Two-dimensional vanadium carbide for simultaneously tailoring the hydrogen sorption thermodynamics and kinetics of magnesium hydride

Chenglin Lu, Hizhen Liu,*, Li Xu, Hui Luo, Shixun He, Xingqing Dun,Xintun Hung, Xinhu Wng, Zhiqing Ln, Jin Guo

a Guangxi Novel Battery Materials Research Center of Engineering Technology, Guangxi Key Laboratory of Processing for Non-ferrous Metallic and Featured Materials, Guangxi Colleges and Universities Key Laboratory of Novel Energy Materials and Related Technology, School of Physical Science and Technology, Guangxi University, Nanning, 530004, China

b State Key Laboratory of Advanced Power Transmission Technology, Global Energy Interconnection Research Institute Co., Ltd., Beijing, 102209, China

cDepartment of Materials Science and Engineering, Baise College, Baise, 533000, Guangxi, China

d Department of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, China

Abstract Magnesium hydride (MgH2) is a potential material for solid-state hydrogen storage.However, the thermodynamic and kinetic properties are far from practical application in the current stage.In this work, two-dimensional vanadium carbide (V2C) MXene with layer thickness of 50-100nm was fis synthesized by selectively HF-etching the Al layers from V2AlC MAX phase and then introduced into MgH2 to improve the hydrogen sorption performances of MgH2.The onset hydrogen desorption temperature of MgH2 with V2C addition is significantl reduced from 318°C for pure MgH2 to 190°C, with a 128°C reduction of the onset temperature.The MgH2+10 wt% V2C composite can release 6.4 wt% of H2 within 10min at 300°C and does not loss any capacity for up to 10 cycles.The activation energy for the hydrogen desorption reaction of MgH2 with V2C addition was calculated to be 112kJ mol-1 H2 by Arrhenius's equation and 87.6kJ mol-1 H2 by Kissinger's equation.The hydrogen desorption reaction enthalpy of MgH2+10 wt% V2C was estimated by van't Hoff equation to be 73.6kJ mol-1 H2,which is slightly lower than that of the pure MgH2 (77.9kJ mol-1 H2).Microstructure studies by XPS, TEM, and SEM showed that V2C acts as an efficien catalyst for the hydrogen desorption reaction of MgH2.The first-principle density functional theory (DFT) calculations demonstrated that the bond length of Mg-H can be reduced from 1.71 °A for pure MgH2 to 2.14 °A for MgH2 with V2C addition, which contributes to the destabilization of MgH2.This work provides a method to significantl and simultaneously tailor the hydrogen sorption thermodynamics and kinetics of MgH2 by two-dimensional MXene materials.

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Keywords: Hydrogen storage; MgH2; V2C; Catalyst; DFT.

1.Introduction

Hydrogen is acknowledged as an ideal carrier to store renewable energy in a large scale.The green hydrogen can be used as an industrial chemical, or as the hydrogen source for hydrogen refueling stations, or converted to electricity by full cell to power the vehicles or connect to the power grid[1-11].However, hydrogen storage in a safe, dense and efficien manner still remains challenging in the current stage,which needs to be seriously addressed before practical application of hydrogen energy in a large scale is possible[12-14].Hydrogen storage in materials has long been considered as an excellent method to store large quantity of hydrogen with high hydrogen density and low operating pressure [14-26].Among the various hydrogen storage materials, magnesium hydride (MgH2) is a candidate material for solid-state hydrogen storage due to its high hydrogen density of 7.6 wt% andfavourable reversibility.However, MgH2requires a temperature above 300°C to start releasing hydrogen and its hydrogen sorption kinetics is rather slow, which severely restrains the practical application of MgH2[27-42].

