Shun Wang,Mingxia Gao,Zhihao Yao,Kaicheng Xian,Meihong Wu,Yongfeng Liu,Wenping Sun,Hongge Pan

State Key Laboratory of Silicon Materials,Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province & School of Materials

Science and Engineering,Zhejiang University,Hangzhou 310027,P.R.China

Abstract Magnesium hydride(MgH2)is one of the most promising hydrogen storage materials for practical application due to its favorable reversibility,low cost and environmental benign;however,it suffers from high dehydrogenation temperature and slow sorption kinetics.Exploring proper catalysts with high and sustainable activity is extremely desired for substantially improving the hydrogen storage properties of MgH2.In this work,a composite catalyst with high-loading of ultrafin Ni nanoparticles(NPs)uniformly dispersed on porous hollow carbon nanospheres is developed,which shows superior catalytic activity towards the de-/hydrogenation of MgH2.With an addition of 5 wt% of the composite,which contains 90 wt% Ni NPs,the onset and peak dehydrogenation temperatures of MgH2 are lowered to 190 and 242 °C,respectively.6.2 wt% H2 is rapidly released within 30 min at 250 °C.The amount of H2 that the dehydrogenation product can absorb at a low temperature of 150 °C in only 250 s is very close to the initial dehydrogenation value.A dehydrogenation capacity of 6.4 wt% remains after 50 cycles at a moderate cyclic regime,corresponding to a capacity retention of 94.1%.The Ni NPs are highly active,reacting with MgH2 and forming nanosized Mg2Ni/Mg2NiH4.They act as catalysts during hydrogen sorption cycling,and maintain a high dispersibility with the help of the dispersive role of the carbon substrate,leading to sustainably catalytic activity.The present work provides new insight into designing stable and highly active catalysts for promoting the(de)hydrogenation kinetics of MgH2.? 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:Hydrogen storage materials;Nano-catalysis;Magnesium hydride;Porous hollow carbon nanospheres;Ni nanoparticles.

1.Introduction

Hydrogen is an ideal alternative to fossil fuels for its high energy density and abundant resources[1].However,safe,efficien and economical hydrogen storage is still a challenge for the large-scale application of hydrogen energy.[2]In the last few decades,solid-state hydrogen storage materials,including light metal hydrides[3,4]and complex hydrides[5-8],have attracted considerable attention due to their high hydrogen density and safety.Among them,MgH2,with a capacity of 7.6 wt% H2,is regarded as one of the most promising candidates owing to its high reversibility,low cost and environmental friendliness[9].Unfortunately,it suffers from high thermodynamic stability and slow sorption kinetics[10].

A variety of strategies have been developed to tackle these problems.One is to thermodynamically destabilize MgH2by alloying Mg with other metal elements[11],such as Al[12],Ni[13],and Fe[14].A representative intermetallic hydride is Mg2NiH4,which shows a favorable dehydrogenation enthalpy of 65 kJ mol-1H2,[15]lowered by 10 kJ mol-1H2compared with that of the pristine MgH2.The reduced enthalpy change results in a low dehydrogenation temperature of 255 °C at 1 bar equilibrium H2pressure.[15]However,a main drawback of this strategy is the inevitable capacity loss,where the theoretical hydrogen storage of Mg2NiH4is only 3.6 wt%[16].

Nanostructuring is also an effective way to modify the thermodynamics and especially kinetics of MgH2.[17-21]Theoretical calculations suggest that there is a significan decrease in thermodynamic stability when the grain size of MgH2is reduced to less than 2.0 nm.[22]For MgH2with a grain size of 0.9 nm,the decomposition enthalpy is only 63 kJ mol-1H2,corresponding to a desorption temperature of only 200 °C.[23]The grain refinemen also improves the hydrogen de-/sorption kinetics,owing to abundant diffusion paths and shortened diffusion distance.[24]Nevertheless,it is so far difficul to experimentally synthesize MgH2NPs smaller than 20 nm.[25,26]In this case,attempts turn to confinin MgH2in various porous scaffolds,which makes it possible to obtain MgH2in several nanometers according to the pore size of scaffolds.However,the scaffolds commonly take up considerable content in the confine systems,and hence the available hydrogen storage capacity is significantl lowered.For instance,a confine MgH2system using activated carbon fiber as scaffold shows a low dehydrogenation enthalpy and a low activation energy of 63.8±0.5 kJ mol-1H2and 1438±2 kJ mol-1,respectively,[27]indicating reduced thermal stability and improved kinetics.However,this system can only load 22 wt% MgH2,representing a theoretical hydrogen capacity of merely 1.67 wt%.

