Thermodynamic Property Study of Nanostructured MgH , Mg-NiH , and Mg-CuH Systems by High Pressure DSC Method

Mg, Ni, and Cu nanoparticles were synthesized by hydrogen plasma metal reaction method. Preparation of Mg 2 Ni and Mg 2 Cu alloys from these Mg, Ni, and Cu nanoparticles has been successfully achieved in convenient conditions. High pressure differential scanning calorimetry (DSC) technique in hydrogen atmosphere was applied to study the synthesis and thermodynamic properties of the hydrogen absorption/desorption processes of nanostructured Mg-H, Mg-Ni-H, and Mg-Cu-H systems. Van’t Hoff equation ofMg-Ni-H system as well as formation enthalpy and entropy of Mg 2 NiH 4 was obtained by high pressure DSCmethod.The results agree with the ones by pressure-composition isotherm (PCT) methods in our previous work and the ones in literature.


Introduction
Mg and Mg-based alloys are widely studied as hydrogen storage materials for the advantages such as low price, light weight, high hydrogen capacity, and high abundance of Mg in the earth's crust [1][2][3][4].The hydrides of Mg and the common Mg-based alloys show considerable hydrogen storage content-7.6mass% for MgH 2 and 3.6, 4.5, and 5.4 mass% for Mg 2 NiH 4 , Mg 2 CoH 5, and Mg 2 FeH 6 , respectively.Recently some new hydrogen storage materials have been explored [4,5], but Mg-based materials are still ones of the most promising hydrogen storage candidates to many researchers, especially for heat storage or stationary energy storage [3,6], in which cases, working temperature can be above 500 K.
One serious barrier of Mg-based alloys for hydrogen storage study is synthesis of these alloys by conventional melting method because of the large difference in melting point and vaporization pressure between Mg and Ni, Co, Fe, and so forth.Ball milling/mechanical alloying method has been developed to synthesize Mg-based alloys and it is considered as one effective way to prepare nonequilibrium alloy samples with plenty of defects and samples with grain size in nanometer scale [7].Recently it has almost become the main preparation method by many groups to study Mg-based alloys [8][9][10][11][12][13][14][15][16][17][18][19][20].However, this method faces the disadvantage of possible pollution by steel balls or air during the milling process.Another difficulty in the study of Mg-based materials is poor kinetics of these materials.For example, Mg 2 Ni in micrometer scale produced by conventional melting method needs absorption/desorption temperature higher than 500 K even after several hydrogen absorption and desorption cycles under hydrogen pressure atmosphere.Common Mg metal samples in micrometer scale need much stricter conditions to absorb and desorb hydrogen.Our group and some other researchers successfully prepared nanostructured Mgbased alloy and hydride samples in convenient conditions from metal nanoparticles which were synthesized by plasma metal reaction [21][22][23][24][25][26].These nanostructured samples show excellent hydrogen storage kinetics and properties.They can absorb and desorb hydrogen in convenient conditions without any activation process [23,27,28].This work is to demonstrate that we can study the preparation process, thermodynamic properties, and reaction mechanism of

