Fundamental Impact of Humidity on SOFC Cathode ORR

Abstract

Although solid oxide fuel cells (SOFC) have demonstrated excellent performance, the durability of SOFCs under real working conditions is still an issue for commercial deployment. In particular cathode exposure to atmospheric air contaminants, such as humidity, can result in long-term performance degradation issues. Therefore, a fundamental understanding of the interaction between water molecules and cathodes is essential to resolve this issue and further enhance cathode durability. To study the effects of humidity on the oxygen reduction reaction (ORR), we used in-situ 18 O isotope exchange techniques to probe the exchange of water with two of the most common SOFC cathode materials, (La 0.8 Sr 0.2 ) 0.95 MnO 3±δ (LSM) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF). In this experiment, heavy water, D 2 O (with a mass/charge ratio of m/z = 20), is used to avoid the overlapping of H 2 O and the 18 O 2 cracking fraction, which both provide a peak at m/z = 18. A series of temperature programmed isotope exchange measurements were performed to comprehensively study the interaction of water with the cathode surface as a function of temperature, oxygen partial pressure, and water vapor concentration. The results suggest that water and O 2 share the same surface exchange sites, leading to competitive adsorption. Our findings show that water prefers to exchange with LSCF at lower temperatures, around 300-450 • C. For LSM, O 2 is more favorable than water to be adsorbed on the surface and the presence of O 2 limits water exchange. The experimental data are summarized in a Temperature-PO 2 diagram to help visualize how the exchange of water on each material depends on the operating conditions. © The Author Solid oxide fuel cells (SOFC) electrochemically oxidize fuels for the generation of electricity. Two key advantages of SOFCs are their high efficiency and ability to utilize conventional fuels. This fuel flexibility stems from the dissociation and transport of oxygen from the cathode through the electrolyte to the anode, where the fuels are oxidized. Unfortunately, cathode degradation under real working conditions is a factor that limits SOFC performance. 1-6The long term durability of these materials is a major challenge, due to the high temperature required for the thermally activated oxygen reduction reaction (ORR), 7 as well as the variety of gases present during operation. 8,9 The impurities present in air on the cathode side of the cell can induce undesirable reactions. Some of these impurities, such as Cr or silica, 10-15 arise due to the interconnect and seal materials while some are intrinsic to ambient air, such as humidity and CO 2 . 16 Humidity has been found to degrade the performance of (La 0.8 Sr 0.2 ) 0.95 MnO 3±δ (LSM) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) based cells. 11,[17][18][19][20][21][22][23][24] This degradation can be either reversible or irreversible. 21 However, there is no conclusive evidence showing, fundamentally, how water participates in the degradation process, and it is hard to quan...

In situ study of strontium segregation in La0.6Sr0.4Co0.2Fe0.8O3−δ in ambient atmospheres using high-temperature environmental scanning electron microscopy

“…Gas stream composition and impurities influence cathode degradation, however, it has also been observed that gas-stream composition affects the oxygen reduction reaction as the dominant oxygen containing gas species (O 2 , H 2 O and CO 2 ) exchanging with the surface varies as a function of temperature 10 . It has been shown that the presence of water vapour in particular increases the oxygen exchange rate 11,12 , however, it is not yet clear the role the gas stream has upon strontium segregation 13 .…”

Infiltration, Overpotential and Ageing Effects on Cathodes for Solid Oxide Fuel Cells: La0.6Sr0.4Co0.2Fe0.8O3-δversus Ba0.5Sr0.5Co0.8Fe0.2O3-δ

“…4). Looking at LSCF, it was actually found that its oxygen activity as cathode material is limited by the oxygen surface exchange 40,54 The results observed against the cathodic overpotential (η) appeared very interesting, since the behavior of BSCF and LSCF is contrasting. In Figs.…”

“…The choice of using LSM was due to its proven favorable influence on the LSCF surface chemistry, since it is able to prevent Sr surface enrichment or surface phase segregation, preventing degradation over time. 40 Moreover, LSM possess: (i) good electrochemical activity toward the Oxygen Reduction Reaction (ORR) and high electronic conductivity, [50][51][52] (ii) excellent stability, 53,54 (iii) increased activity under a cathodic overpotential. 40,[55][56][57][58][59] The effect of LSMimpregnation, on the degradation of LSCF and BSCF porous cathodes, was studied through impedance spectroscopy, XRD and SEM-EDX investigations.…”

Influence of Water Vapor on Performance Degradation and Microstructural Change of (La,Sr)(Co,Fe)O3−δ Cathode

“…The influence of water vapor in the cathode gas on (La,Sr)MnO 3+δ and (La,Sr)(Co,Fe)O 3−δ has been investigated so far. [10][11][12][13][14][15][16][17][18] The performance of (La,Sr)MnO 3+δ was deteriorated under humidified conditions. This was ascribable to the decomposition of electrode to form La 2 O 3 , SrO, and Mn 2 O 3 over the electrode surface and at the interface with electrolyte.…”