Hui Yong ,Xin Wei ,Jifan Hu,∗ ,Zeming Yuan ,Shihai Guo ,Dongliang Zhao ,Yanghuan Zhang,,∗∗

aSchool of Materials Science &Engineering, Taiyuan University ofScienceand Technology,Taiyuan030024,China

b Laboratory of Magnetic and ElectricFunctionalMaterials andthe Applications,The KeyLaboratory of ShanxiProvince,Taiyuan030024, China

cDepartmentofFunctionalMaterialResearch,CentralIron and SteelResearchInstitute, Beijing 100081, China

d Key Laboratory of I ntegratedExploitation ofBaiyun Obo Multi-MetalResources,Inner Mongolia University of Science and Technology, Baotou 014010,China

Abstract The composites comprised of Co nanoparticle and C nanosheet were prepared though a high-temperature carbonization reaction.The catalysis of Co@C composites on the hydrogen storage behavior of Mg90Ce5Y5 alloy was investigated in detail by XRD,SEM,TEM,PCI,and DSC method.Because of the synergistic catalytic function of C and Co in C@Co nanocomposites,the Mg90Ce5Y5 alloy with 10 wt.%C@Co shows the excellent hydrogen absorption and desorption performances.Time for releasing hydrogen reduces from 150 min to 11 min with the addition of the C@Co composites at the temperature of 300 °C.Meanwhile,the dehydrogenation activation energy also declines from 130.3 to 81.9 kJ mol−1 H2 after the addition of the C@Co composites.This positive effect attributes to the C layer with the high defect density and the Co nanoparticles,which reduces the energy barriers for the nucleation of Mg/MgH2 phase and the recombination of hydrogen molecule.Besides,the C@Co composites also improve the activation property of the Mg90Ce5Y5 alloy which was fully activated in the first cycle.Moreover,the temperature for initial dehydrogenation and the endothermic peak of the alloy hydride were also decreased.Although the addition of the C@Co composites increases the plateau pressures and decreases the value of the decomposition enthalpy,these differences are so small that the improvement on thermodynamics can hardly be seen.

Keyword: Hydrogen storage;Mg-based alloy;Kinetics;Thermodynamics;Synergistic effect;Nanocomposites.

1.Introduction

With the increasingly serious energy exhaustion and environmental pollution,human urgently needs an economic,environmentally friendly,and sustainable energy to replace the original fossil energy [1,2].However,the primary energy,including wind energy,solar energy,geothermal energy,biological energy,not only has a certain limitation in application but also faces the problem of lacking appropriate energy carrier for the transport and transfer[3].So,the hydrogen energy is recognized as an important clean energy source and will play an important role in future energy demand.However,due to its inflammable,explosive,diffusible characters,and low energy density per unit volume,the storage and transport of hydrogen become a bottleneck that restricts the wide application of hydrogen energy [1,3].So,the key to developing hydrogen energy depends on the effective hydrogen storage technologies.At present,there are essential safety problems in high pressure gaseous and low-temperature liquid hydrogenstorage and their hydrogen storage capacity is also unsatisfied[4].Therefore,developing high capacity solid-hydrogen storage with great security is particularly important [5].The hydrogen storage capacities of the main category solid-hydrogen storage materials were listed in Table 1.It is obvious that Mgbased hydride as the most promising candidate is the only material that can meet the capacity requirements in the field of light-duty vehicles[1,5].However,Mg-based materials still can not fulfill the extensively applied requirement of hydrogen energy considering the sluggish kinetics and unsuitable thermodynamics of them [7].

Table 1 The hydrogen storage capacity of the main category solid hydrogen storage material [1,6].

