Stability of Ni-YSZ composites for solid oxide fuel cells during reduction and re-oxidation

Dissertation

Research output: ThesisDissertationCollection of Articles

Abstract

An operating Ni-based SOFC can be severely damaged by inadvertent oxidation of the nickel. A central way to improve this Achilles' heel is to design and prepare a dimensionally stable anode half cell that does not overload the electrolyte upon re-oxidation. Understanding the mechanisms that lead to the redox expansion, and designing and manufacturing modified anode support structures that improve stability is the core of the present work. The behaviour of Ni-YSZ cermets for SOFCs are characterised under conditions cyclically altered between reducing and oxidising (redox cycling). The main operating conditions that affect redox stability are shown to be temperature and humidity; both affect the growth of Ni particles through sintering. The temperature of re-oxidation also plays a significant role in redox stability; a re-oxidation at a high temperature (850°C or higher) leads to larger expansions. The behaviour of the cermet under redox conditions is highly dependent on microstructure; as porosity of the composite increases, redox stability is improved. A redox cycle at 600°C speeds up the subsequent re-reduction significantly, indicating a change in microstructure due to the re-oxidation; also the electrical conductivity of the cermets improves on such a redox cycle. The redox strains during redox cycles below 700°C are reversible, while cumulating strain and damage is created in the ceramic backbone at elevated temperatures. NiO particle growth during oxidation, combined with low temperature pseudoplasticity is suggested to be a decisive internal factor for redox stability. Redox cycling at high temperatures rapidly leads to irreversible nonelastic strains (cracking, creep) in the YSZ backbone that cause mechanical degradation. The combination of mild operating conditions and redox-improved cells appears to be a plausible solution to circumvent redox failures. An intentional low-temperature redox treatment could lead to an improvement in performance. The durability and stability of the anode can be improved by modifications in the microstructure and the composition of the cermets.
Original languageEnglish
QualificationDoctor Degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Kaiser, Andreas, Supervisor, External person
  • Lund, Peter D., Supervisor, External person
Award date11 Jun 2010
Place of PublicationEspoo
Publisher
Print ISBNs978-951-38-7400-1
Electronic ISBNs978-951-38-7401-8
Publication statusPublished - 2010
MoE publication typeG5 Doctoral dissertation (article)

Fingerprint

Solid oxide fuel cells (SOFC)
Oxidation
Composite materials
Cermets
Anodes
Oxidation-Reduction
Temperature
Microstructure
Cermet Cements
Nickel
Electrolytes
Atmospheric humidity
Creep
Durability
Sintering
Porosity

Keywords

  • fuel cell
  • SOFC
  • Ni-YSZ
  • Ni cermet
  • redox stability
  • thermomechanics
  • sintering
  • continuum mechanics
  • creep
  • viscoelastic
  • NiO reduction
  • Ni oxidation
  • kinetics

Cite this

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title = "Stability of Ni-YSZ composites for solid oxide fuel cells during reduction and re-oxidation: Dissertation",
abstract = "An operating Ni-based SOFC can be severely damaged by inadvertent oxidation of the nickel. A central way to improve this Achilles' heel is to design and prepare a dimensionally stable anode half cell that does not overload the electrolyte upon re-oxidation. Understanding the mechanisms that lead to the redox expansion, and designing and manufacturing modified anode support structures that improve stability is the core of the present work. The behaviour of Ni-YSZ cermets for SOFCs are characterised under conditions cyclically altered between reducing and oxidising (redox cycling). The main operating conditions that affect redox stability are shown to be temperature and humidity; both affect the growth of Ni particles through sintering. The temperature of re-oxidation also plays a significant role in redox stability; a re-oxidation at a high temperature (850°C or higher) leads to larger expansions. The behaviour of the cermet under redox conditions is highly dependent on microstructure; as porosity of the composite increases, redox stability is improved. A redox cycle at 600°C speeds up the subsequent re-reduction significantly, indicating a change in microstructure due to the re-oxidation; also the electrical conductivity of the cermets improves on such a redox cycle. The redox strains during redox cycles below 700°C are reversible, while cumulating strain and damage is created in the ceramic backbone at elevated temperatures. NiO particle growth during oxidation, combined with low temperature pseudoplasticity is suggested to be a decisive internal factor for redox stability. Redox cycling at high temperatures rapidly leads to irreversible nonelastic strains (cracking, creep) in the YSZ backbone that cause mechanical degradation. The combination of mild operating conditions and redox-improved cells appears to be a plausible solution to circumvent redox failures. An intentional low-temperature redox treatment could lead to an improvement in performance. The durability and stability of the anode can be improved by modifications in the microstructure and the composition of the cermets.",
keywords = "fuel cell, SOFC, Ni-YSZ, Ni cermet, redox stability, thermomechanics, sintering, continuum mechanics, creep, viscoelastic, NiO reduction, Ni oxidation, kinetics",
author = "Mikko Pihlatie",
note = "Project code: 41902",
year = "2010",
language = "English",
isbn = "978-951-38-7400-1",
series = "VTT Publications",
publisher = "VTT Technical Research Centre of Finland",
number = "740",
address = "Finland",
school = "Aalto University",

}

Stability of Ni-YSZ composites for solid oxide fuel cells during reduction and re-oxidation : Dissertation. / Pihlatie, Mikko.

