Ling Zeng,Zhiqing Ln,*,Boo Li,Huiren Ling,Xioin Wen,Xintun Hung,

Jun Tanc,*,Haizhen Liua,Wenzheng Zhoua,Jin Guoa

a Guangxi Novel Battery Materials Research Center of Engineering Technology,Guangxi Colleges and Universities Key Laboratory of Novel Energy Materials and Related Technology,School of Physical Science and Technology,Guangxi University,Nanning530004,PR China

b School of Materials Science and Engineering,Baise University,Baise533000,PR China

c National Engineering Research Center for Magnesium Alloys,College of Materials Science and Engineering,Chongqing University,Chongqing400044,PR China

Abstract Carbon materials have excellent catalytic effects on the hydrogen storage performance of MgH2.Here,carbon-supported Ni3S2(denoted as Ni3S2@C)was synthesized by a facile chemical route using ion exchange resin and nickel acetate tetrahydrate as raw materials and then introduced to improve the hydrogen storage properties of MgH2.The results indicated the addition of 10wt.% Ni3S2@C prepared by macroporous ion exchange resin can effectively improve the hydrogenation/dehydrogenation kinetic properties of MgH2.At 100°C,the dehydrogenated MgH2-Ni3S2@C-4 composite could absorb 5.68wt.% H2.Additionally,the rehydrogenated MgH2-Ni3S2@C-4 sample could release 6.35wt.% H2 at 275°C.The dehydrogenation/hydrogenation enthalpy changes of MgH2-Ni3S2@C-4 were calculated to be 78.5 kJ mol-1/-74.7 kJ mol-1,i.e.,11.0 kJ mol-1/7.3 kJ mol-1 lower than those of MgH2.The improvement in the kinetic properties of MgH2 was ascribed to the multi-phase catalytic action of C,Mg2Ni,and MgS,which were formed by the reaction between Ni3S2 contained in the Ni3S2@C catalyst and Mg during the firs hydrogen absorption-desorption process.? 2021 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:Magnesium;Hydrogen storage performance;Catalytic effect;Carbon-based catalyst.

1.Introduction

Hydrogen energy is one of the most promising candidates to replace traditional fossil energy due to its cleanliness,abundant resources,and environmentally friendly features[1-6].Among the various solid hydrogen storage materials,Mg-based materials have attracted extensive attention worldwide because of their high hydrogen storage capacity,low price,and rich natural resources[7].However,poor hydrogenation/dehydrogenation kinetic performances and high dehydrogenation temperature are the two main drawbacks of Mg-based materials[8].

We show here that the addition of catalysts such as doping with transition metals(TM)[9-16],metal oxides[17-22],or metal sulfide[23-27]is a highly effective and simple way to improve the thermodynamic and kinetic properties of Mgbased materials.Cui et al.[10]introduced the TM(Ti,Nb,V,Co,Mo,Ni)into MgH2by coating and suggested that TM with lower electro-negativity had a better catalytic action on the hydrogenation performance of MgH2.Khatabi et al.[13]and Yu et al.[14]studied the catalytic effect of the TM on the hydrogen storage properties of MgH2and found that the addition of TM could weaken the Mg-H bond and decrease the energy barrier for dehydrogenation from MgH2.Lu et al.[15]prepared a core-shell Mg@Pt nanocomposite and revealed that Pt transformed H-stabilized Mg3Pt,which acted as a“hydrogen pump”for the dehydrogenation of Mg and enhanced the hydrogen storage properties of Mg.Valentoniet al.[17]introduced a VNbO5catalyst into MgH2by ball milling and revealed that hydrogen(＞5.0wt.%)resulted in negligible degradation of a 15wt.% VNbO5-doped sample after 70 cycles of hydrogen absorption-desorption.Zhang et al.[18]reported that the Mg-H bonds of MgH2could be elongated and weakened under the catalytic reaction of Mn3O4;a MgH2+10wt.% Mn3O4composite could release hydrogen at 200 °C.Xie et al.[23]introduced NiS into Mg by ball milling and found that NiS reacted with Mg to form Ni,MgS,and Mg2Ni after the firs absorptiondesorption cycle.These multi-phase catalysts formed in situ greatly improved the hydrogenation kinetics of Mg,and the apparent activation energies of hydrogenation and dehydrogenation for NiS-doped Mg decreased to 45.45 kJ mol-1and 64.71 kJ mol-1,respectively.However,the gradual aggregation of Mg particles after hydrogenation and dehydrogenation cycles is inevitable,leading to a decrease in hydrogen storage performance.To solve such problems,carbon materials are often compounded with TM to improve the thermodynamic and kinetic properties of Mg-based materials[28-40].Here,1D porous Ni@C nanostructures were adopted to enhance the dehydrogenation and hydrogenation performance of MgH2based on An et al.[31],whose study showed that a Ni@C-doped MgH2composite showed outstanding hydrogen storage performance.At 300 °C,the 5wt.% Ni@C-doped MgH2sample could release 6.4wt.% H2in 10 min whereas bare MgH2could not release any hydrogen under the same condition.Liu et al.[38]synthesized a nano-V2O3@C composite and introduced it into Mg through ball milling.The dehydrogenated MgH2-V2O3@C composite could absorb hydrogen at ambient temperature and completely rehydrogenate at 150°C after only 700s.Theoretical studies suggested that the presence of V was responsible for the improvement in the hydrogen absorption and desorption performances of MgH2.

