Experimental study of the microstructures and hydrogen storage properties of the LaNi₄Mn_0·5Co_0.5 alloys

Abstract

In the present study, the absorption and desorption kinetics of hydrogen and the isotherm (P–C-T)) of the LaNi₄Mn_0·5Co_0.5 alloy were measured at values of 283 K, 303 K, and 313 K. The morphological states of this sample were examined using characterization techniques, including X-ray diffraction and scanning electron microscopy. The thermodynamic functions for the absorption-desorption of hydrogen by hydrides, such as enthalpy (H) and entropy (S), were calculated from the experimental data or by using a model that exists in the literature and is premised on the adjustment of isotherm curves at various temperatures. This model is based on an integrated form of the Van't Hoff equation and a simultaneous examination of the isotherms. According to the experimental results, the amount of hydrogen absorbed or desorbed by the sample is significantly affected by the partial substitution of the nickel atom by the elements Mn and Co. However, this substitution increased the absorption or de-sorption plateau pressure.

1. Introduction

At present, hydrogen is viewed in the literature as a crucial component of a sustainable energy system that can deliver affordable and non-polluting energy in the quest for obtaining renewable energy alternatives. As a result, energy experts recommend hydrogen as a practical substitute fuel for several uses [[1], [2], [3], [4]]. In the same way, these types of renewable energy depend on the time factor and the situation. Therefore, it is very important to improve their storage techniques and also the means of transport. Since, these two factors are very expensive and environmentally friendly [[5], [6], [7]]. Hydrogen is the most abundant element in the environment and has a good energy density (147 MJ/kg). Consequently, it is considered one of the ideal energy carriers and used in various applications [8].

Currently, several countries have launched national strategies and studies to develop hydrogen energy. In parallel, research is focused on solving problems related to production techniques, storage, and transport of hydrogen. Hydrogen storage technology is still the major problem in this cycle. Generally, hydrogen can be stored in gas, liquid, and solid forms. Each storage technique has advantages and disadvantages. Hydrogen storage by metal alloys has several advantages over traditional storage techniques (hydrogen gas or liquid).

Since they have a high storage density and favorable kinetics reaction, metals, and intermetallic alloys are metals that can store hydrogen in a reversible manner at temperatures and pressures close to ambient [[9], [10], [11], [12]]. AB₅-type alloys have been the subject of various investigations published in the literature in recent years [[13], [14], [15], [16], [17], [18], [19], [20], [21]]. For LaNi₅-based alloys, some studies assert that a partial element substitution of either La or Ni by other elements can enhance their hydrogen storage capacity.

Nitin and all [22] performed a novel method developed to estimate the gravimetric density or weight percentage of hydrogen. Zhu al [23]. was studied the hydrogen storage performance of LaNi_5-xCo_x alloys (x = 0, 0.25, 0.50, and 0.75). Smith and Goudy [24]. examined the impact of composition substitution in the LaNi_5-xCo_x–H system (x = 0, 1, 2, and 3) on the thermodynamic and kinetic properties. the results obtained reported that the rate of dehydration is inversely corresponding to the hydride stability. They demonstrate that the volume of the cell increases with the increase in the content of the element Co, which results in a reduction of the equilibrium pressure of absorption/desorption of hydrogen and makes the hydride phase more stable. Asano and associates [[25], [26], [27], [28]] studied the stability of the hydride phase as well as the impact of substitutional Cr, Fe, Al, and M elements on the phenomena of hydrogen atom diffusion in the compound VH. They concluded that the stability of the hydride phase as well as the hydrogen atom diffusion occurring at interstitial sites were significantly affected by the concentration of these components.

LaNi₅-type alloys are observed to be the most promising H₂ storage materials when compared to other materials because of their large volumetric capacity, pleasant cycling behavior, and reaction kinetics. In this regard, it should be noted that the low thermal conductivity of these alloys presents a major challenge to the requirements for hydrogen storage [20]. Numerous studies have determined the production of alloys by replacing Ni with other elements in the combination of LaNi₅ (Fe, Co, Al, Mn, etc.) Simultaneously, alloys based on LaNi₅ can improve their overall performance regarding the H₂ storage technique by the inclusion of metals or transition elements [21,25], and [26]. Similarly, the hydrogenation or dehydrogenation cycles' stability and storage capacity is balanced by the substitution principle of hydrides. Despite this, elemental substitution continues to be a significant issue in research that requires further attention from academics [[21], [22], [23], [24], [25], [26], [27], [28]].

