Durability Testing of Solid Oxide Cell Electrodes with Current Switching

Abstract

A method for durability testing of solid oxide cell electrodes in current-switching operation is presented. The aim is to obtain correlated time-dependent electrochemical and microstructural information by simultaneously testing multiple identical cells connected in series. By periodically removing cells from the circuit during the life test, microstructural changes were observed after different times under current using Focused Ion Beam-Scanning Electron Microscope (FIB-SEM) tomography. Initial tests were done on symmetrical cells with (La 0.8 Sr 0.2 ) 0.98 MnO 3 -Zr 0.84 Y 0.16 O 2 (LSM-YSZ) electrodes at 800 • C in air with a current of up to 1.5 Acm −2 . The current direction was switched every 30 minutes. Electrochemical Impedance Spectroscopy (EIS) measurements showed stable operation at low currents, but a gradual degradation of both ohmic and polarization resistance at 1.5 Acm −2 . Little microstructural change was observed except that silver from current collectors was found to migrate into the electrodes near the YSZ electrolyte during fuel cell operation. The silver content increased with time at current and presumably explained the observed increase in polarization resistance.Life testing of solid oxide cells (SOCs) presents significant challenges, not only because of long desired operation times (>40,000 h) but because of the materials complexity of SOCs. There are a number of examples of SOC stack tests carried out for >10,000 h, and they invariably show that a number of factors contribute to degradation. 1,2 A different strategy is to carry out tests focused on a specific cell component, using conditions that accelerate degradation such that a large amount of data can be obtained in a relatively short time. When the electrochemical measurements are correlated with microstructural and microchemical measurements, they can provide a more fundamental understanding of specific degradation mechanisms and ultimately lead to mechanistic degradation models that can be used to make quantitative long-term durability predictions. 3 One problem with such measurements is that artifacts, such as impurity accumulation, 4 can obfuscate the observation of the intrinsic degradation mechanism.The specific application addressed here is a SOC alternated between fuel cell and electrolysis operation, of interest for storing excess electrical energy from intermittent renewable sources such as wind and solar, and then converted back to electricity. 5 Prior work has focused on observing degradation mechanisms in SOCs operated in either fuel cell 6,7 or electrolyzer 8-10 mode. The ability of SOCs to work reliably over many cycles of switched current direction, i.e., between fuel cell and electrolysis modes, is unknown. This paper describes a novel method for coupled electrochemical and microstructural life testing, applied to air:LSM-YSZ | YSZ | LSM-YSZ:air symmetrical SOCs in current-switching operation. These tests allowed a more straightforward interpretation of EIS data compared to full cells, especially since the current...

Life testing of LSM–YSZ composite electrodes under reversing-current operation

“…LSM-YSZ electrode functional layers were screen printed on dense YSZ pellets and fired at 1175 1C for 1 hour to produce symmetrical cells, similar to a previous report. 24 The YSZ electrolyte pellets were first sintered at 1400 1C for 4 hours with a final thickness of approximately 0.6 mm. The LSM-YSZ electrode ink was produced by ball milling a 1 : 1 weight ratio of A-site deficient (La 0.8 Sr 0.2 ) 0.98 MnO 3Àd (Praxair) with Zr 0.84 Y 0.16 O 2Àg (Tosoh) in ethanol for 24 h. This mixture was then dried, passed through a 125 mm mesh sieve before adding 1.18 wt% Heraeus vehicle, and homogenized through three roll milling.…”

Degradation of (La0.8Sr0.2)0.98MnO3−δ–Zr0.84Y0.16O2−γ composite electrodes during reversing current operation

“…3 Delamination of the oxygen electrode (electrolysis anode), typically (La 0.8 Sr 0.2 ) 0.98 MnO 3Àd -Zr 0.84 Y 0.16 O 2Àg (LSM-YSZ), is the dominant degradation mechanism. 4 Furthermore, reversing current operation does not introduce new degradation mechanisms, and in fact reduces the delamination degradation when compared with DC electrolysis operation. 4,5 It was possible to essentially eliminate degradation at 1.0 A cm À2 by using a cycle with very short periods of electrolysis separated by fuel cell operation, 5 but such a cycle would not be practical for most storage applications.…”

Linking Initial Microstructure to ORR Related Property Degradation in SOFC Cathode: A Phase Field Simulation

“…24 So it is hard to conclude if there is densification or not in the cathode during coarsening. In contrast, there are several experiments on anode showing that there isn't any observable change in the porosity in anode after coarsening for up to 500 hours.…”