Key issues in the manufacturing of solid oxide fuel cells with nanometric powders

The climate change, the decreasing of petroleum supplies and the abrupt increase of the energy demand due to the emerging countries and to an energy-hungry society, has driven the interest towards new energy and more efficient devices of energy production. Only a strong acceleration of alternative devices of energy production and an increase of renewables, can succeed in reducing pollution, improving the climate and at the same time assuring the energetic autonomy and competitiveness. In this scenario, electrochemical cells show several economic and environmental advantages compared to the conventional industrial processes. Fuel cells are an excellent alternative to the conventional systems of energy production in terms of CO2 emissions, low noise and flexibility of fuels and generated power. Solid oxide fuel cells (SOFC) in particular, are one of the most promising energy devices for their high efficiency, modularity, low emissions and the possibility to be directly fuelled with natural gas, GPL and alcohols. Lot of efforts are however necessary to develop commercially available generators and to increase their stability lowering at the same time their costs. These hurdles can be partially overcome lowering the operating temperature but also using more economic and easily scalable manufacturing techniques. These objectives can be reach deepened the knowledge on the relationships between SOFC materials and the main industrial production processes (tape casting and screen printing) necessary to obtain cell of dimensions close to the commercial ones with easily scalable processes. In this work the main issues related to tape casting and screen printing of nanopowders for SOFC ceramic devices is presented. Nano-powders represent the forefront of materials for SOFC. Nano-structured powders exhibit in fact important size-dependant properties such as high catalytic activity, low sintering temperatures and therefore high performances. Aim of this study is to find the correlation that link the process parameters with the nano-materials properties in order to enhance the performances and the durability both of the materials and of the final device. One of the most critical issue is to produce homogeneous and stable ceramic suspensions of nanopowders. The process optimization can be obtained merging the surface and morphological properties of the nanopowders considered (shape, dimensions\u27 distribution, surface area, etc.) to its behavior in suspension (viscosity, zeta potential, etc.) either organic of water-based, in order to obtain a well dispersed and homogenous system. With this kind of control, it is possible to produce large area reliable devices with the necessary reproducibility and reliabilit

One step production process for large area supporting cathode

Tape casting is a cheap and easily scalable shaping technique used to produce large-area, flat ceramic substrate for SOFC applications. To obtain elements with the desired porosity it is necessary adding a pore forming agent in the tape casting slurry. In this work, the possibility to obtain porous La0.8Sr0.2MnO3-Ce0.8Gd0.2O2 (LSM-GDC) supporting cathode without the use of pore formers was evaluated. The reactive sintering approach was therefore considered to exploit the porosity induced by the precursor decomposition during a single thermal treatment of calcining-debonding-sintering. Through this approach the La0.8Sr0.2MnO3 phase was formed directly during the sintering step. This process allowed to obtain 10x10 cm2 LSM-GDC tapes with values of mechanical strength, porosity and permeability suitable for fuel cells applications without pore former addition and in a single thermal step. To the author knowledge this is the first time that a large area supporting cathode has been produced by tape casting using the reactive sintering approac

Elastic and dielectric measurements of the structural transformations in the ferroelectric perovskite (Na1/2Bi1/2)1-xBaxTiO3

NBT is a perovskite undergoing a series of structural and polar transitions starting from the high temperature paraelectric phase: tetragonal antiferroelectric, rhombohedral and finally ferroelectric. In solid solution with BaTiO3 the ferroelectric phase changes from rhombohedral to tetragonal, at the so-called morphotropic phase boundary, and the phases at higher temperature become ill-defined, also because of the large lattice disorder induced by the coexistence of differently charged cations in the same sublattice. Combined dielectric and anelastic spectroscopy measurements are presented, which clarify some issues related to the phase transitions in NBT-BT. The influence of Ba substitution on the tetragonal antiferroelectric phase is determined for the first time, and the possibility that a monoclinic phase, although with very short coherence length, exists near the morphotropic phase boundary is discussed in view of a large maximum of the elastic complianc

