SOFC Cathodes, Supports and Contact Layers. Alan Atkinson Department of Materials Imperial College London SW7 2AZ, UK

Transcription

1 SOFC Cathodes, Supports and Contact Layers Alan Atkinson Department of Materials Imperial College London SW7 2AZ, UK

2 Contents for cathodes Requirements for application in SOFCs LSM based cathodes defects in TM perovskites temperature limitations Mixed conducting cathodes Oxygen diffusion and exchange Drive for lower operating temperatures Contamination and durability issues Cr poisoning water vapour sensitivity surface segregation Review: S. B. Adler, "Factors governing oxygen reduction in solid oxide fuel cell cathodes", Chemical Reviews, 104, (2004)

3 Oxygen reduction reaction adsorption dissociation ionisation incorporation e- Oxygen vacancies are required for surface reaction as well as for bulk diffusion Electrons are required for ionisation

4 Triple phase boundary electrodes O 2 e - Electrocatalyst has high electronic conductivity, but low ionic conductivity Reaction site is at triple phase boundary LSM cathodes typical example Composites with good ionic conductor used to improve performance (e.g. LSM/YSZ cathodes) Capacitance is low (e.g F cm -2 ) O 2- electrolyte Cathode reaction requires both electronic and (oxygen) ionic defects In oxides this means transition metal constituents and oxygen nonstoichiometry (vacancies or interstitials) Working overpotentials can change oxide stability or properties

5 Theory for porous single phase cathode e - Adler, Lane, and Steele, J. Electrochem. Soc., 143 (1996) 3554 porosity tortuosity volume specific surface area O 2 O 2- electrolyte R chem Oxygen ion concentration in solid RT = 2F 1 ( ε ) τ 2 2 O Oxygen tracer diffusion coefficient c ad k Oxygen surface exchange rate constant Microstructure is very important (a, ε, τ) Only gives ASR close to open circuit conditions (when gas and solid are close to being in equilibrium).

6 Targets Cathode ASR = 0.15 ohm cm 2 For τ = 2, ε = 0.3, a = 10 7 m -1, T = 1000K Then D*k* > cm 3 s -1 Chem RT τ = 4F ( 1 )ac D k 2 2 ε 0 (Note: an ionic conductivity of 0.01 S/cm corresponds to D* = 2x10-8 cm 2 s -1 ) e.g. D* > 10-8 cm 2 s -1 and k* > 10-6 cm s -1 R For current collection need high electronic conductivity (e.g. 100s S/cm) 20µ 5 mm channels σ>1,000 S cm -1 All require chemical and thermo-mechanical stability (CTE match)

7 Defects in TM perovskites e.g. La 1-x Sr x MnO 3 (LSM) Acceptor doping 2SrO 2Sr + 2O + V La O O Reduction 1 Electronic Schottky O V +2e + O (g) O O 2 2 null e + h La Mn O null V + V + 3V Mn valency change LSM in this range

8 Properties of LSM [Sr]=0.2 D too low for single phase cathode Need electrolyte (YSZ) to provide ionic conductivity and vacancies for oxygen reaction

9 LSM-based cathodes Review: S.P. Jiang, J. Mat. Sci. 43 (2008) 6799 Microstructure is very important (particle size, mixing, sintering temperature) Compatible with YSZ (if A-site deficient) Good CTE match to YSZ Lowest operating T about 800C Conditioning on initial polarization (initial improvement in performance) probably due to changes in surface composition

10 Oxygen diffusion in TM perovskites Temperature ( C) La 0.9 Sr 0.1 CoO 3 La 0.9 Sr 0.1 FeO 3 La 0.5 Sr 0.5 (Mn 0.8 Co 0.2 ) 0.6 Ni 0.4 O 3 La 0.6 Ca 0.4 Co 0.8 Fe 0.2 O 3 Sm 0.6 Sr 0.4 CoO 3 D T (cm 2 /s) Sm 0.5 Sr 0.5 CoO 3 Sm 0.5 Sr 0.5 CdO 3 La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 La 0.8 Sr 0.2 Mn 0.5 Co 0.5 O 3 La 0.8 Sr 0.2 Mn 0.8 Co 0.2 O La 0.8 Sr 0.2 MnO /T (K -1 )

11 Co-containing cathodes O vacancies that give high diffusivity also give high CTE Ullmann et al., Solid State Ionics (2000) Ln 0.4 Sr 0.6 Co 0.8 Fe 0.2 O 3-δ React with zirconia electrolytes Need an intermediate barrier layer (e.g. CGO) Tu et al., Solid State Ionics 117 (1999)

12 Defects in La 1-x Sr x CoO 3-δ LSC in this range Sr increases conductivity and induces metallic behaviour A.N. Petrov et al., Solid State Ionics (1995)

