N.E.Kiratzis 1, P. Tragias 1, L. Yiamouridis 1 and E. Papastergiadis 2. Department of Food Technology, ATEI of Thessaloniki

Transcript

1 FABRICATION OF ELECTRODES AND ELECTROLYTES FOR SOFCS VIA SPRAY PYROLYSIS AND CONVENTIONAL METHODS N.E.Kiratzis 1, P. Tragias 1, L. Yiamouridis 1 and E. Papastergiadis 2 2 Department of Food Technology, ATEI of Thessaloniki

2 LABORATORY FOR ADVANCED MATERIALS AND ELECTROCHEMICAL TECHNOLOGY (LAMET) SOFCs and Electrochemical Testing SOFCs component fabrication via Spray pyrolysis Equipment GC, MS TGA DSC HT Furnaces Planetary ball mill Electrochemical instrumentation

3 GENERAL OVERVIEW OF SOFC Electrical energy FUEL e - e - O 2- - AIR + CO + O 2- CO 2 + 2e H 2 +O 2- H 2 O + 2e Ni YSZ LSM-YSZ YSZ electrolyte 1/2O 2 + 2e - O 2- anode LSM: Sr-doped LaMnO 3 YSZ: (8-10mole% Y 2 O 3 )-ZrO 2 cathode O 2- MAIN CHARACTERISTICS AND ADVANTAGES Typical operating temperatures C High thermodynamic efficiencies Low pollutant emissions High quality heat in CHP cycles and/or gas turbine applications Fuel flexibility (including biofuels) Potential for internal reforming of hydrocarbon fuels Operation with industrially important catalytic reactions

4 CHALLENGES FOR SOFCS Lowering the operating temperature to C relaxing of the material selection requirements for sealing and interconnection Cost reduction promote application of SOFCs in small units such as in dispersed power generation systems and transport Operating on conventional hydrocarbon (e.g.ng) fuels and/or biofuels Use existing infrastructure Elimination or minimization of external fuel reforming Elimination or minimization of internal fuel reforming

5 LOWERING THE OPERATION TEMPERATURE Very thin electrolytes and electrodes (i.e. less than 5 µ) with lower ohmic resistances Innovative fabrication method based on a molecular approach Use the alternative electrolyte CeO 2 (10% Gd 2 O 3 ) (CGO) which shows higher conductivities than YSZ in the C range

6 SPRAY PYROLYSIS AS FABRICATION METHOD FOR SOFCS COMPONENTS Precursor Torch Hot plate Precursor solution spray Air (1-3 atm) Nozzle Substrate TI MAIN ADVANTAGES Low cost Low temperature operation Open atmosphere Precise stoichiometry control at droplet level Better control on final particle size Suitable for mixed metal oxide synthesis Allows in situ fabrication of all SOFC components and composites Might allow integrating precipitation, calcination and sintering in a single process

7 Cps COMPOSITE ELECTROLYTES Bar corresponds to 10μm CGO SEM of deposited film YSZ CGO YSZ T sinter/ o C CGO YSZ CGO YSZ YSZ CGO Bar corresponds 2θ/deg to 20μm YSZ Flow rate = 24 cm 3 /min Total ion concentration = 0.5 M T sub = 360C T sintering = 700C CGO MAIN CONCLUSIONS Polycrystalline CGO films of 0.5-2μm were successfully prepared on dense YSZ substrate with EtOH:H2O 7:3 solvent Substrate temperature is significantly lowered due to the cooling effect of the spraying Initial substrate temperatures for uniform films were C Sintering temperature is optimal at 700 C for improved crystallinity Optimal solution flow rates were >10 cm 3 /min Reduction of film thickness below 1 μm causes significant reduction of micro-cracks on film surface No micro porosity present throughout the whole film thickness Average growth rates were of the order of μm/h Kiratzis et al., Fabrication of ceramic electrolytic films by the method of solution aerosol thermolysis (SAT) for solid oxide fuel cells (SOFC), 2009

8 ANODE SUPPORTED ELECTROLYTES Bar corresponds to 10μm T sub = 300C T sintering = 700C SEM of deposited film MAIN CONCLUSIONS Deposits are not dense on porous anodic substrates although adhesion is good Particle packing is difficult to control Cu from the substrate seems to segregate at the interface More optimization is required in terms of substrate temperature, sintering temperature and combination of solvent/precursor Different types of porous cermet substrates containing other combination of metals such as Ag or Au and ceramics should be investigated Bar corresponds to 20μm 6 o ΠΑΝΕΛΛΗΝΙΟ ΣΥΜΠΟΣΙΟ ΠΟΡΩΔΩΝ SEM ΥΛΙΚΩΝ of deposited film