In our previous work,carbon materials with TM or their oxides such as Y2O3@rGO[41],V2O3@rGO[42],Ni@rGO[43],and Ni-TiO2@rGO[44]were successfully synthesized using graphene oxide(GO)and TM compounds.These materials significantl improved the hydrogenation and dehydrogenation performances of Mg-based materials.This showed that catalysts based on nickel compounded with carbon materials can greatly improve the thermodynamic and kinetic properties of Mg-based materials[31,35,37].Here,carbon-supported nickel sulfid(Ni3S2@C)composites were prepared using cheap cation exchange resins and Ni(CH3COO)2.Moreover,the impacts of Ni3S2@C on the hydrogenation and dehydrogenation kinetics and thermodynamics of MgH2were discussed.

2.Experimental section

2.1.Synthesis of Ni3S2@C

Four different cation exchange resins(Table 1),AmberliteIR-120(H)(resin-1;Alfa Aesar),Amberlite?IRN77(H)(resin-2;Alfa Aesar),Dowex Marathon MSC(H)(resin-3;Sigma-Aldrich),and Amberlyst? 15(H)(resin-4;Alfa Aesar),were used to synthesize Ni3S2@C composites.The preparation of Ni3S2@C is shown schematically in Fig.1.First,fi e grams of resin was dispersed in 100ml of hydrochloric acid solution at a concentration of 15%(by weight)and magnetically stirred for fi e hours at ambient temperature.The resin was then washed with deionized water to remove excess acid and impurities.A nickel ion(Ni2+)solution was obtained by dissolving 2.5g of Ni(CH3COO)24H2O(Analytical reagent;XILONG SCIENTIFIC)in 100ml of deionized water followed by magnetic stirring for one hour.The as-treated resin was then added to the solution containing nickel ions.The exchange process between Ni+ions and the resin was completed through magnetic stirring for 24h.The exchanged resin was then cleaned and dried for 24h in a frozen drying oven.Subsequently,the as-dried resin was heated to 500 °C for fi e hours under a nitrogen atmosphere.The obtained samples were milled for two hours with a ball-to-powder weight ratio of 40:1 at 500 revolutions per minute(rpm).The four milled samples corresponding to resin-1,resin-2,resin-3,and resin-4 were named Ni3S2@C-1,Ni3S2@C-2,Ni3S2@C-3,and Ni3S2@C-4,respectively.In addition,5g of the treated resin-4 was dried,carbonized,and milled under the same conditions,yielding carbon(C).