Experimental Details
The Mg, Ni, and Cu nanoparticles were synthesized from bulk metals by hydrogen plasma metal reaction method.Bulk Mg, Ni, and Cu metals (purity > 99.7%) were melted and vaporized by hydrogen-plasma-metal reaction in the chamber and the gaseous metals were taken to the collecting room by the circulating gas and were deposited on the filter wall, where we obtained the Mg, Ni, and Cu metal nanoparticles.To prepare Mg-Ni and Mg-Cu system alloys and hydrides, Mg and Ni (Cu) nanoparticles were weighted at a 2 : 1 molar ratio and mixed in acetone by an ultrasonic homogenizer.Then the mixture was dried and pressed into small pieces, from which, Mg-Ni (Mg-Cu) hydrides were synthesized in 4 MPa hydrogen atmosphere at 623 K (673 K) for 2 h (9 h) and alloys were obtained after the evacuation of the hydrides at the same temperature.
The composition and structure analysis of the samples were carried out by X-ray diffraction at an automatic Rigaku X-ray diffractometer with monochromatic Cu K radiation at a scanning rate of 4 ∘ /min.
The synthesis and thermodynamic properties of Mg-H, Mg-Ni-H, and Mg-Cu-H systems were studied from DSC measurements using a NETZSCH DSC 204 HP apparatus starting from metal nanoparticles.Mg, 2Mg+Ni and 2Mg+Cu nanoparticle mixture samples with about 10 mg in weight were put into the chamber of the DSC apparatus.After closure of the system, evacuation and supply of 0.5 MPa hydrogen cycles were conducted to fresh the system.For the synthesis of Mg-Ni-H and Mg-Cu-H system, a flowing and constant hydrogen atmosphere of 4 MPa pressure was provided to the DSC chamber system.A special device is equipped in this DSC machine to keep the flowing atmosphere constant.The temperature was increased from room temperature to 823 K at a heating rate of 20 K/min.The heating and cooling processes were repeated two more cycles after the first synthesis cycle.After the production of Mg 2 NiH 4 , the sample was taken through several heating and cooling cycles between 453 K and 823 K at 5, 10, and 20 K/min in various flowing and constant hydrogen pressure value (1 MPa, 2 MPa, and 4 MPa).

Results and Discussion
Figures 1 and 2 show the XRD curves of the metal nanoparticle samples by hydrogen plasma metal reaction method and Mg-Ni-H, Mg-Cu-H system samples prepared from these metal nanoparticles.Figure 1(a) shows the metal nanoparticle sample prepared from bulk Mg contains pure Mg phase.Mg nanoparticle sample shows hexagonal structure and the space group is P6 3 /mmc.The size is in the range of a few hundred nm.There is a small reflection peak at 42.9 ∘ , which is due to the small amount of MgO impurity (less than 1 mass%).All of the other peaks are indexed to Mg phase.This thin layer MgO actually is very helpful to prevent the Mg particles (orthorhombic, space group: Fddd) was obtained.A small amount of MgCu 2 impurity is detected.Figure 3 shows the DSC curve of the Mg nanoparticles in an initial hydrogen pressure of 4 MPa, at a scan rate of 20 K/min.The upper dash line indicates the temperature trend and the solid line shows the DSC curve.The first peak is a small exothermic peak at 662.6 K, which is due to the Mg In the second heating cycle, the hydrogenated MgH 2 desorbs hydrogen to form Mg phase and shows an endothermic peak with a peak temperature of 744.6 K (2).The formed Mg absorbs hydrogen again during the second cooling process and shows a peak temperature of 672.6 K (1). ( The Mg 2 Ni phase absorbs hydrogen and shows an exothermic peak at 682.6 K in the first cooling cycle, which is ascribed to (4).The exothermic peaks at 683.3 K in the second cooling cycle and 683.1 K in the third cycle have the same attribution: A small exothermic peak at 503.7 K in the first cooling cycle is due to the transformation of high temperature (HT) Mg 2 NiH 4 phase to low temperature one (LT) (5).The exothermic peak at 503.9 K in the second cooling cycle and the one at 503.
The high and sharp peak at 729.7 K at the second heating cycle and the one at 729.0 K at the third cycle are due to desorption of Mg 2 NiH 4 phase to form Mg 2 Ni and hydrogen as follows: Figure 5 indicates the DSC curve of 2Mg+Cu nanoparticle mixture in 4 MPa hydrogen atmosphere.The heating and cooling rate is 20 K/min.The first exothermic peak occurs at 434.5 K, which is corresponding to the hydrogen absorption of Mg nanoparticles with Cu nanoparticles as the catalyst.This is similar to Mg-Ni-H system in Figure 4.With Cu catalyst, the hydrogenation peak temperature is lowered about 230 K.After the first exothermic peak, there are several exothermic and endothermic peaks in the temperature range of 660 K to 740 K in the first heating period.The attribution of these peaks remains unclear currently.After these peaks, the next peak at 617.4 K in the first cooling cycle could be easily defined as hydrogen absorption peak of Mg 2 Cu phase according to DSC results of Mg-H system in Figure 3 and pressure-composition isotherm (PCT) results reported by us before [28].The reactions of ( 1), (7), and ( 8) are thought to contribute to these peaks between 600 K and 740 K in the first heating process as follows: The sharp exothermic peaks at 617.4 K, 616.2 K, and 614.7 K in the cooling period are due to the hydrogen absorption of Mg 2 Cu expressed as ( 9), which could be easily confirmed by the XRD results after the DSC measurement.The large sharp endothermic peaks at 682.1 K in the second and third cycles are attributed to desorption of MgH 2 and MgCu 2 mixture to form Mg 2 Cu and hydrogen (10).The same reaction mechanism during cycling of Mg-Cu-H system was reported by other groups [1,29]: Figure 6 presents the DSC curve started from low temperature Mg 2 NiH 4 phase in 2 MPa hydrogen, which is obtained from 2Mg+Ni nanoparticle mixture by DSC methods in 4 MPa hydrogen for one heating and cooling cycle.The upper dash line shows the temperature program.The middle solid line indicates the pressure change with time.During each heating and cooling cycle with different scan rates (5, 10, and 20 K/min), there are two large peaks and two small ones.These peaks have the same attribution with the ones in the second and third cycles of the DSC curve in Figure 4.They are also ascribed to ( 5), ( 6), (4), and (5) in the appearance order.From the figure, we may see that the desorption reaction of Mg Using the data shown in Table 1, we could obtain the van't Hoff equations at different scan rates, which are given in Table 2.The van't Hoff equations vary with the scan rates because the peak temperatures for hydrogen desorption reaction from DSC technique differ at different scan rates.The higher the scan rate, the higher the temperature difference between peak temperature and real equilibrium temperature.
If we want to obtain the van't Hoff equation at complete equilibrium state, we should take the DSC measurement at zero scan rate, which is impossible.However, in our case, the kinetics of the absorption and desorption reaction of nanostructured Mg-Ni-H system is superior.This means we can approach almost equilibrium state of the sorption reactions at low scan rates.After we measured the system at 5 K/min rate, we made the plot.We obtain a van't Hoff equation of ln(/0.

