Enhanced hydrogen storage properties of Mg by the synergistic effect of grain refinemen and NiTiO3 nanoparticles

Ninhu Yn,Xiong Lu,Zhiyu Lu,Hijie Yu,Fuying Wu,Jigung Zheng,Xiuzhen Wng,Liuting Zhng,*

a School of Energy and Power,Jiangsu University of Science and Technology,Zhenjiang,212003,China

b Analysis and Testing Center,Jiangsu University of Science and Technology,Zhenjiang,212003,China

Abstract As a promising hydrogen storage material,the practical application of magnesium is obstructed by the stable thermodynamics and sluggish kinetics.In this paper,three kinds of NiTiO3 catalysts with different mole ratio of Ni to Ti were successfully synthesized and doped into nanocrystalline Mg to improve its hydrogen storage properties.Experimental results indicated that all the Mg-NiTiO3 composites showed prominent hydrogen storage performance.Especially,the Mg-NiTiO3/TiO2 composite could take up hydrogen at room temperature and the apparent activation energy for hydrogen absorption was dramatically decreased from 69.8±1.2(nanocrystalline Mg)kJ/mol to 34.2±0.2kJ/mol.In addition,the hydrogenated sample began to release hydrogen at about 193.2°C and eventually desorbed 6.6 wt% H2.The desorption enthalpy of the hydrogenated Mg-NiTiO3-C was estimated to be 78.6±0.8kJ/mol,5.3kJ/mol lower compared to 83.9±0.7kJ/mol of nanocrystalline Mg.Besides,the sample revealed splendid cyclic stability during 20 cycles.No obvious recession occurred in the absorption and desorption kinetics and only 0.3 wt%hydrogen capacity degradation was observed.Further structural analysis demonstrates that nanosizing and catalyst doping led to a synergistic effect on the enhanced hydrogen storage performance of Mg-NiTiO3-C composite,which might serve as a reference for future design of highly effective hydrogen storage materials.? 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;NiTiO3;Nanocrystalline Mg;Reversibility;Synergistic effect.

1.Introduction

The rapid development of modern industrial civilization has caused more serious environmental pollution and energy shortage than ever before[1].Thus,findin a clean and environment-friendly energy carrier has been a research hotspot worldwide[2,3].Among various alternative energy sources,hydrogen is considered one of the most optimal carriers owing to the advantages of its rich reserves,high calorifi value,and wide application[4-7].To store and deliver hydrogen in a safe and efficien way,Solid-state hydrogen storage technology greatly caught scientists’attention[8-10].Different hydrogen storage materials have been intensively studied,such as MgH2,MgNiH4,LiBH4,NaBH4,Mg(BH4)2,NaAlH4,etc.[11-16].Among them,MgH2has received wide attention in recent years due to its low cost,high capacity,and good reversibility[17-19].However,the stable thermodynamics and sluggish kinetics remain key problems to its practical application in hydrogen storage[20-22].

Various modificatio methods for Mg-H system have been reported in literature,mainly involving nanosizing[23-26],catalyst doping[27-31]and alloying[32-34].Refinin particle size to nanoscale by ball milling is one of the most common methods for improving the kinetic properties of Mg-H system due to the high surface-to-volume ratio and short hydrogen diffusion paths.For instance,Xiao et al.[35]reported Mg2FeH6@MgH2core-shell nanostructure with particle size of 40-60nm could start to release H2at 220°C and desorb over 5.0 wt% H2within 50min at 280°C.Moreover,the sample exhibited excellent cycling properties due to the core-shell nanostructure.In the meanwhile,Calizzi et al.[36]successfully prepared Mg-Ti nanoparticles(Mg-Ti NPs)by inert gas condensation and in situ hydrogenation at 150°C.The calculation results showed that the values of apparent activation energy for absorption and desorption of the Mg-Ti NPs were sharply decreased to 68kJ/mol and 78kJ/mol,respectively.