2.2.Synthesis of MgH2-Ni3S2@C

Commercial MgH2powder(98%;Langfang Beide Trading)was mixed with 10wt.% of C,Ni3S2(99.9%;Jiuding Chemical),Ni3S2@C-1,Ni3S2@C-2,Ni3S2@C-3,and Ni3S2@C-4.The resulting six mixtures were milled for fi e hours,and the ball-to-powder weight ratio was maintained at 40:1 at a speed of 500rpm.The six milled composites were denoted as MgH2-C,MgH2-Ni3S2,MgH2-Ni3S2@C-1,MgH2-Ni3S2@C-2,MgH2-Ni3S2@C-3,and MgH2-Ni3S2@C-4,respectively.

2.3.Characterization

The phase structure of the samples was det ermined by Xray diffraction(XRD;Minfl x 600;Cu-Kαradiation,40kV,and 200mA)and X-ray photoelectron spectroscopy(XPS,Thermo Fisher Scientific USA;Al-KαX-ray source).All XRD tests were performed by scanning the sample from 2θ=10° to 2θ=90° with a scanning speed of 5° min-1.The microstructure of the samples was determined by field emission scanning electron microscopy(FE-SEM;SU8020,HITACHI)and transmission electron microscopy(TEM;FEI Tecnai G2,f20 s-twin,200kV).The distributions of Ni,S,and C in the samples were determined using an energydispersive X-ray spectrometer(EDS)attached to an FE-SEM.The Bruner-Emmett-Teller(BET)surface areas were measured with a Micromeritics Tristar II instrument at 77.3K.The pressure-composition-temperature(PCT)was measured on an automatic Sievert-type device with a hydrogen pressure of 35 atm for the hydrogenation process and the lowest pressure of 0.06 atm for the dehydrogenation process.The hydrogen absorption and desorption properties of the samples were determined using a Sievert-type device.The nonisothermal hydrogenation tests were performed by heating the dehydrogenated samples from ambient temperature to 390 °C at 1 °C min-1under 60 atm of H2.The non-isothermal dehydrogenation tests were performed by heating the rehydrogenated samples from ambient temperature to 390 °C at 0.5 °C min-1under 0.001 atm of H2.The isothermal hydrogenation/dehydrogenation measurements were performed at various temperatures under 60 atm of H2for hydrogen absorption and 0.001 atm of H2for hydrogen desorption.The dehydrogenation performances of the composites were evaluated by differential scanning calorimetry(DSC,Mettler Toledo).The samples were heated from ambient temperature to 500 °C under an Ar atmosphere(fl w rate:75 ml min-1)at a heating rate of 5 °C min-1.

Table 1Main parameters of the cation exchange resins.

Fig.1.Schematic representation of the preparation of Ni3S2@C.

