Magnesium hydride for energy storage applications: The kinetics of dehydrogenation under different working conditions

TL;DR: In this paper, a new approach to the kinetics of magnesium hydride dehydrogenation is considered, where a combined kinetic analysis method, which considers the thermodynamic of the process according to the microreversibility principle, has been used for performing the kinetic analysis of data obtained under different thermal schedules at hydrogen pressures ranging from high vacuum up to 20 bar.

About: This article is published in Journal of Alloys and Compounds.The article was published on 2016-10-05 and is currently open access. It has received 22 citations till now. The article focuses on the topics: Magnesium hydride & Dehydrogenation.

Summary

1. Introduction

• Magnesium hydride is a material of the most interest for a number of technical applications, mainly as hydrogen storage material for PEM fuel cells, due to its large reversible storage capacity (7.6 mass%) of high purity hydrogen [1-5], and as a thermal energy storage system in thermosolar plants due to the high enthalpy of the hydrogenation-dehydrogenation reactions [6-10].
• Moreover, magnesium has a relatively high abundance in earth.
• For any of these two applications, the kinetics of the dehydrogenation-hydrogenation reactions is of paramount importance.
• A very large number of studies of absorption and desorption of hydrogen in MgH2 and related compounds have been performed employing either volumetric analysis under isothermal conditions or thermogravimetry (TG) and differential scanning calorimetry (DSC) at linear rising temperature under a flow of hydrogen or an inert gas [11, 17-18, 21-28].
• A proper kinetic analysis of reversible thermal decomposition reactions under a given constant pressure of the gas self-generated in the reaction would imply to consider the thermodynamic of the process by taking into account the microreversibility principle [26, 30, 34-35].

2. Experimental

• Commercially available magnesium hydride purchased from Aldrich (product number 683043, with average particle size of 50 μm) was used for performing the study.
• Typical sample size was ~85 mg, which was placed in alumina pan.
• The system was connected to a mass flow controller and a pressure controller in order to carry out the experiments under 50 cm3 min-1 hydrogen flow and at constant pressure.
• Moreover, the recorded melting point of this standard is compared to the known melting point and the difference is calculated for temperature calibration.
• The α-T plots were obtained by numerical integration of the normalized DSC plots.

3. Theoretical

• The dehydrogenation of magnesium hydride takes place according to the following reversible reaction: ←→ (1) According to the microreversibility principle, in a reversible reaction the mechanism in one direction is exactly the reverse of the mechanism in the other direction.
• If the reaction is carried out under high vacuum in such a way that the pressure p is extremely low with regard to the equilibrium pressure, the term p/p* would be close to zero and equation (2a) becomes: 2 3.1.
• The equation for the combined kinetic analysis is obtained rearranging the equation (2a) in logarithmic form after replacing the f(α) function by the Sestak-Berggren equation: /1 1 ∗ ln 5 or /1 ln 5 , 8    when far from equilibrium (i.e.; p/p* = 0).
• The set of experimental data, corresponding to different heating schedules, is substituted either into equation (5a) or (5b) and the left-hand side of the equations is plotted versus the inverse of temperature.
• The Pearson linear correlation coefficient is set as an objective function for optimization, and the values of the parameters n and m that provide the best linear fit to the plot are determined.

4. Results and discussion

• 1. Kinetics of magnesium hydride dehydrogenation in vacuum.
• It is noteworthy to point out that it has been theoretically demonstrated in literature that the β-MgH2 phase is capable of accommodating only very small concentrations of hydrogen vacancies that are mainly isolated rather than forming clusters [44].
• The original experimental curves were reconstructed by simulating a set of curves using the kinetic parameters resulting from the above analysis.
• Thus, equation 4b, without accounting for (1- p/p*), was employed for the calculations, and the activation energy values obtained are presented in Table 2.
• For MgH2 dehydrogenation under 20 bar the mean activation energy is even higher if calculated without considering the pressure term (271 kJ mol-1) and a high deviation in the activation energy of 40 kJ mol-1 is obtained between the maximum and minimum values of α.

5. Conclusions

• Kinetics of MgH2 dehydrogenation has been studied in three different experimental conditions.
• The reliability of the calculated kinetic parameters has been tested by comparing simulated and experimental curves.
• A unified theory that explains this behavior is given.
• The activation energy of the reaction is less influenced by the experimental conditions, and the values obtained agree with the value reported for MgH bond energy, which suggests that the breaking of this bond could be the rate limiting step of the MgH2 thermal dehydrogenation.

Figures

Figure 5. DSC experimental curves for thermal dehydrogenation of magnesium hydride, registered at different hydrogen pressures: a) 10 bar; b) 20 bar.

Figure 9. Comparison of the f(α) functions (lines) normalized at α = 0.5 corresponding to some of the ideal kinetic models with the f(α) functions resulting from the combined analysis of MgH2 dehydrogenation in (a) 10 bar of hydrogen pressure and (b) 20 bar of hydrogen pressure.

Figure 3. Experimental curves (dotted lines) corresponding to the thermal dehydrogenation of magnesium hydride under high vacuum registered at 0.2, 0.5 and 1 K min-1. The curves reconstructed using the kinetic parameters obtained from the analysis are plotted as solid lines.

Figure 2. (a) Integral TG (mass loss) and differential TG curves for the thermal dehydrogenation of magnesium hydride recorded under ~5 × 10-5 mbar at 0.2 K min-1. (b) Corresponding H2 signals registered by mass spectrometry as a function of temperature.

Table 2. Activation energy values as a function of α, obtained from the Friedman isoconversional analysis of the experimental curves shown in Figure 7 without accounting for the pressure term (1-p/p*).

Table 3. Activation energy values as a function of α, obtained from the Friedman isoconversional analysis of the experimental curves shown in Figure 7 accounting for the pressure term (1-p/p*).

Figure 4. (a) Combined kinetic analysis plot of the experimental curves presented in Figure 3. (b) Comparison of the f(α) functions (lines) normalized at α = 0.5 corresponding to some of the ideal kinetic models with the f(α) function resulting from the combined analysis, f(α) = (1-α)0.94 (dots).

Figure 8. Combined kinetic analysis plots of the experimental curves presented in Figure 7, recorded in 10 bar and 20 bar of hydrogen pressure.

Figure 6. Temperature dependence of the pressure correction term (1-p/p*) at pressures of 10 bar and 20 bar.

Figure 7. Normalized dα/dt plots for the dehydrogenation of magnesium hydride, calculated from the DSC traces shown in figure 5 (dotted lines). a) Hydrogen pressure of 10 bars; b) hydrogen pressure of 20 bars. The curves reconstructed using the kinetic parameters obtained from the kinetic analysis are plotted as solid lines.

Table 1. Activation energy values as a function of α, obtained from the Friedman isoconversional analysis of the experimental curves shown in Figure 3.

Figure 1. XRD pattern of the as received magnesium hydride recorded in vacuum at room temperature.

Frequently asked questions

Q1. What have the authors contributed in "Magnesium hydride for energy storage applications: the kinetics of dehydrogenation under different working conditions" ?

Thus, the reaction follows a first order kinetics, equivalent to an Avarmi-Erofeev kinetic model with an Avrami coefficient equal to 1, when carried out under high vacuum, while a mechanism of tridimensional growth of nuclei previously formed ( A3 ) is followed under hydrogen pressure. It has been shown that the activation energy is closed to the Mg-H bond breaking energy independently of the hydrogen pressure surrounding the sample, which suggests that the breaking of this bond would be the rate limiting step of the process.