Priynk Meen,Rmvir Singh,V.K.Shrm,I.P.Jin

a Centre for Non-Conventional Energy Resources,University of Rajasthan,Jaipur,India

b Metallurgical and Material Engineering Department,MNIT,Jaipur,India

c Physics Department,University of Rajasthan,Jaipur,India

Abstract Hydriding and dehydriding properties of MgH2-x wt.%NiMn9.3Al4.0 Co14.1Fe3.6(x=10,25,50)nanocomposites have been investigated in present work.Doped alloy was prepared by arc melting method and ball milled with MgH2 to get nanocomposites.Onset temperature as low as 180 °C was observed for MgH2-50 wt%system which is 80°C lower than the as-milled MgH2 giving 131.34 KJ/mol activation energy.Structural analysis shows tetragonal,orthorhombic and monoclinic phases for MgH2,Al60Mn11Ni4 and Mg2NiH4.Morphology by SEM were undertaken to investigate the effect of hydrogenation on nanostructured alloy.DSC studies show a single broad exothermic peak in the temperature range 48 °C–353 °C after alloy addition in MgH2.These results indicate that the hydrogenation properties of MgH2 nanocomposite have been improved compared to the as-milled MgH2.

Keywords:Hydrogen storage;Activation energy;Mg based nanocomposites;Phase structure;TGA.

1.Introduction

World over scientists,politicians and administrators are worried over the present energy scenario couple with the depletion of fossil fuels and environmental degradation.This led them to work for clean energy and hydrogen has been identifie as an ideal candidate for a sustainable energy future.Hydrogen storage is one of the most important key issues for its becoming a viable solution for depleting energy resources[1].Hydrogen can be stored as compressed gas,liquid form and in solids using reversible metal hydrides which are considered to be safe and practical materials[2].AB5type intermetallic hydrides are considered as an efficien method of hydrogen storage.MgH2is an attractive energy storage material due to high theoretical hydrogen storage capacity of about 7.6 wt%,light weight,high abundance and low cost.Unfortunately MgH2suffers from high temperature hydrogen discharge,slow hydrogen sorption kinetics and a high reactivity with oxygen which makes it unsuitable for commercial applications[3].Ball-milling/mechanical millin are the most popularly adopted method which creates a fresh surface and structural defects in the material[4,5].The formed defects and grain boundaries allow easy diffusion of hydrogen from the surface to the bulk material[6].However,this method has limitations to achieve nanocrystallite size that is required(＜5 nm)for destabilization of MgH2[7].

Recently solid state hydrogen storage in metal hydrides constitutes a major research activity[8–10]because of its importance in renewable energy sources.Li et al.[11]investigated LaNi3.8Al1.0Mn0.2alloy and found decrease in hydrogen storage capacity.LaNi3.6Mn0.3Al0.4Co0.7alloy was improved by the addition of Co,Mn and Al by Briki et al.[12].which led to a significan reduction of hysteresis in hydriding and dehydriding(H/D).(H/D)cycles on the structural and morphological properties of MmNi4.22Co0.48Mn0.15Al0.15alloy was studied by Zareii et al.[13]who found and absorption plateau pressures to be～0.51,1.22 and 2.49 bars at 293,313 and 333 K with a maximum hydrogen storage capacity of about 5.78 at 293 K.Exchange and limit current density and diffusion capability of La-Mg-Ni-based hydrogen storage alloys were improved by proper Mn and Ni content[14].

