Development of High Temperature Thermoelectrics
Thermoelectric generators are solid state devices which convert heat into electricity (Seebeck effect). The use of solar radiation or waste heat from thermal combustion processes (e.g. machines, engines, living beings) as energy source for a thermoelectric generator is an attractive and sustainable way to cover the increasing auxiliary electrical power demand. In order to compete as energy converters, solid state thermoelectric generators must be more efficient and/or cheaper and lighter than the today’s systems. The research on thermoelectricity at Empa aims at the development and characterisation of novel functional materials suitable for the direct and efficient thermoelectric conversion of heat into electricity. The material should exhibit high stability, a large Seebeck coefficient, good electrical conductivity, and a small thermal conductivity. The challenge for the materials design is that these transport are interdependent- changing one alters the other-, making the optimization difficult. One way to reduce thermal conductivity and therefore enhance the figure of merit, Z, without affecting thermopower and electrical conductivity is to design nano-scaled materials.
Complex transition metal- oxides are potential candidates for thermoelectric devices. Perovskite type oxides display an interesting range of properties including thermoelectricity. Among these structures, materials with p-type conductivity as well as compounds with n-type conductivity are identified. The tuning of the thermoelectric properties of the perovskites-type candidates is based on controlled anionic and cationic substitutions as well as on changes in the (morphological and structural) dimensionality from 1D to 3D.
To fulfil this task, soft-chemistry processes are developed at Empa which allow to control the cation-substitution from ppm level to 50% in e.g. cobalt oxide systems as La1-xCaxCo1-yMyO3 (M= Ti, Ni, ...) in order to modify the Co valency and therefore the transport properties. Moreover, these processes enhance the ability for particle size control.
(see e.g. Robert, R., Romer, S., A. Reller and A. Weidenkaff, Adv.Eng. Mat., 7 (2005) 303-308.)
Poster
Thermoelectric Conversion of Low Temperature Geothermal Heat
The use of geothermal or waste heat as energy source is an attractive and environmentally clean (CO2-free) way to generate electrical power. For an efficient conversion of low temperature geothermal heat the specific power output (W/cm2) of the device has to be enhanced by the increased surface area of Thermoelectric Oxide Microdevices (TOMs).
(see e.g. Bocher, L., Robert, R., Aguirre, M. H., Schlapbach, L. and Weidenkaff, A., Proc. of the 4nd European Conference on Thermoelectrics, 2006)
Synthesis and Characterisation of Perovskite-type Oxynitrides
In comparison to cationic substitutions, anionic substitutions in perovskites are rarely examined to improve the properties of a given perovskite-type phase. However the exchange of the oxide ions for halide or nitride ions can have a substantial influence on the structural and physical characteristics of perovskites.
Poster
Noble Metal Free Materials for Catalytical Exhaust Gas Treatment
Future legislation of automotive exhaust gas emissions can only be met when the catalysts are mounted closer to the engine in order to shorten the cold start phase. As a consequence, higher temperatures of up to 1200K in changing redox atmospheres can be reached in the catalytic system during normal operation. Common noble-metal catalysts, as they are used in the present systems, are sensitive to reagglomeration and recristallisation of the dispersed metals at high temperatures.
With the development of novel complex metal oxide materials, mainly of the perovskite type, suitable for the catalytical treatment of exhaust gases, a higher resistance of the catalyst against elevated temperatures (>1000K), as well as sufficient catalytic activity at lower tempera­tures, will be possible. The flexibility of the perovskites allows tuning of the properties of the oxides to approach the desired targets concerning reactivity and selectivity in exhaust gas treatment.
The reversible Alkaline Fuel Cell (rAFC)
Low temperature fuel cells will become essentially for the utilisation of hydrogen as clean fuel in a few hundred W to a few kW devices. Furthermore the production of hydrogen from water in Electrolysers with a renewable electricity source (e.g. solar energy) has to be permitted when peak energy is provided. The stored chemical potential (hydrogen) will be available for the consumer when required. Such a reversible hydrolyser-fuel cell system can be operated as rechargeable battery as well.
At present, neither of the two low temperature fuel cell types - the polymer electrolyte membrane fuel cell (PEFC) and the alkaline fuel cell (AFC) - allows commercialisation while alkaline electrolysers are a well established technology. In comparison both fuel cells have specific advantages and disadvantages.
The advantages of the AFC are in summary a higher cell voltage, lower costs, near-atmospheric operation of the system and less sensitivity to impurities like iron or SO2. Electrochemical reaction kinetics of several conversion processes are generally faster and more efficient in alkaline electrolytes compared to acidic systems. Alkaline Fuel Cells (AFC) and Alkaline Hydrolysers (AH) can reach extraordinarily high system efficiency (60-65%) for the reversible chemical conversion processes of water to hydrogen and oxygen and vice versa (hydrogen + oxygen <-> water).
The disadvantage of the AFC - the need for nearly CO2-free process gases – is by far less severe as it was claimed in the past. Challenges in materials research are to develop an electrode material as an alternative to noble metals that is low in cost and highly efficient, which is bi-functional for the water generating process (during fuel cell operation) and the water splitting process (during electrolyser operation).
Today’s electrode materials in alkaline fuel cells are made of expensive, precious-metal electro catalysts (e.g. Raney-Ag) and large-surface carbon as a conductive support. This composite material is used in a highly alkaline solution which causes corrosion problems concerning mainly the carbon support. In order to optimise the interface between the carbon and the electro catalyst, as well as the stability of the electrode, a new composite material based on multiwalled carbon
nanotubes (MWCNT) as replacement for carbon black will be developed at Empa. The tubular structure of these graphitic materials offers interesting electronic properties combined with low weight, enhanced chemical and thermal stability, good electrical conductance and a large surface area.
Conductive and alkaline stable perovskite-type oxides (ABO3) with rare earth and/or alkaline earth ion in A and transition metals in B position are promising candidates to be used as cathode catalysts in AFCs. Since January 2001 new electrode materials for rechargeable zinc air batteries have been developed in co-operation with the PSI. These air-electrodes work in alkaline media. The requirements on the material are similar to those in alkaline fuel cell: The oxygen evolution and the oxygen reduction processes have to be catalysed on the air electrode. For the Anodes LaNi5-MWCNT materials will be used to provide the possibility of catalysing the reversible electrode reaction in both direction and the required hydrogen storage capability as well.
(see e.g. Weidenkaff, A., Adv. Eng. Mat., 6 (2004) 709.)
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Services
We offer measurement services on:
- Morphology and composition of solid materials by electron microscopic methods.
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Surface area determination of solids by the BET method.
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Thermal properties of Materials including thermo gravimetric-mass spectrometric (TGA-MS) studies on gas solid reactions, phase transitions, specific heat, and thermal diffusivity in a wide temperature range.
- Measurements of thermoelectric properties of materials up to 1000°C, including Seebeck coefficient, electrical resistivity, thermal conductivity , and Figure-of-merit.
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