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The research and development of new Solid Oxide Fuel Cell cathode materials is an area of intense activity. The kinetic coefficients describing the O2-reduction mechanism are the O-ion diffusion ( D chem ) and the O-surface exchange coefficients ( k chem ). These parameters are strongly dependent on the nature of the material, both on its bulk and surface atomic and electronic structures. This review discusses the method for obtaining the kinetic coefficients through the combination of electrochemical impedance spectroscopy with focused ion-beam 3D tomography measurements on porous electrodes (3DT-EIS). The data, together with oxygen non-stoichiometry thermodynamic data, is analysed using the Adler-Lane-Steele model for macro-homogeneous porous electrodes. The results for different families of oxides are compared: single- and double-layered perovskites with O-vacancies defects, based on La-Sr cobalt ferrites (La0.6Sr0.4Co1-xFexO3-δ , x = 0.2 and 0.8) and La/Pr-Ba cobaltites (La0.5-xPrxBa0.5CoO3-δ , x = 0.0, 0.2 and 0.5), as well as Ruddlesden-Popper nickelates (Nd2NiO4 +δ ) with O-interstitial defects. The analysis of the evolution of molar surface exchange rates with oxygen partial pressure provides information about the mechanisms limiting the O2-surface reaction, which generally is dissociative adsorption or dissociation-limited. At 700 °C in air, the La-Ba cobaltite structures, La0.5-xPrxBa0.5CoO3-δ , feature the most active surfaces ( k chem ≃0.5–1 10−2 cm.s−1), followed by the nickelate Nd2NiO4 +δ and the La-Sr cobalt ferrites, with k chem ≃1–5 10−5 cm.s−1. The diffusion coefficients D chem are higher for cubic perovskites than for the layered ones. For La0.6Sr0.4Co0.8Fe0.2O3-δ and La0.6Sr0.4Co0.2Fe0.8O3-δ , D chem is 2.6 10−6 cm2.s−1 and 5.4 10−7 cm2.s−1, respectively. These values are comparable to D chem = 1.2 10−6 cm2.s−1, observed for La0.5Ba0.5CoO3-δ . The layered structure drastically reduces the O-ion bulk diffusion, e.g. D chem = 1.3 10−8 cm2.s−1 for the Pr0.5Ba0.5CoO3-δ double perovskite and D chem ≃2 10−7cm2.s−1 for Nd2NiO4 +δ . Finally, the analysis of the time evolution of the electrodes shows that the surface cation segregation affects both the O-ion bulk diffusion and the surface exchange rates.

This work presents the study of the O2-Reduction Reaction (ORR) by electrochemical impedance spectroscopy of La0.6Sr0.4Co0.8Fe0.2O3-δ and La0.6Sr0.4Co0.2Fe0.8O3-δ cathodes as a function of temperature and pO2. The combination of the impedance data, modeled with a Transmission Line Model, with the microstructural data obtained by FIB-SEM tomography, allowed to obtain and compare the chemical diffusion coefficients (Dchem), O2 equilibrium molar exchange rates (R0) and the oxygen surface exchange rates (kchem) for both compounds. The obtained values were, at 700°C in air, Dchem = 5.4.10-7 cm2 .s-1 and kchem = 1.4.10-6 cm. s-1 for La0.6Sr0.4Co0.2Fe0.8O3-δ, while Dchem = 2.6.10-6 cm2 .s-1 and kchem = 3.1.10-6 cm. s-1 were obtained for La0.6Sr0.4Co0.8Fe0.2O3-δ. The detailed analysis of these parameters as a function of pO2 (10-4 < pO2 ≤ 1) and temperature (500°C ≤ T ≤ 700°C) by means of the Adler-Lane-Steele model, adapted to a finite length porous electrode, allowed identifying the O-ion diffusion and surface exchange as processes co-limiting the ORR. From this analysis, a predominantly surface limited ORR was found for La0.6Sr0.4Co0.2Fe0.8O3-δ, changing to a more bulk limited ORR for La0.6Sr0.4Co0.8Fe0.2O3-δ, which has higher oxygen-vacancy concentration. Fil: Ascolani Yael, Julian Esteban. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; Argentina Fil: Montenegro Hernandez, Alejandra. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche | Comision Nacional de Energia Atomica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche.; Argentina Fil: Liu, Qinyuan. Northwestern University; Estados Unidos Fil: Barnett, Scott A.. Northwestern University; Estados Unidos Fil: Mogni, Liliana Verónica. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche | Comision Nacional de Energia Atomica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche.; Argentina

