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

Abstract

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 ...

A detailed kinetic model for the reduction of oxygen on LSCF-GDC composite cathodes

“…On the other hand, consistently with the presence of a potential barrier against electronation, the larger χ eq is, the smaller the coverage fraction of charged oxygen adatoms becomes at the equilibrium. The coverage fraction of charged oxygen adatoms (Table 5) always maintains around 1% under all the investigated conditions, in line with the values reported in the literature by other authors [38,39].…”

“…The main differences concern the adoption of holes or electrons as active species, and the description of the electronation process. On LSM-YSZ cathodes, Zhang et al [38] propose the same ORR mechanism, considering diffusion of charged oxygen toward the TPB. Still with reference to LSM-YSZ cathodes, Banerjee and Deutschmann [27] compare a series of alternative mechanisms, introducing holes: the authors discuss different routes for the surface electronation process, coming to the conclusion that the most comprehensive mechanism includes the presence of a super-oxo ܱ ଶ ି species, which decomposes into two charged oxygen atoms.…”

“…These four detailed models share some important features. The surface charge-transfer rates include a surface potential, first proposed by Fleig [36,37] and Liu [38][39][40], whose nature is related to the specific capability of MIEC materials to create an additional capacitive interface between charged oxygen adsorbates and positive electronic countercharges [36], other than the electrode/electrolyte capacitive interface. In Refs.…”

“…Following the definition provided by Fleig [36] and Liu [38,40], the surface overpotential Δχ is defined as the difference between the potential ߶ ெ in the bulk of LSCF and the potential ߶ ெ ௦ at the surface of LSCF. According to the Poisson's law, it is expressed as a function of the surface site density, of the surface capacitance and of the coverage fraction of the adsorbed charged species, i.e.…”

“…23). As pointed out by Zhang et al [38], the relation stems from the fact that the electrode potential associated to the 2PB pathway must be equal to that associated to the TPB pathway, irrespective of which of the two prevails, for self-consistency. An additional consequence of this relationship is that the equilibrium potential of the TPB step ߂߶ ் can be derived as a linear combination of the equilibrium potentials ∆ϕ eq and χ eq (Eq.…”