No abstract

Geometrically well-defined model electrodes have been employed to unambiguously elucidate the individual resistive and capacitive processes of various solid oxide fuel cell cathodes by means of impedance spectroscopy. The measurements were performed on dense, thin film-type microelectrodes of normalLa1−xnormalSrxnormalCo1−ynormalFeynormalO3−δ and related perovskite-type materials prepared by pulsed laser deposition and photolithography. It was found that the substitution of the A-site cation La in normalLa1−xnormalSrxnormalCo1−ynormalFeynormalO3−δ by Sm and especially by Ba leads to a strong enhancement of the surface exchange kinetics, whereas a variation of the Co∕Fe ratio between 0 and 1 has only little effect on this quantity at temperatures around 750°C . Furthermore, it has been studied how the electrochemical activation effect, i.e., the strong reduction of the surface exchange resistance after application of a large dc bias, depends on composition.

It is shown that the electrochemical resistance of mixed conducting solid oxide fuel cell ͑SOFC͒ model cathodes can be reduced drastically by a short but strong dc polarization of the cell. The samples investigated are dense thin-film microelectrodes of La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3−␦ on a yttria-stabilized zirconia solid electrolyte. Relative performance improvements of more than two orders of magnitude can be achieved with a cathodic dc bias of the order of 1 V, applied for a few minutes at fuel cell operating temperature. The positive effect on the electrode performance corresponds to an acceleration of the oxygen surface exchange reaction, initially the resistance determining process. For this surface-related resistance, absolute values as low as 0.065 ⍀ cm 2 at 700°C have been obtained. After such an activation, the resistance slowly increases again on a much larger time scale, indicating the possibility of a steady performance enhancement by a periodic activation with short dc pulses. X-ray photoelectron spectra show that a strong cathodic polarization severely changes the cation concentrations within the outermost surface layer of the electrode, and these field-induced surface compositional changes are assumed to be the main cause of the performance improvement.High-temperature solid oxide fuel cells ͑SOFCs͒ receive considerable attention because they enable a highly efficient conversion of chemical into electric energy. A major goal in SOFC research is the development of cathodes with a sufficiently low electrochemical resistance ͑ϳ0.15 ⍀ cm 2 ͒ at operating temperatures substantially below 800°C. 1-3 It is generally accepted that this is hardly achievable with lanthanum strontium manganite ͑LSM͒-based oxygen electrodes, which are the most thoroughly studied and technically developed SOFC cathodes at the moment. Focus has been shifted toward other materials that offer a sufficient electrochemical performance already in the intermediate-temperature range, and particularly the mixed conducting perovskites of the ͑La,Sr͒͑Co,Fe͒O 3−␦ family are considered as promising candidates.In the literature, activation effects upon dc polarization have been reported for porous LSM and LSM/YSZ ͑yttria-stabilized zirconia͒ composite electrodes, 4-11 as well as for Pt electrodes. 11-13 However, a general agreement on the causes of these activation effects has not been achieved yet, and the reported magnitudes of the effect and the suggested mechanisms strongly vary in the literature. Measurements on morphologically and crystallographically well-defined electrodes are not available but are highly desirable in this context. 11 Moreover, to the best of the authors' knowledge, a significant performance improvement by electrochemical polarization has not been observed for the technologically important class of mixed conducting electrode materials.In this work, a strong electrochemical activation effect is reported for mixed conducting La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3−␦ ͑LSCF͒ electrodes. The performance improvement is achieved...

No abstract

No abstract