Production and characterisation of molecular hydrogen storage materials

Abstract

The subject of the present work is the characterization of potential materials for molecular hydrogen storage with a stress on the use of Small Angle Neutron Scattering (SANS).This research is dedicated mainly to a study of the porous structure of activated carbon. Employment of the contrast matching technique together with SANS led to an understanding of the process of pore filling resulting from an increase of partial pressure of contrast matching liquid (deuterated toluene). The Author has designed a special Al alloy sample cell for the variable vapour pressure SANS experiments. The accessible empty pores fraction at each p/po is calculated using the Porod Invariant. The data obtained matches the results from gravimetric measurements of D-toluene adsorption. The fractal nature of activated carbon is determined via application of the neutron scattering technique. The density of activated carbon - an important characteristic affecting the total hydrogen uptake - is found to be Q-dependent with an average saturation limit of 1.85 g/cc. The effect of the carbon activation process, i.e. the formation of micropores, is illuminated by SANS. Finally, a novel model, implying exponential decay of the pore size distribution with a lower cut-off, is proposed and the minimum pore radius is calculated for each experimental partial pressure of wetting liquid. These results are compared with those derived from the standard Guinier approximation. The proposed model yields the exact value of D-toluene surface tension when the derived pore radii are associated to the corresponding pressures via the Kelvin equation (within the range of its applicability), whereas the Guinier approximation gives an average value of the D-toluene surface tension approximately twice the tabulated value. Thus, it is concluded that the novel model presented in this thesis is an improved approximation to the porous structure of activated carbon.Additionally, a complimentary double-Gaussian pore size distribution model is suggested. It highlights the presence of ultramicro- and microporosity and shows a good agreement with the SANS data for dry activated carbon