In recent years, a new family of two-dimensional transition metal carbides and nitrides discovered in 2011, which we call MXenes, received considerable attentions due to their fascinating chemical and physical properties [43-46].MXenes have shown promising application in energy conversion and storage [47-57].In 2016, Liu et al.[58]firs used twodimensional Ti3C2as an excellent catalyst to improve the hydrogen storage properties of MgH2.The MgH2+5 wt%Ti3C2can release 6.2 wt% of hydrogen within 1min at 300°C and absorb 6.1 wt%of hydrogen within 30s at 150°C[58].This is the firs attempt to utilize MXene to improve the performance of hydrogen storage materials.After that, many efforts have been made to study the effects of MXenes or their derivatives on the hydrogen storage properties of MgH2and other hydrogen storage materials.Shen et al.[59]prepared a solidsolution MXene (Ti0.5V0.5)3C2to reduce the onset hydrogen desorption temperature of MgH2from 266°C to 196°C.The MgH2+10 wt% (Ti0.5V0.5)3C2can release 5.0 wt% of hydrogen within 20min at 250°C.Wang et al.[60]synthesized a layered NbTiC solid-solution MXene to ball mill with MgH2.NbTiC transformed to NbTi nanocrystals of 5nm after ball milling and reduced the onset hydrogen desorption temperature of MgH2to 195°C.DFT calculation showed a lower adsorption energy of H2on NbTi, which was believed to benefit the detachment of H2from NbTi surfaces [60].An effort by Liu et al.[61]demonstrated that the onset hydrogen desorption temperature of MgH2+5 wt% Nb4C3Txcould be reduced to 150.6°C, which they thought should be due to the layered structures of in situ formed NbHxphase.Zhang et al.[62]oxidized the solid-state (Ti0.5V0.5)3C2MXene at 300°C to prepare TiVO3.5with layered structures and coarse surfaces.After addition of TiVO3.5, MgH2starts to release hydrogen at 197°C and gives a hydrogen desorption capacity of 5 wt% in 10min at 250°C [62].Kong et al.carried out an alkali treatment of the Ti3C2MXene and obtained a Hamamelis-like structure of K2Ti6O13.The onset hydrogen desorption temperature of MgH2+5 wt% K2Ti6O13can be reduced to 175°C, which is attributed to the catalytic effects of the in situ formed KMgH3, TiO, and Ti from K2Ti6O13[63].In summary, MXenes and their derivatives have exhibited excellent catalytic effects on the hydrogen sorption properties of MgH2, which was believed to origin from the unique layered structure and the active species of the in situ formed metal or metal hydrides.In addition to MgH2, MXenes also shows fascinating effects on the hydrogen storage properties of other hydrogen storage materials such as NaAlH4, LiBH4,MgH2-LiAlH4, 2LiH+MgB2, etc.[64-67].

In the present work, another MXene, vanadium carbide(V2C), was introduced for the firs time to improve the hydrogen storage properties of MgH2.V2C MXene was firs synthesized by selectively etching the Al layers from V2AlC MAX phase with hydrofluoric-aci (HF) and then introduced into MgH2by ball milling to tailor the hydrogen storage performances of MgH2.The following will demonstrate that the as-synthesized two-dimensional V2C MXene improves both the hydrogen desorption thermodynamics and the kinetics of MgH2.The MgH2+10 wt% V2C behaves excellent cycling stability with no any capacity loss for up to 10 cycles.Experimental and theoretical studies will be carried out jointly to reveal the detailed hydrogen storage properties and enhancing mechanisms of MgH2with V2C addition.

2.Experimental details and calculation methods

2.1.Sample preparations

All solvents and reagents were used as received without any further purifications High-purity V2AlC MAX phase powder with particle size of 400 mesh was purchased from Laizhou Kai Kai Ceramic Materials Co., Ltd.HF with a concentration of 40% was purchased from Aladdin.MgH2with a purity of 98% was purchased from Langfang Beide Commerce and Trade Co., Ltd.

Two-dimensional V2C MXene was prepared by firs selectively HF-etching the Al layers from the V2AlC MAX phase at 60°C for 60h.After that, the solution was centrifuged at 3500rpm for 5 times and then the precipitate was washed by using deionized water till the pH reached 6.Finally, the washed precipitate was dried in a freezer dryer for 24h and the product was the multilayer V2C MXene.The multilayer V2C MXene was further subject to ultrasonic exfoliation, to delaminate the multilayer V2C, which leads to the formation of lesslayer V2C MXene.Experimentally, 1g of multilayer V2C powders were firstl dispersed in 50mL of 1 wt%tetramethylammonium hydroxide (TMAOH) solution at room temperature for 8h.Then the resultant sediments were washed via centrifugation of 3500rpm and a V2C suspension was obtained.Finally, the V2C suspension was subject to highspeed centrifugation at 8000rpm to obtain the lesslayer V2C.It should be noted that the multilayer V2C was employed to tailor MgH2since the yield of multilayer V2C is much higher than lesslayer V2C.The MgH2-V2C composites with various ratios were prepared by ball milling on a planetary ball mill (Fritsch, Pulverisette 7).The rotation speed was 400rpm and the milling duration was set to be 1h.A ball-to-powder ratio of 40:1 was used for all ball millings.For comparisons,MgH2and MgH2-V2AlC were also milled under the same conditions.For the preparation of Mg-related materials, all handlings were conducted in a glovebox (Etelux) with the H2O and O2concentrations both below 1ppm.

