<figure><img src="image_25.png" /><figcaption>**Figure 22.** Thermodynamics and kinetics of complex hydrides at the nanoscale. a) Diagram of equilibrium plateau pressure shift observed for NaAlH₄ upon nanosizing. Adapted with permission.[331] Copyright 2010, Elsevier; b) reaction map indicating the thermodynamically favored decomposition paths for nanosized NaAlH₄ as a function of the particle size and the temperature. Adapted with permission.[332a] Copyright 2010, ACS; evolution of c) the hydrogen desorption temperature and d) the activation energy (Eₐ) of nanoconfined NaAlH₄ through pore impregnation[309q] and melt infiltration.[572]</figcaption></figure>

solvent evaporation with Ti doping also decomposed to LiH, Al and H₂ directly via a single step (LiAlH₄ → LiH + Al + 3/2 H₂). [315]

However, a single step decomposition is not an inevitable outcome of nanosizing. For instance, high surface area graphite (HSAG) supported LiAlH₄ nanoparticles with the size ranging from 3–15 nm still followed the three decomposition steps of bulk LiAlH₄.[256] Once again, the thermodynamic evolution upon particle size reduction is affected by the chemical/physical environment of the nanoparticles. But if such a concept could be controlled, nanosizing could potentially lead to a one step decomposition of the complex hydrides at low decomposition temperatures. Another benefit of the nanosizing approach is the suppression of side products. For example, the elimination of diborane (B₂H₆) release upon borohydride or ammonia borane nanoconfinement has been reported.[309i,333]

Evolutions of ΔH and ΔS for complex hydrides upon nanosizing are barely reported in the literature due to the difficulty of measuring the full equilibrium plateau pressure at varying temperatures and the poor stability of nanoconfined complex hydrides upon hydrogen cycling. Therefore, the reported thermodynamic alterations are often embodied in the better reversibility and changed pathways. Hence, the reversible hydrogen storage reported for various complex hydrides, e.g., LiAlH₄, LiBH₄, NaBH₄, Mg (BH₄)₂, Ca (BH₄)₂,[256,313] upon nanosizing assumes an alternation of their thermodynamics.

It should be noted again that the main approach to nanosize complex hydrides is through the use of an inorganic porous host, and thus the reversible hydrogen capacity of these nanoconfined

hydride is often too low for application (<1 mass%). This means that better approaches to stabilize and confine the nanoparticles need to be developed. One possible strategy is via a core-shell method.[186] For example, a remarkable reversible capacity of 5 mass% was achieved for NaBH₄–Ni core shell structure.[334]

### 8.1.3. The Enthalpy–Entropy Compensation Effect

Enthalpy–entropy compensation effect is a phenomenon observed for homologous reactions with ΔH and ΔS evolution following the same trend along the proposed linear correlation[320a,335]

$$ \Delta H = \alpha + \eta \Delta S \qquad (82) $$

where α and η are constants. As discussed above, similar compensation effects have been observed for metal hydrides (Figure 21d) and understanding this phenomenon is critical to achieve any significant improvement in the hydrogen sorption thermodynamics of nanosized hydrides.

Different hypotheses have been advanced to explain the enthalpy/entropy compensation effect observed in chemical systems involving solvents or variations in surfactant chain length, for example.[320b-d,321a,336] But one approach to better describe the observations made to date is by considering the interrelation between ΔH, ΔS, and ΔG.[320a] Hence, according to Pan et al. by comparing (16) and (82), it can be proposed that α and η should have the same dimension as that of ΔG and T,
respectively.[320a] Under these conditions, ΔG and T should have distinct values and be constant.

Following this principle, by varying a parameter experimentally (e.g., the particle size of the absorbing hydrogen material), the observation of small differences in ΔG at a constant temperature upon particle size variation, should then lead to a positive slope (η) corresponding to a compensation effect. The intercept (α) would correspond to the average of all the small variations in ΔG values (Figure 20a). However, if the variations in ΔG are large, i.e., very dissimilar values of ΔG are obtained at a constant temperature for different particle sizes; a nonlinear or curved correlation will be observed (Figure 20a). At larger variations of ΔG in comparison to ΔH this would result in an anticomensation effect (Figure 20a).

Observation of such enthalpy/entropy anticomensation effects has been reported for molecular pairs in solutions and the complexation of metal ions,[337] so these are not uncommon. Applying this concept to nanoscale hydrides, this means that variations of ΔG should be related in the first instance to variations in ΔGsurf and the correlation between ΔH and ΔS upon nanosizing. Achieving anticomensation effects for nanosized hydrides is highly desirable because it would lead to a significant improvement in the thermodynamics upon hydrogen cycling, but to date, methods to achieve this are unknown.

## 8.2. Nanosizing for Better Hydrogen Sorption Kinetics

While the scenario for nanosizing induced thermodynamics evolution is complicated, the kinetics has been universally reported to be improved for both metal hydrides[311a,338] and complex hydrides[309j,v,316,339] upon nanosizing. Often, the lower desorption temperatures reported for nanosized hydrides are claimed to be the results of kinetic enhancements. However, it should also be noted that the experimental hydrogen desorption temperatures determined by thermal gravimetric methods coupled with mass spectrometry are the result of combined effects of thermodynamic and kinetic evolutions and thus not conclusive.

Nanosizing has the potential to lead to a reduction of kinetic barriers by facilitating the dissociation/recombination of hydrogen at the surface and the diffusion of hydrogen (Figures 8 and 10c). Through nanosizing, the surface-to-volume ratio of the material is remarkably increased, which means that more atoms are exposed to the surface, and this results in an increased number of adsorption and dissociation sites, in the form of steps, kinks, corners and edge atoms.[27e,126b] As for the penetration and diffusion process, the easiness for hydrogen atoms or ionic species to diffuse in the material depends upon the nature of the atomic organization within the nanoparticle core but assuming very small particles a significant reduction of diffusion lengths should improve hydrogen kinetics.[195a,338b] For relatively small particles, the number of nucleation points for the growth of the hydride phase should be limited and thus this should also contribute to faster kinetics since the likelihood of forming a blocking MH/M layer through which the reactive phases have to diffuse is reduced (Figure 8b). Hence, if all these barriers are removed, the kinetics of the reaction should be solely controlled by the nucleation and growth of the hydride or dehydrogenated

phase. This last step mainly depends upon the material structure and composition as well as the temperature and pressure conditions used to drive the hydrogen absorption/desorption process.[44] In complex hydrides, this assumes that during hydrogen cycling the decomposition products are easily recombined and thus "active" toward hydrogen and ionic mobility as well as tightly confined and restricted from diffusing away.

Similar to enthalpy evolution for thermodynamics, the kinetics improvement is not strictly determined by the particle size and often the experimentally obtained E_a values may significantly vary depending on the synthesis methods and experimental conditions (Figure 23a). However, when the chemical/physical environment of the nanoparticles is controlled and restricted, E_a appears to depend upon particle size reduction, i.e., smaller particles display a lower E_a that favors faster reaction rates and lower temperatures (Figure 22c,d). More remarkably, this seems to hold for both metal hydrides and complex hydrides (Figures 22d and 23a).

However, it should be noted that this correlation may be affected by the pre-exponential factor (A) (from the Arrhenius equation $k(T) = Ae^{-E_a/RT}$) that may vary for different materials. The correlation between the pre-exponential factor (A) and E_a for hydrogen storage reactions is barely evaluated in literature. With Mg as an example, Figure 23b,c summarizes selected experimental results from the literature and existing discrepancies. Accordingly the validity of a pre-exponential factor-activation energy compensation effect is still to be determined.

## 8.3. Looking Beyond the Particle Size Reduction to Enable Practical Hydrogen Storage

There is now much evidence suggesting that nanosizing is an approach that could potentially lead to the design of practical hydrogen storage materials with high hydrogen capacity and cycling capability near the ambient. These observed improvements in hydrogen properties are not the direct consequence of particle size reduction, but the result of interrelated parameters including: i) the nanosized hydride surface/host interaction, ii) the structural and atomic order/disorder of the nanosized hydride core and surface, and iii) the nanostructure/hydrogen properties relationships including atomic coordination states, hydrogen bonding sites, and diffusion mechanisms. Many of these factors are currently not very well understood, including the interaction of hydrogen with nanosized materials and this constitutes an important barrier for progress. An obvious limitation is also in the effective achievable hydrogen storage capacity through the nanosizing approach,[309c,311a,313a,324,340] and alternative approaches must emerge to enable the synthesis of nanosized hydrides via optimum stabilization methods, where weight, volume, heat convection, diffusions paths and distances are appropriately managed. Reactant residuals, surfactants or stabilizers, as well as the scaffold or support materials for nanoconfinement are a dead mass to the hydride material and this leads to a decrease of the effective hydrogen storage capacity. The additional sensitivity of hydrides to oxidation and exacerbated reactivity at the nanoscale is also a challenge, not only in the synthesis of the nanosized hydrides but also upon material handling and
<figure><img src="image_26.png" /><figcaption>**Figure 23.** Activation energy (E_a) and pre-exponential factor (A) of Mg at the nanoscale. a) Size dependence E_a for Mg synthesized by chemical reduction[53a,311a] and electrochemical deposition;[183] b) correlation between A and E_a for Mg nanoparticles. Values were extracted from the literature.[53a,183]</figcaption></figure>

hydrogen cycling. For example, the reversible hydrogen storage capacity demonstrated for some complex hydrides through nanosizing tends to decrease with cycling because of the side reactions of the hydrides with the host material and/or a loss of nanoconfinement. Ideally, the method of confinement should retain the nanosized features of the hydride upon hydrogen uptake and release, and protect it from any “aggressive” environment. Synthetic paths to achieve this have been reviewed elsewhere.[126b,186] Along this, a narrowing of the equilibrium plateau pressure has also been observed owing to the lower number of hydrogen binding sites in metal hydride nanoparticles,[309g,325] and this would mean that nanosized hydrides could potentially display low hydrogen storage capacities because of the small “bulk” volume of nanoparticles. Such behavior has not currently been observed with complex hydrides, and may be overcome by designing nanostructures that can accept more hydrogen atoms per site than their bulk counterparts.

## 9. Designing Novel Hydrogen Storage Materials with Modeling Guidance

The outcomes of computer simulations may also contribute greatly to the design of hydrogen-storage materials. Computer simulations can be used to rationalize the causes behind unresolved observations as the systems and processes of interest can be accessed at the atomic scale under controlled conditions.

Indeed, key properties for the design of hydrogen-storage materials, which may be difficult to measure in practice, e.g., molecular absorption/adsorption energies and kinetic energy barriers, can often be estimated accurately with theory. In addition, from a resource point of view, computer simulations are inexpensive. For instance, by using open-source software with a standard office computer, one can already simulate complex gas-adsorption phenomena and assess the thermodynamic stability of energetically competitive crystal polymorphs – provided that fine interaction models are in use. Consequently, modeling of materials can be done systematically and ahead of the experiments in order to provide useful analytical guidance.

The reliability of computer simulations in mimicking real systems, however, strongly relies on the simplifications made in the adopted methods and structural models. In materials modeling there is a trade-off between structural model reliability and computational method accuracy and this is due to practical limitations. Hence, if the approach is to employ accurate first-principles methods (see next sections) then the simulations are likely to be performed at zero temperature and by considering perfectly ordered atomic/molecular systems, which may result in unrealistic conditions as compared to the experimental conditions. Likewise, to study gas-absorption phenomena at high temperatures on systems containing large concentrations of crystalline defects, the calculations will be preferentially performed with classical interatomic potentials, which may suffer from critical transferability issues and in general low predictive
power. Fortunately, there are ways of achieving a suitable balance between computational accuracy and model reliability and it is the responsibility of the modeler to balance these requirements in order to provide meaningful guidance. In the next section, some relevant quantities that can be estimated efficiently with computer simulation methods in order to boost and further rationalize the design of hydrogen-storage materials are discussed. Then, the energy-force methods that can be used to calculate such quantities are briefly reviewed by emphasizing some of their advantages and limitations.

## 9.1. Modeling Key Physical and Chemical Properties of Hydrides

### 9.1.1. Hydrogen Bonding Strength

The strength of hydrogen molecule binding to a material serves as a measure of its potential for hydrogen storage applications. The hydrogen binding energy is defined as

$$E_{\text{bind}}(\text{H}_2) = E_{\text{el}}(\text{M}) + n/2E_{\text{el}}(\text{H}_2) - E_{\text{el}}(\text{MH}_n) \quad (83)$$

where $E_{el}(X)$ represents the energy of system X calculated by considering all atoms perfectly still at their equilibrium positions, M the material under study, and $MH_n$ the same material but with n adsorbed hydrogen atoms.

The more (less) positive $E_{bind}$ is, the stronger (weaker) hydrogen is bound to M. In order to meet the target of low temperatures for hydrogen release and uptake, the hydrogen binding energy of a material should lie within the interval $0.7 > E_{bind} > 0.2$ eV/H$_2$.[341] Such a binding energy interval, from which thermal effects are explicitly excluded, holds both for hydrogen absorption and hydrogen adsorption.

Adsorption of hydrogen molecules (that is, H$_2$ binding to the surface of a material) also can be simulated directly at finite temperatures with Monte Carlo (MC) methods.[342] In MC simulations random Markovian moves are used to sample a particular statistical mechanical ensemble in order to provide averaged quantities (e.g., equilibrium gas uptake and enthalpies). Adsorption isotherms, for instance, can be estimated in grand canonical MC simulations as the average number of uptake molecules, where the temperature and volume of the system are maintained fixed while its chemical potential is allowed to equilibrate with a gas phase that is not explicitly simulated. MC simulations, however, involve the use of large simulation cells (=1000 atoms) and numerous sampling moves (>1 000 000), hence normally they are performed with semiempirical interaction models, e.g., force fields (see next section).

### 9.1.2. Enthalpy of Hydride Formation

As previously discussed, the enthalpy of hydride formation indirectly determines the hydrogen absorption/desorption temperatures and kinetics (assuming a constant entropy). The simplest way of hydrogenating a metal (M) is through direct dissociative chemisorption (Figure 8a).

The enthalpy of formation $\Delta H_f$ of the resulting metal hydride ($MH_n$) then can be expressed as

$$\Delta H_f = (E + pV)_{\text{MH}_n} - (E + pV)_{\text{M}} - \frac{n}{2}(E + pV)_{\text{H}_2} \quad (84)$$

where the dependence on temperature is implicitly introduced via the pV terms.

Since the molar volumes of the metal hydride and metal in general are small as compared to that of the gas, the expression above can be approximated as[343]

$$\Delta H_f \approx \Delta H_{\text{el}} + \Delta H_{\text{ZPE}} + \delta\Delta H_{\text{T}} \quad (85)$$

where

$$\Delta H_{\text{el}} = E_{\text{el}}(\text{MH}_n) - E_{\text{el}}(\text{M}) - \frac{n}{2}E_{\text{el}}(\text{H}_2) \quad (86)$$

$$\Delta H_{\text{ZPE}} = E_{\text{ZPE}}(\text{MH}_n) - E_{\text{ZPE}}(\text{M}) - \frac{n}{2}E_{\text{ZPE}}(\text{H}_2) \quad (87)$$

$$\delta\Delta H_{\text{T}} = E_{\text{vib}}(\text{MH}_n) - E_{\text{vib}}(\text{M}) - \frac{n}{2} \left[ \frac{7}{2} k_{\text{B}} T + E_{\text{vib}}(\text{H}_2) \right] \quad (88)$$

$E_{el}$ represents the energy of the system calculated by considering the atoms perfectly still at their equilibrium positions; $E_{ZPE}$ and $E_{vib}$ are respectively the zero-point and vibrational energies of the material, which are estimated as

$$E_{\text{ZPE}} = \frac{1}{N_q} \sum_{q,s} \frac{1}{2} \hbar \omega_{qs} \quad E_{\text{vib}} = \frac{1}{N_q} \sum_{q,s} \frac{\hbar \omega_{qs}}{e^{\hbar \omega_{qs} / k_B T} - 1} \quad (89)$$

where the summations run over wave vectors q and phonon branches s, $N_q$ is the total number of wave vectors used for integration within the first Brillouin zone, and $\omega_{qs}$ are the phonon frequencies of the system.

By following Equations (85)–(89) and using first-principles methods for the calculation of zero-temperature energies and phonon frequencies (see Section 8.2.), it is possible to estimate the formation enthalpy of metal hydrides with reasonable accuracy (Figure 24). We note, however, that in this case the level of agreement with the experiments may depend somewhat on the approximation used for the exchange-correlation energy. This effect is explicitly shown for alkaline earth metal hydrides in Figure 24, where the enthalpy values obtained with the LDA and GGA approximations tend to bound the experimental data either by below or above.[343]

### 9.1.3. Thermodynamic Stability of Polymorphs

The calculation of phonon frequencies also may provide some valuable information on the thermodynamic and vibrational stability of crystal polymorphs. Phonon frequencies and vibrational modes can be calculated efficiently with several techniques, including linear response theory[344] and the small displacement method.[345] For a crystal to be vibrationally stable all the phonon frequencies at any arbitrary wave vector belonging to its Brillouin zone must be real, i.e., nonimaginary. Once the phonon frequencies of a given system are known, one
<figure><img src="image_27.png" /><figcaption>**Figure 24.** Enthalpy of formation of several alkaline earth metal hydrides at room temperature. LDA and GGA points refer to first-principles results obtained with local and semi-local density functional theory techniques, respectively. The sketched atomic configurations correspond to the rutile (left) and contunnite (right) structures rendered by the stable phases of small-radius and large-radius metal atom hydrides, respectively. Hydrogen (metal) atoms are represented with small (large) spheres. Reproduced with permission.[343] Copyright 2008, IOP Publishing Ltd.</figcaption></figure>

can use the quasi-harmonic approximation to estimate the corresponding Gibbs free energy

$$G = F_{hz} + pV = H - TS \qquad (90)$$

where $F_{hz}$ represents the Helmholtz free energy.

In the quasi-harmonic approximation it can be assumed that the potential energy of a crystal can be estimated as a quadratic atomic-displacement expansion around the equilibrium configuration.[346] The system is then mapped onto a collection of noninteracting harmonic oscillators with frequencies $\omega_{qs}$ and energy levels $E^{N}_{qs} = (1/2 + n) \cdot \omega_{qs}$. The Helmholtz free energy of such a system $F_{harm}$ is analytically known and reads

$$F_{harm} = \frac{1}{N_q} k_B T \sum_{qs} \ln \left[ 2 \sinh \left( \frac{\hbar \omega_{qs}}{2k_B T} \right) \right] \qquad (91)$$

By further considering that hydrostatic pressure in a quasi-harmonic crystal can be estimated as

$$p = - \frac{\partial(E_{el} + F_{harm})}{\partial V} \qquad (92)$$

the corresponding Gibbs free energy can be determined. Consequently, the thermodynamic stability between different crystal polymorphs can be accurately determined upon calculation and comparison of their $G_s$.

The quasi-harmonic approach is particularly well-suited for studying materials with a relatively small number of particles in their unit cells (≈10 atoms) with first-principles simulation methods (see next section). In **Figure 25**, we show quasi-harmonic free-energy results obtained for the hydride $LiBH_4$, a promising hydrogen-storage and ionic conductor material.[347] At ambient pressure, $LiBH_4$ undergoes a first-order phase

<figure><img src="image_28.png" /><figcaption>**Figure 25.** Calculated low temperature and pressure polymorphs stability in $LiBH_4$. a) Density of vibrational states corresponding to the experimentally determined and theoretically predicted low temperature and pressure phases, SLT[348] and TLT[349] respectively; b) Gibbs free-energy difference between the SLT and TLT structures expressed as a function of temperature. The results were obtained within the quasi-harmonic approximation using the small-displacement method.</figcaption></figure>

transition from a low-temperature (LT) phase to a high-temperature (HT) phase around 108 °C:[348] upon further heating, it is possible to liberate 13.1 mass% $H_2$ via the two-step reaction

$$12LiBH_4 \leftrightarrow Li_2B_{12}H_{12} + 10LiH + 13H_2 \leftrightarrow 12LiH + 12B + 18H_2 \qquad (93)$$

The exact crystal structure of the LT and HT phases in $LiBH_4$, however, remain contentious. Experimentally, the LT phase has been ascribed by Soulié et al. to an orthorhombic *Pnma* structure and the HT to an hexagonal $P6_3mc$.[348] Tekin et al. have proposed new candidate structures for the LT and HT phases of $LiBH_4$ based on standard first-principles calculations.[349] At zero temperature, the structures predicted by Tekin et al. are energetically competitive with respect to the ones resolved in the experiments; however, it has been subsequently demonstrated by Shevlin et al. that after considering entropy contributions to the Gibbs free energy, the LT and HT phases are most likely to be the structures proposed by Soulié et al.[348] (Figure 24). This case illustrates the importance of considering thermal effects when assessing the relative stability between
different structures and assigning the correct crystal symmetry to experimentally observed phases.

Another way of assessing the thermodynamic stability of a particular structure or system is to simulate it directly with molecular dynamics (MD) techniques.[350] In MD simulations the trajectories of atoms are determined numerically by solving the corresponding Newton's equations of motion; thermostat- ting and barostatting techniques can be used along the way in order to reproduce specific thermodynamic constraints, e.g., pressure and temperature. In contrast to quasi-harmonic approaches, anharmonic effects are fully taken into consid- eration in MD simulations. Nonetheless, large system sizes (≈1000 atoms) and long simulation times (≈1 ns) normally are required in MD simulations so as to obtain well-converged quantities, hence semiempirical interaction models are cus- tomarily employed. It is worth noting that Gibbs free ener- gies of crystal polymorphs cannot be accessed directly in MD simulations.

### 9.1.4. Hydrogen Release and Uptake Kinetics

Besides favorable hydrogen binding energy and thermody- namic stability, it is desirable for hydrogen-storage materials to display quick charge and discharge kinetics. In order to assess such dynamic features, the harmonic transition-state theory[351] can be used to calculate the hydrogen diffusion rate (11). The quantity governing the hydrogen kinetics generally is $E_a$ as it enters Equation (11) via an exponential. The less (more) positive $E_a$ is, the quicker (slower) the uptake/discharge reaction occurs. In practice, $E_a$ is calculated by determining minimum energy reaction pathways. This is commonly done by using the nudged- elastic band (NEB) method,[352] in which a number of interme- diate configurations between the initial (e.g., free H₂ molecules and unloaded system) and final (e.g., H₂-loaded system) states are defined and made to interact with their nearest-neighbor images through fictitious harmonic springs. The minimum energy reaction pathway is then determined by minimizing the perpendicular forces along the configuration path. It is relatively easy to perform NEB calculations for materials in which hydrogen molecules are adsorbed since appropriate inter- mediate configurations can be guessed easily; by contrast, NEB calculations of hydrogen absorption processes involving changes in the stoichiometry of the materials may be convoluted.

## 9.2. Computational Methods Overview

In materials modeling the interactions between atoms normally are described with energy-force methods that can be classified within two major categories: semiempirical and first-principles. In semiempirical approaches, the forces between particles are modeled via physically inspired analytical functions known as force fields or classical potentials. Force fields contain a number of parameters that are set so as to reproduce a certain amount of experimental data or the results of highly accurate quantum-chemistry calculations. The low computational cost associated with semiempirical approaches makes it possible to address the study of hydrogen-storage materials within ample

thermodynamic intervals without sacrificing complexity in the adopted structural models. It is even feasible to reproduce the breaking and forming of bonds during chemical reactions through the use of special reactive force fields.[353] Nevertheless, classical potentials may suffer from impeding transferability issues as due to the impossibility of mimicking the systems of interest at conditions different from those in which the force- field setups were performed. In this context, first-principles methods, also known as ab initio, turn out to be preferable. In first-principles approaches no empirical information is generally assumed on the derivation of the atomic interactions, as those are directly obtained from applying quantum mechanics princi- ples to the electrons and nuclei in the system. Nonetheless, due to their great accuracy, ab initio methods are very demanding in terms of computational expense. Consequently, first-princi- ples calculations generally are restricted to zero-temperature conditions and the involved structural models are highly ideal, i.e., contain a relatively small number of particles ≈100–1000. A third general materials modeling approach exists that is gaining a lot of attention at the moment and which is based on machine learning techniques. Machine learning modeling may combine the best of the first-principles and semiempirical worlds, namely, great accuracy at affordable computational cost. This third approach is entirely mathematically oriented, that is, it employs algorithms and statistical models while leaving aside any chemical intuition. For machine learning techniques to provide reliable information, the constitutive mathematical models need to be built upon extensive “training data” sets usu- ally generated with first-principles methods; how to construct those training sets and generate the desired mathematical models, however, is not always a straightforward and self-con- tained task. Next, we briefly review the basic ingredients in first-principles, semiempirical, and machine learning modeling of hydrogen-storage materials.[342,354]

### 9.2.1. First-Principles Methods

The starting point in most first-principles methods is the non- relativistic Born–Oppenheimer electronic Hamiltonian

$$H_{\text{el}}^{\text{BO}} = -\frac{1}{2} \sum_{i} \nabla_{i}^{2} - \sum_{i} \sum_{j \neq i} \frac{Z_{i}}{|\mathbf{R}_{i} - \mathbf{r}_{i}|} + \sum_{i} \sum_{j \neq i} \frac{1}{|\mathbf{r}_{i} - \mathbf{r}_{j}|} \quad (94)$$

where $Z_i$ represent nuclear charges, $\mathbf{r}_i$ the positions of elec- trons, and $\mathbf{R}_i$ the positions of nuclei (which are considered to be fixed).

