Mark R. De Guire

The overall goals of the program were: • To understand how selected operational parameters affect the performance of solid oxide fuel cells (SOFCs), with a focus on parameters affecting cathodes (air electrodes) comprised of lanthanum strontium manganite (LSM, (La1-xSrx)1-yMnO3±delta) and yttria-stabilized zirconia (YSZ, (Zr1–zYz)O2±delta). • To relate the observed relationships between operational parameters and cell performance to the microstructural changes that can be observed in the cells after operation. This project shared these overall goals with its predecessor, DE-0023476, “Long Term Degradation of LSM Based SOFC Cathodes: Use of a Proven Accelerated Test Regimen.” That project compared the effects of operating cells under “accelerated conditions” (elevated current density and temperature) versus more conventional conditions for (typically) up to 500 h, but always with humidified air as the cathode atmosphere. The current program added reduced oxygen partial pressure pO2 as an operating variable for the cathode conditions. It also included aging tests — long-term operation of the cells under open circuit voltage (i.e. zero current density) as a way to separate the effects of ambient conditions (temperature and pO2) from the effects of operation under load on how the cells’ performance and microstructures change with time. Both the prior project andmore » the current project looked at three different LSM cathode compositions that varied slightly in their degree of manganese excess, i.e. the ratio 1/(1–y) in (La1-xSrx)1-yMnO3±delta. The program completed several tests under various combinations of the selected operating variables and LSM compositions. Significant findings can be summarized as follows: • Reduced oxygen partial pressure (pO2 ≤ 0.15) is a significant operating stress on solid oxide fuel cells, greatly decreasing their operating lifetime and leading to earlier appearance of certain microstructural changes that may be relevant in degradation of cell performance. • Cell-to-cell variations can lead to differences in long-term performance that are as large as or larger than the effects of cathode composition, over the range of LSM compositions studied here. • Trends in the area specific resistance (ASR) of these SOFCs with time could be traced using periodic electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) as well as direct observations from cell output voltage versus time. • Distribution of relaxation times (DRT) analysis of the EIS results showed that electrode loss mechanisms, with characteristic frequencies in the ranges of 0.1 to 1,000 Hz, can be associated with reaction and transport processes at the anode and cathode of SOFCs. « less

The overall goals of the program were: • To understand the microstructural basis of long-term performance loss in solid oxide fuel cells (SOFCs) with cathodes based on lanthanum strontium manganite (LSM, (La1–xSrx)1–yMnO3±z); • To develop strategies for optimizing LSM–based cathodes that achieve improved long-term performance, while exhibiting microstructural and chemical stability, for commercial fuel cell systems. The research project carried out accelerated testing of SOFCs with LSM-based cathodes of selected compositions. After intervals of accelerated testing (and control tests using non-accelerated conditions), the cells underwent extensive microstructural characterization to determine the physical and chemical changes in the cells. By analysis of a) the results of the testing, b) the observed microstructural changes, and c) the differences in these correlations between different cathode compositions and test durations, the research: • Identified a correlation between microstructural change and performance loss: specifically, between decrease of three-phase boundary (TPB) density and increase of area specific re-sistance (ASR) during operation; • Identified a cathode composition parameter — Mn excess, defined as 1/(1–y) in (La1–xSrx)1–yMnO3±z) — and how it affects TPB density, ASR and change of cell performance over time.

Microstructural changes in solid oxide fuel cell anodes after long‐term operation have been characterized by sequential sectioning with a focused ion beam, followed by scanning electron microscopy imaging and three‐dimensional reconstruction. The anodes were porous composites of Ni and Y ₂ O ₃ ‐stabilized ZrO ₂ ( YSZ ). The cells were operated at 800°C for 2, 4, and 8 kh, and at 925°C for 2 and 4 kh. For each specimen, the volume fraction, surface area, particle diameter, and tortuosity have been calculated for each phase (Ni, YSZ , and pores). The dependence of these microstructural parameters on the volume of sample analyzed was monitored; sufficiently large volumes were analyzed so as to eliminate any effect of sample volume. Gradients in volume fraction of Ni and porosity developed during fuel cell operation, with Ni fraction increasing, and pore fraction decreasing, at the electrolyte/anode interface. The magnitudes of these gradients increased with time.

Treatments with organic surfactants, followed by the deposition of nanocrystalline ceria coatings from aqueous solution, were applied to anodes of solid oxide fuel cells. The cells were then operated in hydrogen/nitrogen fuel streams with H₂S contents ranging from 0 to 500 ppm. Two surfactant treatments were studied: immersion in dodecanethiol, and a multi-step conversion of a siloxy-anchored alkyl bromide to a sulfonate functionality. The ceria coatings deposited after the thiol pretreatment, and on anodes with no pretreatment, were continuous and uniform, with thicknesses of 60–170 nm and 100–140 nm, respectively, and those cells exhibited better lifetime performance and sulfur tolerance compared to cells with untreated anodes and anodes with ceria coatings deposited after the sulfonate pretreatment. Possible explanations for the effects of the treatments on the structure of the coatings, and for the effects of the coatings on the performance of the cells, are discussed.

The performance of lab-scale solid oxide fuel cells (SOFCs) was evaluated under several types of non-ideal conditions affecting the anode (fuel electrode) including: inadequate exhaust of the combustion products; exposure to hydrogen-nitrogen fuel streams containing up to 500 ppm H 2 S; interruption of the fuel stream; and leakage of air into the anode atmosphere. The cells were supported by yttria-stabilized zirconia (YSZ) electrolytes (3.8 cm diameter) and had lanthanum strontium manganite (LSM) cathodes and anodes of either Ni/YSZ or Ni/gadolinia-doped ceria (GDC). In addition, the anodes of some cells had been infiltrated with a 50- to 100-nm thick layer of nanocrystalline cerium oxide before testing. Cell performance was evaluated by monitoring output voltage and area-specific resistance (ASR) during galvanostatic operation. The non-ideal operating conditions produced distinctive changes in output voltage and ASR. These were attributed to microstructural changes in the anodes. The performance changes can potentially be used to diagnose, and possibly rectify, adverse operating conditions in the field.

Lanthanum strontium manganite (LSM) is widely used in cathodes of solid oxide fuel cells (SOFCs) because of its long-term stability and performance at higher operating temperatures (850–900°C). The conductivity of LSM depends on the Mn valence, which in turn depends on the level of strontium doping, the ratio of (La+Sr) to Mn, and the operating environment of the cell (temperature and atmosphere). Consequently, determining Mn valences is a key issue in understanding the relationship between conductivity and operating conditions. Although the defect chemistry of LSM has been investigated extensively, this study is the first that compares new measurements of Mn valence from two direct, independent techniques: X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS). Furthermore, we have examined surfaces and interfaces of LSM in cathode-symmetric SOFCs, and XPS results from surfaces in LSM exposed to a range of atmospheres and temperatures.