2.2.Characterization methods

X-ray diffractions (XRD) were carried out on an X-ray diffractometer (Rigaku, Minifl x 600, Cu Kαradiation, 40kV and 200mA)to study the phase structures of the samples.The samples were sealed with a tape to prevent the influenc of air and moisture.The morphologies and microstructures were characterized by fiel emission scanning electron microscopy(FE-SEM.Hitachi SU8020 and Zeiss Sigma 300) and high-resolution transmission electron microscopy (HRTEM.FEI TECNAI G2 F20/F30).The element distributions were determined by the attached energy dispersive X-ray detector(EDX).For SEM measurements, sample transfers were conducted carefully to avoid contamination from the atmosphere.For TEM measurements, the samples were firs dispersed in acetone and then dropped onto the Cu grid.The nitrogen absorption and desorption measurements were carried out at the liquid nitrogen temperature on a physicochemical adsorption apparatus (Micromeritics TriStar II 3020).The specifi surface areas were estimated by the BET method.X-ray photoelectron spectroscopy (XPS.Thermofishe 250XI) were used to determine the valence states of the atoms.The XPS data were all calibrated by using the signal of C 1s at 284.8eV as the reference.

The hydrogen desorption and absorption measurements were performed on a lab-built Sievert-type apparatus.For the non-isothermal hydrogen desorption measurements, the samples were heated from room temperature to 400°C with a heating rate of 2°C min-1.For the isothermal hydrogen desorption measurements, the samples were firs heated to the target temperature under 1MPa H2and then held at this temperature.The hydrogen was vent to about 0.01MPa to start the hydrogen desorption of the samples.The desorbed samples were exposed to 6MPa H2at the constant desired temperature for the hydrogen absorption.The thermal decomposition behaviours of the samples were studied by a differential scanning calorimeter (DSC.Setaram Labsys Evo) with an argon fl w of 30mL min-1as the carrier gas.The pressureconcentration-temperature curves (PCT) were collected by a lab-built automatic Sievert-type apparatus.

2.3.Calculation methods

The first-principle density functional theory (DFT) calculations were performed with the Vienna ab initio simulation package (VASP) software package [68].The generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof(PBE) exchange correlation functional and the projector augmented wave (PAW) method were used to describe the structural and electronic properties of the systems[69].The energy cutoff for the plane waves was set to be 500eV.The convergence for the energy calculation was 0.0001eV, and the structural optimization without any symmetry constraint was performed until the force is smaller than 0.02eV/°A.The Brillouin zone is sampled using the 5×5×1 grid in the Monkhorst-Pack scheme.To include the van der Walls (vdW) interactions, the vdW-DF3 functional was used [70].The binding energies (Ebe) of MgH2and MgH2with V2C addition could be estimated by the following equation:

whereEMgH2(+V2C)are the energy of MgH2without or with V2C addition,EH(= -1.11709010eV) andEMg(=-0.03808424eV) are the energies of the isolate H and Mg atom in bulk state, respectively.