In this context,it is significant to explore methods to improve the hydrogen storage properties of Mg-based alloy.These approaches are comprehensively integrated into Refs.[8–12].The ball-milling,regarded as the most commonly used modification method to ameliorate the hydrogen storage properties of Mg-based materials,could refine alloy particles,reduce crystallite size,and generate defects and inner stress,which is favorable to the diffusion of hydrogen and the nucleation of Mg/MgH2phase,thus decreasing the activation energy and improving hydrogen storage performance [13,14].In addition,the various catalytic or additives can be introduced through ball milling technology to further enhance the hydrogen storage performance [15].Recently,the composites containing nano-sized metal clusters deposited on various carbon materials possess the most efficient catalytic action and show the advanced catalytic properties[16].The carbon-based materials with high specific surface area,such as the activated carbon,graphite,graphene as well as carbon nanotube and so on,are usually used as the support of metal nanoparticle because metal clusters can be evenly deposited on them and do not agglomerate during the preparation or phase transformations,which ensures the stability of active catalyst [17,18].In addition,the defects generated in the carbon-based materials during ball-milling or preparation also can act as the active sites for the nucleation of Mg/MgH2phase and show the synergistic catalyst effect in consort with metal clusters to positively reduce the energy barrier of hydrogen recombination and decomposition [19–21].Wu et al.[22]investigated the function of metallic catalysts and SWNT(Single-wall carbon nanotube)on the hydrogen desorption property of unmodified MgH2,and according to the result,the co-doped SWNT and metallic catalysts possess more efficient catalytic action than the single SWNT or metallic catalysts.Tarasov et al.[23]also reported that the hydrogen storage behavior of Mg-based alloy catalyzed by nickel-graphene nanocomposites shows a significant enhancement in comparison with the Mg/MgH2without catalyst.The enhancement is attributed to the changes in the mechanism of nucleation and growth and the alteration of rate-limiting steps in the reversible reaction.Besides,Veron et al.[24]reported that the energy barrier of hydrogenation of Mg-based alloy could be reduced by the addition of carbon nanotubes combined with Co particles,and the Co@CNT catalyst still retained high catalytic properties even after several cycles.In general,the graphene and graphene-like materials are the promising materials as the support of nanostructured metals or metal oxide catalysts [21,24–27].However,neither graphene nor carbon nanotube is affordable enough for the large-scale applications.The challenges still exist in the preparation of economical and efficient catalysts.

On account of our previous investigation,the Mg-based alloy,Mg90Ce5Y5,presents the great hydrogenation kinetics and superior cycling stability [28].However,the hydrogen desorption kinetics remains sluggish for practical applications,which is attributed to the limited reaction rate of hydrogen desorption by the recombination of H2on the Mg surface.According to the above-mentioned studies,one can conclude that the catalysts prepared by combining the transition metals and carbon can improve the hydrogen storage properties of Mg-based materials.In the present study,we have investigated a simple route to synthesize a composite catalyst composed of carbon and cobalt.Then,the hydrogen storage behavior of the Mg90Ce5Y5alloy catalyzed by carbon-cobalt composites were studied in detail.

2.Material and methods

2.1.Synthesis of C@Co nanocomposites

1.0 g Co(NO3)3·6H2O and 1.5 g Polyvinylpyrrolidone(PVP K30)were dissolved into 30 mL deionized water sequentially.The entire dissolution process was carried out under the condition of ultrasonic vibration.The uniform solution obtained entirely dried,following was heated to 800 °C using the tube furnace and maintained for two hours in the atmosphere of high-purity argon(99.999%).After that,the C@Co composites were obtained.All the above raw-materials with purity exceeding 99.7% were purchased from Aladdin Co.,Ltd.

2.2.Preparation of C@Co-doped Mg-Y-Ce composites

The as-cast Mg-Y-Ce alloy powders,Mg90Ce5Y5,were prepared in our previous work [28].The C@Co-doped Mg-YCe composites(defined as C10)composed of 10 wt.% C@Co and Mg-Y-Ce alloy powders were fabricated by utilizing the mechanical milling for 5 h duration at a ball to powder ratio of 40:1 and 300 rpm milling rate.The ball-milling process needed argon as a protective gas to avoid the oxidation of the sample,but other processes not studiously avoid the oxidation.Meanwhile,for comparison,the microstructure and hydrogen storage performance of as-milled Mg-Ce-Y powders(defined as C0)also were investigated.It’s worth mentioning that the addition of 10 wt.% was designed according to the reports of the relevant literatures [29,30].The excessive addition will reduce ball-milling efficiency [31],which result into that the number of interfaces and defects in alloy particles has not grown significantly,making nucleation step and hydrogen diffusion more difficult.

