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

Single electrode characterisation / Peter Holtappels / / Folie 2 Content why single electrode measurements? setups involving reference electrodes supported electrolyte cells concluding remarks Outlook to new EMPA projects

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

Single electrode characterisation / Peter Holtappels / / Folie 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 ff

Single electrode characterisation / Peter Holtappels / / Folie 5 Misalignment problems P.V. Hendriksen et al., 17th Risoe Int. Symp. Materials Science (1996)

Single electrode characterisation / Peter Holtappels / / Folie 6 The misalignment problem  i center rim real The supporting electrode always has the lower overpotential ! false Risoe statement ESF-OSSEP workshop: Electrode processes and kinetics in SOFCs, Frascati, IT, Feb 2004

Single electrode characterisation / Peter Holtappels / / Folie 7 Edge effects ! Real electrodes (e.g. screen printed) are flattened towards the rim Special cases (low conductive electrodes): CAN diminish the problems:

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

Single electrode characterisation / Peter Holtappels / / Folie 9 Criteria for reference electrodes CriteriaLiquid systems Secondary systems Hg/HgO reference Solid State Systems Pt/air Defined EC potentialHg/HgO Luggin capillary p(O 2 ) Stable current potential distribution WE and CE Non-polarisablehigh mass/surface „secondary electrode“ area dependent, porous Pt paste instead of Pt-bead Constant temperaturereference independent / outside of the test cell T:4K-298K: no influence! not realised so far

Single electrode characterisation / Peter Holtappels / / Folie 10 Wat we did:

Single electrode characterisation / Peter Holtappels / / Folie 11 Nano-structured cathodes T : 750°C gases: H 2 /air cell area: 50 cm2 cells uf: ≈ 70%

Single electrode characterisation / Peter Holtappels / / Folie 12 Anode supported cells Ni/YSZ//YSZ/CGO/LSCF-cell after 300 h operation

Single electrode characterisation / Peter Holtappels / / Folie 13 3-cell stack: I/V Characteristics Experimental: v NG : 4 g/h v air : 200 g/h T= 800°C f.u. 90%  el : 61% Max. Power output: 0.44 W/cm 0.78 V

Single electrode characterisation / Peter Holtappels / / Folie 14 3-cell stack: performance Experimental: v NG : 20 g/h v air : 1000 g/h T= 800°C

Single electrode characterisation / Peter Holtappels / / Folie 15 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?

Single electrode characterisation / Peter Holtappels / / Folie 16 Porosity graded anode substrates

Single electrode characterisation / Peter Holtappels / / Folie 17 Anode supports No pore former 18% porosity 20 vol% pore former 34% porosity 30 vol% pore former 40% porosity

Single electrode characterisation / Peter Holtappels / / Folie 18 Performance of graded anode supports Electrical performance Impedance measurements non-sealed set-up ± 880 mV (theoretical 1 V)  Residual porosity 1 – 2  cm 2 instead of 0.1  cm 2

Single electrode characterisation / Peter Holtappels / / Folie 19 What we intend to do:

Single electrode characterisation / Peter Holtappels / / Folie 20 Understanding Ageing in SOFCs Anodes: –C(H 2 O, O 2, OH - ) –Structural stability = f(U) – Cathodes: –Cation diffusion –Structural stability = f(U) –Cr-poisoning = f(U) –  Single electrode information on supported electrolyte cells LENI:cell testing LPI: anode development EMPA: cathode development ETHZ: cathode modelling

Single electrode characterisation / Peter Holtappels / / Folie 21 AnodeCathode Air O 2 reduction O 2 + 4e +2V.. O -> 2O x O -> O 2- Fuel H 2 H 2 oxidation -> electrons + water O 2- Anode Air O 2 reduction -> water Cathode H+H+ Fuel H 2 H 2 oxidation -> electrons + H + H 2 O + V.. O + O x O -> 2OH. O Concentration cell Gas I, Pt / electrolyte / Pt, Gas II Conduction type Proton/Oxid ion ? Source: H. Iwahara et al., Solid State Ionics 61 (1993) H 2 + 1/2O 2  H 2 O

Single electrode characterisation / Peter Holtappels / / Folie 22 Source: T. Norby, Solid State Ionics 125 (1995) 1-11 Proton conductivity / Total conductivity Source: H. Iwahara, Solid State Ionics 77 (1995) In H atmosphere

Single electrode characterisation / Peter Holtappels / / Folie 23 Cathodes for Solid Oxide Electrolysis Cell (SOEC) O 2- water vapourhydrogen oxygen + - cathode anode electrolyte carbon dioxidecarbon monoxide 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?

Single electrode characterisation / Peter Holtappels / / Folie 24 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! ?

Single electrode characterisation / Peter Holtappels / / Folie 25 Acknowledgment Graded SOFC Anodes Proton conducting Fuel Cells BFE-Projekt: / BFE-Projekt: / at EMPA: Hansjürgen Schindler Simone Zürcher Ulrich Vogt Mogens Mogensen, Risø National Laboratory Former colleagues Forschungszentrum Jülich, IEF (IWV-3)

Single electrode characterisation / Peter Holtappels / / Folie 26 Wet (saturation with water at room T) Dry (passing through phosphoruspentoxide powder) Hydrogen and/or Oxygen concentration cell: H 2 (1 atm), Pt/ electrolyte / Pt, H 2 (pH 2 ) -> Nernst Emf mesurement: H 2 O diluted H 2 -> emf decrease -> oxid ion cond. H 2 O diluted air exces -> no emf diminution -> proton cond. H permeation: H 2 (1 atm), Pt/ electrolyte / Pt, Ar -> Faraday Or H permeation with a radioactive isotop Cell: H 2, Pt/ electrolyte / Pt, O 2 (wet hydrogen – constant current) Pressure mesurements: H 2 O at cathode -> proton cond. Conduction type Proton/Oxid ion ? Source: N. Bonanos, Solid State Ionics (1992)