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Single electrode characterisation in supported electrolyte cells

Single electrode characterisation in supported electrolyte cells. Peter Holtappels Laboratory for High Performance Ceramics EMPA Dübendorf peter.holtappels@empa.ch With contributions from: Carsten Sorof (FH Koblenz, D) Maarten Verbraeken (Uni Twente, NL) Sophie Duval (EMPA)

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Single electrode characterisation in supported electrolyte cells

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  1. Single electrode characterisation in supported electrolyte cells Peter Holtappels Laboratory for High Performance Ceramics EMPA Dübendorf peter.holtappels@empa.ch With contributions from: Carsten Sorof (FH Koblenz, D) Maarten Verbraeken (Uni Twente, NL) Sophie Duval (EMPA) Jörg Richter (EMPA)

  2. Content • why single electrode measurements? • setups involving reference electrodes • supported electrolyte cells • concluding remarks • Outlook to new EMPA projects

  3. Why single electrode charactersiation ? Cell / stack Modelling Gas atmosphere „Button cells“ Kinetic modelling Reaction mechanisms Kinetic parameters (micro) structure Materials development SOFC SOEC

  4. Three-electrode setups • Electrolyte as support • Design: • Foils • Pellets • Electrode sintered seperately • Reference Pt/Air Source: W. Winkler et al, J. Electrochemical Soc. 145 (1998) p. 1184 ff

  5. Misalignment problems P.V. Hendriksen et al., 17th Risoe Int. Symp. Materials Science (1996)

  6. The misalignment problem real i center false rim h The supporting electrode always has the lower overpotential ! Risoe statement ESF-OSSEP workshop: Electrode processes and kinetics in SOFCs, Frascati, IT, Feb 2004

  7. Edge effects ! • Real electrodes (e.g. screen printed) are flattened towards the rim • Special cases (low conductive electrodes): CAN diminish the problems:

  8. „Safe“ Pellet geometries • safe according to simplified modelling • Rs/Rp ~ 1/0.5 O.A. Marina, Proc 5th Eur. Symp. SOFC Lucerne (2002)

  9. Criteria for reference electrodes

  10. Wat we did:

  11. Nano-structured cathodes • T : 750°C • gases: H2/air • cell area: 50 cm2 cells • uf: ≈ 70%

  12. 3-cell stack: performance Experimental: vNG: 20 g/h vair: 1000 g/h T= 800°C

  13. Electrodes cofired with supported electrolytes? • state of the art SOFC cell concept • How to analyse: • Gas conversion • Gas transport • Interface contributions • Micro-structure performance relations • How to determine: • data for kinetic modelling • data for cell and stack modelling Difficult from full cell tests?

  14. Porosity graded anode substrates

  15. Anode supports 20 vol% pore former 34% porosity 30 vol% pore former 40% porosity No pore former 18% porosity

  16. Performance of graded anode supports • Electrical performance • non-sealed set-up • ± 880 mV (theoretical 1 V) • Residual porosity • Impedance measurements 1 – 2 cm2 instead of 0.1  cm2

  17. What we intend to do:

  18. Understanding Ageing in SOFCs • Anodes: • C(H2O, O2, OH-) • Structural stability • = f(U) • ....... • Cathodes: • Cation diffusion • Structural stability • = f(U) • Cr-poisoning • = f(U) • .......... LENI: cell testing LPI: anode development EMPA: cathode development ETHZ: cathode modelling • Single electrode information on supported electrolyte cells

  19. O2- Air O2reduction O2 + 4e +2V..O -> 2OxO ->O2- Fuel H2 H2 oxidation -> electrons + water Anode Cathode Fuel H2 H2 oxidation -> electrons + H+ H+ Air O2 reduction -> water H2O + V..O + OxO -> 2OH.O Anode Cathode Conduction type Proton/Oxid ion ? Concentration cell Gas I, Pt / electrolyte / Pt, Gas II H2 + 1/2O2 H2O Source: H. Iwahara et al., Solid State Ionics 61 (1993) 65-69

  20. Proton conductivity / Total conductivity In H atmosphere Source: H. Iwahara, Solid State Ionics 77 (1995) 289-298 Source: T. Norby, Solid State Ionics 125 (1995) 1-11

  21. oxygen anode + electrolyte O2- O2- - cathode water vapour hydrogen carbon dioxide carbon monoxide Cathodes for Solid Oxide Electrolysis Cell (SOEC) • Electrolyte • gas tight (prevent recombination) • as thin as possible (minimize the voltage drop) • Electrodes • stable in respective atmospheres • good electronic conductors • porous • Anode • oxidizing atmosphere: electronically conducting mixed oxides? • Cathode • reducing but korrosive atmosphere • mixture of ceramic and metal ? Pure ceramic?

  22. Summary • single electrode characterisation using reference electrodes • electrolyte supported designs (ES) are feasible • modelling approach is not fully consistent ! • More work is needed! • single electrode charaterisation in supported electrolyte designs (SE) • limited to full cell tests • technical relevance: • HIGH for performance evaluations • MEDIUM for materials development • scientific relevance: • LOW: non sealed set-up, “undefined conditions” • compromises • symmetrical electrodes, ….. • preparation/co-sintering is key! ?

  23. Acknowledgment Mogens Mogensen, Risø National Laboratory Former colleagues Forschungszentrum Jülich, IEF (IWV-3) • at EMPA: • Hansjürgen Schindler • Simone Zürcher • Ulrich Vogt

  24. Conduction type Proton/Oxid ion ? • Cell: H2, Pt/ electrolyte / Pt, O2 (wet hydrogen – constant current) Pressure mesurements: • H2O at cathode -> proton cond. Emf mesurement: H2O diluted H2 -> emf decrease -> oxid ion cond. H2O diluted air exces -> no emf diminution -> proton cond. • H permeation: H2 (1 atm), Pt/ electrolyte / Pt, Ar -> Faraday Or H permeation with a radioactive isotop • Hydrogen and/or Oxygen concentration cell: H2 (1 atm), Pt/ electrolyte / Pt, H2 (pH2) -> Nernst Wet (saturation with water at room T) Dry (passing through phosphoruspentoxide powder) Source: N. Bonanos, Solid State Ionics 53-56 (1992) 967-974

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