Besides,Catalyst doping is also an effective strategy to reduce the high temperature requirement for Mg-H system.Based on previous investigations,transition metals,compounds and alloys have shown excellent catalysis to accelerate the de/rehydrogenation kinetics of MgH2[37,38].Among vast transition metal catalysts been investigated,Ti-based catalysts exert superior performance.For instance,Zhang et al.[39]evidenced that 5 wt% TiO2nanosheet modifie MgH2composite started to release hydrogen at 180.5°C and the dehydrogenation activation energy was decreased to 67.6kJ/mol.In addition,the MgH2-5 wt% Ti3C2composite prepared by Liu et al.[40]began to liberate hydrogen at 185°C and approximately 7.1 wt% H2was released.Simultaneously,the dehydrogenated sample absorbed about 3.0 wt% hydrogen within 150s at low temperature of 50°C under 5MPa hydrogen pressure.

As another transition metal element,Ni-based materials have also been demonstrated effective for Mg-based materials.Yao et al.[41]uniformly dispersed Ni nanoparticles on reduced graphene(Ni@rGO)and doped them into MgH2.Experimental results showed that the onset dehydrogenation temperature of MgH2+10 wt% Ni4@rGO6was sharply decreased to 190°C and the dehydrogenated composite could take up 5.0 wt% hydrogen in 20min at 100°C.Li et al.[42]also proved that nanocrystalline Ni@C greatly improved the hydrogen storage properties of MgH2.The peak hydrogen desorption temperature of MgH2-Ni@C composite can be reduced to 283°C,74°C lower compared with that of MgH2.

In addition,Ti-based or Ni-based bivariate materials were reported to show outstanding catalysis for MgH2on hydrogen storage[43-45].Zhang et al.[43].discovered the onset dehydrogenation temperature of MgH2-7 wt% TiNb2O7was reduced to 177°C from 300°C(MgH2)and about 4.5wt%H2was absorbed in 3min at 150°C.Chen et al.[45]found MgH2-9 NiMoO4absorbed 6.7 wt% hydrogen within 60s and release 6.7wt% hydrogen within 10min at 300°C.In addition,the MgH2-9 NiMoO4composite exhibited excellent cycling stability and low-temperature hydrogen storage performance.

Enlightened by the previously reported modificatio strategies on MgH2including nanocrystallization and catalyst doping,three kinds of NiTiO3nanoparticles with different ratios of Ni to Ti were synthesized via a facile hydrothermal method and then doped into nanocrystalline Mg in this work.The Mg-NiTiO3composites were systematically investigated through a range of composition characterization,morphologies and hydrogen storage properties.On the basis of above experimental results and analysis,the catalytic mechanism of NiTiO3catalysts on MgH2hydrogen desorption and absorption were discussed in detail.

Table 1Detailed information for preparing NiTiO3 nanoparticles.

2.Experimental

2.1.Synthesis of NiTiO3 nanoparticles

A typical synthesis of the NiTiO3catalysts were as follows:a certain amount of nickel acetate tetrahydrate(C4H14NiO8?4H2O,SCR,purity＞99%)and titanium tetraisopropanolate(C12H28O4Ti,SCR,purity＞95%)were dissolved in 500mL deionized water.Ammonium hydroxide(SCR,purity＞99.5%)was adopted to adjust the pH of the above solution to 11.Subsequently,the prepared solution was stirred for 24 h at room temperature.The resulting suspension was evaporated at 80°C to get the green sediment.After the centrifugation wash(8000rpm,3min)by deionized water and ethyl alcohol for several times,the green sediment was then maintained at 80°C for 6 h to obtain the green powders.After being calcined at 800°C for 6 h,the yellow NiTiO3powders were obtained.Three different NiTiO3nanoparticles were prepared via the above method,detailed information collected in Table 1.

2.2.Synthesis of nanocrystalline MG and MG-NiTiO3 composite

The nanocrystalline Mg was prepared via a wet chemical ball milling method.3g Mg(SCR,purity＞99.9%),0.15mL oleamine(Sinopharm,purity＞98.5%),0.45mL oleic acid(Sinopharm,purity＞90%)and 6mL n-heptane were mixed in a steel jar and subsequently milled for 40 h with the speed of 400rpm(QM-3SP4,Nanjing Chi Shun Technology Development Co.,Ltd,China).The mixture was washed with n-heptane for 3 times and the powers were collected by centrifugation(9000rpm,10min).Finally,nanocrystalline Mg powers were obtained after been vacuumed at 250°C for 3 h.7 wt% as-prepared NiTiO3powers were doped into the nanocrystalline Mg by ball milling to get the composites(4 h;400rpm),named as Mg-NiTiO3-A,Mg-NiTiO3-B and Mg-NiTiO3-C,respectively.

