NANOMATERIALS FOR MICRO - PLANAR MP SOFC

Transcription

1 NANOMATERIALS FOR MICRO - PLANAR MP SOFC A. Sobolev 1, A. Lipilin 2, E. Chernov 3. 1 Zimin advanced materials laboratory, department of chemical engineering, biotechnology and materials, Ariel University, Ariel 40700, Israel; sobolev@ariel.ac.il. 2 IEP UD RAS (Institute of Electrophysics of the Ural Division of the Russian Academy of Sciences), RF, , Yekaterinburg, Sverdlovsk Region, Amundsen Street 106; Lipilin@iep.uran.ru 3 ZAO ECON (Closed Joint Stock Company), RF, Obninsk, Kaluga Region, Lesnaya Street 9 ABSTRACT In this paper, we have obtained poorly aggregated nanopowders by the method of laser evaporation. The resulting aggregates were investigated for microhardness and fracture toughness. The dependence of the conductivity of grain boundaries of 9.5YSZ ceramics on the size of crystallites, the activity of nanocrystallites with an increase / decrease in the temperature of heat treatment is studied. The dynamics of aging of solid YSZ electrolyte with different crystallite sizes at 800 C was studied. 1. INTRODUCTION The basic component of solid oxide fuel cells (SOFC) is ceramics, usually ceramics based on yttrium dioxide stabilized zirconium 8,5YSZ. The electric properties of this solid electrolyte have been thoroughly studied, the electrolyte is produced on an industrial basis, but its properties are still studied and admired. The earlier experiments of contact between grains of electrolyte oxide ceramic samples and micron-sized crystallites [1] were performed depending on various factors: electrolyte production technology, grain size, concentration, nature of impurities, etc. This data confirms basic regularities for the unit-type model of this ceramics, namely: electrolyte volume resistance does not depend on the grain size, and contact resistance is reduced as the grain size gets larger. 2. Results and discussion As we continued these works in the direction of nanometer-sized crystallites, we were able to form nanostructured ceramics from unique weakly aggregated nanopowders of electrolyte obtained by laser evaporation (specific surface was 62 m 2 /g, average particle size -16 nm). The mechanical properties of high-density YSZ 305

2 ceramics with a structure scale of less than 700 nm were investigated by method of indentation. Typical values of microhardness and crack resistance were 15 GPa and 3.7 MPa m -1, respectively, which is approximately 2 times higher than those described in the literature for microcrystalline ceramics. It was also found that a decrease in grain size in the submicron range leads to an even greater increase in the crack resistance of the material [2]. When going to nanoscale, the basic characteristics of solid electrolyte also improve [3] (Figure 1). First, with the decrease in the size of crystallites down to 50 nm, the total ionic conductivity of the sintered ceramics increases, approaching the conductivity of a single crystal, and at crystallite sizes less than 50 nm, the ionic conductivity of the boundaries begins to predominate over the conductivity of the volume, and the nanostructured polycrystalline solid electrolyte begins to exceed the conductivity of a single crystal. A similar effect of nanostructure was observed for other promising solid electrolytes, for example, on the basis of gallate (La 0.88 Sr 0.12 Ga 0.82 Mg 0.18 O 2.85 ). It is interesting that the aging of YSZ nanostructured solid electrolyte also has its own characteristics in comparison with microcrystalline samples. As the size of crystallites decreases, the depth of aging decreases, i.e. a more stable structure of crystallites is achieved in the nanometer range [4] Figure 2. Studies of SOFC electrodes showed a general dependence - their activity decreases with increasing heat treatment temperature, i.e. to increase the activity of the electrode, it is necessary to lower the temperature of heat treatment, and to decrease the electrical resistance along the SOFC electrode, the temperature should be increased. This contradiction can be solved; a compromise has been found which suggests using additives from nanopowder components. The formation of the traditional material of the Ni-cermet SOFC anode using NiO nanopowder (with particle size of most particles nm), obtained by the electric wire explosion method, in the electrode suspension, significantly changes both the electrode manufacturing technology and its activity. During the manufacturing process, synthesizing baking with subsequent milling is eliminated and the sintering temperature is reduced to 1200 C due to an increase in the adhesion of the electrode material to the solid electrolyte. At the same time, it became possible to reduce the content of nano NiO to % by weight, while reducing the specific resistivity to (4 13) 10-4 Ohm cm, which is unattainable within the traditional technology. However, the anode activity at 750 C and overvoltage up to 100 mv provides current density of 0.7 A/cm 2 [4]. Besides, a significant increase in the activity of the electrodes can be achieved by introducing nanoparticles of mixed-conductivity oxides into the electrode material onto the three-phase boundary line. For oxidative gas media, as a rule, Pr 2 О 3 -Х is used, for reducing media - mainly СеО 2-306

3 Figure 1. The dependence of the conductivity of 9.5YSZ ceramic grain boundaries on the crystallite size at 431 C (1) and 450 C (2) Figure 2. Aging of solid YSZ electrolytes with different crystallite sizes at 800 C 307

