Application of FIB-TOF-SIMS and FIB-SEM-EDX ...

ECS Transactions, 57 (1) 581-587 (2013) 10.1149/05701.0581ecst ©The Electrochemical Society

Application of FIB-TOF-SIMS and FIB-SEM-EDX Methods for the Analysis of Element Mobility in Solid Oxide Fuel Cells. R. Kanarbik, P. Möller, I. Kivi, and E. Lust Institute of Chemistry, University of Tartu, Ravila str. 14a, 50411 Tartu, Estonia

The solid oxide fuel cell single cells with porous Pr0.6Sr0.4CoO3-δ and La0.6Sr0.4CoO3-δ (PSCO, LSCO respectively) cathodes on compact Ce0.9Gd0.1O2-δ|Zr0.85Y0.15O2-δ or Ce0.9Gd0.1O2-δ|Zr0.85Sc0.15O2-δ bilayered electrolytes deposited onto Ni-Zr0.85Y0.15O2-δ (Ni-ZYO) or Ni-Ce0.9Gd0.1O2-δ (Ni-CGO) supporting anode were prepared for ion (element) mobility studies. Focused ion beam - time of flight secondary ion mass spectrometry (FIB-TOF-SIMS) method in addition to FIB-SEM, SEM-EDX and XRD methods has been used for analysis of mass-transfer (interlayer diffusion) of cathode electrode elements, demonstrating that during PSCO and LSCO sintering at 1100°C on to CGO|ZYO or CGO|ZScO bilayered electrolyte, noticeable mass-transfer of Sr2+ cations through the partially microporous CGO has been verified using FIB-TOF-SIMS and SEM-EDX methods. The single cells have been additionally studied using cyclic voltammetry, electrochemical impedance and chronoamperometry methods and high power densities have been demonstrated.

Introduction The intermediate temperature solid oxide fuel cells (IT-SOFC) based on Pr1-xSrxCoO3-δ and La1-xSrxCoO3-δ cathodes are at interest for the efficient and environmentally friendly conversion of chemical energy into electricity and heat energy (1-5). However, the time stability of single cells based on Ce1-xGdxO2-δ or Ce1-xSmxO2-δ electrolyte deposited onto Ni-Zr0.85Y0.15O2-δ or Ni-Ce0.9Gd0.1O2-δ supporting anodes is rather low. One of the reasons may be the electronic conductivity of Ce1-xGdxO2-δ or Ce1-xSmxO2-δ electrolyte under reducing conditions (ie. electroreduction of Ce4+ to Ce3+). For the elimination of this effect, usually the additional protective layer of Zr1-xYxO2-δ has been depositied onto NiZr1-xYxO2-δ supporting anode. However, the experimental data obtained using cyclic voltammetry and impedance methods indicate to the small increase of very high series resistance of single cell after long lasting polarization. Therefore in this paper more systematic FIB-TOF-SIMS and FIB-SEM-EDX analysis have been conducted to study the possible mass transfer processes inside partially meso-macroporous Ce1-xGdxO2-δ or Ce1-xSmxO2-δ, observed and discussed in our earlier papers(1-5).

Preparation of Single Cells The corresponding single cells were constructed for the studies of element mobility: supporting Ni-/yttrium stabilized zirconia (YSZ) anode and bi-layered

581

Downloaded on 2015-10-22 to IP 220.142.89.63 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

ECS Transactions, 57 (1) 581-587 (2013)

YSZ|GDC electrolyte layers were screen-printed and co-sintered at temperature 1300 °C for 10 hours. For comparison Ni-Ce0.9Gd0.1O2-δ anode and different co-sintered bi-layered electrolytes (Zr0.85Sc0.15O2-δ | Ce0.9Gd0.1O2-δ and Zr0.85Y0.15O2-δ | Ce0.9Gd0.1O2-δ with different thickness of electrolyte layers, discussed later, have been prepared and sintered at the same thermal conditions. Thereafter the Pr0.6Sr0.4CoO3-δ or La0.6Sr0.4CoO3-δ cathode was screen printed onto the gadolinium doped ceria (GDC) electrolyte and sintered at T=1100 °C for 3 hours. Nitrate-glycine combustion method was used for preparation of the raw porous cathode powders (4, 5, 7-10, 12-14). Electrochemical measurements have been conducted in 20%, 50% and 97% O2 in N2 oxidizer mixture and 20%, 50% and 97% H2 in Ar, humidified at room temperature has been used as fuel. After electrochemical tests have been conducted, the single cell was cooled to room temperature under working atmospheres and finally the samples were mechanically broken or FIB-milled to expose the cross sections of all electrode and electrolyte layers for determining of specific element diffusion through the electrolyte and possibly through the electrolyte protective Zr1-xYxO2-δ layers. The single cells were tested in ProboStat alumina tube sample holder (Norwegian Electro Ceramics AS) using 99.9% pure gold ring on the electrolyte as a gasket which has been softened at 1020°C for complete separation of the anode and cathode compartments. To ensure gastight separation, the gold ring was, prior installation, generously coated with gold paste (FuelCell Materials) and leaks were tested using high flow of air with slight overpressure on the cathode compartment, while checking the absence of any gas flow on the gas output line of the cathode compartment. All electrochemical tests were conducted on electrochemical analyser set Solartron 1287A potentiostat with 1260A impedance analyser while, accurately controlling gas dosing with gas mass flow controllers (Bronkhorst High-Tech B.V). The gas purities were or exceeded level 5.0 (99.999%).

Physical Characterization of Single Cells For the detailed analysis of element distribution throughout the single cell under study SEM-EDX(Zeiss EVO with Oxford X-Max 80mm2), FIB-TOF-SIMS(Ulvac-PHI), and XRF(Rigaku) methods have been applied. For porosity measurements FIB-SEM (Fei Helios) method has been applied to cathode, anode and CGO electrolyte layers separately. Also XRD method has been applied (Fig. 1), demonstrating that well crystallized NiZYO anodes, CGO electrolyte or Pr0.6Sr0.4CoO3-δ and La0.6Sr0.4CoO3-δ cathodes have been prepared. The calculated total porosity, Vtot, values were high for the specially designed very porous cathode and anode. The calculations of the total porosity were performed using Amira software package (Visualization Sciences Group) and calculations on the pore size distributions (PSD) were performed using Fiji image processing package with Beats plugin. Less porous structure for CGO electrolyte was calculated from FIB-SEM data. Using the same dataset, the continuous pore size distribution curves (Fig 2) were also calculated. For anode and cathode, the dataset was constructed using 250 images, but 350 images were used for less porous CGO electrolyte. The images were collected with FEI Helios dualbeam instrument with AutoSliceandView automation software using

582

Downloaded on 2015-10-22 to IP 220.142.89.63 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

ECS Transactions, 57 (1) 581-587 (2013)

100nm Slice spacing. Vtot values calculated were: Vtot>60% for anode, Vtot>65% for cathode and only Vtot