9 OPERATING ON CONVENTIONAL HYDROCARBON (E.G.NG) FUELS AND/OR BIOFUELS Replace Ni with Cu to avoid carbon deposition and keep electronic conductivity Combine Cu with a ceramic (e.g. CeO 2 or LSCM* or YZT**) exhibiting mixed conductivity to expand TPB and active electrocatalytical properties Comparison of conventional ceramic techniques (i.e. wet slurries) with SP * LSCM : (La 0.75 Sr 0.25 )Cr 0.5 Mn 0.5 O 3- ** YZT : Y 0.2 Ti 0.18 Zr 0.62 O 1.9

10 CONVENTIONAL ANODIC FILM FABRICATION METHOD (LIQUID SLURRY) CERMET PREPARATION CuO Dispersant CeO 2 or LSCM* Muffle furnace High shear milling ½-1 h Sintering C 5h Binder High shear milling ½-1 h Application on YSZ * Combustion synthesis

11 CU-CEO2 ON YSZ Bar corresponds to 10μm SP Cu:CeO2=7/3 (w/w) SEM of deposited film MAIN CONCLUSIONS Different morphologies are obtained for Cu-CeO 2 films on YSZ by SP and conventional methods Better adhesion is obtained by SP For conventional films, adhesion requires sintering temperatures of 1100 C with 5% (w/w) YSZ in the CeO 2 powder Addition of YSZ seems also to prevent Cu- agglomeration in conventional cermets In SP films Cu seems to segregate more on the pellet surface rather than on the YSZ interface but more work is required to verify it Bar corresponds to 20μm CM Cu:CeO2=65/35 (w/w)

12 CONCLUSIONS-FUTURE WORK The technique of Spray pyrolysis was used to fabricate electrolytes and electrodes for SOFCs Cermet electrodes based on Cu showed better adhesion to YSZ dense electrolyte pellets when made by SP than by the conventional wet slurry method Electrolyte films of CGO were also successfully made by SP but the substrate type (i.e. porous or dense) plays a significant role on morphology Making dense films by SP proves a more difficult challenge due to less control in particle packing and the effect of subsequent sintering steps Electrochemical testing is required to SOFCs consisting of SP made components

13 ACKNOWLEDGEMENTS This research has been co-financed by the European Union (European Social Fund ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funding Program: ARCHIMEDES III. Investing in a knowledge society through the European Social Fund. TEIWM for continuing support for consumables Undergraduates: V. Machairas, A. Keramitzi, A. Baxevanos Prof. A. Triantafyllou and A. Krestou and the Laboratory of Atmospheric Pollution and Environmental Physics for use of their particle size analyzer CERTH (XRD, SEM)

14 APPENDICES

15 COPPER-BASED ANODES FOR DIRECT HYDROCARBON OXIDATION IN SOFCS Requirements for DHCO based anodes: Suitable for operation in the absence of large amounts of steam in the fuel High electrocatalytic* activity without undesirable side reactions (e.g. carbon deposition) Good adhesion to the electrolyte surface Ionic conductivity for an expanded Triple Phase Boundary (TPB) region from Porous cermets composites Single phase mixed conducting (i.e. permitting both O 2 and e mobility) *i.e. high current densities and low overpotentials or polarization resistances)

16 Ne, mol/cm^2/s USE OF CGO AS ALTERNATIVE TO YSZ SOLID ELECTROLYTE FOR SOFCS Effect of cathode porosity on oxygen flux MAIN CONCLUSIONS Performance requirements for cathode supported CGO* electrolytes : Current density : 0,7 A.cm -2 Oxygen flux above 2E-6 mol/cm 2 /s Simulation showed that performance requirements can be met for thickness even 500μm as long as porosities are above 30% for cathode supported electrolytes Fabricated pellets of LSCF** as cathode supports possessed no microporosity at all, with typical porosities of 30% and thickness ranging μm 1.000E E E mi 500mi 1000mi 0.7A/cm^2 * CGO : Ce 0.9 Gd 0.1 O 1.95 in its 10% Gd 2 Ο 3 stabilized form (CGO10) Gd 2 Ο 3 2Gd Ce + V o + 3O o x Higher ionic conductivities Ο ο x 1/2Ο 2 (g) + V o + 2e > Mixed conductivity (ionic and electronic) > Lattice expansion 1.000E porosity Kiratzis et al. Proceedings of the 2nd European Solid Oxide Fuel Cell Forum, 1996 a=1000å, t=400 C ** LSCF : La 1-x Sr x Co 1-y Fe y O 3, with selected combination for x = 0.4 and y = 0.8 made by the Pechini method (mean pore radius=0.67 μm) sintered at 1200 C, 5h.

