SUPERHALOGENS AND THEIR ROLE IN STORING HYDROGEN

摘要

The success of a hydrogen economy depends on our ability to find a material suitable for storing hydrogen. For application in the mobile industry this material must be able to store hydrogen with large gravimetric and volumetric density and operate at near ambient pressure and temperature. Unfortunately, such materials do not exist. Those that can store hydrogen in large quantities either bind to hydrogen strongly so that it is hard for hydrogen to desorb or bind to hydrogen weakly so that it desorbs at low temperatures [1]. To overcome these difficulties, it will be ideal to find materials that are light weight and the bond strength of hydrogen is intermediate between physisorption and chemisorption. Two mechanisms for this kind of bonding was suggested more than twenty years ago by Kubas [2] and hena and coworkers [3]. According to the Kubas mechanism, a transition metal atom can bind to hydrogen quasi-molecularly where the donation of electrons from the H2 molecule to the unfilled d-orbitals of the transition metal atom and back donation to the antibonding orbital of the H2 molecule leaves the H-H bond slightly stretched and bind energy of the order of 0.5 eV/H2 molecule. The mechanism proposed by Jena and coworkers, on the other hand, accomplishes the same task through the use of a metal cation which binds to H2 molecule through charge polarization. Much work in the past few years have concentrated in finding the suitable metal atom to dope. In this work I will discuss a class of materials that are composed of alkali metal cations compensated by the superhalogen anions for storing hydrogen. For the later, we use borane derivatives. In particular, Li2(B6H6) is able to reversibly store up to 12 wt % hydrogen.