Thin compact La-Sr-Co-O composite films consisting of (La,Sr)CoO 3-δ and (La,Sr) 2 CoO 4+δ crystallites were prepared by compositional phase separation during pulsed laser deposition. While the total La:Sr ratio is kept constant at 70:30, different phase ratios and crystallite sizes are investigated. The effect of hetero-interfaces on the oxygen reduction reaction (ORR) kinetics was investigated by impedance spectroscopy. The enhancement relative to the appropriate reference activity is quantified in terms of the triple phase boundary (TPB) length specific activity k TPB , resulting in k TPB = 2-9 · 10 −13 cm 2 /s at 600 • C. A metastable Co-deficient perovskite phase La 0.7 Sr 0.3 Co 0.9 O 3-δ formed in La-Sr-Co-O films under certain conditions was found to be highly active with an effective rate constant k q = 3 · 10 −6 cm/s at 600 • C, representing one possible source for the activity enhancement in these composite films. (La,Sr)CoO 3-δ /(La,Sr) 2 CoO 4±δ (perovskite phase, P/RuddlesdenPopper phase, R) two-phase materials were reported in literature to show enhanced activity for the oxygen reduction reaction (ORR) compared to reference single-phase samples 1-11 making them a promising new material for highly active cathodes in intermediate temperature solid oxide fuel cells. This enhancement was attributed to effects at the P/R/O 2 triple phase boundary (TPB). It is important to note that -in contrast to the traditional use -here TPB does not refer to the contact of gas phase, electrode and electrolyte material, but to locations where hetero-grain boundaries of two components of the cathode material are exposed to the atmosphere. The literature approaches to investigate this ORR enhancement at P/R hetero-interfaces (additional ORR activity related to TPBs, which in this work is simply denoted as "TPB effect") can be roughly divided into local ORR activity measurements by 18 O exchange and SIMS experiments, 1-4 and integral activity measurements of macroscopic two-phase electrodes with high TPB density by impedance spectroscopy.
5-11With a local measurement, the activity of the surface at a single TPB as well as that at neighboring P and R phases (reference activities) can be determined. If sufficiently high lateral resolution and vertical sampling depth is available, the strength of the TPB effect can be quantified in terms of a TPB length specific contribution to the activity. With an integral measurement of a whole electrode which contains a large amount of P and R grains and TPBs, an average activity is measured with contributions from the grains (surfaces with effective rate constants k P (x) in P La 1-x Sr x CoO 3-δ with Sr fraction x and k R (y) in R (La 1-y Sr y ) 2 CoO 4±δ with Sr fraction y) and from TPBs, which has to be compared to an appropriate reference activity and related to the TPB density for quantification.In literature, local activity measurements were performed either on ceramic samples where P/R hetero-interfaces formed sporadically by spontaneous phase separation, 1,3 or as illustrated ...
Several recent publications have claimed that the oxygen exchange surface reaction is accelerated at hetero grain boundaries betwen La1-xSrxCoO3-
d and (La1-ySry)2CoO4+
d. To investigate this effects in a greater detail, PLD films with a high density of such hetero grain boundaries are prepared from a two-phase target, and characteristic crystal morphologies are observed. The issue of preferential Sr accumulation in the (La1-ySry)2CoO4+
d Ruddlesden-Popper phase is exemplarily quantified for two-phase powder samples based on X-ray diffraction. The implications of this uneven Sr distribution (occurring similarly in the two-phase films) for the analysis of the oxygen exchange kinetics and quantification of the specific hetero grain boundary effect are discussed
This study presents the temperature and pO 2 -dependence of the effective oxygen reduction rate constant k q at the surface of dense films with a high density of (La,Sr)CoO 3-δ /(La,Sr) 2 CoO 4+δ heterointerfaces. These films were prepared by self-assembly during PLD from a single target, yielding a grain size on the order of 50 nm. While their activity is higher than the reference activity (in absence of a specific effect of the hetero-interfaces on kinetics), the variation in the pO 2 -dependence for different samples prevents a simple mechanistic interpretation.
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