The composition and microstructure of cathode materials has a large impact on the performance of solid oxide fuel cells (SOFCs). Rational design of materials composition through controlled oxygen nonstoichiometry and defect aspects can enhance the ionic and electronic conductivities as well as the catalytic properties for oxygen reduction in the cathode. Cell performance can be further improved through microstructure optimization to extend the triple-phase boundaries. A major degradation mechanism in SOFCs is poisoning of the cathode by chromium species when chromium-containing alloys are used as the interconnect material. This article reviews recent developments in SOFC cathodes with a principal emphasis on the choice of materials. In addition, the reaction mechanism of oxygen reduction is also addressed. The development of Cr-tolerant cathodes for intermediate temperature solid oxide fuel cells, and a possible mechanism of Cr deposition at cathodes are briefly reviewed as well. Finally, this review will be concluded with some perspectives on the future of research directions in this area.

/pdf/cathode-materials-for-solid-oxide-fuel-cells-a-review-4bog9hmhl1.pdf

Cathode materials for solid oxide fuel cells: a review

Solid oxide fuel cells: fundamental aspects and prospects

This work is focused on the comparative analysis of electrochemical and transport properties in the major families of cathode and anode compositions for intermediate-temperature solid oxide fuel cells (SOFCs) and materials science-related factors affecting electrode performance. The first part presents a brief overview of the electrochemical and chemical reactions in SOFCs, specific rate-determining steps of the electrode processes, solid oxide electrolyte ceramics, and effects of partial oxygen ionic and electronic conductivities in the SOFC components. The aspects associated with materials compatibility, thermal expansion, stability, and electrocatalytic behavior are also briefly discussed. Primary attention is centered on the experimental data and approaches reported during the last 10–15 years, reflecting the main challenges in the field of materials development for the ceramic fuel cells.

Electrode materials and reaction mechanisms in solid oxide fuel cells: a brief review

Ferrite-based perovskites as cathode materials for anode-supported solid oxide fuel cells. Part I. Variation of composition

Bismuth ferrite (BiFeO3), a perovskite material, rich in properties and with wide functionality, has had a marked impact on the field of multiferroics, as evidenced by the hundreds of articles published annually over the past 10 years. Studies from the very early stages and particularly those on polycrystalline BiFeO3 ceramics have been faced with difficulties in the preparation of the perovskite free of secondary phases. In this review, we begin by summarizing the major processing issues and clarifying the thermodynamic and kinetic origins of the formation and stabilization of the frequently observed secondary, nonperovskite phases, such as Bi25FeO39 and Bi2Fe4O9. The second part then focuses on the electrical and electromechanical properties of BiFeO3, including the electrical conductivity, dielectric permittivity, high-field polarization, and strain response, as well as the weak-field piezoelectric properties. We attempt to establish a link between these properties and address, in particular, the macroscopic response of the ceramics under an external field in terms of the dynamic interaction between the pinning centers (e.g., charged defects) and the ferroelectric/ferroelastic domain walls.

BiFeO3 Ceramics: Processing, Electrical, and Electromechanical Properties

Barium borosilicate glass – a potential matrix for immobilization of sulfate bearing high-level radioactive liquid waste

Chemical compatibility of the LaFeO3 base perovskites (La0.6Sr0.4)zFe0.8M0.2O3 − δ (z = 1, 0.9; M = Cr, Mn, Co, Ni) with yttria stabilized zirconia

A sodium barium borosilicate glass matrix with a higher solubility of sulfate has been developed recently at Bhabha Atomic Research Centre for vitrification of sulfate bearing high-level nuclear waste. We report here the studies carried out to understand the influence of sulfate ion on the three-dimensional borosilicate network. Experiments were carried out with sodium barium borosilicate base glass samples loaded with varying amounts of SO 2- 4 (0-5 mol%). Phase separation studies on the samples revealed that as much as 3 mol% of SO 2- 4 can be loaded within the base glass without any phase separation, however, beyond this limit BaSO 4 (barite) crystallizes within the matrix. Thermal analyses of the samples indicated a shift in glass transition temperature from 534° (0 mol% SO 2- 4 ) to 495°C (3 mol% SO 2- 4 ) and it remained more or less unaltered afterwards even with high SO 2- 4 loading. A similar observation of structure stabilization was obtained from 29 Si MAS-NMR studies also, which showed that with 2 mol% of SO 2- 4 loading, the Q 2 :Q 3 ratio changed from 59:41 (for samples with 0 mol% SO 2- 4 loading) to 62:38 and it remained almost the same afterwards even with higher SO 2 4 - loading. 11 B MAS NMR patterns of the glass samples, however, remained unchanged with SO 2- 4 loading ((BO 4 ]:[BO 3 ]= 38:62). Based on 29 Si and 11 B MAS NMR studies, the authors propose two different ways of interaction of SO 2- 4 ions with the borosilicate network: (i) the network modifying action of SO 2- 4 ions with -Si-O-Si- linkages, at low SO 2- 4 ion concentration ( 2 mol%).

Role of Sulfate in Structural Modifications of Sodium Barium Borosilicate Glasses Developed for Nuclear Waste Immobilization

Orthoferrite-based perovskites are of interest as materials for the cathode in solid oxide fuel cells (SOFCs). Therefore, the chemical compatibility between perovskites of the composition (La1−xSrx)zFe1−yMnyO3−δ (0 # x # 0.3; 0.2 # y # 1; z = 0.90, 0.95, 1.00) and the solid electrolyte zirconia (ZrO2) doped with 8 mol% yttria (Y2O3) (8YSZ) has been investigated. Powder mixtures of the two materials have been annealed at different temperatures. The formation of monoclinic ZrO2 at 1000°C, as well as of La2Zr2O7 and SrZrO3 at 1400°C, has been determined in some samples. The reactions that are observed are discussed, with respect to the thermodynamic activities, tolerance factor, and oxygen-ion migration energies. Some perovskite compositions seem to be compatible with Y2O3-stabilized ZrO2 (YSZ), thereby offering the possibility to use orthoferrite-based perovskites in SOFCs with a solid electrolyte made of YSZ.

Dasarathi Das

Papers

Barium borosilicate glass – a potential matrix for immobilization of sulfate bearing high-level radioactive liquid waste

Chemical compatibility of the LaFeO3 base perovskites (La0.6Sr0.4)zFe0.8M0.2O3 − δ (z = 1, 0.9; M = Cr, Mn, Co, Ni) with yttria stabilized zirconia

Role of Sulfate in Structural Modifications of Sodium Barium Borosilicate Glasses Developed for Nuclear Waste Immobilization

Chemical Interactions between La-Sr-Mn-Fe-O-Based Perovskites and Yttria-Stabilized Zirconia

Standard enthalpy of formation and heat capacity of compounds in the pseudo-binary Bi2O3–Fe2O3 system