By solving the corresponding Schrödinger equation one has access to the many-electron wave function of the system $Ψ$, and hence to all its static properties. In real materials, however, such a many-electronic wave function is a tremendously com- plex function that cannot be determined in practice. The mis- sion of any ab initio method is to find out a good approximation for $Ψ$, or related quantity, that while being accurate still allows for carrying out practical calculations. Examples of first-prin- ciples methods include quantum Monte Carlo, Moller–Plesset perturbation theory, coupled clusters with perturbative excita- tions, and density functional theory. Of these techniques den- sity functional theory (DFT) probably presents the best balance
between accuracy and computational expense. For this reason, the vast majority of computational works done on modeling of hydrogen-storage materials involve DFT methods, which we briefly review next.

In the sixties, Kohn and Sham developed a pioneering theory[355] showing that the exact ground-state energy $E_{el}$ and electronic density $n(\mathbf{r})$ of a system can be determined by solving effective one-electron Schrödinger equations involving the Hamiltonian

$$H_{el}^{eff} = -\frac{1}{2} \nabla_i^2 + \int \frac{n(\mathbf{r}')}{|\mathbf{r} - \mathbf{r}'|} d\mathbf{r}' + V_{xc}(\mathbf{r}) \quad (95)$$

where

$$n(\mathbf{r}) = \sum_{i,\sigma} |\psi_{i\sigma}(\mathbf{r})|^2 \quad (96)$$

and $V_{xc}(\mathbf{r})$ represents the exchange-correlation potential.

The exchange-correlation energy has a genuine quantum origin and, despite representing a small portion of $E_{el}$, has a crucial role on the bonding between atoms. For most systems $V_{xc}$ is unknown and needs to be approximated: this is the only source of fundamental error in DFT methods.

In local DFT approaches (e.g., LDA), the exchange-correlation energy is approximated as

$$E_{xc}^{l}[n] \approx \int \epsilon_{xc}^{l}(\mathbf{r}) n(\mathbf{r}) d\mathbf{r} \quad (97)$$

where the adopted exchange-correlation energy functional $\epsilon_{xc}^{l}$ is taken equal to that in a gas of noninteracting electrons at density $n(\mathbf{r})$, which is numerically known from exact quantum Monte Carlo calculations.[356] In semilocal approaches, e.g., GGA, the $\epsilon_{xc}^{l}$ functional is made to depend on the electronic density and also its gradient in order to fulfil a series of analytical constraints known from slowly varying $n(\mathbf{r})$ systems. Both local and semilocal DFT approaches work reasonably well for most bulk systems at equilibrium conditions. However, they cannot reproduce long-range electronic exchange or correlation interactions, e.g., dispersion forces.

Dispersion DFT methods have been recently developed that allow to reproduce satisfactorily long-range correlation effects in materials while still keeping the accompanying computational loads within moderate levels. One illustrative example is the suite of DFT-D techniques,[357] in which an attractive energy term reproducing the asymptotic $C_6/r^6$ interaction between two particles in a gas is added to $H_{el}^{eff}$ (see Equation (95)). Several improvements have been proposed that go beyond DFT-D, in which the values of the dispersion coefficients $C_6$ and other higher-order terms are obtained by taking into account the specific environment of the atoms.[358] Other more elaborate and computationally intensive dispersion DFT approaches exist in which many-electron long-range interactions can be estimated seamlessly without the need of introducing ad hoc parameters, e.g., vdW-DF[359] and the random phase approximation.[360]

We should note that the choice of the exchange-correlation approximation in DFT studies of gas-storage materials

may have an important effect on the final results.[354a,361] As explained in previous sections, molecular physisorption represents a promising hydrogen-storage route; van der Waal's interactions are critical in this context and thus any DFT work attempting to describe $H_2$ physisorption should take into account dispersion forces. $Ca^+(H_2)_4$, a system for which several computational benchmark studies have been reported (see, for instance[362]) is an illustrative example of how crucial the choice of the right exchange-correlation approximation may be. It turns out that standard DFT approaches, e.g., LDA and GGA, tend to overestimate the cohesion energy of the system by ≈0.1 eV/H$_2$ as compared to the results obtained with advanced dispersion DFT and quantum chemistry methods. Overall, for arriving at meaningful conclusions on modeling of hydrogen-storage materials the blind use of local and semilocal DFT approaches is not recommended owing to their inherent limitations in describing long-range exchange and correlations in many-electron systems.[354a]

### 9.2.2. Semiempirical Methods

In semiempirical approaches, the interaction between atoms is normally modeled with simple Lennard-Jones (LJ) plus Coulomb potentials of the form

$$V_{ij}^{FF}(r_{ij}) = 4\epsilon_{ij} \left[ \left( \frac{\sigma_{ij}}{r_{ij}} \right)^{12} - \left( \frac{\sigma_{ij}}{r_{ij}} \right)^{6} \right] + \frac{q_i q_j}{r_{ij}^2} \quad (98)$$

where $r_{ij}$ represents the separation between atoms i and j having charges $q_i$ and $q_j$, and $\sigma_{ij}$ and $\epsilon_{ij}$ are potential parameters to be determined.

In semiempirical approaches, pairwise additivity normally is assumed when computing total energies and forces. The first term in Equation (98) accounts for the short-ranged atomic repulsion deriving from the overlapping between electron clouds; the second term represents long-ranged atomic attraction due to dispersive van der Waals forces, and the third is the usual Coulomb interaction between atomic point charges. In ionic materials the atomic point charges $q_i$ typically adopt the value of the oxidation state of the ions, whereas in molecular systems they are set to reproduce the polarity of the gas molecules, e.g., $H_2$ quadrupole moment, and internal electric fields. The value of the LJ potential parameters $\epsilon$, $\sigma$, and $q$ can be obtained from different sources, including the fit to thermodynamic data or highly accurate quantum chemistry calculations. Arithmetic averages such as the Lorentz-Berthelot mixing rules are usually employed to straightforwardly obtain the value of LJ potential parameters corresponding to crossed gas-material interactions. Further and more complex terms can be added to Equation (98) in order to account for higher-order dispersion terms and molecule torsional degrees of freedom.

The low computational expense associated with semiempirical potentials allows explicit consideration of thermal and structural disorder/nanostructuring effects, e.g., slit-like pores, in the simulation of hydrogen-storage materials (see, for instance[342]). Due to the usual light atomic masses and weak intermolecular interactions involved in hydrogen-storage
materials, quantum nuclear effects (QNE) may also significantly affect their gas-uptake capacity at room temperature.[346] This is the case, for instance, of the adsorption and diffusion of hydrogen molecules on the surface of graphene and other related carbon-based nanomaterials.[363] Indeed, by using semi-empirical potentials and quantum simulation techniques, e.g., path-integral Monte Carlo methods, it has been possible to assess quantitatively the non-negligible role of QNE on the adsorption of hydrogen and deuterium molecules to the interior of carbon-based nanotubes with varying radii.[364] Likewise, QNE have been shown to be key for understanding the proton dynamics in lithium imide Li₂NH,[365] a promising hydrogen-storage material that is able to liberate more than 5 wt% H₂ through the decomposition reaction

$$ \mathrm{Li_2NH} + \mathrm{LiH} + \mathrm{H_2} \leftrightarrow \mathrm{Li_3N} + 2\mathrm{H_2} \quad (99) $$

### 9.2.3. Machine Learning Techniques

As explained in previous sections, many physical quantities that are relevant to modeling of hydrogen storage materials (e.g., free energy of hydrogen storage/release reactions and materials decomposition processes) can be calculated accurately with first-principles methods. However, in order to determine the most favorable reaction pathways, adsorption sites, chemical structures, and other properties, a number of candidate processes and configurations need to be guessed first. The guessing of such candidate systems relies entirely on chemical intuition hence may be erroneous and/or incomplete depending on the complexity of the problem. To deal with such “open” problems, one would like to have access to theoretical frameworks that automatically, and with acceptable precision, could predict the thermodynamically most favorable reaction paths, states, and configurations by simply providing the overall chemical composition of the system or similar certain information. Machine learning techniques come to fill such a modeler’s need, leaving aside any chemical intuition in favor of computational affordability and objectiveness.

In ML, algorithms and statistical models are employed to steadily improve the computational performance on a specific task; in the context of hydrogen-storage materials those tasks can be, for example, the prediction of lowest-energy configurations and decomposition reaction pathways. The analytic approaches that are used to construct ML algorithms and models include permutation invariant polynomials, the modified Shepard method using Taylor expansions, Gaussian processes, interpolating moving least squares, and artificial neural networks.[366] ML models, however, need to be trained on specific and comprehensive sets of data, usually generated with first-principles methods and sometimes difficult to be completed. Theoretical ML-based studies of hydrogen storage materials have started to appear in the literature and we foresee that in the next few years their number will grow steadily due to the great versatility that they offer when it comes to screen vast numbers of different systems and processes. Examples of such ML studies include, the analysis of catalytic activity of the hydrogen evolution reaction on nanoclusters,[367] generation of thousands of new alloy combinations presenting

favorable enthalpies of formation for the design of high pressure hydrogen compressor materials,[368] and identification of the performance limits of physical hydrogen storage of 850 000 different nanoporous materials.[369]

## 10. Design of Hydride Tanks toward Practical Application

Assuming that materials are designed with the right hydrogen storage properties, their practical application relies on their integration at the kg scale into a storage tank meeting practical requirements for the rate of charging and discharging. Depending on the reversibility of the hydrogen release, two types of systems can be designed: i) one shoot tanks containing hydrides that are hydrolyzed to produce hydrogen, and ii) reversible tanks that contain hydrides that can be simply regenerated by applying hydrogen pressure. In both cases, tank design poses several challenges including careful heat management, appropriate choice of the tank material capable of withstanding pressure, stresses and hydrogen embrittlement, and an appropriate management of the slurry (hydrides and water mix) as well as the decomposition products in the case of hydrolysis tanks.

### 10.1. One Shoot Hydrogen Delivery Tanks

Chemical hydrides like the much-investigated sodium borohydride NaBH₄ and ammonia borane NH₃BH₃ are traditionally not considered for reversible hydrogen storage because of the formation of thermodynamically stable residues (i.e., byproducts). Instead, to extract hydrogen from these compounds significant efforts have been devoted to developing methods to control their hydrolysis.[370] As discussed below, prototypes have been developed for hydrolysis of NaBH₄, but to our knowledge hydrolysis of NH₃BH₃ has not been scaled up to date. One may, however, cite Keskin et al.’s work that can be seen as a first step toward scaling up.[371] They developed NH₃BH₃ composite pellets (10 mm diameter and 1 mm thickness) and based on their hydrolysis results, they made recommendations for fuel cartridge.

The first prototype using hydrolysis of NaBH₄ for on-board H₂ production was developed by the now-bankrupted company Millennium Cell (USA).[372] With an apparently basic design (Figure 26), the prototype consisted of a storage tank filled with alkaline aqueous NaBH₄; for H₂ generation, the solution had to be pumped onto a tubular catalyst bed.[373] A similar design was later developed by other groups. Kojima et al. worked on a device of 80 kg with a body size of about 200 L (Figure 27), mainly made up of a 2 L stainless steel tank for storing aqueous NaBH₄ (8.8 mol L⁻¹) and a 4 L stainless steel tank for storing the spent fuel.[374] Using a similar concept, Richardson et al. focused their attention on the performance of a ruthenium-based catalytic reactor.[375] Wang and co-workers’ prototype was able to feed a 3000 W fuel cell to power an electric vehicle.[376] Recently, Gang and Kwon adapted this H₂ generator (5.9 L H₂ min⁻¹) for a portable electric power plant with a specific energy density of 211 Wh kg⁻¹.[377] Kim et al. powered a small
<figure><img src="image_29.png" /><figcaption>**Figure 26.** The hydrogen generator developed by Millennium Cell.[372] The generator is based on the use of alkaline aqueous solution of NaBH₄ (stored in a tank) that is pumped onto a tubular catalytic bed (ruthenium-based). H₂ generated by hydrolysis is then separated by the spent fuel (aqueous solution of borates) to feed a PEM-type fuel cell. The as-separated spent fuel is stored in another tank. Reproduced with permission.[373a] Copyright 2018, WILEY-VCH.</figcaption></figure>

unmanned aircraft (requiring 80 W for cruising and 300 W for take-off) with 360 g of a 15 wt% NaBH₄ solution using a cobalt catalyst.[378] For the same application, Jung et al. reported a NaBH₄ hydrogen generator based on a cobalt-phosphorus/ nickel foam catalyst. Their system, coupled to a 100 W PEMFC, produced H₂ at a constant rate for 3 h with a conversion efficiency of 97.8% and a maximum power and energy density of 95.96 W and 185.2 Wh kg⁻¹, respectively.[379] Gang and Kwon took the technology further by successfully constructing an integrated hybrid electric power system based on a NaBH₄-PEMFC (300 W h⁻¹) system, a solar cell (80 W h⁻¹) and a battery (117 W h⁻¹).[380]

For more compact technologies, miniaturization has also been targeted.[381] For example, Lee and Kim developed a H₂

<figure><img src="image_30.png" /><figcaption>**Figure 27.** The hydrogen generator based on NaBH₄ developed by Kojima et al.[374] The two stainless steel tanks used to store the fuel (alkaline aqueous solution of NaBH₄, 8.8 mol L⁻¹) and the spent fuel (borates in aqueous solution) are shown. The system has a body size of about 200 L and a weight of 80 kg. With this prototype, the effective hydrogen storage capacities would be at best of 2 mass% H₂ and 15 kg (H₂) m⁻³, which would be comparable to a compressed H₂ system at 25 MPa. Reproduced with permission.[374] Copyright 2004, Elsevier B.V.</figcaption></figure>

generator for a 1 W fuel cell to be used in nanosatellites.[382] According to the authors, the specific energy density of their system is three times higher than that of a Li-ion battery. The device is inspired from the aforementioned prototype but it is different by the absence of a spent fuel tank (Figure 28). Indeed, for compactness, the borates are stored, upon reaction, in the fuel tank. They confirmed the superiority of the miniaturised NaBH₄ cartridges over Li-ion batteries for a specific mission of a small unmanned vehicle, which had to flight more than 5 h with, e.g., a cruise speed of 50 km h⁻¹.[383]

The power system based on NaBH₄ as H₂ generator coupled to a H₂/air PEMFC (gravimetric power density of 400 W kg⁻¹) showed an endurance of 7 h versus the 4.8 h achievable with a Li-ion battery of 290 Wh kg⁻¹ gravimetric energy density. It is worth mentioning that a lithium-sulfur battery (gravimetric energy density of 420 Wh kg⁻¹) was found to be a bit more enduring with 7.1 h. On a larger scale, Rivarolo et al. compared, from thermodynamic and economic standpoints, traditional H₂ storage in pressurized tanks with H₂ storage in the from of NaBH₄.[384] The model was based on a plant where H₂ is produced from solar (power peak of 1 MW) coupled with water electrolysis (alkaline electrolyzer of 1 MW delivering hydrogen at 3 MPa). The as-generated H₂ is then used to synthesize NaBH₄ (600 °C, 3 MPa H₂; capacity of 171 kg h⁻¹) from its spent fuel NaBO₂·4H₂O using Mg as catalyst. In this model, trucks are used to transport NaBH₄ to the local usage zone and a ruthenium-catalyzed hydrolysis at 80 °C is considered to regenerate 36 kg H₂ h⁻¹. The spent fuel is recovered and transferred back to the pant. In this cycle of using NaBH₄ as the hydrogen carrier, the total cost of the method using NaBH₄ was found to be 15.5 € kg⁻¹ instead of 22.5 € kg⁻¹ for the approach based on compressed H₂. Transportation of NaBH₄, which is safer, is one of the positive features of the technology and this makes distribution of hydrogen easy.

The widespread development of the NaBH₄-based H₂ generator discussed above has, however, been hindered by several technical problems. Most of them have recently been summarized in an excellent paper authored by Lapeña-Rey et al. from Boeing R&D Europe who tested the technology with a fuel cell-powered unmanned aerial vehicle (200 W) using an alkaline solution (1 L) of NaBH₄ (20 wt%) as a H₂ generator (900 Wh).[385] In this trial, they used the commercial device sold by the company Horizon Energy Systems (Singapore), a device that is comparable to the millennium cell's generator.[386] The main drawbacks were found to be that: i) the catalytic activity unavoidably deteriorates after about 8 h of use; ii) the NaBH₄ fuel is irritant; iii) the fuel requires rapid deployment because of spontaneous hydrolysis and H₂ generation; iv) the fuel cost is high; v) precipitation of borates in the piping takes place, resulting in system clogging; vi) the device thus requires water flushing and tedious maintenance after each use to remove the precipitated borate byproducts. Few other problems were reported elsewhere. Kim et al. detected the presence of Na⁺ in the water vapor generated because of the exothermicity of the hydrolysis reaction, and such Na⁺ deteriorates the performance of the Nafion membrane
<figure><img src="image_31.png" /><figcaption>**Figure 28.** NaBH₄-based nanosatellite hydrogen generator. Schematic representation and photography of the power system developed for nanosatellites by Lee and Kim: A membrane is used to separate H₂ from the spent fuel that is finally stored in the cartridge of NaBH₄. Reproduced with permission.[382] Copyright 2014, Elsevier Ltd.</figcaption></figure>

of a PEM-type fuel cell.[387] Petit et al. found that the generated H₂ carries borates, and is thus not as pure as what is often claimed.[388] This may explain why Kim and Kim used a third tank (filled with water), called a “wash tank” (Figure 29),[389] in which the generated H₂ was bubbled to be cooled down and to favor the precipitation-solubilization of the evolving borates. Taking into account the aforementioned technical issues, there is thus a clear need to mature this technology, as it could be ideal for operation in remote areas while avoiding storage systems based on high pressures.[385]

## 10.2. Regeneration of Hydrides from One Shoot Tanks

As discussed above, hydrogen can effectively be released from many hydrides through their hydrolysis. However, the drawback of such an approach is that once reacted with water, the hydrides are fully oxidized and thus cannot be simply regener-

ated. The only option to close the hydrogen cycle is to regenerate the oxides through effective chemical synthetic approaches.[373]

### 10.2.1. Recycling of Hydrolytic Boron Hydrides

The attractive feature of NaBH₄ is its ability to spontaneously hydrolyze at ambient conditions, the process being accelerated and completed with the help of an accelerator like an acid or a metal-based catalyst, following the reaction[390]

$$ \mathrm{NaBH_4} + 4\mathrm{H_2O} \rightarrow \mathrm{NaBH(OH)_4} + 4\mathrm{H_2} \quad (100) $$

The byproduct forming at ambient conditions (<80 °C) is sodium tetramethoxyborate NaB(OH)₄, one of the hydrated forms of sodium metaborate NaBO₂.[391] Because the B–O bond is as energetic as the C–O bond of CO₂, the formation of B–H bonds from B–O bonds requires harsh conditions and the use of reducing agents. Another issue with NaB(OH)₄ and more widely with borates (including NaBO₂) is the absence of suitable solvents (besides the protic solvents like water and ethanol into which NaBH₄ is not stable). For example, Kojima and Haga showed that the ball milled mixture of NaBO₂ and MgH₂ can generate NaBH₄ (with a yield of almost 100%) if heated at 550 °C for 2 h under 7 MPa of H₂.[392] With the objective of decreasing the overall cost of the process, further works focused on improvements by using the couple Mg–Si or Mg₃La hydrides instead of MgH₂,[392a,393] ball milling under inert atmosphere,[394] and/or substituting Mg by another element like Al or Ti.[395]

From a commercial point of view, it is preferable to recycle the hydrated form of the borate as this is the species that forms directly upon hydrolysis of NaBH₄. Furthermore the formation of NaBO₂ by

<caption>**Figure 29.** NaBH₄-based hydrogen generator developed by Kim and Kim. An additional tank called a “wash tank” is used to bubble the generated H₂ in order to cool the gas stream down and trap the evolving borates (by solubilization in water). Reproduced with permission.[389] Copyright 2010, Elsevier Ltd.</caption>
dehydration of NaB(OH)₄ requires temperatures higher than 250 °C and is consequently less cost-effective.[396] Adapting Kojima et al.'s approach, Chen et al. showed that NaB(OH)₄ can be used instead of NaBO₂.[397] It reacts with MgH₂ by ball milling with a yield of almost 90%

$$NaBO_2 \cdot 2H_2O + 4MgH_2 \rightarrow NaBH_4 + 4MgO + 4H_2 \quad (101)$$

Zhong et al. also used NaB(OH)₄ that was reacted with Mg₂Si by ball milling at room temperature and under inert atmosphere to regenerate NaBH₄.[398] NaBH₄ formed with a maximum yield of 74%, and was isolated by dissolution in ethylenediamine. To date, chemical recycling of NaBO₂ or NaB(OH)₄ using a metallic reducing agent like Mg or MgH₂ has been the most investigated method.[399]

An alternative approach is to use sodium tetramethoxyborate NaB(OCH₃)₄. Kemmitt and Gainsford synthesized it by methoxylation of NaBO₂.[400] To regenerate NaBH₄ (with a yield higher than 80%), NaB(OCH₃)₄ was refluxed with NaAlH₄ (in diglyme)

$$NaB(OCH_3)_4 + NaAlH_4 \rightarrow NaBH_4 + NaAl(OCH_3)_4 \quad (102)$$

Although the process appears not to be viable because of the high cost of the alanate (Table 3), it can be adapted to the “classical” industrial synthesis path of NaBH₄, namely the Brown-Schlesinger process (Figure 30).[399a,401] The step 6 of this process could be adapted to use directly B(OCH₃)₄⁻ and a cheap hydride source (cheaper than the aforementioned NaAlH₄), or even better, to use the acidic counterpart B(OCH₃)₃ and NaH. Note that the aforementioned recycling processes are all based on solid-phase reactions; an intimate mixture of the borate and of the reducing agent is therefore much important, which can only be done by ball milling.

A last regeneration approach that is worth mentioning is electrochemical reduction[402]

$$B(OH)_4^- + 4H_2O + 8e^- \rightarrow BH_4^- + 8OH^- \quad (103)$$

Similar to the electrochemical reduction of CO₂, this is a high energy-demanding process. The standard potential (E°) of the

<figure><img src="image_32.png" /><figcaption>**Figure 30.** The Brown-Schlesinger process for the stepwise industrial production of NaBH₄: from methane CH₄ as hydrogen H₂ source (step 1) and sodium chloride NaCl as metallic sodium Na source (step 2), via the intermediate sodium hydride NaH (step 5); from borax (step 3) and methanol as sources of trimethoxyborate B(OCH₃)₃ (step 4). NaBH₄ is obtained by reaction of NaH and B(OCH₃)₃ (step 6) and the by-product NaOCH₃ is recovered to be recycled into CH₃OH (step 7). Reproduced with permission.[401] Copyright 2007, Elsevier B.V.</figcaption></figure>

reaction (103) is +1.24 V.[403] Hence, all the recycling processes developed so far suffer from the need of a significant energy input to regenerate NaBH₄, and thus the only viable solution is to use excess electricity from renewable sources.[399a,404]

Like NaBH₄, NH₃BH₃ has also been considered as a hydrolytic hydrogen carrier. To our knowledge, the recycling of its hydrolysis byproducts has not been reported so far. Upon hydrolysis, the species B(OH)₃ (104) and B(OH)₄⁻ (105) form if an excess of water is used[405]

$$NH_3BH_3 + 3H_2O \rightarrow NH_3 + B(OH)_3 + 3H_2 \quad (104)$$

$$B(OH)_3 + 2H_2O \leftrightarrow B(OH)_4^- + H_3O^+ \quad (105)$$

Polyborates (B₄⁻/B₅⁻-based compounds) form when water is used in stoichiometric conditions or if the excess of water is extracted.[406] It is preferable to keep the byproducts in aqueous solution, and especially at acidic pH values in order to favor the predominance of B(OH)₃. Kemmitt and Gainsford's approach[400] could be adapted in order to methoxylate the byproduct B(OH)₃ to form B(OCH₃)₃ one of the key reactants of the Brown-Schlesinger process (Figure 30). In other words, the hydrolysis byproducts of NH₃BH₃ could be recycled to form NaBH₄, and then NH₃BH₃ since NaBH₄ is its precursor.[373]

### 10.2.2. Recycling of Ammonia Borane's Thermolytic Residue

To release hydrogen for ammonia borane, a thermolysis process whereby ammonia borane is heated up to 200 °C can also be used.[24a,b,300–301,303] In this case, ammonia borane cannot be regenerated by simply applying hydrogen pressure on the thermolysis. To close the hydrogen cycle, it is essential not to totally dehydrogenate the borane into a hydrogen-free residue like boron nitride. To achieve its chemical regeneration, the dehydrogenation of NH₃BH₃ by thermolysis should be conceived so that less than 2.5 equivalents of H₂ are released under heating. Under these conditions, a polymeric residue of complex composition (mixture of polyaminoborane, polyiminoborane, o-polyborazylene, and/or graphitic cross-linked polyborazylene) is formed but its exact composition is still unknown.[407]

Regeneration of NH₃BH₃ has thus been investigated while considering only one of the aforementioned polymers.[408] Gordon and co-workers considered polyborazylene as a model residue.[409] The regeneration method they then developed is multistep (Figure 31). By using benzenedithiol, polyborazylene is digested and the as-forming reaction intermediate is reduced with Bu₃SnH or Bu₂SnH₂ into NH₃BH₃ with an overall yield of 67%. Although efficient, this multistep method is complex, requires the use of several chemicals, and results in an average yield. All of these are detrimental to the overall cost of the process. Later, the same group reported a much simpler approach (Figure 31),[410] whereby polyborazylene was rehydrogenated with the participation of the reducing agent N₂H₄ in liquid NH₃ through a one-step protocol. In this case, NH₃BH₃ formed with a yield of 100%. By slightly modifying the conditions, the formation of N₂H₄BH₃ was even obtained and this is clearly the most efficient regeneration method reported so far.
<figure><img src="image_33.png" /><figcaption>**Figure 31.** The possible ammonia borane (NH<sub>3</sub>BH<sub>3</sub>) hydrogen cycle. According to the processes proposed by Gordon and co-workers: a) from ref. [409] and b) from ref. [410]. Both processes use polyborazylene as model residue (denoted [BNH<sub>x</sub>] in the scheme, and with the structure associated with the *) of the thermolytic decomposition of NH<sub>3</sub>BH<sub>3</sub>. Reproduced with permission.<sup>[373b]</sup> Copyright 2015, WILEY-VCH.</figcaption></figure>

However, the N<sub>2</sub>H<sub>4</sub>-assisted rehydrogenation can be applied to polyborazylene, only.

