Development of solid oxide fuel cell stack components and characterization methods

Johan Tallgren

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

Solid oxide fuel cells (SOFCs) are a promising technology to reduce energy-related carbon dioxide emissions and combat climate change. Benefits include high efficiency, low emissions and flexibility in fuel choice. However, market penetration is delayed by higher cost levels compared to conventional energy production methods and further development is needed. This thesis focuses on selected development needs: protective coatings for metallic interconnects to migitigate chromium migration, hybrid compressible stack sealing materials, and characterization methods to replicate stack conditions. Stack characterization should be performed with methods that replicate the real operation conditions as closely as possible, including fuels and reforming conditions. A fuel feeder for a SOFC test station was constructed for pre-reforming of natural gas, with the aim of being a versatile research tool. The fuel feeder was able to operate with several reforming techniques and in a wide temperature and gas flow range. The outlet gas composition can be predicted by the outlet gas temperature. Other developed characterization tools include a measurement method to determine fuel utilization and temperature distributions in stacks in situ with electrochemical impedance spectroscopy. The method was demonstrated on a real stack and its results were in accordance with analysis based on cell voltage measurements. The characterization method could aid in validating stack design. Manganese-cobalt oxide coatings for metallic interconnects were developed and characterized with a novel measurement method that replicates the stack conditions. The developed method offers the same contacting materials, electrical contacts, chemical interactions and chromium migration mechanisms as in stacks. Additionally, the setup enables post-test analysis of chromium migration through the coating with the same sample. Mn-Co oxide coatings deposited with various techniques were evaluated with respect to area specific resistance (ASR), coating microstructure and chromimum retention. Coatings deposited with physical vapour deposition (PVD) showed lowest ASR and a high chromium retention. These could be applied as such in stacks. The best performing coatings were found to be dense and well adhered to the substrate. Hybrid seals with a compressible core and surface glass layers were developed and tested in stacks. The seal material showed lower leak rates at lower compressible stress than uncoated compressible seals. It was found that the leak rate through hybrid compressible seals depends mainly on gas composition instead of overpressure and is mostly diffusive by nature. Interactions between hybrid seals and stack components were studied in an in situ stack test. No significant material interactions were found and the developed material was considered to be suitable for SOFCs. The results have been commercialized by Flexitallic Ltd.
Translated title of the contributionKiinteäoksidipolttokennoston komponenttien ja karakterisointimenetelmien kehitys
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Lund, Peter, Supervising Professor
  • Himanen, Olli, Thesis Advisor, External person
Publisher
Print ISBNs978-952-64-0119-5
Electronic ISBNs978-952-64-0120-1
Publication statusPublished - 2020
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • solid oxide fuel cell
  • protective coatings
  • sealant materials
  • characterization methods

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