Elsevier

Journal of Power Sources

Volume 326, 15 September 2016, Pages 331-340
Journal of Power Sources

Chromium poisoning in (La,Sr)MnO3 cathode: Three-dimensional simulation of a solid oxide fuel cell

https://doi.org/10.1016/j.jpowsour.2016.06.110Get rights and content

Highlights

  • 3D numerical model of SOFC considering Cr poisoning on the cathode are developed.

  • Simulation results reveal micro-scale phenomena inside the complex porous electrodes.

  • Developed model reproduces trends of Cr poisoning reported in experimental studies.

  • Cr poisoning increases not only activation overpotential but also ohmic loss.

Abstract

A three-dimensional numerical model of a single solid oxide fuel cell (SOFC) considering chromium poisoning on the cathode side has been developed to investigate the evolution of the SOFC performance over long-term operation. The degradation model applied in the simulation describes the loss of the cathode electrochemical activity as a decrease in the active triple-phase boundary (TPB) length. The calculations are conducted for two types of cell: lanthanum strontium manganite (LSM)/yttria-stabilized zirconia (YSZ)/Ni-YSZ and LSM-YSZ/YSZ/Ni-YSZ. Their electrode microstructures are acquired by imaging with a focused ion beam scanning-electron microscope (FIB-SEM). The simulation results qualitatively reproduce the trends of chromium poisoning reported in the literature. It has been revealed that the performance degradation by chromium is primarily due to an increase in the cathode activation overpotential. In addition, in the LSM-YSZ composite cathode, TPBs in the vicinity of the cathode–electrolyte interface preferentially deteriorate, shifting the active reaction site towards the cathode surface. This also results in an increase in the cathode ohmic loss associated with oxide ion conduction through the YSZ phase. The effects of the cell temperature, the partial pressure of steam at the chromium source, the cathode microstructure, and the cathode thickness on chromium poisoning are also discussed.

Introduction

The performance degradation of solid oxide fuel cells (SOFCs) due to impurities is a critical issue that must be overcome for the commercialization of SOFC systems. Chromium, which is contained in the alloy separators and flow channel walls of SOFC systems, causes severe degradation in SOFC cathodes [1]. A number of experimental studies [2], [3], [4], [5], [6], [7], [8], [9] and a thermodynamic analysis [10] have led to the proposal of the following two main reactions for chromium poisoning in lanthanum strontium manganite/yttria-stabilized zirconia (LSM-YSZ) cathodes.2CrO2OH2g+6eCr2O3s+2H2O+3O2La0.8Sr0.2MnO3+0.1CrO3g1.1La0.8Sr0.20.909Mn0.909Cr0.091O3

Eq. (1) is an electrochemical reaction, where gaseous chromium in the supplied air is electrochemically reduced near the active reaction sites to form solid chromium oxide (III), covering up the reaction sites at which the electrochemical reduction reaction of oxygen can occur [9]. On the other hand, Eq. (2) is a chemical reaction, where chromium replaces manganese atoms in the B-site of the perovskite LSM crystal [10], which may reduce the electronic conductivity within the phase.

Konysheva et al. [5], [6] reported the distribution of chromium in an LSM-YSZ composite cathode after exposure to chromium-containing air as observed by an energy-dispersive X-ray (EDX) mapping. Under open-circuit voltage (OCV) conditions, chromium was distributed randomly in the entire cathode, whereas under a current load of 200 mA cm−2, chromium was detected in the vicinity of the cathode–electrolyte interface. In addition, the increase in the activation overpotential under a current load was significantly larger than that under OCV conditions. Horita et al. [9] reported a similar tendency in an LSM cathode; chromium deposition mainly occurred near the triple-phase boundary (TPB) region under a current load while a uniform chromium distribution was observed under OCV conditions. Most of the degradation in the cathode was found to be due to an increase in the activation overpotential, although a slight increase in the ohmic resistance was also observed. These findings indicate that the main factor causing degradation by chromium is not the chemical reaction between LSM and chromium but the electrochemical reaction of chromium oxide at the TPBs. In this case, higher activation overpotential may promote the deposition of chromium.