2.3.Characterization and measurements

In order to detect the phase compositions of the samples,X-ray diffraction(XRD)measurement was carried out on an X’Pert Pro-X-ray diffractometer(PANalytical,the Netherlands)with Cu Kαradiation at 40kV,40mA.Meanwhile,the morphology and microstructure of the samples were characterized by Scanning Electron Microscopy(SEM,Hitachi SU-70)and Transmission Electron Microscope(TEM,Tecnai G2 F30 working at 300kV).

Fig.1.XRD patterns of as-prepared NiTiO3-A,NiTiO3-B and NiTiO3-C.

A homemade Sievert’s type device was employed to measure hydrogen absorption and desorption properties.For the non-isothermal mode,each sample was heated from room temperature to 400°C under 3MPa hydrogen pressure with a heat-up rate of 1°C/min for hydrogen absorption.In hydrogen desorption experiment,the hydrogenated sample was heated from room temperature to 430°C under static vacuum with a heat-up rate of 2°C/min.With respect to the isothermal tests,each sample was rapidly heated to the desired temperature and kept for 60min under 3MPa hydrogen pressure(hydrogen uptake)or static vacuum environment(hydrogen release).In order to minimize errors caused by the test instruments,each experiment was repeated at least twice.

3.Results and discussion

XRD analysis of the as-prepared NiTiO3samples are carried out to explore the phase composition,presented in Fig.1.It is apparent that the characteristic peaks of three samples matched well with NiTiO3(PDF#33-0960)at 23.8°,32.7°,35.4°,40.5°,49.1°,53.4°,62.0°and 63.9°.Besides,two peaks at 37.2° and 43.3° belonging to NiO occurred in NiTiO3-A,which is due to the overdose of C4H14NiO8.Similarly,two TiO2peaks at 27.3° and 36.1° occurred in NiTiO3-C sample due to the excess amount of titanium tetraisopropanolate.Although the molar ratio of C4H14NiO8to C12H28O4Ti is 1 to 1,the TiO2peak(at 27.3°)and NiO peaks(at 37.2° and 43.3°)still appeared in the NiTiO3-B sample,which might result from the incomplete reaction during the hydrothermal or calcination process.To further reveal the morphology of the as-prepared NiTiO3samples,TEM measurement was employed.The TEM images in Fig.2 indicated that the particle size of all the three NiTiO3was about 50-100nm.Besides,it’s obvious that comparing to aggregated NiTiO3-A and NiTiO3-B,well scattered NiTiO3-C particles were found by TEM observations,and this difference in dispersity may help the homogeneous dispersion of NiTiO3-C nanoparticles on the surface of Mg to exert better catalytic effect.

Fig.2.TEM images of as-prepared NiTiO3-A(a),NiTiO3-B(b)and NiTiO3-C(c).

Fig.3.XRD patterns of as-prepared nanocrystalline Mg,Mg-NiTiO3-A,Mg-NiTiO3-B and Mg-NiTiO3-C.

In order to test the catalytic effect of as-prepared NiTiO3,7 wt% as-prepared NiTiO3powers were doped into the nanocrystalline Mg by ball milling.Fig.3 shows the XRD patterns of different NiTiO3doped Mg samples.No detectable impurities can be found in the diffraction peaks of nanocrystalline Mg,which means no detectable oxidation of Mg appeared during the preparation process.For the Mg-NiTiO3-A,Mg-NiTiO3-B and Mg-NiTiO3-C composites,both the strong Mg phase and obvious reflectio signals of NiTiO3could be detected.Besides,some weak signals from NiO and TiO2can also be observed,which are in consist with Fig.2.Based on the Scherrer formula and XRD data,the average crystallite size of the above samples can be calculated to be 64.4nm(nanocrystalline Mg),54.6nm(Mg-NiTiO3-A),52.5nm(Mg-NiTiO3-B)and 47.5nm(Mg-NiTiO3-C),respectively.This indicates that doping of NiTiO3,especially NiTiO3-C with higher Ti amount,may significantl reduce the grain size of nanocrystalline Mg.