3.Results and discussion

The microstructures of the Ni3S2@C-1,Ni3S2@C-2,Ni3S2@C-3,and Ni3S2@C-4 composites examined by FESEM are displayed in Fig.2.All four Ni3S2@C samples prepared by different precursors possessed a spherical structure before ball milling as shown in Fig.2(A-D).These spherical samples were broken into fin particles after milling for two hours(Fig.2(A1,B1,C1,and D1)).EDS mapping revealed that all four Ni3S2@C composites contained C,S,and Ni.The XRD patterns of the resultant Ni3S2@C samples are presented in Fig.3A.There were two wide peaks at about 22° and 44°from the respective(002)and(100)planes of carbon[32].The diffraction peaks of Ni3S2@C-1,Ni3S2@C-2,Ni3S2@C-3,and Ni3S2@C-4 appeared at 21.76°,31.10°,37.80°,44.35°,49.73°,50.11°,and 55.16° from the respective(010),(-110),(111),(020),(120),(-120),and(-121)planes of the Ni3S2phase(JCPDS card no.85-1802).The diffraction peak of C could not be detected in the XRD profile of the Ni3S2@C composites because the diffraction peak intensity of C was weaker than that of Ni3S2.However,the EDS analysis results in Fig.2 confir that Ni3S2@C was successfully synthesized using cation exchange resin and nickel acetate as raw materials.Furthermore,the specifi surface areas and pore size distributions of the Ni3S2@C composites were investigated by nitrogen adsorption and desorption isotherms at 77.3K(Figs.3B and C).The BET surface areas of Ni3S2@C-1,Ni3S2@C-2,Ni3S2@C-3,and Ni3S2@C-4 were calculated as 273.9 m2g-1,319.5 m2g-1,331.7 m2g-1,and 330.3 m2g-1,respectively.All four synthesized catalysts had a large BET surface area.The BET surface areas of the composites prepared by macroreticular resins were noticeably larger than those prepared by gel resins.Based on the desorption isotherm curves,the Barret-Joyner-Halenda(BJH)desorption average pore diameters of Ni3S2@C-1,Ni3S2@C-2,Ni3S2@C-3,and Ni3S2@C-4 were calculated as 37.3 °A,36.0 °A,39.5 °A,and 40.1 °A,respectively(Fig.3C),indicating that all four catalysts were mesoporous.To further study Ni3S2@C,the Ni3S2@C-4 composite was selected as a typical representative and was investigated by the XPS method.Fig.3(D-F)displays the high-resolution XPS spectra of Ni3S2@C-4.The peaks observed at 855.9eV and 873.6eV were assigned to the respective Ni 2p3/2and Ni 2p1/2orbitals of Ni3S2[45,46],and the other two peaks at 861.0eV and 879.4eV corresponded to the accompanying satellite peaks of Ni 2p3/2and Ni 2p1/2(Fig.3D).In addition,weak peaks at 853.1eV and 870.8eV appeared in the Ni 2p3/2and Ni 2p1/2orbitals of NiO[47]due to the long exposure of the sample to air during fabrication.The peaks at 163.6eV and 164.8eV corresponded to the respective S 2p3/2and S 2p1/2orbitals of Ni3S2[48](Fig.3E).In the C 1s spectrum,the peak located at 284.8eV was assigned to the C-C bond[29,31,32].The results indicated that the Ni3S2@C-4 composite was successfully prepared using ion exchange resin and nickel acetate as raw materials.

Fig.2.FE-SEM images of Ni3S2@C-1(A),Ni3S2@C-2(B),Ni3S2@C-3(C),and Ni3S2@C-4(D)composites before ball milling;FE-SEM images and EDS mapping of ball-milled Ni3S2@C-1(A1-A4),Ni3S2@C-2(B1-B4),Ni3S2@C-3(C1-C4),and Ni3S2@C-4(D1-D4)composites.

Fig.3.(A)XRD patterns of C,Ni3S2,and Ni3S2@C composites.(B)Nitrogen adsorption and desorption isotherms of Ni3S2@C composites at 77.3K.(C)Pore size distributions curves of Ni3S2@C composites.(D-F)High-resolution XPS spectra of the respective Ni 2p,S 2p,and C 1s orbitals of Ni3S2@C-4.

The microtopographies of the as-synthesized Ni3S2@C and MgH2-Ni3S2@C-4 composites were further investigated by TEM,and the corresponding results are displayed in Fig.4.The particle size of Ni3S2ranged between 5nm and 20nm(Fig.4A and B),and the d-spacing of 0.410nm corresponded to the(010)plane of Ni3S2(Fig.4C).The SAED image in the inset of Fig.4D displays MgH2with crystal indices of(111)and(002)planes.The MgH2-Ni3S2@C-4 composite particles were distributed dispersively as shown in Fig.4(D,E).For the MgH2-Ni3S2@C-4 composite,the interplanar spacings of 0.207nm and 0.219nm were well matched with the(020)plane of Ni3S2and the(111)plane of MgH2,respectively(Fig.4F).

