Current Activities on Solid Oxide Cells at DLR

Current Activities on Solid Oxide Cells at DLR Asif Ansar, Rémi Costa, Michael Hörlein, and Günter Schiller German Aerospace Center Institute of Engineering Thermodynamics Stuttgart, Germany

Outline Brief Introduction of DLR-ITT Metal Supported Cell Concepts for SOC - Plasma spray concept - EVOLVE concept SOEC activities - Hi2H2 project - Degradation study - Solar fuels Conclusion

DLR German Aerospace Center Aeronautics Space Transport Energy Research Institution Space Agency Project Management Agency > 7500 employees across 32 institutes and facilities

Institute of Engineering Thermodynamics Prof. Dr. André Thess Administration Jörg Piskurek Electrochemical Energy Technology Prof. A. Friedrich Computational Electrochemistry Prof. A. Latz Thermal Process Technology Dr. A. Wörner System Analysis and Techn. Assessment Dr. C. Schillings / C. Hoyer-Klick (komm.) Staff: About 190 in Stuttgart, Köln, Hamburg and Ulm Yearly budget: About 18 Mio. EUR including 50% third party funding... innovative solutions for sustainable and environmentally friendly energy storage and conversion processes...

Electrochemical Energy Technology R&D of efficient electrochemical energy storage and conversion 5 Fields of Research Solid Oxide Cells Realization of durable, powerful and cost effective SOFC-stacks Polymer Electrolyte Fuel Cells Improvement of longevity and reliability with regard to electric mobility and residential power supply Lithium Metal and Lithium Ion Batteries Development of mobile energy storages Modeling & Simulation Improvement of the efficiency factor, longevity and costs of fuel cells and batteries Electrochemical Systems Development of efficient and effective, multifunctional fuel cells systems for stationary and mobile applications

Generations of planar SOFCs 1 st gen. 2 nd gen. 3 rd gen. ESC Cathode ASC MSC Electrolyte Anode Limited power density Fuel flexibility Robustness High power density Fuel flexibility Sulfur poisoning Thermal cycling Redox Cycling High power density Fuel flexibility Sulfur poisoning Thermal cycling Redox Cycling Stationary Transportation Stationary Transportation Stationary Transportation

Metal Supported SOFC Plasma Spray for Functional Layers Compact design with thin sheet ferritic substrates and interconnects 100 cm² foot print Counter flow stamped gas manifold air channel fuel channel oxygen/air not used air Bipolar plate protective coating contact layer cathode current collector cathode active layer electrolyte anode porous metallic substrate Bipolar plate Welded substrate with the interconnect Brazing or Glass Seal as joining of repeat units fuel brazing not used fuel + H O 2 (not in scale) Cathode 20 µm Electrolyte 35 µm Anode 35 µm Substrate

Functional Principle of DC Plasma Plasma Gases Ar H 2 N 2 He Plasma Gun Particle Melting and Acceleration Powder Jet Coating Substrate Particle Injection Particle Impingement Splat Layering

Metal Supported SOFC - Performance MSC Cell 12.5 cm² cell at 800 C; H 2 /N 2 and air MSC Cell with Suspension Plasma Spray Electrodes Power density above 800 mw/cm² at 0.7 V 12.5 cm² cell at 800 C; H 2 /N 2 and air Cell voltage / V 1,1 1 0,9 0,8 0,7 0,6 A B C D 1000 750 500 250 0 0 400 800 1200 1600 Current density / ma*cm -2 Power density / mw*cm -2 610 mw/cm² @ 0.7V (2009- G6) 10-Cell Stack 100 cm² cells at 800 C; H 2 /N 2 ; air Zellspannung U [V] 1,2 1,1 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 Stromdichte@ 700mV Zelle 10: 173 mw/cm² Zelle 9: 315 Zelle 8: 324 Zelle 7: 325 mw/cm² Zelle 6: 318 Zelle 5: 324 mw/cm² Zelle 4: 329 Zelle 3: 320 mw/cm² Zelle 2: 319 Zelle 1: 320 mw/cm² MSC-10-31, 800 C, 27h 10H2+10N2/ 20 Luft (SLPM) P1: Kennlinie nach Aufheizen, KL1 OCV Zelle 10: 1,019 V Zelle 9: 1,006 V Zelle 8: 1,007 V Zelle 7: 1,001 V Zelle 6: 1,006 V Zelle 5: 1,016 V Zelle 4: 1,022 V Zelle 3: 1,005V Zelle 2: 1,013V Zelle 1: 1,014 V U @ 7,0 V Pstack = 250 W FU = 24,8 mol% 800 600 400 200 Leistungsdichte p [mw/cm²] 0,1 p 0,0 0 100 200 300 400 500 Stromdichte i [ma/cm²] 0 Vortrag > Autor > Dokumentname > Datum