Conclusions
The main conclusions of this work are as follows: (

Figure 1 :Figure 2 :Figure 3 :
Figure 1: XRD patterns of (a) Mg nanoparticles, (b) Ni nanoparticles, (c) Mg 2 NiH 4 sample prepared from Mg and Ni nanoparticles at 623 K in 4 MPa hydrogen for 2 hours, and (d) Mg 2 Ni sample prepared after evacuation of Mg 2 NiH 4 .

Figure 4 :
Figure 4: DSC curve of Mg-Ni-H system started from 2Mg+Ni nanoparticle mixture in 4 MPa hydrogen pressure.

Figure 4
presents the DSC result of starting sample-2Mg+Ni nanoparticle mixture in 4 MPa hydrogen, at a scan rate of 20 K/min.The first exothermic peak at 413.2 K is attributed to the hydrogen absorption of part of the Mg nanoparticles (1) with Ni as catalyst.The hydrogen absorption peak of Mg nanoparticles is about 250 K lowered (compared to Figure 3) when there are Ni nanoparticles as catalyst, which indicates the excellent catalytic effect of Ni nanoparticles to the hydrogen absorption of Mg phase.At about 430∼ 450 K, there are several exothermic peaks, which are also due to hydrogen absorption of the rest Mg nanoparticles.The formed MgH 2 reacts with Ni to form Mg 2 Ni and shows an endothermic peak in the first heating period at 728.0 K.The reaction is expressed as follows:

Figure 5 :
Figure 5: DSC curve of Mg-Cu-H system started from 2Mg+Cu nanoparticle mixture in 4 MPa hydrogen pressure.