13 Oxygen diffusion in La 0.6 Sr 0.4 CoO 3 at p O2 =0.21 atm 10-7 T ( C) 10-6 La 0.6 Sr 0.4 CoO 3 (Imperial, IEDP) La 0.6 Sr 0.4 CoO 3 (ULeoben, CR) La 0.6 Sr 0.4 CoO 3 (Inoue et al.[1], GPA) (La 0.6 Sr 0.4 ) 0.99 CoO 3 (Søgaard et al. [2], CR) Sm 0.6 Sr 0.4 CoO 3 (Fullarton et al. [3], IEDP) D * (cm 2 s -1 ) E act = ev x x x x x10-3 1/T (K -1 ) (1) Inoue, T., Kamimae, J., Ueda, M., Eguchi, K. & Arai, H. Journal of Materials Chemistry 3, (1993). (2) Sogaard, M., Hendriksen, P. V., Mogensen, M., Poulsen, F. W. & Skou, E. Solid State Ionics 177, (2006). (3) Fullarton, I. C., Kilner, J. A., Steele, B. C. H. & Middleton, P. H. in 2nd International Symposium on Ionic and Mixed Conducting Ceramics (eds. Ramanarayanan, T. A., Worrell, W. L. & Tuller, H. L.) 9-26 (The Electrochemical Soviety, San Francisco, 1994).

14 ALS model for MIEC porous electrode l δ = D k * * ( 1 ε ) aτ Effective cathode thickness increases with D and decreases with k and is usually < 10 µm Z chem = R chem Gerischer impedance 1+ 1 jωr chem C chem t = c ( ε ) 1 1 V chem co A a * k C chem 2 ( ε ) 2F 1 = ART cl V δ Large chemical capacitance controlled by deviation from stoichiometry, c V (e.g.1f/cm 2 ) A is a thermodynamic factor for defects and is close to unity Data for La 0.6 Ca 0.4 Co 0.2 Fe 0.8 O 3 on SDC at 700 C (Adler et al. J. Electrochem Soc., 1996)

15 Surface exchange in La 0.6 Sr 0.4 CoO 3 and Sm 0.6 Sr 0.4 CoO 3 at p O2 =0.21 atm 10-5 T ( C) La 0.6 Sr 0.4 CoO 3 (Imperial, IEDP) La 0.6 Sr 0.4 CoO 3 (ULeoben, CR) Sm 0.6 Sr 0.4 CoO 3 (Fullarton et al. [3], IEDP) Sensitive to rare-earth cation K (cm s -1 ) E act = ev Predicted E a for R p =1.4eV Measured is 1.1eV x x x x x10-3 1/T (K -1 )

16 D* in LSC at 600 C 1E-5 1E-6 * D * bulk (cm2 s -1 ) 1E-7 1E-8 1E-9 1E-10 1E-11 1E-12 * 1E-13 1E-14 1E-15 * La 1-x Sr x CoO 3-d Sm 1-x Sr x CoO 3-d * Extrapolated from higher T Sr doping (x)

17 k* at 600 C 1E-4 1E-5 * 1E-6 K (cm s -1 ) 1E-7 1E-8 1E-9 1E-10 1E-11 * * La 1-x Sr x CoO 3-d Sm 1-x Sr x CoO 3-d Sr doping (x) Need high Sr content

18 Low temperature composite cathodes 2 log10 (ASR/ohm cm 2 ) % La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 30% Ce 0.8 Gd 0.2 O 2 Composite 1000/T (K -1 ) Esquirol, Kilner and Brandon, Solid State Ionics, 2004

19 Conductivity of LSC/CGO composites 4 3 Material CTE ppm/k 2 CGO 12.5 logσt (s.cm -1 ) LSC 20%LSC /CGO 30%LSC /CGO %0.4LSC 20%0.4LSC 30%0.4LSC LSC CGO /T (K -1 )

20 Low temperature composite cathodes 50% Gd 0.8 Sr 0.2 CoO 3 50% Ce 0.9 Gd 0.1 O 2 Composite Huang et al. J. Power Sources (2008)

21 Oxygen surface exchange on thin film cathode materials Thickness is much less than D*/k*, so rate is controlled by surface exchange k = RT 2 4 s F Rc O Dense thin film electrodes BSCF cathodes have excellent initial performance but degrade rapidly in air F.S. Baumann et al., J Electrochem. Soc. (2007) Surface carbonation forming BaCO 3 is a major problem

22 Cr poisoning Cr released from steel components as CrO(OH) 2 vapour At cathode this is reduced to Cr 2 O 3 : 2CrO(OH) 2 +2e - Cr 2 O 3 +2H 2 O+O 2- Reacts with Sr in cathode to give SrCrO 4 Need coating on steel to prevent Cr evaporation I. Vinke, SOFC IX, 2007