Fig.4.The curves of no-isothermal hydrogen uptake for Mg-NiTiO3-A,Mg-NiTiO3-B,Mg-NiTiO3-C,nanocrystalline Mg and bulk Mg(a);the curves of no-isothermal hydrogen release for hydrogenated Mg-NiTiO3-A,Mg-NiTiO3-B,Mg-NiTiO3-C,nanocrystalline Mg and MgH2(b).

Fig.4a shows the curves of no-isothermal hydrogen uptake for the Mg-NiTiO3-A,Mg-NiTiO3-B,Mg-NiTiO3-C,nanocrystalline Mg and bulk Mg samples.The bulk Mg started to take up H2at about 146.5°C while the nanocrystalline Mg could begin to absorb H2at about 87.6°C.Both samples could obtain a hydrogen absorption capacity of approximate 7.2 wt%.It could be told that nano-sized Mg show better hydrogen uptake performance.After NiTiO3-A,NiTiO3-B and NiTiO3-C were doped,all the three Mg-NiTiO3samples started to absorb H2from room temperature with a fina capacity of 6.6 wt%.Besides,the distinctions on hydrogen absorption rate for the three doped samples were also clearly observed as follows:Mg-NiTiO3-C＞Mg-NiTiO3-B＞Mg-NiTiO3-A.In addition,the non-isothermal hydrogen release curves of the hydrogenated samples shown in Fig.4b revealed that bulk Mg started to desorb hydrogen at about 347.5°C while the hydrogenated nanocrystalline Mg began to release H2at about 262.4°C,with a hydrogen desorption capacity of 7.2 wt% for both samples.As expected,the three NiTiO3doped samples started to desorb hydrogen at about 206.1°C(Mg-NiTiO3-A),199.6°C(Mg-NiTiO3-B)and 193.2°C(Mg-NiTiO3-C),respectively.Besides,the dehydrogenation kinetics of hydrogenated Mg-NiTiO3-C was better than that of hydrogenated Mg-NiTiO3-B and Mg-NiTiO3-A,which is in agreement with the absorption kinetics.Above investigation results on the hydrogen absorption and desorption demonstrated that the formation of nanocrystalline Mg and further doping of NiTiO3nanocatalysts remarkably enhanced the hydrogen storage performance in Mg-MgH2system.In addition,the Mg-NiTiO3-C composite shows superior hydrogen absorption and desorption properties compared with those of Mg-NiTiO3-A and Mg-NiTiO3-B composites,thus,the Mg-NiTiO3-C composite was chosen in the following experiments and discussions.

The hydrogen absorption kinetic of Mg-NiTiO3-C sample was further investigated by isothermal hydrogenation measurements.Fig.5a and Fig.5b reveal the isothermal hydrogen uptake curves for nanocrystalline Mg and Mg-NiTiO3-C,respectively.Nanocrystalline Mg showed partial hydrogen absorption capacities(1.5 wt% at 150°C;3.7 wt% at 175°C;6.3 wt% at 200°C)within the firs 10min.Encouragingly,2.7 wt% and 1.7 wt% hydrogen was absorbed in Mg-NiTiO3-C composite within the firs 10min at temperatures as low as 125°C and 100°C,respectively,superior than that of nanocrystalline Mg at 150°C.Moreover,when the hydrogenation temperature was decreased to 75°C,the Mg-NiTiO3-C sample could still absorb about 1 wt%hydrogen within the firs 10min and over 2 wt% hydrogen was taken up in 30min.At 150°C,the nanocrystalline Mg absorbed about 5 wt% H2in 60min while over 6 wt% H2was recharged by the Mg-NiTiO3-C in identical time.When the preset temperature reached 175°C and 200°C,both the two samples showed remarkable hydrogen absorption kinetics,resulting from the high temperature.

Moreover,the apparent activation energy(Ea)for hydrogen absorption was calculated to further study the kinetic properties of the nanocrystalline Mg and Mg-NiTiO3-C composite.The data during the isothermal hydrogen uptake for each sample(Fig.5a and Fig.5b)was simulated through the Johnson-Mehl-Avrami-Kolmogorov(JMAK)equation[46].The expression and relevant parameters for the JMAK equation are shown in the following equation.

