Oxygen exchange measurements on perovskites as cathode materials for solid oxide fuel cells

The present study reports thermal and electrical properties of Sr 1-x Y x CoO 2.5+δ (x = 0-0.40) as a promising cathode for intermediatetemperature solid oxide fuel cells. The results show that x = 0.10 is the best composition possessing a single primitive cubic perovskite structure, stable conductivity and the lowest polarization resistance. Thermogravimetric analysis indicates an oxygen intake from RT to ∼375 • C, above which oxygen loss occurs. The oxygen gain-loss behavior corresponds well with the conductivity increase-decrease trending, reflecting that oxygen-nonstoichiometry controls the hole-concentration (or oxidation-state of Co-ions). Electrochemical impedance spectroscopy analysis further reveals that the overall ORR polarization consists of a faster charge-transfer and a slower surface oxygen exchange.

mentioning

confidence: 99%

Thermal and Electrical Stability of Sr_0.9Y_0.1CoO_2.5+δas a Promising Cathode for Intermediate-Temperature Solid Oxide Fuel Cells

Jiang

Wang

Xiong

et al. 2016

“…Furthermore, it is often difficult to accurately separate k * and D * from fitting the depth profiles. Conductivity and curvature relaxation methods [4][5][6][7] face similar challenges with fabrication of bulk pellets or thin films, and separating both coefficients unambiguously. Here, the response to an applied chemical potential gradient is measured, yielding the chemical diffusion coefficient (D chem ) and the chemical surface reaction coefficient (k chem ).…”

mentioning

confidence: 99%

Determination of Electrode Oxygen Transport Kinetics Using Electrochemical Impedance Spectroscopy Combined with Three-Dimensional Microstructure Measurement: Application to Nd₂NiO_4+δ

Yakal-Kremski

Mogni

Montenegro-Hernández

et al. 2014

Oxygen reduction kinetic parameters -oxygen ion diffusion D δ , molar surface exchange rate O and surface exchange coefficient k -were determined for porous Nd 2 NiO 4+δ solid oxide fuel cell cathodes as a function of temperature and oxygen partial pressure by analyzing electrochemical impedance spectroscopy data using the Adler-Lane-Steele model. Electrode microstructural data used in the model calculations were obtained by three-dimensional focused ion beam-scanning electron microscope tomography. Cathodes were fabricated using Nd 2 NiO 4+δ powder derived from a sol-gel method and were tested as symmetrical cells with LSGM electrolytes. The oxygen surface exchange rate exhibited a power-law dependency with oxygen partial pressure, whereas the oxygen diffusivity values obtained varied only slightly. The present analysis suggests that the O-interstitial diffusion has a bulk transport path, whereas the surface exchange process involves dissociative adsorption on surface sites followed by O-incorporation. For Nd 2 NiO 4+δ at 700 • C and 0.2 atm oxygen pressure, D δ = 5.6 · 10 −8 cm 2 s −1 , O = 2.5 · 10 −8 mol · cm −2 s −1 . The present D δ and O values and their activation energies are slightly different to those previously reported for Nd 2 NiO 4+δ using other measurement methodologies, and lower than typical state-of-the-art Co-rich perovskites. However, the average k δ = 1.0 10 −5 cm · s −1 at 700 • C is comparable to those of fast oxygen exchange rate perovskites.

“…[42][43][44] The model assumes that the oxygen surface exchange reaction obeys first-order kinetics, and that both kinetic parameters are constant during the pO 2 -step. Multiple measurements of k chem and D chem were performed at various temperatures in order to estimate the experimental errors (see error bars in Fig.…”

Section: Methodsmentioning

confidence: 99%

Impact of SO₂on the Oxygen Exchange Kinetics of the Promising SOFC/SOEC Air Electrode Material La_0.8Ca_0.2FeO_3-δ

Berger

Bucher

Gspan

et al. 2017

The mass and charge transport properties of La 0.8 Ca 0.2 FeO 3-δ (LCF82) were determined at 600-800 • C and pO 2 = 0.1 bar. The electronic conductivity is in the range of 112 S cm −1 at 700 • C. The chemical surface exchange coefficient (k chem ) and the chemical diffusion coefficient of oxygen (D chem ) were measured. LCF82 shows exceptionally fast oxygen exchange kinetics in pure O 2 -Ar at 700 • C. Long-term measurements at 700 • C in pure O 2 -Ar showed an excellent stability of the kinetic parameters during 1000 h. However, when 2 ppm of sulfur dioxide were added, k chem decreased by a factor of 40 within the first 24 h. After further 1000 h, the total decrease in k chem amounted to two orders of magnitude. Post-test analyses of LCF82 were performed by SEM-EDXS, XPS, and STEM. Ca-enrichment and La-/Fe-depletion of the LCF82 surface occurred during the first 1000 h without SO 2 . At the surface of the sample exposed to 2 ppm SO 2 for additional 1000 h, CaSO 4 crystals with diameters of 1-2 μm and an underlying layer of Fe 2 O 3 (thickness approx. 300-450 nm) were found. Remarkably, LCF82 shows faster oxygen exchange kinetics than La 0.6 Sr 0.4 CoO 3-δ even in the degraded state. Therefore, LCF82 is suggested as a promising material for SOFC and SOEC air electrodes.