Effect of partial substitution of Co or Al for Ni in La2Mg(Ni0.8-xCo0.2Alx)9(x=0–0.03)alloys was studied by Liao et al.[15]who found that increase of Al content leads to an increase in the cell volume and the hydride stability which causes the decrease in cell volume expansion on hydriding.In case of MmNi3.55Co0.75Mn0.7-xAlxalloy Al substitution for Mn can lower the hydride formation pressure and reduce the hysteresis between the hydrogen absorption and desorption pressure[16].The effect of iron substitution on the electrochemical behavior of LaNi3.55Mn0.4Al0.3Co0.75-xFex(0-x-0.55)compounds was reported by Khaldi et al.[17]who found that the value of hydrogen diffusion coefficien decreases when increasing iron content in the alloy.Cheng et al.[18]in the year 2008 studied partial substitution of Al for Ni in LaNi4.25Al0.75alloy improving the cyclic performance and decreases plateau pressure of the hydride.Influenc of 1–10 at.%Co or 5 at.%Pd additives in LaMg2Ni and its hydrogen reactivity in La25Mg50Ni25alloys was studied by Teresiak et al.[19].Influenc of 1–10 at.%Co or 5 at.%Pd additives in LaMg2Ni and its hydrogen reactivity in La25Mg50Ni25alloys was studied by Teresiak et al.[19].

Zhang et al.[20]found that the Mn partial substitution for Ni decreases the plateau pressure without reducing its hydrogen storage capacity.It has been reported by Jiang et al.[21]that appropriate substitution of Ni with Mn could increase the electrochemical discharge capacity and the hydrogen absorption rate of ReNi2.6-xMnxCo0.9(x=0–0.9)alloys.Zhang et al.[22]investigate the kinetic mechanism of hydriding reaction inαphase(solid solution)region for cubic Laves Ho1-xMmxCo2alloys.The results indicate that the hydrogen absorption kinetics inαphase region is controlled by hydrogen diffusion into the bulk.The activation energies calculated using Chou model with the least square method are in the range of 29.4–53.5 kJ/mol H2.Li et al.[23]studied the Mg–3 mol%LaNi3composition by hydrogen combustion synthesis(HCS)method and investigated effects on the hydrogenation properties,analyzed changes from the relationship between the microstructure,crystalline state and the energy injected into the composite from the external high magnetic field Rate-controlling step of hydrogen diffusion in Mg–30 wt.%LaNi5at temperatures ranging from 302 to 573 K,while that of Mg–50 wt.%LaNi5changed from surface penetration to hydrogen diffusion with increasing initial hydrogen pressure ranging from 0.2 to 1.5 MPa[24].It was found that activation energies calculated by Chou model for hydrogen absorption in Mg–30 wt.%LaNi5were 25.2 and 28.0 kJ/mol.H2calculated by Chou model.Li et al.[25]studied the hydriding/dehydriding of(LaNi5)1-xMgx(x=0,0.018,0.041,0.063)and the results indicate that the hydriding kinetics of(LaNi5)1-xMgxalloys are controlled by diffusion of hydrogen atom in hydride.The addition of small amount of magnesium improves the kinetic performance of LaNi5slightly.

In present work NiMn9.3Al4.0Co14.1Fe3.6,prepared for the firs time is added to MgH2to form nanocomposite and studied its structural and hydrogen storage properties.However alloys suffer from disadvantages like difficul activation treatment,poor-kinetics and high desorption temperature.

2.Experimental

2.1.Alloy preparation

NiMn9.3Al4.0Co14.1Fe3.6alloy was prepared using Arcmelting method in argon atmosphere by re-melting 3 times the stoichiometric amounts of 99.5%purity of elements to get homogeneous composition.The ingot was then annealed in a sealed evacuated quartz tube for one week at 1173 K after which it was crushed to get alloy for study.

2.2.Preparation of MgH2-alloy composite

The MgH2-x wt%NiMn9.3Al4.0Co14.1Fe3.6(x=10,25,50)composites were milled mechanically under 0.1 MPa Ar pressure for 10 h(300 rpm)using a FRITSCH P7 ball milling machine.This process was completed on the basis of 5 min rest and 15 min work pattern and the 10:1 ratio of ball to powder and MgH2was fied for mechanical milling.The high purity Ar gas(99.99%)was used to avoid the oxidation and hydrogenation during the mechanical milling.

