Modified Lithium Borohydrides for Reversible Hydrogen Storage

Abstract

In an attempt to develop lithium borohydrides as reversible hydrogen storage materials with high hydrogen storage capacities, the feasibility of reducing the dehydrogenation temperature of the lithium borohydride and moderating rehydrogenation conditions was explored. The lithium borohydride was modified by ball milling with metal oxides and metal chlorides as additives. The modified lithium borohydrides released 9 wt % hydrogen starting from 473 K. The dehydrided modified lithium borohydrides absorbed 7-9 wt % hydrogen at 873 K and 7 MPa. The modification with additives reduced the dehydriding starting temperature from 673 to 473 K and moderated the rehydrogenation conditions from 923 K/15 MPa to 873 K/7 MPa. XRD and SEM analysis revealed the formation of an intermediate compound that might play a key role in changing the reaction path, resulting in the lower dehydriding temperature and reversibility. The reversible hydrogen storage capacity of the oxide-modified lithium borohydrides decreased gradually during hydriding/dehydriding cycling. One of the possible reasons for this effect might be the loss of boron during dehydrogenation, but this can be prevented by changing the dehydriding path using appropriate additives. The additives reduced the dehydriding temperature and improved the reversibility, but they also reduced the hydrogen storage capacity. The best compromise can be reached by selecting appropriate additives, optimizing the additive loading, and using new synthesis processes other than ball milling.

Improved reversible dehydrogenation of 2LiBH4+MgH2 system by introducing Ni nanoparticles

“…Ni phases are also observed after heating to 330°C, and no additional peaks corresponding to the Mg 2 Ni phase were observed, indicating that the enhancement of the first-step dehydrogenation can be attributed to the catalytic effect of nano-Ni, which is highly active in decreasing the activation energy of MgH 2 for hydrogen desorption by surface activation, as reported previously. 29 On further heating to 500°C, LiBH 4 disappeared and LiH phase was observed, indicating that the system is fully dehydrogenated at 500°C, as the boron phase in dehydrogenated samples of LiBH 4 -relevant samples is usually amorphous, 22 agreeing well with the TPD results ( Fig. 1).…”

“…As is known, the boron phase in the dehydrogenated samples of LiBH 4 -relevant is usually amorphous and therefore can not be detected by means of XRD. 22 Similar products corresponding to Mg or Li-Mg, as well as some LiH and MgB 2 were also identified in 2LiBH 4 -MgH 2 sample after dehydrogenated at 500°C. The observed MgH 2 phase in the dehydrogenated state of 2LiBH 4 -MgH 2 or 2LiBH 4 -MgH 2 -0.05Ni sample is probably from the hydrogenation of Mg or Li-Mg during the temperature of 500°C cooling down to room temperature because about 1 bar hydrogen pressure was observed in the vessel after dehydrogenated at 500°C.…”

“…10 Various attempts have been undertaken to overcome the kinetic and thermodynamic limitations, and some improvements have been achieved that enhance the dehydrogenation and rehydrogenation properties of LiBH 4 , such as reducing the particle size, doping with catalysts such as carbon, metals, oxides, and halides, and reacting with metal hydrides or hydride mixtures. [17][18][19][20][21][22][23] Among these attempts, one of the most promising has been proposed by Vajo et al, 24 that LiBH 4 can be effectively destabilized by MgH 2 , reducing the dehydrogenation temperature of LiBH 4 to near 350°C and allowing complete decomposition below 500°C. The reaction of LiBH 4 with MgH 2 is proposed as follows:…”

The role of Ni in increasing the reversibility of the hydrogen release from nanoconfined LiBH4

“…11 However later research showed that SiO 2 reacts with LiBH 4 to form stable silicates 23,24 while irreversibly releasing hydrogen, as is the case with most metal oxide additives. 25,26 A large range of materials, including metal halides, 12,14,27 Al, Pt, carbon nanotubes, fullerene and Ni, 13,[15][16][17]28 has been investigated in an effort to identify an effective catalyst for LiBH 4 de/rehydrogenation. Addition of these materials generally led to a decrease in the dehydrogenation temperatures but most of the systems still required temperatures above 400 C and hydrogen pressures above 40 bar for only achieving partial rehydrogenation of the desorbed material.…”

LiSc(BH4)4: A Novel Salt of Li+ and Discrete Sc(BH4)4− Complex Anions

“…In view of their high theoretical hydrogen storage capacities, metal borohydrides have recently been the subject of intensive investigation. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] The dehydrogenation of lithium and Group II borohydrides is plagued by severe kinetic limitations and irreversibility that precludes the utilization of these compounds under practical conditions, [2][3][4][5][6][7][8][9][10] even as components of binary hydride mixtures. [12][13][14][15][16][17][18] Many transition metal borohydride complexes also have suitable gravimetric hydrogen densities.…”