Introduction of catalysts is another effective way to reduce the dehydrogenation temperature and improve the reaction kinetics of MgH2,where transition metals and their compounds are the commonly used catalysts[28-33].Among them,Ni has attracted considerable attention due to its active role in the dissociation of hydrogen,where a vacantd-orbital of Ni firs accepts electron of hydrogen and then the bind is stabilized by back-donation of electrons from the filled-orbital to the anti-bonding orbital(σ*)of H2,thus facilitating the break of H-H bond[34,35].The combination of H atoms to form H2molecular is just the inverse process.Especially nanosized Ni-based catalysts are highly effective since they can provide large contact surface area and abundant active sites[36].This helps to decrease their addition amount for effective catalysis,hence minimizing the loss of the theoretical hydrogen storage capacity.However,nanosized Ni particles are dimensionally unstable and easily agglomerate during hydrogen sorption cycling due to the high surface energy[37].Recent studies show that the size stability and catalytic activity of Ni NPs can be further enhanced by loading them on substrates like carbonbased materials[37,38].Moreover,carbonaceous materials are also favorable for the nucleation of Mg or/and MgH2and provide additional channels for hydrogen diffusion,exhibiting a synergistic catalytic effect with Ni[39].The reported carbonaceous substrates for supporting nano-sized Ni particles include graphene nanosheets[24,39,40],carbon aerogel[41],mesoporous carbon CMK-3[42],hard carbon spheres[16,43],and so forth.Nevertheless,the Ni loading in the currently reported carbon-supported catalysts is commonly low in order to stably disperse Ni NPs and maximize their catalytic effectiveness.Challenges remain on achieving small size,high loading and uniform dispersion of nano-Ni particles on substrates simultaneously.Although the hydrogen storage properties of MgH2are improved by these carbon-supported nickel catalysts to date,developing highly effective catalysts is still highly imperative to further improve the overall hydrogen storage properties of MgH2at milder conditions.

In the present study,a type of porous hollow carbon nanospheres(PHCNSs)that we previously reported[44]was used as the substrate to prepare the Ni-incorporated PHCNSs composites,and their catalytic effect on MgH2was investigated.The merits of PHCNSs as the substrate are as follows.First,the PHCNSs possess high specifi surface area,large pore volume and hierarchical pore structure,which can provide large amounts of dispersive sites for Ni NPs.Second,the PHCNSs have a highly amorphous structure with massive defects,brought by the pores generated during the CO2activation process.As reported,surface defects can change the electronic structures of carbon surfaces,creating trapping centers for metal atoms[45].Lastly,except for C,the PHCNSs also contain 18.2 at% O and minor amounts of N and H,which are originated from the surface functional groups of PHCNSs.The heteroatom-induced coordination sites in carbon substrates can anchor metal precursors via chelation[45].It is thus believed that the abundant dispersive sites,surface defects and heteroatoms of PHCNSs favor the dispersion of metal Ni NPs.In order to optimize the catalytic effect,different amounts of Ni are incorporated to the PHCNSs substrate.Up to 90 wt%Ni NPs can be highly dispersed on to the PHCNSs without agglomeration.With an addition of only 5 wt%of the 90 wt% Ni-incorporated PHCNSs,the system shows evidently lowered dehydrogenation temperature,improved kinetics and superior cycling stability.The evolution of phase compositions and microstructures of the system during cycling is investigated for identifying the reasons for the improved hydrogen storage properties.

2.Experimental section

2.1.Materials synthesis

The detailed synthesis process and structural characteristic methods of the PHCNSs were as reported in our previous work[44].The PHCNSs have intact spherical morphology with a uniform size distribution of ca.90 nm,and with inner cavities in size of ca.30 nm and hence a shell thickness of 30 nm.The specifi surface area and pore volume of the PHCNSs are up to 2609 m2g-1and 2.275 cc g-1,respectively.The volume of the inner cavities is not included in the measured value as the size is beyond the measurement range of the analyzer.The nanopores generated by CO2activation in the carbon shell are mostly smaller than 4.5 nm.In addition,the PHCNSs contain 18.2 at% O and minor amounts of N and H.

