Mixed conductivity and electrochemical behavior of (La_0.75Sr_0.25)_0.95Cr_0.5Mn_0.5O_{3 −} _δ

The electronic and oxygen-ionic transport in (La0.75Sr0.25)(0.95)Cr0.5Mn0.5O3-delta, a member of promising family of solid oxide fuel cell (SOFC) anode materials, was studied at 1023-1273 K in the oxygen partial pressure range from 10(-2)0 to 0.5 atm. In oxidizing and moderately reducing atmospheres, this perovskite exhibits a predominant p-type electronic conductivity, which ties in the range 20-35 S/cm and is essentially p(O-2)-independent. Reducing p(O-2) below 10(-16)-10(-12) atm leads to a drastic increase in the oxygen vacancy concentration, ionic conductivity and oxygen permeability, whilst the total conductivity decreases down to 1-3 S/cm. The ion transference numbers, calculated from the oxygen permeation data and measured by the faradaic efficiency technique controlling oxygen pressures at both sides of dense ceramic membranes, vary in the range 9 x 10(-7) to 8 x 10(-5) at 1223-1273 K, increasing with temperature. The average thermal expansion coefficients in air increases from 10.8 x 10(-6) K-1 at 373-923 K up to 14.1 x 10(-6) K-1 at 1223-1523 K. Under both oxidizing and reducing conditions, the electrochemical behavior of porous (La-0.75 Sr-0.25)(0.95)Cr0.5Mn0.5O3-based electrodes applied onto (La0.9Sr0.1)(0.98)Ga0.8Mg0.2O3-delta solid electrolyte suggests a key role of electronic transport-related processes. As a result, the electrode performance can be significantly enhanced by optimizing current collector and/or by introducing an additional electronically-conductive component, such as metallic Ni or Ag. Further decrease of overpotentials may be achieved via incorporation of electrocatalytically active additions, including praseodymium oxide in oxidizing atmospheres and ceria at low P(O-2). (c) 2006 Elsevier B.V. All rights reserved.</p

Behavior of (La,Sr)CoO3- and La2NiO4-based ceramic anodes in alkaline media: compositional and microstructural factors.

The behavior of dense ceramic anodes made of perovskite-type La1-x-ySrxCo1-zAlzO3-δ (x=0.30-0.70; y=0-0.05; z=0-0.20) and K2NiF4- type La2Ni1-xMexO4+δ (Me=Co, Cu; x=0-0.20) indicates significant influence of metal hydroxide formation at the electrode surface on the oxygen evolution reaction (OER) kinetics in alkaline solutions. The overpotential of cobaltite electrodes was found to decrease with time, while cyclic voltammetry shows the appearance of redox peaks characteristic of Co(OH)(2)/CoOOH. This is accompanied with increasing effective capacitance estimated from the impedance spectroscopy data, because of roughening of the ceramic surface. The steady-state polarization curves of (La,Sr)CoO3-δ in the OER range, including the Tafel slope, are very similar to those of model Co(OH)(2)-La(OH)(3) composite films where the introduction of lanthanum hydroxide leads to decreasing electrochemical activity. La2NiO4-based anodes exhibit a low electrochemical performance and poor stability. The effects of oxygen nonstoichiometry of the perovskite-related phases are rather negligible at high overpotentials but become significant when the polarization decreases, a result of increasing role of oxygen intercalation processes. The maximum electrocatalytic activity to OER was observed for A-site-deficient (La0.3Sr0.7)(0.97)CoO3-δ, where the lanthanum content is relatively low and the Co4+ concentration determined by thermogravimetric analysis is highest compared to other cobaltites. Applying microporous layers made of template-synthesized nanocrystalline (La0.3Sr0.7)(0.97)CoO3-δ leads to an improved anode performance, although the effects of microstructure and thickness are modest, suggesting a narrow electrochemical reaction zone. Further enhancement of the OER kinetics can be achieved by electrodeposition of cobalt hydroxide- and nickel hydroxide- based films. © 2008, Springer. The original publication is available at www.springerlink.co