Hydrogen Energy is in great need to realize a low-carbon society. Hydrogen can be produced using renewable energies, can be stored/transported in gaseous, liquid and solid ways, and can generate power via fuel cell or combustion without the emission of CO2 gas. Hydrogen, as a clean energy carrier, therefore, has been attracting increasing interest. The energy density of hydrogen per unit volume at ambient temperature and pressure is as low as 1/3400 of gasoline, which means that storage of hydrogen in a limited space is a big challenge. Therefore, development of hydrogen storage technologies is a key issue for realizing the sustainable hydrogen society. The energy density of hydrogen can be increased largely by using compression or liquefaction techniques, nevertheless, the volumetric density can not exceed 70 kg H2/m3 in principle due to the repulsive forces between hydrogen molecules. Distinct from these technologies, hydrides store hydrogen in an atomic way, reveals high potential use for hydrogen storage.
Hydrides are a fast expanding class of materials, approaching multi-functionality, in particular within energy storage. They can not only store hydrogen in the solid state, but also act as novel battery materials, both as electrolytes and electrode materials, or store solar heat in a more efficient manner as compared to traditional heat storage materials. Metal boranes M(BxHy)n, a typical example of hydrides, have been attracting increasing interest from the energy storage point of view, especially in the context of solid-state hydrogen storage and superionic conductivity. Lower boranes such as metal tetrahydroborate M(BH4)n, with hydrogen gravimetric density higher than 10 mass%, have been extensively investigated for high density hydrogen storage. M2(B12H12)n with a stable icosahedral cage structure, on the other hand, favors its potential application as superionic conductor. Recently, we found that the ionic conductivity of a bimetallic closo-borane LiNaB12H12 reaches 0.79 S/cm at 550 K above its order-disorder phase transition. This value is 10 times higher than those of its single counterpart of Li2B12H12 and Na2B12H12 at the same temperature.
In the presentation, we will give an overview of the state-of-the-art research progress on hydrides for energy applications, following the introduction of current Japan's basic hydrogen strategy and the related activities.