Ni(NO3)2?6H2O(98%,Aladdin)was used as the Ni source.Different amounts of the PHCNSs and Ni(NO3)2?6H2O corresponding to mass ratios of Ni:PHCNSs of 5:5,7:3,9:1 and 9.5:0.5,respectively,were added to a medium of ethanol and stirred for 30 min.The mixed suspension was then subjected to ultrasonication under dynamic vacuum for 6 h to ensure a full wetting of the Ni(NO3)2solution to the outer and inner surface of the PHCNSs,including the pores in the shell.During this process,most of ethanol medium is removed by vacuum extraction,leaving a viscous mixture,which is then dried at 60 °C for 12 h in a vacuum drying oven to remove the residual ethanol.Thereafter,the completely dried mixture was heat-treated at 500 °C for 4 h under a fl wing mixture gas of 10 vol% H2/Ar for the thermal reduction of Ni precursor to metal Ni.The fina products were denoted as Nix@PHCNSs,wherexrepresents the mass fraction of Ni in the composites.

MgH2used in the present study was synthesized in-house by hydriding a commercial Mg powder(Macklin,purity 98%,20-100 mesh)at 340 °C under 20 bar H2for 12 h.The obtained Nix@PHCNSs composites were introduced to MgH2at an addition amount of 5 wt% by ball-milling under 50 bar H2for 24 h using a planetary ball mill(QM-3SP4,Nanjing).A ball-to-sample weight ratio of 120:1 and a rotating speed of 500 rpm were used.The mill rotated for 0.3 h in one direction,and paused for 0.1 h,and then revolved in the reverse direction for 0.3 h,to minimize the temperature increase during ball milling.The obtained systems were denoted as MgH2/Nix@PHCNSs.In addition,the MgH2/PHCNSs system was also prepared under the same experimental procedure for comparison.All operations were conducted in a glove box(MBRAUN 200B,Germany)fille with pure Ar(H2O＜1 ppm;O2＜1 ppm)to avoid the moisture and air contamination.

2.2.Structural and morphological characterizations

The crystal structures of the samples were analyzed by X-ray diffraction(XRD,MiniFlex 600 X-ray diffractometer,Rigaku).The samples were sealed in a custom-designed container covered with a Scotch tape to prevent air and moisture contamination during operation.The Ni content of the Nix@PHCNSs composites was measured by thermogravimetry analysis(Netzsch,TG 209 F3).A NOVA-1000e automated surface area analyzer(Quantachrome,USA)was used to conduct N2sorption measurement,and the specifi surface area and pore size distribution of the samples were calculated based on the Brunner-Emmet-Teller(BET)and density functional theory(DFT)methods.Morphologies of the samples were analyzed by scanning electron microscopy(SEM,Hitachi,S-4800)and transmission electron microscopy(TEM,FEI,Tecnai G2 F20 S-TWIN).The distribution of C,Ni and Mg elements in the samples was identifie with an energy-dispersive X-ray spectrometer(EDS)attached to the TEM facility.X-ray photoelectron spectroscopy(XPS,Thermo Scientific ESCALAB 250Xi)was used to determine the chemical valence state of Ni in the samples.XPS spectra were recorded using monochromatic an Al Kα(1486.6 eV)X-ray source with a base pressure of 5×10-10 Torr of air.The adventitious C 1 s peak centered at 284.8 eV was used as a reference to calibrate the obtained XPS data,and the binding energy spectra were fitte by XPSPEK software.

2.3.Hydrogen storage property measurement

The temperature-dependent dehydrogenation properties of the catalyzed systems were evaluated by a custom-designed temperature-programmed desorption(TPD)instrument coupled with a mass spectrometer(MS).The TPD-MS curves at different heating rates(1,2,4,8 °C min-1)were collected to evaluate the apparent dehydrogenation activation energies(Ea)of the systems based on the Kissinger method[46]:

whereβis the heating rate,Tpis the absolute temperature corresponding to the maximum desorption rate andRis the gas constant.In this work,Tpis the peak temperature of the TPD-MS curves with different heating rates.Dehydrogenation and hydrogenation of the systems were quantitatively measured by a volumetric method on a custom-designed Sievert-type apparatus.For the non-isothermal testing,the systems were heated from room temperature to 400 °C at a heating rate of 2 °C min-1under static vacuum for dehydrogenation and heated from room temperature to 250 °C at a heating rate of 1 °C min-1under 50 bar H2for hydrogenation.For the isothermal testing,the systems were rapidly heated to the pre-set temperature and dwelled for 1 h under static vacuum for dehydrogenation,while for hydrogenation,the systems were firs heated to the pre-set temperature under static vacuum and then rapidly loaded with 50 bar H2,dwelling for 10 min.A heating rate of 10 °C min-1was used for both isothermal dehydrogenation and hydrogenation.Cyclic performance of the optimized system was evaluated with a regime of dehydrogenation to 350 °C under static vacuum and hydrogenation to 250 °C under 50 bar H2.

