Doctoral defence: Riinu Härmas “The structure and H2 diffusion in porous carbide-derived carbon particles”

On 15 March at 12:15 Riinu Härmas will defend her doctoral thesis “The structure and H2 diffusion in porous carbide-derived carbon particles” for obtaining the degree of Doctor of Philosophy (in Chemistry).

Supervisors:
Research Fellow Rasmus Palm, University of Tartu
Professor Enn Lust, University of Tartu

Opponent:
Professor Dr.-Ing. Bastian J.M. Etzold, Technische Universität Darmstadt (Germany)

Summary

Activated carbon can be found in the pharmacy and is used to cure food poisoning. It consists mostly of carbon and contains numerous minute holes and tunnels, which are called pores. There pores are formed in between the curved graphene (i.e. one-layered graphite) layers, which contain many defects. Carbon materials with similar properties to activated carbons have many different applications. One of these lies in the field of energy (or hydrogen) storage devices. Namely, since porous carbons are very stable, yet cheap materials, which conduct electricity, these materials are widely used as electrode materials in energy storage/conversion devices such as batteries, polymer electrolyte fuel cells and supercapacitors.

In the chair of Physical Chemistry of the University of Tartu, numerous different porous carbon materials have been synthesized and used as electrode materials. One of the most widely studied types of porous carbons has been carbide-derived carbons (CDCs). Carbides are chemical compounds, that typically consist of two elements, one of which is carbon. In order to synthesize a CDC, the reaction between a precursor carbide and chlorine gas at high temperature (i.e. the synthesis temperature) is typically conducted. As a result of this reaction, only pure carbon material particles are left in the reaction vessel, since other products of the reaction are washed away with excess gas. In a way the synthesized carbon can be seen as the „skeleton“ of the precursor carbide. Actually, the Estonian company Skeleton, that produces supercapacitors, partly uses CDCs in its products.

In this PhD thesis, the differences in the microstructure of CDCs, with respect to the synthesis temperature and/or the precursor carbide, was studied. It was seen that higher synthesis temperatures resulted in the formation of wider platelets of graphene, which contained less defects. In the case of some precursor carbides (Mo2C, VC), also the average height of the stack of graphene platelets increased, but for most studied CDC materials the height of the stack remained independent of the synthesis temperature.

The porous structure of molybdenum carbide derived carbon is highly dependent on the synthesis temperature and this was studied in detail with small-angle scattering methods. It was seen that as the synthesis temperature of the CDC increased, the pores in the CDC became smoother and the average shape of the pores became more slit-like.

In addition, the diffusion of hydrogen in the pores of three different CDC materials was studied with quasi-elastic neutron scattering. It was established, that a small amount of hydrogen is very strongly confined (i.e. practically immobile) in the subnanometer pores of silicon carbide derived carbon up to relatively high temperature of 120 K (hydrogen liquefies at 20 K). However, the mobility of a small amount of H2 in the pores of titanium carbide derived carbon showed similar characteristics to liquid hydrogen up to temperature of 70 K. The third CDC, in which the mobility of H2 was studied was derived from molybdenum carbide. The pores in molybdenum carbide derived carbon were not very effective in confining hydrogen, since the diffusion of hydrogen was seen to be quite quick already in the case of low temperatures and low H2 amounts. It was seen that a large amount of subnanometer pores is paramount for the successful confinement of H2 in a porous carbon material, since the carbon derived from silicon carbide contained the most of subnanometer pores. In addition, the shape of the pore also impacts the success of the confinement of H2. Namely, the amount of subnanometer pores was similar for both silicon and titanium carbide derived carbon materials, but H2 was more strongly confined in silicon carbide derived carbons, in which the average shape of the pore is spherical.

 

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