2.3.Activation of nanocomposites

Two grams of MgH2+x wt%(x=10,25,50)nanocomposite was kept in sample holder of 1′′diameter Cu tube with thermocouple and heater.Initially the composite was evacuated to 10-5mbar vacuum,flushe with 99.95%pure hydrogen at 3 bar pressure,again evacuated to 10-5mbar vacuum and heated to 398 K for 3 h.At this point cool the sample introduced hydrogen and heats it to 398 K for 3 h in hydrogen environment.Considerable time was allowed to heat the sample till an equilibrium temperature is reached.Now evacuate alloy to 10-5mbar pressure and cool it to room temperature.

In second cycle after cooling the composite material to room temperature hydrogen at 3 bar was introduced in the cell where the composite start absorbing hydrogen resulting in decrease in pressure in the cell.This process was repeated for fie cycles till the activation process completes and the pressure in the reactor comes to a constant value showing the formation of the hydride material with full saturation[26].

2.4.Characterization of composite

Fig.1.XRD pattern of MgH2-x wt%NiMn9.3Al4.0Co14.1 Fe3.6(10,25,50)nanocomposites.

Structural characterization was performed with the help of X-Ray Diffraction(XRD)technique with Cu-Kαradiation(operated at 45KV,λ=1.54°A in 2θrange of 20–90°).SEM analysis was carried out using Nova Nano FE-SEM operated at 30 keV to 50 eV.The FE-SEM is coupled to EDX detector for measuring the elemental chemical composition of materials.The Differential Scanning Calorimetry(DSC)was used to study thermal performance of samples for absorption processes and heated at a different rate of 5,10,150C/min to 4500C.TGA(STA 6000-Perkin Elmeris)studies was performed for the study of desorption properties at room temperature to 4500C under 0.1 MPa argon atmosphere at a heating rate of 5,10,150C/min.

3.Results and discussion

3.1.Structural characterization by XRD

Fig.1 shows XRD patterns of the Mg-based nanocomposites containing 10,25,50 wt.%NiMn9.3Al4.0Co14.1Fe3.6as additive after 10 hrs of ball milling.In these XRD patterns various phases were identifie e.g.MgH2,Mg2NiH4and Al60Mn11Ni4.The presence of MgO hydride phases containing additives species were not observed in the XRD patterns of the powder mixtures processing.

The increase in unit cell volume on activation is due to hydrogen absorption which decreases density of alloy resulting in decrease in lattice constant.While desorption of hydrogen the material takes original shape showing the increase in density that is decrease in lattice constant.The peak intensities decrease with the broadening of peaks which indicates accumulation of mechanical strains and reduction in particle size during milling[27].

It was observed that the broadening of peak in as-milled MgH2samples and it could be attributed to the refinemen of crystallite size and the presence of lattice strain.

The average crystallite size was calculated using Debye Scherer formula,

Whereλis the X-ray wavelength,βis the line broadening at half the maximum intensity in radian,θis the Bragg angle.The instrumental broadening was corrected by using standard single crystal Si sample.Using Debye Scherer formula,the values of average crystallites size for the MgH2-x wt%alloy(10,25,50)nano composites are estimated to be about 32 nm,33 nm and 36 nm respectively(Table 1).