3.Results and discussion

3.1.Structures and morphologies of the Nix@PHCNSs composites and their catalyzed MgH2 systems

The preparation process of the MgH2/Nix@PHCNSs systems is illustrated in Scheme 1.The porous hollow carbon nanospheres are firstl fully impregnated with the ethanol solution of nickel nitrate under dynamic vacuum.After heat reduction,the Ni NPs-loaded PHCNSs composites are obtained,which are then introduced to MgH2by ball-milling.Fig.1 presents the XRD patterns of the Nix@PHCNSs composites with different Ni contents.The diffraction peaks of Ni(JCPDS no.04-0850)are clearly seen for all composites,indicating the successful formation of elemental Ni with high crystallinity.Thermogravimetric analysis(TGA)of the Nix@PHCNSs composites(Fig.S1)conducted in air shows that there are weight gains for the composites with high Ni contents while weight losses for the composites with low Ni contents,which is ascribed to the competitive result of the oxidation of Ni to NiO and the combustion of the PHCNSs to carbon dioxide during the heating process.Based on the TGA results,the Ni contents for the composites with weight ratios of Ni:PHCNSs of 5:5,7:3,9:1 and 9.5:0.5 are calculated to be 54.4,72.2,90.7 and 96.7 wt%,respectively,very close to the designed values.

Scheme 1.Schematic illustration of the preparation process of the MgH2/Nix@PHCNSs systems.

Fig.1.XRD patterns of the Nix@PHCNSs composites.

Fig.2 shows the SEM images of the Nix@PHCNSs composites.It is seen that the spherical structure of the PHCNSs remains stable after the incorporation of Ni NPs.For the composite with Ni contents of 50 and 70 wt%,the morphology is almost the same as that of the original PHCNSs,without other evidently different morphology,suggesting that the Ni NPs are mostly deposited in the inner cavities and nanopores of the PHCNSs.With the Ni content increasing to 90 wt%,there are numerous bright dots corresponding to Ni NPs observed,which are uniformly deposited on the surface of the PHCNSs.The corresponding EDS mapping(Fig.S2)shows that Ni element is well distributed in the C substrate without obvious aggregation,demonstrating the ultrafin size and the well dispersion of Ni NPs.However,when the Ni content reaches 95 wt%,some bulk Ni aggregates with the size of several hundred nanometers are observed,as marked by white dashed circles in Fig.2d.

Further TEM observation of the Nix@PHCNSs composites is shown in Fig.3,where the bright area is the PHCNSs substrate and the dark dots represent Ni NPs.The stable spherical structure of the PHCNSs is demonstrated by white dashed circles,and the Ni NPs with an average size of ca.10 nm are uniformly dispersed on the surface of the PHCNSs and also in their inner cavities.The latter is generated from the nickel nitrate solution infiltratin into the inner cavities of the PHCNSs during the vacuum impregnation process.As seen from Fig.3a-d,with increasing Ni content from 50 to 95 wt%,the density of the dispersive Ni NPs is gradually increased.What’s notable is that the Ni NPs up to a high loading of 90 wt% maintain fin size and high dispersibility,with almost no agglomeration.Although there is aggregated Ni observed at a higher loading of 95 wt%,most Ni particles are still dispersive,in consistence with the SEM observation.In addition,in the case that the samples for TEM characterization are subjected to strong ultrasonic treatment during the preparation process,the Ni NPs are still immobilized on the PHCNSs,suggesting a strong bonding between the Ni NPs and the PHCNSs substrate.Such a strong metal-substrate interaction is helpful for suppressing the aggregation of metal NPs and tailoring the geometric structures and electronic configu rations of catalytic active sites[45].