Espoo : VTT Technical Research Centre of Finland, 2010. 99 p.

Research output: ThesisDissertationCollection of Articles

TY - THES

T1 - Stability of Ni-YSZ composites for solid oxide fuel cells during reduction and re-oxidation

T2 - Dissertation

AU - Pihlatie, Mikko

N1 - Project code: 41902

PY - 2010

Y1 - 2010

N2 - An operating Ni-based SOFC can be severely damaged by inadvertent oxidation of the nickel. A central way to improve this Achilles' heel is to design and prepare a dimensionally stable anode half cell that does not overload the electrolyte upon re-oxidation. Understanding the mechanisms that lead to the redox expansion, and designing and manufacturing modified anode support structures that improve stability is the core of the present work. The behaviour of Ni-YSZ cermets for SOFCs are characterised under conditions cyclically altered between reducing and oxidising (redox cycling). The main operating conditions that affect redox stability are shown to be temperature and humidity; both affect the growth of Ni particles through sintering. The temperature of re-oxidation also plays a significant role in redox stability; a re-oxidation at a high temperature (850°C or higher) leads to larger expansions. The behaviour of the cermet under redox conditions is highly dependent on microstructure; as porosity of the composite increases, redox stability is improved. A redox cycle at 600°C speeds up the subsequent re-reduction significantly, indicating a change in microstructure due to the re-oxidation; also the electrical conductivity of the cermets improves on such a redox cycle. The redox strains during redox cycles below 700°C are reversible, while cumulating strain and damage is created in the ceramic backbone at elevated temperatures. NiO particle growth during oxidation, combined with low temperature pseudoplasticity is suggested to be a decisive internal factor for redox stability. Redox cycling at high temperatures rapidly leads to irreversible nonelastic strains (cracking, creep) in the YSZ backbone that cause mechanical degradation. The combination of mild operating conditions and redox-improved cells appears to be a plausible solution to circumvent redox failures. An intentional low-temperature redox treatment could lead to an improvement in performance. The durability and stability of the anode can be improved by modifications in the microstructure and the composition of the cermets.

AB - An operating Ni-based SOFC can be severely damaged by inadvertent oxidation of the nickel. A central way to improve this Achilles' heel is to design and prepare a dimensionally stable anode half cell that does not overload the electrolyte upon re-oxidation. Understanding the mechanisms that lead to the redox expansion, and designing and manufacturing modified anode support structures that improve stability is the core of the present work. The behaviour of Ni-YSZ cermets for SOFCs are characterised under conditions cyclically altered between reducing and oxidising (redox cycling). The main operating conditions that affect redox stability are shown to be temperature and humidity; both affect the growth of Ni particles through sintering. The temperature of re-oxidation also plays a significant role in redox stability; a re-oxidation at a high temperature (850°C or higher) leads to larger expansions. The behaviour of the cermet under redox conditions is highly dependent on microstructure; as porosity of the composite increases, redox stability is improved. A redox cycle at 600°C speeds up the subsequent re-reduction significantly, indicating a change in microstructure due to the re-oxidation; also the electrical conductivity of the cermets improves on such a redox cycle. The redox strains during redox cycles below 700°C are reversible, while cumulating strain and damage is created in the ceramic backbone at elevated temperatures. NiO particle growth during oxidation, combined with low temperature pseudoplasticity is suggested to be a decisive internal factor for redox stability. Redox cycling at high temperatures rapidly leads to irreversible nonelastic strains (cracking, creep) in the YSZ backbone that cause mechanical degradation. The combination of mild operating conditions and redox-improved cells appears to be a plausible solution to circumvent redox failures. An intentional low-temperature redox treatment could lead to an improvement in performance. The durability and stability of the anode can be improved by modifications in the microstructure and the composition of the cermets.

KW - fuel cell

KW - SOFC

KW - Ni-YSZ

KW - Ni cermet

KW - redox stability

KW - thermomechanics

KW - sintering

KW - continuum mechanics

KW - creep

KW - viscoelastic

KW - NiO reduction

KW - Ni oxidation

KW - kinetics

M3 - Dissertation

SN - 978-951-38-7400-1

T3 - VTT Publications

PB - VTT Technical Research Centre of Finland

CY - Espoo

ER -