(Invited) Operando Measurement of Redox-Originated Stress in Anode Supported SOFC

Abstract

Mechanical failures of an anode supported SOFC implies crack formation in the electrolyte, delamination of the electrode/electrolyte interfaces, and contact loss caused by cell deformation. To find the risk of those failures, it is essential to know the complicated behavior of stress state which is affected not only by thermal expansion mismatch but also by chemical change of the components. Structural analyses have been made by various groups by using mechanical property data accumulated in these decades. However, effects of chemical strain are not always taken into consideration, and even when it is properly included in the calculation, actually observed behavior does not always follow the expectation. It is thus important to make in-situ or operando measurement of the stress in cell components. Several in-situ measurement methods were tested, and cosα method of X-ray diffraction was chosen as the best suited method for the operando application [1]. With cosα method, focused X-ray beam irradiates the sample surface with a certain angle to the normal, and the Debye ring is monitored with a co-axially aligned imaging plate. The stress of the sample is estimated from the deformation of the Debye ring from the stress-free state. A specially designed one- or two-chamber cell holder was developed to be used with a portable X-ray stress analyzer (μ-X360s, Pulstec Industrial. Co. Ltd.). Anode-supported cells purchased from multiple sources were analyzed. Measurements were made focusing on the initial reduction of the nickel oxide support, re-oxidation at operation temperatures, and re-oxidation at medium temperature ranges. In the initial reduction at 700˚C, highly compressive stress of about -300 MPa appeared on the electrolyte. It, however, was gradually mitigated down to around -100 MPa in 3 hours. This behavior was explained with the reduction contraction of NiO to Ni and subsequent softening of the support layer. Similarly, as is expected, reoxidation at 700˚C caused the shift of the residual stress to the tensile direction. The transient behavior, however, was different with the cells from different manufacturers. This could be due to the difference of the microstructure of the anode support layer which causes different bending behavior. Another interesting behavior was on the reoxidation of the support at intermediate temperature range. As reported elsewhere [2,3], nickel metal shows oxidation “contraction” when it is oxidized at temperatures around 400˚C to 500˚C, which is accompanied by enhancement of creep deformation rate. The mechanism is interpreted as the formation of slightly oxidized layer which promotes mass transport at the particle surfaces and grain boundaries. It is coupled with the driving force of oxidation that makes the nickel ions move from the inside to the outside. Consequently, neck growth takes place and the separation of the particles are reduced. The same mechanism can work with Ni-YSZ cermet anode and can cause compressive stress on the electrolyte. With this expectation, the reduced cell without cathode was re-oxidized at 400˚C. Actually, increase in the compressive stress was found on the electrolyte. The magnitude of the stress, however, was unexpectedly large as -600 MPa. It is even larger than that on the initial reduction, and the stress mitigation did not work despite the enhanced creep rate. This is probably because the driving force for the contraction is the oxidation that continues to exist before all nickel is re-oxidized. In an actual cell, this large compressive stress may cause buckling delamination of the electrolyte. Reoxidation during shutdown operation must be carefully avoided. As was found in several experiments, unexpected stress state may appear in an actual SOFC operation, which emphasizes the importance of operando analyses.AcknowledgementThis study was supported by the New Energy and Industrial Technology Development Organization (NEDO) .Acknowledgement[1] K. Oshima et al.,ECS Trans. 103(1), 1251-1260 (2021)[2] K. Yashiro et al., 12th European SOFC&SOE Forum, Lucerne, Switzerland, 05-08 July (2016)[3] Y. Morishita et al. , ECS Trans., 91(1) 1979 (2019).