3.2.Morphology of composite by SEM

The morphology of the NiMn9.3Al4.0Co14.1Fe3.6observed by Scanning Electron Microscopy is shown in Fig.2.It is obvious that increasing the concentration from 10 wt.%to 50 wt.%alloy content increases.Mechanical milling is effective in decreasing the size of particles and providing more fresh active surface.It has been reported that the kinetics of MgH2can be enhanced by decreasing the particle size which can lead to a reduction in the hydrogen diffusion pathway[28].The inhomogeneous contact of the powders and steel balls during the ball-milling process could be the cause of this inhomogeneous particle size distribution[29].It can be observed that the addition of alloy to the MgH2and 10 h of ball milling reduce the particle size which can have the benefiof more free surface and probably result in increasedhydrogen desorption[30].It is seen that the alloy powder consisted of particles varying in size 10–30 mm with irregular shapes and relatively smooth surfaces.The change in the structure of particle metal powders is due to the different sizes of all the substituted elements.The emergence of nano-sized particles in the sample shows that high-energy ball milling is an ideal method to decrease the particle size of the powder particles down to the nano-scale and,consequently,increase the surface area and potential sites of hydrogen desorption.

Table 1 Characteristics of the phases in the MgH2-x wt%(10,25,50)NiMn9.3Al4.0Co14.1Fe3.6..

Fig.2.SEM image of(a)as-milled MgH2(b)pure alloy(c)MgH2-10%alloy(d)MgH2-25%alloy(e)MgH2-50%alloy.

Fig.3.EDS analysis of the alloy.

Table 2 Chemical composition NiMn9.3Al4.0Co14.1Fe3.6 by EDX analysis.

The elemental composition of the NiMn9.3Al4.0Co14.1Fe3.6has been checked by EDS technique at different sites of the ingots and thee elemental composition was found almost same at all the sites as shown in Fig.3.The elemental percentage is shown in Table 2.

3.3.Kinetic study of the dehydrogenation process

3.3.1.TGA analysis

TGA studies on as-milled MgH2resulted in release of 7.3 wt%at 360 °C hydrogen with onset temperature of 260°C.Fig.4 shows the TGA profile of as-milled and MgH2+xwt%NiMn9.3Al4.0Co14.1Fe3.6(x=10,25,50)nanocomposites at a heating rate of 10°C/min under 0.1 MPa Ar atmospheres.The total relative weight loss of 10 hrs milled MgH2and MgH2–x wt%NiMn9.3Al4.0Co14.1Fe3.6is respectively about 7.3 wt%,5.4,3 and 2 wt%as shown in Fig.4(a and b).

DTA curve to analyze peak temperature of the as-milled MgH2and MgH2-x wt.%NiMn9.3Al4.0Co14.1Fe3.6nanocomposites under different heating rate 5,10,15°C/min are shown in Fig.5(a).The as-milled MgH2without the catalysts starts to decompose at 260°C.With the addition of NiMn9.3Al4.0Co14.1Fe3.6the starting temperature of hydrogen desorption decreases from 260°C to 220°C,210°C and 180°C for 10 wt.%,25 wt.%and 50 wt.%respectively.There is slight reduction in the desorption temperature as the concentration increase to 25 wt.%to 50 wt.%.The lowest onset temperature value is observed for the 50 wt.%alloy doped sample,followed by the 10 wt.%doped and the 25 wt.%doped samples,while the pure MgH2sample gives the highest value.It indicates that alloy significantl improves desorption properties of MgH2.

The activation energy of dehydrogenation for the MgH2-NiMn9.3Al4.0Co14.1Fe3.6nanocomposites was evaluated to understand the effect of alloy on the dehydrogenation of MgH2.Calculating activation energy of dehydrogenation at three different heating rates(5,10 and 15°C/min).

The activation energy can be calculated by plotting a curve between ln k and 1/RTPusing the following equation[31]:

Fig.5(b)Shows Kissinger plot of the hydrogen desorption reaction for catalyzed MgH2samples.The activation energies were estimated from the slope of the straight line to be 177.90 KJ/mol for as-milled MgH2,200.62 KJ/mol,148.85 KJ/mol,131.34 KJ/mol for MgH2-xwt%NiMn9.3Al4.0Co14.1Fe3.6(x=10,25,50)respectively.From the data,it is clear that the addition of NiMn9.3Al4.0Co14.1Fe3.6to MgH2lower its activation energy.It is thought that the activation energy is not directly related to the hydrogen desorption kinetics;it is a barrier that must be overcome to start the release of hydrogen.But the speed of hydrogen release also depends on other factors,such as alloy effect,surface area and particle size.There was also a slight correlation between activation energy and desorption temperature as well.Samples with high activation energies tended to have high thermal stabilities.