This approach was also applied for hydrogenating the ther-
molysis residues of LiNH<sub>2</sub>BH<sub>3</sub> and BN-confined NH<sub>3</sub>BH<sub>3</sub>.<sup>[411]</sup>
For example through this approach, Tang et al. regenerated
LiNH<sub>2</sub>BH<sub>3</sub> from its dehydrogenated counterpart and the con-
version back to the targeted borane was found to be of only
63%.<sup>[411b]</sup> The incomplete conversion is explained by the pres-
ence of a fraction of polymers different from the polyborazylene
structure.<sup>[373]</sup> Though targeted, the thermolytic dehydrocoupling
of B-N-H compounds does not lead to the selective formation of
polyborazylene,<sup>[407]</sup> and other polymers (e.g., polyaminoborane,
polyiminoborane) are formed. With this in mind, Mertens and
co-workers developed a generic digestion procedure based on
the use of the superacid HCl(4 MPa)/AlCl<sub>3</sub>/CS<sub>2</sub> solvent system
at 80 °C for 12 h.<sup>[412]</sup> This digestion was successfully applied
to borazine, polyaminoborane, and crosslinked polyborazylene.
More recently these authors proposed a more effective super-
acid digestion system based on the trio AlBr<sub>3</sub>/HBr/CS<sub>2</sub>.<sup>[413]</sup>
Linear polyaminoborane was digested at room temperature
within 4 h and polyborazylene required a temperature of 80 °C
(for >12 h). All of these important achievements demonstrate
that B-N-H-based hydrogen compounds can potentially be
used as one-shoot hydrogen generation systems and that a new
chemical industry could be built to support their commerciali-
zation and regeneration.

**10.3. Reversible Hydrogen Storage Tanks**

Typically the design of an appropriate storage tank for reversible
hydrides involves choosing appropriate bed geometries, and
methods for the simulation of the tank performances to enable
its optimization in terms of heat management and hydrogen
flows. The absorption of hydrogen in metal hydride tanks is an

exothermic process, and the release of hydrogen endothermic.
As a result, without proper heat management the driving force
toward the progress of the hydrogen uptake or release reac-
tion decreases as P<sup>eq</sup> is shifting with temperature (Figure 14a).
Hence cooling must be provided during hydrogen charging and
heating during hydrogen discharging to achieve sufficient rates
of hydrogen uptake and release. The latter can be achieved by
coupling the low heat generated by a fuel cell to the hydride via
a heat transfer media.[414] However, the heat transfer across the
hydride/heat transfer media interface must also be optimized
in addition to the heat across the hydride bed because of the
low thermal conductivity ≈0.1 W m<sup>-1</sup> K<sup>-1</sup> of metal-hydride
powders.[415] The nonregular shape of the hydride bed particles
results in additional thermal resistance (contact resistance).[416]
In order to achieve close contact between the hydride bed and
the heat transfer fluid, the bed is often compacted with some
additional powder material enhancing heat conduction.[415,417]
Heat conduction rather than radiation has been identified as the
main issue in low temperature hydride (e.g., LaNi<sub>5</sub>) tanks.[417a]
In contrast, heat exchange by radiation plays an important role
in tanks based on high temperature hydrides such as Mg-based
hydrides.[418] Several strategies can be used to improve heat
transfer. This includes inserting solid pieces of metals of high
thermal conductivity including aluminum foams.[419] Finally,
heat losses can be minimized during hydrogen sorption by
insulating the tanks walls.[420] Based on this understanding,
prototypes are then built to evaluate the performance of the
overall system, i.e., tank and hydrogen storage materials cou-
pled to the desired application, e.g., fuel cells, and validate the
simulation work for further optimization.

The bed geometry can be planar or cylindrical (Figure 32); however, if the storage tank is to be subjected to extensive hydrogen pressures, only a cylindrical design, which allows a uniform distribution of the stress and heat/mass transfer, is possible. For planar tanks, the geometry can be rectangular[421]
<figure><img src="image_34.png" /><figcaption>**Figure 32.** Reversible hydrogen tanks. a) Possible geometry of the hydride tanks (planar/rec- tangular, cylindrical) and hydrogen diffusion; b) Al-alloy hydrogen tank for NaAlH₄. Reproduced with permission.[554] Copyright 2014, Elsevier Ltd.</figcaption></figure>

or that of a disc;[422] however, this comes with the constraint of a low degree of freedom for the integration of a heat exchanger. In contrast, cylindrical shaped reactors, provide more freedom in terms of design and integration of the heat exchanger with the metal hydride powder. Bottle-shaped tanks can also be envis- aged. However, this may be restricted to small scale application owing to the difficulty of introducing a heat exchanger within the restricted opening of the bottle neck. The use of toroidal tanks can also be considered, however, such a design has so far been limited to gaseous storage owing to the complexity of the design and the intrinsic difficulty of filling such a tank with hydride powder.[423]

For modeling, several parameters need to be considered. These include: i) hydrogen diffusion through the material's bed, ii) the chemical reaction driving the temperature, pressure and rate of hydrogen release, and iii) the effective transfer of heat across the hydride bed because this is one of the main fac- tors affecting the overall performance of storage tanks.[424] To build the kinetic model, the dehydrogenation/hydrogenation mechanism needs first to be determined. This is often done by evaluating the hydrogen sorption kinetics of materials with a Sievert's apparatus and by fitting the reacted fraction (β) to determine the general equation rate g(β) (Figure 9). Then by using the Equation (106) the kinetic model of the storage tank can be determined.[425]

$$d\beta/dt = (Ae^{-E_a/RT}) \times f(p, p_{eq}) \times g(\beta) \quad (106)$$

where A is the pre-exponential factor, E_a is the activation energy, R is the universal gas constant and f(p, p_{eq}) is usually described as ln(p/p_{eq}).

Using this methodology, various kinetic models have been developed for tanks based on NaAlH₄, for example. This includes the optimization of shell, tube, fin-based hydride bed systems, and multitubular reactors equipped with fins.[426] For example, Na Ranong et al. simulated the effect of the storage tank diameter,[427] and scoping models for the evaluation of the tank design were also proposed.[428] Additional models were also proposed to evaluate the heat removal efficiency of a design and couple reaction kinetics with heat and mass transfer across the storage tank.[416d,429] Models were also developed to evaluate the effect of the compaction of the hydride bed including the

addition of elements, e.g., expanded natural graphite or metal foams, to improve heat con- duction across the bed.[430] 3D models have also been proposed to simulate the effects of pressure drop, heat propagation as well as the evolution of the equilibrium plateau pressure through the sorption process.[431] One diffi- culty in the simulation of solid state hydrides tanks is the prediction of the behavior of the hydride bed thermal conductivity, because it evolves with many parameters including the packing density, bed porosity, particle size distribution, hydrogen pressure,[432] and hydrogen concentration in the hydride.[433] Based on these simulations, various tank prototypes have been built by utilizing many different types of hydrides such as intermetal-

lics, Mg, and NaAlH₄. These are summarized in Table 5, and demonstrate that upon appropriate heat management, relatively fast filling can be achieved and good overall hydrogen cycling performances are maintained.

Practically, the outer diameter of solid state store tanks is limited to ensure adequate heat management of the bed, and this results in long tanks with small cross section and aspect ratios larger than 10.[434] Single tanks can then be bundled to reach the required hydrogen storage capacity.[435] The heat exchanger module (spiral coils, tube bundles, heat pipes) can then be embedded in the metal hydride bed within the tank. A phase change material can also be used to collect the heat generated during charging and reuse that heat during dis- charging.[436] To date, the application of reversible solid-state hydrogen tanks remains limited despite their vast potential in stationary application for long term energy storage or even load levelling. Toshiba has commenced demonstration and opera- tion of the H₂OOne units based on an AB₅ alloy (mischmetal nickel alloy).[437] The potential of the McPhy system based on Mg/MgH₂ has also been demonstrated across several projects, but large scale implementation remains limited.[438] The rem- nant barriers are in the lack of understanding of the technology by the general community and the misconception around bat- tery storage and its competitiveness as a mature technology.

## 11. Conclusion and Outlook

Despite being a young area of research as compared to the more traditional fields, e.g., catalysis or battery research, significant progress has been made by many groups on hydrogen storage materials since the early developments in the 1970s. Focus was initially on the interaction of hydrogen with metals and from this, fundamental understanding of hydrogen in metal hydrides has rapidly emerged. However, with the finding of hydrogen reversibility in NaAlH₄ significant progress has also been made over the last two decades with complex hydrides.

Today, the “art” of material design for hydrogen storage is still based on this early understanding of the evolution of the hydrogen properties of metal hydrides upon their alloying. Ele- ments are first selected for their capacity to bind large amounts of hydrogen and their properties modified by combining
<table><caption>**Table 5.** Example of solid state hydrogen storage tanks designed.</caption><thead><tr><th>Storage material</th><th>Weight of storage material [kg]</th><th>Design</th><th>Capacity of the tank [wt% H<sub>2</sub>]</th><th>Operation conditions</th><th>Kinetics/cycles</th><th>Purpose/limitations</th></tr></thead><tbody><tr><td colspan="7"><strong>Sodium alanate-based tank system</strong></td></tr><tr><td>NaAlH<sub>4</sub> doped with 2 mol% (TiCl<sub>3</sub>-0.3AlCl<sub>3</sub>) and 5 mol% carbon</td><td>8</td><td>Tubular reactor with porous sintered metal tube</td><td>3.7</td><td>Charging at 125 °C and 10 MPa H<sub>2</sub>. Discharging at 160-175 °C</td><td>80% of the capacity reached in 10 min</td><td>Large scale and stationary applications<sup>[551]</sup></td></tr><tr><td>NaAlH<sub>4</sub> doped with 4 mol% CeCl<sub>3</sub></td><td>0.087</td><td>Hydride bed reactor with flow-thru mode</td><td>3.9</td><td>Charging at 130 °C and 10 MPa H<sub>2</sub>. Discharging at 180 °C</td><td>—</td><td>Investigation on operational principles<sup>[552]</sup></td></tr><tr><td>NaAlH<sub>4</sub> with Al and 10 wt% graphite</td><td>4 × 21.5</td><td>Modular system of 12 tubular vessels</td><td>3.2 in 10 min</td><td>Charging at 120-150 °C and 5.52-6.89 MPa H<sub>2</sub>.</td><td>40 cycles</td><td>To be refueled in 10 min and to deliver H<sub>2</sub> up to 2.0 g s<sup>-1</sup>[430b]</td></tr><tr><td>NaAlH<sub>4</sub> doped with 4 mol% TiCl<sub>3</sub></td><td>2.7</td><td>Stainless steel tank with double wounded helical coil heat exchanger</td><td>2.24</td><td>Charging at 135 °C and 10 MPa H<sub>2</sub>. Discharging at 120-180 °C</td><td>—</td><td>To couple with HT-PEM fuel cell. Poor thermal conductivity of the sodium alanate leads to low heat transfer<sup>[553]</sup></td></tr><tr><td>Na<sub>3</sub>AlH<sub>6</sub> doped with 4 mol% TiCl<sub>3</sub></td><td>0.213</td><td>Al-alloy tank with bayonet heat exchanger</td><td>1.7</td><td>Charging at 2.5 MPa. Discharging at 177-180 °C</td><td>10 cycles</td><td>To develop and test lightweight Al-alloy storage tank<sup>[554]</sup></td></tr><tr><td>Na<sub>3</sub>AlH<sub>6</sub> doped with 4 mol% TiCl<sub>3</sub>, 8 mol% Al and 8 mol% activated carbon</td><td>1.9</td><td>Al-alloy tank with corrugated heat exchanger</td><td>2.1</td><td>Charging at 160 °C and 2.5 MPa H<sub>2</sub>. Discharging at 180 °C</td><td>31 cycles</td><td>To develop the lightweight Al-tank<sup>[555]</sup></td></tr><tr><td>NaAlH<sub>4</sub> doped with 2 mol% (TiCl<sub>3</sub>-0.3AlCl<sub>3</sub>) and 5 mol% expanded graphite</td><td>4.4</td><td>Ti-alloy tube-and-shell system tank</td><td>4</td><td>Charging at 124 °C and 10 MPa H<sub>2</sub>. Discharging at 120-170 °C</td><td>33 cycles</td><td>Slow hydrogen sorption because of the large diameter of tank<sup>[556]</sup></td></tr><tr><td colspan="7"><strong>Metal amide-based tank system</strong></td></tr><tr><td>Mg(NH<sub>2</sub>)<sub>2</sub>-2LiH-0.07KOH with 9 wt% ENG</td><td>0.098</td><td>—</td><td>N/A</td><td>Charging at 220 °C and 8 MPa H<sub>2</sub>. Discharging at 220 °C</td><td>Desorption duration of 79.5 min at 0.6 L H<sub>2</sub> min<sup>-1</sup></td><td>To investigate influence of graphite content and compaction pressure on the desorption rate.<sup>[557]</sup></td></tr><tr><td>LiNH<sub>2</sub>-MgH<sub>2</sub>-LiBH<sub>4</sub> 3 wt% ZrCoH<sub>3</sub> (in annulus)<br>LaNi<sub>4</sub>.3Al0.4Mn0.3 (in core)</td><td>0.6</td><td>Tubular reactor with two materials separated by a gas permeable layer</td><td>—</td><td>Charging: 165-170 °C up to 0.17 MPa; discharging at constant and periodic H<sub>2</sub> mass flow.</td><td>10% hydrogen desorption in 30 min and full release in 1 h</td><td>To validate a model and study effects of the reactor concept on desorption performance.<sup>[558]</sup></td></tr><tr><td colspan="7"><strong>LaNi<sub>5</sub>-based materials</strong></td></tr><tr><td>LaNi<sub>5</sub></td><td></td><td>Concentric triple-tube tank.</td><td>—</td><td>Hydrogen absorption at 3.5 MPa, and 20 °C. Desorption at 0.8 MPa, and 49 °C</td><td>The tank is fully charged with hydrogen after 20 s.</td><td>Low-temperature hydrogen storage<sup>[559]</sup></td></tr><tr><td colspan="7"><strong>Ti-based hydrogen storage material</strong></td></tr><tr><td>Alloy tank based on Ti-Cr-V-Mo</td><td>225</td><td>Multicylinder hydrogen-absorbing alloy tanks (13 L)</td><td>2.5</td><td>—</td><td>75% of capacity reached in 5 min</td><td>For high-pressure hydrogen storage<sup>[560]</sup></td></tr><tr><td>Alloy tank based on Ti-Cr-Mn</td><td>207</td><td>Multicylinder alloy tanks</td><td>2.5-3</td><td>—</td><td>60% of storage capacity reached in 5 min</td><td>For high-pressure hydrogen storage.<sup>[560]</sup></td></tr><tr><td>Ti<sub>25</sub>Cr<sub>50</sub>V<sub>20</sub>Mo<sub>5</sub></td><td>225</td><td>Cylindrical tank</td><td>2.5</td><td>Hydrogen can be supplied at low temperature e.g., -30 °C.</td><td>72% of capacity reached in 5 min</td><td>For fuel cell vehicles<sup>[561]</sup></td></tr><tr><td colspan="7"><strong>Mg-based hydrogen storage material</strong></td></tr><tr><td>MgH<sub>2</sub> doped with 5 wt% TiF<sub>4</sub> and activated carbon</td><td></td><td>Cylindrical tank tightly packed with four beds of MgH<sub>2</sub>-TiF<sub>4</sub>-AC (≈60.5 g)</td><td>4.4</td><td>Hydrogen absorption at 250 °C and 1-1.5 MPa H<sub>2</sub>. Hydrogen desorption at 300 °C with H<sub>2</sub> pressure of 0.1 MPa.</td><td>—</td><td>Gravimetric hydrogen capacity decreasing from 4.46 wt to 3.42-3.62 wt% after 15 cycles<sup>[562]</sup></td></tr></tbody></table>
<table><caption>**Table 5. Continued.**</caption><thead><tr><th>Storage material</th><th>Weight of storage material [kg]</th><th>Design</th><th>Capacity of the tank [wt% H<sub>2</sub>]</th><th>Operation conditions</th><th>Kinetics/cycles</th><th>Purpose/limitations</th></tr></thead><tbody><tr><td>MgH<sub>2</sub></td><td>11.7</td><td>Cylindrical 316 stainless steel tank</td><td>6</td><td>Hydrogen absorption: starting from 90 °C with 0.6 MPa H<sub>2</sub>.<br>Hydrogen desorption: from 370 °C with H<sub>2</sub> pressure of 0.1–0.3 MPa</td><td>50 cycles</td><td>For cheap and high hydrogen storage capacity. Needs effective heat management<sup>[563]</sup></td></tr><tr><td colspan="7">Other alloy materials</td></tr><tr><td>Lm<sub>0.85</sub>Ca<sub>0.15</sub>Ni<sub>5</sub> (Lm: La-rich mischmetal)</td><td>4,3</td><td>Hydrogen tank using four shunted reactors, made of aluminum alloy tubes of 50 mm outer diameter and 3 mm wall thickness</td><td>—</td><td>Equilibrium pressure controlled in the range of 0.8–1.0 MPa at 25 °C</td><td>Full hydrogen release in 120 min</td><td>For electric bicycles<sup>[564]</sup></td></tr><tr><td>Ti<sub>0.9</sub>Zr<sub>0.15</sub>Mn<sub>1.6</sub>Cr<sub>0.2</sub>V<sub>0.2</sub></td><td>2.9</td><td>Hydrogen tank made of an aluminum alloy tube of 60 mm outer diameter and 3.6 mm wall thickness.</td><td>2.1</td><td>Desorption pressure 0.7 MPa at 25 °C</td><td>90% hydrogen released in 60 min</td><td>For electric bicycles<sup>[564]</sup></td></tr><tr><td>Lm<sub>1.06</sub>Ni<sub>4.96</sub>Al<sub>0.04</sub> (Lm: La-rich mischmetal)</td><td>15</td><td>Horizontal vessel divided into compartments by metal plates</td><td>—</td><td>—</td><td>Refueling time: 2 h with 2 MPa H<sub>2</sub></td><td>For hydrogen storage onboard city transit bus<sup>[565]</sup></td></tr><tr><td>Fe<sub>0.9</sub>Mn<sub>0.1</sub>Ti</td><td>—</td><td>Horizontal vessel divided into compartments by metal plates</td><td>—</td><td>50% loading at 50 °C and 0.75 MPa H<sub>2</sub></td><td>90% full in 60 min</td><td>For a commercial John Deere Gator utility vehicle<sup>[565]</sup></td></tr><tr><td>MmNi<sub>5</sub> metal hydride alloy</td><td>85</td><td>Horizontal tank</td><td>—</td><td>Hydrogen absorption 1.5–2.5 MPa<br>Hydrogen desorption 0.5–1 MPa</td><td>Absorption rate of 11.8 NL min<sup>-1</sup><br>Desorption rate of 8.1 NL min<sup>-1</sup></td><td>[566]</td></tr><tr><td>Li–Mg–N–H system</td><td></td><td>Cylindrical tank with heating and cooling jacket</td><td>2.45</td><td>Hydrogen filling pressure 6.7 MPa.<br>Hydrogen release pressure 0.15 MPa</td><td>Maximum filling time 15 min.<br>70% of hydrogen released at 160 °C in 1 h</td><td>Integrated with HT-PEM fuel cell.<sup>[567]</sup></td></tr></tbody></table>

additional elements/compounds that may lead to intermediate hydrogen binding strengths toward room temperature sorption. The modification of the lattice parameters of the hydride or the addition of a catalyst are then used to speed-up the rate of hydrogen sorption. This approach has been central to the emergence of Ni–MH batteries,[439] getters, hydrogen sensors,[440] hydrides for hydrogen purification and compressors,[441] and it remains highly relevant in the current context of energy storage for renewable energy. Indeed, many of the room temperature hydrides that have been discovered have the potential to effectively store renewable energy assuming that their properties can be adapted for this application. This includes the development of: i) advanced manufacturing methods to reduce production cost, ii) methods to improve the initial activation for hydrogen cycling, iii) a better understanding of the materials’ evolution upon repeated hydrogen cycles to ensure long cycling life (>20 000 cycles), iv) stable hydrogen capacity, resistance to oxidation and hydrogen impurities, and v) facile reactivation upon maintenance of the storage tank and associated valving system. The preparation methods of alloys by metallurgical synthesis, mechanical milling or high pressure torsions

and cold rolling[184] can provide new paths to achieve an easy manufacturing process. Better control over the properties of high entropy alloys could also help in the design of alloys with higher hydrogen storage capacities, i.e., >2 mass%.[442]

From this basic understanding of the hydrogen behavior in metals, novel ideas have also appeared toward the design of new hydrogen storage materials mainly based on light elements and complex hydrides. This has been particularly facilitated by the emergence of more comprehensive mechanochemistry tools, the approach of chemical destabilization and more recently nanosizing. Destabilization has the potential to lead to new forms of hydrides with adequate properties, assuming that the adjustment of a particular hydride’s hydrogen storage properties is not done at the expense of the storage capacity. This loss of hydrogen storage capacity currently remains the main barrier as the approach has demonstrated its potential on many hydrides and in particular the amide systems.[443] New ideas to achieve chemical destabilization without affecting the final hydrogen storage capacity also need to emerge as well as strategies to maintain a long cycling life and avoid hydrogen uptake/release alongside reaction paths.
Depending on the hydride of interest, a known approach cannot simply be translated to another hydride to improve its hydrogen storage properties, and this remains one of the major difficulties in the field. To date, there are no general rules that can be used to unify the behavior of hydride materials. Even, within a single family, e.g., borohydrides, different hydrogen behaviors are observed. Such diversity significantly hinders the translation of concepts from one system to the other, and this currently limits progress. In particular, light complex hydrides, at the basis of future applications including vehicles, require the development of a new fundamental understanding to grasp this complexity. New thinking along the control of ionic transport in complex hydrides is needed in addition to effective concepts to release/absorb hydrogen along predetermined thermodynamic paths. Understanding of the hydrogen interaction in ionic hydrides and the stabilization of "active" forms of light elements resulting from the decomposition of the complex hydrides is also needed to achieve low temperature hydrogen sorption.

The approach of nanosizing could potentially help to establish general rules since the behavior of hydrides at the nanoscale seems to be more consistent. Through a nanosizing approach both the thermodynamics and kinetics of hydrides can be modified at the same time and this offers new opportunities to effectively design hydrogen storage materials. However, one of the challenges is in the synthesis and accurate characterization of the properties of nanosized hydrides with respect to hydrogen. Current synthetic methods are not well established in particular in the making of nanosized hydrides stable upon successive hydrogen cycles, although some progress has been made over the last few years.[126b,186] Precise control in the synthetic approach is often overlooked, but this is a prerequisite for the exact determination of the behavior of hydrogen in the nanostructures without additional artefacts due to the oxidation, degradation, and partial reaction of the confined hydride within the host material. Beyond this, the effective hydrogen storage capacity achievable by the nanosizing approach is also limited because of the dead mass of the mesoporous host material nanoconfining the hydrides. As discussed above, this is currently a recurring problem that must be tackled through radical concepts. In this respect, theoretical modeling can significantly help but alternative methods to run more complex calculations to determine enthalpy and entropy evolutions across length scales as well as ways to take into account the chemical environment of the hydrides must emerge.

New thinking is also needed in the way hydrogen is loaded and unloaded in materials. Currently, the preferred method relies on variations in hydrogen pressure and temperature to shift the equilibrium plateau to the required pressure. This not only complicates the implementation of materials in tanks but also brings additional engineering challenges for the heat management of the hydride bed to ensure reasonable charging/discharging times. If alternative methods could be developed where the loading/unloading of hydrogen in the hydride bed proceeded without significant heat evolution, tank engineering would be simplified and hydrogen storage capacities at the system level improved. We have started to work on alternatives

where the plasmonic effect of Au can be used to locally control the release of hydrogen.[38,444] Although this solution is not viable for large solid state storage tanks, it may provide a path for a simpler design of small, portable and light hydrogen storage devices.

To date, the early vision of a hydrogen economy is more than ever pertinent. Enabling the use of hydrogen as the universal energy vector will be disruptive but also benefit society from multiple perspectives including new means for many to locally access energy. Technologies for water electrolysis and fuel cells are market ready and further reduction in cost will accelerate their uptake. Hydrogen vehicles are now a commercial reality, and this coupled with the sharp price fall of solar panels are undoubtedly the first enablers of a transition to the hydrogen economy. The challenge for the scientific community is to facilitate this transition by accelerating the pace of thinking along the behavior of hydrogen in materials, in particular those composed of light elements, and by unlocking creativity in the design of new strategies and approaches to synthesize, control and engineer hydrogen storage materials.

## Acknowledgements

Financial support by UNSW Internal Research Grant program is gratefully acknowledged along with the Office of Naval Research (Award No: ONRG-NICOP-N62909-16-1-2155).

## Conflict of Interest

The authors declare no conflict of interest.

