Interpreting impedance data of SOFC electrodes: the benefits of variation in microstructure and composition

International audienc

Use of a distribution function of relaxation times (DFRT) in impedance analysis of SOFC electrodes

International audienceElectrochemical Impedance Spectroscopy (EIS) is a frequently used method to characterize electrodes for Solid Oxide Fuel Cells (SOFC) or Electrolyzer Cells (SOEC). The porous microstructures, use of composite structures and sometimes extra functional layers in an electrode, result often in impedance spectra that are difficult to analyze. Transformation of the impedance into a distribution function of relaxation times (DFRT) is about to become a new standard in EIS analysis.This inversion to the τ-domain requires solving a Fredholm integral of the second kind, which is known as an ‘ill-posed inverse problem’. Hence the resulting DFRT's should not be trusted directly. In cases were impedance data can be modelled satisfactory with an Equivalent Circuit (EqC), built of known dispersion relations (e.g. (RQ), Gerischer, Finite Length Warburg) an analytic distribution function, G(τ), can be constructed. This can be compared with the inversion results obtained from Fourier Transform (FT), Tikhonov Regularization (TR) and multi-(RQ) CNLS fits (m(RQ)fit), thus allowing evaluation and validation of these methods This is illustrated in this contribution with four examples of SOFC cathodes with quite different properties. The results apply equally well to SOFC anodes (or SOEC cathodes)

Développement d’électrodes innovantes pour piles à combustible réalisées sur support métallique poreux

National audienc

Oxide ion conductors for solid oxide cells

International audienc

Développement d’électrodes innovantes pour piles à combustible réalisées sur support métallique poreux

National audienc

Red-ox behaviour in the La0.6Sr0.4CoO3 +/-delta-CeO2 system

The compositions in the (100 - x)La0.6Sr0.4CoO3-delta-xCeO(2) (LSCC) system with 5 &lt; x &lt; 76 are two-phase at room temperature. They consist of the modified perovskite with rhombohedral symmetry (R (3) over barc) and modified ceria with fluorite structure (Fm (3) over barm). The cross-dissolution of La, Sr, Co and Ce cations between the initial La0.6Sr0.4CoO3-delta(LSC) and CeO2 takes place and results in the modification of the initial phases. This is particularly important for the modified ceria. The lattice parameter of the modified ceria increases due to the dissolution of La and Sr cations with larger ionic radii, thereby changing noticeably the oxygen sublattice in the fluorite structure. Above 300 degrees C LSCCx composites are three-phase due to the reversible change in the symmetry from rhombohedral (R (3) over barc) to cubic (Pm (3) over barm) within the perovskite phase. Red-ox behaviour of the LSCC composites has been explored under air and argon atmospheres in terms of evolution of the chemical composition at the grain's surface and phase interfaces, formation of oxygen vacancies and thermochemistry of this process. Reversible red-ox behaviour was observed in LSCCx with x - 8-37 most probably due to an observed high surface concentration of Co cations, that can be easily involved in the reduction/re-oxidation cycle. The increase in the surface concentration of Ce4+ cations together with the decrease in surface concentration of Co cations seems to result in the differences in the reduction and oxidation behaviour under air in LSCCx with x = 57-76. Formation of oxygen vacancies in LSC, LSCC02 and LSCCx with x = 5-76 in air was not accompanied by any distinct thermal events. This process becomes more endothermic with further increase in oxygen nonstoichiometry (delta) above certain values: delta &gt; 0.08 in LSC, delta &gt; 0.13 in LSCC02, and LSCC with x = 5-76. The LSCCx with x = 5-37 and with x = 57-76 show slightly different reduction behaviour under a(o(2)) = 7.4 x 10(-5). In the composites with a relatively low CeO2 content, the extent of the reduction is proportional to the Co content in a composition, whereas the reduction of the LSCCx with x = 57-76 was more significant than expected. The changes in the enthalpy of oxygen vacancy formation and the kinetics of reduction have been discussed.</p