Whereαis the proportion of Mg transformed into MgH2,k represents an effective kinetic parameter,t means t moment and n denotes the Avrami exponent,respectively.And the values of n and nlnk were obtained by fittin the JMAK plots,shown in Fig.5c and Fig.5d.Subsequently,the values ofEafor hydrogen absorption were acquired according to the Arrhenius equation[47],which is displayed as follows:

As depicted in Fig.5e,the calculatedEavalues for hydrogen absorption of the nanocrystalline Mg and Mg-NiTiO3-C were 69.8±1.2kJ/mol and 34.2±0.2kJ/mol,much lower than that of MgH2(121.5kJ/mol).TheEaof hydrogen absorption for the Mg-NiTiO3-C composite was dramatically reduced about 51%(35.6kJ/mol),compared with that of the nanocrystalline Mg,which embodied the remarkable improvement of the NiTiO3-C on hydrogen absorption kinetics for Mg-H system.

Fig.5.Isothermal hydrogen uptake curves for the nanocrystalline Mg(a)and the Mg-NiTiO3-C(b);isothermal hydrogenation JMAK curve plots of nanocrystalline Mg(c)and Mg-NiTiO3-C(d);and the corresponding Arrhenius plots(e)of nanocrystalline Mg and Mg-NiTiO3-C.

To validly manifest the improvements on hydrogenation kinetics of Mg-NiTiO3-C,the apparent activation energy(Ea)values of hydrogen absorption for different modifie Mg-H system in recent literature were listed in Table 2.It is simply found thatEaof hydrogen absorption can be significantl reduced by various modificatio methods.Interestingly,the Mg-NiTiO3-C in this paper is at state-of-the-art level of reportedEa,indicative of superior synergistic catalytic effect of NiTiO3-C on the hydrogen absorption kinetics property for the Mg-H system.

In addition,the hydrogen desorption kinetic property of the Mg-NiTiO3-C composite was investigated.Fig.6a and Fig.6b display the isothermal hydrogen release curves for the hydrogenated nanocrystalline Mg and Mg-NiTiO3-C,respectively.Obviously,at 325°C,the hydrogenated nanocrystalline Mg could release 6.5 wt% H2within firs 15min while it only took 5min for the hydrogenated Mg-NiTiO3-C to desorb the same amount of hydrogen.At 300°C,the hydrogenated Mg-NiTiO3-C could desorb about 6.4 wt% hydrogen within the firs 10min while less than 3 wt% hydrogen was released for the hydrogenated nanocrystalline Mg at identical time.In addition,the isothermal hydrogen release curves at 275°C for the two samples showed a tremendous distinction.Specifi cally,within the firs 30min,the hydrogenated Mg-NiTiO3-C released about 6 wt% H2,more than three times that of the hydrogenated nanocrystalline Mg.The above analysis demonstrated that NiTiO3-C can serve as a bidirectional catalyst on improving the de/rehydrogenation properties of Mg.

Table 2The apparent activation energy(Ea)of hydrogen absorption for different Mgbased samples.

Fig.6.The isothermal hydrogen release curves for hydrogenated nanocrystalline Mg(a)and Mg-NiTiO3-C(b).

To investigate the thermodynamic properties of hydrogenated Mg-NiTiO3-C composite,PCT desorption curves at 300°C,325°C and 350°C for the hydrogenated nanocrystalline Mg and Mg-NiTiO3-C were measured.In the meanwhile,apparent pressure plateaus were detected at preset temperatures,presented in Fig.7a and 7b.The plateau pressures of hydrogenated Mg-NiTiO3-C were determined to be 1.52,3.00 and 5.75bar while that of hydrogenated nanocrystalline Mg was measure to be 1.36,2.83 and 5.67bar for 300°C,325°C and 350°C,respectively.As a result,the desorption enthalpy of the hydrogenated Mg-NiTiO3-C was estimated to be 78.6±0.8kJ/mol through the fittin the van’t Hoff plot(Fig.7c and 7d),5.3kJ/mol lower compared to 83.9±0.7kJ/mol of nanocrystalline Mg.The decreased value in the desorption enthalpy manifests that the addition of NiTiO3-C could also tune the thermodynamic properties of nanocrystalline Mg.