## Keywords

complex hydrides, hydrogen storage, metal hydrides

Received: March 22, 2019

Revised: May 11, 2019

Published online: June 26, 2019

<footnote>[1] a) S. Brenner, *Science* **1998**, *282*, 1411; b) S. Şener, E. Sarıdoğan, *Procedia—Soc. Behav. Sci.* **2011**, *24*, 815.</footnote>

<footnote>[2] M. F. Perutz, in *Science and Technology Education and Future Human Needs* (Eds: J. L. Lewis, P. J. Kelly), Pergamon, Exeter **1987**, pp. 17–56.</footnote>

<footnote>[3] G. Kallis, C. Kerschner, J. Martinez-Alier, *Ecol. Econ.* **2012**, *84*, 172.</footnote>

<footnote>[4] Ö. Esen, M. Bayrak, *J. Econ. Financ. Admin. Serv.* **2017**, *22*, 75.</footnote>

<footnote>[5] A. Züttel, A. Remhof, A. Borgschulte, O. Friedrichs, *Philos. Trans. R. Soc.*, A **2010**, *368*, 3329.</footnote>

<footnote>[6] a) I. Ridjan, B. V. Mathiesen, D. Connolly, N. Duic, *Energy* **2013**, *57*, 76; b) S. H. Jensen, P. H. Larsen, M. Mogensen, *Int. J. Hydrogen Energy* **2007**, *32*, 3253.</footnote>

<footnote>[7] J. B. S. Haldane, *Daedalus, or, Science and the Future*, Kegan Paul, Trench, Trubner & Co, Cambridge **1923**.</footnote>

<footnote>[8] M. Hirscher, *Handbook of Hydrogen Storage: New Materials for Future Energy Storage*, Wiley-VCH Verlag GmbH & Co, Anguilla **2009**.</footnote>
[9] C. Yang, J. Ogden, *Int. J. Hydrogen Energy* **2007**, *32*, 268.

[10] E. Akiba, H. Iba, *Intermetallics* **1998**, *6*, 461.

[11] D. P. Broom, *Hydrogen Storage Materials: The Characterisation of Their Storage Properties*, Springer Science & Business Media, London **2011**.

[12] a) J. M. Carrasco, L. G. Franquelo, J. T. Bialasiewicz, E. Galván, R. C. PortilloGuisado, M. M. Prats, J. I. León, N. Moreno-Alfonso, *IEEE Trans. Ind. Electron.* **2006**, *53*, 1002; b) T. Zhou, B. François, *IEEE Trans. Ind. Electron.* **2011**, *58*, 95.

[13] a) S. M. Schoenung, W. V. Hassenzahl, *Long-vs. Short-Term Energy Storage Technologies Analysis: A Life-Cycle Cost Study: A study for the DOE Energy Storage Systems Program*, Sandia National Laboratories, Albuquerque and Livermore **2003**; b) B. D. James, D. A. DeSantis, *Manufacturing Cost and Installed Price—Analysis of Stationary Fuel Cell Systems, Strategic Analysis Inc.* Arlington VA **2015**; c) S. Satyapal, J. Petrovic, C. Read, G. Thomas, G. Ordaz, *Catal. Today* **2007**, *120*, 246; d) M. P. de Wit, A. P. C. Faaij, *Int. J. Hydrogen Energy* **2007**, *32*, 4859; e) G. Gahleitner, *Int. J. Hydrogen Energy* **2013**, *38*, 2039; f) C. Darras, M. Muselli, P. Poggi, C. Voyant, J. C. Hoguet, F. Montignac, *Int. J. Hydrogen Energy* **2012**, *37*, 14015; g) C. Ziogou, D. Ipsakis, C. Elmasides, F. Stergiopoulos, S. Papadopoulou, P. Seferlis, S. Voutetakis, *J. Power Sources* **2011**, 196, 9488; h) L. Ntziachristos, C. Kouridis, Z. Samaras, K. Pattas, *Renewable Energy* **2005**, *30*, 1471.

[14] A. P. Côté, A. I. Benin, N. W. Ockwig, M. O'Keeffe, A. J. Matzger, O. M. Yaghi, *Science* **2005**, *310*, 1166.

[15] K. M. Thomas, *Dalton Trans.* **2009**, 1487, 1505.

[16] a) Q. Lai, M. Paskevicius, D. A. Sheppard, C. E. Buckley, A. W. Thornton, M. R. Hill, Q. Gu, J. Mao, Z. Huang, H. K. Liu, Z. Guo, A. Banerjee, S. Chakraborty, R. Ahuja, K.-F. Aguey-Zinsou, *ChemSusChem* **2015**, *8*, 2789; b) Y. B. He, F. L. Chen, B. Li, G. D. Qian, W. Zhou, B. L. Chen, *Coord. Chem. Rev.* **2018**, *373*, 167.

[17] L. Morris, J. J. Hales, M. L. Trudeau, P. Georgiev, J. P. Embs, J. Eckert, N. Kaltsoyannis, D. M. Antonelli, *Energy Environ. Sci.* **2019**, *12*, 1580.

[18] a) P. Preuster, C. Papp, P. Wasserscheid, *Acc. Chem. Res.* **2017**, *50*, 74; b) T. He, P. Pachfule, H. Wu, Q. Xu, P. Chen, *Nat. Rev. Mater.* **2016**, *1*, 16059; c) N. Onishi, G. Laurenczy, M. Beller, Y. Himeda, *Coord. Chem. Rev.* **2018**, *373*, 317.

[19] L. Klebanoff, *Hydrogen Storage Technology: Materials and Applications*, CRC Press, Boca Raton **2013**.

[20] a) A. J. Maeland, A. F. Andersen, *J. Chem. Phys.* **1968**, *48*, 4660; b) K. Christmann, *Surf. Sci. Rep.* **1988**, *9*, 1.

[21] D. A. Hutchinson, *Can. J. Chem.* **1966**, *44*, 2711.

[22] W. Grochala, P. P. Edwards, *Chem. Rev.* **2004**, *104*, 1283.

[23] K. H. J. Buschow, P. C. P. Bouten, A. R. Miedema, *Rep. Prog. Phys.* **1982**, *45*, 937.

[24] a) M. G. Hu, R. A. Geanangel, W. W. Wendlandt, *Thermochim. Acta* **1978**, *23*, 249; b) M. Bowden, T. Autrey, I. Brown, M. Ryan, *Curr. Appl. Phys.* **2008**, *8*, 498; c) Q. Lai, A. Rawal, M. Z. Quadir, C. Cazorla, U. B. Demirci, K.-F. Aguey-Zinsou, *Adv. Sustainable Sys.* **2018**, *2*, 1700122.

[25] G. Sandrock, K. Gross, G. Thomas, C. Jensen, D. Meeker, S. Takara, *J. Alloys Compd.* **2002**, 330-332, 696.

[26] G. Sandrock, *J. Alloys Compd.* **1999**, 293-295, 877.

[27] a) A. Schneemann, J. L. White, S. Kang, S. Jeong, L. F. Wan, E. S. Cho, T. W. Heo, D. Prendergast, J. J. Urban, B. C. Wood, M. D. Allendorf, V. Stavila, *Chem. Rev.* **2018**, *118*, 10775; b) I. Sreedhar, K. M. Kamani, B. M. Kamani, B. M. Reddy, A. Venugopal, *Renewable Sustainable Energy Rev.* **2018**, *91*, 838; c) S. Garroni, A. Santoru, H. J. Cao, M. Dornheim, T. Klassen, C. Milanese, F. Gennari, C. Pistidda, *Energies* **2018**, *11*, 1027; d) L. Pasquini, *Crystals* **2018**, *8*, 106; e) V. Bérubé, G. Radtke, M. Dresselhaus, G. Chen, *Int. J. Energy Res.* **2007**, *31*, 637; f) A. Pundt, R. Kirchheim, *Annu. Rev. Mater. Res.* **2006**, *36*, 555; g) G. Walker, *Solid-State Hydrogen Storage: Materials and Chemistry*, Elsevier, Cambridge **2008**; h) S. Qiu, H. Chu, Y. Zou, C. Xiang, F. Xu, L. Sun, *J. Mater. Chem. A* **2017**, *5*, 25112; i) R. A. Varin, T. Czujko, Z. S. Wronski, *Nanomaterials for Solid State Hydrogen Storage*, Springer Science & Business Media, New York **2009**; j) H. Wipf, *Hydrogen in Metals III: Properties and Applications*, Springer, Berlin **1997**; k) G. AlefeldJohann, J. Völkl, *Hydrogen in Metals I*, Springer, Berlin **1978**; l) G. Alefeld, J. Völkl, *Hydrogen in Metals II*, Springer-Verlag, Berlin **1978**; m) Y. Fukai, *The Metal-Hydrogen System: Basic Bulk Properties*, 2nd rev. and updated ed., Springer, Berlin **2005**.

[28] J. A. Menéndez, A. Arenillas, B. Fidalgo, Y. Fernández, L. Zubizarreta, E. G. Calvo, J. M. Bermúdez, *Fuel Process. Technol.* **2010**, *91*, 1.

[29] A. Zlotorzynski, *Crit. Rev. Anal. Chem.* **1995**, *25*, 43.

[30] a) Y. Nakamori, S.-I. Orimo, T. Tsutaoka, *Appl. Phys. Lett.* **2006**, *88*, 112104; b) M. Matsuo, Y. Nakamori, K. Yamada, S.-I. Orimo, *Appl. Phys. Lett.* **2007**, *90*, 232907; c) M. Matsuo, Y. Nakamori, K. Yamada, T. Tsutaoka, S. Orimo, *Adv. Mater. Res.* **2008**, *10*, 157.

[31] J. R. Ares, F. Leardini, P. Díaz-Chao, J. Bodega, J. F. Fernández, I. J. Ferrer, C. Sánchez, *Ultrason. Sonochem.* **2009**, *16*, 810.

[32] M. Schwickert, E. Carpene, K. P. Lieb, M. Uhrmacher, P. Schaaf, H. Gibhardt, *Phys. Scr.* **2004**, *T108*, 113.

[33] *Executive Summary of Laser Hydrides-Laser Induced Adsorption of Hydrogen from Magnesium Hydride*, P. K. Corporation, Scottsdale **2008**.

[34] a) J. G. McCaffrey, J. M. Parnis, G. A. Ozin, W. Breckenridge, *J. Phys. Chem.* **1985**, *89*, 4945; b) D. Dougherty, P. J. Herley, *J. Less Common Met.* **1980**, *73*, 97.

[35] P. J. Herley, D. H. Spencer, *J. Phys. Chem.* **1979**, *83*, 1701.

[36] a) P. J. Herley, O. Christofferson, *J. Phys. Chem.* **1981**, *85*, 1887; b) I. E. Gabis, A. P. Voyt, I. A. Chernov, V. G. Kuznetsov, A. P. Baraban, D. I. Elets, M. A. Dobrotvorsky, *Int. J. Hydrogen Energy* **2012**, *37*, 14405; c) P. Herley, O. Christofferson, *J. Phys. Chem.* **1981**, *85*, 1882.

[37] P. Herley, P. Levy, *J. Chem. Phys.* **1967**, *46*, 627.

[38] Y. Sun, K. F. Aguey-Zinsou, *ChemPlusChem* **2018**, *83*, 904.

[39] P. Verma, Y. Kuwahara, K. Mori, H. Yamashita, *Chem. - Eur. J.* **2017**, *23*, 3616.

[40] a) M. Matuschek, D. P. Singh, H.-H. Jeong, M. Nesterov, T. Weiss, P. Fischer, F. Neubrech, N. Liu, *Small* **2018**, *14*, 1702990; b) W. L. Watkins, Y. Borensztein, *Sens. Actuators, B* **2018**, *273*, 527; c) F. A. A. Nugroho, I. Darmadi, V. P. Zhdanov, C. Langhammer, *ACS Nano* **2018**, *12*, 9903.

[41] Y. Zhu, C. Yang, J. Zhu, L. Li, *J. Alloys Compd.* **2011**, *509*, 5309.

[42] a) F. Cuevas, J. M. Joubert, M. Latroche, A. Percheron-Guégan, *Appl. Phys. A: Mater. Sci. Process.* **2001**, *72*, 225; b) J. J. G. Willems, *Philips J. Res.* **1984**, *39*, 1.

[43] a) B. Panella, M. Hirscher, *Adv. Mater.* **2005**, *17*, 538; b) Y. Jia, C. Sun, Y. Peng, W. Fang, X. Yan, D. Yang, J. Zou, S. S. Mao, X. Yao, *J. Mater. Chem. A* **2015**, *3*, 8294.

[44] K.-F. Aguey-Zinsou, J.-R. Ares-Fernandez, *Energy Environ. Sci.* **2010**, *3*, 526.

[45] M. G. Nijkamp, J. Raaymakers, A. J. Van Dillen, K. P. De Jong, *Appl. Phys. A: Mater. Sci. Process.* **2001**, *72*, 619.

[46] a) J. Huot, G. Liang, S. Boily, A. Van Neste, R. Schulz, *J. Alloys Compd.* **1999**, 293-295, 495; b) K. F. Aguey-Zinsou, J. R. A. Fernandez, T. Klassen, R. Bormann, *Int. J. Hydrogen Energy* **2007**, *32*, 2400; c) K. F. Aguey-Zinsou, J. Ares-Fernandez, T. Klassen, R. Bormann, *Int. J. Hydrogen Energy* **2007**, *32*, 2400.

[47] C. Nishimura, M. Komaki, M. Amano, *J. Alloys Compd.* **1999**, 293-295, 329.

[48] J. Philibert, *Defect Diffus. Forum* **2006**, 249, 61.

[49] C. Wert, C. Zener, *Phys. Rev.* **1949**, *76*, 1169.
[50] a) J. G. P. Binner, N. A. Hassine, T. E. Cross, *J. Mater. Sci.* **1995**, *30*, 5389; b) J. Vilčáková, P. Sáha, V. Křesálek, O. Quadrat, *Synth. Met.* **2000**, *113*, 83; c) K. Park, D. Lee, A. Rai, D. Mukherjee, M. R. Zachariah, *J. Phys. Chem. B* **2005**, *109*, 7290.

[51] N. Katada, S. Sota, N. Morishita, K. Okumura, M. Niwa, *Catal. Sci. Technol.* **2015**, *5*, 1864.

[52] a) J. Huot, J. F. Pelletier, L. B. Lurio, M. Sutton, R. Schulz, *J. Alloys Compd.* **2003**, *348*, 319; b) G. Liang, J. Huot, S. Boily, A. Van Neste, R. Schulz, *J. Alloys Compd.* **1999**, *291*, 295.

[53] a) N. S. Norberg, T. S. Arthur, S. J. Fredrick, A. L. Prieto, *J. Am. Chem. Soc.* **2011**, *133*, 10679; b) P. Heitjans, S. Indris, *J. Mater. Sci.* **2004**, *39*, 5091; c) S. D. Benedetto, M. Carewska, C. Cento, P. Gislon, M. Pasquali, S. Scaccia, P. P. Prosini, *Thermochim. Acta* **2006**, *441*, 184.

[54] J. Wilhelm, *Z. Anorg. Allg. Chem.* **1927**, *163*, 1.

[55] a) W. A. Johnson, R. F. Mehl, *Trans. Metall. Soc. AIME* **1939**, *135*, 396; b) M. Avrami, *J. Chem. Phys.* **1940**, *8*, 212; c) M. Avrami, *J. Chem. Phys.* **1941**, *9*, 177; d) M. Avrami, *J. Chem. Phys.* **1939**, *7*, 1103.

[56] a) K.-C. Chou, Q. Li, Q. Lin, L.-J. Jiang, K.-D. Xu, *Int. J. Hydrogen Energy* **2005**, *30*, 301; b) M. Martin, C. Gommel, C. Borkhart, E. Fromm, *J. Alloys Compd.* **1996**, *238*, 193; c) Y. Pang, Q. Li, *Int. J. Hydrogen Energy* **2016**, *41*, 18072; d) G. Stepura, V. Rosenband, A. Gany, *J. Alloys Compd.* **2012**, *513*, 159.

[57] a) F. J. Castro, G. Meyer, *J. Alloys Compd.* **2002**, 330–332, 59; b) E. A. Evard, I. E. Gabis, A. P. Voyt, *J. Alloys Compd.* **2005**, 404–406, 335.

[58] G. Liang, J. Huot, S. Boily, A. Van Neste, R. Schulz, *J. Alloys Compd.* **1999**, *292*, 247.

[59] N. Hanada, T. Ichikawa, H. Fujii, *J. Phys. Chem. B* **2005**, *109*, 7188.

[60] M. Pourabdoli, S. Raygan, H. Abdizadeh, D. Uner, *Int. J. Hydrogen Energy* **2013**, *38*, 11910.

[61] a) A. Borgschulte, M. O. Jones, E. Callini, B. Probst, S. Kato, A. Zuttel, W. I. F. David, S. Orimo, *Energy Environ. Sci.* **2012**, *5*, 6823; b) S. Nayebossadri, K. F. Aguey-Zinsou, *Phys. Chem. Chem. Phys.* **2011**, *13*, 17683.

[62] a) L. L. Shaw, W. Osborn, T. Markmaitree, X. Wan, *J. Power Sources* **2008**, *177*, 500; b) G. Amica, F. Cova, P. Arneodo Larochette, F. C. Gennari, *Int. J. Hydrogen Energy* **2017**, *42*, 6127.

[63] B. Bogdanović, M. Felderhoff, S. Kaskel, A. Pommerin, K. Schlichte, F. Schüth, *Adv. Mater.* **2003**, *15*, 1012.

[64] J. M. Bellosta von Colbe, W. Schmidt, M. Felderhoff, B. Bogdanović, F. Schüth, *Angew. Chem., Int. Ed.* **2006**, *45*, 3663.

[65] a) B. Bogdanović, R. A. Brand, A. Marjanović, M. Schwickardi, J. Tölle, *J. Alloys Compd.* **2000**, *302*, 36; b) G. J. Thomas, K. J. Gross, N. Y. C. Yang, C. Jensen, *J. Alloys Compd.* **2002**, 330–332, 702; c) M. Felderhoff, K. Klementiev, W. Grunert, B. Spliethoff, B. Tesche, J. M. Bellosta von Colbe, B. Bogdanovic, M. Hartel, A. Pommerin, F. Schuth, C. Weidenthaler, *Phys. Chem. Chem. Phys.* **2004**, *6*, 4369.

[66] F. Schuth, B. Bogdanovic, M. Felderhoff, *Chem. Commun.* **2004**, 2249.

[67] a) H. Gunaydin, K. N. Houk, V. Ozoliņš, *Proc. Natl. Acad. Sci. USA* **2008**, *105*, 3673; b) G. K. P. Dathar, D. S. Mainardi, *J. Phys. Chem. C* **2010**, *114*, 8026.

[68] G. Sandrock, K. Gross, G. Thomas, *J. Alloys Compd.* **2002**, *339*, 299.

[69] E. G. Maksimov, O. A. Pankratov, *Phys. Usp.* **1975**, *18*, 481.

[70] a) G. Schaumann, J. Völki, G. Alefeld, *Phys. Status Solidi B* **1970**, *42*, 401; b) R. L. Corey, T. M. Ivancic, D. T. Shane, E. A. Carl, R. C. Bowman, J. M. Bellosta von Colbe, M. Dornheim, R. Bormann, J. Huot, R. Zidan, A. C. Stowe, M. S. Conradi, *J. Phys. Chem. C* **2008**, *112*, 19784.

[71] O. Palumbo, R. Cantelli, A. Paolone, C. M. Jensen, S. S. Srinivasan, *J. Alloys Compd.* **2005**, 404–406, 748.

[72] R. Hempelmann, *J. Less-Common Met.* **1984**, *101*, 69.

[73] S. Hao, D. S. Sholl, *Appl. Phys. Lett.* **2008**, *93*, 251901.

[74] D. G. Hunt, D. K. Ross, *J. Less-Common Met.* **1976**, *49*, 169.

[75] J. Bergsma, J. A. Goedkoop, *Physica* **1960**, *26*, 744.

[76] E. Wicke, H. Brodowsky, H. Züchner, in *Hydrogen in Metals II: Application-Oriented Properties* (Eds: G. Alefeld, J. Völkl), Springer, Berlin **1978**, pp. 73–155.

[77] T. L. Hill, *An Introduction to Statistical Thermodynamics*, Dover Publication, Inc., New York **1986**.

[78] D. W. Taylor, *Phys. Rev. B* **1980**, *21*, 5096.

[79] E. Wimmer, W. Wolf, J. Sticht, P. Saxe, C. B. Geller, R. Najafabadi, G. A. Young, *Phys. Rev. B* **2008**, *77*, 134305.

[80] C. Wagner, *Acta Metall.* **1971**, *19*, 843.

[81] a) O. J. Kleppa, P. Dantzer, M. E. Melnichak, *J. Chem. Phys.* **1974**, *61*, 4048; b) R. Johansson, R. Ahuja, O. Eriksson, B. Hjörvarsson, R. H. Scheicher, *Sci. Rep.* **2015**, *5*, 10301.

[82] a) V. V. Sumin, H. Wipf, B. Coluzzi, A. Biscarini, R. Campanella, G. Mazzolai, F. M. Mazzolai, *J. Alloys Compd.* **2001**, *316*, 189; b) A. Blomqvist, G. K. Pálsson, C. M. Araújo, R. Ahuja, B. Hjörvarsson, *Phys. Rev. Lett.* **2010**, *105*, 185901.

[83] A. Driessen, P. Sanger, H. Hemmes, R. Griessen, *J. Phys.: Condens. Matter* **1990**, *2*, 9797.

[84] Y.-L. Liu, Y. Zhang, G. N. Luo, G.-H. Lu, *J. Nucl. Mater.* **2009**, *390–391*, 1032.

[85] G. M. Evgenii, O. A. Pankratov, *Phys. Usp.* **1975**, *18*, 481.

[86] R. Khoda-Bakhsh, D. K. Ross, *J. Phys. F: Met. Phys.* **1982**, *12*, 15.

[87] V. A. Somenkov, *Ber. Bunsen-Ges. Phys. Chem.* **1972**, *76*, 733.

[88] L. R. Lichty, J. W. Han, R. Ibanez-Meier, D. R. Torgeson, R. G. Barnes, E. F. W. Seymour, C. A. Sholl, *Phys. Rev. B* **1989**, *39*, 2012.

[89] I. S. Anderson, J. J. Rush, T. Udovic, J. M. Rowe, *Phys. Rev. Lett.* **1986**, *57*, 2822.

[90] O. Blaschko, G. Krexner, L. Pintschovius, P. Vajda, J. N. Daou, *Phys. Rev. B* **1988**, *38*, 9612.

[91] V. Koteski, J. Belosevic-Cavor, K. Batalovic, J. Radakovic, A. Umicevic, *RSC Adv.* **2015**, *5*, 34894.

[92] J. Zhang, H. Qu, S. Yan, G. Wu, X. F. Yu, P. Peng, *J. Alloys Compd.* **2017**, *694*, 687.

[93] a) J. L. Souquet, *Annu. Rev. Mater. Sci.* **1981**, *11*, 211; b) K. Funke, *Solid State Ionics* **1986**, 18–19, 183.

[94] R. Kirchheim, T. Mütschele, W. Kieninger, H. Gleiter, R. Birringer, T. D. Koblé, *Mater. Sci. Eng.* **1988**, *99*, 457.

[95] a) K. J. Gross, S. Guthrie, S. Takara, G. Thomas, *J. Alloys Compd.* **2000**, *297*, 270; b) W. Lohstroh, M. Fichtner, *Phys. Rev. B* **2007**, *75*, 184106.

[96] P. Martelli, A. Remhof, A. Borgschulte, P. Mauron, D. Wallacher, E. Kemner, M. Russina, F. Pendolino, A. Züttel, *J. Phys. Chem. A* **2010**, *114*, 10117.

[97] C. S. Sunandana, P. S. Kumar, *Bull. Mater. Sci.* **2004**, *27*, 1.

[98] a) R. P. Buck, *Sens. Actuators* **1981**, *1*, 137; b) J. B. Goodenough, *Annu. Rev. Mater. Res.* **2003**, *33*, 91.

[99] T. Mezaki, Y. Kuronuma, I. Oikawa, A. Kamegawa, H. Takamura, *Inorg. Chem.* **2016**, *55*, 10484.

[100] J. C. Bachman, S. Muy, A. Grimaud, H.-H. Chang, N. Pour, S. F. Lux, O. Paschos, F. Maglia, S. Lupart, P. Lamp, L. Giordano, Y. Shao-Horn, *Chem. Rev.* **2016**, *116*, 140.

[101] a) S. Hoshino, *Solid State Ionics* **1991**, *48*, 179; b) C. M. Araújo, A. Blomqvist, R. H. Scheicher, P. Chen, R. Ahuja, *Phys. Rev. B* **2009**, *79*, 172101.

[102] A. Borgschulte, A. Jain, A. J. Ramirez-Cuesta, P. Martelli, A. Remhof, O. Friedrichs, R. Gremaud, A. Züttel, *Faraday Discuss.* **2011**, *151*, 213.

[103] a) K. Jimura, S. Hayashi, *J. Phys. Chem. C* **2012**, *116*, 4883; b) A. V. Skripov, A. V. Soloninin, O. A. Babanova, R. V. Skoryunov, *J. Alloys Compd.* **2015**, *645*, S428; c) A. V. Skripov, A. V. Soloninin,
M. B. Ley, T. R. Jensen, Y. Filinchuk, *J. Phys. Chem. C* **2013**, *117*, 14965.

[104] R. L. Corey, D. T. Shane, R. C. Bowman, M. S. Conradi, *J. Phys. Chem. C* **2008**, *112*, 18706.

[105] a) M. Matsuo, Y. Nakamori, S.-I. Orimo, H. Maekawa, H. Takamura, *Appl. Phys. Lett.* **2007**, *91*, 224103; b) A. V. Soloninin, A. V. Skripov, A. L. Buzlukov, A. P. Stepanov, *J. Solid State Chem.* **2009**, *182*, 2357.

[106] N. Verdal, T. J. Udovic, J. J. Rush, H. Wu, A. V. Skripov, *J. Phys. Chem. C* **2013**, *117*, 12010.

