Advanced supporting anodes for Solid Oxide Fuel Cells Maarten Verbraeken M. Sc. Thesis Faculty of Science and Technology Inorganic Materials Science MESA+, EMPA Dübendorf Enschede, March 2005 Advanced supporting anodes for Solid Oxide Fuel Cells M. Sc. Thesis By Maarten Verbraeken Enschede, March 2005 Graduation Committee Prof. Dr. ing. D.H.A. Blank (chairman) Dr. B.A. Boukamp (supervisor, IMS) Dr. P. Holtappels (supervisor, EMPA) Dr. H.J.M. Bouwmeester Dr. B.L. Mojet Advanced supporting anodes for Solid Oxide Fuel Cells Summary Three different ceramic materials for nickel-cermet anodes have been used to prepare and characterise supporting anodes for Solid Oxide Fuel Cells. Symmetrical cells with nominally identical supporting anodes were prepared and electrochemically tested. The three different cermets were nickel/yttria stabilised zirconia (YSZ), nickel/gadolinium doped ceria (CGO) and nickel/titania and yttria doped zirconia (YZT). The Ni/YSZ anodes contained different ratios of fine and coarse YSZ. Their polarisation resistances varied from 2.0 – 7.4 Ωcm2; the resistance increased with an increasing amount of coarse YSZ. The anodes suffered a lot from degradation, which makes a good analysis and comparison of the measurement data hard. Therefore it is hard to ascribe the impedances to certain electrochemical processes. The lowest polarisation resistance was found for the Ni/CGO anodes, with a polarisation resistance of 1.1 Ωcm2. In the first place, the fine microstructure plays an important role for the relatively low impedance. Secondly, the stability of this anode in contrast to the other materials is thought to be due to the high purity powder that was used. From the electrochemical characterisation, it is believed that an adsorption process and oxidation/reduction of the ceria cause the main impedances. The last material, Ni/YZT, performed a bit worse than the Ni/YSZ anodes. The impedances of these Ni/YZT anodes were fitted with a Finite Length Fractal Gerischer, resulting in consistent fit data. This Gerischer describes a diffusion process coupled to a side reaction, which limits the amount of diffusing species. A proper electrochemical explanation has yet to be formulated, but the mixed ionic and electronic conductivity of YZT is almost certainly involved. In any case, the total polarisation impedances for this material amounted 5.9 – 10 Ωcm2. Like the Ni/YSZ anodes, the impedance decreased with an increasing amount of fine YZT. Advanced supporting anodes for Solid Oxide Fuel Cells Contents 1. Introduction ..................................................................................................................................... 6 1.1. Fuel cell characteristics............................................................................................................... 7 1.2. State-of-the-art SOFC................................................................................................................. 8 1.2.1. Electrolyte ............................................................................................................................ 8 1.2.2. Cathode ............................................................................................................................... 9 1.2.3. Anode................................................................................................................................... 9 1.3. Objectives ................................................................................................................................. 10 2. Theoretical background ................................................................................................................ 11 2.1. The anode ................................................................................................................................. 11 2.1.1. Hydrogen oxidation............................................................................................................ 11 2.1.2. Polarisation ........................................................................................................................ 12 2.1.3. Anode structure.................................................................................................................. 14 2.1.4. Alternative materials: mixed ionic-electronic conductors (MIEC) ...................................... 15 2.1.5. Coke formation................................................................................................................... 16 2.2. Impedance spectroscopy ..........................................................................................................17 2.2.1. Cell design ......................................................................................................................... 17 2.2.2. Equivalent circuits.............................................................................................................. 19 3. General set-up considerations...................................................................................................... 20 3.1. Electrochemical set-up.............................................................................................................. 20 3.1.1. Gas tightness of the set-up/position of the sample in the furnace..................................... 20 4. General cell preparation ............................................................................................................... 22 4.1. Experimental ............................................................................................................................. 22 4.1.1. Anode substrates............................................................................................................... 22 4.1.2. Functional anode layers..................................................................................................... 22 4.1.3. Electrolyte layers................................................................................................................ 23 4.1.4. Symmetrical cells...............................................................................................................24 4.2. Results ...................................................................................................................................... 25 4.2.1. Symmetrical cells – dilatometer tests ................................................................................ 25 4.2.2. Microstructure .................................................................................................................... 28 5. Ni/YSZ functional anodes ............................................................................................................. 30 5.1. Theoretical background............................................................................................................. 30 5.2. Experimental ............................................................................................................................. 32 5.2.1. Materials ............................................................................................................................ 32 5.2.2. Slurry preparation ..............................................................................................................33 5.2.3. Impedance spectroscopy................................................................................................... 33 5.3. Results & discussion................................................................................................................. 34 5.3.1. Microstructure .................................................................................................................... 34 5.3.2. Electrochemical characterisation....................................................................................... 35 5.4. Concluding remarks .................................................................................................................. 41 5.4.1. Effect of microstructure on electrochemical performance ................................................. 42 6. Ni/CGO anodes ............................................................................................................................ 43 6.1. Ceria.......................................................................................................................................... 43 6.2. Experimental ............................................................................................................................. 44 6.2.1. Materials ............................................................................................................................ 44 6.2.2. Slurry preparation ..............................................................................................................45 6.2.3. Impedance spectroscopy................................................................................................... 45 6.3. Results ...................................................................................................................................... 45 6.3.1. Microstructure .................................................................................................................... 45 6.3.2. Electrochemical measurements......................................................................................... 46 6.4. Discussion................................................................................................................................. 49 6.4.1. Microstructure ...................................................................................................................