Cyclability of Metal Supported Cells cell voltage U [mv] 1200 1000 800 600 400 200 0 U1_start U2_start U1_end U2_end p1_start p2_start p1_end p2_end Thermal cycles MSC-02-17, 800 C 2 H 2 +2 N 2 / 4 Air (SLPM) 458 / 1227 h 0 50 100 150 200 250 300 current density i [ma/cm²] 15 thermal cycles performed, 12 down to 350 C and 3 to ambient temperature Degradation after thermal cycles was 10.3 % U p 400 350 300 250 200 150 100 50 0 power density p [mw/cm²] cell voltage U [mv] 1200 1000 800 600 400 200 0 Redox_start cell2: 157 mw/cm² Redox_5 cell2: 177 mw/cm² Redox_20 cell2: 149 mw/cm² Redox cycle MSC-02-17, 800 C 2 H 2 +2 N 2 / 4 Air (SLPM) 1227 / 1517 h 0 50 100 150 200 250 300 current density i [ma/cm²] U p Redox start @ 1,4 V Pstack = 28,5 W FU = 14,1 mol% U2_start U2_Redox_5 U2_Redox_20 p2_start p2_redox_5 p2_redox_20 Redox 5 @ 1,4 V Pstack = 33,3 W FU = 16,6 mol% Redox 20 @ 1,4 V Pstack = 26,0 W FU = 12,9 mol% 20 forced redox cycles performed with 50 ml/min O 2 on the anode side per layer Increase of power density after 5 cycles (Improving contact Ni-YSZ?) Degradation of the stack was 9.1 % after 20 redox cycles 400 350 300 250 200 150 100 50 0 power density p [mw/cm²]

Measurement Setup for Segmented Cells 16 galvanically isolated segments Local and global i-v characteristics Local and global impedance measurements Local temperature measurements Local fuel concentrations Flexible design: substrate-, anode-, and electrolyte-supported cells Co- and counter-flow

Experimental Setup for Raman Spectroscopy Measurements

Beyond the 3 rd Gen. SOFC: Issues to be addressed for improving MSCs Cr-poisoning at the cathode side > Protective coating required Improve tolerance toward sulfur poisoning Lifetime of metal substrate if stationary applications are considered Hermetic electrolyte Which materials and architecture for the next generation SO(F)C?

Beyond the 3 rd Generation SOFC Metal substrate resistant toward oxidation Formation of an Al 2 O 3 layer as a durable protective coating Al rich alloys, on the basis of MCrAl(Y) with M being Fe, Ni, Co or a mixture

Nickel-free Hybrid Metal-Ceramic Supported SOFC Infiltration with an electronic conductor (ideally a ceramic) Dense La 0.1 Sr 0.9 TiO 3 (800 C): sintering in H 2 : σ tot 150 S/cm O. Marina et al. Solid State Ionics, 149 (2002) 21-28. S. Hashimoto et al. Journal of Alloys and Compounds, 397 (2005) 245-249. Y. Tsvetkova et al. Materials and Design, 30 (2009) 206-209. Hybrid current collector mechanically and chemically stable in both oxidant and reducing atmosphere

Beyond the 3 rd Generation SOFC Use of perovskite materials at the anode and cathode, being modified by addition of suitable catalysts High power density, sulfur resistant, fuel flexibility, thermal cycling, redox cycling Stationary applications