In the present study, a new LaNi₄Mn_0·5Co_0.5 alloy is created. In-depth experimental research is also conducted on the hydrogen absorption and desorption capacities of isotherms and kinetics. Considerable focus is placed on the impact of partial Ni substitution with Co and Mn on gaseous hydrogen storage capabilities. Furthermore, an adjustment based on the Van't Hoff equation was used to calculate the enthalpy and hydride entropy values of LaNi₄Mn_0·5Co_0.5.

2. Experiment

From the raw material (99.97% purity for La, Ni, Mn, and Co), a sample of 1 g of metal powder was created in a glove box in an argon environment. An extremely fine powder was created by mechanically grinding the alloy. Using an experimental Sievert apparatus, the behavior of the alloy's H₂ absorption and desorption processes was investigated. Its schematic is illustrated in Fig. 1. The activation operations of this sample were conducted across multiple cycles of cooling at T_fluid = 298 K and applying a pressure of 15 bars for the absorption state, and heating at T_fluid = 333 K and applying a pressure of 2 mbar for the desorption state of pure H₂ gas (99.99%). The experimental readings were examined in a hydrogen pressure range of P_ap = 8 bar to P_ap = 15 bar after the sample was fully activated. The isotherms (P–C-T) were then measured at temperatures ranging from T_Fluid = 298 K to T_Fluid = 333 K.

Fig. 1.

Schematic view of experimental installation [29].

A diffractometer was used to calculate the X-ray diffraction (XRD) measurements. This measurement technique determines the phase structure of the sample, the network parameters, and the volume of the phase cell. Similarly, the shape and chemical composition of the alloy can be determined using a scanning electron microscope.

3. Results and discussion

3.1. Structural analysis, and morphological behavior

The XRD patterns for the LaNi₄Mn_0·5Co_0.5 alloy are presented in Fig. 2. The curve reveals that the alloy has a single-phase structure of the CaCu₅-type and a space group of the P6/mmc-type. The lattice parameter values and unit lattice volume values of a sample can be determined by analyzing its X-ray diffraction pattern through the Rietveld refinement method.

Fig. 2.

XRD patterns of the LaNi₄Mn_0·5Co_0.5 alloy.

The lattice parameter values can be obtained from the refined atomic positions and the crystal symmetry of the sample. The results show that the mesh parameters have values a = 5.0330 Å and c = 3.9967 Å corresponding to a cell volume V = 87.678 Å³ for the LaNi₄Mn_0·5Co_0.5 alloy.

According to the results found by Briki et all [29,30], it is clear that the partial substitution of the element Ni in the compound LaNi₅ can increase both the lattice parameters (a and c) and the mesh volume of the alloy. Similarly, the LaNi₅ compound has the same structure and same lattice parameters a = 5.019 Å and c = 3.982 Å corresponding to V = 86.913 Å³ [29,30] as the LaNi₄Mn_0·5Co_0.5 alloy. Therefore, the addition of the element Cobalt (Co) and manganese (Mn) does not induce the formation of new phases but leads to an increase in the lattice parameters “a” and “c” and consequently the increase in the volume of the crystal lattice. This result is in agreement with the sequence of the atomic radii of the metals: R_Ni (1.24 Å) < R_Co (1.25 Å) < R_Mn (1.37 Å) which indicates that the cell volume of the alloy would increase when Ni was partially replaced by a metal of greater atomic radius.

The LaNi₄Mn_0·5Co_0.5 alloy has a cubic crystal structure, the (hkl) indices of its crystal planes can be determined using the Bragg equation (eq (1)):

$$\mathbf{n}\times \mathbf{\lambda }=\mathbf{2}\mathbf{d}\times \mathbf{sin}\mathbf{\theta }$$ (1)

where n is an integer, λ is the wavelength of the X-ray radiation used for the diffraction experiment, d is the inter-planar spacing, and θ is the angle of diffraction.