[107] a) W. Li, G. Wu, Z. Xiong, Y. P. Feng, P. Chen, *Phys. Chem. Chem. Phys.* **2012**, *14*, 1596; b) L. Q. He, H. J. Lin, H. F. Li, Y. Filinchuk, J. J. Zhang, Y. Liu, M. Y. Yang, Y. Hou, Y. H. Deng, H. W. Li, H. Y. Shao, L. P. Wang, Z. G. Lu, *J. Power Sources* **2018**, *396*, 574; c) S. Kim, N. Toyama, H. Oguchi, T. Sato, S. Takagi, T. Ikeshoji, S. I. Orimo, *Chem. Mater.* **2018**, *30*, 386; d) H. Wang, H. J. Cao, W. J. Zhang, J. Chen, H. Wu, C. Pistidda, X. H. Ju, W. Zhou, G. T. Wu, M. Etter, T. Klassen, M. Dornheim, P. Chen, *Chem. - Eur. J.* **2018**, *24*, 1342; e) Z. P. Yao, S. Kim, K. Michel, Y. S. Zhang, M. Aykol, C. Wolverton, *Phys. Rev. Mater.* **2018**, *2*, 065402; f) R. Mohtadi, S.-I. Orimo, *Nat. Rev. Mater.* **2017**, *2*, 16091.

[108] M. Matsuo, S.-I. Orimo, *Adv. Energy Mater.* **2011**, *1*, 161.

[109] M. Matsuo, A. Remhof, P. Martelli, R. Caputo, M. Ernst, Y. Miura, T. Sato, H. Oguchi, H. Maekawa, H. Takamura, A. Borgschulte, A. Züttel, S.-I. Orimo, *J. Am. Chem. Soc.* **2009**, *131*, 16389.

[110] a) A. Holt, E. C. Edgar, J. B. Firth, *Z. Phys. Chem.* **1913**, *82U*, 513; b) E. Wicke, J. Blaurock, *J. Less-Common Met.* **1987**, *130*, 351.

[111] a) H. Inui, T. Yamamoto, M. Hirota, M. Yamaguchi, *J. Alloys Compd.* **2002**, *330–332*, 117; b) J. M. Joubert, M. Latroche, R. Černý, A. Percheron-Guégan, K. Yvon, *J. Alloys Compd.* **2002**, *330–332*, 208.

[112] J. J. Vajo, S. L. Skeith, F. Mertens, *J. Phys. Chem. B* **2005**, *109*, 3719.

[113] J. F. Mao, X. B. Yu, Z. P. Guo, H. K. Liu, Z. Wu, J. Ni, *J. Alloys Compd.* **2009**, *479*, 619.

[114] R. M. van Essen, K. H. J. Buschow, *Mater. Res. Bull.* **1980**, *15*, 1149.

[115] T. B. Flanagan, *Interstitial Intermetallic Alloys*, Vol. 281, Springer, Netherlands **1994**.

[116] a) C. Złotea, M. Latroche, *Colloids Surf. A* **2013**, *439*, 117; b) A. Pundt, *Adv. Eng. Mater.* **2004**, *6*, 11.

[117] A. Kianvash, I. R. Harris, *J. Mater. Sci.* **1985**, *20*, 682.

[118] a) S.-I. Orimo, Y. Nakamori, J. R. Eliseo, A. Züttel, C. M. Jensen, *Chem. Rev.* **2007**, *107*, 4111; b) B. Sakintuna, F. Lamari-Darkrim, M. Hirscher, *Int. J. Hydrogen Energy* **2007**, *32*, 1121.

[119] J. J. Reilly Jr., R. H. Wiswall Jr., *Inorg. Chem.* **1967**, *6*, 2220.

[120] A. Andreasen, M. Sørensen, R. Burkarl, B. Møller, A. Molenbroek, A. S. Pedersen, J. W. Andreasen, M. M. Nielsen, T. R. Jensen, *J. Alloys Compd.* **2005**, *404–406*, 323.

[121] S. Bouaricha, J. P. Dodelet, D. Guay, J. Huot, S. Boily, R. Schulz, *J. Alloys Compd.* **2000**, *297*, 282.

[122] J. R. Ares, K. F. Aguey-Zinsou, F. Leardini, I. J. Ferrer, J. F. Fernandez, Z. X. Guo, C. Sanchez, *J. Phys. Chem. C* **2009**, *113*, 6845.

[123] J. Yang, S. Hirano, *Adv. Mater.* **2009**, *21*, 3023.

[124] M. Fichtner, O. Fuhr, O. Kircher, *J. Alloys Compd.* **2003**, *356–357*, 418.

[125] S. Yamaguchi, H. Miyaoka, T. Ichikawa, Y. Kojima, *Int. J. Hydrogen Energy* **2017**, *42*, 5213.

[126] a) Y. Nakamori, G. Kitahara, S. Orimo, *J. Power Sources* **2004**, *138*, 309; b) Y. Sun, C. Shen, Q. Lai, W. Liu, D.-W. Wang, K.-F. Aguey-Zinsou, *Energy Storage Mater.* **2018**, *10*, 168.

[127] A. Züttel, P. Wenger, S. Rentsch, P. Sudan, P. Mauron, C. Emmenegger, *J. Power Sources* **2003**, *118*, 1.

[128] T. Graham, *J. Chem. Soc.* **1869**, 22, 419.

[129] A. Kagawa, E. Ono, T. Kusakabe, Y. Sakamoto, *J. Less-Common Met.* **1991**, 172-174, 64.

[130] T. Nambu, H. Ezaki, M. Takagi, H. Yukawa, M. Morinaga, *J. Alloys Compd.* **2002**, *330-332*, 318.

[131] S. Bowerman Biays, C. A. Wulff, B. Flanagan Ted, *Z. Phys. Chem.* **1979**, *116*, 197.

[132] a) A. R. Miedema, P. F. de Châtel, F. R. de Boer, *Cohesion in Alloys—Fundamentals of a Semi-Empirical Model*. *Physica B+C* **1980**, *100*, 1; b) M. Dornheim, in *Thermodynamics—Interaction Studies—Solids, Liquids and Gases* (Ed: J. C. Moreno-Pirajan), InTech, Rijeka **2011**.

[133] R. Griessen, T. Riesterer, in *Hydrogen in Intermetallic Compounds I*, Springer, Berlin **1988**, pp. 219–284.

[134] A. Djellouli, K. Benyelloul, H. Aourag, S. Bekhechi, A. Adjadj, Y. Bouhadda, O. Elkedim, *Int. J. Hydrogen Energy* **2018**, *43*, 19111.

[135] a) M. H. Mintz, Z. Hadari, *J. Appl. Phys.* **1981**, *52*, 463; b) F. A. Kuijpers, H. H. van Mal, *J. Less-Common Met.* **1971**, *23*, 395.

[136] J. J. Reilly Jr., R. H. Wiswall Jr., *Inorg. Chem.* **1968**, *7*, 2254.

[137] a) N. Terashita, K. Kobayashi, T. Sasai, E. Akiba, *J. Alloys Compd.* **2001**, *327*, 275; b) N. Hanada, S.-I. Orimo, H. Fujii, *J. Alloys Compd.* **2003**, *356–357*, 429.

[138] H. H. Van Mal, K. H. J. Buschow, F. A. Kuijpers, *J. Less-Common Met.* **1973**, *32*, 289.

[139] T. Gamo, Y. Moriwaki, N. Yanagihara, T. Yamashita, T. Iwaki, *Int. J. Hydrogen Energy* **1985**, *10*, 39.

[140] a) W. G. Moffat, *The Handbook of Binary Phase Diagrams*, General Electric Company, Corporate Research and Development, Shenecdaty, NY **1977**; b) T. B. Massalski, *Binary Alloy Phase Diagrams*, ASM International, **1990**.

[141] a) Z. Q. Zhou, H. Y. Zhang, D. Tang, in *6th World Hydrogen Energy Conference, Vol. Hydrogen Energy Progress VI* (Eds: T. N. Veziroğlu, N. Getoff, P. Weinzierl), Pergamon Press, Vienna, Austria **1986**; b) G. D. Sandrock, *Proc. 12th Intersociety Energy Conversion Engineering Conference*, American Nuclear Society, **1977**, pp. 951–958.

[142] W. A. Oates, T. B. Flanagan, *Mater. Res. Bull.* **1984**, *19*, 1397.

[143] J. C. Achard, A. Percheron-Guégan, H. Diaz, in *2nd International Congress on Hydrogen in Metals*, Pergamon, Paris, France **1977**.

[144] C. Lartigue, A. Percheron-Guégan, J. C. Achard, F. Tasset, *J. Less-Common Met.* **1980**, *75*, 23.

[145] J. Lamloumi, A. Percheron-Guegan, J. C. Achard, G. Jehanno, D. Givord, *J. Phys.* **1984**, *45*, 1643.

[146] J. Chen, S. X. Dou, H. K. Liu, *J. Power Sources* **1996**, *63*, 267.

[147] G. Inden, *Z. Metallkd.* **1975**, *66*, 648.

[148] J. S. Benjamin, *Metall. Trans.* **1970**, *1*, 2943.

[149] a) E. Ivanov, I. Konstanchuk, A. Stepanov, V. Boldyrev, *J. Less-Common Met.* **1987**, *131*, 25; b) M. Y. Song, E. Ivanov, B. Darriet, M. Pezat, P. Hagenmuller, *J. Less-Common Met.* **1987**, *131*, 71; c) A. Stepanov, E. Ivanov, I. Konstanchuk, V. Boldyrev, *J. Less-Common Met.* **1987**, *131*, 89.

[150] L. Zaluski, A. Zaluska, J. O. Ström-Olsen, *J. Alloys Compd.* **1995**, *217*, 245.

[151] a) A. Zaluska, L. Zaluski, J. O. Ström-Olsen, *J. Alloys Compd.* **1999**, *288*, 217; b) L. Zaluski, A. Zaluska, J. O. Ström-Olsen, *J. Alloys Compd.* **1997**, 253–254, 70.

[152] J. Huot, D. Ravnsbæk, J. Zhang, F. Cuevas, M. Latroche, T. R. Jensen, *Prog. Mater. Sci.* **2013**, *58*, 30.

[153] G. Gorrasi, A. Sorrentino, *Green Chem.* **2015**, *17*, 2610.

[154] G. Kaupp, *CrystEngComm* **2009**, *11*, 388.

[155] F. Cuevas, D. Korablov, M. Latroche, *Phys. Chem. Chem. Phys.* **2012**, *14*, 1200.

[156] F. Delogu, G. Gorrasi, A. Sorrentino, *Prog. Mater. Sci.* **2017**, *86*, 75.

[157] a) N. Z. A. Khafidz, Z. Yaakob, K. L. Lim, S. N. Timmiati, *Int. J. Hydrogen Energy* **2016**, *41*, 13131; b) G. Friedlmeier, M. Groll, *J. Alloys Compd.* **1997**, 253–254, 550.
[158] T. P. Yadav, R. M. Yadav, D. P. Singh, *Nanosci. Nanotechnol.* **2012**, *2*, 22.

[159] C. Vasilius-Oprea, F. Dan, *Macromolecular Mechanochemistry: Polymer Mechanochemistry*, Vol. 1, Cambridge International Science Publishing Ltd, Cambridge, UK **2006**.

[160] a) C. F. Burmeister, A. Kwade, *Chem. Soc. Rev.* **2013**, *42*, 7660;
b) B. K. Mishra, *Int. J. Miner. Process.* **2003**, *71*, 73; c) B. K. Mishra,
*Int. J. Miner. Process.* **2003**, *71*, 95; d) T. H. Courtney, D. Maurice,
*Scr. Mater.* **1996**, *34*, 5; e) E. J. Kreuzer, *Math. Modell.* **1987**, *8*, 37;
f) R. Venugopal, R. K. Rajamani, *Powder Technol.* **2001**, *115*, 157.

[161] I. Colombo, G. Grassi, M. Grassi, *J. Pharm. Sci.* **2009**, *98*, 3961.

[162] C. Suryanarayana, N. Al-Aqeeli, *Prog. Mater. Sci.* **2013**, *58*, 383.

[163] a) I. Bonadies, M. Avella, R. Avolio, C. Carfagna, G. Gentile,
B. Immirzi, M. E. Errico, *Polym. Test.* **2012**, *31*, 176; b) R. Avolio,
I. Bonadies, D. Capitani, M. E. Errico, G. Gentile, M. Avella, *Carbo-
hydr. Polym.* **2012**, *87*, 265.

[164] a) C. C. Koch, J. D. Whittenberger, *Intermetallics* **1996**, *4*, 339;
b) B. S. Murty, S. Ranganathan, *Int. Mater. Rev.* **1998**, *43*, 101.

[165] J. Joardar, S. K. Pabi, B. S. Murty, *Scr. Mater.* **2004**, *50*, 1199.

[166] a) A. Tonejc, M. Stubicar, A. M. Tonejc, K. Kosanović, B. Subotić,
I. Smit, *J. Mater. Sci. Lett.* **1994**, *13*, 519; b) A. Tonejc, A. M. Tonejc,
D. Bagović, C. Kosanović, *Mater. Sci. Eng.*, A **1994**, 181–182, 1227.

[167] D. B. Witkin, E. J. Lavernia, *Prog. Mater. Sci.* **2006**, *51*, 1.

[168] J. R. Ares Fernandez, F. Aguey-Zinsou, M. Elsaesser, X. Z. Ma,
M. Dornheim, T. Klassen, R. Bormann, *Int. J. Hydrogen Energy*
**2007**, *32*, 1033.

[169] J. R. Ares, K. F. Aguey-Zinsou, T. Klassen, R. Bormann, *J. Alloys
Compd.* **2007**, 434–435, 729.

[170] C. Suryanarayana, G.-H. Chen, F. H. Froes, *Scr. Metall. Mater.* **1992**, *26*, 1727.

[171] a) J. H. Weber, in *TMS Fall Meeting* (Eds: A. H. Clauer,
J. J. deBarbadillo), Warrendale, Pa.: Minerals, Metals & Materials
Society, Indianapolis, Indiana **1990**; b) H. Imamura, N. Sakasai,
Y. Kajii, *J. Alloys Compd.* **1996**, *232*, 218; c) H. Imamura, N. Sakasai,
T. Fujinaga, *J. Alloys Compd.* **1997**, 253–254, 34; d) M. A. Lillo-Rodenas,
Z. X. Guo, K. F. Aguey-Zinsou, D. Cazorla-Amoros, A. Linares-Solano,
*Carbon* **2008**, *46*, 126–137; e) M. A. Lillo-Rodenas, K. F. Aguey-Zinsou,
D. Cazorla-Amoros, A. Linares-Solano, Z. X. Guo, *J. Phys. Chem. C*
**2008**, *112*, 5984.

[172] O. Friedrichs, J. C. Sánchez-López, C. López-Cartes, M. Dornheim,
T. Klassen, R. Bormann, A. Fernández, *Appl. Surf. Sci.* **2006**, *252*,
2334.

[173] a) H. Imamura, M. Kusuhara, S. Minami, M. Matsumoto,
K. Masanari, Y. Sakata, K. Itoh, T. Fukunaga, *Acta Mater.* **2003**, *51*,
6407; b) H. Fujii, S. Orimo, *Phys. B* **2003**, *328*, 77.

[174] a) K. F. Aguey-Zinsou, J. R. Ares Fernandez, T. Klassen, R. Bormann,
*Mater. Res. Bull.* **2006**, *41*, 1118; b) J. R. Ares-Fernández,
K. F. Aguey-Zinsou, *Catalysts* **2012**, *2*, 330.

[175] a) M. S. El-Eskandarany, K. Sumiyama, K. Aoki, K. Suzuki,
*J. Mater. Res.* **1992**, *7*, 888; b) K. Sikdar, S. Chakravarty, D. Roy,
R. O. Scattergood, C. C. Koch, *J. Alloys Compd.* **2017**, *717*, 219;
c) C. C. Koch, K. M. Youssef, R. O. Scattergood, K. L. Murty, *Adv.
Eng. Mater.* **2005**, *7*, 787.

[176] a) J. Huot, J. F. Pelletier, G. Liang, M. Sutton, R. Schulz, *J. Alloys Compd.* **2002**, 330–332, 727; b) J. Huot, M. L. Tremblay, R. Schulz, *J. Alloys Compd.* **2003**, 356–357, 603; c) J. Huot, G. Liang, R. Schulz, *Appl. Phys. A: Mater. Sci. Process.* **2001**, *72*, 187.

[177] B. Bogdanović, M. Schwickardi, *J. Alloys Compd.* **1997**, 253–254, 1.

[178] a) G. Barkhordarian, T. Klassen, R. Bormann, *J. Alloys Compd.* **2004**, *364*, 242; b) G. Liang, J. Huot, S. Boily, R. Schulz, *J. Alloys Compd.* **2000**, *305*, 239.

[179] C. Zhu, S. Hosokai, I. Matsumoto, T. Akiyama, *Cryst. Growth Des.* **2010**, *10*, 5123.

[180] a) S. Barcelo, M. Rogers, C. P. Grigoropoulos, S. S. Mao, *Int. J. Hydrogen Energy* **2010**, *35*, 7232; b) H. Fujii, K. Higuchi,

K. Yamamoto, H. Kajioka, S. Orimo, K. Toiyama, *Mater. Trans.* **2002**, *43*, 2721; c) J. Qu, Y. Liu, G. Xin, J. Zheng, X. Li, *Dalton Trans.* **2014**, *43*, 5908.

[181] a) E. J. Setijadi, C. Boyer, K.-F. Aguey-Zinsou, *Phys. Chem. Chem. Phys.* **2012**, *14*, 11386; b) P. E. de Jongh, R. W. Wagemans, T. M. Eggenhuisen, B. S. Dauvillier, P. B. Radstake, J. D. Meeldijk, J. W. Geus, K. P. de Jong, *Chem. Mater.* **2007**, *19*, 6052; c) C. Zlotea, C. Chevalier-César, E. Léonel, E. Leroy, F. Cuevas, P. Dibandjo, C. Vix-Guterl, T. Martens, M. Latroche, *Faraday Discuss.* **2011**, *151*, 117.

[182] a) W. Liu, K.-F. Aguey-Zinsou, *J. Mater. Chem.* A **2014**, *2*, 9718;
b) S. B. Kalidindi, B. R. Jagirdar, *Inorg. Chem.* **2009**, *48*, 4524.

[183] C. Shen, K.-F. Aguey-Zinsou, *Energies* **2016**, *9*, 1073.

[184] a) J. Huot, *Metals* **2012**, *2*, 22; b) K. Edalati, E. Akiba, Z. Horita,
*Sci. Technol. Adv. Mater.* **2018**, *19*, 185.

[185] C. Suryanarayana, C. C. Koch, *Hyperfine Interact.* **2000**, *130*, 5.

[186] Q. Lai, T. Wang, Y. Sun, K. F. Aguey-Zinsou, *Adv. Mater. Technol.* **2018**, *3*, 1700298.

[187] E. Hellstern, H. J. Fecht, C. Garland, W. L. Johnson, *Mater. Res. Soc.* **1989**, *132*, 137.

[188] R. A. Varin, C. Chiu, *J. Alloys Compd.* **2005**, *397*, 276.

[189] a) J. Huot, D. B. Ravnsbaek, J. Zhang, F. Cuevas, M. Latroche,
T. R. Jensen, *Prog. Mater. Sci.* **2013**, *58*, 30; b) C. Mottillo, T. Friscic,
*Molecules* **2017**, *22*, 144.

[190] S. Li, K. Wang, L. Sun, Z. Wang, *Scripta Metall Mater* **1992**, *27*, 437.

[191] a) F. A. Mohamed, *Acta Mater.* **2003**, *51*, 4107; b) F. A. Mohamed,
Y. Xun, *Mater. Sci. Eng.*, A **2003**, *354*, 133.

[192] C. C. Koch, *Nanostruct. Mater.* **1997**, *9*, 13.

[193] A. R. Yavari, *Mater. Trans.*, **JIM 1995**, *36*, 228.

[194] A. A. Rostislav, *Phys.-Usp.* **2007**, *50*, 691.

[195] a) A. Oudriss, J. Creus, J. Bouhattate, E. Conforto, C. Berziou,
C. Savall, X. Feaugas, *Acta Mater.* **2012**, *60*, 6814; b) T. Mütschele,
R. Kirchheim, *Scr. Metall.* **1987**, *21*, 135; c) S.-M. Lee, J.-Y. Lee,
*Metall. Trans.* A **1986**, *17*, 181.

[196] H. G. Schimmel, M. R. Johnson, G. J. Kearley, A. J. Ramirez-Cuesta,
J. Huot, F. M. Mulder, *J. Alloys Compd.* **2005**, *393*, 1.

[197] a) H. J. Fecht, *Acta Metall. Mater.* **1990**, *38*, 1927; b) H. J. Fecht,
*Phys. Rev. Lett.* **1990**, *65*, 610.

[198] a) K.-F. Aguey-Zinsou, J. A. Fernandez, T. Klassen, R. Bormann,
*Mater. Res. Bull.* **2006**, *41*, 1118; b) M. Dornheim, N. Eigen,
G. Barkhordarian, T. Klassen, R. Bormann, *Adv. Eng. Mater.* **2006**, *8*, 377; c) M. Dornheim, S. Doppiu, G. Barkhordarian,
U. Boesenberg, T. Klassen, O. Gutfleisch, R. Bormann, *Scr. Mater.* **2007**, *56*, 841; d) B. Paik, A. Walton, V. Mann, D. Book, I. Jones,
I. Harris, *Int. J. Hydrogen Energy* **2010**, *35*, 9012.

[199] O. Friedrichs, T. Klassen, J. C. Sánchez-López, R. Bormann,
A. Fernández, *Scr. Mater.* **2006**, *54*, 1293.

[200] B. Zahiri, M. Danaie, X. Tan, B. S. Amirkhiz, G. A. Botton,
D. Mitlin, *J. Phys. Chem. C* **2012**, *116*, 3188.

[201] J. Lu, Y. J. Choi, Z. Z. Fang, H. Y. Sohn, E. Rönnebro, *J. Am. Chem. Soc.* **2009**, *131*, 15843.

[202] a) O. Friedrichs, F. Aguey-Zinsou, J. R. A. Fernández,
J. C. Sánchez-López, A. Justo, T. Klassen, R. Bormann, A. Fernández,
*Acta Mater.* **2006**, *54*, 105; b) O. Friedrichs, A. F. Camacho,
J. C. S. López, T. Klassen, R. Bormann, *EMPA Activities*, **2006**,
pp. 70–71; c) O. Friedrichs, J. C. Sánchez-López, C. López-Cartes,
T. Klassen, R. Bormann, A. Fernández, *J. Phys. Chem. B* **2006**, *110*,
7845.

[203] K. Lu, *Mater. Sci. Eng.*, R **1996**, *16*, 161.

[204] M. Danaie, D. Mitlin, *J. Alloys Compd.* **2009**, *476*, 590.

[205] a) H. Gleiter, *Prog. Mater. Sci.* **1989**, *33*, 223; b) S. R. Phillipot,
D. Wolf, H. Gleiter, *J. Appl. Phys.* **1995**, *78*, 847.

[206] a) R. W. Siegel, *Materials Interfaces: Atomic Level Structure and Properties*, Chapman & Hall, London, UK **1992**; b) Z. Horita,
D. J. Smith, M. Furukawa, M. Nemoto, R. Z. Valiev, T. G. Langdon, *J. Mater. Res.* **1996**, *11*, 1880.

[207] a) S. D. House, J. J. Vajo, C. Ren, A. A. Rockett, I. M. Robertson, *Acta Mater.* **2015**, *86*, 55; b) P. Larsson, C. M. Araújo, J. A. Larsson, P. Jena, R. Ahuja, *Proc. Natl. Acad. Sci. USA* **2008**, *105*, 8227.

[208] a) M. Y. Song, J.-L. Bobet, B. Darriet, *J. Alloys Compd.* **2002**, *340*, 256; b) W. Oelerich, T. Klassen, R. Bormann, *J. Alloys Compd.* **2001**, *315*, 237; c) K. F. Aguey-Zinsou, T. Nicolaisen, J. R. Ares Fernandez, T. Klassen, R. Bormann, *J. Alloys Compd.* **2007**, 434-435, 738.

[209] a) L. Guoxian, W. Erde, F. Shoushi, *J. Alloys Compd.* **1995**, *223*, 111; b) M. Terzieva, M. Khrussanova, P. Peshev, *J. Alloys Compd.* **1998**, *267*, 235; c) C. Zhou, Z. Z. Fang, C. Ren, J. Li, J. Lu, *J. Phys. Chem. C* **2013**, *117*, 12973.

[210] a) M. Ponthieu, F. Cuevas, J. F. Fernández, L. Laversenne, F. Porcher, M. Latroche, *J. Phys. Chem. C* **2013**, *117*, 18851; b) M. Porcu, A. K. Petford-Long, J. M. Sykes, *J. Alloys Compd.* **2008**, *453*, 341.

[211] H. G. Schimmel, J. Huot, L. C. Chapon, F. D. Tichelaar, F. M. Mulder, *J. Am. Chem. Soc.* **2005**, *127*, 14348.

[212] a) B. Bogdanović, M. Felderhoff, G. Streukens, *J. Serb. Chem. Soc.* **2009**, *74*, 183; b) P. Wang, X. D. Kang, H. M. Cheng, *J. Phys. Chem. B* **2005**, *109*, 20131; c) C. P. Baldé, A. M. J. van der Eerden, H. A. Stil, F. M. F. de Groot, K. P. de Jong, J. H. Bitter, *J. Alloys Compd.* **2007**, 446-447, 232; d) C. M. Andrei, J. C. Walmsley, H. W. Brinks, R. Holmestad, S. S. Srinivasan, C. M. Jensen, B. C. Hauback, *Appl. Phys. A* **2005**, *80*, 709.

