INS Spectra taken from the experiment on TOSCA
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Researchers at the Department of Chemical Engineering, University of Bath, the Rutherford Appleton Laboratories (RAL), the USA and Germany have uncovered some unexpected behavior of hydrogen gas when it is contained in optimally-sized sub-nanometer pores. Using the TOSCA inelastic scattering instrument at ISIS, the team observed direct evidence for solid-like behaviour of hydrogen gas, indicating hydrogen densities almost 1,000 times the density of gaseous hydrogen at ambient temperatures and pressures.
This research has implications for the use of hydrogen as a sustainable, low-carbon alternative to fossil-based transport fuels, as a major technological barrier to use hydrogen as a transport fuel is its storage.
As an alternative to storage of hydrogen as a high pressure compressed gas, nanoporous materials such as activated carbons, zeolites, metal-organic framework materials and certain porous polymers can spontaneously adsorb hydrogen and compress it within the pores, allowing the hydrogen to be stored at much higher densities.
Evaluation and identification of the most promising materials on which to focus further development is challenging for researchers. The total amount of hydrogen stored in these molecular sponge materials is highly dependent on the conditions of pressure and temperature of storage and difficult to directly probe. Mathematical modelling of experimental data (hydrogen uptake as a function of pressure and temperature) enables prediction of the storage characteristics of a material under different operating conditions. However, current modelling methods generally rely on estimates of the density of hydrogen inside the pores, with the maximum, limiting density being assumed to approximate liquid hydrogen.
Inelastic neutron scattering is one of the few experimental techniques that can be used to obtain direct information on the state of the hydrogen inside a solid material. In-situ gas dosing experiments on TOSCA by the Bath–led team on a tailored nanoporous activated carbon hydrogen storage material resulted in characteristic spectral fingerprints for solid-like hydrogen.
Carbon surfaces interact only weakly with hydrogen, indicating that the over-riding cause of the enhancement of gas storage in this material, supplied by collaborators at MAST Carbon International, was likely to be the pore geometry. This signals that the primary design parameter for new porous hydrogen storage materials should not be solely based on ever-increasing surface areas but must also include optimised pore dimensions.
The discovery of solid-like adsorbed hydrogen at unexpectedly low pressures and high temperatures necessitates the development a new, partitioned model for absorptive hydrogen storage.
University of Bath researcher Dr Valeska Ting says of the work: “Greater understanding of how the nanoscale structure of the storage material can influence gas storage capacities is expected to lead to more accurate evaluation methods for existing porous hydrogen storage materials. This, in turn, should have an impact on the design and evaluation of new hydrogen storage materials for future automotive applications.”
Dr Tim Mays, Head of Department of Chemical Engineering and a co-author on the paper says ”This work is a key step in making hydrogen storage in porous materials a practical reality. As well as involving important collaborations, this has been a real team effort at Bath including contributions from Chemical Engineering student Antonio Noguera-Diaz, a recent graduate in the Department Dr Jessica Sharpe, and research officer Dr Nuno Bimbo. We are very grateful for research funding provided by the University, STFC and EPSRC in the last case via hydrogen SUPERGEN projects.”
Research date: July 2015
The full article, published in ACS Nano, is freely available here.
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