Solid oxide fuel cells (SOFCs) have the potential to be one of the cleanest and most efficient energy technologies for direct conversion of chemical fuels to electricity. Economically competitive SOFC systems appear poised for commercialization, but widespread market penetration will require continuous innovation of materials and fabrication processes to enhance system lifetime and reduce cost. One early technical opportunity is minimization of resistance to the oxygen reduction reaction (ORR) at the cathode, which contributes the most to performance degradation and efficiency loss in the existing SOFCs, especially at temperatures <700 °C. Detailed study over the past 15 years has revealed the positive impact of catalyst infiltration on SOFC cathode performance, both in power density and durability metrics. However, realizable performance improvements rely upon strongly-coupled relationships in materials and morphology between the infiltrate and the backbone, and therefore efficacious systems cannot be simply generated with a set of simple heuristics. This article reviews recent progress in enhancing SOFC cathode performance by surface modification through a solution-based infiltration process, focusing on two backbone architectures – inherently functional and skeletal – infiltrated using wet-chemistry processes. An efficient cathode consists of a porous mixed-conducting backbone and an active coating catalyst; the porous backbone provides excellent ionic and electronic conductivity, while the infiltrated surface coating possesses high catalytic activity and stability. As available, performance comparisons are emphasized and reaction schematics for specific infiltrate/backbone systems are summarized. While significant progress has been achieved in enhancing surface catalytic activity and durability, the detailed mechanisms of performance enhancement are insufficiently understood to obtain critical insights and a scientific basis for rational design of more efficient catalysts and novel electrode architectures. Recent progress in characterization of surfaces and interfaces is briefly discussed with challenges and perspectives in surface modification of SOFC electrodes. Surface modification through infiltration is expected to play an increasingly important role in current and next-generation commercial SOFC development, and this review illustrates the sophisticated technical considerations required to inform judicious selection of an infiltrate for a given SOFC system.

Enhancing SOFC cathode performance by surface modification through infiltration

https://hal-univ-rennes1.archives-ouvertes.fr/hal-01216294/document

Recent advances in perovskites: Processing and properties

Solid oxide electrolysis cells (SOECs) have attracted increasing attention as a promising device for the electrochemical CO2 reduction reaction (CO2RR) due to their high efficiency and fast kinetics. Exploring active cathode catalysts for the CO2RR is highly desirable for the research and development of SOECs. Herein, in situ exsolved FeNi3 nanoparticles on a Sr2Fe1.35Mo0.45Ni0.2O6−δ (SFMN) double perovskite substrate (FeNi3@SFMN) is developed to efficiently catalyze the CO2RR in SOECs. The SOEC with the FeNi3@SFMN–GDC (Gd0.2Ce0.8O1.9) cathode shows a current density of 0.934 A cm−2 at 1.6 V and 800 °C, as well as high stability and no coke deposition for 40 h at 1.2 V. CO2-temperature programmed desorption and quasi in situ Fourier-transform infrared spectroscopy measurements verify the intensive adsorption of CO2 on the FeNi3@SFMN–GDC cathode. Distribution of relaxation time analysis combined with density functional theory calculations discloses the stimulative activation of CO2 at the interface between the exsolved FeNi3 nanoparticles and the SFMN substrate with abundant oxygen vacancies, which improves the CO2RR performance at the FeNi3@SFMN–GDC cathode.

In situ exsolved FeNi3 nanoparticles on nickel doped Sr2Fe1.5Mo0.5O6−δ perovskite for efficient electrochemical CO2 reduction reaction