[213] a) E. Raju, C. G. Aneesh, M. Joydev, V. Manvendra, S. Pratibha, *Mater. Res. Express* **2014**, 1, 046502; b) B. Bogdanović, M. Felderhoff, A. Pommerin, F. Schueth, N. Spielkamp, *Adv. Mater.* **2006**, *18*, 1198; c) D. Sun, T. Kiyobayashi, H. T. Takeshita, N. Kuriyama, C. M. Jensen, *J. Alloys Compd.* **2002**, *337*, L8; d) R. H. Scheicher, S. Li, C. M. Araujo, A. Blomqvist, R. Ahuja, P. Jena, *Nanotechnology* **2011**, 22, 335401; e) Z. Qian, M. S. L. Hudson, H. Raghubanshi, R. H. Scheicher, B. Pathak, C. M. Araújo, A. Blomqvist, B. Johansson, O. N. Srivastava, R. Ahuja, *J. Phys. Chem. C* **2012**, *116*, 10861.

[214] A. Leon, O. Kircher, J. Rothe, M. Fichtner, *J. Phys. Chem. B* **2004**, *108*, 16372.

[215] A. Léon, D. Schild, M. Fichtner, *J. Alloys Compd.* **2005**, 404-406, 766.

[216] C. P. Balde, H. A. Stil, A. M. van der Eerden, K. P. de Jong, J. H. Bitter, *J. Phys. Chem. C* **2007**, *111*, 2797.

[217] C. Weidenthaler, A. Pommerin, M. Felderhoff, B. Bogdanović, F. Schüth, *Phys. Chem. Chem. Phys.* **2003**, *5*, 5149.

[218] C. M. Araújo, R. Ahuja, J. M. Osorio Guillén, P. Jena, *Appl. Phys. Lett.* **2005**, *86*, 251913.

[219] a) I. P. Jain, C. Lal, A. Jain, *Int. J. Hydrogen Energy* **2010**, *35*, 5133; b) T. Hongo, K. Edalati, M. Arita, J. Matsuda, E. Akiba, Z. Horita, *Acta Mater.* **2015**, *92*, 46.

[220] K. Aoki, A. Memezawa, T. Masumoto, *J. Mater. Res.* **1993**, *8*, 307.

[221] a) H. Leng, T. Ichikawa, S. Hino, N. Hanada, S. Isobe, H. Fujii, *J. Power Sources* **2006**, *156*, 166; b) Y. Kojima, K. Tange, S. Hino, S. Isobe, M. Tsubota, K. Nakamura, M. Nakatake, H. Miyaka, H. Yamamoto, T. Ichikawa, *J. Mater. Res.* **2009**, *24*, 2185; c) T. Ichikawa, N. Hanada, S. Isobe, H. Leng, H. Fujii, *J. Phys. Chem. B* **2004**, *108*, 7887.

[222] T. Ichikawa, H. Leng, S. Isobe, N. Hanada, H. Fujii, *J. Power Sources* **2006**, *159*, 126.

[223] Y. Song, Z. Guo, *Phys. Rev. B* **2006**, *74*, 195120.

[224] P. Chen, Z. Xiong, G. Wu, Y. Liu, J. Hu, W. Luo, *Scr. Mater.* **2007**, *56*, 817.

[225] a) E. Rönnebro, E. H. Majzoub, *J. Phys. Chem. B* **2006**, *110*, 25686; b) J. Graetz, Y. Lee, J. Reilly, S. Park, T. Vogt, *Phys. Rev. B* **2005**, *71*, 184115; c) J. Zhang, M.-A. Pilette, F. Cuevas, T. Charpentier, F. Mauri, M. Latroche, *J. Phys. Chem. C* **2009**, *113*, 21242.

[226] M. Mamatha, B. Bogdanović, M. Felderhoff, A. Pommerin, W. Schmidt, F. Schüth, C. Weidenthaler, *J. Alloys Compd.* **2006**, *407*, 78.

[227] a) A. Remhof, Y. Yan, O. Friedrichs, J. Kim, P. Mauron, A. Borgschulte, D. Wallacher, A. Buchsteiner, A. Hoser, K. Oh, in *Journal of Physics: Conference Series*, Vol. 340, IOP Publishing, 2012, p. 012111; b) S. W. Linehan, A. A. Chin, N. T. Allen, R. Butterick, N. T. Kendall, I. L. Klawiter, F. J. Lipiecki, D. M. Millar, D. C. Molzahn, S. J. November, *Low-Cost Precursors to Novel Hydrogen Storage Materials*, Rohm and Haas Company, Philadelphia, PA 2010.

[228] O. Friedrichs, A. Remhof, A. Borgschulte, F. Buchter, S. Orimo, A. Züttel, *Phys. Chem. Chem. Phys.* **2010**, *12*, 10919.

[229] D. Ravnsbæk, Y. Filinchuk, Y. Cerenius, H. J. Jakobsen, F. Besenbacher, J. Skibsted, T. R. Jensen, *Angew. Chem.* **2009**, *121*, 6787.

[230] R. Černý, P. Schouwink, Y. Sadikin, K. Stare, L. u. Smrčok, B. Richter, T. R. Jensen, *Inorg. Chem.* **2013**, *52*, 9941.

[231] a) R. Liu, D. Book, *Int. J. Hydrogen Energy* **2014**, *39*, 2194; b) M. Sharma, E. Didelot, A. Spyratou, L. M. Lawson Daku, R. Černý, H. Hagemann, *Inorg. Chem.* **2016**, *55*, 7090; c) G. L. Soloveichich, M. Andrus, Y. Gao, J. C. Zhao, S. Kniajanski, *Int. J. Hydrogen Energy* **2009**, *34*, 2144.

[232] T. Noritake, M. Aoki, S. Towata, A. Ninomiya, Y. Nakamori, S. Orimo, *Appl. Phys. A* **2006**, *83*, 277.

[233] Y. S. Chua, G. Wu, Z. Xiong, P. Chen, *J. Solid State Chem.* **2010**, *183*, 2040.

[234] a) H. Wu, W. Zhou, F. E. Pinkerton, M. S. Meyer, Q. Yao, S. Gadipelli, T. J. Udovic, T. Yildirim, J. J. Rush, *Chem. Commun.* **2011**, 47, 4102; b) Y. S. Chua, H. Wu, W. Zhou, T. J. Udovic, G. Wu, Z. Xiong, M. W. Wong, P. Chen, *Inorg. Chem.* **2012**, *51*, 1599; c) F. Leardini, J. R. Ares, J. Bodega, M. J. Valero-Pedraza, M. A. Bañares, J. F. Fernández, C. Sánchez, *J. Phys. Chem. C* **2012**, *116*, 24430.

[235] a) N. Biliškov, A. Borgschulte, K. Užarević, I. Halasz, S. Lukin, S. Milošević, I. Milanović, J. G. Novaković, *Chem. - Eur. J.* **2017**, *23*, 16274; b) H. Wu, W. Zhou, Q. Yao, S. Gadipelli, T. Udovic, T. Yildirim, J. J. Rush, F. E. Pinkerton, M. S. Meyer, *Chem. Commun.* **2011**, 47, 4102; c) Y. S. Chua, W. Li, G. Wu, Z. Xiong, P. Chen, *Chem. Mater.* **2012**, *24*, 3574; d) H. Chu, S. Qiu, L. Sun, F. Xu, in *Advanced Materials for Renewable Hydrogen Production, Storage and Utilization*, InTech, 2015; e) K. T. Møller, M. Jørgensen, J. G. Andreassen, J. Skibsted, Z. Łódziana, Y. Filinchuk, T. R. Jensen, *Int. J. Hydrogen Energy* **2018**, *43*, 311.

[236] Z. Xiong, C. K. Yong, G. Wu, P. Chen, W. Shaw, A. Karkamkar, T. Autrey, M. O. Jones, S. R. Johnson, P. P. Edwards, *Nat. Mater.* **2008**, *7*, 138.

[237] K. J. Fijalkowski, R. V. Genova, Y. Filinchuk, A. Budzianowski, M. Derzsi, T. Jarorí, P. J. Leszczyński, W. Grochala, *Dalton Trans.* **2011**, *40*, 4407.

[238] T. Hügle, M. F. Kühnel, D. Lentz, *J. Am. Chem. Soc.* **2009**, *131*, 7444.

[239] a) R. Moury, U. B. Demirci, V. Ban, Y. Filinchuk, T. Ichikawa, L. Zeng, K. Goshome, P. Miele, *Chem. Mater.* **2014**, *26*, 3249; b) R. Moury, U. B. Demirci, T. Ichikawa, Y. Filinchuk, R. Chiriac, A. van der Lee, P. Miele, *ChemSusChem* **2013**, *6*, 667; c) Y. S. Chua, Q. Pei, X. Ju, W. Zhou, T. J. Udovic, G. Wu, Z. Xiong, P. Chen, H. Wu, *J. Phys. Chem. C* **2014**, *118*, 11244; d) S. Ould-Amara, D. Granier, R. Chiriac, F. Toche, P. Yot, U. Demirci, *Materials* **2017**, *10*, 750.

[240] W. Li, R. H. Scheicher, C. M. Araújo, G. Wu, A. Blomqvist, C. Wu, R. Ahuja, Y. P. Feng, P. Chen, *J. Phys. Chem. C* **2010**, *114*, 19089.

[241] R. B. Schwarz, W. L. Johnson, *Phys. Rev. Lett.* **1983**, *51*, 415.

[242] P. Wang, A. M. Wang, B. Z. Ding, Z. Q. Hu, *J. Alloys Compd.* **2002**, *334*, 243.
[243] U. Bösenberg, D. B. Ravnsbæk, H. Hagemann, V. D'Anna, C. B. Minella, C. Pistidda, W. Van Beek, T. R. Jensen, R. d. Bormann, M. Dornheim, *J. Phys. Chem. C* **2010**, *114*, 15212.

[244] X. Xiao, Z. Liu, S. Saremi-Yarahmadi, D. H. Gregory, *Phys. Chem. Chem. Phys.* **2016**, *18*, 10492.

[245] K. Aguey-Zinsou, S. Chaoqi, *J. Mater. Chem. A* **2017**, *5*, 8644.

[246] a) G. Liang, R. Schulz, *J. Mater. Sci.* **2003**, *38*, 1179; b) A. R. Yavari, P. J. Desré, T. Benameur, *Phys. Rev. Lett.* **1992**, *68*, 2235.

[247] G. X. Liang, R. Schulz, *J. Metastable Nanocryst. Mater.* **2002**, *12*, 93.

[248] G. Liang, *J. Alloys Compd.* **2004**, *370*, 123.

[249] C. X. Shang, M. Bououdina, Y. Song, Z. X. Guo, *Int. J. Hydrogen Energy* **2004**, *29*, 73.

[250] a) M. Scott, *J. Mater. Sci.* **1975**, *10*, 269; b) A. Inoue, T. Masumoto, *Mater. Sci. Eng., A* **1993**, *173*, 1.

[251] E. J. Lavernia, T. S. Srivatsan, *J. Mater. Sci.* **2010**, *45*, 287.

[252] a) K. Edalati, J. Matsuda, H. Iwaoka, S. Toh, E. Akiba, Z. Horita, *Int. J. Hydrogen Energy* **2013**, *38*, 4622; b) Y. Kusadome, K. Ikeda, Y. Nakamori, S. Orimo, Z. Horita, *Scr. Mater.* **2007**, *57*, 751.

[253] L. Zaluski, P. Tessier, D. H. Ryan, C. B. Doner, A. Zaluska, J. O. Ström-Olsen, M. L. Trudeau, R. Schulz, *J. Mater. Res.* **1993**, *8*, 3059.

[254] J. Graetz, *ISRN Mater. Sci.* **2012**, 2012, 1.

[255] L. Wang, A. Rawal, K.-F. Aguey-Zinsou, *Chem. Eng. Sci.* **2019**, *194*, 64.

[256] L. Wang, A. Rawal, M. Z. Quadir, K.-F. Aguey-Zinsou, *Int. J. Hydrogen Energy* **2017**, *42*, 14144.

[257] A. J. Maeland, L. E. Tanner, G. G. Libowitz, *J. Less-Common Met.* **1980**, *74*, 279.

[258] A. Santoro, A. Maeland, J. J. Rush, *Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem.* **1978**, *34*, 3059.

[259] K. Samwer, *J. Less-Common Met.* **1987**, *131*, 210.

[260] K. Aoki, X. G. Li, T. Masumoto, *Acta Metall. Mater.* **1992**, *40*, 1717.

[261] J. J. Vajo, F. Mertens, C. C. Ahn, R. C. Bowman, B. Fultz, *J. Phys. Chem. B* **2004**, *108*, 13977.

[262] a) G. Barkhordarian, T. R. Jensen, S. Doppiu, U. Bösenberg, A. Borgschulte, R. Gremaud, Y. Cerenius, M. Dornheim, T. Klassen, R. Bormann, *J. Phys. Chem. C* **2008**, *112*, 2743; b) G. Barkhordarian, T. Klassen, M. Dornheim, R. Bormann, *J. Alloys Compd.* **2007**, *440*, L18.

[263] J. Rumble, *CRC Handbook of Chemistry and Physics*, 98th ed., CRC Press LLC, **2017**.

[264] M. Van Schilfgaarde, W. A. Harrison, *J. Phys. Chem. Solids* **1985**, *46*, 1093.

[265] E. Nishibori, M. Takata, M. Sakata, H. Tanaka, T. Muranaka, J. Akimitsu, *J. Phys. Soc. Jpn.* **2001**, *70*, 2252.

[266] a) T. E. Price, D. M. Grant, V. Legrand, G. S. Walker, *Int. J. Hydrogen Energy* **2010**, *35*, 4154; b) G. S. Walker, D. M. Grant, T. C. Price, X. Yu, V. Legrand, *J. Power Sources* **2009**, *194*, 1128; c) X. Yu, D. Grant, G. Walker, *Chem. Commun.* **2006**, 3906.

[267] F. E. Pinkerton, M. S. Meyer, G. P. Meisner, M. P. Balogh, J. J. Vajo, *J. Phys. Chem. C* **2007**, *111*, 12881.

[268] U. Bösenberg, J. W. Kim, D. Gosslar, N. Eigen, T. R. Jensen, J. M. Bellosta Von Colbe, Y. Zhou, M. Dahms, D. H. Kim, R. Günther, Y. W. Cho, K. H. Oh, T. Klasseng, R. Bormanna, M. Dornheim, *Acta Mater.* **2010**, *58*, 3381.

[269] D. T. Shane, R. C. Bowman Jr., M. S. Conradi, *J. Phys. Chem. C* **2009**, *113*, 5039.

[270] S. Kato, A. Borgschulte, M. Bielmann, A. Zuttel, *Phys. Chem. Chem. Phys.* **2012**, *14*, 8360.

[271] J. Li, A. Oudriss, A. Metsue, J. Bouhattate, X. Feaugas, *Sci. Rep.* **2017**, *7*, 45041.

[272] P. Mauron, F. Buchter, O. Friedrichs, A. Remhof, M. Bielmann, C. N. Zwicky, A. Züttel, *J. Phys. Chem. B* **2008**, *112*, 906.

[273] R. Gosalawit-Utke, C. Milanese, T. K. Nielsen, F. Karimi, I. Saldan, K. Pranzas, T. R. Jensen, A. Marini, T. Klassen, M. Dornheim, *Int. J. Hydrogen Energy* **2013**, *38*, 1932.

[274] F. Pendolino, P. Mauron, A. Borgschulte, A. Züttel, *J. Phys. Chem. C* **2009**, *113*, 17231.

[275] a) U. Bösenberg, S. Doppiu, L. Mosegaard, G. Barkhordarian, N. Eigen, A. Borgschulte, T. R. Jensen, Y. Cerenius, O. Gutfleisch, T. Klassen, *Acta Mater.* **2007**, *55*, 3951; b) H.-Q. Kou, X.-Z. Xiao, L.-X. Chen, S.-Q. Li, Q.-D. Wang, *Trans. Nonferrous Met. Soc. China* **2011**, *21*, 1040.

[276] E. Deprez, A. Justo, T. C. Rojas, C. López-Cartés, C. B. Minella, U. Bösenberg, M. Dornheim, R. Bormann, A. Fernández, *Acta Mater.* **2010**, *58*, 5683.

[277] C. Li, P. Peng, D. Zhou, L. Wan, *Int. J. Hydrogen Energy* **2011**, *36*, 14512.

[278] Y. Nakamori, K. Miwa, A. Ninomiya, H. Li, N. Ohba, S.-i. Towata, A. Züttel, S.-I. Orimo, *Phys. Rev. B* **2006**, *74*, 045126.

[279] a) C. Kim, S.-J. Hwang, R. C. Bowman Jr., J. W. Reiter, J. A. Zan, J. G. Kulleck, H. Kabbour, E. Majzoub, V. Ozolins, *J. Phys. Chem. C* **2009**, *113*, 9956; b) H. Hagemann, M. Longhini, J. W. Kaminski, T. A. Wesolowski, R. Cerny, N. Penin, M. H. Sørby, B. C. Hauback, G. Severa, C. M. Jensen, *J. Phys. Chem. A* **2008**, *112*, 7551; c) R. Cerny, G. Severa, D. B. Ravnsbæk, Y. Filinchuk, V. D'Anna. H. Hagemann, D. r. Haase, C. M. Jensen, T. R. Jensen, *J. Phys. Chem. C* **2010**, *114*, 1357; d) M. Chong, E. Callini, A. Borgschulte, A. Züttel, C. M. Jensen, *RSC Adv.* **2014**, *4*, 63933.

[280] a) H.-S. Lee, Y.-S. Lee, J.-Y. Suh, M. Kim, J.-S. Yu, Y. W. Cho, *J. Phys. Chem. C* **2011**, *115*, 20027; b) P. Javadian, T. R. Jensen, *Int. J. Hydrogen Energy* **2014**, *39*, 9871.

[281] Z. Zhao-Karger, R. Witter, E. G. Bardají, D. Wang, D. Cossement, M. Fichtner, *Energy Procedia* **2012**, *29*, 731.

[282] J. Lu, Z. Z. Fang, H. Y. Sohn, *Inorg. Chem.* **2006**, *45*, 8749.

[283] P. Chen, Z. Xiong, J. Luo, J. Lin, K. L. Tan, *Nature* **2002**, *420*, 302.

[284] P. Chen, Z. Xiong, J. Luo, J. Lin, K. L. Tan, *J. Phys. Chem. B* **2003**, *107*, 10967.

[285] Y. H. Hu, E. Ruckenstein, *J. Phys. Chem. A* **2003**, *107*, 9737.

[286] Y. Chen, C.-Z. Wu, P. Wang, H.-M. Cheng, *Int. J. Hydrogen Energy* **2006**, *31*, 1236.

[287] a) Z. Xiong, J. Hu, G. Wu, P. Chen, W. Luo, K. Gross, J. Wang, *J. Alloys Compd.* **2005**, *398*, 235; b) Z. Xiong, G. Wu, J. Hu, P. Chen, W. Luo, J. Wang, *J. Alloys Compd.* **2006**, *417*, 190; c) Y. Nakamori, G. Kitahara, A. Ninomiya, M. Aoki, T. Noritake, S.-I. Towata, S.-I. Orimo, *Mater. Trans.* **2005**, *46*, 2093; d) J. Wang, T. Liu, G. Wu, W. Li, Y. Liu, C. M. Araújo, R. H. Scheicher, A. Blomqvist, R. Ahuja, Z. Xiong, *Angew. Chem., Int. Ed.* **2009**, *48*, 5828; e) C. M. Araújo, R. H. Scheicher, R. Ahuja, *Appl. Phys. Lett.* **2008**, *92*, 021907.

[288] a) M. Aoki, K. Miwa, T. Noritake, G. Kitahara, Y. Nakamori, S. Orimo, S. Towata, *Appl. Phys. A* **2005**, *80*, 1409; b) F. E. Pinkerton, G. P. Meisner, M. S. Meyer, M. P. Balogh, M. D. Kundrat, *J. Phys. Chem. B* **2005**, *109*, 6.

[289] G. P. Meisner, M. L. Scullin, M. P. Balogh, F. E. Pinkerton, M. S. Meyer, *J. Phys. Chem. B* **2006**, *110*, 4186.

[290] a) M. Somer, S. Acar, C. Koz, I. Kokal, P. Hoehn, R. Cardoso-Gil, U. Aydemir, L. Akselrud, *J. Alloys Compd.* **2010**, *491*, 98; b) Y. Bai, L.-L. Zhao, Y. Wang, X. Liu, F. Wu, C. Wu, *Int. J. Hydrogen Energy* **2014**, *39*, 13576; c) A. V. Soloninin, O. A. Babanova, E. Y. Medvedev, A. V. Skripov, M. Matsuo, S.-I. Orimo, *J. Phys. Chem. C* **2014**, *118*, 14805.

[291] X. Yu, Y. Guo, D. Sun, Z. Yang, A. Ranjbar, Z. Guo, H. Liu, S. Dou, *J. Phys. Chem. C* **2010**, *114*, 4733.

[292] H. Chu, Z. Xiong, G. Wu, J. Guo, T. He, P. Chen, *Dalton Trans.* **2010**, *39*, 10585.

[293] L. Liu, G. Wu, W. Chen, Z. Xiong, T. He, P. Chen, *Int. J. Hydrogen Energy* **2015**, *40*, 429.
[294] a) Y. Nakamori, A. Ninomiya, G. Kitahara, M. Aoki, T. Noritake, K. Miwa, Y. Kojima, S. I. Orimo, *J. Power Sources* **2006**, *155*, 447;
b) Z. Xiong, G. Wu, J. Hu, P. Chen, *J. Power Sources* **2006**, *159*, 167;
c) J. Lu, Z. Z. Fang, *J. Phys. Chem. B* **2005**, *109*, 20830.

[295] J.-W. Jang, J.-H. Shim, Y. W. Cho, B.-J. Lee, *J. Alloys Compd.* **2006**, *420*, 286.

[296] a) L. Zaluski, A. Zaluska, J. Ström-Olsen, *J. Alloys Compd.* **1999**, *290*, 71; b) V. P. Baleva, V. K. Pecharsky, K. W. Dennis, *J. Alloys Compd.* **2000**, *313*, 69.

[297] Y. Kojima, M. Matsumoto, Y. Kawai, T. Haga, N. Ohba, K. Miwa, S.-i. Towata, Y. Nakamori, S.-I. Orimo, *J. Phys. Chem. B* **2006**, *110*, 9632.

[298] G. Wolf, J. Baumann, F. Baitalow, F. Hoffmann, *Thermochim. Acta* **2000**, *343*, 19.

[299] a) A. Gutowska, L. Li, Y. Shin, C. M. Wang, X. S. Li, J. C. Linehan, R. S. Smith, B. D. Kay, B. Schmid, W. Shaw, *Angew. Chem., Int. Ed.* **2005**, *44*, 3578; b) M. J. Valero-Pedraza, V. Gascón, M. A. Carreón, F. Leardini, J. R. Ares, Á. Martín, M. Sánchez-Sánchez, M. A. Bañares, *Microporous Mesoporous Mater.* **2016**, *226*, 454.

[300] A. Feaver, S. Sepehri, P. Shamberger, A. Stowe, T. Autrey, G. Cao, *J. Phys. Chem. B* **2007**, *111*, 7469.

[301] a) S. Gadipelli, J. Ford, W. Zhou, H. Wu, T. J. Udovic, T. Yildirim, *Chem. - Eur. J.* **2011**, *17*, 6043; b) G. Srinivas, J. Ford, W. Zhou, T. Yildirim, *Int. J. Hydrogen Energy* **2012**, *37*, 3633; c) G. Srinivas, W. Travis, J. Ford, H. Wu, Z.-X. Guo, T. Yildirim, *J. Mater. Chem. A* **2013**, *1*, 4167.

[302] C. Salameh, G. Moussa, A. Bruma, G. Fantozzi, S. Malo, P. Miele, U. B. Demirci, S. Bernard, *Energy Technol.* **2018**, *6*, 570.

[303] L. Zhang, G. Xia, Y. Ge, C. Wang, Z. Guo, X. Li, X. Yu, *J. Mater. Chem. A* **2015**, *3*, 20494.

[304] a) A. B. E. Zirngiebl, W. Ger. Pat. **1959**, *70*, 148; b) O. Kravchenko, S. Kravchenko, *Zh. Obshch. Khim.* **1989**, *59*, 1935; c) V. N. Konoplev, T. A. Silina, *Zh. Neorg. Khim.* **1985**, *30*, 1125.

[305] Y. Guo, Y. Jiang, G. Xia, X. Yu, *Chem. Commun.* **2012**, *48*, 4408.

[306] a) S. R. Johnson, W. I. F. David, D. M. Royse, M. Sommariva, C. Y. Tang, F. Fabbiani, M. O. Jones, P. P. Edwards, *Chem. - Asian J.* **2009**, *4*, 849; b) Y. Guo, G. Xia, Y. Zhu, L. Gao, X. Yu, *Chem. Commun.* **2010**, *46*, 2599; c) M. Ramzan, F. Silvearv, S. Lebègue, R. Ahuja, *J. Phys. Chem. C* **2011**, *115*, 20036; d) Z. Tang, Y. Tan, Q. Gu, X. Yu, *J. Mater. Chem.* **2012**, *22*, 5312.

[307] a) X. Chen, F. Yuan, Y. Tan, Z. Tang, X. Yu, *J. Phys. Chem. C* **2012**, *116*, 21162; b) H. Chu, G. Wu, Z. Xiong, J. Guo, T. He, P. Chen, *Chem. Mater.* **2010**, *22*, 6021.

[308] W. Sun, X. Chen, Q. Gu, K. S. Wallwork, Y. Tan, Z. Tang, X. Yu, *Chem. - Eur. J.* **2012**, *18*, 6825.

