Simulation of Surface-Potential Driven ORR Kinetics on SOFC Cathode with Parallel Reaction Pathways

Gong, Mingyang; Gemmen, Randall; Mebane, David S.; Gerdes, Kirk; Liu, Xingbo

doi:10.1149/2.104403jes

Cited by 28 publications

(44 citation statements)

References 40 publications

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“…Therefore, detailed understanding of the particle size dependence of the electrode reaction mechanisms and operating performance is indispensable. This research continues previous study 8,16 of the detailed surface contribution on the ORR kinetics of LSM-type cathodes under polarization. Quantitative examination of competition and transition between parallel surface and bulk pathways (i.e.…”

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confidence: 85%

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Surface Transport Mechanism and Bi-Pathway ORR Kinetics for Solid Oxide Fuel Cell Cathode

Zhang¹,

Gong²,

Gerdes³

et al. 2014

J. Electrochem. Soc.

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A one-dimensional continuum model related to close-packing theory has been developed to produce insights into the oxygen reduction reaction (ORR) process in a dense LSM-type cathode system. By incorporating SOFC cathode particle size, the modeling simulations reveal detailed surface processes and parallel reaction pathways responding to polarization-induced changes. A formula to distinguish the contributions of surface diffusion and local reactions to total 3PB is presented, which allows an estimation of the detailed influencing factor for 3PB/2PB transition. The results reveal that the essential correspondence in spatial domains of surface adsorbates and oxygen vacancies implies tight coupling behavior for 3PB/2PB kinetic competition. The kinetic domination of surface reactions shifts between oxygen adsorption and incorporation, demonstrates the importance of harmonization between surface activity and ionic conduction for improving the overall electrochemical performance of SOFC cathode. Such mechanistic studies suggest means of rational design of more active cathodes to optimize the overall electrochemical performance.The oxygen reduction reaction (ORR) taking place at the cathode prominently influences the electrochemical performance and operating efficiency of solid oxide fuel cells (SOFC). Investigation of ORR for a SOFC cathode has revealed a complex multistep process yielding sluggish kinetic response. Explanations of cathodic polarization manifested in impedance spectroscopy and associated with the complicated oxygen transport behavior requires an improved understanding of such ORR mechanisms. Modeling calculations have been reported that illuminate the relationship between charge-transfer reactions, transport phenomena, and interdependency of the ORR processes under different operating conditions. Even when considering basic electrode architectures and simplifying assumptions, materials properties and the corresponding parameters must be considered for model development.Prominent electronically conductive SOFC cathode materials are LSM-type perovskites, such as La 0.8 Sr 0.2 MnO 3-δ (LSM) and La 0.6 Sr 0.4 FeO 3-δ (LSF). Their semiconductor-like behavior and hopping of small polarons on the B-site generally accentuates electronic conductivity over ionic conductivity. Thereby, oxygen reduction and incorporation into the cathode materials first requires oxygen adsorption and dissociation at the interface between gas and cathode. The initial performance of the LSM-type cathodes relies on surface diffusion of oxygen species and a charge-transfer process at the three phase boundary (3PB) where gas, cathode and electrolyte meet. In addition to this surface pathway, a major bulk reaction pathway becomes activated after the LSM-type materials develop mixed ionic and electronic conductivity (MIEC) under electric load. The formation of an oxygen vacancy gradient in the oxygen sublattice facilitates a net flow of oxygen ions into the electrolyte at the interface of cathode/electrolyte. Different models 1 generally ...

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confidence: 85%

“…8,16 As a typical model feature, there are two parallel reaction paths with respect to electrochemical oxygen transport into the SOFC electrolyte. The present interest is to investigate pathway competition, surface reaction kinetics and mass-transport properties associated with the size of SOFC cathode particles.…”

Section: Model Descriptionsmentioning

confidence: 99%

Surface Transport Mechanism and Bi-Pathway ORR Kinetics for Solid Oxide Fuel Cell Cathode

Zhang¹,

Gong²,

Gerdes³

et al. 2014

J. Electrochem. Soc.

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“…The decreased oxygen activity by the cathodic polarization increases the oxide ion conductivity by vacancy mechanism [47,48]. As is well-known for the mixed-ionicelectronic conductor electrode materials, the reaction zone can be extended from the triple phase boundaries by the increased ionic conductivity of LSM [49,50]. This may explain less pronounced deterioration effects in the performance of LSM electrode in SOFC mode and large variation between the data sets as shown in Fig.…”

Section: Resultsmentioning

confidence: 77%

Pinning-down polarization losses and electrode kinetics in cermet-supported LSM solid oxide cells in reversible operation

et al. 2015

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“…The changes in the aging behavior can thus be predicted through pairing the defect chemistry model with an appropriate LSM oxygen reduction model. The authors are currently working toward incorporating the changing defect concentrations into an existing LSM oxygen reduction and corresponding SOFC multiphysics model which can predict the electrochemical performance of a porous LSM electrode similar to the one found in the symmetrical cell samples [39][40][41]. By stepping the defect concentrations in time based upon reasonable cation diffusion coefficients, simulated impedance aging data will be created.…”

Section: Discussionmentioning

confidence: 99%

Examination of the mechanism for the reversible aging behavior at open circuit when changing the operating temperature of (La0.8Sr0.2)0.95MnO3 electrodes

et al. 2015

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The aging behavior of symmetrical cells, consisting of either (La 0.8 Sr 0.2 ) 0.95 MnO 3 (LSM) or La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) electrodes screen printed on either 8 mol% yttria-stabilized zirconia (YSZ) or Ce 0.8 Gd 0.2 O 2 (GDC) electrolyte substrates, is reported as the symmetrical cell is thermally cycled between 700°C and 800°C. For LSM, between 700°C and 850°C, the polarization resistance exhibits slow increases or decreases with time (on the order of days) after a quick change in temperature. When increasing the temperature, the polarization resistance decreases with time, and when decreasing the temperature, the polarization resistance slowly increases with time. In a previous work, the authors had explained these results with LSM by connecting the testing conditions to literature reports of surface analysis of LSM thin films which demonstrated a change in the amount of surface cation segregation as a function of temperature. In this work, TEM/EDS/XPS analysis of dense LSM pellets thermally cycled under the same conditions as the symmetrical cells does not indicate any significant reversible change in the surface composition of the LSM pellet between 700°C and 800°C. An alternative hypothesis is proposed to explain the relationship between polarization resistance and the LSM cation/anion vacancy concentrations controlled by the Schottky reaction. The timescale of aging behavior is related to the time necessary for the cations to move to or from the LSM surface to adjust to the new equilibrium at each temperature. The relevance in understanding the mechanism behind the aging behavior is emphasized with respect to fuel cell sample/stack modeling as well as to proper testing procedures for reaching reliable conclusions when comparing different electrode samples.

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Simulation of Surface-Potential Driven ORR Kinetics on SOFC Cathode with Parallel Reaction Pathways

Cited by 28 publications

References 40 publications

Surface Transport Mechanism and Bi-Pathway ORR Kinetics for Solid Oxide Fuel Cell Cathode

Surface Transport Mechanism and Bi-Pathway ORR Kinetics for Solid Oxide Fuel Cell Cathode

Pinning-down polarization losses and electrode kinetics in cermet-supported LSM solid oxide cells in reversible operation

Examination of the mechanism for the reversible aging behavior at open circuit when changing the operating temperature of (La0.8Sr0.2)0.95MnO3 electrodes

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