Core Research on Solid Oxide Fuel Cells, plus flexible funding project Application of 3D imaging and analysis to the design of improved current collectors for SOFCs. Professor gel Brandon OBE FREng BG Chair in Sustainable Gas Imperial College London Director: Hydrogen and Fuel Cell SUPERGEN Hub (H2FC SUPERGEN) www.h2fcsupergen.com www.imperial.ac.uk/energyfutureslab
Content Core - 3D Imaging and Analysis of Solid Oxide Fuel Cell Electrodes. Flexible - Application of 3D imaging and analysis to the design of improved current collectors for SOFCs Core - New approaches to SOFC electrode fabrication. Summary. Ambition to move to a move towards a design led approach to optimum SOFC electrodes
Typical planar SOFC geometries Brett DJL, Atkinson A, Brandon NP, Skinner SJ, Intermediate temperature solid oxide fuel cells, CHEM SOC REV, 2008, Vol:37, Pages:1568-1578
SOFC Electrode Design Illustration of the effect of extending the TPB using a MIEC electrolyte. (a) Electrolyte / cermet anode with active TPB circled; (b) mechanism of reaction at the TPB; (c) mechanism of reaction at the extended TPB.
Electrode Microstructure in three dimensions TPB 2 TPB 3 TPB 1
Tomography techniques to resolve 3D microstructure >1m 3 Volume Size Analysis Page 6 10 nm 3 1µm 3 100µm 3 10mm 3 3D Atom Probe Electron Tomo Dual Beam FIB Tomo X-ray NCT X-ray Microtomogaphy CT/Synchrotron Mechanical Sectioning 0.1 nm 10 nm 1µm 100µm Voxel Length Scale Combine multiple tomographic techniques Functional Materials Multi-scale Tomography FOV/Resolution We can apply this to SOFC/LIB electrodes And other materials 1mm 1cm >1m Farid Tariq et al, Acta Materialia 59(5),2011 Diagram After Uchic and Holzer, MRS Bulletin, 2007
Tomography of -ScSZ electrodes 30 Vol.% 40 Vol.% 50 Vol.% A B C Pores ScSZ Pores ScSZ Pores 5 µm 5 µm 5 µm Percolation Threshold Allows feature extraction (/ScSZ/Pores) FIBSEM, voxel sizes ~20-50nm 1350ºC sintering, 1 hr at temperature, reduced Percolated Fabrication and characterization of /ScSZ cermet anodes for IT-SOFCs, Somalu MR, Yufit V, Cumming D, Lorente E, Brandon NP, INTERNATIONAL JOURNAL OF HYDROGEN ENERGY, 2011, Vol:36, Pages:5557-5566.
Percolated nickel networks A 30 Vol.% Considered Percolation Threshold B 40 Vol.% Considered Percolated C 50 Vol.% Considered Percolated 5 µm 5 µm 5 µm Preliminary results indicate: 30 65% of is percolated 40 97% of is percolated Surface Area of particles in total volume analysed (x 10 3 m -1 ) 50 90% of nickel is percolated 646 Pores 1317 ScSZ 1345 2481 Pores 2976 ScSZ 4195 1594 Pores 1999 ScSZ 2130
Advanced Analysis: 3D Interface Changes 30-50 A 30 Vol.% B 50 Vol.% ScSZ Pores 8 µm Percolation Threshold Percolated Auriga Zeiss, 5kV, SEI, 1nA 100-200 Images Feature extraction (/ScSZ/Pores) FIBSEM, voxel sizes ~20-30nm 1350ºC sintering, 2 hr at temperature 30% has some particles forming percolated networks and other particles separate content >30% is very well connected Page 2 M.Samalu et al, Intl Journal of Hydrogen Energy 36(9),2011
Advanced Analysis of 3D Microstructure Changes A B C D 5 µm E Page 4 10 µm Example: Particles of ckel Necks between adjacent particles : Percolation, sintering and strain 3D imaging and quantification of interfaces in SOFC anodes, F. Tariq, M.Kishimoto, V.Yufit, G.Cui, M.Somalu and N.Brandon (Journal of European Ceramic Society, In Press & Available May 2014)
3D Interfaces: Structure-property-behaviour Experimental, Analytical and Modeling Results 6 - ScSZ-ScSZ 2 -ScSZ 2 4 2 1 1 0 0 0 N/A Expt Sim Ratio Expt Sim Ratio Expt Sim Ratio Neck Experimentally Measured and Modelled - necks (nm 2 /nm 3 ) ScSZ-ScSZ necks (nm 2 /nm 3 ) -ScSZ necks (nm 2 /nm 3 ) 30ScSZ 50ScSZ Ratio Conductivity Change 2.7x10-4 3.55x10-4 1.32 Resistance:3.5 Expt. 4 Sim. 3.7 Young's Modulus 4.86x10-4 3.22x10-4 1.5 Most (though not Expt. - all) load is passed through ceramic matrix 1.4±0.1 Sim. - 1.1 TPB Density For electrical conductivity any contact (e.g. more necks) would cause a larger expt. conductivity increase 15.7x10-4 19.5x10-4 1.2 Expt. - 1.1 Sim. N/A Page 15 3D imaging and quantification of interfaces in SOFC anodes, F. Tariq, M.Kishimoto, V.Yufit, G.Cui, M.Somalu and N.Brandon (Journal of European Ceramic Society, In Press & Available May 2014)
