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As SOFC technology is moving closer to a commercial break through, methods to measure the “state-of-health” of operating stacks are becoming of increasing interest. This requires application of advanced methods for detailed electrical and electrochemical characterization during operation. An operating stack is subject to compositional gradients in the gaseous reactant streams, and temperature gradients across each cell and across the stack, which complicates detailed analysis. Several experimental stacks from Topsoe Fuel Cell A/S were characterized using Electrochemical Impedance Spectroscopy (EIS). The stack measurement geometry was optimized for EIS by careful selection of the placement of current feeds and voltage probes in order to minimize measurement errors. It was demonstrated that with the improved placement of current feeds and voltage probes it is possible to separate the loss contributions in an ohmic and a polarization part and that the low frequency response is useful in detecting mass transfer limitations. A sequential and a parallel measurement setup was developed for acquisition of impedance measurements. From the sequential to the parallel measurement setup the acquisition time was cut down significantly and it was demonstrated parallel acquisition of 16 repeating units (cells) and the total stack impedance could be made fully automated. The performance and degradation of a 13-cell cross-flow stack was monitored for more than 2500 hours at steady operating conditions using the sequential impedance measurement setup. Impedance measurements was used to examine the long-term behavior and monitor the evolution of the series and polarization resistances for four out of the 13 repeating units during the first 1400 hours of operation. The losses for the four selected repeating units are reported and discussed. The performance and degradation of a 14-cell co-flow stack was monitored for more than 667 hours at steady operating conditions using the sequential impedance measurement setup. The stack was tested galvanostatically (at constant current) with 50% steam in the hydrogen fuel gas supplied to the stack. EIS was used to examine the long-term behavior and monitor the evolution of the impedance of each of the repeating units and the whole stack. The observed impedance was analyzed in detail for one of the repeating units and the whole stack and the losses reported and discussed in the following. Parallel acquisition using electrochemical impedance spectroscopy can be used to detect possible minor differences in the supply of gas to the individual cells, which is important when going to high fuel utilizations. The fuel flow distribution was determined and provides important information about the operating limits of the stack when high electrical efficiency is required. Furthermore, the measured gas concentration impedance was in between the impedances predicted by two different gas concentration impedance models. Total harmonic distortion, THD, experiments were carried out on an experimental 14-cell SOFC stack at varying frequencies and fuel utilizations. The results illustrated that THD can be used to detect increasing non-linearities in the current-voltage characteristics of the stack when the stack suffers from fuel starvation by monitoring the stack sum voltage only.
|Publisher||Department of Energy Conversion and Storage, Technical University of Denmark|
|Number of pages||176|
|Publication status||Published - 2014|