To study the effect of Ni3S2@C on the hydrogen absorption and desorption kinetic performances of MgH2,hydrogenation and dehydrogenation analyses of different MgH2-Ni3S2@C composites were performed.Fig.5A shows the hydrogenation curves of MgH2,MgH2-C,MgH2-Ni3S2,MgH2-Ni3S2@C-1,MgH2-Ni3S2@C-2,MgH2-Ni3S2@C-3,and MgH2-Ni3S2@C-4.For the unmodifie MgH2sample,the onset hydrogenation temperature was about 120 °C,and 7.36wt.% hydrogen was absorbed when the temperature reached～225 °C.The onset hydrogenation temperature of MgH2was greatly reduced after the addition of Ni3S2@C.After the addition of Ni3S2,Ni3S2@C-1,Ni3S2@C-2,Ni3S2@C-3,and Ni3S2@C-4,the resulting MgH2composites absorbed 4.39wt.%,3.74wt.%,2.94wt.%,5.15wt.%,and 5.68wt.% of H2at 100 °C,respectively.The onset hydrogenation temperatures of the MgH2-Ni3S2@C-1 and MgH2-Ni3S2@C-2 composites decreased to 50 °C,and MgH2-Ni3S2,MgH2-Ni3S2@C-3,and MgH2-Ni3S2@C-4 absorbed hydrogen at ambient temperature.The addition of Ni3S2and Ni3S2@C dramatically reduced the initial hydrogenation temperature of MgH2.The MgH2-Ni3S2@C-4 composite exhibited excellent hydrogenation performance.Fig.5B shows that the hydrogen desorption kinetic property of MgH2was also signifi cantly improved when it was modifie by the addition of the Ni3S2@C composites.When MgH2-Ni3S2@C-1,MgH2-Ni3S2@C-2,MgH2-Ni3S2@C-3,and MgH2-Ni3S2@C-4 were heated to 275°C with a heating rate of 0.5°C/min,6.16wt.%,5.61wt.%,5.50wt.%,and 6.35wt.% of H2were released,respectively.However,MgH2could not release any hydrogen at this temperature until the temperature exceeded 300 °C.It is obvious that the catalyst containing Ni3S2promoted the hydrogen desorption of the samples.The Ni3S2@C composite exerted an especially excellent catalytic action on the hydrogenation and dehydrogenation performances of MgH2.This may be due to the presence of C,which can make the Ni3S2with catalytic activity disperse more evenly and inhibit the agglomeration of particles during the milling process[34].The MgH2-Ni3S2@C-4 sample could absorb more hydrogen at 100°C and release more hydrogen at 275°C than any other sample,showing good hydrogen absorption and desorption performance.

Fig.4.TEM and HR-TEM images of(A,B,C)Ni3S2@C-4 and(D,E,F)MgH2-Ni3S2@C-4 composites.

Fig.5.(A)Hydrogenation and(B)dehydrogenation curves of MgH2,MgH2-C,MgH2-Ni3S2,and MgH2-Ni3S2@C composites.

The Ni3S2@C composites showed excellent catalytic activity for the hydrogen storage of MgH2,and the Ni3S2@C-4 composite was selected as a representative to further study its effect on the hydrogen storage performance of MgH2.Fig.6 presents the isothermal hydrogenation and dehydrogenation curves of the MgH2-Ni3S2@C-4 composite.For comparison,the isothermal hydrogenation/dehydrogenation curves of MgH2are also plotted in the figure The MgH2-Ni3S2@C-4 composite could absorb 6.08wt.% H2in 10 min at 150 °C and could absorb 6.0wt.% H2in 157min even at 50 °C(Fig.6A),whereas MgH2could only absorb 0.70wt.% H2at 150 °C(Fig.6C).Hence,the hydrogenation property of MgH2was significantl improved under the catalytic effect of Ni3S2@C-4.The catalytic effect of Ni3S2@C-4 on the hydrogenation performance of MgH2was better than that of Ni-V[12],NiS[23],Ni@rGO[43],Fe3S4[25],and Co@C[29].For example,MgH2-Ni@rGO and MgH2-20wt.% Fe3S4took 100min and 20min to reabsorb 5.94wt.%[43]and 3.41wt.% of H2[25]at 150 °C,respectively.Furthermore,under the same hydrogenation time(10min),Mg-Ni-V[12],Mg-5wt.%NiS[23],and MgH2-Co@C[29]absorbed only 1.0wt.%,3.5wt.%,and 2.71wt.% of H2,respectively.The dehydrogenation kinetics of MgH2-Ni3S2@C-4 were determined at different temperatures(225 °C,250 °C,275 °C,300°C,and 325 °C)(Fig.6B).The sample desorbed 5.61wt.%H2in 160min at 250 °C.When the temperature reached 300 °C,the dehydrogenation capacity increased to 6.15wt.%H2in eight minutes.However,unmodifie MgH2hardly released any hydrogen at temperatures below 275 °C and took 160min to release 4.38wt.% H2at 300 °C(Fig.6D).Mg-Ni-V[12],Mg-5wt.%NiS[23],and MgH2-Co@C[29]only released 3.0wt.%,3.1wt.%,and 5.74wt.%of H2,respectively,in 30min at 300 °C.Hence,versus the abovementioned materials,MgH2-Ni3S2@C-4 could release more hydrogen at a faster rate,showing excellent hydrogen desorption performance.