Fig.1.(a)XRD pattern,(b)Raman spectra,(c)Nitrogen adsorption-desorption isotherms,(d)Pore diameter distribution curve of as-synthesized C@Co composites.

2.3.Structure characterization

The characterization of phase composition was conducted by X-ray diffraction(XRD Rigaku)with a with 2 °/min scanning rate and Cu-Kαradiation.The microstructure observation was accomplished by using a scanning electron microscope(SEM Philips-Q400)with an energy dispersive spectrometer(EDS).High-resolution morphologies and crystal states of samples were observed and estimated by the Tecnai G2F20 transmission electron microscope(TEM)and the selected area electron diffraction(SAED),respectively.The disorder degree information of C@Co was also analyzed by the Raman spectrum with laser excitation length 514.5 nm(Renishaw RM2000).

2.4.Hydrogen storage performance measurements

The gaseous hydrogen storage property was measured by the Sieverts-type constant volumetric equipment equipped with a furnace to control the temperature(accuracy of±1°C).The amount of sample required for each measurement was 0.5 g and the samples were activated at the temperature of 360 °C before the formal measurement.The pressurecomposition-isotherms(PCI)curves at different temperatures(320,340,and 360 °C)were measured with the hydrogen pressure changing from 0.005 MPa to 3.6 MPa(accuracy of 0.0001 MPa).The isothermal hydrogenation kinetics were measured at the temperature of 250,200,150,and 100 °C with 3.0 MPa original hydrogen pressure,respectively.Besides,the isothermal dehydrogenation kinetics also were measured at the temperature of 360,320,300,and 280°C with an initial hydrogen pressure of 0.005 MPa,respectively.It should be noted that the sample after hydrogen desorption needs to be reheated to 360 °C and vacuumed until the air pressure is 0.0001 MPa to remove the residual hydrogen.In addition,the Differential Scanning Calorimetry(DSC NETZSCH STA 449F3)was executed to analyze the dehydrogenation properties of hydride,of which the heating rate was 5 °C/min and the argon flow rate was 100 mL/min.

3.Results and discussion

3.1.Characterization of the C@Co composites

The X-ray diffraction results of the C@Co composites are displayed in Fig.1(a).As seen from the pattern,the C@Co composites are composed of theCphase and Co phase,and theCphase shows a characteristic peak of graphene,indicating this composite is a graphene-like structure.The Raman spectrogram of the C@Co composites is displayed in Fig.1(b).Based on the known Raman spectra of different materials [32],the peak at 675 cm−1corresponds to Co NPs.The characteristic peak at 1335 cm−1as well as 1587 cm−1can be assigned toDandGpeaks of carbon,respectively.TheGband at about 1587 cm−1stands for the in-plane shake of sp2−carbon atoms and theDband at about 1335 cm−1stands for the structure defects inside the carbon [26].Generally,the intensity ratio(ID/IG)ofD-band andG-band reflects the graphitization extent and defect density of carbon samples [33].It is obvious that the calculated intensity ratio(ID/IG)of C@Co composites is about 0.97 and higher than that of all graphitized carbon [24]and the Ni@C fabricated by low-temperature solid-phase reaction [33],suggesting that the C@Co composites have more defects density which can provide catalytic active sites for nucleation of the absorption/desorption process and decrease the activation energy [34].In addition,the relatively broad 2D peak indicates that the C@Co composites consist of a few layers,and the D+G peaks can also be assigned to the defects in carbon[26].The specific surface areas and the pore size distribution of the C@Co composites were measured by nitrogen adsorption/desorption isotherms(Fig.1(c)).The BET of the C@Co composites is 186.4 m2/g.The pore size distribution(Fig.1(d))show the formation of randomly distributed pores with a dominant diameter of 3.88 nm,which belongs to a mesoporous structure [35].Although this mesoporous structure is prone to breakage during ball milling,it results into the formation of small carbon sheets which can be more evenly dispersed in the particles of matrix alloy to provide a large numbers of interface channels for the diffusion of hydrogen.