The chief, believing he had hit him, was looking down for him, when all at once he came up behind and cleft him to the waist and sent him straight to hell

The nitrogen sorption isotherms and pore size distributions of the Nix@PHCNSs composites(Fig.S3a and b)show that both specifi surface area and pore volume value are all extremely lowered compared with those of the PHCNSs.Besides the observed Ni NPs that deposit at the surface and cavity of the PHCNSs,there are also some ultrafin Ni particles with the size of only a few nanometers dispersed in the pore channels of the carbon shell,leading to the reduction of porosity.The unique structure of the PHCNSs not only provides large amounts of dispersive sites for Ni NPs,but also realizes their hierarchical size distribution.

Fig.2.SEM images of the Nix@PHCNSs composites with Ni mass fractions of 50 wt%(a),70 wt%(b),90 wt%(c)and 95 wt%(d).

Fig.3.TEM images of the Nix@PHCNSs composites with Ni mass fractions of 50 wt%(a),70 wt%(b),90 wt%(c)and 95 wt%(d).

Fig.4a shows the XRD patterns of the as-milled MgH2/Nix@PHCNSs systems as well as the pristine MgH2and the MgH2/PHCNSs system.β-MgH2is the main phase for all the systems.Besides,a minor amount of MgO is also identifie from its main diffraction peak at 42.8°(JCPDS no.45-0946)although its intensity is very weak,which is suggested from the chemical reaction between MgH2and the oxygen-containing functional groups of the during ball milling.Notably that there are no diffraction peaks of Ni detected for all the MgH2/Nix@PHCNSs systems,possibly due to its low relative content.Further XPS analysis of the Ni 2p spectrum of the MgH2/Ni90@PHCNSs system,Fig.4b,shows that there are two peaks at 852.9 and 870.0 eV,which are well assigned to the binding energy of Ni 2p3/2and Ni 2p1/2,respectively,[47,48]demonstrating the existence of elemental Ni.Therefore,it is obtained that ball milling is only a physical mixing process.

Fig.4.(a)XRD patterns of the as-milled MgH2/Nix@PHCNSs systems,the MgH2/PHCNSs system and the pristine MgH2;(b)Ni 2p XPS spectrum of the MgH2/Ni90@PHCNSs system.

A representative SEM image of the as-milled MgH2/Ni90@PHCNSs system(Fig.S4a)shows that the ball-milled system is composed of irregular particles with an overall size distribution from tens to hundreds of nanometers.By contrast,the as-milled pristine MgH2shows an obviously large size distribution,where nano-scale and micron-scale MgH2particles coexist(Fig.S4b).The result demonstrates that the introduction of the Nix@PHCNSs composites enhance the ball milling efficien y by serving as grinding agents.Further TEM characterization of the as-milled MgH2/Ni90@PHCNSs system is conducted,as shown in Fig.5a.The original spherical morphology of the Ni-incorporated PHCNSs disappears after ball milling,which is transformed into lamellar carbon with uniformly embedded Ni NPs under stress,covered homogeneously on the surface of MgH2.Although the lamellar carbon is hardly identifie due to its poor contrast relative to MgH2,the superfin Ni NPs in size of ca.10 nm are vaguely seen,as marked by the circles in Fig.5a.EDS analysis(Fig.5b-d)further shows that Ni and C elements are uniformly distributed on the MgH2particles without aggregation.It is thus obtained that the PHCNSs can not only disperse a high loading of Ni in the initial structure,but also preserve the high dispersibility of Ni NPs even after the high energy ball milling,which is likely due to the strong metal-substrate interaction as mentioned above.

3.2.Hydrogen storage properties of the MgH2/Nix@PHCNSs systems

Fig.6a shows the temperature-dependent dehydrogenation behavior of the MgH2/Nix@PHCNSs systems analyzed by TPD-MS measurement,and that of the pristine MgH2and the MgH2/PHCNSs system is also shown for comparison.It is seen that with the introduction of only 5 wt% of the Nix@PHCNSs composite,both onset and peak dehydrogenation temperatures of MgH2are significantl reduced.By contrast,the individual PHCNSs has limited catalytic effect on the dehydrogenation of MgH2,where the onset dehydrogenation temperature is almost the same as that of the pristine MgH2and the peak temperature is only 7 °C lower than 317 °C for the pristine MgH2,suggesting that Ni is central important for the catalysis.Additionally,it should be noted that there is an additional shoulder peak at 360°C for the pristine MgH2,while such peak disappears in the MgH2/PHCNSs system,corresponding to an evidently reduced ending dehydrogenation temperature.As reported previously[49,50],such a shoulder peak is attributed to the nonuniform size distribution of MgH2particle.The result illustrates that the PHCNSs act as the grinding agent during ball-milling,increasing the size homogeneity of the MgH2particles.Moreover,with the Ni loading in the Nix@PHCNSs composites increasing from 50 to 90 wt%,the reduction of the dehydrogenation temperatures is more significant confirmin that Ni plays the main role for the catalysis.Among them,the system introduced with Ni90@PHCNSs exhibits the lowest onset and peak dehydrogenation temperatures of 195 °C and 242 °C,respectively,which are 55 and 75 °C lower than those of the pristine MgH2.Further increasing the Ni loading to 95 wt% reverses the decreasing trend of the dehydrogenation temperatures,which is supposed due to the formation of the Ni aggregates,resulting a slightly lowered catalytic effect compared with the well dispersive Ni NPs in the Ni90@PHCNSs composite.

