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 (manganates, cobaltates, titanates),

·       Fe- and Ni based half Heusler materials,

·       inhomogeneous high temperature phases

·       2D materials: Chalcogenides

·       Nanostructured and hybride materials. 

 

 

Perovskites

Synthesis and Characterisation of Perovskite-type Oxynitrides and Hybride Materials

Tailor-made perovskite-type materials can enable better future technologies or improve existing devices. Cationic and anionic substitutions in perovskites are pursued to improve the properties of a given perovskite-type phase. Particularly 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.

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.

 

Perovskite_HD_1

Current research topics include:

  • Perovskite-type oxynitrides for solar water splitting photoelectrochemical cells
  • High temperature perovskite-type thermoelectrics
  • Perovskite-type materials for solar CO2 conversion
  • Perovskite-type membranes for gas separation
  • Luminescent materials
  • Battery materials
  • Memristor Materials
  • Porous Materials as Substrates for Catalytic Converters
  • Noble Metal Free Materials for Catalytical Exhaust Gas Treatment

 

 

Electrode Materials for Li-ion batteries

 

Cooperations and Sponsorship:

 

Inorganic Materials: AC Uni Stuttgart
Exhaust Gas Catalysis: Empa, Switzerland.
Halfheusler Phases, AK Felser MPI Dresden
PSI: Pulsed Laser Deposition (PLD) of thin perovskite type epitaxial films (T. Lippert) and Neutron Diffraction at Swiss Spallation Neutron Source (SINQ)
ETHZ: Rutherford Backscattering methods (Max Döbeli)
Oxide Thermoelectrics: Laboratory CRISMAT Caen, France; AIST; Japa
Deutsches Elektronen-Synchrotron HASYLAB DESY

 

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