SOFC Tomography and Modelling Unanswered Questions Definition of -YSZ Interface? Self-Contact? Fatigue/Cracking Behaviour? Mechanisms at work Schematic from P.J.Withers, Adv. Eng. Materials, 2011
LSCF Electrode Imaging and Modelling 700 C Porosity 2 µm LSCF Phases Advanced 3D Imaging and Analysis of SOFC Electrodes F.Tariq, M.Kishimoto, S.J Cooper, P.Shearing, N.P.Brandon, ECS Trans, 2013 Microstructural Analysis of an LSCF Cathode using in-situ tomography and simulation S.J Cooper, M.Kishimoto, F.Tariq, R.Bradley, A.Marquis, N.P.Brandon, J.Kilner, P.Shearing, ECS Trans, 2013
Flow Modelling in Porous structures (Pa) Low pressure Higher pressure Fluid Inlet 5 µm - Pressure gradient calculated across microstructure - This can be used to calculate permeability - A measure of how much fluid could pass through this type of structure
Application of 3D imaging and analysis to the design of improved current collectors for SOFCs N Brandon, A Atkinson & Z Chen with Ceres Power The core of the Ceres proposition is its unique metal-supported cell Thin steel substrate with even thinner layers of active SOFC materials coated on top AIR FUEL Cathode Layer Stainless Steel Substrate ELECTRICITY Low temperature electrolyte (ceria) enables operation at <600 o C Ceria Electrolyte Layer Anode Layer Key advantages: Low cost cells Compact, lightweight design Mechanically tough Simple & reliable stack sealing Enables low cost balance of plant Ceres Power 2013 Title: 8 th International Smart Hydrogen and Fuel Cell Conference Rev: 1.0 10
Methodology Experiment Simulation Indentation experiment on bulk/films/cells 3D models with different material constitutives 3D models by FIB/SEM tomography Elastic properties Indentation FEM Compression FEM Response curves Response curves Elastic properties Fracture criteria prediction with varied current collector designs Electrode structure optimisation Electrolyte failure estimation As FEM input parameters Compare and validate the models
Axisymmetric modelling of mechanical indentation into electrodes Indentation process in axisymmetric modelling (a) before indentation, (b) loading to a maximum depth, and (c) complete unloading generated residual depth.
Load (mn) Load (mn) Load (mn) Load (mn) Nano-indentation curves for porous LSCF cathodes 500 300 400 900 C_Experime nt 250 1000 C_Experiment 1000 C_Simulation 300 200 150 200 100 100 50 0 0 800 1600 2400 3200 4000 0 0 400 800 1200 1600 2000 300 Indentation depth (nm) 80 Indentation depth (nm) 250 200 1100 C_Experim ent 70 60 50 1200 C_Experiment 1200 C_Simulation 150 40 100 50 30 20 10 0 0 200 400 600 800 1000 0 0 40 80 120 160 Indentation depth (nm) Indentation depth (nm) Comparison of load vs. depth curves for models with varying porosities resulted from different sintering temperatures. Porous LSCF sintered at different temps, 50 to 30 vol% porous, pellet, spherical indenter, 25 mm radius, RT data
Results: elastic modulus and hardness Comparison of elastic modulus and hardness results determined by experiment and simulation Sintering temperature ( C) 900 1000 1100 1200 Method h max (nm) P max (mn) S (mn/nm) a (nm) E (GPa) H (GPa) Experiment 437.9 1.05 13079.4 34.1 0.83 4008.4 Simulation 409.5 1.13 13146.3 36.1 0.75 241.4 0.89 9221.2 47.2 0.90 1973.4 Simulation 246.4 0.93 9256.5 47.2 0.91 252.2 1.02 6136.2 75.9 2.19 950.1 Simulation 258.2 1.04 6137.0 71.6 2.18 67.5 0.86 2294.7 189.3 4.03 164.1 Simulation 68.2 0.78 2241.1 173.9 4.17 Experiment Experiment Experiment
Electrode fabrication: porous scaffold Tape casting or screen printing Porous CGO Pore former YSZ YSZ Slurry Co-sintering T > 1300 C CGO Mixture of commercial powder and nano-powder (supplied by Prof Jawwad Darr, UCL)
State of the art electrodes: Impregnation of porous scaffolds Porous scaffold Metal nitrate solution Infiltration 550ºC, 1 h + heating & cooling n times Decomposition To oxide University of St Andrews University of Pennsylvania
FIB-SEM: 1 x infiltration Before reduction After reduction CGO O Enhanced triple phase boundary density in infiltrated electrodes for SOFCs, M Kishimoto, M Lomberg, E Ruiz-Trejo and N P Brandon, J Power Sources, 2014, Vol:266, Pages:291-295..