The deposited chromium oxide may block the diffusion pathways of reactant gas species to the electrode reaction sites, i.e, TPBs, and decrease the local exchange current density. Nakajo et al. [11] proposed a model that considers the decrease in the local exchange current density as a decrease in the TPB length owing to the electrochemical reaction of chromium oxide in LSM-YSZ composite cathodes. In this model, the relative decrease in the TPB density is a function of the cathode activation overpotential. They applied this model to a one-dimensional (1D) macroscale simulation and investigated the changes in overall performance over 12,000 h of exposure to chromium.

To improve understanding of the chromium degradation in SOFC cathodes, microscale simulation using actual microstructure data for porous electrodes is useful. Recent developments in tomography techniques, such as imaging with a focused ion beam scanning electron microscope (FIB-SEM) and X-ray computed tomography (CT), enable us to directly observe and virtually reconstruct electrode microstructures, which can be used in microscale simulations. Numerical simulation is an effective approach not only for calculating the overall system performance but also for analyzing microscale phenomena inside complex porous electrodes. Because the microstructure of the electrodes has a significant effect on the cell performance, researchers have been investigating the relationships between the electrode microstructure and the electrochemical performance through experimental and numerical approaches [12], [13], [14], [15], [16], [17], [18].

In this study, a three-dimensional (3D) numerical model of an SOFC considering chromium poisoning on the cathode side has been developed to investigate the evolution of the SOFC performance over long-term operation. As a model for the chromium poisoning in LSM and LSM-YSZ composite cathodes, an empirical relationship proposed by Nakajo et al. [11] is applied. The electrode microstructures are acquired by FIB-SEM imaging. The time evolution of the cathode electrochemical performances, such as the activation overpotential and ohmic loss, is investigated. Also, the effects of the cell temperature, the partial pressure of steam at the chromium source, the cathode microstructure, and the cathode thickness on chromium poisoning are discussed.

Section snippets

3D microstructural data of electrodes

To conduct the microscale simulation, we acquired 3D data of actual electrode microstructures using an FIB-SEM. Since Konysheva et al. [5] reported that the chromium-induced degradation on LSM cathodes was faster than that on LSM-YSZ composite cathodes, we prepared not only an LSM cathode but also an LSM-YSZ composite cathode for comparative purposes. Commercial LSM and LSM-YSZ inks (Fuel Cell Materials, U.S.A.) were painted and sintered on YSZ disks (Tosoh, Japan) at 1150 °C for 5 h. The

Numerical model

The simulation is based on the finite volume method (FVM), where the conservation of electrons, ions, and gas species is considered. The electrochemical reaction is assumed to take place only at the TPBs. In this model, the local transport coefficients in each grid Γieff,local are evaluated asΓieff,local=VilocalΓibulk,where Vi is the volume fraction of phase i in the grid. Details of the model were reported by Kishimoto et al. [16], [17].

Degradation of electrochemical performance

Table 4 shows the calculation conditions in this study. All calculations were conducted under a 200 mA cm−2 current load with an anode gas of 97% H2–3% H2O and a cathode gas of 21% O2–79% N2. In Cases 1–4, the simulation was conducted for the LSM-YSZ composite cathode with different cell temperatures (750 and 850 °C) and molar fractions of steam at the chromium source (0.1 and 2.6%). These results were used to investigate the effects of temperature as well as the partial pressure of CrO2(OH)2

Conclusions

A 3D numerical model of a single SOFC cell considering chromium poisoning on the cathode side has been developed to investigate the evolution of the SOFC performance over long-term operation. The calculation domain consisted of an actual electrode microstructure obtained using an FIB-SEM. The simulation results revealed that the degradation by chromium is mainly caused by an increase in the cathode activation overpotential due to a decrease in the TPB length. In addition, in LSM-YSZ composite

Acknowledgment

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Development of System and Elemental Technology for Solid Oxide Fuel Cell (SOFC) Project and by a Grant-in-Aid for JSPS Fellows. We also thank Mr. Y. Tanaka for helping us to construct the numerical model.

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