The inter-planar spacing of a crystal plane can be calculated using the following equation (eq (2)):

$$\mathit{d}=\frac{\mathit{a}}{\sqrt{{\mathit{h}}^{\mathbf{2}}+{\mathit{k}}^{\mathbf{2}}+{\mathit{l}}^{\mathbf{2}}}}$$ (2)

where a is the lattice parameter and h, k, and l are the Miller indices.

The (hkl) indices for LaNi₄Mn_0·5Co_0.5 alloy are presented in Fig. 2.

Scanning electron microscopy was used to investigate the morphology of the material. The SEM images show that the alloy is formed by nanoparticles of different dimensions. Also, these photos show that the powder has irregularly shaped and sized particles as a result of high-intensity grinding (Fig. 3).

Fig. 3.

SEM images of the LaNi₄Mn_0·5Co_0.5 alloy; (on top: scale 100 μm, down: scale 50 μm).

Therefore, the grain sizes of this sample vary from around 50 to 100 μm. In addition, the grain size of an alloy changes due to this scrambling before hydrogenation and also decreases with the number of cycles of hydrogen absorption/desorption and desorption then stabilizes.

3.2. Absorption/desorption behavior of hydrogen by the LaNi₄Mn_0·5Co_0.5 alloy

Fig. 4 depicts the LaNi₄Mn_0·5Co_0.5 alloy's hydrogen absorption rates at temperatures ranging between T_Fluid = 283 K and T_Fluid = 313 K with an applied H₂ pressure of 15 bar. These curves show that for high temperatures, the amount of hydrogen absorbed at saturation by the LaNi₄Mn_0·5Co_0.5 alloy decreases. The compound LaNi₄Mn_0·5Co_0.5 is found to reach its maximum capacity faster than the compound LaNi₅. This is due to several factors including the crystal structure and electronic properties of the compounds. The crystal structure of LaNi₅ represents a lower hydrogen diffusion coefficient, which limits the rate of hydrogen uptake and release. In contrast, the crystal structure of LaNi₄Mn_0·5Co_0.5 can provide more open sites for hydrogen adsorption, which increases the rate of hydrogen uptake and release. for this, crystal structure, and electronic properties of compounds can also play an important role in their hydrogen storage properties. Adding the elements of Co and Mn to LaNi₅ can change the electronic structure of the compound, which can affect the bond energy between hydrogen and metal atoms. This can result in a higher hydrogen uptake and release rate compared to LaNi₅. These curves show that substitution on the nickel group can improve the absorption/desorption kinetic properties of hydrogen.

Fig. 4.

Hydrogen absorption characteristics of LaNi₄Mn_0·5Co_0.5 alloy (working condition: T_Fluid = 283 K to T_Fluid = 313 K and at P₀ = 15 bar).

Simultaneously, it can be observed that this behavior, in contrast to the reaction kinetics of the LaNi₅ compound's hydrogen absorption capacity, is qualitatively more significant. This is demonstrated by the improvement in the kinetics behavior caused by the partial replacement of the Ni element by the Co and Mn elements. In addition, the cobalt metal presents strong catalytic activity that can aid in the migration of hydrogen atoms inside the alloy's grain boundaries. This justification is supported by the previously con-ducted research that indicates the dissociation of H₂ atoms might be aided by the addition of lower-valence metals to transition metals [[30], [31], [32], [33]].

The LaNi₄Mn_0·5Co_0.5 alloy's desorption process curve is depicted in Fig. 5 at temperatures ranging from T_Fluid = 283 K to T_Fluid = 308 K and at a hydrogen pressure of 2 mbar. It is evident that when the temperature increases, the alloy's hydrogen desorption kinetics considerably accelerate. At an increasing temperature of T_Fluid = 303 K, the LaNi₄Mn_0·5Co_0.5 alloy may release approximately 10⁻² g of hydrogen in approximately 20 min; however, at a warming temperature of T_Fluid = 283 K, the entire dehydrogenation process takes about 40 min.

