Christodoulos Chatzichristodoulou Technical University of Denmark, Department of Energy Conversion and Storage

Transcription

1 Fuel Cell & Hydrogen Technologies JP SP2: Catalyst and Electrodes Borovetz, Bulgaria June 2 nd and 3 rd 2014 The need for localized electrochemical measurements and the promise of Controlled Atmosphere High Temperature scanning probe microscopy EERA FCH2-SP2 WORKSHOP in frame of EIA10 Bridging experimental and numerical research: development and optimization of advanced characterization tools Electrochemical Impedance Spectroscopy Christodoulos Chatzichristodoulou Technical University of Denmark, Department of Energy Conversion and Storage

2 Outline The Danish sustainable energy vision The promise and challenges of SOEC SOEC degradation Trying to understand SOEC degradation Limitations of modeling Localized in-operando probing (CAHT-SPM) CAHT-SPM achievements Future perspectives Conclusions Page 2

3 The Danish sustainable energy vision Danish ambition 2020: 50% Renewable electricity 2035: 100% Renewable power and heat 2050: 100% Renewable energy Challenges Increasing share of fluctuating production Biomass resources are limited! Liquid fuels from where? (Aviation, heavy transport) B. V. Mathiesen et al., Applied Energy 88 (2011) Page 3

4 The promise and challenges of SOEC O 2 electrode electrolyte fuel electrode + support Promise High conversion efficiency (ca. 95%) High production rate (ca. 1 A/cm 2 or 0.5 L/h cm 2 ) Low cost Challenges Durability Page 4

5 SOEC degradation Si poisoning of the fuel electrode (ph 2 O) SiO 2 inclusions in Ni grains at the fuel electrode (po 2 ) Formation of zirconia nanoparticles on Ni grains at the fuel electrode (po 2 ) Evaporation of Ni(OH) 2 and loss of electronic percolation (ph 2 O) Formation of O 2 bubbles in the electrolyte (po 2 ) Delamination at the electrolyte/o 2 -electrode interface (po 2 ) Delamination at the electrolyte/electrolyte interface (po 2 ) Kinetic demixing and formation of Kirkendall voids (V, po 2 )... M. Chen et al., J. Electrochem. Soc. 160 (2013) F883. R. Knibbe et al., J. Electrochem. Soc. 157 (2010) B1209. F. Tietz et al., J. Power Sources 223, (2013) 129. Page 5

6 SOEC degradation - Formation of zirconia nanoparticles on Ni grains at the fuel electrode 1 μm n-zro 2 Ni-YSZ electrode YSZ electrolyte ( ) 29 3 ZrO Ni Zr in Ni O g 2 2 1ppmZr in Ni p 1 10 C O 2 M. Chen et al., J. Electrochem. Soc. 160 (2013) F883. Page 6

7 SOEC degradation -SiO 2 inclusions in Ni grains at the fuel electrode Ni-YSZ electrode Si/Zr/Al Si/Zr/Al 2 μm ( ) 19 SiO Ni Si in Ni O g 2 2 1ppmSi in Ni p 1 10 C O 2 Page 7

8 SOEC degradation is location specific M. Chen et al., J. Electrochem. Soc. 160 (2013) F883. R. Knibbe et al., J. Electrochem. Soc. 157 (2010) B1209. Page 8

9 Trying to understand SOEC degradation EIS EIS C. Chatzichristodoulou et al., to be submitted Page 9

10 Trying to understand SOEC degradation model validation i ion = -1.5 A cm -2 Ni-YSZ/YSZ(10μm)/LSM-YSZ T = 850 C p = 1 atm in in j 200Nml min, j 200Nml min j H in O Nml min -1-1 H O -1 Page 10

11 Trying to understand SOEC degradation polarization resistance of the O 2 electrode Ni-YSZ/YSZ(10μm)/LSM-YSZ T = 800 C p = 1 atm in in j 200Nml min, j 200Nml min H H O i ion = -1 A cm -2 Page 11

12 Trying to understand SOEC degradation inhomogeneous fuel electrode response Ni-YSZ/YSZ(7.5μm)/CGO10(2.5μm)/LSC-CGO10 T = 800 C p = 1 atm in H in -1 H O j 200Nml min, j 200Nml min i ion = -1 A cm -2 Page 12

