transfer of ions across the lattice structure. As a consequence, the effects of the leakage current are not confined to the electrolyte, but extend to the electrodes, and entail a complex relationship with the current drawn from the cell. 6 Deeper insight can therefore be achieved by mathematical modeling.In the literature, comprehensive models can be found, which take into account the MIEC properties of Ce-based electrolytes. 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-l...

Fabrication and Testing of Copper/Gadolinium‐Doped Ceria‐Based Solid Oxide Fuel Cells Operating at Intermediate Temperature

“…The output data, i. e. OCV, power density, ohmic and polarization resistance are summarized in Table . The OCV values are comparable for R5, R10 and R15 cells, fluctuating from 0.79 V to 0.76 V. Nevertheless, the values measured at 650 °C are slightly lower than those previously reported in the literature, probably due to microstructural imperfections in the cells, although an electric leakage can not be totally excluded . Interestingly, the highest power density values are comparable with those recorded on other Cu/CeO 2 anode based cells operating at higher temperatures (800 °C) but realised with multi step procedure involving impregnation methods .…”

“…The OCV values are comparable for R5, R10 and R15 cells, fluctuating from 0.79 V to 0.76 V. Nevertheless, the values measured at 650°C are slightly lower than those previously reported in the literature, probably due to microstructural imperfections in the cells, although an electric leakage can not be totally excluded. [18] Interestingly, the highest power density values are comparable with those recorded on other Cu/CeO 2 anode based cells operating at higher temperatures (800°C) but realised with multi step procedure involving impregnation methods. [19] The R5 power output (94 mW cm À 2 ) is significantly lower than in the other cells (227 and 181 mW cm À 2 for R15 and R10, respectively), clearly indicating that the partial sintering of R5 cell electrolyte (and to a minor extent of the anode) severely affects the overall electrochemical performance.…”

“…From EIS measurements at OCV, R Ω is more than twice the corresponding value measured in H 2. We ascribed this to the less reducing atmosphere of biogas mixture (compared to H 2 ) that increases the resistance of the electrolyte . R pol dramatically increases in CH 4 /CO 2 due to the reduced electrocatalytic activity of the anode.…”

“…We ascribed this to the less reducing atmosphere of biogas mixture (compared to H 2 ) that increases the resistance of the electrolyte. [18] R pol dramatically increases in CH 4 /CO 2 due to the reduced electrocatalytic activity of the anode. The significant increase of the low frequency arch in the Nyquist plot is related to the methane diffusion into the substrate.…”

Development of a Multiscale SOFC Model and Application to Axially‐Graded Electrode Design

“…The microscale model simulates the distributed mass and charge transport along the z ‐direction i.e., throughout the thickness of the positive‐electrolyte‐negative electrode (PEN) . The complete description of the equations and of the integration methods is found in and : for the sake of clarity, the main details are reported and commented here. The mass transport in the electrodes porous structure is modeled with a reactive dusty gas model (DGM) , as reported in Eqs.…”