Effect of pore geometry on ultra-densified hydrogen in microporous carbons

Mi Tian,Matthew J Lennox,Alexander J O’Malley,Alexander J Porter,Benjamin Krüner,Svemir Rudić,Timothy J Mays,Tina Düren,Volker Presser,Lui R Terry,Stephane Rols,Yanan Fang,Zhili Dong,Sebastien Rochat,Valeska P Ting

doi:10.1016/j.carbon.2020.11.063

Abstract

Our investigations into molecular hydrogen (H2) confined in microporous carbons with different pore geometries at 77 K have provided detailed information on effects of pore shape on densification of confined H2 at pressures up to 15 MPa. We selected three materials: a disordered, phenolic resin-based activated carbon, a graphitic carbon with slit-shaped pores (titanium carbide-derived carbon), and single-walled carbon nanotubes, all with comparable pore sizes of <1 nm. We show via a combination of in situ inelastic neutron scattering studies, high-pressure H2 adsorption measurements, and molecular modelling that both slit-shaped and cylindrical pores with a diameter of ∼0.7 nm lead to significant H2 densification compared to bulk hydrogen under the same conditions, with only subtle differences in hydrogen packing (and hence density) due to geometric constraints. While pore geometry may play some part in influencing the diffusion kinetics and packing arrangement of hydrogen molecules in pores, pore size remains the critical factor determining hydrogen storage capacities. This confirmation of the effects of pore geometry and pore size on the confinement of molecules is essential in understanding and guiding the development and scale-up of porous adsorbents that are tailored for maximising H2 storage capacities, in particular for sustainable energy applications.

Highlights

Molecular hydrogen (H2) has received much attention as a potentially sustainable, zero-carbon energy vector due to its global abundance in the form of water and biomass, its relative ease of production, for example via water electrolysis or thermochemicalM
The selected samples were a TE7 activated carbon consisting of randomly-ordered graphitic layers, a titanium carbide-derived carbon (TiC-carbidederived carbons (CDCs)-800) with slit pore geometry and a sample of single-walled carbon nanotubes (SWCNTs) with cylindrical pore geometry
Through the combination of in-situ inelastic neutron scattering (INS), high-pressure gas sorption experiments and simulations, we have systematically investigated the effects of pore geometry and pore size on the density and mobility of H2 in the microporous carbon materials, contrasting and comparing the slit-like pores found in TiC-CDC-800 with the cylindrical pores found in SWCNTs and the disordered structure of TE7 carbon

Summary

Introduction

Molecular hydrogen (H2) has received much attention as a potentially sustainable, zero-carbon energy vector due to its global abundance in the form of water and biomass, its relative ease of production, for example via water electrolysis or thermochemicalM. In the case of hydrogen, confinement in porous Vycor glass with the mean pore diameter of ~6 nm has been shown to prompt liquefaction at temperatures above the triple point of bulk H2 [17], while the freezing transition temperatures of H2 and molecular deuterium (D2) confined inside a range of porous aerogels were shown to be lower [9,18] compared to the bulk gas Each of these deviations from the classical phase behaviours of the bulk properties observed for fluids in tight confinement is a result of the combination of interactions between the fluids and the walls and geometrical or steric limitations, and hydrogen densification and phase behaviours are expected to be heavily dependent on the size and shape of the confining space

Methods

Results

Conclusion