23 K 2 NiF 4 -structured oxides with oxygen excess Oxygen diffusion Oxygen surface exchange S.J. Skinner and J.A. Kilner, Solid State Ionics 135 (2000)709. With no Sr or Ba hope to avoid some of the degradation issues Because defects are oxygen interstitials performance under polarisation is not enhanced (as it is with oxygen deficient oxides)

24 K 2 NiF 4 -structured oxides with oxygen excess (2) Boehm et al., Solid State Ionics 176 (2005) 2717

25 Performance of LNO cathodes F. Chauveau et al, ECS Transactions, 25, 2557 (2009) LSM Nd 2 NiO 4+δ La 2 NiO 4+δ

26 Final comments on cathodes LSM needs ionic conductor (YSZ) to give composite with TPBs and percolating ionic pathways. LSM composites good for higher T (e.g. > 800 C) Transition metal perovskites good mixed conductors and some can be used as single phase cathodes E.g. LSCF (with CGO barrier layer) at T > 700 C. Below 700 C needs composite to boost ionic conductivity Composites also allow better CTE matching In general, the higher the intrinsic activity the lower the stability (chemical and mechanical) High Ba, Sr and/or Co contents bring problems Would like better tolerance of Cr, H 2 O and CO 2

27 Thank you for your attention! Questions on cathodes?

28 Supports and contact/protective layers

29 Cell supports Requirements Mechanical support for cell structure Sufficient gas transport to electrode (conflicts with previous) Sufficient electronic conductivity CTE match to cell Stable to chemical changes in environment Easy and cheap to manufacture Options Anode Cathode Electrolyte Interconnect (metal) Inert material

30 Anode support (Ni-based) 10 µm electrolyte (e.g.ysz) 30 µm fine anode (e.g.ni-ysz) 500 µm coarse support (e.g.ni-ysz) Advantages Popular (e.g. Juelich, Versa, Topsoe) Good performance Internal reforming Disadvantages Expensive Ni content (but better than cathode support) Poor redox tolerance C-deposition problems

31 Anode support (ceramic) 10 µm electrolyte (e.g.ysz) 30 µm impregnated anode (e.g.pd-ceria) 500 µm support (e.g.nb-doped SrTiO 3 ) Advantages No redox or C-deposition problems Disadvantages Poor electronic conduction (difficult current collection) Unreliable impregnation of catalyst Still at research stage

32 Electrolyte support (e.g. Hexis) 50 µm cathode (e.g.lsm/ysz) 150 µm electrolyte support (e.g. 3YSZ) 50 µm anode (e.g.ni-ysz) Advantages Flexible choice for anode Some redox tolerance even with Ni-based anodes Disadvantages Limited to high operating T (e.g.900c) Fragile Expensive interconnector (e.g. CrFeY)

33 Metallic support (e.g. Ceres Power, DLR) Stainless steel Stainless steel-supported Advantages Mechanically robust Cheap material Good for lower temperatures Easy to seal Disadvantages Tricky processing

34 Inert supported (e.g. LGFCS) Air Fuel Advantages Cheap material Some redox tolerance Disadvantages Porous support Ni-YSZ anode In-plane design has difficult current collection LSM-YSZ cathode YSZ electrolyte

35 Need for contact and protective coatings Used with metallic interconnects/bipolar plates To improve (lower) contact resistance between metal and cathode Target contribution to ASR < 20 mω cm 2 Corresponds to approx. 1% reduction in power density To reduce contamination of cathode by Cr-containing vapour evaporating from metal Target for cathode ASR to increase by < 1%/kh Applied as a coating(s) on the interconnect on cathode side Juelich cell cross-section

36 Corrosion scales on interconnect alloys Often there are 2 layers: inner Cr 2 O 3 and outer (Mn,Cr) 3 O 4 spinel Evaporation of Cr species poisons cathode M. Zahid

37 Protective coatings

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39 Electroplated or PVD Co coating on interconnect Q.-X. Fu, et al., Solid State Ionics (2010) Available commercially as Sandvik Sanergy HT

40 Coatings improve performance MnCo 1.9 Fe 0.1 O 4 protective coating on Crofer 22APU

41 Final comments on coatings Protective and contact coatings need to be optimised for particular interconnect alloys and cathodes Protective coatings Ideally should be dense Should be able to absorb Cr in solid solution Can reduce alloy corrosion rates Should not react to produce insulating phases Transition metal spinels currently favoured for stainless steel interconnects If not used, then need Cr-tolerant cathodes Contact coatings Ideally should be porous (compliant) Transition metal perovskites currently favoured

42 Thank you for your attention! Questions on supports and coatings?