By the method of SEM(Fig.2(a)and(b)),AFM(Illustration)and TEM(Fig.2(c)–(e)),the morphology observation and the grain size measure of C@Co composites can be realized.Fig 2(a)indicates that the C@Co composites consist ofCnanosheets whose thickness is less than 13 nm.Besides,it can be observed that the Co nanoparticles with smaller than 50 nm in size uniformly distribute on the carbon nanosheets,as shown in the SEM micrograph(Fig.2(b)).This phenomenon can also be seen in TEM morphology(Fig.2(c)and(d)).The HRTEM morphology showed in Fig.2(e)indicates that the Co grains with 0.204 nm for(111)interplanar spacing are embedded on theCnanosheets with highly defective.In addition,a small amount of amorphous carbon can also be observed in Fig.2(e),which is consistent with XRD results.Moreover,several thin graphiticClayers can be observed around the Co grain owing to the high-temperature catalytic graphitization [36],which the structure of the carbon shell had a transformation from disordered to graphitic by the aid of catalysis of Co core.

Fig.2.SEM morphology(a),(b)and TEM micrograph(c)–(e)of as-synthesized C@Co composites.

Fig.3.The X-ray diffraction patterns of the ball-milled and hydrogen absorption/desorption samples for Mg90Ce5Y5 and Mg90Ce5Y5+10 wt.% C@Co alloys:(a)ball-milled,(b)hydrogen absorption,(c)hydrogen desorption.

3.2.The evolution of the phase and microstructure

The X-ray diffraction patterns of the ball-milled and hydrogen absorption/desorption samples for Mg90Ce5Y5(C0)and Mg90Ce5Y5+10 wt.% C@Co(C10)alloys are shown in Fig.3.It can be seen from Fig.3(a)that the diffraction peaks of the ball-milled C0 and C10 alloys are broadened,which reflect the phenomena of grain refinement and amorphization caused by mechanical milling [37].In addition,the C0 alloy is comprised of the main phase CeMg12(PDF-#19–0289),the minor phase Y5Mg24(PDF-#31–0817),the minor phase Ce2Mg17(PDF-#17–0400)as well as the remaining Mg(PDF-#35–0821)phase.However,no new phase appears in the XRD pattern of C10 alloy except the extra Co(PDF-#89–7094)phase,indicating no reaction occurs between the C@Co nanocomposites and Mg90Ce5Y5alloy in the process of mechanical milling.This is due to the parcel effect of theClayers on the Co grain so that there is no contact with Mg during the ball milling process.It’s worth mentioning that the diffraction peaks ofCare invisible,which is derived from the appearance of amorphous carbon during the ball milling process.

The XRD patterns of hydrogenated samples are displayed in Fig.3(b)from which it can be seen that the diffraction peaks in the ball-milled samples have disappeared and been replaced by those of hydrides,including MgH2(PDF-#74–0934),CeH2.73(PDF-#31–0327)and YH2(PDF-#12–0388)phase.Besides,owing to the oxidizable characteristic of Ce element,the phase CeO2(PDF-#75–0076)appears in the hydrogenated alloys.The XRD patterns of dehydrogenated samples are displayed in Fig.3(c)which includes Mg(PDF-#35–0821),CeH2.73(PDF-#75–0076),YH2(PDF-#75–0076)and CeO2(PDF-#75–0076)phases.Obviously,the CeMg12,Y5Mg24and Ce2Mg17phases in as-milled alloy do not regenerate after re-dehydrogenation,which are mainly attributed to the CeH2.73,YH2and CeO2phases also can’t be decomposed in the process of hydrogen desorption [38],which suggests that there is only a reversible reaction of Mg/MgH2in subsequent cycles.In addition,the Co(PDF-#89–7094)phase still exists after absorption and desorption of hydrogen,indicating the C@Co composites are also not involved in the de/hydrogenation reaction.In summary,the C@Co composites belong to an inert catalyst which has not changed the reaction path,but its unique porous and lamellar structure provide channels for the diffusion of hydrogen and the multitude of defects onClayer play the activated“catalytic sites”role in the nucleation of Mg/MgH2[39].

