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Research Topics

The group has been active in the synthesis and investigation of nanoscale conversion materials for H storage and for batteries.

 

New Battery Concepts

 

The Fluoride Ion Battery

In search of new concepts to build batteries with high energy densities, electrochemical cells based on metal fluorides may be promising.

We have demonstrated the first reversibly working battery cells based on fluoride shuttle. In secondary fluoride batteries, fluoride anion acts as charge transfer ion between a metal/ metal fluoride pair where it will react with metal or evolve from metal fluoride depending on the flow of current. The theoretical capacity can be several times higher than that of conventional Li ion batteries, depending on the combination of metal and metal fluoride (see also: A.M. Reddy and M. Fichtner, J. Mater. Chem. 21, 17059 (2011) ).

 

 Cover

 Sketch of the principle architecture of a Fluoride Ion Battery.

See also http://www.rsc.org/chemistryworld/News/2011/October/13101102.asp

 

 

Cathode Materials

 

Nanocomposites based on transition metal compounds may serve as high energy cathode materials in case the solid state reaction between the nonmetal and Li can be made fully reversible on the microscale.

Current research focuses on:

  • Synthesis of nanocomposites
  • Investigation of the transformation mechanism

  • Improvement of reversibility




We have developed a new solid state synthesis method for fluoride based cathode materials by pyrolysis of metal organic precursors. The new material allows a stable cycling behavior over hundreds of cycles on a high capacity level (approximately 250-280 mAh/g), at room temperature. This is probably the first example of a conversion material for cathodes with high cyclic stability.

  •  Ferrocen processFeF3

 

A novel single step Reactive Intercalation Process allows synthesis of carbon-FeF2 nanocomposites where the active material is clamped between sheetes of graphite. Graphite fluoride and iron carbonyl are used as a metal source. FeF2 nanoparticles form inside a matrix which initially acts as an F- donor and, at the same time, transforms from a non-conducting graphite derivative into an electrically well conducting graphitic matrix. The synthesis proceeds in one step and leaves no solid or liquid by-products so that the product can be used as electrode material in a battery cell without further purification or post-treatment.

Scheme of the process and cyclic properties of product:

TOC
















cycling




Anode Materials

 Fe3O4XRD

 

Nanoconfined Fe3O4 in graphitic carbon produced by a simple pyrolitic one-step synthesis of Fe(CO)5.

The composite has a high cyclic stability as anode and works close to the theoretical capacity of 926 mAh/g.

 

 

The battery activities in different media

on TV:

in English

 

On the radio (in German):

http://digibib.ubka.uni-karlsruhe.de/diva/2011-170/

 

 

 

Conversion Materials for Hydrogen Storage

 

Alanate Systems for Hydrogen Storage

 

Alanate

 

Tetrahydroaluminates or alanates are complex hydrides containing AlH4 groups. They have been studied in the context of hydrogen storage since it was discovered that the hydrogen de- and absorption can be catalyzed by doping with titanium and other transition metal and rare earth metal species.

Pure Mg alanate was synthesized in our group and the crystal structure of the compound was determined. We also synthesized Ca alanate and investigated its hydrogen storage behavior. In isothermal kinetic studies we showed for the first time that mass transport is the rate limiting step in the kinetics of the hydrogen ab- and desorption of alanates. In kinetic isotope experiments, AlHx was found to be the mobile species in the materials transformation. We also found that Ti13 clusters are world-record catalysts for alanate materials. X ray absorption experiments (EXAFS, XANES) shed light on the development of the near order around the Ti atoms and showed that the dopant forms small Ti-Al clusters during cycling.

 

 

 

 

 

Boranate Systems for Hydrogen Storage

 

Boronate

 

Tetrahydroborates or boranates are complex hydrides containing BH4 anions, which have high volumetric and gravimetric hydrogen content. A method for direct synthesis of pure light metal boranate was developed in our group. Pure and adduct free magnesium boranate was synthesized and the hydrogen desorption reaction was studied. Moreover, the solid state reaction and catalysis of LiBH4 with Al, Ce and other reactants is a matter of research. Current research focuses on:

 

  • synthesis of new boranates
  • structure of boranates
  • tuning the thermodynamics

  • enhancing the kinetics

  • mechanisms of hydrogen exchange

 

 

 

 

Amide-Imide Systems for Hydrogen Storage

 

imideamideAmide-imide systems have shown promising results for hydrogen storage. Most importantly, various systems have been shown to be reversible. We have been investigating various combinations of hydrides and contributed to the understanding of the role of dopants and ball milling. Current research focuses on:

 

  • new materials combinations
  • reaction kinetics                                             
  • reaction pathways

  • crystal structure

  •  upscale production

 

 

 

 

 

Nanoconfined Hydrides for Hydrogen Storage

 

In many hydrogen storage systems it has been shown that using nano sizednano powders is essential for high reaction kinetics. Infiltrating hydrides into nano structures can be used to control the particle diameter and study size effects of the materials. Infiltration may considerably enhance the kinetics of ab- and desorption. Thermodynamic effects have been found, too. Current research focuses on:

 

  • infiltration and encapsulation methods
  • synthesis and investigation of nanocarbon templates
  • thermodynamic and kinetic properties

  • microstructural effect

  •  upscale production

 

 

 

Safety Tests

 

safetyFor the first time, scoping studies were performed in order to assess the behaviour of nanoscale Na-alanate powder in contact with various environments. The experiments were small scale device failure tests of alanate-filled tubular tanks, monitored by high speed and IR cameras, fast p and T sensors.
Hot powder under 10 bar H2 was ejected into various environments: dry and humid air; with and without ignition sources; into hot water and into hot mineral oil.
We could identify more and less risky or even safe scenarios. The propagation of the flame front of the powder-dust cloud was found to be much slower compared to the one of pure hydrogen.

 

 

 



 


 

Computational Methods

Computational

Combining computational techniques like density theory and experimental work has in the past shown to greatly benefit the research. currently in our group DFT is used to help to understand the decomposition of Mg(BH4)2. Current research focuses on:

 

  • Thermodynamics of the decomposition of Mg(BH4)2
  • Identification of intermediate species during decomposition of Mg(BH4)2