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A time transient technique for performance characterization and degradation diagnostics in solid oxide fuel cells Hoff, Brian David

Abstract

Solid oxide fuel cells (SOFCs) have demonstrated higher fuel-to-electricity conversion efficiency than any competing device. However, their commercialization is hindered by poor reliability, poor durability, and high manufacturing and materials costs. Degradation diagnostics in operating SOFCs by non-invasive methods remains a challenge. Diagnostic technique developments might cultivate improvements in SOFC reliability and durability. Ohmic-, activation- and concentration-related power losses in SOFCs are determined using transient techniques. Electrochemical impedance spectroscopy (EIS) is the leading nondestructive technique for this purpose. Time domain transient techniques including current step and galvanostatic current interruption (GCI) have been applied to some extent in SOFC research. Currently, no technique exists to elucidate SOFC power losses from time transient data with resolution comparable to EIS. If such a technique is made available, time domain techniques might transcend EIS with reduced testing durations and equipment costs. In this aspect, time transient techniques might be more suitable candidates for onboard degradation monitoring of commercial SOFC stacks; simplification of auxiliary electronics and reduction of diagnostic testing times might yield significant cost savings and operational benefits. This work consisted of two parts: 1. the assessment of perturbation via load resistance switching as a means of electrochemical characterization, and 2. the evaluation of spectroscopic interpretation of fuel cell power losses via Laplace inversion of time domain data. The electrochemical characteristics of an SOFC were evaluated via DC polarization and EIS at temperatures ranging from 600-900 °C. A circuit was constructed to rapidly switch the fuel cell’s load. Current and voltage responses to load switching were acquired at 800 °C. Rather than inducing a voltage response to a current step, load switching yielded mutual relaxations of both current and voltage. Due to this complication, neither over potential decays nor the complex impedance response was derived using this technique. Nevertheless, responses to small load steps yielded ohmic data comparable to that of EIS. Analysis of the Laplace inversion of simulated transient data suggested that even with extremely small noise levels, a lack of confidence exists in the resolution of electrochemical constituents in time transient responses.

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