The SEM patterns of the ball-milled samples for Mg90Ce5Y5(C0)and Mg90Ce5Y5+10 wt.% C@Co(C10)alloys are presented in Fig.4(a)and(b),respectively.The particles of C10 samples(Fig.4(b))are significantly smaller than that of the C0 sample(Fig.4(a))and appear an irregular spherical shape,nor flat shape like C0 sample.The results indicate that the C@Co composites can improve the efficiency of ball milling and reduce the size of the alloy particles.The smaller the particle size is,the larger the surface area is.This will lead to more contact with the catalyst to create more effective active sites.Fig.4(c)shows the EDS mapping of the C10 sample,where the green,blue,and red stand for Mg,C and,Co elements distribution,respectively.Conspicuously,the carbon layer covers on the particles of samples,and the cobalt grains are dispersed in the carbon layer.

Fig.4.The SEM patterns of the ball-milled samples for Mg90Ce5Y5 and Mg90Ce5Y5+10 wt.% C@Co alloys:(a)C0 alloy,(b)C10 alloy,(c)the EDS mapping of the C10 alloy(For interpretation of the references to color in this figure,the reader is referred to the web version of this article.).

Fig.5.TEM images and corresponding SAED patterns of the ball-milled(a,b)and dehydrogenated(c,d)Mg90Ce5Y5+10 wt.% C@Co alloy.

The microstructure and SAED pattern of the as-milled C10 sample is deeply researched by utilizing a high-resolution TEM.As seen in Fig.5(a),the ball-milled C10 sample mainly consists of the RE-Mg IMC(intermetallic compound)phases and the randomly distributed nano-crystalline particles.In addition,the CeMg12(PDF-#19–0289),Y5Mg24(PDF-#31–0817),Ce2Mg17(PDF-#17–0400),and Co(PDF-#89–7094)phases all can be identified by the SAED pattern,which is consistent with XRD analysis.As shown in the highresolution TEM image(Fig.5(b)),the inner microstructure of the C10 sample consists of phase CeMg12(202)(interplanar distance is 0.258 nm),phase Y5Mg24(330)(interplanar distance is 0.265 nm)and many imperfections like dislocations and amorphous.Moreover,the carbon layers adhered to the surface of the particle also was observed in TEM images.As for the dehydrogenated C10 sample in Fig.5(c),there is a great number of nano-particles whose average size was about 10–50 nm in the sample.The grains are believed to be CeH2.73(PDF-#31–0327)and CeO2(PDF-#75–0076),which is confirmed by the elevant SAED pattern.In Fig.5(d),in situformed CeH2.73are inserted in the matrix Mg surrounded by carbon layers.The interplanar spacing of the nano-spheres was about 0.318 nm corresponding to the(111)plane of the CeH2.73phase.The interplanar spacing of the basal body was about 0.245 nm corresponding to the(101)plane of the Mg phase.

Fig.6.The activation hydrogenation curves of Mg90Ce5Y5(a)and Mg90Ce5Y5+10 wt.% C@Co(b)alloys.

3.3.Absorption and desorption kinetic performances

Fig.6 shows the activation hydrogenation curves of Mg90Ce5Y5(Fig.6(a))and Mg90Ce5Y5+10 wt.% C@Co(Fig.6(b))alloys at 360 °C.Obviously,the hydrogenated curves of C0 alloy are almost overlapping from the 2nd to 3rd cycles,which suggests that the C0 alloy can be completely activated after the third hydrogenation cycle.However,the C10 alloy can be completely activated at the first hydrogenated cycle.This is mainly due to theClayers wrapped in the alloy particles inhibit the oxidation and sintering of alloy particles during heating.Besides,as for the hydrogen absorption rate,the C10 alloy is better than the C0 alloy,especially in the second part.General speaking,there are two main reaction controlling stages in the procedure of hydrogenation.The first stage is mainly controlled by nucleation and the second stage is mainly controlled by diffusion [40].Because high temperature is conducive to nucleation,the nucleation effect of the C@Co composite catalyst is not obvious at high temperatures.However,the addition of the catalyst reduced the time of the second stage from 66min to 11min.This is mainly due to the carbon nanosheets embedded in the alloy particles provide a channel for the diffusion of hydrogen in MgH2.

