Hydrogen energy is a kind of high efficiency, clean and renewable secondary energy, which provides an ideal solution to solve the problems of climate, environmental degradation and resource depletion caused by the current massive use of fossil fuels. However, the large-scale commercial application of hydrogen energy faces a series of technical challenges in hydrogen production, storage, transportation and application. The development of efficient and safe hydrogen storage and transportation technology faces the most prominent challenges due to hydrogen's inflammable, explosive, easy diffusion and low volume energy density at room temperature and pressure, which is the bottleneck link that restricts hydrogen energy utilization. The research on hydrogen storage materials began in the late 1960s, when Brookhaven National Laboratory in the United States and Philips Company in the Netherlands reported that hydrogen storage alloys could absorb large amounts of hydrogen. After years of development, the system has been continuously expanded, including hydrogen storage alloys, coordination metal hydrides, low dimensional nano structural materials, new porous adsorption materials, boranes and borohydrides. Among them, boranes and borohydrides have the advantages of light weight, high capacity and adjustable structure, and they have become the main research object in the field of hydrogen storage in recent years.
- Boranes: The representative material of boranes is ammonia borane. Ammonia borane is a unique molecular complex with melting point between 110℃ to 114℃, which is soluble in ammonia, water, tetrahydrofuran and other polar solvents. The electron-enriched NH3 in ammonia borane combines with electron-poor BH3 to form the molecule NH3BH3. Ammonia borane has a para-cross conformation and forms a strong dihydrogen bond network due to the interaction of dipole bonds, which makes it a very stable colorless solid crystal at room temperature and pressure. Boron atoms and nitrogen atoms in ammonia borane are bonded by coordination bonds. The hydrogen atom connected with nitrogen atom is positive, while the hydrogen atom connected with boron atom is electronegative. The electrostatic interaction between them makes ammonia borane possess certain chemical properties and thermal stability. The synthesis methods of ammonia borane include borane method and sodium borohydride method. Moreover, the hydrogen release performance of ammonia borane can be improved by changing the dehydrogenation environment, using catalysts and adding accelerators.
Figure 1. Schematic diagram of the crystal structure of NH3BH3.
- Borohydrides: Borohydrides mainly refers to light metal coordination borohydride. Its general structure formula is Mn+[BH4]n, which is a kind of representative new high capacity hydrogen storage material. The weight hydrogen storage density of most alkali or alkaline earth borohydrides exceeds 10wt%. In Mn+[BH4]n, H atoms mainly bond with B through covalent bond, forming [BH4]- and the formed [BH4]- connects with Mn+ by ionic bond. Due to the unique chemical structures, borohydrides have a wide range of applications in the field of hydrogen storage.
Figure 2. An example of borohydrides used in hydrogen storage.
- Bo P, Chen J. Ammonia borane as an efficient and lightweight hydrogen storage medium[J]. Energy & Environmental Science, 2008, 1.
- Wu R, Zhang X, Liu Y, et al. A unique double-layered carbon nanobowl‐confined lithium borohydride for highly reversible hydrogen storage [J]. Small, 2020:2001963.