Fig.6.(A,C)Isothermal hydrogenation and(B,D)dehydrogenation curves of the MgH2-Ni3S2@C-4 composite and MgH2.

To further explore the effect of Ni3S2@C-4 on the hydriding reaction of MgH2,the Johnson-Mehl-Avrami-Kolmogorov(JMAK)and Arrhenius equations were employed to calculate the apparent activation energy(Ea).The JMAK equation can be written as follows[12,18]:

wheref(t)is the time-dependent function,nis the Avrami exponent,andkis an effective kinetic parameter.Based on the isothermal hydrogenation/dehydrogenation curves at different temperatures,the values ofnandnlnkwere obtained by fittin the JMAK plots between ln[-ln(1-f(t))]and lnt(Fig.7(AD).Based on these JMAK plots,the corresponding apparent activation energies were calculated by the Arrhenius equation:

Fig.7.(A-D)JMAK and(E,F)Arrhenius plots of the MgH2-Ni3S2@C-4 composite and MgH2:(A,C,E)hydrogenation and(B,D,F)dehydrogenation.

Table 2Empirical hydrogenation apparent activation energies of some common MgH2 systems.

whereEa,R,T,andk0represent the apparent activation energy,the gas constant(8.314 J K-1mol-1),the absolute temperature,and the Arrhenius pre-exponential factor,respectively.The Arrhenius plots of MgH2-Ni3S2@C-4 and MgH2are displayed in Fig.7(E,F).The hydrogenation apparent activation energy of the dehydrogenated MgH2-Ni3S2@C-4 composite was calculated to be 39.6 kJ mol-1,which is lower than that of MgH2(93.6 kJ mol-1).Table 2 presents the hydrogenation apparent activation energies of some common Mgbased composites.It is obvious that in comparison with the catalysts listed in Table 2,Ni3S2@C-4 can more effectively reduce the hydrogenation reaction potential barrier and improve the hydrogenation kinetics performance.Moreover,the dehydrogenation apparent activation energy of the rehydrogenated MgH2-Ni3S2@C-4 composite was calculated as 115.2 kJ mol-1(lower than that of MgH2)according to the slope of the straight line shown in Fig.7F.The hydriding reaction potential barrier for the hydrogen absorption and desorption processes of MgH2was significantl reduced by the addition of Ni3S2@C-4.The decrease in the hydriding reaction potential barrier promoted the diffusion of hydrogen atoms in the Mg matrix and improved the hydrogenation and dehydrogenation kinetics of MgH2.

DSC analysis was employed to further study the dehydrogenation performance of the MgH2-Ni3S2@C-4 composite.Fig.8 displays the DSC curves of the MgH2-Ni3S2@C-4 composite and MgH2.The endothermic peak of MgH2appeared at 434.3 °C;when Ni3S2@C-4 was added,the endothermic peak temperature was reduced to 296.3°C(Fig.8),which was 138.0 °C lower than that of MgH2.The endothermic peak temperature significantl decreased with the addition of Ni3S2@C-4,indicating an improvement in dehydrogenation performance.