Fig.7 shows the hydrogenation kinetic curves of the asmilled Mg90Ce5Y5(Fig.7(a))and Mg90Ce5Y5+10 wt.%C@Co(Fig.7(b))alloys at 250,200,150,and 100 °C,respectively.It can be found that the temperature as a crucial factor restricts the kinetics of hydrogenation reaction.The hydrogen absorption kinetics slides down significantly with decreasing the test temperature,which is closely related to activation energy that are the most obvious barrier for nucleation and diffusion during hydrogen release process.In addition,the addition of the C@Co composites brings on a decline in the maximal value of hydrogen absorption capacity since the C@Co composites does not absorb hydrogen under this experimental condition.For example,the hydrogen storage capacity reduces from 5.09 wt.% to 4.56 wt.% with the addition of the C@Co composites at 250 °C.However,the kinetic properties of hydrogen absorption have been greatly improved.The C10 sample can reach the maximal hydrogen absorption capacity in 100 min at 200 °C,while the C0 sample only can reach 95% of the maximal hydrogen absorption capacity under the same experimental conditions.This indicates that the addition of the C@Co composites is conducive to the full absorption of hydrogen,which is consistent with the analysis results about the action of carbon nanosheets in Fig.6.

Aiming at making sense of the influence of C@Co composites on the hydrogenation kinetics,the saturation degree(R10min)is introduced to estimate the hydrogen absorption rate and defined as:

WhereC10minis the hydrogen-absorbing capacity at 10 min and theCmaxis the maximum hydrogen-absorbing capacity at the temperature of 360 °C.TheR10minvalues of the C0 and C10 samples at 250,200,150,and 100 °C are presented in Fig.7(c).Obviously,theR10minvalue of C10 sample is higher than that of C0 sample at any temperature,indicating the Co@C composites are beneficial to the enhancement of hydrogen absorption kinetics of the Mg-based alloy.It should be noted that the first ten minutes belongs to the first period of hydrogen absorption of alloy,which is mainly controlled by nucleation.Therefore,the Co@C composites are advantageous to promote the nucleation of MgH2,which is mainly attributed to the carbon nanosheets with high defect density.

Fig.8 shows the dehydrogenation kinetic curves of Mg90Ce5Y5(Fig.8(a))and Mg90Ce5Y5+10 wt.% C@Co(Fig.8(b))alloys at 360,320,300,and 280 °C,respectively.Obviously,the Co@C composites are favorable to improve the kinetics of hydrogen desorption.For example,at the temperature of 300 °C,it needs 150 min for the C0 sample to entirely release hydrogen whereas it takes only 11 min for the C10 sample to reach the same extent.Thus,the C@Co composites have a great catalytic function and can significantly enhance dehydrogenation kinetics of Mg-based alloy.

Fig.7.The hydrogenation kinetic curves of Mg90Ce5Y5(a)and Mg90Ce5Y5+10 wt.% C@Co(b)alloys as well as the saturation degree R10min(c).

Fig.8.The dehydrogenation kinetic curves of Mg90Ce5Y5(a),and Mg90Ce5Y5+10 wt.% C@Co(b)alloys.

In general,the Johnson-Mehl-Avrami(JMA)formula can determine the solid-state reaction mechanism of hydrogen desorption [41].Thus,the JMA formula can be used to further demonstrate the catalytic effect of C@Co composites on the dehydrogenation kinetics of the samples through fitting the desorption curves in Fig.8(a)and(b),as follows:

