The main obstacle to building a ‘hydrogen economy’ – this much touted vision of a society where the main energy carrier is hydrogen – is the lack of efficient hydrogen storage. The research conducted in the hydrogen storage scientific community is aimed towards mobile applications. Hydrogen is a gas at ambient conditions and takes up a lot of space. For stationary storage facilities, for which available space is not an issue, hydrogen gas can be kept in large tanks at moderate pressures using already known technology. However, in order to utilize hydrogen for mobile applications i.e. to produce and be able to sell hydrogen fuelled cars on a large scale, it must be stored in a compact, safe, cheap and efficient way.
In 2009, the U.S Department of Energy (DOE) proposed on-board hydrogen storage system performance targets that have become widely accepted. So far, researchers haven’t been able to successfully demonstrate a material that is capable of simultaneously meeting all of the requirements and criteria set out by the DOE.
A European research team has now reported on a new concept for hydrogen storage using nanoconfined reversible chemical reactions. They demonstrate that nanoconfined hydride has a significant hydrogen storage potential. Research at the Nano Energy-Materials research group at Interdisciplinary Nanoscience Center (iNANO) at Aarhus University in Denmark, led by Flemming Besenbacher and Torben R. Jensen, focuses on the utilization of nanoporous materials as scaffolds for preparation and confinement of nanosized metal hydrides. This bottom-up approach limits the particle size of the hydride to the average pore size of the scaffold material, which allows for the direct production of smaller particles than obtainable mechanically. Furthermore, particle growth and agglomeration may be hindered by the compartmentalization of the nanoparticles within the scaffold material. Nanoconfinement may also mediate improved re-hydrogenation properties of complex metal hydrides.
“Nanoconfinement of metal hydrides is receiving increasing interest in the field of hydrogen storage and this principle has already been applied to a number of promising hydrogen storage materials,” the researchers explain. In their work published in the online edition of ACS Nano, the team introduces an alternative bottom-up approach where nanoparticles of hydrides are synthesized or melt infiltrated in a nanoporous inert scaffold material, which has several advantages:
- increased surface area of the reactants
- nanoscale diffusion distances
- increased number of grain boundaries, which facilitate release and uptake of hydrogen and enhance reaction kinetics
Lithium borohydride (LiBH4) and magnesium hydride (MgH2) nanoparticles are embedded in a nanoporous carbon aerogel scaffold with a maximum pore size of 21 nm and react during release of hydrogen and form magnesium diboride. The hydrogen desorption kinetics is significantly improved compared to bulk conditions, and the nanoconfined system has a high degree of reversibility and stability and possibly also improved thermodynamic properties. The purpose of this work was to further develop the concept of nanoconfinement by investigating a system of higher complexity. Lithium borohydride and magnesium hydride have been studied intensively in the past due to their high theoretical hydrogen densities.
“However” explains Nielsen, “the use of lithium borohydride as a solid-state hydrogen storage material is hampered by its unfavorable high thermal stability; that is, release of hydrogen takes place at temperatures above 400°C and, importantly, uptake of hydrogen only occurs under extreme conditions. Similarly, application of the abundant and cheap metal magnesium is also impeded by unfavorable thermodynamic properties.” Jensen adds that, “fortunately, both the kinetic and thermodynamic properties of potential hydrogen storage materials can be significantly improved by combining exothermic and endothermic chemical reactions. A more favorable total enthalpy change may be obtained by the introduction of a new dehydrogenated state which may facilitate hydrogenation. This concept is referred to as reactive hydride composites (RHC), and it helps to preserve a high gravimetric hydrogen storage capacity.”
By studying the effect of nanoconfinement on the hydrogen storage properties of a lithium-borohydride/magnesium-hydride system, the team (which included scientists from the Institute of Material Research in Germany and Lund University in Sweden) found that it possesses a high degree of reversible stability and improved hydrogen desorption kinetics as compared to the bulk. Furthermore, the concept of nanoconfined chemical reactions may develop to become an important tool within the emerging area of nanotechnology for the improvement of the properties and reaction yield of a wide range of chemical reactions. This new scheme of nanoconfined chemistry may have a wide range of interesting applications in the future, for example, within the merging area of chemical storage of renewable energy.
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