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HỘI THẢO QUỐC TẾ ATiGB LẦN THỨ CHÍN - The 9 ATiGB 2024 51

2. OVERVIEW OF METAL HYDRIDES 2.1. Equilibrium State of Metal Hydrides
Metal hydrides (MH) are solid-state hydrogen The equilibrium state of reaction (1) is governed by
storage compounds formed through absorption and the interplay of hydrogen pressure, hydrogen
desorption processes according to the following concentration within the solid phase, and temperature.
equation: This relationship is unique to each hydride-forming
material and is a reflection of the thermodynamic
x properties governing its interaction with hydrogen gas.
M(s)+ H (g)  MH (s)+Q (1)
2 2 x Despite differences in composition, metal-hydrogen
systems share common thermodynamic characteristics
In this context, M represents a metal or alloy, while [19].
H refers to a hydrogen ion, with (s) and (g) denoting
the solid and gas phases, respectively. The formation At low hydrogen concentrations, hydrogen atoms
of metal hydrides through hydrogen absorption is an are dissolved and distributed homogeneously within the
exothermic reaction, releasing heat (Q). Conversely, metal lattice, adhering to Henry's law and Sievert's law.
the decomposition of the hydride to release hydrogen Nevertheless, upon exceeding the saturation limit,
requires the input of an equivalent amount of heat. hydride precipitation commences. Under constant
pressure conditions, an increment in hydrogen
The characteristics of metal hydrides in the gas concentration induces a sudden structural
phase provide distinct advantages for compact and transformation within the material, resulting in the
efficient hydrogen storage, mainly due to the high formation of a hydrogen solid solution. As the hydrogen
volumetric density of atomic hydrogen (approximately concentration approaches its maximal value, the
100 gH/L) within the crystal structure of the metal equilibrium pressure exhibits an asymptotic increase.
hydride matrix. A simple and efficient absorption and In the high-pressure region, the equilibrium state of
desorption system is based on pressure and reaction (1) is described by the Van't Hoff equation:
temperature fluctuations. The reversibility coupled
with the large heat effect (about 20 kJ/mol H 2) of this  P   S 0  H 0
reaction opens up potential for numerous applications ln  P 0  = + (2)
in energy conversion using MH [10-16].  P  R RT
0
In this context, P denotes the standard pressure,
0
0
while ΔS and ΔH represent the standard entropy and
enthalpy changes of hydride formation, respectively. R
refers to the ideal gas constant
2.2. Kinetics of Hydrogen Absorption and Release
Although the theoretical efficiency of a hydrogen
compression cycle using metal hydrides (MH) can be
calculated based on thermodynamic principles, the
actual rate of hydrogen absorption and desorption
processes, as well as the kinetic efficiency of the
hydrogen compression system, depend on the kinetics
of these reactions. The rates of hydrogen absorption and
desorption reactions can vary significantly depending
Figure 1. Provides a general overview of on the composition and structure of the metal hydride
materials capable of storing hydrogen according to (MH) alloy.
their volumetric and gravimetric density [17]. Many MH alloys have the ability to absorb and
Based on Figure 1, it can be observed that metal release hydrogen very quickly, however, the rates of
hydrides have the ability to release hydrogen at high these processes are often limited by heat transfer [20],
levels and have a medium hydrogen storage density. [21]. Especially when operating at low temperatures or
Some typical examples include: MgH 2 with 7.6%, in environments containing impurities, kinetic factors
TiMn2 with 1.8%, LaNi5 with 1.4%, NaAlH4 with become paramount and can determine system
7.5%, TiFe with 1.89%, and LiBH 4 with 18.5%. performance. Therefore, to accurately model the
hydrogen absorption and desorption process in MH,
Additionally, the composition, structure, and kinetic expressions that reliably describe the rates of
morphology of the solid phases (M and MHx) involved these reactions must be incorporated into the heat and
in this process are crucial factors. These properties, mass transfer model [22], [23].
which are fundamentally tied to the material science of
metal hydrides, will be thoroughly analyzed in this One of the most critical kinetic factors is the rate of
section [18]. these reactions near equilibrium. Førde, Yartys, and

ISBN: 978-604-80-9779-0