The oxygen transport kinetics of heterogeneous gas−solid exchange has been investigated on the basis of a two-step reaction mechanism, linking surface catalysis to solid-state self-diffusion, via gas phase isotopic oxygen exchange on the mixed ionic electronic conductor (MIEC) La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−x (LSCF) and electronic conductor (La 0.8 Sr 0.2 ) 0.95 MnO 3±x (LSM). The catalytic activity of LSCF is higher than that of LSM toward the elementary step of oxygen dissociation, likely caused by a higher vacancy concentration. The apparent activation energy for surface exchange of LSCF is lower than values obtained from bulk characterization techniques. The diffusion coefficient (D) for LSM at different temperatures shows a huge deviation from literature values, and an alternate exchange mechanism has been proposed. The fast transport pathway is attributed to the substoichiometry of LSM in the near surface region. These results have significant implications for the improvement of the oxygen reduction reaction for the design of higher-performance materials and the importance and limitations of isotope exchange experimental design.

Oxygen dissociation on metal oxides is a key reaction step, limiting the efficiency of numerous technologies. The complexity of the multi-step oxygen reduction reaction (ORR) makes it difficult to investigate the oxygen dissociation step independently. Direct observation of the oxygen dissociation process is described, quantitatively, on perovskites La Sr Co Fe O and (La Sr ) MnO , using gas-phase isotope-exchange with a 1:1 O : O ratio. Oxygen transport mechanisms between gas-surface reactions and surface-bulk exchange are deconvoluted. Our findings show that regardless of participation of lattice oxygen, La Sr Co Fe O is better at oxygen dissociation than (La Sr ) MnO . Heteroexchange, involving lattice oxygen, dominates on La Sr Co Fe O . In contrast, (La Sr ) MnO shows both homoexchange and heteroexchange, with the latter only happening above 600 °C. Using a 1:1 isotope mixture, a simple method is presented for separation of the oxygen dissociation step from the overall ORR.

Although solid oxide fuel cells (SOFC) have demonstrated excellent performance, the durability of SOFCs under real working conditions is still an issue for commercial deployment. In particular cathode exposure to atmospheric air contaminants, such as humidity, can result in long-term performance degradation issues. Therefore, a fundamental understanding of the interaction between water molecules and cathodes is essential to resolve this issue and further enhance cathode durability. To study the effects of humidity on the oxygen reduction reaction (ORR), we used in-situ 18 O isotope exchange techniques to probe the exchange of water with two of the most common SOFC cathode materials, (La 0.8 Sr 0.2 ) 0.95 MnO 3±δ (LSM) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF). In this experiment, heavy water, D 2 O (with a mass/charge ratio of m/z = 20), is used to avoid the overlapping of H 2 O and the 18 O 2 cracking fraction, which both provide a peak at m/z = 18. A series of temperature programmed isotope exchange measurements were performed to comprehensively study the interaction of water with the cathode surface as a function of temperature, oxygen partial pressure, and water vapor concentration. The results suggest that water and O 2 share the same surface exchange sites, leading to competitive adsorption. Our findings show that water prefers to exchange with LSCF at lower temperatures, around 300-450 • C. For LSM, O 2 is more favorable than water to be adsorbed on the surface and the presence of O 2 limits water exchange. The experimental data are summarized in a Temperature-PO 2 diagram to help visualize how the exchange of water on each material depends on the operating conditions. © The Author Solid oxide fuel cells (SOFC) electrochemically oxidize fuels for the generation of electricity. Two key advantages of SOFCs are their high efficiency and ability to utilize conventional fuels. This fuel flexibility stems from the dissociation and transport of oxygen from the cathode through the electrolyte to the anode, where the fuels are oxidized. Unfortunately, cathode degradation under real working conditions is a factor that limits SOFC performance. 1-6The long term durability of these materials is a major challenge, due to the high temperature required for the thermally activated oxygen reduction reaction (ORR), 7 as well as the variety of gases present during operation. 8,9 The impurities present in air on the cathode side of the cell can induce undesirable reactions. Some of these impurities, such as Cr or silica, 10-15 arise due to the interconnect and seal materials while some are intrinsic to ambient air, such as humidity and CO 2 . 16 Humidity has been found to degrade the performance of (La 0.8 Sr 0.2 ) 0.95 MnO 3±δ (LSM) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) based cells. 11,[17][18][19][20][21][22][23][24] This degradation can be either reversible or irreversible. 21 However, there is no conclusive evidence showing, fundamentally, how water participates in the degradation process, and it is hard to quan...

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The presence of Cr has already been reported in literature to cause severe degradation to LaSrCoFeO (LSCF). However, fundamental understanding of Cr effects on the surface exchange kinetics is still lacking. For the first time, in situ gas phase isotopic oxygen exchange was utilized to quantitatively determine Cr effect on oxygen exchange kinetics of LSCF powder as a function of temperature and water vapor. Our investigations revealed that the formation of secondary phases such as SrCrO, CrO, Cr-Co-Fe-O, and La-Co-Fe-O can affect both the oxygen dissociation step and overall surface exchange. Specifically, Cr-containing secondary phases on the surface not only decrease the active sites for surface reactions but also alter the nearby stoichiometry of the LSCF matrix, thereby limiting surface oxygen transport. In addition, water molecules actively participate in the surface reactions and can further block the active sites.