[309] a) S. Zhou, X. Zhang, T. Li, N. Wang, H. Chen, T. Zhang, H. Yu, H. Niu, D. Liu, *Int. J. Hydrogen Energy* **2014**, *39*, 13628; b) M. Konarova, A. Tanksale, J. Norberto Beltramini, G. Qing Lu, *Nano Energy* **2013**, *2*, 98; c) Z. Zhao-Karger, J. Hu, A. Roth, D. Wang, C. Kubel, W. Lohstroh, M. Fichtner, *Chem. Commun.* **2010**, *46*, 8353; d) Y. S. Au, M. Ponthieu, R. van Zwienen, C. Zlotea, F. Cuevas, K. P. de Jong, P. E. de Jongh, *J. Mater. Chem. A* **2013**, *1*, 9983; e) W. Liu, K.-F. Aguey-Zinsou, *Int. J. Hydrogen Energy* **2016**, *41*, 1679; f) L. Wang, M. Z. Quadir, K.-F. Aguey-Zinsou, *Int. J. Hydrogen Energy* **2016**, *41*, 18088; g) W. Liu, K.-F. Aguey-Zinsou, *Int. J. Hydrogen Energy* **2016**, *41*, 14429; h) C. Zlotea, F. Cuevas, V. Paul-Boncour, E. Leroy, P. Dibandjo, R. Gadiou, C. Vix-Guterl, M. Latroche, *J. Am. Chem. Soc.* **2010**, *132*, 7720; i) X. Liu, D. Peaslee, C. Z. Jost, T. F. Baumann, E. H. Majzoub, *Chem. Mater.* **2011**, *23*, 1331; j) A. F. Gross, J. J. Vajo, S. L. Van Atta, G. L. Olson, *J. Phys. Chem. C* **2008**, *112*, 5651; k) Z.-Z. Fang, X.-D. Kang, P. Wang, *Int. J. Hydrogen Energy* **2010**, *35*, 8247; l) Z. Fang, P. Wang, T. Rufford, X. Kang, G. Lu, H. Cheng, *Acta Mater.* **2008**, *56*, 6257; m) Z.-Z. Fang, X.-D. Kang,

P. Wang, H.-M. Cheng, *J. Phys. Chem. C* **2008**, *112*, 17023;
n) S. Cahen, J.-B. Eymery, R. Janot, J.-M. Tarascon, *J. Power Sources* **2009**, *189*, 902; o) Y. Zhang, W.-S. Zhang, A.-Q. Wang, L.-X. Sun, M.-Q. Fan, H.-L. Chu, J.-C. Sun, T. Zhang, *Int. J. Hydrogen Energy* **2007**, *32*, 3976; p) C. P. Baldé, B. P. C. Hereijgers, J. H. Bitter, K. P. de Jong, *Angew. Chem., Int. Ed.* **2006**, *45*, 3501; q) C. P. Baldé, B. P. C. Hereijgers, J. H. Bitter, K. P. d. Jong, *J. Am. Chem. Soc.* **2008**, *130*, 6761; r) P. A. Berseth, A. G. Harter, R. Zidan, A. Blomqvist, C. M. Araújo, R. H. Scheicher, R. Ahuja, P. Jena, *Nano Lett.* **2009**, *9*, 1501; s) J. Gao, P. Adelhelm, M. H. W. Verkuijlen, C. Rongeat, M. Herrich, P. J. M. van Bentum, O. Gutfleisch, A. P. M. Kentgens, K. P. de Jong, P. E. de Jongh, *J. Phys. Chem. C* **2010**, *114*, 4675;
t) M. S. Leo Hudson, H. Raghubanshi, D. Pukazhselvan, O. N. Srivastava, *Int. J. Hydrogen Energy* **2010**, *35*, 2083;
u) T. K. Nielsen, P. Javadian, M. Polanski, F. Besenbacher, J. Bystrzycki, T. R. Jensen, *J. Phys. Chem. C* **2012**, *116*, 21046;
v) R. D. Stephens, A. F. Gross, S. L. Van Atta, J. J. Vajo, F. E. Pinkerton, *Nanotechnology* **2009**, *20*, 204018.

[310] Y. Jia, C. Sun, S. Shen, J. Zou, S. S. Mao, X. Yao, *Renewable Sustainable Energy Rev.* **2015**, *44*, 289.

[311] a) K.-J. Jeon, H. R. Moon, A. M. Ruminski, B. Jiang, C. Kisielowski, R. Bardhan, J. J. Urban, *Nat. Mater.* **2011**, *10*, 286; b) L. H. Rude, T. K. Nielsen, D. B. Ravnsbæk, U. Bösenberg, M. B. Ley, B. Richter, L. M. Arnbjerg, M. Dornheim, Y. Filinchuk, F. Besenbacher, *Phys. Status Solidi A* **2011**, *208*, 1754.

[312] a) C. Zlotea, Y. Ournellat, S. J. Hwang, C. M. Ghimbeu, P. E. de Jongh, M. Latroche, *J. Phys. Chem. C* **2015**, *119*, 18091; b) P. Ngene, P. Adelhelm, A. M. Beale, K. P. de Jong, P. E. de Jongh, *J. Phys. Chem. C* **2010**, *114*, 6163.

[313] a) Q. Lai, M. Christian, K.-F. Aguey-Zinsou, *Int. J. Hydrogen Energy* **2014**, *39*, 9339; b) Q. Lai, K.-F. Aguey-Zinsou, *Sustainable Energy Fuels* **2017**, *1*, 1308.

[314] a) L. Wang, K.-F. Aguey-Zinsou, *Inorganics* **2017**, *5*, 38;
b) M. Christian, K.-F. Aguey-Zinsou, *Nanoscale* **2010**, *2*, 2587.

[315] L. Ouyang, Z. Cao, H. Wang, R. Hu, M. Zhu, *J. Alloys Compd.* **2017**, *691*, 422.

[316] M. Fichtner, *Nanotechnology* **2009**, *20*, 204009.

[317] G. O. Piloyan, N. S. Bortnikov, N. M. Boeva, *J. Mod. Phys.* **2013**, *4*, 16.

[318] a) T. Frolov, Y. Mishin, *Phys. Rev. B* **2009**, *79*, 045430; b) Q. Zhao, Y. Liu, E. W. Abel, *J. Colloid Interface Sci.* **2004**, *280*, 174;
c) L. Sandoval, C. Reina, J. Marian, *Sci. Rep.* **2015**, *5*, 17251.

[319] H. Omid, H. Delavari, H. R. Madaah Hosseini, *J. Phys. Chem. C* **2011**, *115*, 17310.

[320] a) A. Pan, T. Kar, A. K. Rakshit, S. P. Moulik, *J. Phys. Chem. B* **2016**, *120*, 10531; b) L.-J. Chen, S.-Y. Lin, C.-C. Huang, *J. Phys. Chem. B* **1998**, *102*, 4350; c) A. Cooper, C. M. Johnson, J. H. Lakey, M. Nöllmann, *Biophys. Chem.* **2001**, *93*, 215; d) R. Lumry, S. Rajender, *Biopolymers* **1970**, *9*, 1125.

[321] a) E. B. Starikov, B. Nordén, *J. Phys. Chem. B* **2007**, *111*, 14431;
b) K. Sharp, *Protein Sci.* **2001**, *10*, 661.

[322] K. C. Kim, B. Dai, J. K. Johnson, D. S. Sholl, *Nanotechnology* **2009**, *20*, 204001.

[323] a) S. Cheung, W.-Q. Deng, A. C. van Duin, W. A. Goddard, *J. Phys. Chem. A* **2005**, *109*, 851; b) J.-C. Crivello, B. Dam, R. Denys, M. Dornheim, D. Grant, J. Huot, T. R. Jensen, P. de Jongh, M. Latroche, C. Milanese, D. Milčius, G. S. Walker, C. J. Webb, C. Zlotea, V. A. Yartys, *Appl. Phys. A* **2016**, *122*, 97;
c) R. W. P. Wagemans, J. H. van Lenthe, P. E. de Jongh, A. J. van Dillen, K. P. de Jong, *J. Am. Chem. Soc.* **2005**, *127*, 16675;
d) Z. Wu, M. D. Allendorf, J. C. Grossman, *J. Am. Chem. Soc.* **2009**, *131*, 13918.

[324] Y. Liu, J. Zou, X. Zeng, X. Wu, H. Tian, W. Ding, J. Wang, A. Walter, *Int. J. Hydrogen Energy* **2013**, *38*, 5302.

[325] M. Yamauchi, R. Ikeda, H. Kitagawa, M. Takata, *J. Phys. Chem. C* **2008**, *112*, 3294.
[326] a) W. Liu, K. F. Aguey-Zinsou, *J. Mater. Chem. A* **2014**, *2*, 9718;
b) M. Paskevicius, D. A. Sheppard, C. E. Buckley, *J. Am. Chem. Soc.* **2010**, *132*, 5077.

[327] W. Luo, W. Hu, *Phys. B* **2013**, *425*, 90.

[328] W. H. Qi, *Phys. B* **2005**, *368*, 46.

[329] a) P. E. De Jongh, P. Adelhelm, *ChemSusChem* **2010**, *3*, 1332;
b) T. K. Nielsen, F. Besenbacher, T. R. Jensen, *Nanoscale* **2011**, *3*, 2086.

[330] Q. Lai, C. Milanese, K.-F. Aguey-Zinsou, *ACS Appl. Energy Mater.* **2018**, *1*, 421.

[331] J. J. Vajo, *Curr. Opin. Solid State Mater.* **Sci. 2011**, *15*, 52.

[332] a) T. Mueller, G. Ceder, *ACS Nano* **2010**, *4*, 5647; b) E. H. Majzoub, F. Zhou, V. Ozolins, *J. Phys. Chem. C* **2011**, *115*, 2636.

[333] X. Liu, D. Peaslee, C. Z. Jost, E. H. Majzoub, *J. Phys. Chem. C* **2010**, *114*, 14036.

[334] M. L. Christian, K.-F. Aguey-Zinsou, *ACS Nano* **2012**, *6*, 7739.

[335] a) J. F. Perez-Benito, M. Mulero-Raichs, *J. Phys. Chem. A* **2016**, *120*, 7598; b) A. Pan, T. Biswas, A. K. Rakshit, S. P. Moulik, *J. Phys. Chem. B* **2015**, *119*, 15876.

[336] a) R. R. Krug, *Ind. Eng. Chem. Fundam.* **1980**, *19*, 50;
b) E. B. Starikov, *Chem. Phys. Lett.* **2013**, *564*, 88.

[337] a) C. Piguet, *Dalton Trans.* **2011**, *40*, 8059; b) D. De Marco, A. Giannetto, W. Linert, *Thermochim. Acta* **1996**, *286*, 387.

[338] a) C. Langhammer, V. P. Zhdanov, I. Zorić, B. Kasemo, *Phys. Rev. Lett.* **2010**, *104*, 135502; b) X. Yao, Z. H. Zhu, H. M. Cheng, G. Q. Lu, *J. Mater. Res.* **2008**, *23*, 336.

[339] P. Ngene, R. van den Berg, M. H. W. Verkuijlen, K. P. de Jong, P. E. de Jongh, *Energy Environ. Sci.* **2011**, *4*, 4108.

[340] E. J. Setijadi, C. Boyer, K.-F. Aguey-Zinsou, *Int. J. Hydrogen Energy* **2013**, *38*, 5746.

[341] a) S.-H. Jhi, Y.-K. Kwon, *Phys. Rev. B* **2004**, *69*, 245407;
b) S. A. Shevlin, Z. X. Guo, *Chem. Soc. Rev.* **2009**, *38*, 211.

[342] R. B. Getman, Y.-S. Bae, C. E. Wilmer, R. Q. Snurr, *Chem. Rev.* **2012**, *112*, 703.

[343] J. L. G. Hector, J. F. Herbst, *J. Phys.: Condens. Matter* **2008**, *20*, 064229.

[344] S. Baroni, S. de Gironcoli, A. Dal Corso, P. Giannozzi, *Rev. Mod. Phys.* **2001**, *73*, 515.

[345] D. Alfe, *Comput. Phys. Commun.* **2009**, *180*, 2622.

[346] C. Cazorla, J. Boronat, *Rev. Mod. Phys.* **2017**, *89*, 035003.

[347] S. A. Shevlin, C. Cazorla, Z. X. Guo, *J. Phys. Chem. C* **2012**, *116*, 13488.

[348] J. P. Soulié, G. Renaudin, R. Černý, K. Yvon, *J. Alloys Compd.* **2002**, *346*, 200.

[349] A. Tekin, R. Caputo, A. Züttel, *Phys. Rev. Lett.* **2010**, *104*, 215501.

[350] T. Yildirim, S. Ciraci, *Phys. Rev. Lett.* **2005**, *94*, 175501.

[351] P. Hänggi, P. Talkner, M. Borkovec, *Rev. Mod. Phys.* **1990**, *62*, 251.

[352] G. Henkelman, B. P. Uberuaga, H. Jónsson, *J. Chem. Phys.* **2000**, *113*, 9901.

[353] a) A. C. T. van Duin, S. Dasgupta, F. Lorant, W. A. Goddard, *J. Phys. Chem. A* **2001**, *105*, 9396; b) D. W. M. Hofmann, L. Kuleshova, B. D'Aguanno, *Chem. Phys. Lett.* **2007**, *448*, 138.

[354] a) C. Cazorla, *Coord. Chem. Rev.* **2015**, *300*, 142; b) E. F. George, *J. Phys.: Condens. Matter* **2002**, *14*, R453.

[355] W. Kohn, L. J. Sham, *Phys. Rev.* **1965**, *140*, A1133.

[356] D. M. Ceperley, B. J. Alder, *Phys. Rev. Lett.* **1980**, *45*, 566.

[357] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, *J. Chem. Phys.* **2010**, *132*, 154104.

[358] A. Tkatchenko, M. Scheffler, *Phys. Rev. Lett.* **2009**, *102*, 073005.

[359] M. Dion, H. Rydberg, E. Schröder, D. C. Langreth, B. I. Lundqvist, *Phys. Rev. Lett.* **2004**, *92*, 246401.

[360] J. F. Dobson, T. Gould, *J. Phys.: Condens. Matter* **2012**, *24*, 073201.

[361] C. Cazorla, S. A. Shevlin, *Dalton Trans.* **2013**, *42*, 4670.

[362] a) M. Bajdich, F. A. Reboredo, P. R. C. Kent, *Phys. Rev. B* **2010**, *82*, 081405; b) J. Cha, S. Lim, C. H. Choi, M.-H. Cha, N. Park, *Phys. Rev. Lett.* **2009**, *103*, 216102.

[363] H. Tanaka, H. Kanoh, M. Yudasaka, S. Iijima, K. Kaneko, *J. Am. Chem. Soc.* **2005**, *127*, 7511.

[364] P. Kowalczyk, A. P. Terzyk, P. A. Gauden, S. Furmaniak, K. Kaneko, T. F. Miller, *J. Phys. Chem. Lett.* **2015**, *6*, 3367.

[365] C. Zhang, M. Dyer, A. Alavi, *J. Phys. Chem. B* **2005**, *109*, 22089.

[366] J. Behler, *Int. J. Quantum Chem.* **2015**, *115*, 1032.

[367] M. O. J. Jäger, E. V. Morooka, F. Federici Canova, L. Himanen, A. S. Foster, *npj Comput. Mater.* **2018**, *4*, 37.

[368] J. R. Hattrick-Simpers, K. Choudhary, C. Corgnale, *Mol. Syst. Des. Eng.* **2018**, *3*, 509.

[369] A. W. Thornton, C. M. Simon, J. Kim, O. Kwon, K. S. Deeg, K. Konstas, S. J. Pas, M. R. Hill, D. A. Winkler, M. Haranczyk, B. Smit, *Chem. Mater.* **2017**, *29*, 2844.

[370] a) M. Ley, M. Meggouh, R. Moury, K. Peinecke, M. Felderhoff, *Materials* **2015**, *8*, 5891; b) U. Demirci, *Energy Technol.* **2018**, *6*, 470; c) S. S. Muir, X. D. Yao, *Int. J. Hydrogen Energy* **2011**, *36*, 5983; d) H. L. Jiang, S. K. Singh, J. M. Yan, X. B. Zhang, Q. Xu, *ChemSusChem* **2010**, *3*, 541.

[371] E. Keskin, B. Coşkuner Filiz, S. Kılıç Depren, A. Kantürk Figen, *Int. J. Hydrogen Energy* **2018**, *43*, 20354.

[372] a) S. C. Amendola, S. L. Sharp-Goldman, M. S. Janjua, N. C. Spencer, M. T. Kelly, P. J. Petillo, M. Binder, *Int. J. Hydrogen Energy* **2000**, *25*, 969; b) S. C. Amendola, S. L. Sharp-Goldman, M. S. Janjua, M. T. Kelly, P. J. Petillo, M. Binder, *J. Power Sources* **2000**, *85*, 186.

[373] a) U. B. Demirci, *Energy Technol.* **2018**, *6*, 470; b) R. Chamoun, U. B. Demirci, P. Miele, *Energy Technol.* **2015**, *3*, 100.

[374] Y. Kojima, K.-i. Suzuki, K. Fukumoto, Y. Kawai, M. Kimbara, H. Nakanishi, S. Matsumoto, *J. Power Sources* **2004**, *125*, 22.

[375] B. S. Richardson, J. F. Birdwell, F. G. Pin, J. F. Jansen, R. F. Lind, *J. Power Sources* **2005**, *145*, 21.

[376] a) S.-C. Li, F.-C. Wang, *Int. J. Hydrogen Energy* **2016**, *41*, 3038; b) F.-C. Wang, W.-H. Fang, *Int. J. Hydrogen Energy* **2017**, *42*, 10376.

[377] B. G. Gang, S. Kwon, *Int. J. Hydrogen Energy* **2018**, *43*, 6331.

[378] a) K. Kim, T. Kim, K. Lee, S. Kwon, *J. Power Sources* **2011**, *196*, 9069; b) T. Kim, S. Kwon, *Int. J. Hydrogen Energy* **2012**, *37*, 615.

[379] E. S. Jung, H. Kim, S. Kwon, T. H. Oh, *Int. J. Green Energy* **2018**, *15*, 385.

[380] B. G. Gang, S. Kwon, *Int. J. Hydrogen Energy* **2018**, *43*, 9787.

[381] a) D. Gervasio, S. Tasic, F. Zenhausern, *J. Power Sources* **2005**, *149*, 15; b) S. Galli, M. De Francesco, G. Monteleone, R. Oronzio, A. Pozio, *Int. J. Hydrogen Energy* **2010**, *35*, 7344; c) T. Kim, *Int. J. Hydrogen Energy* **2011**, *36*, 1404.

[382] J. Lee, T. Kim, *Acta Astronaut.* **2014**, *101*, 165.

[383] T. H. Oh, *Energy Convers. Manage.* **2018**, *176*, 349.

[384] M. Rivarolo, O. Improta, L. Magistri, M. Panizza, A. Barbucci, *Int. J. Hydrogen Energy* **2018**, *43*, 1606.

[385] N. Lapeña-Rey, J. A. Blanco, E. Ferreyra, J. L. Lemus, S. Pereira, E. Serrot, *Int. J. Hydrogen Energy* **2017**, *42*, 6926.

[386] *Fuel Cells Bull.* **2007**, *2007*, 6.

[387] H. J. Kim, K.-J. Shin, H.-J. Kim, M. K. Han, H. Kim, Y.-G. Shul, K. T. Jung, *Int. J. Hydrogen Energy* **2010**, *35*, 12239.

[388] E. Petit, P. Miele, U. B. Demirci, *ChemSusChem* **2016**, *9*, 1777.

[389] J. Kim, T. Kim, *Appl. Energy* **2015**, *160*, 945.

[390] P. Brack, S. E. Dann, K. G. U. Wijayantha, *Energy Sci. Eng.* **2015**, *3*, 174.

[391] J. Andrieux, L. Laversenne, O. Krol, R. Chiriac, Z. Bouajila, R. Tenu, J. J. Counioux, C. Goutaudier, *Int. J. Hydrogen Energy* **2012**, *37*, 5798.

[392] a) Y. Kojima, T. Haga, *Int. J. Hydrogen Energy* **2003**, *28*, 989; b) C. Lang, Y. Jia, J. Liu, H. Wang, L. Ouyang, M. Zhu, X. Yao, *Int. J. Hydrogen Energy* **2017**, *42*, 13127.

[393] L. Z. Ouyang, H. Zhong, Z. M. Li, Z. J. Cao, H. Wang, J. W. Liu, X. K. Zhu, M. Zhu, *J. Power Sources* **2014**, *269*, 768.

[394] Ç. Çakanyıldırım, M. Gürü, *Renewable Energy* **2010**, *35*, 1895.
[395] a) B. H. Liu, Z. P. Li, J. K. Zhu, N. Morigasaki, S. Suda, *Energy Fuels* **2007**, *21*, 1707; b) B. H. Liu, Z. P. Li, N. Morigasaki, S. Suda, *Energy Fuels* **2008**, *22*, 1894.

[396] A. M. Beaird, P. Li, H. S. Marsh, W. A. Al-Saidi, J. K. Johnson, M. A. Matthews, C. T. Williams, *Ind. Eng. Chem. Res.* **2011**, *50*, 7746.

[397] W. Chen, L. Z. Ouyang, J. W. Liu, X. D. Yao, H. Wang, Z. W. Liu, M. Zhu, *J. Power Sources* **2017**, *359*, 400.

[398] a) H. Zhong, L. Ouyang, J. Liu, C. Peng, X. Zhu, W. Zhu, F. Fang, M. Zhu, *J. Power Sources* **2018**, *390*, 71; b) H. Zhong, L. Z. Ouyang, J. S. Ye, J. W. Liu, H. Wang, X. D. Yao, M. Zhu, *Energy Storage Mater.* **2017**, *7*, 222.

[399] a) C.-H. Liu, B.-H. Chen, *Materials* **2015**, *8*, 3456; b) L. Ouyang, W. Chen, J. Liu, M. Felderhoff, H. Wang, M. Zhu, *Adv. Energy Mater.* **2017**, *7*, 1700299; c) L. Ouyang, H. Zhong, H.-W. Li, M. Zhu, *Inorganics* **2018**, *6*, 10; d) İ. Ar, Ö. U. Güler, M. Gürü, *Int. J. Hydrogen Energy* **2018**, *43*, 20214.

[400] T. Kemmitt, G. J. Gainsford, *Int. J. Hydrogen Energy* **2009**, *34*, 5726.

[401] U. B. Demirci, *Turk. J. Chem.* **2018**, *42*, 193.

[402] A. E. Sanli, I. Kayacan, B. Z. Uysal, M. L. Aksu, *J. Power Sources* **2010**, *195*, 2604.

[403] M. Monteverde, L. Magistri, *Int. J. Hydrogen Energy* **2012**, *37*, 5452.

[404] M. Chandra, Q. Xu, *J. Power Sources* **2007**, *168*, 135.

[405] G. Moussa, R. Moury, U. B. Demirci, P. Miele, *Int. J. Hydrogen Energy* **2013**, *38*, 7888.

[406] J.-F. Petit, P. Miele, U. B. Demirci, *Int. J. Hydrogen Energy* **2016**, *41*, 15462.

[407] O. T. Summerscales, J. C. Gordon, *Dalton Trans.* **2013**, *42*, 10075.

[408] T. Q. Hua, R. K. Ahluwalia, *Int. J. Hydrogen Energy* **2012**, *37*, 14382.

[409] B. L. Davis, D. A. Dixon, E. B. Garner, J. C. Gordon, M. H. Matus, B. Scott, F. H. Stephens, *Angew. Chem., Int. Ed.* **2009**, *48*, 6812.

[410] A. D. Sutton, A. K. Burrell, D. A. Dixon, E. B. Garner, J. C. Gordon, T. Nakagawa, K. C. Ott, J. P. Robinson, M. Vasiliu, *Science* **2011**, *331*, 1426.

[411] a) G. Moussa, U. B. Demirci, S. Malo, S. Bernard, P. Miele, *J. Mater. Chem. A* **2014**, *2*, 7717; b) Z. Tang, Y. Tan, X. Chen, X. Yu, *Chem. Commun.* **2012**, *48*, 9296.

[412] S. Hausdorf, F. Baitalow, G. Wolf, F. O. R. L. Mertens, *Int. J. Hydrogen Energy* **2008**, *33*, 608.

[413] C. Reller, F. Mertens, *ChemPlusChem* **2018**, *83*, 1013.

[414] P. Muthukumar, A. Singhal, G. K. Bansal, *Int. J. Hydrogen Energy* **2012**, *37*, 14351.

[415] K. J. Kim, B. Montoya, A. Razani, K. H. Lee, *Int. J. Hydrogen Energy* **2001**, *26*, 609.

[416] a) A. Jemni, S. B. Nasrallah, *Int. J. Hydrogen Energy* **1995**, *20*, 881; b) Z. Guo, H. J. Sung, *Int. J. Heat Mass Transfer* **1999**, *42*, 379; c) F. Laurencelle, J. Goyette, *Int. J. Hydrogen Energy* **2007**, *32*, 2957; d) B. J. Hardy, D. L. Anton, *Int. J. Hydrogen Energy* **2009**, *34*, 2992.

[417] a) F. Askri, A. Jemni, S. Ben Nasrallah, *Int. J. Hydrogen Energy* **2003**, *28*, 537; b) E. Bershadsky, Y. Josephy, M. Ron, *J. Less-Common Met.* **1989**, *153*, 65; c) M. Ron, D. Gruen, M. Mendelsohn, I. Sheft, *J. Less-Common Met.* **1980**, *74*, 445; d) A. R. Sánchez, H. P. Klein, M. Groll, *Int. J. Hydrogen Energy* **2003**, *28*, 515.

[418] F. S. Yang, G. X. Wang, Z. X. Zhang, X. Y. Meng, V. Rudolph, *Int. J. Hydrogen Energy* **2010**, *35*, 3832.

[419] a) I. Sheft, D. M. Gruen, G. J. Lamich, *J. Less-Common Met.* **1980**, *74*, 401; b) F. Laurencelle, Z. Dehouche, F. Morin, J. Goyette, *J. Alloys Compd.* **2009**, *475*, 810.

[420] R. C. Bowman Jr., *J. Alloys Compd.* **2003**, *356-357*, 789.