13 Trying to understand SOEC degradation Ni-YSZ/YSZ(10μm)/LSM-YSZ: (0 900 h) T = 850 C in in in j 200 min, 200 min, 400 min p = 1 atm H Nml j 2 H2O Nml jo Nml i = -1.5 A cm -2 R elc (t) R s (t) L Rs R LSM Hi R TPB R LSM Low R Diff. R Conv. LSM Hi CPE TPB CPE LSM low CPE Diff. CPE Conv. Page 13

14 Trying to understand SOEC degradation Ni-YSZ/YSZ(10μm)/LSM-YSZ T = 850 C p = 1 atm in in j 200Nml min, j 200Nml min j H in O Nml min -1-1 H O -1 i ion = -1.5 A cm -2 Page 14

15 Trying to understand SOEC degradation - demixing Ni-YSZ/YSZ(10μm)/LSM-YSZ T = 800 C p = 1 atm in in j 200Nml min, j 200Nml min H H O bulk diffusion: τ st.st. ~ years grain boundary diffusion: τ st.st. ~ 1,6 years YSZ Ni-YSZ/YSZ(10μm)/CGO(5μm)/LSCF T = 780 ± 6 C p = 1 atm t = 1 year F. Tietz, D. Sebold, A. Brisse, J. Schefold, J. Power Sources 223 (2013) Page 15

16 Limitations of modeling A lot of complexity is ignored/simplified: grain boundaries/interfaces segregation/deposition of impurities microstructural heterogeneities microstructural changes compositional changes mechanical strain Localized in-operando probing is needed Page 16

17 Localized in-operando probing (CAHT-SPM) K. V. Hansen et al., Rev. Sci. Instrum. 84 (2013) Max temperature Atmosphere 850 C Humidity % po 2 measurement Probes N 2 -O 2, 9% H 2, CO 2, from bottle Risø po 2 monitor Commercial AFM probes for RT/LT Home made Pt-Ir or ceramic probes for HT Page 17

18 Localized in-operando probing (CAHT-SPM) Possible electrical/electrochemical techniques Counter electrode Probe Sample Microelectrode Surface ac conductance Impedance spectroscopy (+ potential sweep etc) Kelvin probe microscopy/ Scanning surface potential microscopy Micropotentiometry/ Scanning voltage microscopy (being developed) Scanning tunneling microscopy and spectroscopy (iv) Page 18

19 Tapping mode 40.0 µm 40.0 µm 40.0 µm 40.0 µm CAHT-SPM - Topography Area 40x40 m, polished and heat treated YSZ Topography [ 597 nm ] 990 nm nm Topography [ 274 nm ] 466 nm nm Topography [ 139 nm ] 238 nm nm Topography [ 134 nm ] 247 nm nm RT, AC C, AC C,AC C,AC(after scanning 1h) µm 40.0 µm furnace:500c, AC 25 C 500 C furnace:850c,ac 40.0 µm 40.0 µm furnace:850c,ac 850 C 850 C after 1 h Page 19

20 65.0 µm 65.0 µm 15.0 µm CAHT-SPM AC conductance (La,Sr)MnO 3 -YSZ at 800 C Black: high conductance White: low conductance Topography [ 190 nm ] 373 nm nm Ext. Input 1 [ 8.56 V ] 8.61 V V Ext. Input 1 [ 8.50 V ] 8.60 V V c, lsm-ysz 65.0 µm Topography 65x65 m 800c, lsm-ysz 65.0 µm Conductance 65x65 m 50.7 µm 800c, lsm-ysz 15.0 µm Conductance 15x15 m Page 20

21 CAHT-SPM EIS of (La 0.75 Sr 0.25 ) 0.95 MnO 3 μelectrodes Microelectrodes Can be dense Have a well defined area Have a favourable geometry for TPB calculations Many electrodes in a small area Easy to vary thickness and size (and composition?) Produced by photo-lithography CE Impedance spectroscopy 20, 50 and 100 m electrodes Performed in oxygen, air, and nitrogen (po 2 = 10-4 atm) C Cathodic and anodic polarisation Gamry potentiostat femtostat 50 m diameter Y. Wu et al., ECS Trans. 57 (2013) July 2014 Page 21

22 CAHT-SPM EIS of (La 0.75 Sr 0.25 ) 0.95 MnO 3 μelectrodes Temperature dependence in air: ±0.03 ev 19 lnr p / Impedance spectra of a 50 µm diameter LSM microelectrode in air /T /K -1 Arrhenius plot of the polarisation resistance in air. Ea is in good agreement with literature Page 22