3D reconstruction x 1 -GDC GDC -GDC 4.2 μm (with GDC) TPB TPB (with GDC) Enhanced triple phase boundary density in infiltrated electrodes for SOFCs, M Kishimoto, M Lomberg, E Ruiz-Trejo and N P Brandon, J Power Sources, 2014, Vol:266, Pages:291-295..
3D reconstruction (10)-GDC GDC -GDC (with GDC) 7.5 μm TPB TPB (with GDC) Enhanced triple phase boundary density in infiltrated electrodes for SOFCs, M Kishimoto, M Lomberg, E Ruiz-Trejo and N P Brandon, J Power Sources, 2014, Vol:266, Pages:291-295..
Quantification GDC scaffold (1)-GDC (10)-GDC Conventional -YSZ Volume fraction [%] Particle/pore size [μm] TPB density [μm/μm 3 ] 0.00 1.29 19.8 25.3 GDC 57.1 56.9 60.2 25.1 Pore 42.9 41.8 20.1 49.6 N/A 0.102 0.354 1.38 GDC 0.844 0.748 0.706 0.730 Pore 0.667 0.594 0.300 1.74 N/A 11.0 18.4 2.49 Enhanced triple phase boundary density in infiltrated electrodes for SOFCs, M Kishimoto, M Lomberg, E Ruiz-Trejo and N P Brandon, J Power Sources, 2014, Vol:266, Pages:291-295..
Electrolyte Supported Cell Fabrication and Testing Counter Electrode (CE) (Air) 16mm (Fuel) 11mm Working Electrode (WE) Reference Electrode (RE) 1mm 2M (NO 3 ) 2 10-20μm 270μm 10-20μm Electrolyte 20mm Screen Printed GDC, sintered at 1350 C Commercial electrolyte, YSZ Screen Printed commercial LSCF-GDC 20-80% H 2 550-750 C (NO 3 ) 2 decomposition at 500 C M Lomberg, E Ruiz-Trejo, G Offer and N P Brandon, Characterization of -Infiltrated 26 GDC Electrodes for Solid Oxide Cell Applications, J Electrochem. Soc., 2014, accepted for publication
-Z'' ( cm 2 ) Impedance Spectroscopy Results 10 times -infiltrated GDC electrode, P(H 2 )=0.5atm, 100k-0.1Hz, OCV 0.15 0.10 L1 R_hfi R_h CPE1 R_l CPE2 580 o C 690 o C 750 o C Fitting 0.05 0.00 0.6kHz 2.5kHz 3.4kHz Element Freedom Value Error Error % L1 Free(+) 1.9281E-07 N/A N/A R_hfi Free(+) 1.271 N/A N/A R_h Free(+) 0.24069 N/A N/A CPE1-T Free(+) 1.818 N/A N/A CPE1-P Free(+) 0.54251 N/A N/A R_l Free(+) 0.092636 N/A N/A CPE2-T Free(+) 0.017949 N/A N/A CPE2-P Free(+) 0.59862 N/A N/A 0.00 0.05 0.10 0.15 0.20 Data File: Circuit Model File: Z' ( cm 2 ) C:\Users\ml2610\Dropbox\PhD\Sync folders from IC desk\3 On going\experimental Da M Lomberg, E Ruiz-Trejo, G Offer and ta\experimental N P Brandon, 10x-CGO-YSZ-LSCF-CGO\2 Characterization of -Infiltrated 27 GDC Electrodes for Solid 4-01-2013\All data files\fra data\high t Oxide Cell Applications, J Electrochem. Soc., 2014, accepted for publication emperature_2.mdl
Summary Progress continues to be made in the application and interpretation of 3D imaging to understand SOFC electrodes structures, and how these relate to performance. In the next 12 months we will be able to leverage new EPSRC capital investments in imaging and characterisation tools and additive manufacturing. Our ultimate ambition is to move towards a design led approach to SOFC fabrication, and to develop in-silico accelerated ageing methodologies, in order to optimise both performance and lifetime of operating devices.
Acknowledgements 3D imaging and analysis Dr. Farid Tariq, Dr. Masashi Kishimoto, Dr Khalil Rhazoui, Prof Claire Adjiman, Dr Qiong Cai (Surrey), Guansen Cui, Sam Cooper, Dr. Paul Shearing (UCL), Prof. Peter Lee and Dr. Dave Eastwood (Manchester). Scaffold electrodes Dr Enrique Ruiz-Trejo, Dr Paul Boldrin, Marina Lomberg, Zadariana Jamil, Prof Jawwad Darr (UCL). The EPSRC for funding. Current collector Project collaborators Ceres Power.