7 KMg 2
in the third cooling cycle are due to the same transformation reaction.The exothermic peak at 515.1 K in the second heating cycle and the one at 515.4 K at the third heating cycle are due to the transformation of low temperature Mg 2 NiH 4 phase to high temperature one as follows: NiH 4 (HT) ←→ Mg 2 NiH 4 (LT) .

Figure 6 :
Figure 6: DSC curve of Mg-Ni-H system in 2 MPa hydrogen, at different scan rates started from Mg 2 NiH 4 sample.

2
NiH 4 to form Mg 2 Ni (6) in 2 MPa hydrogen shows the peak temperatures of 684.2 K at 5 K/min, 689.2 K at 10 K/min, and 695.1 K at 20 K/min.From the pressure recording data, we obtain the real-time pressure values, which are 2.106 MPa for 684.2K at 5 K/min, 2.106 MPa for 689.2K at 10 K/min, and 2.107 MPa for 695.1 K at 20 K/min.These pressure, temperature and scan rate data in 2 MPa hydrogen are described in Table1and the values in 4 MPa and 1 MPa are

Table 1 :
Desorption temperature and pressure values from DSC measurements of Mg-Ni-H system.
[31]sually takes several days or much more depending on the hydrogen storage kinetics of the samples, measurement parameters, and equilibrium conditions.Also it has not been much noticed that the obtained equilibrium pressure values vary much with different equilibrium conditions during PCT measurements, which could contribute to some difference in the calculated van't Hoff equation and formation enthalpy and entropy results using these plateau pressure data.By DSC technique, we could make a whole van't Hoff plot in a much more time-saving way.By comparing the results from DSC method and former PCT technique, it shows DSC method is an excellent way to obtain van't Hoff equations as well as formation enthalpy and entropy values of nanostructured hydrogen storage systems, which are with good kinetics.After we reported our method[30], Rongeat et al. reported their results about thermodynamic properties of hydrides determined by high pressure DSC method in a much different way[31].We take the middle point of the DSC reaction peak and assume that the thermodynamics values obtained by our method are not equilibrium state while with kinetics factor, so we can compare with other people's results from desorption PCT measurements.Rongeat et al. tried to give a range of thermodynamics values of the equilibrium state between absorption measurements and desorption ones.Table3shows the temperature values of transformation reaction between high temperature Mg 2 NiH 4 phase and low temperature one, in different hydrogen pressure values and at different scan rates.From this table, we can see that the transformation temperature from LT phase to HT phase and the one from HT phase to LT phase do not change much with the different hydrogen pressure.
1) Nanostructured Mg-Ni-H, Mg-Cu-H hydride systems and Mg 2 Ni, Mg 2 Cu alloys can be obtained from Mg, Ni, and Cu nanoparticles by high hydrogen pressure DSC method.(2) The preparation process and hydrogen absorption and desorption properties of nanostructured Mg-H, Mg-Ni-H, and Mg-Cu-H systems were studied by DSC method.With Ni or Cu as catalyst, Mg nanoparticles absorb hydrogen at temperatures about 230-250 K lowered.Nanostructured Mg-H, Mg-Ni-H, and Mg-Cu-H systems show excellent hydrogen storage properties.(3) From the temperature and hydrogen pressure values obtained in DSC measurements of nanostructured Mg-Ni-H system, van't Hoff equation was obtained as ln(/0.1 MPa) = −8123/ + 14.961 at 5 K/min scan rate.The formation enthalpy and entropy are −67.5 kJ/mol H 2 and −124.4J/(K⋅mol H 2 ).These results agree with those by PCT method.

Table 2 :
Van't Hoff equations of Mg 2 NiH 4 at different DSC scan rates.

Table 3 :
Transformation temperatures between high temperature Mg 2 NiH 4 phase and low temperature one in different hydrogen pressure values and scan rate conditions.