Reducing fuel cells degradation and cathode polarization losses are currently two of the most important challenges in order to achieve more efficient and cost competitive SOFC's. One strategy has been to reduce operation temperatures to the range between 600 ºC and 800 ºC (IT-SOFC), which reduces degradation rates but increases activation over potentials as a consequence (mainly the cathode reaction overpotential, due to the high activation energy of oxygen reduction reaction -ORR-). Several approaches have been taken to counteract this problem and recently, promising results [1] have been obtained by modifying the electrodes surface using the impregnation technique. This technique consists of synthesizing catalyst nanoparticles on the electrode surface through a liquid solution-based process [2, 3]. In this work we show the results obtained when decorating La0.6Sr0.4Co0.8Fe0.2O3-δ -LSCF- cathodes with lanthanide oxides (CeOx, Ce1-yGdyOx, PrOx) nanoparticles after its impregnation with nitrate solutions. All systems were characterized using SEM, TEM and XRD, where spatially homogeneous and monodisperse size distributions were observed (See Figure 1a and 1b, where a cathode with and without nanoparticles is shown, respectively). Figure 1c presents the Nyquist plot of Electrochemical Impedance Spectroscopy (EIS) measurements at 600 ºC. These measurements showed that when impregnating with lanthanide oxide nanoparticles the cathode polarization resistance at 600ºC was reduced from 0.6 Ωcm2 for plain LSCF to 0.32, 0.2 and 0.08 Ωcm2 for LSCF decorated with Ce0.8Gd0.2O2, CeO2 and PrO2 oxide nanoparticles, respectively. The reduction due to the Pr oxide impregnation allows, in principle, a decrease of 100 ºC (from 700ºC to 600ºC) of the cathode's operation temperature, which could be considered as a candidate for low temperature SOFC (LT-SOFC)[4]. Figure 1. a) plain LSCF b) GDC impregnated LSCF c) Nyquist plots measured at 600 ºC of plain LSCF and LSCF impregnated with CeO2, GDC and PrO2. The EIS measurements as a function of temperature and oxygen partial pressure enable the proposal of a mechanism for the oxygen reduction reaction and an explanation about the effect of lanthanide oxides nanoparticles decoration. For example, when impregnating with GDC nanoparticles the impregnation is observed to modify the surface resistance of the ORR in the cathode’s surface but leaves the O2- ion conduction resistance in bulk LSCF unmodified [5]. Finally, the EIS measurements as a function of time during the aging of the samples exposed for periods between 300 h and 500 h to the operation temperatures was observed to produce an irreversible degradation (melting) of the impregnated nanoparticles. References: [1] A. Jacobson. Materials for Solid Oxide Fuel Cells. Chem. Mater. 22, 660, (2010). [2] D. Dong. Enhancing SOFC cathode performance by surface modification through infiltration. Energy Environ. Sci. 7,552, (2014). [3] S. P. Jiang. Nanoscale and nano-structured electrodes of solid oxide fuel cells by infiltration: Advances and challenges. Int J Hydrogen Energy 37, 1, (2012). [4] S. Barnett L. Gao, L. Mogni. A perspective on low-temperature solid oxide fuel cells. Energy Environ. Sci., 9 (5):1602–1644, (2016). [5] J. Ascolani-Yael, et. al. Study of the Rate Limiting Steps and Degradation of a GDC Impregnated La0. 6Sr0. 4Co0. 8Fe0. 2O3-δ cathode. ECS Transactions, 78(1), 795-805, (2017). Figure 1

Reducing fuel cells degradation and cathode polarization losses are currently two of the most important challenges in order to achieve more efficient and cost competitive SOFC's. One strategy has been to reduce operation temperatures to the range between 600 ºC and 800 ºC (IT-SOFC), which reduces degradation rates but increases activation over potentials as a consequence (mainly the cathode polarization resistance Rp,c, due to the oxygen reduction reaction -ORR-). Several approaches have been taken to counteract this problem and, recently, promising results [1] have been obtained by modifying the electrode surface using the impregnation technique. This technique consists of synthesizing catalyst nanoparticles on the electrode surface through a liquid solution-based process [2, 3]. The mechanisms through which the impregnated catalysts reduce the polarization resistance and even enhance durability are not completely well understood yet, especially when non conventional catalysts are impregnated (i.e.: electrolyte nanoparticles into mixed ionic and electronic conductors –MIECs- electrodes), as the performance improvement relies upon the microstructure and coupled interactions between the materials. In this work, a cathode surface modification by nanoparticle impregnation was carried out. La0.6Sr0.4Co0.8Fe0.2O3-δ –LSCF- ceramic cathodes impregnated with Ce0.8Gd0.2O3-δ –GDC- nanoparticles were characterized with electronic scanning –SEM- and transmission –TEM- microscopy. The electrochemical response as a function of temperature, oxygen partial pressure -pO2-(10-4 < pO2< 1 atm) and time was evaluated by electrochemical impedance spectroscopy –EIS. Reproducible and homogeneous impregnation were obtained, which reduce the Rp,c around 30% in the temperature range between 400ºC and 800ºC and during 500 h at 700 ºC. Fig. 1 shows a SEM image of the LSCF sample with (Fig 1b) and without impregnation (Fig 1a), the time evolution of the Rp,c for both samples (Fig 1c) and the EIS spectra of both samples at pO2 = 10-2 atm and at 700 ºC (Fig 1d). Through the pO2 measurements, the effect of the impregnation on the rate-limiting steps involved was analyzed. The EIS spectra show at least two arcs, where the pO2 dependence of the low frequency arc (µ(pO2)-n, n ~ 0.7) suggests a complex mechanism where O2-gas diffusion is not the unique driving process. The complexity of this mechanism is also verified by comparing EIS spectra collected at several pO2 by using Ar and He as gas carriers, where the observed reduction of impedance as a consequence of lower diffusion of O2 in He, was lower than expected. On the other hand, the high frequency arc, with a Gerisher-type impedance, is almost independent of pO2. Fig. 1d shows how that the surface modification by GDC-impregnation mainly affects the low frequency contribution. Although a clear effect has been observed in the Rp,c values as a consequence of impregnation, no significant difference was detected in the time evolution behavior at 700 C in air. This means that GDC impregnation reduces the cathode polarization resistance but seems not to alter significantly the origin of the degradation processes. References: [1] A. Jacobson. Materials for Solid Oxide Fuel Cells. Chem. Mater. 22 (2010), 660 [2] D. Dong. Enhancing SOFC cathode performance by surface modication through inltration. Energy Environ. Sci. 7 (2014),552 [3] S. P. Jiang. Nanoscale and nano-structured electrodes of solid oxide fuel cells by infiltration: Advances and challenges. Int J Hydrogen Energy 37 (2012), 1. Figure 1