Apart from the performances on hydrogen absorption and desorption,the reversibility is also considered as one of the most essential factors for the practical application of hydrogen storage materials[55,56].Thus,the cycling performance of the doped sample was measured through repeating the hydrogen absorption and desorption tests for 20 cycles at 300°C under 3MPa hydrogen pressure(absorption)and static vacuum(desorption).Fig.8a depicts the hydrogen absorption and desorption curves in 20 cycles,which indicates inconspicuous recession occurred in the kinetic and capacity after 20 cycles.In order to observe the recession of hydrogen storage capacity more intuitively for the Mg-NiTiO3-C composite in 20 cycles,the graph about the reversible hydrogen capacity of uptake or release for the 20 cycles were drawn and shown in Fig.8b.Obviously,the results certify that the hydrogen capacity of the Mg-NiTiO3-C sample after 20 cycles was 6.38 wt%,equivalent to 95.7% of the original hydrogen storage capacity(6.6 wt%).In general,MgH2particles tend to grow and aggregate during the thermolysis,which leads to the degenerating cycling properties[57,58].The above results manifested that the Mg-NiTiO3-C composite showed a superior cyclic stability.Previous studies have reported that doping Ti-based materials can improve the cycling stability for Mg-H system.Acosta et al.[59]measured the cycling hydrogen absorption and desorption curves within the firs 15min for MgH2+5mol% Early Transition Metals(ETM=Sc,Y,Ti,Zr,V,and Nb)during 20 cycles at 300°C.The results showed that the absorption and desorption kinetics of Ti doped sample remained the most stable on cycling among the above samples.In this paper,thanks to the element Ti in NiTiO3-C catalyst,the cyclic stability on hydrogen absorption and desorption of the Mg-NiTiO3-C composite was similarly enhanced.

To shed light on the catalytic mechanism behind the dramatically improved hydrogen storage properties of Mg-NiTiO3-C composite,XRD,TEM,HRTEM and EDS were carried out.Fig.9a and Fig.9b present the XRD profile of as-prepared nanocrystalline Mg and Mg-NiTiO3-C in ball milled state,hydrogenated state and dehydrogenated state,respectively.It could be apparently observed that the diffraction peaks of Mg completely dominated the ball milling state as well as the dehydrogenation state,and all the intense peaks in the hydrogenation state were belonging to MgH2.In addition,Mg2+(1305.3eV in Mg1s)was detected from XPS profil of Mg-NiTiO3-C after ball milling(Fig.S1),manifesting the existence of MgO in the surface of Mg-NiTiO3-C.The phase of NiTiO3stably existed in above three states while the weak peak of TiO2disappeared after hydrogen absorption,which may due to the strong peak of MgH2at 27.9° covered the weak peak of TiO2at 27.3° In the meanwhile,the lattice spacing of the(103)plane of TiO2was confirme in the TEM(Fig.10a)and HRTEM(Fig.10b)of hydrogenated Mg-NiTiO3-C,which demonstrated TiO2still existed in hydrogenated Mg-NiTiO3-C.Besides,TEM and HRTEM images(Fig.10a and Fig.10b)of the hydrogenated Mg-NiTiO3-C confirme the lattice spacing of the(110)plane of MgH2and(104)plane of NiTiO3,which agrees well with the XRD results.Furthermore,the EDS mapping results revealed that the elements of Ni,Ti,and O were evenly dispersed over MgH2,manifesting the homogeneous distribution of NiTiO3-C on the surface of the hydrogenated nanocrystalline Mg.

Fig.7.PCT curves of hydrogenated nanocrystalline Mg(a)and Mg-NiTiO3-C(b),van’t Hoff plot for hydrogenated nanocrystalline Mg(c)and Mg-NiTiO3-C(d).

Fig.8.The curves of hydrogen absorption and desorption during 20 cycles for Mg-NiTiO3-C at 300 °C(a)and corresponding capacity retention graph(b).

Fig.9.X-ray diffraction patterns of as-prepared nanocrystalline Mg(a)and Mg-NiTiO3-C(b)in the ball milling,hydrogenation and dehydrogenation states;TEM(c)and HRTEM(d)of hydrogenated Mg-NiTiO3-C.

Fig.10.TEM(a),HRTEM(b),STEM-HAADF(c)and corresponding EDS mapping results(d)of hydrogenated Mg-NiTiO3-C.

