ABSTRACT
This work deals with the effect of sulfur incorporation into model-type GDC thin films on their in-plane ionic conductivity. By means of impedance measurements, a strongly deteriorating effect on the grain boundary conductivity was confirmed, which additionally depends on the applied electrochemical polarisation. To quantify the total amount of sulfur incorporated into GDC thin films, online-laser ablation of solids in liquid (online-LASIL) was used as a novel solid sampling strategy. Online-LASIL combines several advantages of conventional sample introduction systems and enables the detection of S as a minor component in a very limited sample system (in the present case 35 µg total sample mass). To reach the requested sensitivity for S detection using an inductively coupled plasma-mass spectrometer (ICP-MS), the reaction cell of the quadrupole instrument was used and the parameters for the mass shift reaction with O2 were optimised. The combination of electrical and quantitative analytical results allows the identification of a potential sulfur incorporation pathway, which very likely proceeds along GDC grain boundaries with oxysulfide formation as the main driver of ion transport degradation. Depending on the applied cathodic bias, the measured amount of sulfur would be equivalent to 1-4 lattice constants of GDC transformed into an oxysulfide phase at the material's grain boundaries.
ABSTRACT
Online-laser ablation of solids in liquid (online-LASIL) coupled with ICP-MS detection was used as a new sampling strategy for the analysis of complex metal oxide (CMO) thin films. The in-house built and optimized online-LASIL ablation cell provides the unique possibility to correlate the signal intensities with the spatial origin of the signal at the sample. For that purpose a particle transport with as little particle dispersion as possible is crucial. To demonstrate the 2D imaging capability of this technique, geometrically structured samples with varying composition were prepared by pulsed laser deposition (PLD) and ion beam etching procedures. These thin films with a thickness of 220 nm were spatially resolved analysed. As a result, 2D intensity maps obtained by online-LASIL can be reported for the first time. Additionally a new approach for simultaneous online quantification was developed by adopting the standard addition concept allowing to correct for instrumental drifts of long time measurements.
ABSTRACT
Model-type sputter deposited platinum microelectrodes with different grain sizes were investigated on single crystalline yttria stabilized zirconia (YSZ) by means of impedance spectroscopy. Measurements on single platinum microelectrodes could be continuously performed for > 100 h and from 250 to 800 °C without losing contact. From the temperature dependence, two parallel reaction pathways for oxygen reduction could be identified. Above 450 °C, a surface path with a rate determining step located at the three phase boundary is predominant. Its polarization resistance is independent of the Pt grain size and exhibits an activation energy of ca. 1.8 eV. In the low temperature regime (< 450 °C) a bulk path through Pt was verified, with an electrode polarization resistance depending on the Pt grain size. This resistance is only slightly thermally activated and the rate limiting step is most probably oxygen diffusion along Pt grain boundaries.
ABSTRACT
For application of acceptor-doped mixed conducting oxides as solid oxide fuel cell (SOFC) anodes, high electrochemical surface activity as well as acceptable electronic and ionic conductivity are crucial. In a reducing atmosphere, particularly the electronic conductivity of acceptor-doped oxides can become rather low and the resulting complex interplay of electrochemical reactions and charge transport processes makes a mechanistic interpretation of impedance measurements very complicated. In order to determine all relevant resistive and capacitive contributions of mixed conducting electrodes in a reducing atmosphere, a novel electrode design and impedance-based analysis technique is therefore introduced. Two interdigitating metallic current collectors are placed in a microelectrode, which allows in-plane measurements within the electrode as well as electrochemical measurements versus a counter electrode. Equivalent circuit models for quantifying the spectra of both measurement modes are developed and applied to simultaneously fit both spectra, using the same parameter set. In this manner, the electronic and ionic conductivity of the material as well as the area-specific resistance of the surface reaction and the chemical capacitance can be determined on a single microelectrode in a H2-H2O atmosphere. The applicability of this new tool was demonstrated in SrTi0.7Fe0.3O(3-δ) (STFO) thin film microelectrodes, deposited on single-crystalline yttria-stabilized zirconia (YSZ) substrates. All materials parameters that contribute to the polarization resistance of STFO electrodes in a reducing atmosphere could thus be quantified.
