On 26 August at 10:15, Miriam Koppel will defend her doctoral thesis “The diffusion of H2 adsorbed in carbide-derived carbons: a quasi-elastic neutron scattering study”.
Supervisors:
Assoc. Prof. Rasmus Palm, PhD, University of Tartu
Riinu Härmas, PhD, Destiny Energy Estonia OÜ
Prof. Enn Lust, PhD, University of Tartu
Opponent:
Alexander O’Malley, PhD, University of Bath (UK)
Summary:
Hydrogen mobility in carbon materials is of great interest for many applications, such as energy conversion using electrolysers and fuel cells as well as energy storage as hydrogen in carbon materials through adsorption of hydrogen. These applications have different requirements for the porosity and orderliness of carbon materials. Pores are small cavities and tunnels that form between curved graphene sheets, forming the carbon material. Graphene sheets can be relatively well-ordered like books on a shelf, but at the same time, there can be regions where the graphene layers are almost randomly oriented with respect to each other, like books in a pile. For hydrogen storage, pores with a width of 7 Å (angstrom) (1 Å = 10⁻¹⁰ m) are needed. For catalyst support materials in electrolysers and fuel cells, pores with widths of 20–500 Å are needed as these enable hydrogen to move to and from active sites. To develop better materials for these applications, it is important to investigate and describe the mobility of hydrogen in materials with various structures.
From the gas adsorption, wide-angle X-ray scattering, and quasi-elastic neutron scattering results, it can be concluded that pores with widths below 7 Å can confine hydrogen so strongly that it is effectively immobile. Also, the roughness of the pore wall has an effect – pores with rougher walls can restrict hydrogen motions more strongly than pores with smoother surfaces. In addition, the orderliness in carbon materials plays a role, where the order in carbon materials can be described by the number of graphene layers in a stack. This is comparable to the number of books in a stack of books. In case of 2–3 layered stacks, the hydrogen molecules cannot fit between the graphene layers but go into larger cavities. The confinement of hydrogen is weaker in such materials. When the graphene layers are not in ordered stacks, but are oriented randomly like books in a pile, then hydrogen can fit in the cavities and is strongly confined. The strong confinement of hydrogen is important for hydrogen storage. In pores larger than 7 Å, two hydrogen components can be distinguished – a very restricted and a more mobile hydrogen component. The very restricted hydrogen component could be disadvantageous in catalyst support materials, as it can result in slower transport of hydrogen to and from active sites.