Fig.9(A,B)displays the pressure-compositiontemperature(PCT)curves of the MgH2-Ni3S2@C-4 composite and MgH2.For MgH2-Ni3S2@C-4,a complete reversible hydrogenation and dehydrogenation cycle occurred at 275°C.In contrast,only the hydrogenation process occurred for MgH2,and the dehydrogenation process did not occur at 325 °C.Based on the PCT curves in Fig.9,the hydriding reaction thermodynamic behavior was described by the Van’t Hoff equation,which can be expressed as follows[23]:

Fig.8.DSC curves of the MgH2-Ni3S2@C-4 composite and MgH2.

wherePH2,ΔH,andΔSrepresent the plateau pressure of hydrogenation/dehydrogenation,the reaction enthalpy change,and the reaction entropy change,respectively.Generally,enthalpy change is regarded as an important parameter of the thermodynamic behavior of a reaction process.The hydrogenation/dehydrogenation enthalpy changes for the MgH2-Ni3S2@C-4 composite and MgH2were obtained from the Van’t Hoff plots shown in Fig.9(C,D).The hydrogenation and dehydrogenation reaction enthalpy changes of MgH2were calculated as-82.0 kJ mol-1H2and 89.5 kJ mol-1H2,respectively(Fig.9D).When MgH2was composited with Ni3S2@C-4,the hydrogenation and dehydrogenation enthalpy changes of MgH2-Ni3S2@C-4 decreased to-74.7 kJ mol-1and 78.5 kJ mol-1,respectively(Fig.9C).These results further demonstrate that the addition of Ni3S2@C-4 decreased both the hydrogenation potential barrier of Mg and the stability of MgH2,thereby improving the reversible hydrogen storage property of MgH2.

To study the reversible hydrogenation and dehydrogenation cycle stability of MgH2-Ni3S2@C-4,30 cycles of hydrogen absorption-desorption were performed for MgH2-Ni3S2@C-4 at 300 °C and for pure MgH2at 375 °C(Fig.10).The maximum hydrogen storage capacities of MgH2-Ni3S2@C-4 and pure MgH2were 6.52wt.% and 6.92wt.%,respectively.After 30 cycles of hydrogen absorption-desorption,the hydrogen storage capacity of the MgH2-Ni3S2@C-4 composite had no decay.The hydrogen storage capacity and the capacity retention ratio of pure MgH2decreased to 6.45wt.%and 93.2%,respectively,implying that the addition of the Ni3S2@C-4 composite enhanced the stability of the hydrogen absorption-desorption cycle.

Fig.9.PCT curves and Van’t Hoff plots of(A,C)MgH2-Ni3S2@C-4 and(B,D)MgH2.

Fig.10.Hydrogenation and dehydrogenation cycles of(A)the MgH2-Ni3S2@C-4 composite and(B)pure MgH2.

Fig.11.XRD patterns of(A)pure MgH2 and(B)the MgH2-Ni3S2@C-4 composite hydrogenatedat 380°C under 60 atm H2 and dehydrogenated at 380°C under 0.001 atm H2,and XRD patterns for the fully activated of MgH2-Ni3S2@C-4 composite,(C)hydrogenated at 100°C,200°C,300°C and 380°C under 60 atm H2 and(D)dehydrogenated at 250°C,300°C,350°C and 380°C under 0.001 atm H2.