Fig. 5.

Hydrogen desorption characteristics of LaNi₄Mn_0·5Co_0.5 alloy (working condition: T_Fluid = 283 K, T_Fluid = 298 K, T_Fluid = 308 K and at P_ap = 2 mbar).

The temperature variation that occurs during the hydrogen absorption/desorption cycle is illustrated in Fig. 6. A temperature probe was used inside the reactor to measure the temperature evolution that occurred during the absorption/desorption process of hydrogen. It can be observed that at the rate of insertion of the hydrogen atoms, the temperature inside the hydride bed increases until a saturation temperature of the order of T_sat = 319.5 K is expected, and then decreases until it reaches the operating temperature.

Fig. 6.

Evolution of the temperature of the LaNi₄Mn_0·5Co_0.5 alloy during the absorption and de-sorption processes of hydrogen.

To understand the effect of substitution on the performance of LaNi₄Mn_0·5Co_0.5 alloy reactive kinetics, The hydrogen absorption curves for the LaNi5 and LaNi₄Mn_0·5Co_0.5 alloys under a temperature T = 283 K and a pressure P = 10 bar are shown in Fig. 7. It can be seen that the LaNi₄Mn_0·5Co_0.5 alloy has a very fast kinetics as the LaNi₅ alloy.

Fig. 7.

Evolution of the mass of hydrogen absorbed by the alloys LaNi₅ and LaNi₄Mn_0·5Co_0.5 for P = 10 bar and T = 283 K.

The substitution of the nickel atom (Ni) with manganese (Mn) and cobalt (Co) elements in the LaNi5 alloy leads to an improvement in the kinetics of hydrogen absorption Fig. 7. This is explained by changes in network parameters that cause variations in the crystalline structure. These factors influence the speed at which hydrogen atoms diffuse into the LaNi₄Mn_0·5Co_0.5 alloy.

Furthermore, the changes occurring in the absorbed or desorbed hydrogen are affected by the increase and decrease in temperature levels (see Fig. 4, Fig. 5). It is clear that the hydride bed rapidly tends towards the maximum hydrogen capacity, causing a decrease in the rate of heat generation, and thus the rapid cooling of the hydride bed.

Fig. 8 depicts the pressure-concentration-temperature curves for the LaNi₄Mn_0·5Co_0.5 alloy at values of T = 298, T = 303, and T = 313 K. The experimental values are grouped in Table 1 as [H/M]_abs and [H/M]_des, which demonstrate the greatest capabilities for hydrogen absorption and desorption processes, respectively; P_eq is the equilibrium pressure in the center of the plateau zone; and H_ys is the hysteresis factor.

Fig. 8.

Pressure-concentration-temperature curves of the LaNi₄Mn_0·5Co_0.5 alloy at different temperatures.

Table 1.

Experimental hydrogen storage parameters of the LaNi₄Mn_0·5Co_0.5 alloy.

Temperature (K)	Hydrogen desorbed		Hydrogen absorbed		Hf(Jmol−1)
	[H/M]des	Peq(bar)	[H/M]abs	Peq(bar)
298	0.62	0.63	0.63	0.71	148
303	0.65	0.81	0.67	0.9	132
313	0.64	0.98	0.66	1.11	162

The mechanism for assessing a metal's resistance to sputtering during hydrogenation/dehydrogenation cycles is the hysteresis phenomenon. The Van 't Hoff equation, which represents the change in pressure at the plateau's midpoint over an absorption/desorption cycle of hydrogen H₂, was used to calculate the experimental values of hysteresis.

The outcomes are presented in Table 1. This table exhibits that, despite the increase in the temperature, there is a minor fluctuation occurring in the maximum quantity of hydrogen that our sample could absorb and desorb. On the other hand, it can be observed that when the temperature increases, the plateau pressure also increases. This is explained by how weakly our hydride stabilizes at higher temperatures. Therefore, it is evident that increasing temperatures hinder the phenomena of hydrogen absorption by metallic alloys, while favoring the process of H₂ desorption [[33], [34], [35]].

