A novel approach for analyzing electrochemical properties of mixed conducting solid oxide fuel cell anode materials by impedance spectroscopy

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.

The Electrochemical Properties of Sr(Ti,Fe)O3-δfor Anodes in Solid Oxide Fuel Cells

“…Circuit models elaborated in Ref. 4 for different current collector positions may explain the slightly different shape of the two impedance spectra due to different ion conduction paths. The following measurements, however, show that also the mechanism of oxygen exchange depends on the placement of the current collector.…”

“…Equivalent circuit models for such microelectrodes were derived in Ref. 4. The width and distance between the current collecting fingers were both varied between 5, 15 and 30 μm, resulting in the 9 different current collector geometries shown in Figure 1e.…”

“…[1][2][3] Some of these materials are chemically stable and mixed conducting also in humidified hydrogen atmosphere, 4 which makes them applicable in SOFC anodes. Several mixed conductors, such as acceptor doped (La,Sr)(Cr,Mn)O 3 , [5][6][7][8] donor-doped SrTiO 3 9,10 and Sr(Ti,Fe)O 3-δ -Ce 0.9 Gd 0.1 O 2-δ (STF-GDC) composites 11 were investigated in form of porous SOFC anodes.…”

“…17,[27][28][29][30][31][32] Lithographically patterned Pt current collectors were applied beneath the STF thin films to compensate the comparatively low electronic conductivity in reducing conditions. 4,32 Pt current collectors on top were tested as well and a high electro-catalytic activity was observed at the Pt-STF-atmosphere triple phase boundary. Also increased electronic and ionic conductivity and decreased polarization resistance were found on electrodes with higher Fe content.…”

“…Recently, a special electrode and current collector geometry was thus developed for investigating such weakly electron conducting materials. 4 Fitting of the impedance spectra to a proper equivalent circuit model enabled separation of resistive contributions from the surface reaction and in-plane charge transport. The circuit model proposes that electrochemically driven oxygen exchange is only possible within a characteristic distance (L C ) from the current collectors and indeed this could be visualized by 18 O isotope exchange.…”