Fig.5.TEM image(a)and the corresponding EDS maps of Mg(b),Ni(c)and C(d)elements of the as-milled MgH2/Ni90@PHCNSs system.

Fig.6.TPD-MS(a)and volumetric dehydrogenation(b)curves of the MgH2/Nix@PHCNSs systems,the MgH2/PHCNSs system and the pristine MgH2.

The volumetric dehydrogenation curves of the MgH2/Nix@PHCNSs systems as well as the pristine MgH2and the MgH2/PHCNSs system are shown in Fig.6b.For the MgH2/Nix@PHCNSs systems,the main dehydrogenation temperature range is evidently down-shifted compared with the pristine MgH2,and dehydrogenation almost accomplishes at ca.310 °C.The reduction on the dehydrogenation temperature is extremely small for the MgH2/PHCNSs system,but the system does not contain the two-step dehydrogenation process as in the pristine MgH2.The result further demonstrates that nano Ni particles play important role in improving the dehydrogenation kinetics while the PHCNSs contribute to the uniform size distribution of the MgH2particles,which are in good agreement with the results from the TPD-MS measurement.The main non-isothermal dehydrogenation properties of the investigated systems are summarized in Table 1.The dehydrogenation capacities of the MgH2/Nix@PHCNSs systems with different Ni loadings upon heating to 300 °C are very close,which are 6.3,6.4,6.5 and 6.4 wt%,respectively,for Ni contents of 50,70,90 and 95 wt%.Further heating to 400 °C only results in less than 0.5 wt% H2released.By taking overall consideration of the dehydrogenation temperature and capacity,the MgH2/Ni90@PHCNSs system shows the optimized performance among all the systems,which is performed further studies on isothermal kinetics and reversibility.

Table 1The non-isothermal dehydrogenation properties of the MgH2/Nix@PHCNSs systems,the MgH2/PHCNSs system and the pristine MgH2.

Fig.7.Isothermal dehydrogenation curves of the MgH2/Ni90@PHCNSs system as well as the pristine MgH2 at different temperatures(a)and the corresponding isothermal hydrogenation curves at different temperatures of the dehydrogenation products performed at 275 °C(b).

Fig.7a shows the isothermal dehydrogenation curves of the MgH2/Ni90@PHCNSs system as well as the pristine MgH2at 225,250 and 275 °C.It is seen that the dehydrogenation rate of the catalyzed system is significantl increased compared with that of the pristine MgH2.There are 3.8 and 6.2 wt% H2released for the MgH2/Ni90@PHCNSs system dwelling at 225 and 250 °C for 30 min,respectively,while for the pristine MgH2,less than 0.5 wt% H2is released at the same condition.When the isothermal temperature is set to a slightly higher temperature of 275 °C,there is 6.4 wt%H2already released when the temperature is just approached to 275 °C,which is almost the stable value of the dehydrogenation capacity at this temperature.By contrast,there is only 1.4 wt% H2desorbed for the pristine MgH2upon heating to 275 °C,and the dehydrogenation capacity is only 2.2 wt% H2even after dwelling for 20 min.The dehydrogenation products performed at 275 °C is further used for isothermal hydrogenation property testing.The corresponding hydrogenation curves at 100,150 and 200 °C under 50 bar H2are shown in Fig.7b,where hydrogen pressure is loaded only when the pre-set temperature is reached.It is seen that the MgH2/Ni90@PHCNSs system achieves a saturated hydrogen capacity of 6.3 wt% within only 100 s at 200 °C.When the isothermal temperature is decreased to 150 °C,a capacity of 6.2 wt% H2is also achieved within 250 s.Even at a low temperature of 100 °C,there is still 5.3 wt% H2absorbed with a dwelling period of 600 s.Whereas for the pristine MgH2,the hydrogenation capacities are only 0.6,1.9 and 5.0 wt% at 100,150 and 200 °C,respectively,even for a dwelling period of 600 s.It is concluded that the superfin PHCNSs-supported Ni NPs show highly bifunctional effect on both dehydrogenation and hydrogenation kinetics of MgH2.