Microwave-assisted synthesis of cerium oxide nanoparticles

Cerium oxide has recently received a lot of attention as a consequence of its catalytic properties that make it attractive for a wide range of applications ranging from solid oxide fuel cell, to three way catalysts gas sensors, etc. Although several methods have been proposed for the synthesis of ultrafine powders, the majority of them do not allow the production of powder with high specific area and they all generally require a calcination step for the crystallisation of the amorphous phase produced. Nanocrystalline ceria particles were successfully produced by one-step microwave-assisted synthesis from a glycol solution of metal nitrates under mild conditions (140?C, 1 atm). The as-prepared powder showed a good crystallinity and nanometric particle size. This simple and economic soft chemical method leads to nanometric cerium oxide with an high specific surface area suitable for catalytic applications

Synthesis of Nanometric Oxide Powders for SOFC Applications

Fuel Cell are electrochemical devices that convert the chemical energy of a fuel (generally hydrogen) and oxygen in electrical energy producing at the same time water and heat. These systems are interesting not only for the possibility of producing "clean" energy but also for the benefit linked to the high conversion efficiency. The Solid Oxide Fuel Cells (SOFC) in particular, are considered the most promising among the new systems of energy production especially for their intrinsic fuel flexibility (hydrocarbon, hydrogen, biogas, etc.). For these reasons, basic as well as technological studies focused on the improvement of the materials and production paths are of paramount importance to obtain SOFC competitive with the traditional energy production systems. Part of ISTEC research is devoted to the development of chemical synthesis able to produce tailored nano-oxides with characteristics suitable for SOFC applications. In particular soft-chemical synthesis routs were optimized to synthesize ceria and gadolinium-doped ceria nano-powders. Nano-structured powders exhibit in fact several size-dependant properties; among those, their high reactivity allows milder sintering conditions and as a consequence, better performances and lower production costs. Cerium oxide has been extensively used in a wide range of applications ranging from three way catalysts to gas sensors. When doped with gadolinum oxide, ceria becomes an alternative electrolyte for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFC). Nano-structured ceria has recently attracted extensive attention because of its properties which were found to be size, shape and orientation-dependent. Although several methods were proposed for the synthesis of ultrafine powders, most of them generally require a subsequent calcination step. This thermal treatment is known to promote the crystallisation of the amorphous phase; however it also induces aggregation, reducing the specific surface area of the powder. The aim of this work was to produce ultrafine, pure and Gd-doped CeO2 powders using standard chemical routes coupled with non-conventional heating processes. Nano-crystalline ceria and Ce1-xGdxO2- (GDC) particles were successfully produced under mild conditions with two different methods: i) applying infrared heating to a common sol-gel process (IR-SG); ii) assisting with microwaves a polyol precipitation method (MW-PP). The correlation of the synthesis parameters with the thermodynamic and kinetic factors involved, allowed the control of fundamental properties such as size distribution, purity and morphology. Nano-structured ceria of particle size in the micron range with complex morphology and high specific surface area was prepared by adjusting the MW-PP synthesis conditions (temperature, time and templating agents). These mesoporous aggregates were found to be active in the catalytic oxidation of toluene. Moreover the GDC obtained through the optimization of the IR-SG parameters exhibited values of ionic conductivity higher than the ones showed by commercial and conventional sol-gel produced powders of similar compositio

MW-Assisted polyol mediated synthesis of gadolinium-doped ceria nanopowders

Gadolinia-doped ceria (GDC) is one of the most promising electrolyte for intermediate temperature solid oxide fuel cells (SOFCs). In particular, the production of GDC as nanopowders leads to an higher reactivity that allows better performances, milder sintering conditions and lower production costs. However, nanopowders can be produced only by carefully tailoring their production process. The choice and optimization of the synthesis process is therefore a key step for the production of powders suitable for efficient SOFC components. In this work nanocrystalline GDC (Ce0.8Gd0.2O2-delta ) particles were successfully obtained by one-step microwave-assisted synthesis from a diethylene glycol solution of metal nitrates under mild conditions (170?C, 1 atm). The as-prepared powder showed good crystallinity with specific surface area of 50 m2/g. The sintering and electrochemical properties were compared with a nanometric commercial powder. The MW-produced powder showed an improved sintering behaviour and a uniform sub-micronic microstructure. Electrochemical tests for the MW-produced GDC showed at 600?C twice the conductivity of the corresponding commercial sampl