Bi and Sn co-doped perovskite BaFe0.8−XSn0.2BiXO3−δ materials have been designed and characterized as a series of new cathodes for proton-conducting solid oxide fuel cells (SOFCs), providing a new life for the traditional BaFeO3-based cathodes. Proton uptake of the cathode increases significantly with bismuth-doping, favoring the application of Bi and Sn co-doped perovskite BaFe0.8−XSn0.2BiXO3−δ materials in proton-conducting SOFCs. The density functional theory (DFT) calculation also suggests that the bismuth-doping leads to a dramatic increase of hydration energy and acceleration of proton migration for the BFS material. The XPS results show that the oxygen reduction reaction (ORR) activity of the cathode is enhanced after bismuth is added, which is consistent with the experimental results of the power density of the single cell. The maximum power density of the single cells with the structure of NiO-BaZr0.1Ce0.7Y0.2O3−δ|BaZr0.1Ce0.7Y0.2O3−δ| BaFe0.5Sn0.2Bi0.3O3−δ (BFSBi0.3) reached 1277 mW cm−2 at 700 °C, which is a record-high performance for proton-conducting SOFCs using cobalt-free cathodes, compared with previous reports. The outstanding fuel cell performance and good long term stability indicate that bismuth-doping is an effective way of promoting proton-conduction in traditional cathodes. This study opens a new door to the design of high performance cathodes for proton-conducting SOFCs.

A novel cobalt-free cathode with triple-conduction for proton-conducting solid oxide fuel cells with unprecedented performance

XPS study on La0.67Ca0.33Mn1−xCoxO3 compounds

X-ray photoelectron spectroscopy investigation on chemical states of oxygen on surfaces of mixed electronic–ionic conducting La0.6Sr0.4Co1−yFeyO3 ceramics

Structure, electrical conducting and thermal expansion properties of Ln0.6Sr0.4Co0.2Fe0.8O3 (Ln = La, Pr, Nd, Sm) perovskite-type complex oxides

Fine and uniform Ln 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (Ln = La, Pr, Nd, Sm) powders with a perovskite phase were produced using a glycine–nitrate process. The structure, electrical conducting and thermal expansion properties of the resulting ceramics were investigated. The results indicate that replacing La 3+ by smaller lanthanide cations led to a change in crystal structure from rhombohedral to orthorhombic symmetry and a decrease of the pseudo-cubic lattice constant. It was found that the electrical conducting properties decrease with reducing lanthanide cation size. Compared with La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 , the degradation of the electrical conducting properties in Pr 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 and Nd 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 is not pronounced, whereas the electrical conducting properties of Sm 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 decline to an unsatisfactory level. There is an evident increase in thermal expansion at high temperatures for all the compositions. This is attributed to a chemically induced lattice expansion due to oxygen lose and formation of oxygen vacancies. Pr 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 and Nd 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 present relatively low thermal expansion coefficient values of 14.2 × 10 −6 and 13.2 × 10 −6  K −1 averaged between 100–750 and 100–700 °C, respectively.

Structure, Electrical Conducting and Thermal Expansion Properties of Ln0.6Sr0.4Co0.2Fe0.8O3 (Ln: La, Pr, Nd, Sm) Perovskite-Type Complex Oxides.

The sintering and electronic conducting properties of La0.8Ca0.2CrO3 synthesized by a glycine-nitrate process (GNP) were investigated in comparison with that synthesized by the conventional solid state reaction (SSR) method. The results demonstrate the advantage of the GNP method in producing La0.8Ca0.2CrO3 ceramic. Compared with the powder synthesized by the SSR method, that synthesized by the GNP method shows a higher sinterability due to its fine morphology. The relative densities of the ceramics made by the GNP and SSR methods attain 96.5 % and 96.0 % when sintering at 1450°C and 1550°C, respectively. In the case of similar relative densities, the ceramic made by the GNP method (sintered at 1450°C) exhibits superior electronic conducting properties to that made by the SSR method (sintered at 1550°C). This is attributed to a desired microstructure of the ceramic made by the GNP method.

Bi-tao Wang

Papers

X-ray photoelectron spectroscopy investigation on chemical states of oxygen on surfaces of mixed electronic–ionic conducting La0.6Sr0.4Co1−yFeyO3 ceramics

Structure, electrical conducting and thermal expansion properties of Ln0.6Sr0.4Co0.2Fe0.8O3 (Ln = La, Pr, Nd, Sm) perovskite-type complex oxides

Structure, Electrical Conducting and Thermal Expansion Properties of Ln0.6Sr0.4Co0.2Fe0.8O3 (Ln: La, Pr, Nd, Sm) Perovskite-Type Complex Oxides.

Sintering and Electronic Conducting Properties of La0.8Ca0.2CrO3 Perovskite-Type Complex Oxides Synthesized by Different Methods