Effect of Chromium Poisoning on Performances of Lscf-SDC Composite Oxygen Electrodes Under Reversible Operation of Solid Oxide Cells

Abstract

Solid oxide cells (SOCs) are high-temperature electrochemical energy conversion and storage devices, which can be operated under solid oxide electrolysis cell (SOEC) mode to produce pure hydrogen from water vapor with high efficiency. SOCs can also be operated in reverse under solid oxide fuel cell (SOFC) mode, providing high-energy conversion efficiency to convert hydrogen fuel to electricity. Similar to SOFCs, cost-effective metallic (alloy) can be used as interconnects for planar-type SOCs operated at reduced temperatures (≤ 800 °C). However, without an effective protective coating on the Cr-containing alloy, volatile Cr-containing species are generated at high temperatures in oxidizing atmosphere. It has been recognized that several oxygen electrodes such as Sr-doped LaMnO3 (LSM) or La0.6Sr0.4Co0.2Fe0.8O3 - δ (LSCF) suffer from a serious Cr-poisoning.1–3 We have recently succeeded in improving the reversible performances of the single-layer LSCF-Ce0.8Sm0.2O1.9 (SDC) composite O2 electrode with SDC interlayer at 800 and 900 °C,4–6 compared with those of an identical-type electrode in our previous work.7 Though the degradation of the LSM or LSCF electrodes by Cr-poisoning has been studied under either SOFC or SOEC operation mode, separately, to the best of our knowledge, the Cr-poisoning have not been investigated on SOCs under simultaneously reversible operation of SOFC/SOEC modes. Here, we examined for the first time the effect of Cr-poisoning on the performances of LSCF-SDC oxygen electrodes under simultaneously cathodic and anodic polarization conditions in the presence of Fe-Cr alloy. A symmetrical cell [LSCF-SDC ǀSDC interlayerǀ ScSZ electrolyte ǀSDC interlayerǀ LSCF-SDC] was prepared.4,5 Here, we briefly describe the protocol. The SDC interlayer was deposited on both sides of the scandia-stabilized zirconia (ScSZ, (ZrO2)0.89(Sc2O3)0.10(CeO2)0.01) electrolyte by spin coating of Ce(III) 2-ethylhexanoate and Sm(III) 2-ethylhexanoate mixed solution, followed by heat-treatment at 1050 oC for 2 h. The LSCF-SDC paste (60:40 volume ratio) was tape-casted onto the SDC interlayer and heat-treated at 1050 oC for 1 h. The air reference electrode consisted of a Pt wire wound around the ScSZ disk. A commercial Fe-Cr alloy (RA446, 23–27 wt% Cr, Rolled Alloy, Canada) was used as the metallic interconnect without any protective coating. The interconnect plate (12 × 12 × 4 mm) was machined with gas channels on one side. Air was supplied to both electrodes at a flow rate of 100 mL min−1. The as-prepared symmetrical cell was sandwiched with two Fe-Cr interconnect plates, which were also acting as the current collectors. After a constant current density operation at 0.5 A cm−2 and 800 oC for 100 h, the IR-free polarization curves of both electrodes were measured. Hereinafter, the electrodes polarized anodically (O2 evolution) and cathodically (O2reduction) for 100 h will be denoted as electrode A and electrode C, respectively. Figure 1 shows the polarization curves of electrode A and electrode C with and without Fe-Cr interconnect plate at 800 oC. When operated with Pt current collectors (without any Fe-Cr alloy), very high performance of both electrode A and electrode C was observed even after 100 h (negligibly small degradation). In contrast, after operation with Fe-Cr alloy interconnects, the overpotential at both electrodes significantly increased due to Cr-poisoning. While the difference in the anodic overpotential between electrode A and C was relatively small in the O2 evolution reaction, electrode A showed much larger overpotential than that of electrode C in the O2 reduction, specifically at high current densities. This could suggest that Cr-containing oxides deposited in the pores2 might block the O2diffusion path. Analyses of the electrode morphology are under progress. This work was supported by the funds for “ALCA” from Japan Science and Technology Agency.