4 Most international researchers use the obtained characteristics of SOFC to confirm great importance of the microstructure of the core (the reaction zone), with respect to specific currents and power values (A/cm 2, W/cm 2 ). Many have engaged in mathematical and 3D modeling of these zones from the anodic and cathodic sides. High SOFC properties are anticipated and confirmed experimentally, even with traditional components, which have been known for a long time: YSZ electrolyte and gas diffusion electrodes LSM, Ni - cermet. Such studies herald the use of dynamically developing 3D printing technology in the field of industrial production of both SOFC and batteries based on them. WATT Fuel Cell company, while developing a micro-tubular structure, was perhaps the first one to apply this technology in manufacturing SOFC. Accordingly, new designs of SOFC are already emerging, which are no longer manufactured by conventional ceramic technologies of the twentieth century, which can only be manufactured by 3D technologies, apparently using "selective laser sintering". At the same time, it is logical to use such technologies in manufacturing batteries of both micro-tubular and micro-planar structures [5]. Almost intuitively, the developers at Solid Power and Rolls - Royce came to optimize their planar designs. The latter gave them a descriptive name: Integral Cells of Rolls-Royce (IC RR). Both structures can rightfully be referred to as MICRO - PLANAR SOFC. Figure 3. Micro-planar SOFC structures and batteries based on them 308

5 Figure 4. Dependence of power density for micro-tubular a) and micro-planar b) SOFC Mobile power engineering requires a further increase in the specific power of SOFC, therefore new "multilayer ceramic structures" are emerging the design of the core zone of the cell, the reaction zone, transferring it from the "inner" threephase boundary line (electrode / electrolyte / gas) of gas diffusion electrodes with a dense solid electrolyte to the outer surface of a dense mixed conductor, thus excluding limiting currents, as a consequence of gas diffusion difficulties. In this way, the specific power of power plants can be increased by an order of magnitude or more. In Russia, researchers at the IEP UD RAS (Institute of Electrophysics of the Ural Division of the Russian Academy of Sciences) are developing micro-tubular SOFC, and, joining their efforts with ZAO ECON (CLOSED JOINT STOCK COMPANY) production company, have commenced experiments with a microplanar design of solid oxide devices elements, testing them both in current generation mode (fuel cells) and in the mode of oxygen generation from air (solid oxide oxygen pumps - SOOP) [5,6]. Figure 5 shows ceramic blanks for the microplanar design of electrochemical elements. 309

6 Figure 5. YSZ solid electrolyte, thin-film plates and structural frames for SOFC and SOOP. 3. Conclusions 1. Nanostructured ceramics were obtained by the method of laser evaporation 2. High values of microhardness and wear resistance, in comparison with other samples obtained by similar methods. 3. With the decrease in the size of crystallites down to 50 nm, the total ionic conductivity of the sintered ceramics increases, approaching the conductivity of a single crystal, and at crystallite sizes less than 50 nm, the ionic conductivity of the boundaries begins to Predominate over the conductivity of the volume, and the nanostructured polycrystalline solid electrolyte begins to exceed the conductivity of the single crystal. 4. The aging of solid electrolyte at a temperature of 800 Depending on the size of the crystallites. REFERENCES 1. Joffe A.J., Inosemzev M.V., Lipilin A.S., Perfilev M.V., Karpachov S.V.: Effect of the Grain Size on the Conductivity of Нigh - Purity Pore-Free Ceramics ZrO2 Y2O3., Phys. Stat. Sol V. 29, 2. P Kaygorodov A.S.: Investigation of physical properties of oxide ceramics, obtained from weakly aggregating nanopowders using magnetic-pulse pressing: dissertation

7 Candidate of Physical and Mathematical Sciences: / Anton Sergeyevich Kaygorodov. Yekaterinburg, pages. 3. Ivanov V.V., Shkerin S.N., Rempel A.A., Khrustov V.R., Lipilin A.S., Nikonov A.V.: Electrical Conductivity of Zirconia - Based Solid Electrolyte with Submicron Grain Size., Doklady Physical Chemistry Vol Pt. 1, P Lipilin A.S.: Developing and manufacturing high - temperature solid oxide devices using nanotechnologies: dissertation., Candidate of Technical Sciences: /Aleksandr Sergeyevich Lipilin.- Yekaterinburg, pages. 5. Lipilin A.S., Chernov M.E., Nikonov A.V., Spirin A.V.: SOFC mission: Reality and future. "Energy of the future: innovative approaches and methods of their realization" - Report at the Fourth All-Russian Conference with International Participation "FUEL CELLS AND FUEL CELL BASED POWER PLANTS, June 25-29, 2017, Russia, Suzdal, Vladimir Region. 6. Lyalin D.A., Chernov E.I.: "Creation of the oxygen mixture generators based on the high-temperature electrochemical devices" - Report at the Fourth All - Russian Conference with International Participation "FUEL CELLS AND FUEL CELL BASED POWER PLANTS, June 25-29, 2017, Russia, Suzdal, Vladimir Region. 311