After having a total realization on the hydrogen storage property and structural changes during de/rehydrogenation in nano scale,the overall catalyzing effect of NiTiO3-C nanoparticles could be explained in three aspects.First,TiO2and NiTiO3themselves,as transition metal based catalysts,have previously been certifie to dramatically improve the hydrogen storage performance for Mg-H system in many reports[39,49,60].Second,the grain refinemen has been reported to play a significan role for facilitating fast hydrogen absorption and desorption[61,62].Recently,Karst et al.[63]found that nucleation of MgH2started at Mg grain boundaries and an increased grain boundary density should speed up the overall hydrogenation process.In this paper,the Mg-NiTiO3-C composite has large grain boundary density due to the small grain size of 47.5nm,benefitin the process of hydrogen absorption/desorption.

Fig.11.XRD pattern(a),SEM images(b-c),STEM-HAADF image(d)and corresponding EDS mapping results(e)of Mg-NiTiO3-C composite after 20 cycles.

In the last aspect of micro morphology,Fig.11 shows the XRD,SEM and EDS mapping of the Mg-NiTiO3-C composite after 20 cycles.From the XRD pattern in Fig.11a,the strong peaks of Mg and NiTiO3are clearly observed in the Mg-NiTiO3-C composite after 20 cycles.In addition,a weak peak of TiO2is detected at about 27.3° in the composite,indicating TiO2still existed after 20 cycles.According to the SEM images,it is clearly observed that numerous small particles around 100nm anchored on the surface of big particles.The EDS data verifie that the small particles were NiTiO3and elements of Ni,Ti,and O were still uniformly distributed on the surfaces of Mg particles after 20 cycles,which prevented the sintering and agglomeration of Mg/MgH2during cycling,resulting in the enhanced cycling stability of Mg-NiTiO3-C composite.

Through the above analysis and discussion,it can be concluded that the significantl enhanced hydrogen storage of Mg-NiTiO3-C composite could be attributed to a synergistic catalyzing effect between the small grain sizes of nanocrystalline Mg and the superior catalytic effect of uniformly dispersed NiTiO3-C.In cycling,a multivalent multielement catalytic environment consisted of Ti and Ni was formed in the Mg/MgH2nano system,which facilitated the dissociation and recombination of H-H and Mg-H[43,45].Hence,the catalytic mechanism of NiTiO3-C on hydrogen storage process for nanocrystalline Mg is proposed in Fig.12:Numerous NiTiO3-C particles are homogeneously anchored on the surface of Mg particles during the ball milling process.In the hydrogenation process,H2molecules are easily dissociated to H atoms through NiTiO3-C and combined with Mg at a large scale of grain boundaries to promote the formation of MgH2.

Fig.12.Schematic diagram of catalytic mechanism on hydrogen storage for the Mg-NiTiO3-C composite.

4.Conclusion

In summary,three kinds of NiTiO3particles were synthesized via a facile hydrothermal method and then applied in catalyzing the as-prepared nanocrystalline Mg.The NiTiO3-C with average particle size ranging from 50 to 100nm and optimal particle dispersity was demonstrated to exhibit superior catalysis in facilitating the hydrogen storage properties of nanocrystalline Mg.The hydrogen uptake tests results revealed that the Mg-NiTiO3-C composite started to absorb hydrogen at room temperature and over 4 wt% H2was absorbed even at 100°C in 60min under 3MPa hydrogen pressure.Moreover,theEaof hydrogen absorption for the composite was calculated to be 34.2kJ/mol,51% and 72% lower compared to that of the nanocrystalline Mg and bulk Mg,respectively.In addition,the hydrogenated Mg-NiTiO3-C composite could release H2at 193.2°C and complete dehydrogenation at 325°C in 5min with a decreased desorption enthalpy of 78.6±0.8kJ/mol.Besides,the superficia NiTiO3helped prevent the sintering and agglomeration of Mg/MgH2during de/rehydrogenation,resulting in a relatively stable cycling stability during the 20 cycles.The significantl enhanced hydrogen storage performance of Mg-NiTiO3-C composite could be attributed to the synergistic effect between catalyzing and grain refinement In brief,our finding may provide reference for the investigation on synergistic modificatio of Mg/MgH2system in future.

Declaration of competing interest

There is no conflic to declare.

Acknowledgements

The authors would like to acknowledge financia support from the National Natural Science Foundation of China(Grant No.51801078),the Natural Science Foundation of Jiangsu Province(Grant No.BK20180986)and the Postgraduate Research & Practice Innovation Program of Jiangsu Province(SJCX19_0614).

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.jma.2021.03.014.