Beyond the 3 rd Generation SOFC Source: Alantum Europe GmbH Metal Foam: NiCrAl Composition of the anode: Ce 1-x Gd x O 2-α / La 0,1 Sr 0,9 TiO 3-α Electrolyte: 8-YSZ / 10-GDC Cathode : La 0,4 Sr 0,6 Co 0,2 Fe 0,8 O 3-α

Beyond the 3 rd Generation SOFC Perovskite based anode Thin film electrolyte (1 µm YSZ+ 2 µm CGO) with improved hermiticity (in collaboration with Ceraco GmbH) 2010 SoA MSCs DLR Manufactured in air (except PVD layer) 2014 2015

Beyond the 3 rd Generation SOFC Replacement of Nickel still remains challenging! 50 mm x 50 mm with active surface 16 cm² P @ 0.7V and 750 C 340 mw /cm² Redox cycles tested: 10 But with addition of Nickel!!! AFL: LST-CGO 50:50 modified with 5 wt% Nickel Current collector: NiCrAl + LST 50vol% - Ni 50vol% Issue with sulfur poisoning still expected

Solid Oxide Electrolysis Advantages: P el High temperature (600-900 C) Fast reaction kinetics Low overvoltage High efficiency & high current densities No noble metals as catalysts Fuel versatility: CO 2 electrolysis Co-electrolysis of H 2 O/CO 2 possible Syn-gas production External (or internal) hydrocarbon formation Air O 2 + e e O 2 H 2 O H 2 Electrolyte Anode Oxygen electrode Cathode Fuel electrode

Hi2H2: I-V Curves of a VPS Cell in SOFC and SOEC Mode as a Function of Temperature 1,4 750 1,3 500 1,2 p(i) 250 cell voltage/v 1,1 1 0,9 0,8 0,7 1-192h, 800 C 2-195h, 750 C 3-199h, 850 C U(i) 0-250 -500-750 -1000 power density/mw cm -2 0,6-1250 gas flow : 40/16//160 ml min -1 cm -2 H 2 /H 2 O//air (30% steam) 0,5-1500 -1200-1000 -800-600 -400-200 0 200 400 600 800 1000 1200 current density/ma cm -2

Hi2H2: Complete Test Run of a VPS Cell in Electrolysis Mode 1,6 temperature 800 1,4 700 1,2 600 voltage/v, ph2o/atm 1 0,8 0,6 0,4 0,2 0 varied electrolysis voltage -0.3 A cm - ² electrolysis +26 mv /1000h (2,1%/1000hr) +46 mv /1000h (3,9%/1000hr) H 2 O-ratio U/V ph2o/bar T/ C 0 288 576 864 1152 1440 1728 2016 2304 time/hr 500 400 300 200 100 0 temperature/ C Lit: G.Schiller et al., J. Appl. Electrochem., 39, 293-301, 2009

Challenges in SOEC technology Improvement of performance and particularly durability (reduced degradation) by - Development of improved materials (cathodes and anodes) for steam electrolysis - Study of degradation behaviour and elaboration of mitigation strategies Integration of high-temperature heat (solar heat, waste heat from industrial processes) Development of operation strategies with use of integrated heat (heat management) Development of co-electrolysis process (steam + CO 2 ) for production of synthetic fuels - Development of cells for co-electrolysis operation - Development of electrocatalysts for synthesis of liquid and gaseous fuels - Operation at elevated pressure conditions > 10 bar - Development of system concepts and demonstration of functionality Efficient operation in bi-functional mode Development work at DLR in the frame of internal project ( Future Fuels ) and in POF III phase of Helmholtz Association (HGF) with a transportable research platform ( e-xplore )

Motivation and Objective of Systematic Degradation Study Problem: low longevity degradation A variety of long-term degradation data about 3-5% / 1000h at 800 C and 80% absolute humidity (AH) about 35% / a Different studies hard to compare! Systematic parameter study of: Temperature T Humidity AH Current density i Test setup quadruple cell measurement Four different current densities simultaneously Identical temperature, gas supply (and also incidents)

Solid Oxide Fuel Cells: Planar Design Materials Cathode: (La,Sr)(Fe,Co)O 3 Diffusion barrier: CGO 1-5 µm Electrolyte: 8YSZ 5-10 µm Anode: Ni/YSZ Anode Substrate: Ni/YSZ Interconnect: ferritic steel