17 APPLICATION OF CGO-BASED SOFCS FOR TRANSPORTATION DEVICES Steele, Kiratzis, Christie et al., Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells, 1995 e - CH 3 OH + 3O 2- CO H 2 O+6e Ni YSZ Electrical O energy 2- O 2- e - + LSCF 3/2O 2 + 6e - 3O 2- anode cathode 500 C < T < 700 C MAIN CONCLUSIONS Anode polarization resistance is below 1Ω.cm 2 at 570 C, with no carbon deposition for H 2 O/CH 3 OH ratio greater than 0.1 in symmetrical Ni-YSZ / CGO10 / Ni-YSZ cells Excellent power output performance was achieved in LSCF / CGO10 / Ni-YSZ cells between 600 C and 700 C under H 2 /3% H 2 O atmosphere Low polarization resistance was recorded on CGO10 in contact with Ni-YSZ or stainless steel based anodes between 450 C and 600 C in moist H 2

18 COMPARISON OF PERFORMANCE BETWEEN LSCM, CU CEO2 AND NI YSZ ANODES* Cu CeO 2 and LSCM based anodes compare favourably with the established Ni YSZ anodes in terms of polarization resistances power densities More testing is needed in terms of their long term performance Good performance for direct utilization of hydrocarbons in a SOFC without promoting carbon deposits Performance under CO or synthesis gas can be exceptional and in some cases exceeds performance under H 2 In terms of sulfur tolerance, Cu CeO 2 anodes also show improved performance with respect to Ni YSZ cermets *Kiratzis NE, Connor P, Irvine JTS (2010) J Electroceram 24 (4):

19 PELLETS FILMS COPPER-BASED ANODES FOR DIRECT HYDROCARBON OXIDATION IN SOFCS Current results based on YSZ electrolyte example on Cu-LCSM and Cu-CeO 2 : *Kiratzis NE, et al., (2010) J Electroceram Cu-LSCM Cu-CeO 2 Comparison of film polarization resistances Cu-LSCM bulk pellet re-dox conductivity K po log( log(po 2 ) time,hours -25

20 PELLETS FILMS COPPER-BASED ANODES FOR DIRECT HYDROCARBON OXIDATION IN SOFCS Current results based on YSZ electrolyte : Cu-YZT Cu-LSCM Cu-CeO 2 Cu causes a slight tetragonal distortion to the cubic lattice of YZT Good electrochemical performance under H 2 up to 800 C Copper agglomeration at higher temperatures Better porosity and Cu particles interconnectivity than in Cu-CeO 2 cermets for reduction temperature lower than 750 C After reduction, the Cu LSCM electrode exhibited better adhesion to the YSZ electrolyte surface than the Cu CeO 2 electrode Possibly higher degree of Cu mobility or segregation occurring on the Cu CeO2 electrode Reduction temperature has to be 750 C for good adhesion with the YSZ electrolyte and for the formation of a suitable microstructure Greater stability is exhibited by the pellets than the thin electrodes Conductivities of the order of 2000 S/cm were measured between K Good redox cycling behavior exhibited Good thermal and red-ox behavior Mechanical integrity is preserved after redox process Conductivities reach about 300 S/cm under reducing conditions No significant volume change after reduction (34.4% v/v Cu) Lower porosity was observed in Cu-CeO 2 pellets than in Cu- LSCM Both types of pellets retain their mechanical strength and fracture resistance after reduction Pellets of these materials are very promising as anodic supports and exhibit better structural stabilities than thin films N. Kiratzis et al., Fuel. Cells. (Weinh.). 1, 211 (2001) Kiratzis NE, et al., (2010) J Electroceram

