PhD, Technical University of Denmark
Investigation of steam as stressor for accelerated lifetime testing of Ni-YSZ electrodes
To date, huge effort has been dedicated to the development of highly performing and stable Solid Oxide Fuel Cells (SOFC). Consequently, obtaining durability/lifetime data under nominal conditions involves prolonged testing periods and high costs, as the lifetime requirements for different applications vary from 2 to 10 years. This critical issue is mainly responsible for the delay in commercialization of the technology. In order to shorten experimental testing times different accelerated testing methods (ALT) are implemented for so called reliability engineering. For ALT testing stressors need to be identified to activate and fasten the targeted failure mode and minimize unexpected effects. However, in a fuel cell stack, this is challenging as the system comprises various components with different dependencies of degradation mechanisms upon various operation conditions, i.e. steam, current density and temperature. Thus, the work on ALT testing for fuel cells and in particular SOFC is rather limited. This study focusses therefore on possible ways for accelerated lifetime testing of SOFCs, in particular on stressors affecting degradation of Ni-Yttria-stabilized zirconia (Ni/YSZ) cermet electrodes – the most frequently used fuel electrode in SOFC. It appears to be the electrochemically limiting cell component in state-of-the-art cells and so should be of particular concern for the developers of accelerated testing methods. At present, operating parameters such as steam content and temperature are considered to mainly affect the degradation of Ni-YSZ electrodes. This study concentrates on experimentally identifying the acceleration impact of steam and its limits for the fuel electrode via electrochemical impedance spectroscopy analysis. Furthermore, the responses of two different initial microstructures of the Ni-YSZ electrode towards steam as accelerating impactor are evaluated. This data is needed to establish a lifetime prediction model for industrial purposes.
Abstract: A method of controlling a fuel cell system includes applying alternating current (AC) signals to an individual fuel cell. The AC signals have a plurality of different frequencies. A voltage across the individual fuel cell is determined at each of the plurality of different frequencies. An impedance characteristic of the individual fuel cell is determined based at least in part on the voltage across the individual fuel cell at each of the plurality of different frequencies. The individual fuel cell is controlled based at least in part on the impedance characteristic.
Pub.: 24 Mar '15, Pinned: 30 Jun '17
Abstract: Various embodiments provide systems and methods for detecting defects in components of a fuel cell. Embodiment methods and systems include directing acoustic energy into the component, detecting acoustic energy from the component, analyzing a characteristic of the detected acoustic energy, and based on the analyzing, determining the presence or absence of a defect in the component.
Pub.: 20 Oct '15, Pinned: 30 Jun '17
Abstract: Electrochemical measurements on solid oxide fuel cells (SOFC) often uncover unusual readings. These are then ascribed to unknown material properties or newly discovered physical effects. However, a closer look shows that these are simply errors − mostly test artefacts. Our group has isolated many causes of error and developed reliable techniques to avoid them, focusing on open circuit voltage (OCV) measurement, current/voltage (i/v) curves and electrochemical impedance spectroscopy (EIS). This paper aims to help researchers avoid errors through balanced choice of apparatus and test procedure. Simple to implement test-sequences are set out to avoid errors. First, test-bench wiring, hardware limitations and apparatus selection are detailed to avoid error sources. Secondly, measurement settings and how they impact measurement results are discussed. Our developed procedures avoid the majority of errors; this is exhaustively tested at operationally relevant conditions.
Pub.: 29 Dec '16, Pinned: 30 Jun '17
Abstract: Solid oxide fuel cell (SOFC) applications require lifetimes of several years on the system level. A big challenge is to demonstrate such exceptionally long lifetimes in ongoing R&D projects. Accelerated or compressed testing are alternative methods to obtain this. Activities in this area have been carried out without arriving at a generally accepted methodology. This is mainly due to the complexity of degradation mechanisms on the single SOFC components as function of operating parameters. In this study, we present a detailed analysis of approx. 180 durability tests regarding degradation of single SOFC components as function of operating conditions. Electrochemical impedance data were collected on the fresh and long-term tested SOFCs and used to de-convolute the individual losses of single SOFC cell components – electrolyte, cathode and anode. The main findings include a time-dependent effect on degradation rates and the domination of anode degradation for the evaluated cell types and operating conditions. Specifically, the steam content as determined by fuel inlet composition, current density and fuel utilization was identified as major parameter, more important than for example operating temperature. The obtained knowledge is adopted to identify optimal operation profiles in order to acquire accelerated testing for lifetime investigation of SOFCs.
Pub.: 17 May '17, Pinned: 30 Jun '17