[421] M. Botzung, S. Chaudourne, O. Gillia, C. Perret, M. Latroche, A. Percheron-Guegan, P. Marty, *Int. J. Hydrogen Energy* **2008**, *33*, 98.

[422] H. Choi, A. F. Mills, *Int. J. Heat Mass Transfer* **1990**, *33*, 1281.

[423] a) L. Zu, S. Koussios, A. Beukers, *Int. J. Hydrogen Energy* **2012**, *37*, 14343; b) L. Zu, D. Zhang, Y. Xu, D. Xiao, *Int. J. Hydrogen Energy* **2012**, *37*, 1027.

[424] G. A. Lozano, J. M. Bellosta von Colbe, T. Klassen, M. Dornheim, *Int. J. Hydrogen Energy* **2014**, *39*, 18952.

[425] S. Vyazovkin, A. K. Burnham, J. M. Criado, L. A. Pérez-Maqueda, C. Popescu, N. Sbirrazzuoli, *Thermochim. Acta* **2011**, *520*, 1.

[426] a) M. Bhouri, J. Goyette, B. J. Hardy, D. L. Anton, *Int. J. Hydrogen Energy* **2011**, *36*, 621; b) M. Bhouri, J. Goyette, B. J. Hardy, D. L. Anton, *Int. J. Hydrogen Energy* **2012**, *37*, 1551.

[427] C. Na Ranong, M. Höhne, J. Franzen, J. Hapke, G. Fieg, M. Dornheim, N. Eigen, J. M. Bellosta von Colbe, O. Metz, *Chem. Eng. Technol.* **2009**, *32*, 1154.

[428] B. J. Hardy, D. L. Anton, *Int. J. Hydrogen Energy* **2009**, *34*, 2269.

[429] M. Raju, S. Kumar, *Int. J. Hydrogen Energy* **2011**, *36*, 1578.

[430] a) G. A. Lozano, C. N. Ranong, J. M. Bellosta von Colbe, R. Bormann, J. Hapke, G. Fieg, T. Klassen, M. Dornheim, *Int. J. Hydrogen Energy* **2012**, *37*, 2825; b) T. A. Johnson, M. P. Kanouff, D. E. Dedrick, G. H. Evans, S. W. Jorgensen, *Int. J. Hydrogen Energy* **2012**, *37*, 2835.

[431] J. Nam, J. Ko, H. Ju, *Appl. Energy* **2012**, *89*, 164.

[432] a) S. Srinivasa Murthy, *J. Heat Transfer* **2012**, *134*, 031020; b) Y. Ishido, M. Kawamura, S. Ono, *Int. J. Hydrogen Energy* **1982**, *7*, 173; c) E. Suissa, I. Jacob, Z. Hadari, *J. Less-Common Met.* **1984**, *104*, 287; d) S. Da-Wen, D. Song-Jiu, *J. Less-Common Met.* **1990**, *160*, 387; e) S. Suda, N. Kobayashi, K. Yoshida, *Int. J. Hydrogen Energy* **1981**, *6*, 521.

[433] a) A. Kempf, W. R. B. Martin, *Phys.-Usp.* **1986**, *11*, 107; b) J. Kapischke, J. Hapke, *Exp. Therm. Fluid Sci.* **1998**, *17*, 347.

[434] a) B. Delhomme, P. De Rango, P. Marty, M. Bacia, B. Zawilski, C. Raufast, S. Miraglia, D. Fruchart, *Int. J. Hydrogen Energy* **2012**, *37*, 9103; b) H. Bjurstrom, Y. Komazaki, S. Suda, *J. Less-Common Met.* **1987**, *131*, 225; c) A. Kumar Phate, M. Prakash Maiya, S. S. Murthy, *Int. J. Hydrogen Energy* **2007**, *32*, 1969; d) S. G. Lee, H. H. Lee, K. Y. Lee, J. Y. Lee, *J. Alloys Compd.* **1996**, *235*, 84.

[435] a) J. G. Park, K. J. Jang, P. S. Lee, J. Y. Lee, *Int. J. Hydrogen Energy* **2001**, *26*, 701; b) A. Isselhorst, M. Groll, *J. Alloys Compd.* **1995**, *231*, 888; c) D. Mori, N. Kobayashi, T. Shinozawa, T. Matsunaga, H. Kubo, K. Toh, M. Tsuzuki, *J. Jpn. Inst. Met.* **2005**, *69*, 308.

[436] S. Garrier, B. Delhomme, P. De Rango, P. Marty, D. Fruchart, S. Miraglia, *Int. J. Hydrogen Energy* **2013**, *38*, 9766.

[437] a) *Fuel Cells Bull.* **2016**, *2016*, 6; b) *Fuel Cells Bull.* **2015**, 2015, 1.

[438] a) M. Jehan, D. Fruchart, *J. Alloys Compd.* **2013**, *580*, S343; b) *Fuel Cells Bull.* **2015**, 2015, 13.

[439] K. H. Young, J. Nei, *Materials* **2013**, *6*, 4574.

[440] a) C. Wadell, S. Syrenova, C. Langhammer, ACS Nano **2014**, *8*, 11925; b) M. M. H. Bhuiya, A. Kumar, K. J. Kim, *Int. J. Hydrogen Energy* **2015**, *40*, 2231.

[441] M. V. Lototskyy, V. A. Yartys, B. G. Pollet, R. C. Bowman, *Int. J. Hydrogen Energy* **2014**, *39*, 5818.

[442] a) I. Kunce, M. Polanski, J. Bystrzycki, *Int. J. Hydrogen Energy* **2014**, *39*, 9904; b) M. Sahlberg, D. Karlsson, C. Zlotea, U. Jansson, *Sci. Rep.* **2016**, *6*, 36770.

[443] J. Wang, H.-W. Li, P. Chen, *MRS Bull.* **2013**, *38*, 480.

[444] J.-R. Ares, R. Nevshupa, E. Muñoz-Cortés, C. Sánchez, F. Leardini, I.-J. Ferrer, V. Minh Huy Tran, F. Aguey-Zinsou, J.-F. Fernández, *ChemPhysChem* **2019**, *20*, 1.

[445] J. Jepsen, *Technical and Economic Evaluation of Hydrogen Storage Systems Based on Light Metal Hydrides*, Germany, **2014**.

[446] V. C. Y. Kong, F. R. Foulkes, D. W. Kirk, J. T. Hinatsu, *Int. J. Hydrogen Energy* **1999**, *24*, 665.

[447] B. Bogdanović, G. Sandrock, *MRS Bull.* **2002**, *27*, 712.

[448] H. Schlesinger, H. C. Brown, *J. Am. Chem. Soc.* **1940**, *62*, 3429.
[449] K. Miwa, M. Aoki, T. Noritake, N. Ohba, Y. Nakamori, S.-I. Towata, A. Züttel, S.-I. Orimo, *Phys. Rev. B* **2006**, *74*, 155122.

[450] A. Züttel, S. Rentsch, P. Fischer, P. Wenger, P. Sudan, P. Mauron, C. Emmenegger, *J. Alloys Compd.* **2003**, *356–357*, 515.

[451] W. D. Davis, L. Mason, G. Stegeman, *J. Am. Chem. Soc.* **1949**, *71*, 2775.

[452] N. Webbook.

[453] K. Miwa, N. Ohba, S.-i. Towata, Y. Nakamori, S.-I. Orimo, *Phys. Rev. B* **2004**, *69*, 245120.

[454] B. Laversenne, L. Bonnetot, B. Bonnetot, *Entropy* **2005**, *184*, 183.

[455] O. Knacke, O. Kubaschewski, K. Hesselmann, Springer, Berlin **1991**.

[456] T. Matsunaga, F. Buchter, P. Mauron, M. Bielman, Y. Nakamori, S. Orimo, N. Ohba, K. Miwa, S. Towata, A. Züttel, *J. Alloys Compd.* **2008**, *459*, 583.

[457] H.-W. Li, K. Kikuchi, Y. Nakamori, N. Ohba, K. Miwa, S. Towata, S. Orimo, *Acta Mater.* **2008**, *56*, 1342.

[458] J. Mao, Z. Guo, C. K. Poh, A. Ranjbar, Y. Guo, X. Yu, H. Liu, *J. Alloys Compd.* **2010**, *500*, 200.

[459] H. Schlesinger, R. T. Sanderson, A. Burg, *J. Am. Chem. Soc.* **1940**, *62*, 3421.

[460] D. Pukazhselvan, V. Kumar, S. Singh, *Nano Energy* **2012**, *1*, 566.

[461] F. H. Stephens, V. Pons, R. T. Baker, *Dalton Trans.* **2007**, 25, 2613.

[462] B. Bonnetot, G. Chahine, P. Claudy, M. Diot, J. M. Letoffe, *The J. Chem. Thermodyn.* **1980**, *12*, 249.

[463] M. Arroyo y de Dompablo, G. Ceder, Z. Anorg. Allg. Chem. **2005**, *637*, 1982.

[464] S. Hino, T. Ichikawa, Y. Kojima, *J. Chem. Thermodyn.* **2010**, *42*, 140.

[465] L. H. Jepsen, P. Wang, G. Wu, Z. Xiong, F. Besenbacher, P. Chen, T. R. Jensen, *Phys. Chem. Chem. Phys.* **2016**, *18*, 25257.

[466] H. Imamura, K. Masanari, M. Kusuhara, H. Katsumoto, T. Sumi, Y. Sakata, *J. Alloys Compd.* **2005**, *386*, 211.

[467] Y. Chen, J. Williams, *J. Alloys Compd.* **1995**, 217, 181.

[468] S. Doppiu, L. Schultz, O. Gutfleisch, *J. Alloys Compd.* **2007**, *427*, 204.

[469] A. Zaluska, L. Zaluski, J. Ström-Olsen, *J. Alloys Compd.* **1999**, *289*, 197.

[470] M. Ponthieu, M. Calizzi, L. Pasquini, J. Fernández, F. Cuevas, *Int. J. Hydrogen Energy* **2014**, *39*, 9918.

[471] F. Hosseini-Gourajoubi, M. Pourabdoli, D. Uner, S. Raygan, *Adv. Powder Technol.* **2015**, *26*, 448.

[472] V. Fuster, G. Urretavizcaya, F. J. Castro, *J. Alloys Compd.* **2009**, *481*, 673.

[473] M.-H. Grosjean, L. Roué, *J. Alloys Compd.* **2006**, *416*, 296.

[474] D. Mirabile Gattia, G. Gizer, A. Montone, *Int. J. Hydrogen Energy* **2014**, *39*, 9924.

[475] D. A. Small, G. R. MacKay, R. A. Dunlap, *J. Alloys Compd.* **1999**, *284*, 312.

[476] H. Shao, M. Felderhoff, F. Schüth, *Int. J. Hydrogen Energy* **2011**, *36*, 10828.

[477] K. Prashanth, *Mater. Manuf. Processes* **2010**, 25, 974.

[478] S.-I. Orimo, F. Kimmerle, G. Majer, *Phys. Rev. B* **2001**, *63*, 094307.

[479] S. Orimo, H. Fujii, T. Yoshino, *J. Alloys Compd.* **1995**, 217, 287.

[480] M. S. El-Eskandarany, H. Ahmed, K. Sumiyama, K. Suzuki, *J. Alloys Compd.* **1995**, *218*, 36.

[481] K. Aoki, A. Memezawa, T. Masumoto, *J. Mater. Res.* **1994**, *9*, 39.

[482] H. Fujii, S. Munehiro, K. Fujii, S. Orimo, *J. Alloys Compd.* **2002**, *330–332*, 747.

[483] J. Huot, H. Hayakawa, E. Akiba, *J. Alloys Compd.* **1997**, 248, 164.

[484] J. Huot, S. Boily, E. Akiba, R. Schulz, *J. Alloys Compd.* **1998**, *280*, 306.

[485] J. Chen, H. Takeshita, D. Chartouni, N. Kuriyama, T. Sakai, *J. Mater. Sci.* **2001**, *36*, 5829.

[486] M. G. Verón, A. M. Condó, F. C. Gennari, *Int. J. Hydrogen Energy* **2013**, *38*, 973.

[487] X. Liu, Y. Zhu, L. Li, *J. Alloys Compd.* **2008**, *455*, 197.

[488] J. Huot, E. Akiba, T. Takada, *J. Alloys Compd.* **1995**, *231*, 815.

[489] J. Zhang, F. Cuevas, W. Zaïdi, J.-P. Bonnet, L. Aymard, J.-L. Bobet, M. Latroche, *J. Phys. Chem. C* **2011**, *115*, 4971.

[490] S. Orimo, H. Fujii, K. Ikeda, *Acta Mater.* **1997**, *45*, 331.

[491] W. Kalisvaart, H. Wondergem, F. Bakker, P. Notten, *J. Mater. Res.* **2007**, *22*, 1640.

[492] L. Lu, Y. F. Zhang, *J. Alloys Compd.* **1999**, *290*, 279.

[493] K. Ninez, M. Junaidi, H. Purwaningsih, *Int. J. Mechan. Prod. Eng.* **2014**, *2*, 26.

[494] B. Srinivasarao, C. Suryanarayana, K. Oh-ishi, K. Hono, *Mater. Sci. Eng.*, *A* **2009**, *518*, 100.

[495] W. Lee, S. Kwun, *J. Alloys Compd.* **1996**, *240*, 193.

[496] J. Keskinen, A. Pogany, J. Rubin, P. Ruuskanen, *Mater. Sci. Eng.*, *A* **1995**, *196*, 205.

[497] Q. Zhang, H. Enoki, E. Akiba, *J. Alloys Compd.* **2007**, *427*, 153.

[498] C. Fan, X. Ju, C. Wan, *Int. J. Hydrogen Energy* **2010**, *35*, 8044.

[499] J. Gu, Y. Zhu, L. Li, *Int. J. Hydrogen Energy* **2008**, *33*, 2970.

[500] X. D. Li, O. Elkedim, M. Nowak, M. Jurczyk, *Int. J. Hydrogen Energy* **2014**, *39*, 9735.

[501] Y. Kojima, Y. Kawai, T. Haga, M. Matsumoto, A. Koiwai, *J. Alloys Compd.* **2007**, *441*, 189.

[502] M. Mamatha, C. Weidenthaler, A. Pommerin, M. Felderhoff, F. Schüth, *J. Alloys Compd.* **2006**, *416*, 303.

[503] J. M. Bellosta von Colbe, M. Felderhoff, B. Bogdanović, F. Schüth, C. Weidenthaler, *Chem. Commun.* **2005**, 4732.

[504] N. Eigen, M. Kunowsky, T. Klassen, R. Bormann, *J. Alloys Compd.* **2007**, *430*, 350.

[505] X. Xiao, K. Yu, X. Fan, Z. Wu, X. Wang, C. Chen, Q. Wang, L. Chen, *Int. J. Hydrogen Energy* **2011**, *36*, 539.

[506] J. Huot, S. Boily, V. Gürther, R. Schulz, *J. Alloys Compd.* **1999**, *283*, 304.

[507] H. Morioka, K. Kakizaki, S.-C. Chung, A. Yamada, *J. Alloys Compd.* **2003**, *353*, 310.

[508] A. Bhatnagar, S. K. Pandey, R. R. Shahi, M. S. L. Hudson, M. Shaz, O. Srivastava, *Cryst. Res. Technol.* **2013**, *48*, 520.

[509] R. A. Varin, C. Chiu, T. Czujko, Z. Wronski, *J. Alloys Compd.* **2007**, *439*, 302.

[510] V. Iosub, T. Matsunaga, K. Tange, M. Ishikiriyama, *Int. J. Hydrogen Energy* **2009**, *34*, 906.

[511] Y. Kim, E.-K. Lee, J.-H. Shim, Y. W. Cho, K. B. Yoon, *J. Alloys Compd.* **2006**, *422*, 283.

[512] Y. Liu, Y. Pang, X. Zhang, Y. Zhou, M. Gao, H. Pan, *Int. J. Hydrogen Energy* **2012**, *37*, 18148.

[513] C. Li, X. Xiao, P. Ge, J. Xue, S. Li, H. Ge, L. Chen, *Int. J. Hydrogen Energy* **2012**, *37*, 936.

[514] Z. P. Li, B. H. Liu, N. Morigasaki, S. Suda, *J. Alloys Compd.* **2003**, *354*, 243.

[515] Z. P. Li, N. Morigazaki, B. H. Liu, S. Suda, *J. Alloys Compd.* **2003**, *349*, 232.

[516] H. W. Li, K. Kikuchi, Y. Nakamori, K. Miwa, S. Towata, S. Orimo, *Scr. Mater.* **2007**, *57*, 679.

[517] E. Jeon, Y. Cho, *J. Alloys Compd.* **2006**, *422*, 273.

[518] R. Liu, D. Reed, D. Book, *J. Alloys Compd.* **2012**, *515*, 32.

[519] R. Černý, N. Penin, V. D'Anna, H. Hagemann, E. Durand, J. Růžička, *Acta Mater.* **2011**, *59*, 5171.

[520] H. Leng, T. Ichikawa, S. Isobe, S. Hino, N. Hanada, H. Fujii, *J. Alloys Compd.* **2005**, *404–406*, 443.

[521] S. Hino, T. Ichikawa, H. Leng, H. Fujii, *J. Alloys Compd.* **2005**, *398*, 62.

[522] Z. Xiong, G. Wu, J. Hu, P. Chen, *J. Alloys Compd.* **2007**, *441*, 152.

[523] J. Hu, G. Wu, Y. Liu, Z. Xiong, P. Chen, K. Murata, K. Sakata, G. Wolf, *J. Phys. Chem. B* **2006**, *110*, 14688.

[524] H. Wu, *J. Am. Chem. Soc.* **2008**, *130*, 6515.
[525] J. Hu, Y. Liu, G. Wu, Z. Xiong, P. Chen, *J. Phys. Chem. C* **2007**, *111*, 18439.

[526] X. Kang, Z. Fang, L. Kong, H. Cheng, X. Yao, G. Lu, P. Wang, *Adv. Mater.* **2008**, *20*, 2756.

[527] H. Wu, W. Zhou, T. Yildirim, *J. Am. Chem. Soc.* **2008**, *130*, 14834.

[528] D. Tabor, *Proc. Phys. Soc. Sect. B* **1954**, *67*, 249.

[529] D. Tabor, *The Hardness of Metals*, Oxford University Press, **2000**.

[530] L. Peña-Parás, J. Taha-Tijerina, L. Garza, D. Maldonado-Cortés, R. Michczewski, C. Lapray, *Wear* **2015**, *332–333*, 1256.

[531] L. O'Bannon, *Dictionary of Ceramic Science and Engineering*, Springer Science & Business Media, **2012**.

[532] G. V. Samsonov, *Handbook of the Physicochemical Properties of the Elements*, Springer Science & Business Media, **2012**.

[533] a) M. Lotoskyy, R. Denys, V. A. Yartys, J. Eriksen, J. Goh, S. N. Nyamsi, C. Sita, F. Cummings, *J. Mater. Chem. A* **2018**, *6*, 10740; b) F. C. Cowlard, J. C. Lewis, *J. Mater. Sci.* **1967**, *2*, 507.

[534] T. Hryniewicz, K. Rokosz, S. Gaiaschi, P. Chapon, R. Rokicki, D. Matysek, *Mater. Lett.* **2018**, *218*, 299.

[535] A. Patah, A. Takasaki, J. S. Szmyd, *Int. J. Hydrogen Energy* **2009**, *34*, 3032.

[536] T. Sadhasivam, K. Gurunathan, *Adv. Sci.* **2014**, *6*, 1.

[537] M. E. Fleet, C. M. De Almeida, N. Angeli, *Canadian Mineral.* **2002**, *40*, 341.

[538] M. Khaghani-Dehaghani, R. Ebrahimi-Kahrizsangi, N. Setoudeh, B. Nasiri-Tabrizi, *Int. J. Refract. Met. Hard Mater.* **2011**, *29*, 244.

[539] M. Lototskyy, M. W. Davids, J. M. Sibanyoni, J. Goh, B. G. Pollet, *J. Alloys Compd.* **2015**, *645*, S454.

[540] M. H. Tavakoli, T. A. Abasi, E. H. Ali, *Cryst. Res. Technol.* **2013**, *48*, 130.

[541] N. Yasuda, T. Tsuchiya, N. Okinaka, T. Akiyama, *Int. J. Hydrogen Energy* **2012**, *37*, 9706.

[542] K. Suganuma, T. Fujita, N. Suzuki, K. Niihara, *J. Mater. Sci. Lett.* **1990**, *9*, 633.

[543] D. Chartouni, F. Meli, A. Züttel, K. Gross, L. Schlapbach, *J. Alloys Compd.* **1996**, *241*, 160.

[544] M. S. Yahya, M. Ismail, *Int. J. Hydrogen Energy* **2018**, *43*, 6244.

[545] R. L. Smith, J. W. Miser, **1963**.

[546] V. Shukla, A. Bhatnagar, S. K. Pandey, R. R. Shahi, T. P. Yadav, M. A. Shaz, O. N. Srivastava, *Int. J. Hydrogen Energy* **2015**, *40*, 12294.

[547] F. P. Bowden, T. Hughes, *Proc. R. Soc. London, Ser. A* **1937**, *160*, 575.

[548] K. Yada, T. Tanji, I. Sunagawa, *Phys. Chem. Miner.* **1981**, *7*, 47.

[549] D. L. Whitney, M. Broz, R. F. Cook, *Am. Mineral.* **2007**, *92*, 281.

[550] M. Mungole, K. Rai, R. Balasubramaniam, K. Singh, *Int. J. Hydrogen Energy* **1992**, *17*, 607.

[551] J. M. Bellosta von Colbe, O. Metz, G. A. Lozano, P. K. Pranzas, H. W. Schmitz, F. Beckmann, A. Schreyer, T. Klassen, M. Dornheim, *Int. J. Hydrogen Energy* **2012**, *37*, 2807.

[552] I. Utz, M. Linder, N. Schmidt, J. J. Hu, M. Fichtner, A. Wörner, *Int. J. Hydrogen Energy* **2012**, *37*, 7645.

[553] R. Urbanczyk, S. Peil, D. Bathen, C. Heßke, J. Burfeind, K. Hauschild, M. Felderhoff, F. Schüth, *Fuel Cells* **2011**, *11*, 911.

[554] R. Urbanczyk, K. Peinecke, M. Felderhoff, K. Hauschild, W. Kersten, S. Peil, D. Bathen, *Int. J. Hydrogen Energy* **2014**, *39*, 17118.

[555] R. Urbanczyk, K. Peinecke, M. Meggouh, M. Felderhoff, S. Peil, D. Bathen, *J. Power Sources* **2015**, *324*, 589.

[556] J. M. Bellosta von Colbe, G. Lozano, O. Metz, T. Bücherl, R. Bormann, T. Klassen, M. Dornheim, *Int. J. Hydrogen Energy* **2015**, *40*, 2984.

[557] M.-Y. Yan, F. Sun, X.-P. Liu, J.-H. Ye, S.-M. Wang, L.-J. Jiang, *J. Alloys Compd.* **2015**, *628*, 63.

[558] I. Bürger, C. Luetto, M. Linder, *Int. J. Hydrogen Energy* **2014**, *39*, 7346.

[559] A. Chibani, C. Bougriou, *Int. J. Hydrogen Energy* **2017**, *42*, 23035.

[560] K. Komiya, M. Daigoro, M. Shinpei, H. Norihiko, Y. Kousei, W. Shintaro, T. Keiji, K. Hidehito, M. Mituo, M. Seiichiro, T. Makoto, *High-Pressure Hydrogen-Absorbing Alloy Tank for Fuel Cell Vehicles*, SAE International, **2010**.

[561] D. Mori, N. Haraikawa, T. Shinozawa, T. Matsunaga, K. Toh, K. Fujita, *Trans. Jpn. Soc. Mech. Eng. Ser. B* **2007**, *73*, 1236.

[562] P. Thongtan, P. Dansirima, S. Thiangviriya, N. Thaweelap, A. Suthummapiwat, P. Plerdsranoy, R. Utke, *Int. J. Hydrogen Energy* **2018**, *43*, 12260.

[563] S. Garrier, A. Chaise, P. de Rango, P. Marty, B. Delhomme, D. Fruchart, S. Miraglia, *Int. J. Hydrogen Energy* **2011**, *36*, 9719.

[564] Y. Chen, C. A. C. Sequeira, C. Chen, X. Wang, Q. Wang, *Int. J. Hydrogen Energy* **2003**, *28*, 329.

[565] L. K. Heung, T. Motyka, W. A. Summers, *Proc. of Hypothesis IV*, CRC Press, Boca Raton **2001**, pp. 336–340.

[566] S. A. Alabbadi, *Energy Procedia* **2012**, *29*, 12.

[567] M. Baricco, M. Bang, M. Fichtner, B. Hauback, M. Linder, C. Luetto, P. Moretto, M. Sgroi, *J. Power Sources* **2017**, *342*, 853.

[568] T. Ogawa, M. Takeuchi, Y. Kajikawa, *Sustainability* **2018**, *10*, 478.

[569] A. A. Akhil, G. Huff, A. B. Currier, B. C. Kaun, D. M. Rastler, S. B. Chen, A. L. Cotter, D. T. Bradshaw, W. D. Gauntlett, *Electricity Storage Handbook in Collaboration with NRECA*, Sandia National Laboratories, Albuquerque, New Mexico and Livermore, California **2013**.

[570] U. S. Department of Energy, *DOE Technical Targets for Onboard Hydrogen Storage for Light-Duty Vehicles*, Office of Energy Efficiency & Renewable Energy, **2012**.

[571] J. Maier, W. Münch, Z. Anorg. *Allg. Chem.* **2000**, *626*, 264.

[572] X. Fan, X. Xiao, J. Shao, L. Zhang, S. Li, H. Ge, Q. Wang, L. Chen, *Nano Energy* **2013**, *2*, 995.