Research Activities at the Chair of Chemical Materials Synthesis

Our main goals and objectives

Synthesis of Advanced Materials

 

Materials Development for Energy Conversion Technologies
  • High temperature thermoelectrics (Heusler and complex oxide phases)
  • Photocatalysts for solar water splitting reaction (Oxynitride Perovskites)
  • Electro catalysts for Alkaline Fuel Cell and Electrolyser and noble metal free car exhaust gas catalysts
  • Reactivity and thermal properties of perovskite-type oxides and oxynitrides

 

Bioinspired Materials
  • Biomineralisation
  • Biotemplates

 

Materials Development for Energy Conversion Technologies

Development of High performance 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, and 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 aims at the development and characterisation of novel 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 properties are interdependent- changing one alters the other-, making the optimization difficult.

The performance of a TE material is quantified by the dimensionless figure of merit ZT = a2sT/k, where a is the Seebeck coefficient (thermopower), σ the electrical conductivity, and k the thermal conductivity (in the simplest case k = ke + kph, where ke is the carrier thermal conductivity and kph the phonon thermal conductivity.). For convenience, a2s is called the power factor (PF). To date, degenerate semiconductors constitute the corner stone of state-of-the-art TE materials; and these materials have maximum ZT~1-2 in their respective temperature range of operation. Per the definition of ZT and in the context of G. Slack’s “phonon glass electron crystal” concept, the strategy of TE research is two-fold: (i) reducing the phonon thermal conductivity (kph) toward a “phonon glass”, while (ii) enhancing the power factor (PF) toward an “electron crystal”.

Our research focus on developing high performance thermoelectric materials by manipulating microstructure and composition, which control the phonon and electron transport properties. The research topics include complex transition metal oxides (SrTiO3, Ca3Co4O9, (Na, Ca)CoO2 and so on), Heusler phase materials, and organic-inorganic hybrid materials.  

 

Synthesis and Characterisation of Perovskite-type Oxynitrides and Hybride Materials

 

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.

The introduction of nitrogen into the anionic sublattice under formation of oxynitrides has a direct influence on the band structure of the respective compounds. Owing a lower electronegativity nitrogen’s 2p orbitals are higher in energy than oxygen 2p orbitals resulting in a hybridisation of both bands forming a new valance band higher in energy. While cationic substitution leads to a decrease of the conduction band in energy. In both cases the band gap of the compound is reduced. Therefore, an adjustment of the band gap is possible by a variation of the N/O ratio, which might be accompanied by an additional cationic substitution to keep charge balance and to avoid the formation of vacancies. A tailoring of the band gap is strongly recommended to shift the absorption edge of the compound from UV to visible light allowing a significant increase in efficiency for the splitting of water into hydrogen and oxygen. A further option to avoid the formation of anionic vacancies is the simultaneous substitution of oxygen by nitrogen and halogenides.

Moreover, Perovskite hybrid materials have been intensively developed for solar applications recently. Especially, organometal halide perovskites were reported as a promising material with outstanding electric properties in the 1990s. The structure of organometal halide perovskite could be represented as ABX3, where B is metal cation (e.g. Pb2+) and A is an organic cation to neutralize the total charge. The organic cation must fit into a strong cuboctahedral frame. However, the structure would be distorted if the organic cation is too large. Here we study the organic or inorganic atoms substitution. On the other hand, the environmental safety of the material is another problem of this new material due to the toxic properties of Lead. Although it is claimed that the amount of lead for perovskite-based PV will only account for 0.1 %, it is still important to explore Pb-free materials. Therefore, we will focus on the way of tuning the polarizability by replacing Pb with other metal cations (e.g. Sn, Cu, Co, Ag, Bi, or Sb). To synthesize high quality perovskite materials, we adopt two-step sequential deposition by using the metal halide MI2 and organic ammonium iodide solution, which could obtain better morphology and grain size of the materials.

Noble Metal Free Materials for Catalytical Exhaust Gas Treatment

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 (T>1000K), as well as sufficient catalytic activity at lower temperatures, will be possible. The flexibility of the perovskite structure 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 essential 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. Electrochemical reaction kinetics of several conversion processes are generally faster and more efficient in alkaline electrolytes compared to acidic systems. Alkaline Fuel Cells (AFC) 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).

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. 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 catalysts in AFC to catalyse the oxygen evolution and the oxygen reduction processes on the air electrode.

 Electrode Materials for Li-ion batteries

 

Co-operations and Sponsorship:

We offer measurement services on:

   

  Determination of phase content using X-ray powder diffraction

   

Investigation of thermal properties (reactivity,thermal expansion and stability) of materials including gas-solid reactions, phase transformations, heat capacity, using thermogravimetry-mass spectrometry over 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.

 

  Thermal diffusivity and conductivity of materials using the Laser-flash method up to 1000°C

  Determination of electric conductivity and thermoelectric properties of materials (resistivity, Seebeck coefficient, Figure of merit) up to 1000°C

Prices and Awards

Nina Stitz: Startup Grant of DFG SPP 1569 - Generation of Multifunctional Inorganic Materials by Molecular Bionics.

 

Bioinspired synthesis

The main research area of the department is the further development of oxide ceramics and organic-inorganic hybrid materials including synthesis mechanisms of biomineralization processes for developing new materials with broad application fields.
Oxide ceramics and inorganic materials are an important part of cutting edge technologies, like fuel cells, transparent electrodes in solar cells or flexible touch screens as well as coatings for heat protection, mechanical strains, and self-cleaning surfaces. However, extreme synthesis conditions and elaborate equipment limit the technical application of such materials. In addition the generation of highly defined inorganic structures in the nanometer range is challenging.
On the other hand, in nature highly structured inorganic materials in nanometer dimensions are generated under apparently simple conditions by process called biomineralization. Here, organic molecules (e.g. proteins, polysaccharides) which serve as template or catalyst are important key factors in the biomineralization process. The department aims the transfer of biomineralization mechanisms for the synthesis of technical ceramics in vitro and in vivo.

A broad range of methods for the characterization of new materials and the determination of organic-inorganic interactions at interfaces in hybrid structures are applied.

  • X-ray diffraction, energy dispersive X-ray spectroscopy and chemical analysis
  • Light microscopy, electron microscopy (SEM), atomic force microscopy (AFM), dynamic light scattering (DLS), photo luminescence spectroscopy, UV-Vis spectroscopy
  • Nanoindentation, zeta potential measurements, viscosity measurements
  • Quartz crystal microbalance (QCM), surface plasmon resonance (SPR), nuclear magnetic resonance (NMR)
biomin_small Biomineralization
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Phage display & Biotemplates
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Bioinspired Mineralization
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nano_small Nanomechanical Characterization
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keramik_small Precursor-Derived Ceramics
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