Therefore,the comprehensive hydrogen storage performance of MgH2was significantl enhanced by the addition of Ni3S2@C-4.To explore the catalytic mechanism of Ni3S2@C-4,the XRD patterns of hydrogenation and dehydrogenation for both pure MgH2and MgH2-Ni3S2@C-4 were investigated(Fig.11).Fig.11(A,B)shows that the as-prepared MgH2was completely dehydrogenated into Mg and H2at 380°C,and the rehydrogenated product was MgH2.For the MgH2-Ni3S2@C-4 sample,both Mg2Ni and MgS phases appeared in the firs dehydrogenation cycle.In addition,the MgH2-Ni3S2@C-4 composite was fully activated at 380°C and then subjected to hydrogen absorption and desorption at different temperatures to obtain the diffraction spectra(Fig.11(C,D)).Fig.11(C)shows that the rehydrogenated MgH2-Ni3S2@C-4 sample was mainly composed of Mg2Ni,MgS,β-MgH2,andγ-MgH2when it hydrogenated at 100°C,200°C,and 300°C,respectively.Additionally,some Mg2Ni reacted with hydrogen to form Mg2NiH4at 300°C(Fig.11C).As the hydrogenation temperature increased to 380°C,Mg2Ni andγ-MgH2were completely converted to Mg2NiH4andβ-MgH2,respectively.In the dehydrogenation process(Fig.11D),Mg2NiH4was completely dehydrogenated at 250°C.However,someβ-MgH2could not release hydrogen until the temperature reached 300°C.The MgS phase remained unchanged during the hydrogenation and dehydrogenation process.Apparently,in the firs dehydrogenation cycle,Ni3S2reacted with Mg to generate Mg2Ni and MgS,thus forming multiphase in situ catalysts(Mg/Mg2Ni,Mg/MgS,and Mg/C).This multi-phase interface provided more active sites and diffusion paths for hydrogen atoms to enhance the hydrogenation/dehydrogenation properties of Mg/MgH2[50].During the dehydrogenation process,Mg2NiH4was dissociated into Mg2Ni and H2at the interface of MgH2/Mg2NiH4and caused MgH2to break down[11].Hence,Mg2NiH4acted as a“hydrogen pump”to drive MgH2to dissociate,thus reducing dehydrogenation temperatures and improving the hydrogen desorption performance of the MgH2-Ni3S2@C-4 composite.Furthermore,carbon prevented Mg,Mg2Ni,and MgS grains from agglomeration and also provided more active sites for hydrogen atoms[31].The synthesis and catalytic mechanism of Ni3S2@C-4 during hydrogenation/dehydrogenation processes are schematically presented in Fig.12.MgH2particle surfaces were covered by high-activity Ni3S2@C-4 after ball milling.Mg2Ni and MgS were formed by the reaction of Ni3S2and Mg during the firs dehydrogenation process.MgS remained unchanged during the hydrogenation and dehydrogenation reactions.Hence,the onset temperature and the apparent activation energy of Mg decreased with the formation of multi-phase catalysts.Consequently,the integrated hydrogen storage performance of MgH2was significantl improved under the catalytic action of Ni3S2@C.

Fig.12.Schematic representation of the catalytic mechanism of the Ni3S2@C-4 composite during hydrogenation and dehydrogenation processes.

4.Conclusion

Here,Ni3S2@C catalysts were successfully prepared using four different cation exchange resins:AmberliteIR-120(H)(resin-1),Amberlite? IRN77(H)(resin-2),Dowex Marathon MSC(H)(resin-3),and Amberlyst? 15(H)(resin-4).The nitrogen adsorption and desorption isotherm measurement results indicated that all four types of Ni3S2@C catalysts were mesoporous materials with a large BET surface area.The comprehensive hydrogen storage performance of MgH2was significantl improved by the addition of Ni3S2@C.The catalytic effects of the Ni3S2@C composites prepared using macroreticular resins(resin-3 and resin-4)on the hydrogen storage performance of MgH2were better than those prepared using gel resins(resin-1 and resin-2).The MgH2-Ni3S2@C-4 composite exhibited excellent comprehensive hydrogen storage performance.It absorbed hydrogen at ambient temperature,took only 10 min to absorb 6.08wt.% H2at 150 °C,and took eight minutes to release 6wt.%H2at 300°C.The hydrogenation and dehydrogenation apparent activation energies of MgH2-Ni3S2@C-4 were 39.6 kJ mol-1and 115.2 kJ mol-1,respectively,which were much lower than those of MgH2(93.6 kJ mol-1and 141.5 kJ mol-1,respectively).Ni3S2of Ni3S2@C reacted with Mg to form Mg2Ni and MgS during the firs desorption process.The multi-phase(Mg/Mg2Ni,Mg/MgS,and Mg/C)interface provided more active sites to improve the hydrogen storage performance of MgH2.

Declaration of Competing Interest

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

Acknowledgements

We thank LetPub(www.letpub.com)for linguistic assistance during the preparation of this manuscript.This work was supported by the National Natural Science Foundation of China(grant number 51571065),the Natural Science Foundation of Guangxi Province(grant numbers,2018GXNSFAA294125,2018GXNSFAA281308,2019GXNSFAA245050),the Innovation-Driven Development Foundation of Guangxi Province(grant number AA17204063),and the Innovation Project of Guangxi Graduate Education(grant number YCSW2020046).