I-V Curves at 750 C as a Function of Steam Content (Flow rates: 2 l/min H 2 /H 2 O, 3 l/min air) 1,4 Cell Voltage / V 1,3 1,2 1,1 1 0,9 0,8 0,7 0,6 0,5 750 C 7%AH 750 C 40%AH 750 C 60%AH 750 C 80%AH 0,4 1,5 1 0,5 0 0,5 1 1,5 Current density / A*cm 2

I-V Curves at 800 C as a Function of Steam Content (Flow rates: 2 l/min H 2 /H 2 O, 3 l/min air) Cell Voltage / V 1,4 1,3 800 C 7%AH 1,2 800 C 40%AH 1,1 800 C 60%AH 1 800 C 80%AH 0,9 0,8 0,7 0,6 0,5 0,4 1,5 1 0,5 0 0,5 1 1,5 Current density / A*cm 2

Degradation Measurements

Degradation Measurements Impedance analysis equivalent circuit High freq. (10 5 Hz) Low freq. (0.5 Hz) L 1 : High frequency interference (~ 10 5 Hz) R 0 : Ohmic resistance (~ 10 5 Hz) R 1 : Fuel electrode process A (~ 10 4 Hz) R 2 : Fuel electrode process B (~ 10 3-10 4 Hz) R 3 : Oxygen electrode process (~ 10 2 Hz) R 4 : Fuel electrode mass transport (~ 10 1 Hz) R 0 R 1 R 3 R 2 R 4 L 1 R 0 R 1 R 2 R 3 R 4 A. Leonide, V. Sonn, A. Weber, and E. Ivers Tiffée Journal of The Electrochemical Society, 155 (1) B36 B41 (2008)

Degradation Measurements Change of each resistive process with time Bias: 0.5 A/cm 2

Degradation Measurements Time / d Relative change in resistance / A.U.

Post mortem Analysis (SEM) Solely reduced 1000 h @ OCV 1000 h @ 1.5 A/cm 2 Ohmic resistance: Weakening of YSZ CGO LSCF interface Visible cracks probably formed during sample preparation along weakened microstructure

Post mortem Analysis (SEM) Solely reduced 1000 h @ OCV Fuel electrode: Change in microstructure Likely due to Ni-coarsening decrease TPB Decreased percolation? 1000 h @ 1.5 A/cm2

Post mortem Analysis (SEM) Solely reduced 1000 h @ OCV 1000 h @ 1.5 A/cm 2 Oxygen electrode: No obvious change in microstructure Appearance of brittleness Degradation due to change in perovskite stoichiometry close to surface Sr-rich surface layer? Difficult to measure surface sensitively with high focus XPS

Post mortem Analysis (SEM) Solely reduced 1000 h @ OCV 1000 h @ 1.5 A/cm 2 Diffusion barrier layer: No reaction between Sr and Zr (or generally LSFC with YSZ) General Sr-enrichment at CGO YSZ interface Ce depletion at CGO YSZ interface

Future Fuels: TP 3 Solar Fuels DLR Internal Project: SF, TT, VT, AT Objective: Production of synthetic liquid hydrocarbons through solarthermal and solarelectrical processes Duration: 2015-2017

Solar Fuels Syn-gas Water, CO 2 Analysis Modelling Radiation Steam CO 2 SOEC-Stack Syn-Fuels High performance radiator Receiver Heat Simulation Evaluation Application Latent heat storage

Conclusion o Plasma sprayed metal supported cell technology has been developed o Stability of ferritic stainless steel against corrosion remains an issue for long-term operation. o Use of perovskite as current collector and anodic electro-catalysts are still challenging because of the reduced electronic conductivity and the stringent manufacturing parameters required for high performance o Current and future SOEC activities in cell degradation, coelectrolysis and synthetic fuel production

Acknowledgment I d like to thank my co-workers Dr. Asif Ansar, Dr. Rémi Costa and PhD student Michael Hörlein for their scientific work as well as all other members of my group for their strong effort. Financial support from Helmholtz Association in the frame of the Helmholtz Energy Alliance Stationary electrochemical solid state storage and conversion is gratefully acknowledged.