23 logrp Z'' / Ω Z'' / Z'' / Ω Ω CAHT-SPM EIS of (La 0.75 Sr 0.25 ) 0.95 MnO 3 μelectrodes Atmosphere dependence at 856 C -3e7-2e7 20 µm 50 µm 100 O 2-7.5e7 20 Air µm µm -5.0e7 µm µm -3e8-2e8 20 µm 50 µm 100 µm N 2-1e7-2.5e7-1e e7 2e7 3e7 Z' / Ω O 2, 1 bar, slope: Air, 0.21bar, slope: N 2,1E-4 bar, slope: e7 5.0e7 7.5e7 1.0e8 Z' / Ω Air and Oxygen Rp scales with 1/d surface reaction path Nitrogen Rp scales more with 1/d 2 bulk path contributes also 0 0 1e8 2e8 3e8 Z' / Ω logd Page 23

24 Temperature CAHT-SPM In-situ reduction of NiO-YSZ-Al 2 O 3 Samples: polished anode support of symmetrical cells (NiO/YSZ/alumina) 13 reduction experiments were carried out C (the possible window due to reduction time) Experimental procedure Air N 2 9% H 2 in N 2 (H 2 O) Time K. V. Hansen et al., J. Solid State Electrochem. (2014) DOI /s Page 24

25 CAHT-SPM In-situ reduction of NiO-YSZ-Al 2 O 3 Unreduced NiO-YSZ-Al 2 O 3 at 500 C: Topography Conductance Conductivity of NiO is due to Ni 3+ Conductance depends on the size of the NiO particles and nature of contact Page 25

26 CAHT-SPM In-situ reduction of NiO-YSZ-Al 2 O 3 Reduction at 500 C in 9% H 2 and 3 % H 2 O: H 2 N 2 Conductance in air Conductance during reduction Conductance after reduction Page 26

27 CAHT-SPM In-situ reduction of NiO-YSZ-Al 2 O 3 Reduction at 500 C in 9% H 2 and 3 % H 2 O: Section B Introduction of hydrogen Conductance decrease. Incubation period H 2 N 2 Section C Growth of Ni areas. Connectivity increases. Conductance cross sections at 0, 5, 10, 15, and 20 m Section D Maximum conductance reached. All surface Ni is connected. Page 27

28 CAHT-SPM In-situ reduction of NiO-YSZ-Al 2 O 3 Arrhenius plot of incubation period length: Retarding effect of water (longer incubation time) Low T and high T regions for dry reductions Two activation energies two different rate limiting steps Two different microstructures Page 28

29 Micropotentiometry: Future perspectives μpotentiometry Initial tests on a symmetrical cell at 650 C in 9% H 2 in N 2 (3% H 2 O) mv -450 mv Page 29

30 LSC on YSZ at 500 C 60.9 µm Ni-YSZ at RT 19.9 µm 19.9 µm 19.9 µm Future perspectives Kelvin probe microscopy Surface potential by Kelvin Probe Microscopy: Topography [ 67.5 nm ] 91.8 nm nm Ext. Input 1 [ 1.43 V ] 1.74 V mv Ext. Input 1 [ 5.14 V ] 5.15 V mv µm Topography (20x20 m) 19.9 µm Surface potential Ext. Input 1 [ 552 mv ] 2.47 V mv 19.9 µm Surface potential and conductance YSZ LSC Topography (60x60 m) 60.9 µm Surface potential Page 30

31 Conclusions SOEC degradation is location specific 2D modeling + EIS + post-mortem EM can yield valuable insight into SOEC degradation Important aspects of real SOEC are ignored/simplified in numerical models In-operando local probes of the electrochemistry, chemistry, microstructure,... are needed CAHT-SPM can yield local electrical, electrochemical and microstructural information in-operando with sub-μm resolution Ongoing developments will provide even more versatility and higher spatial resolution in the future Page 31

32 Acknowledgements This work was supported financially by: The Programme Commission on Sustainable Energy and Environment, The Danish Council for Strategic Research, via the Strategic Electrochemistry Research Center (SERC) ( contract no ( ) The Catalysis for Sustainable Energy (CASE) initiative funded by the Danish Ministry of Science, Technology and Innovation. Energinet.dk through the projects ForskEL plansoec-r&d and commercialization roadmap for SOEC electrolysis and ForskEL Development of SOEC cells and stacks Page 32