Performance of proton-solid oxide fuel cells (H+-SOFC) is governed by ion transport through solid/gas interfaces. Major breakthroughs are then intrinsically linked to a detailed understanding of how parameters tailoring bulk proton conductivity affect surface chemistry in situ, at an early stage. In this work, we studied proton and oxygen transport at the interface between H+-SOFC electrolyte BaCe x Zr0.9–x Y0.1O2.95 (x = 0; 0.2; 0.9) thin films and the gas (100 mTorr of H2O and O2) by using synchrotron-based ambient pressure X-ray photoelectron spectroscopy at operating temperature (>400 °C). We developed highly textured BaCe x Zr0.9–x Y0.1O2.95 epitaxial thin films, which exhibit high level of in-plane proton conductivity, that is, up to 0.08 S cm–1 at 500 °C for x = 0.9. Upon applying 100 mTorr water partial pressure above 300 °C, major changes are observed only in the O 1s and Y 3d core level spectra, with a clear Zr/Ce ratio dependency. OH– formation is favored by Ce content while initiated near Y. Hydration is also associated with surface secondary phase growth comprising oxygen-under-coordinated yttrium and/or yttrium hydroxide. With BaCe0.2Zr0.7Y0.1O2.95, high levels of ionic conductivities and chemical stability are obtained as a result of the optimized surface reaction kinetics, with low activation energy barrier for proton transport while restraining formation of OH–/SO4 2– adsorb species.

The high activity of oxide catalysts toward the oxygen reduction reaction (ORR) attracts unwanted interactions with other gaseous oxygen-containing species in air. Understanding the interaction between oxygen-containing species, mainly water and carbon dioxide, and oxides is important for many energy applications. However, the oxygen self-exchange process and the high-temperature operating conditions limit the investigation of these concurrent reactions. Here we report a direct observation of the effects of water and carbon dioxide on dissociation rates of ionically c o n d u c t i n g c a t a l y s t s , L a 0 . 6 S r 0 . 4 C o 0 . 2 F e 0 . 8 O 3 − δ ( L S C F ) a n d (La 0.8 Sr 0.2 ) 0.95 MnO 3±δ (LSM), using gas-phase isotope exchange. The concurrent heterogeneous reactions of oxygen and other oxygen-containing species on oxide catalysts can either promote or hinder oxygen dissociation rates, depending on the participation of lattice oxygen. LSCF appears to be much more active in exchange with these oxygen-containing species, while LSM shows relatively little exchange. Oxygen-containing species exhibit site-blocking effects and inhibit the reaction on LSCF. In contrast, water and CO 2 promote the oxygen dissociation rate on LSM, likely due to the prominence of homoexchange, where intermediate surface species play an important role. Our study provides insights into the reaction mechanism of oxygen dissociation and the effect of coexisting ambient air oxygen species.

The impact of sintering temperature on Cr-poisoning of solid oxide fuel cell (SOFC) cathodes was systematically studied. La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ -Ce 0.9 Gd 0.1 O 2-δ symmetric cells were aged at 750 • C in synthetic air with the presence of Crofer 22 APU, a common high temperature interconnect, over 200 hours and electrochemical impedance spectroscopy (EIS) was used to determine the degradation process. Both the ohmic resistance (R ) and polarization resistance (R P ) of LSCF-GDC cells, extracted from EIS spectra, for different sintering temperatures increase as a function of aging time. Furthermore, the Cr-related degradation rate increases with decreased cathode sintering temperature. The polarization resistance of cathode sintered at lower temperature (950 • C) increases dramatically while aging with the presence of Cr and also significantly decreases the oxygen partial pressure dependence after aging. The degradation rate shows a positive correlation to the concentration of Cr. The results indicate that decreased sintering temperature increases the total surface area, leading to more available sites for Sr-Cr-O nucleation and thus greater Cr degradation. The growing demand for robust, inexpensive, clean, secure, and sustainable energy have stimulated great interest in fuel cells. Among all types, solid oxide fuel cells (SOFCs) are one of the most promising candidates due to high efficiency and fuel flexibility.1-4 Recently, significant effort has been devoted to the development of intermediate to low temperature (400-800• C) SOFCs. Lowing operating temperature not only decreases the degradation rates but also broadens the selections of materials. The mixed ionic and electronic conducting (MIEC) perovskite La 0.6 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ (LSCF) and its composite with Ce 0.9 Gd 0.1 O 2-δ (GDC) is a promising cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs) because of its high intrinsic electrocatalytic activity to the oxygen reduction reaction (ORR) and acceptable thermal expansion coefficient. However, one of the challenges for the commercialization is the durability during long-term operation. [5][6][7][8][9] One of the most severe issues for cathode degradation is Cr-poisoning from Cr-containing stainless steel interconnects at high temperatures. Significant efforts have been given to understand the Cr-degradation mechanisms on SOFC cathodes. Jiang et al. [10][11][12] shows that gaseous Cr species can be chemically reduced and deposited at triple phase boundaries (TPB) as well as two-phase boundaries, blocking sites active toward the ORR. [10][11][12] The activity of the ORR on cathodes is governed by several factors, such as the atomic structure, surface composition 13-15 and bulk microstructure. In this research, sintering effect on Cr-degradation on LSCF-GDC was studied. Small changes in cathode sintering temperature leads to distinct differences in degradation rates for LSCF-GDC composite cathodes exposed to Cr-containing interconnect materials. Elevated cathode sinteri...