Based on the TPD-MS curves at different heating rates and the Kissinger’s plots(Fig.S5a and b),the dehydrogenation apparent activation energy(Ea)of the MgH2/Ni90@PHCNSs system is estimated to be 98±6 kJ mol-1,corresponding to a reduction of 30% compared with 139 kJ mol-1for the pristine MgH2[4],the value of which is also smaller than those of other recently reported MgH2-catalyst systems[51-57].Therefore,a reduced dehydrogenation energy barrier is achieved with the introduction of the Ni-incorporated PHCNSs composite,which contributes to the improvement in dehydrogenation kinetics.

Fig.8 shows the selected cyclic dehydrogenation(a)and hydrogenation(b)curves of the MgH2/Ni90@PHCNSs system with a regime of dehydrogenation to 350 °C under static vacuum and hydrogenation to 250 °C under 50 bar H2.There is 6.8 wt% H2desorbed in the firs dehydrogenation.The dehydrogenation product is highly reversible in the subsequent hydrogenation.Notable that from the second cycle,the onset and ending dehydrogenation temperatures are further decreased to ca.200 and 290 °C,respectively,and the dehydrogenation and hydrogenation curves are almost overlapped for the subsequent cycles,demonstrating a superior cyclic stability.The practical available hydrogen capacity after 50 cycles remains to be 6.4 wt%,corresponding to a capacity retention of 94.1%.In addition,the expression of cyclic de/rehydrogenation kinetics of time versus hydrogen sorption capacities is shown in Fig.S6a and b.It is seen from that hydrogen is rapidly released during the heating period,especially from the second cycle.The main dehydrogenation occurs in 40 min from ca.220 °C to 300 °C,corresponding to ca.6.2 wt% H2releasing.The dehydrogenation kinetics maintains high from the second cycle.There is ca.6.0 wt%H2released in 40 min from ca.200 to 280 °C for the subsequent cycles.It terms of the hydrogenation curves,Fig.6Sb,the curves of time versus capacity is still overlapped in a high level.There is ca.4.0 wt% H2absorbed in the initial 50 min,corresponding to a temperature range of ca.50 to 100 °C,then the absorption turns to slightly lower rates,where more ca.2.5 wt% H2is absorbed in 150 min from 100 to 250 °C.

Although the hydrogen storage properties from different laboratories cannot be quantitatively compared because of the different testing programs,it is still informative to summarize the progress reported.Table 2 lists the comparison of the hydrogen storage properties of the present MgH2/Ni90@PHCNSs system and the representative MgH2systems added with various carbon-supported nickel catalysts.Obviously,the present system shows lower dehydrogenation temperature,better hydrogen sorption kinetics and higher reversible capacity compared with those of the reported systems.This demonstrates that the ultrafin Ni particles and their high dispersibility is highly effective in improving the reaction kinetics of MgH2.Therefore,even an addition as low as 5 wt% of Ni90@PHCNSs results in significan improvement on the hydrogen storage properties of MgH2.The abundant pores,especially the ultrafin nanopore channels,and the large surface area of the present PHCNSs offer the possibility for the high content and well dispersive distribution of ultrafin Ni NPs,which contribute to a highly active catalysis to MgH2.

Fig.9.XRD patterns of the MgH2/Ni90@PHCNSs system at different dehydrogenation and hydrogenation states.