ln[−ln(1−α)]=ηlnk+ηlnt

a——the reaction fraction

η——the Avram index

k——the dehydrogenation rate constant

t——the reaction time

As displayed in Fig.9(a)and(b),the fitting results of ln[-ln(1-a)]and ln(t)straight lines with the linearity constantR2>0.98 indicates that the JMA model is suitable to be applied in the determination of the hydrogen desorption kinetics.In addition,theηvalue reduced from 1.59 to 1.32 with the addition of C@Co composite catalyst,indicating the C@Co composites changes the mechanism of hydrogen desorption reaction for Mg90Ce5Y5alloy.Given the results of previous studies [28],we can conclude from the fit results of experimental data of dehydrogenation in JMA equation that the ratelimiting step of desorption hydrogen process changes from the surface-controlled process to random nucleation and growth process after adding the C@Co composites.Because the increase of Co nanoparticles makes it easier for the hydrogen atom to recombine and causes that the surface control is no longer the control step in the hydrogen release process.The high defect density of theClayer also promotes nucleation of the Mg phase.There are mainly two rate-limiting steps in the process of hydrogen release[42]:(1)the nucleation of the Mg phase and(2)the recombination ofHatom on the Mg surface correspond to two energy barriers that needs to be overcome,respectively.Therefore,we conclude that the additive C@Co composite catalyst is beneficial to the reduction of activation energy of Mg-RE based alloys,which can be ascribed to the synergistic effect of C and Co.The C nanosheets are mainly helpful for the nucleation and diffusion,while Co nanoparticles have beneficial effects on the recombination of hydrogen atoms.

Fig.9.JMAK plots of the hydrogenated Mg90Ce5Y5(a)and Mg90Ce5Y5+10 wt.% C@Co(b)alloys at various temperature.

The dehydrogenation activation energy(Ea)as the energy barrier needs to be overcome in the procedure of dehydrogenation can be quantitatively computed by adopting the Arrhenius equation [43],as follows:

WhereEais the activation energy of hydrogen desorption,Tis the absolute temperature,Ais the frequency factor,Ris the gas constant(8.314J/mol/K),andkis the hydrogen desorption rate constant estimated based on the intercept(ηlnk)and slope(η)in JMK equation.The Arrhenius plots are displayed in Fig.10 and the activation energy(Ea)was calculated from the slopes of a line in the Arrhenius plots.TheEavalues of Mg90Ce5Y5and Mg90Ce5Y5+10 wt.% C@Co alloys are 130.3 and 81.9 kJ mol−1H2,respectively.As expected,the C@Co composite catalyst remarkably decreases the dehydrogenation activation energy of Mg-based alloy.Meanwhile,theEdevalues of C10 alloys also are smaller than that of MgH2alloy catalyzed by Co@CNTs [18],Fe3O4@GS [26],Ni@C [33]and Co@C [44]composite catalysts as well as the MgH2–10 wt.% NiMn9.3Al4.0Co14.1Fe3.6nanocomposites[45].Clearly,the catalysis of C@Co nanocomposites is more prominent and efficient.

Fig.10.Arrhenius plots of Mg90Ce5Y5 and Mg90Ce5Y5+10 wt.% C@Co alloys.

3.4.PCT curves and thermodynamics performance

Fig.11 shows the PCT curves of Mg90Ce5Y5(Fig.11(a))and Mg90Ce5Y5+10 wt.% C@Co(Fig.11(b))alloys at 360,320,and 320 °C,respectively.There is only one plateau pressure existing in each PCT curves corresponding to the reversible transformation of Mg and MgH2.All the curves exhibit a flat,long plateau without hysteresis,indicating the greatly reversible de/hydrogenation performances and high hydrogen storage capacity these alloys have.The plateau pressures in PCT curves are exhibited in Table 2.The enthalpy change(∆H)and entropy change(∆S)can be estimated based on the date of Table 2 by utilizing the Van’t Hoff equation[46],as follows:

WhereP(H2)stands for the equilibrium hydrogen pressure,P0represents the standard atmospheric pressure(1.01×105Pa),Ris a universal gas constant(8.314 J/mol/K),Tis the sample temperature.Based on the Van’t Hoff equation,the Van’t Hoff plots of C0 and C10 samples are shown in Fig.11(c)and(d),respectively.The∆Hvalues and∆Svalues are calculated according to ratio of the slope and intercept of fitting lines,and are listed in Table 2.It can be observed from Table 2 that the addition of the C@Co composites increases the plateau pressures and decreases the value of the decomposition enthalpy,which is beneficial to reduce the thermodynamic stability of the alloy.However,these differences are so small that the improvement in thermodynamics is not significant.