21 NI-YSZ CERMET *J.-H. Lee et al., SSI 148 (2002) Low cost and high activity for hydrogen oxidation High steam reforming and direct oxidation activity for methane (i.e. internal reforming) Promotes the formation of carbon filaments above 700 C with CH 4 fuel Conventional ceramic fabrication techniques More suitable using CH 3 OH or EtOH fuels

22 SPECIFIC ENERGY AND ENERGY DENSITY OF PORTABLE ENERGY SOURCES Evans, A., et al., Journal of Power Sources, (2009)

23 FUEL-CELL TYPES AND FUEL PROCESSING B. C. H. Steele & A. Heinzel, NATURE VOL NOVEMBER 2001

24 increasing temperature a b c d droplet evaporation precipitate substrate melting Solute vaporization decomposition liquid vapor solid sintering powder

25 CONTRIBUTING PROCESSES TO DIRECT HC OXIDATION J.T.S. Irvine et al., Nature Materials, VOL 3, 2004, p.17

26 GRADUAL INTERNAL CH 4 REFORMING P. Vernoux, J. Guindet, and M. Kleitz J. Electrochem. Soc., Vol. 145, No. 10, October 1998

27 NI-YSZ ANODE TPB *S.McIntosh and R. J. Gorte, Chem. Rev. 2004, 104,

28 CEO 2 LATTICE

29 CU-CEO 2 CERMET * * Replace Ni with Cu which is simply an electronic conductor CeO 2 is a good oxidation catalyst Mixed conductivity at low PO2 Good performance under a variety of HCs and SG Very good sulfur tolerance Formation of non-graphitic carbon improves anode performance Gorte RJ, Park S, Vohs JM, Wang C (2000) Adv. Mater. 12:1465 He H, Gorte RJ, Vohs JM (2005) Electrochem. Solid-State. Lett. 8(6): A279 McIntosh S, Vohs JM, Gorte RJ, JECS (2003), 150, p. A470 Kim H et al., Chem. Commun., (2001) p.2334

30 CU/CEO 2 /YSZ MICROSTRUCTURE WITH TAR FORMATION* *J.T.S. Irvine et al., Nature Materials, VOL 3, 2004, p.17

31 PEROVSKITE ABO 3 STRUCTURE AND UC *Tao, S and Irvine JTS, The Chemical Record, (4), (2004) p. 83 S.McIntosh and R. J. Gorte, Chem. Rev. 2004, 104,

32 LSCM PEROVSKITE ANODE Perovskites based on La 1 x Sr x CrO 3 (higher e conductivities) Stability Substitution of mid-transition metals (e.g. Mn or Fe) on the B-site (La 0.75 Sr 0.25 )Cr 0.5 Mn 0.5 O 3- (LSCM)* Reasonable performance under methane above 700 C promoting full oxidation Chemical compatibility with YSZ is quite good up to 1300 C Mixed conductivity (i.e. ionic +electronic -depending on PO2) total conductivities (<10 S cm 1 ) under reducing conditions *Plint SM, Connor PA, Tao S, Irvine JTS (2006) Solid State Ion. 177 p.2005 Raj ES, Kilner JA, Irvine JTS (2006) Solid State Ion. 177 p.1747 Tao, S and Irvine JTS, The Chemical Record, (4), (2004) p. 83 Tao S, Irvine JTS (2003) Nat.Mater. 2 p320 Ruiz-Morales JC et al., (2007) Electrochim. Acta. 52 p.7217 Bruce MK et al.,(2008) J. Electrochem. Soc. 155 (11) p. B1202

33 YZT Y 0.2 Ti 0.18 Zr 0.62 O 1.9 Kaiser, A.; Feighery, A.J.; Fagg, D.P.; Irvine J.T.S. Ionics 1998, 4, 215. Feighery, A.J.; Irvine, J.T.S.; Fagg, D.P.; Kaiser, A. J Solid State Chem 1999, 143, 273

34 N. Kiratzis et al., Fuel. Cells. (Weinh.). 1, 211 (2001)

35 N. Kiratzis et al., Fuel. Cells. (Weinh.). 1, 211 (2001)

36 FILM MORPHOLOGY WITH EDS *Kiratzis NE, Connor P, Irvine JTS (2010) J Electroceram 24 (4):

37 K po log( log(po 2 ) *Kiratzis NE, et al., (2010) J Electroceram time,hours