Red-ox behaviour in the La0.6Sr0.4CoO3 +/-delta-CeO2 system

The compositions in the (100 - x)La0.6Sr0.4CoO3-delta-xCeO(2) (LSCC) system with 5 &lt; x &lt; 76 are two-phase at room temperature. They consist of the modified perovskite with rhombohedral symmetry (R (3) over barc) and modified ceria with fluorite structure (Fm (3) over barm). The cross-dissolution of La, Sr, Co and Ce cations between the initial La0.6Sr0.4CoO3-delta(LSC) and CeO2 takes place and results in the modification of the initial phases. This is particularly important for the modified ceria. The lattice parameter of the modified ceria increases due to the dissolution of La and Sr cations with larger ionic radii, thereby changing noticeably the oxygen sublattice in the fluorite structure. Above 300 degrees C LSCCx composites are three-phase due to the reversible change in the symmetry from rhombohedral (R (3) over barc) to cubic (Pm (3) over barm) within the perovskite phase. Red-ox behaviour of the LSCC composites has been explored under air and argon atmospheres in terms of evolution of the chemical composition at the grain's surface and phase interfaces, formation of oxygen vacancies and thermochemistry of this process. Reversible red-ox behaviour was observed in LSCCx with x - 8-37 most probably due to an observed high surface concentration of Co cations, that can be easily involved in the reduction/re-oxidation cycle. The increase in the surface concentration of Ce4+ cations together with the decrease in surface concentration of Co cations seems to result in the differences in the reduction and oxidation behaviour under air in LSCCx with x = 57-76. Formation of oxygen vacancies in LSC, LSCC02 and LSCCx with x = 5-76 in air was not accompanied by any distinct thermal events. This process becomes more endothermic with further increase in oxygen nonstoichiometry (delta) above certain values: delta &gt; 0.08 in LSC, delta &gt; 0.13 in LSCC02, and LSCC with x = 5-76. The LSCCx with x = 5-37 and with x = 57-76 show slightly different reduction behaviour under a(o(2)) = 7.4 x 10(-5). In the composites with a relatively low CeO2 content, the extent of the reduction is proportional to the Co content in a composition, whereas the reduction of the LSCCx with x = 57-76 was more significant than expected. The changes in the enthalpy of oxygen vacancy formation and the kinetics of reduction have been discussed.</p

Influence of the Lactotripeptides Isoleucine–Proline–Proline and Valine–Proline–Proline on Systolic Blood Pressure in Japanese Subjects: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

<div><p>Background</p><p>The lactotripeptides isoleucine–proline–proline (IPP) and valine–proline–proline (VPP) have been shown to decrease systolic blood pressure (SBP) in several populations, but the size of the effect varies among studies. We performed a meta-analysis including all published studies to evaluate the SBP-lowering effect of IPP/VPP in Japanese subjects more comprehensively.</p><p>Methods and Findings</p><p>Eligible randomized controlled trials were searched for within four bibliographic databases, including two Japanese ones. Eighteen studies (including a total of 1194 subjects) were included in the meta-analysis. A random effect model using the restricted maximum likelihood (REML) estimator was used for the analysis. The analysis showed that consumption of IPP/VPP induced a significant reduction in SBP as compared with placebo in Japanese subjects, with an estimated effect of -5.63 mm Hg (95% CI, -6.87 to -4.39, P<0.0001) and no evidence of publication bias. A significant heterogeneity between series was evident, which could be explained by a significant influence of the baseline blood pressure status of the subjects, the effect of IPP/VPP on SBP being stronger in hypertensive subjects (-8.35 mm Hg, P<0.0001) than in non-hypertensive subjects (-3.42mm Hg, P<0.0001). Furthermore, the effect of IPP/VPP on SBP remained significant when limiting the analysis to series that tested the usual doses of IPP/VPP consumed daily (below 5 mg/d), with estimated effects of -6.01 mm Hg in the overall population and -3.32 mm Hg in non-hypertensive subjects.</p><p>Conclusions</p><p>Results from this meta-analysis show that IPP/VPP lactotripeptides can significantly reduce office SBP in Japanese subjects with or without overt hypertension, and for doses that can potentially be consumed as an everyday supplement. This suggests that these peptides could play a role in controlling blood pressure in Japanese subjects. The systematic review protocol was published on the PROSPERO register (CRD42014014322).</p></div