Fig. 9 shows the XRD patterns of the MgH2/Ni90@PHCNSs system at different dehydrogenation and hydrogenation states.After the firs dehydrogenation,the diffraction peaks of MgH2disappear and instead,peaks of Mg appear,demonstrating the complete decomposition of MgH2.Moreover,there are three new weak peaks detected at 39.8,40.8 and 44.9°,matching well with Mg2Ni(JCPDS no.35-1225),which suggests that the Ni NPs react with MgH2in the initial dehydrogenation process forming Mg2Ni.The diffraction peaks of MgH2reappear after the firs hydrogenation,indicating the regeneration of MgH2,and besides,there is a minor amount of residual Mg,which may be responsible for the slight decrease of cyclic capacity.Meanwhile,the Mg2Ni phase is also hydrogenated and transformed to Mg2NiH4(JCPDS no.35-1225).The formation of Mg2NiH4is likely to be the reason for the further reduction of the onset dehydrogenation temperature of the system after the firs cycle(Fig.8),as Mg2NiH4is easier to release hydrogen than MgH2[39]and shows better catalytic effect on MgH2than pure Ni[60].The onset dehydrogenation temperature of the catalyzed system from the second cycle(ca.200 °C,Fig.8a)is lower than the theoretical decomposition temperature of Mg2NiH4reported in literature(ca.255 °C at 1 bar equilibrium H2pressure)[15].It is supposed due to the lowH2pressure in the dehydrogenation process,the extra active effect of the carbon interaction and the ultrafin particle size of Mg2NiH4for the present system.Moreover,Mg2NiH4is still detected in the 50th hydrogenated product.It is clearly that the in-situ formed Mg2Ni/Mg2NiH4during the firs cycle exist stably in the subsequent cycles,acting as highly effective catalyst for the hydrogenation and dehydrogenation of MgH2.In addition,the minor MgO derived from the reaction between MgH2and the oxygen-containing functional groups of the PHCNSs during ball milling also remains during the cycling,with no evidently change in the diffraction feature.

Table 2Comparison of the hydrogen storage properties of the present MgH2/Ni90@PHCNSs system and the representative MgH2 systems added with various carbonsupported nickel catalysts.

Fig.10.SEM image(a),TEM image(b)and the EDS maps of Mg(c),Ni(d)and C(e)elements of the MgH2/Ni90@PHCNSs system at the 50th hydrogenation.

Fig.10a shows a SEM morphology of the MgH2/Ni90@PHCNSs system at the 50th hydrogenation cycle.It is seen that the overall morphology is similar as that of its as-milled state,without obvious particle growth.In contrast,the pristine MgH2of the same state shows severe particle agglomeration(Fig.S7).Further EDS analysis of the catalyzed system(Fig.10b-e)shows that Ni and C elements are still homogeneously distributed in the MgH2matrix without aggregation.The extra noises in the carbon map is originated from the carbon support film It is thus concluded that the ultrafin size and the high dispersibility of the in-situ formed Mg2Ni/Mg2NiH4phases maintain during hydrogen sorption cycles.Furthermore,the lamellar carbon covered on MgH2particles act as barrier in suppressing the growth and agglomeration of MgH2particles,both of which contribute to the superior cycling stability of the system.

4.Conclusions

Ultrafin Ni NPs supported hollow carbon nanospheres(PHCNSs)are synthesized as the catalysts for promoting the hydrogen storage performance of MgH2.With a high loading of 90 wt%,the Ni NPs are well dispersed at the outer surface,in the inner cavity and also in the pore channels of the PHCNSs.Introduced to MgH2by ball milling,the Ni NPs are uniformly distributed on the MgH2particles with the help of the excellent dispersive role of the PHCNSs substrate,exhibiting superior bidirectional catalytic activity towards the dehydrogenation and hydrogenation of MgH2.The MgH2system containing only 5 wt% Ni90@PHCNSs shows onset and peak dehydrogenation temperatures as low as 190 °C and 242 °C,respectively,and desorbs 6.5 wt%H2upon heating to 300°C.Moreover,6.2 wt% H2is rapidly released within 30 min at 250 °C and the dehydrogenation product can absorb almost the same amount of hydrogen within 250 s at 150 °C under 50 bar H2.Even at a low temperature of 100 °C,the system can absorb 5.3 wt% H2in 600 s.A reversible dehydrogenation capacity of 6.4 wt% remains after 50 cycles,corresponding to a high capacity retention of 94.1%.The in-situ formed Mg2Ni/Mg2NiH4inherit the superfin size and uniform dispersion of Ni NPs,acting as highly-active catalysts during the dehydrogenation and hydrogenation cycles of MgH2.The present work provides new ideas for developing highly effective and durable catalysts toward enhanced hydrogen storage properties of MgH2.

Declaration of Competing Interests

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

This work is supported by the National Key Research and Development Program of the Ministry of Science and Technology of PR China(No.2018YFB1502103),National Natural Science Foundation of PR China(Nos.52071287,51571175,U1601212,51831009).

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.jma.2021.05.004.