Thin film cathodes in SOFC research: How to identify oxygen reduction pathways?

Abstract

The considerable potential of model-type thin film electrodes for the investigation of oxygen exchange pathways is demonstrated for different electrode materials on yttria-stabilized zirconia (YSZ). In particular, a correlation of voltage-driven 18 O tracer experiments and electrical ac and dc measurements has proven to be helpful when aiming at mechanistic conclusions. For Pt electrodes, two different parallel reaction pathways can be identified under equilibrium conditions. At lower temperatures, a diffusion limited path through the electrode is dominant, whereas at higher temperatures, an electrode surface path with oxygen incorporation at the three-phase boundary determines the electrochemical activity. In addition, for high cathodic polarization, an electrolyte surface path with electron transfer via YSZ outperforms both other pathways. The oxygen incorporation zones of the bulk path as well as the electrolyte surface path can be visualized by 18 O tracer incorporation experiments in combination with time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis. A successful separation of surface and bulk path can also be obtained for La 0.8 Sr 0.2 MnO 3Àd (LSM) electrodes by means of 18 O tracer incorporation at different cathodic overpotentials. Under lower polarization, a surface path with oxygen incorporation at the three-phase boundary is dominant, whereas at higher cathodic overpotential, the bulk path becomes significantly more pronounced. These changes are discussed in terms of polarization-induced changes of the ionic conductivity in the LSM electrode. Measurements on the acceptor-doped perovskite-type materials La 0.6 Sr 0.4 CoO 3Àd (LSC) and La 0.6 Sr 0.4 FeO 3Àd (LSF) illustrate the limitations of the tracer incorporation method. In the case of highly active LSC electrodes with low polarization resistances, the tracer distribution is determined by the electrolyte, and thus the active sites of the electrodes can no longer be visualized. The effect of polarization-induced changes of the electrode's electronic conductivity is demonstrated for LSF. Only a region close to the current collector remains electrochemically active owing to limited lateral electron transport.

Experimental Design for Voltage Driven Tracer Incorporation and Diffusion Studies on Oxide Thin Film Electrodes

“…According to existing defect chemical models of LSM, the voltage or partial pressure range considered here should only lead to minor changes in the electronic charge carrier concentration and thus a significant drop of the electronic conductivity along the LSM film is not expected. The SIMS measurements close to the zero volt stripe electrode lead to tracer profiles that consist of two parts, a steep 18 O decrease close to the surface followed by a shallow 18 O decay (Figs. 3b, 3c at −0.033 V).…”

“…The modified shapes compared to the non-biased situation can again be explained by considering the combination of fast grain boundary diffusion and stoichiometry polarization of the bulk; see below. The larger grains obviously reduce the impact of grain boundary diffusion on the inbound flux of 18 O and moreover, a higher fraction of the driving force drops at the LSM surface due to surface degradation (keeping in mind that the LSM 800 sample was post-annealed). This results in a less pronounced stoichiometry polarization of the grain bulk.…”

“…The resulting 18 O depth profiles were subsequently investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS 5, ION-TOF GmbH, Germany). Measurements were done in the collimated burst alignment (CBA) mode with Bi 3++ primary ions (25 keV).…”

“…Surface charging was compensated with an electron flood gun. The isotope exchange depth profiles (tracer fraction f( 18 O)) were obtained by normalizing integrated intensities I of 18 O and 16 O in the mass spectra according to…”

The Capacitance of Nickel Pattern Electrodes on Zirconia Electrolyte

“…One of the main advantages of pattern electrodes is that their geometry can be controlled in a well-defined manner and thus geometry dependencies of electrochemical parameters can be obtained much easier compared to cermet electrodes. 31,32 So far, most research effort has been put into explaining the resistive behavior of Ni/YSZ model electrodes, while capacitive effects are still much less understood, despite their importance in impedance measurements. In previous literature high area specific capacitance (ASC) values of Ni/YSZ electrodes were reported, which could not be explained by a classical Helmholtz-type double layer.…”