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 In Focus

Investigations of realistic materials

Published 2001-01-11

During last two years, one postdoctoral and one Ph.D. student from South Korea visited Uppsala University and one Ph.D. student from Uppsala has visited South Korea last year. This project is related with different class of energy materials from computational point of view and one of the focused project is hydrogen storage materials which is discussed below.

Our energy-hungry world has become increasingly depending on new methods to store and convert energy for new, environmentally friendly modes of transportation. Mobility, the transport of people and goods, is a socioeconomic reality that will surely increase in the coming years. Nearly 75 per cent of the oil consumption goes to meet the needs of the transportation industry. In addition to the environmental concerns regarding CO2 emission and global warming, the limited supply of these natural resources and the growing worldwide demand for energy make it imperative that we seek alternate energy sources that are safe, secure, abundant, and pollution free.

In this regard there is heightened interest in the study of hydrogen as it is the third most abundant element on earth and produces water when burns. However, there are substantial hurdles to overcome before hydrogen can be considered to be a viable alternative to meet the growing energy need world wide. First, one has to realize that hydrogen is not an energy source but an energy carrier. Since it occurs in nature in association with other atoms such as water and organic molecules, economical means of hydrogen production is necessary. In addition, one must find safe and cost effective means of storing, distributing and using hydrogen. It is generally regarded that safe, efficient, and economical ways of storing hydrogen as well as hydrogen fuel cells are critical factors for a new hydrogen economy.

It is easy to see that for a material to store hydrogen at around 10 wt per cent, it has to consist of light elements such as Li, B, N, C, Na and Mg. For example, the hydrogen storage capacity of methane (CH4), namely, 25 wt per cent is the highest among materials. Unfortunately, hydrogen in methane and in other compounds of light elements is held by strong bonds. Consequently the temperatures for releasing of hydrogen from these materials are high and the speed of hydrogen release are not favourable. The central challenge then is to find materials that can store hydrogen like methane but whose speed and thermodynamics mimic that of metals. This requires a fundamental understanding of the interaction of hydrogen with the light weight materials.

There are two main approaches to store hydrogen that are being pursued by scientific community, namely adsorption of H2 molecules on high-surface area materials and chemical hydrides. A class of light metal hydrides that shows some promise in this regard, particularly when certain catalysts are used, has the formula unit MXH4 (M=Li, Na; X=B, Al). In terms of hydrogen density these are some of most promising materials. Although these materials still have higher than desirable releasing temperature, addition of Ti based catalyst has been found to significantly lower the releasing temperature.

Figure. Li-decorated Metal Organic Framework-5. Li - green, Zn - yellow, O - red, C - blue and H - grey.

Metal-Organic Framework (MOFS - see Figure) is another class of potential materials for hydrogen storage applications. These systems are promising gas storage materials since they display very low atomic density forming pores with high surface area. The hydrogen molecules, inserted in these frameworks, are adsorbed on the pores walls through wan de Walls-like interactions. Such interactions are quite weak requiring low temperatures in order to keep the hydrogen gas inside the systems. This is a drawback toward the mobile applications of such hydrogen storage system. One noteworthy approach to overcome this problem, which is being pursued in our group, consists in decorating the pores walls with Li atoms (as illustrated by the green atoms in the Figure). The latter are adsorbed in the organic rings forming a strong force field around themselves, which interact with the hydrogen molecules raising their adsorption energies and consequently their releasing temperatures. This new insight will be of great help in the future designing of hydrogen storage materials. It opens a new way to design hydrogen fuel based transportation and likely to offset most of the benefits of saving millions barrels of petroleum per day.

Rajeev Ahuja
Department of Physics
Uppsala University

Senast uppdaterad: 07-04-12 09:48

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