Flexible Fuel SOFCs: Survey of Advanced Anodes for Hydrocarbon Fueled SOFCs

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Flexible Fuel SOFCs(?)Survey of Advanced Anodes
for Hydrocarbon Fueled SOFCs

• Literature Presentation
• Gerardo Jose la O

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Basics

• All-ceramic
• High operating temperatures 600-1000C
• Main components and operating principles

Traditional SOFC Materials Cathode LSM
(La1-xSrxMnO3-d) Electrolyte 8YSZ (8mol
Y2O3-stabilized ZrO2) Anode Nickel-8YSZ
Gorte, R. J., Kim, H. Vohs, J. M. Novel SOFC
anodes for the direct electrochemical oxidation
of hydrocarbon. Journal of Power Sources 106,
10-15 (2002).
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Possible Applications

• Stationary
• Residential, Commercial, and Industrial Settings
• 5kW for typical home
• 100kW for small office-building or factory
• UPS (uninterruptible power supply) backup
• Mobile
• Auxiliary Power Units (APU)
• Power for vehicle electronics, climate control,
  and entertainment system for large cars/trucks
• Marine and Aviation
• Reduces pollution and waste from engine idling
• 3-10kW output range
• Military
• Personal/Soldier APU
• Generator systems for field bases
• Vehicle Mounted APU

Now commercially available at www.acumentrics.com
General Electric Stationary SOFC Unit
http//www.seca.doe.gov/
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Advantages

• Use of non-noble metal catalysts
• Ni and LSM
• Heated gas by-product can be scavenged
• Recuperative cycles
• Electrodes more robust to poisoning
• No water management issue
• Fuel flexible system
• H2 and/or CO
• Reformed hydrocarbons (external or internal ?
  steam/POx reform)
• Direct hydrocarbon oxidation of methane, ethane,
  decane, toluene, diesel, gasoline, etc.
• Coal(?)

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Challenges

• Long startup time from high temperature operation
• Severe operating environments
• Exotic/expensive materials
• Materials degradation issues
• Complex gas seals (critical for planar SOFC
  design)
• HC fuels result in coke buildup (anode)
• Ni is a very good coking catalyst
• ? Target 600C SOFC
• - Reduced temperature compatible with HC fueled
  
• SOFCs (less coking at lower
  temperatures)

Delphi APU Stack (planar design)
Acumentrics UPS Tubular Assembly
http//www.seca.doe.gov/
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Literature Presentation

• Novel SOFC Anodes for the Direct Electrochemical
  Oxidation of Hydrocarbons
• Gorte, R. J. Vohs, J. M. Journal of Catalysis
  216, 477-486 (2003).
• 2. Advanced anodes for high-temperature fuel
  cells
• Atkinson, A. et al. Nature Materials 17-27
  (2004).

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1. Novel SOFC Anodes

• Gorte et al.
• Objective Obtain anode that is inert for coke
  formation
• Cu excellent e- conductor
• Cu a poor catalyst for C-C bond formation (likely
  related to coking)
• Test Result Cu surface coated with carbon but no
  active formation found
• ?Replace Nickel with Copper
• Processing Ni-YSZ sintered at 1300C
• CuYOx melts at gt1350C
• ?New method Cu ion Cu(NO3)2 impregnation into
  YSZ

Ni-YSZ
cells heated in denoted atmosphere for 1-2
hours at 700-800C
Cu-YSZ
in 100 H2
in 100 CH4
in 100 CH4
in 40 C7H8
in 40 C7H8
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Gorte et al

• Catalyst still required to oxidize hydrocarbons
• Cu is non-catalytic
• Ceria (CeO2) a widely known good oxidation
  catalyst was used
• ?Cu-Ceria anode performance

CxHy(2xy/2)O2- ? xCO2y/2H2O(4xy)e-
?H2 at 973K ? H2 at 1073K ? n-butane at 973K ?
n-butane at 1073K
Park, S. D., Vohs, J. M. Gorte, R. J. Direct
oxidation of hydrocarbons in a solid-oxide fuel
cell. Nature 404, 265-267 (2000).
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Gorte et al

• -Cu-Ceria anode tested versus different fuel
  types

Similar performance characteristics observed for
fuels tested with exception for CH4

• Sulfur tolerance
• Ceria is most sensitive component to sulfur
• Reasonably high(?) concentrations can be used

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Advanced Anodes

• Atkinson et al.
• -Survey of recent developments in anode
  technologies
• Anode materials requirements
• Site for electrochemical oxidation of fuel
• Material stability for good commercial lifetime
• Good processability
• Ni-YSZ composite anodes
• A 40 year-old innovation (Spacil, US Patent
  3,558,360 1970) and current state-of-the-art
  has not changed much today
• Disadvantage of coking however prevents use in
  multi-fuel SOFCs
• Need external or internal reformation with steam
  ? adds complexity to the system

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Atkinson et al

• Anode Catalytic Properties
• For Ni and coke formation
• Carbon Chemisorption ? Dissolution ?
  Precipitation after supersaturation
• Need sufficient amount of steam to prevent
  buildup
• Steam not good for anode reaction
• Addition of Cu and Ceria has improved performance
• As shown by Gorte et al
• ?However, understanding mechanisms of surface
  reactions is still very poor.
• ?Rate-limiting step and reaction step
  identification would be a first attempt

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Atkinson et al

• Other Materials Surveyed
• Cu/Ni and Cu/Co alloys
• Reduce the catalytic activity of Ni
• Has been shown to reduce coking
• No improvement in HC performance seen so far
• Oxide materials such as La1-XSrxCrO3
• Sufficient electronic conductivity in these
  materials
• Anode performance found to be poor, possibly due
  to low oxygen ion conduction
• Doped with Ni (La1-XSrxCryNi1-yO3) improved
  performance but exsolution of Ni found

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Atkinson et al

• Summary and Recommendations
• No single material currently fits all
  requirements of
• Current collection
• Electrochemical performance
• Catalytic performance
• Need further research to understand oxygen
  transport and surface exchange in reducing
  environments
• Crystal structure, defect structure understanding
  requires fundamental understanding
• ?The current best materials barely meet the
  requirements for long-term performance and search
  for totally new oxides need to be done

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Summary

• Truly fuel flexible SOFCs (no reforming) is
  currently in its infancy
• Pure HC electrochemical oxidation is a
  multi-electron transfer reaction and is very
  complex
• In the near-future, internal or external
  reforming will most likely be the fuel flexible
  route for SOFCs

CxHy(2xy/2)O2- ? xCO2y/2H2O(4xy)e-
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