Fig.4.Hydrogen content measurement(a)as-milled MgH2(b)MgH2-x wt%NiMn9.3Al4.0Co14.1Fe3.6(x=10,25,50)nanocomposites.

The above results indicate a significan improvement in dehydrogenation kinetics of MgH2by the addition of NiMn9.3Al4.0Co14.1Fe3.6.It is interesting to observe that increasing Ni content brings decreases hydrogen desorption activation energy facilitating the improvement of hydrogen desorption kinetics of nanocomposite.Ni may be creating high catalytic alloy surface for the hydrogen reactions during mechanical milling[32].

3.3.2.DSC analysis

DSC investigations were undertaken to investigate the effect of(10,25,50)wt.%NiMn9.3Al4.0Co14.1Fe3.6on hydrogen absorption in MgH2which is shown in Fig.6 and is compared with as-milled MgH2.Hydrogenation was performed at heating rate of 15°C min-1was maintained during DSC studies.

Fig.5.(a)DTA curve(b)Kissinger plot for dehydrogenation of the asmilledMgH2 and MgH2-x wt%NiMn9.3Al4.0Co14.1Fe3.6(x=10,25,50)nanocomposites.

It is observed from DSC curves that 10 hrs milled MgH2consists of two exothermic peaks at 260°C and 356°C with onset at around 220°C,which means that the hydrogenation could be started at as low as 220°C.These peaks may be attributed to the presence of activated and non-activated species in the sample.The addition of alloy reduce the onset temperature to 155°C for MgH2-10 wt.%NiMn9.3Al4.0Co14.1Fe3.6nanocomposites which further reduce to 17°C for 25 wt.%.On increasing the concentration 25 wt.%to 50 wt.%alloy added MgH2the onset temperature remains same.After the addition of alloy a single broad exothermic peak was obtained between the temperature ranges 48°C–353°C.

This study on effect of NiMn9.3Al4.0Co14.1Fe3.6alloy on MgH2has significantl decreased the hydrogen desorption temperature,improved kinetics and moderate content of hydrogen of nanocomposite as shown in Table 3.

Table 3 Kinetics parameters for MgH2-NiMn9.3Al4.0Co14.1Fe3.6 nanocomposites.

Fig.6.DSC curves for 10 hrs milled MgH2 and MgH2–x wt.%NiMn9.3Al4.0Co14.1Fe3.6.

4.Conclusions

(a)NiMn9.3Al4.0Co14.1Fe3.6alloy was synthesized firs time by arc melting furnace and the effect of alloy content on hydrogenation properties of MgH2were studied.

(b)The average crystallites size for the MgH2-x wt.%NiMn9.3Al4.0Co14.1Fe3.6(10,25&50)nanocomposites were estimated to be about 32 nm,33 nm and 36 nm.

(c)Dehydrogenation characterization of MgH2with and without NiMn9.3Al4.0Co14.1Fe3.6showed that the 50 wt.%alloy doped MgH2powder decreases desorption temperature by about 80°C compare to the as-milled MgH2.

(d)TGA result shows the activation energy of nanocomposite is lower by about 46.56 kJ/mol compared to as-milled MgH2and 50%added NiMn9.3Al4.0Co14.1Fe3.6nanocomposites shows best result.

(e)DSC curves show that 10 h milled MgH2consists of two exothermic peaks at 260°C and 356°C with onset at around 220°C,which means that the hydrogenation could be started at as low as 220°C.

Acknowledgment

Priyanka Meena is thankful to Malaviya National Institute of Technology(MNIT),Jaipur for providing Institute Assistant fellowship for PhD work.