Fig. 9 shows a comparison of the isotherms of the LaNi₅ and LaNi₄Mn_0·5Co_0.5 alloys. It is remarkable the effect of substitution in LaNi₅ of the elements Mn and Co on the equilibrium pressures of the plate also on the amount stored at saturation. Several elements can substitute the elements La or Ni giving changes in the hydrogenation properties. But unfortunately, hydrogen storage capacity can never be increased by these substitutions. It is clear that there is a decrease in the amount of hydrogen absorbed at saturation of the order of 0.1 wt%. But the most important effect of substitution on the storage properties of hydrogen is proved by the pressure of the plate, which is dominated by the half to LaNi₅ alloy.

Fig. 9.

The hydrogen absorption isotherms for LaNi₅ and LaNi₄Mn_0·5Co_0.5 alloys at T = 303 K and T = 313 K.

The values of reaction enthalpy “H" and entropy “S" are determined from the equation of Van't Hoff (eq (3)).

$$\mathbf{ln}\left({\mathbf{P}}_{\mathbf{e}\mathbf{q}}\right)=\frac{\mathbf{\Delta }\mathbf{H}}{\mathbf{R}\mathbf{T}}-\frac{\mathbf{\Delta }\mathbf{S}}{\mathbf{R}}$$ (3)

where T(K) is the experiment temperature, P_eq (bar) is the equilibrium pressure, R = 8.314 Jmol⁻¹K⁻¹ is the universal gas constant, and S (Jmol⁻¹K⁻¹) and H (Jmol⁻¹K⁻¹) are the entropy and enthalpy activities of the process, respectively.

Fig. 10 depicts the development of the plateau pressure (P_eq) as a function of the temperature's reciprocity (1/T). These findings demonstrate that the connection between (P_eq) and (1/T) on Van't Hoff plots is linear. By adjusting the experimental data with the parameters of this equation, the ΔH and ΔS values for the absorption and desorption processes of H₂ by the LaNi₄Mn_0·5Co_0.5 alloy are: ΔH = 22225 ± 5 Jmol^-1 and ΔS = ±71 ± 1 Jmol^-1. The isotherms of the LaNi₄Mn_0·5Co_0.5 alloy for different temperatures show a reversible hydrogen sorption capacity of 1.2% by weight and a pressure value of the equilibrium that does not exceed 1 bar. For this reason, the decrease in plateau pressure is mainly related to the stability of the hydride, which has an enthalpy value of 71.1 J/mol and an entropy of 22225 J/mol. It can be noted that the formation enthalpy of the LaN_i5 alloy (ΔH_LaNi5 = 30.1 KJmol^-1 [[36], [37], [38], [39]]) is higher than the value of the H enthalpy of the alloy under study. In this case, it is clear that the partial substitution of the atom of Ni by the elements Mn and Co has a significant effect on the decrease of the plateau and reduces hysteresis.

Fig. 10.

The van't Hoff plot for the LaNi₄Mn_0·5Co_0.5 alloy was obtained using the data from Fig. 7.

4. Conclusion

By using X-ray diffraction techniques and scanning electron microscopy, the mi-cro-structural properties and phase compositions of the LaNi₄Mn_0·5Co_0.5 alloy were inves-tigated in the present study.

Similarly, the LaNi₄Mn_0·5Co_0.5 hydride's' hydrogen storage capabilities were experi-mentally investigated by obtaining measurements of the reaction kinetics and from the isotherms (P–C-T).

The empirical values of the hydride's enthalpy (H) and entropy (S) values were cal-culated by fitting the experimental data to the isotherms using the Van't Hoff equation.

According to the results, the intermetallic compound LaNi₄Mn_0·5Co_0.5 exhibits im-proved hydrogen absorption/desorption kinetics due to the partial substitution of Ni by Mn and Co elements.

Author contribution statement

Chaker Briki: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper.

Sihem Belkhiria: Analyzed and interpreted the data; Wrote the paper.

Maha Almoneef: Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data.

Mohamed Mbarek: Performed the experiments; Contributed reagents, materials, analysis tools or data.

Abdelmajid Jemni: Conceived and designed the experiments; Wrote the paper.

Data availability statement

Data will be made available on request.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.