Fig.11.PCT curves and Van’t Hoff plots for Mg90Ce5Y5(a,c)and Mg90Ce5Y5+10 wt.% C@Co(b,d)alloys.

Table 2 The platform pressure and thermodynamic parameter of as-milled Mg90Ce5Y5 and Mg90Ce5Y5+10 wt.% C@Co alloys.

Fig.12.DSC curves of Mg90Ce5Y5 and Mg90Ce5Y5+10 wt.% C@Co alloys.

Fig.12 shows the DSC curves of hydrogenated Mg90Ce5Y5and Mg90Ce5Y5+10 wt.% C@Co alloys,respectively.A main endothermic peak appears in all curves and corresponds to the decomposition of the MgH2,which is accordant with the analysis results of XRD and PCT.As observed in Fig.12,the addition of the C@Co composites results into the temperatures of endothermic peak decreased from 396.5 to 338°C,which indicates that the C@Co composites significantly promotes the hydrogen desorption property.Similarly,the initial dehydrogenation temperature also dropped from 341.0 to 267.0°C.In addition,when the thermal effect of physical and chemical reactions occurs,the peak area of the DSC curve,namely the area between the DSC curve and baseline,is directly proportional to enthalpy change [47,48].Therefore,it can be used to determine the influence of the C@Co composite catalyst on the thermodynamic properties of Mg-based alloys.As can be seen from Fig.12,the heat required for the reaction of C10 sample is slightly less than that of C0 sample,which is consistent with the analysis of thermodynamic based on PCT curves.Therefore,it is concluded that owing to the unmodified thermodynamics,the excellent activation energy and fast dehydrogenation rate of C10 sample is not attributed to the optimization of thermodynamics but the improvement of kinetics.The working temperature of hydrides is determined by platform pressure thermodynamics and total reaction kinetics.So,further improvements still need to be made in thermodynamics in order to the practical application.

4.Conclusions

The C@Co composites were synthesized by the hightemperature carbonization reaction,which consisted of C nanosheets and Co nanoparticles deposited on C nanosheet and is a mesoporous structure.Then,the Mg90Ce5Y5+10 wt.% C@Co alloy was synthesized by mechanical ball-milling.The hydrogen storage behavior of Mg90Ce5Y5alloy catalyzed by C@Co composites was studied in detail.The following are summarized conclusions:

(1)The C@Co composites belong to an inert catalyst which has not changed the original phase composition and reaction path of the Mg-based alloy,but its unique porous and lamellar structure will provides channels for the diffusion of hydrogen and the multitude of defects on C layer also plays the activated“catalytic sites”role on the nucleation of Mg/MgH2.

(2)The Mg90Ce5Y5+10 wt.% C@Co alloy can be activated entirely at the first hydrogenated cycle,which is mainly due to the C layers wrapped on the alloy particles inhibit the oxidation and sintering of the alloy particles during heating.In addition,the Mg90Ce5Y5+10 wt.% C@Co alloy shows excellent hydrogen absorption and desorption performances because of the synergistic catalysis ofCand Co in C@Co nanocomposites,and only takes 11 min to completely release hydrogen at 300 °C and the dehydrogenation activation energy also was reduced to 81.9 kJ mol−1H2.

(3)The addition of the C@Co composite catalyst increases the plateau pressures and decreases the value of decomposition enthalpy.However,these differences are so small that the improvement on thermodynamics is not significant.Therefore,the excellent activation energy and fast dehydrogenation rate of C10 sample are not attributed to the optimization of thermodynamics but the improvement of kinetics.

(4)The addition of C@Co catalyst has little effect on the phase composition and microstructure of master alloy,but it declines the particle size and ameliorates the activation property and hydrogen absorption/desorption kinetics.In addition,the activation energy and the initial temperature of dehydrogenation are lowered by adding this catalyst.These positive effects can be ascribed to the high defect density,high specific surface area,and the fine metal nanoclusters in the C@Co catalyzer.

Acknowledgments

This work is financially supported by the National Natural Science Foundations of China(51761032 and 51871125),the Natural Science Foundations of Inner Mongolia,China(No.2019BS05005),and the Scientific Research Staring Foundation of Taiyuan University of Science and Technology(20202040).