3.Results and discussion

Two dimensional V2C MXene was synthesized by selectively HF-etching the Al layers from V2AlC MAX phase.Fig.1a schematically shows the synthesis process of V2C.Fig.1b displays the XRD patterns of the raw materials of V2AlC MAX phase, which is identical to that of V2AlC(JCPDS 29-0101).After HF-etching, Al layers in V2AlC were etched away and multilayer V2C MXene was formed(Fig.1c).It can be seen from Fig.1c that only a small amount of V2C is observed and there is still a large quantity of V2AlC left.Therefore, the multiplayer V2C MXene was further subject to ultrasonic exfoliation to delaminate the multiplayer V2C, which leads to the formation of lesslayer V2C MXene(Fig.1d).It can be seen from Fig.1d that V2AlC almost transform to V2C after the ultrasonic exfoliation treatment.The diffraction peaks of the lesslayer V2C correspond to (002),(004), (006), and (008) planes of V2C [71-73].Fig.1e displays the SEM image of V2AlC, which shows a dense structure of the material.Fig.1f and g display the SEM images of multilayer V2C.It can be clearly observed that the assynthesized V2C gets a well-layered structure with a layer thickness of several hundred nanometers.The layers can be further divided into sublayers with a thickness of 50-100nm.The element mapping in the inset of Fig.1g demonstrates that the V, C, and Al elements all distribute uniformly in the materials.Fig.1h shows the TEM image of multilayer V2C,which also has a layered structure with a layer thickness of 159nm.The layers consist of sublayers with a thickness of 52nm, which is consistent with that in Fig.1g.The HRTEM image of region A was displayed in Fig.1i, in which lattice plane belonging to V2C (002) are observed.This suggests the successful synthesis of V2C.Fig.1j displays the electron image and elemental mapping of lesslayer V2C, which shows a sheet-like structure.The content of Al is very low in the lesslayer V2C, which means that the Al layers in V2AlC MAX phase have almost been etched away.It should be noted that due to the low yield of the lesslayer V2C, the multilayer V2C was utilized for enhancing the hydrogen storage performances of MgH2.The nitrogen sorption curves in Fig.1k shows that the BET specifi surface area of the multilayer V2C is 10.7 m2 g-1.

The as-synthesized multilayer V2C was introduced into MgH2by ball milling to prepare the MgH2+xwt% V2C(x=5, 7.5, 10) composites.For comparison, MgH2+10 wt%V2AlC composite and pure MgH2were also ball milled under the same conditions.Fig.2a displays the non-isothermal hydrogen desorption curves of the samples with a heating rate of 2°C min-1.The pure MgH2starts to release hydrogen at about 318°C.After addition of 7.5 wt% V2AlC, the onset hydrogen desorption temperature of MgH2is reduced to 280°C, which is 38°C lower than that of pure MgH2.It is inspiring that the addition of V2C further reduce the onset hydrogen desorption temperature of MgH2to 190°C, which behaves a 128°C reduction than the pure MgH2.These results suggest that V2AlC can slightly reduce the hydrogen desorption temperature of MgH2, while V2C can significantlreduce the temperature of MgH2.The MgH2+xwt% V2C(x=5, 7.5, 10) composites complete their hydrogen desorption after the temperature is increased to about 275°C, with a hydrogen releasing capacity of about 6.5 to 7.0 wt%.Fig.2b displays the DSC curves of V2C, pure MgH2and MgH2+10 wt% V2C composite with a heating rate of 5°C min-1.The peak hydrogen desorption temperature of MgH2+10 wt%V2C composite is 314°C, which is significantl lower than that of pure MgH2(389°C).The DSC curve of MgH2+10 wt% V2C composite consists of two peaks, with the lower-temperature peak at 314 °C and the higher-temperature peak at 399 °C.Such decomposition characteristic of two peaks is not original from V2C since the DSC curve of V2C in Fig.2b has no peak.Therefore, it was suggested that the lower-temperature peak may be related to the decomposition of MgH2contacting with V2C, while the higher-temperature peak, whose temperature is very close to that of pure MgH2,is related to the decomposition of MgH2that does not contact with V2C.Fig.2c displays the isothermal hydrogen desorption curves of pure MgH2,MgH2+10 wt%V2AlC composite,and MgH2+10 wt% V2C composite at 300 °C.At a constant temperature of 300 °C, the MgH2+10 wt% V2C composite can release 6.4 wt% of hydrogen within 10min.However,pure MgH2can hardly release hydrogen at such temperature and the and the MgH2+10 wt% V2AlC composite can only release a small amount of hydrogen (about 0.6 wt%) even within 30min.This suggests that V2C can significantl improve the hydrogen desorption kinetics of MgH2.Fig.2d displays the cycling isothermal hydrogen desorption curves of MgH2+10 wt% V2C composite, which shows no any capacity loss and kinetic decay for up to 10 cycles.Therefore,MgH2+10 wt% V2C composite possesses not only excellent hydrogen desorption kinetics and lower hydrogen desorption temperature, but also quite good cycling stability.

Fig.1.(a) Schematic picture showing the synthesis process of the two-dimensional V2C, (b-d) XRD patterns of V2AlC, multiplayer V2C, and lesslayer V2C,(e) SEM images of V2AlC, (f-g) SEM images and element mapping of V2C, (h) TEM image of V2C, (i) HRTEM image of region A in (h), (j) electron image and element mapping of V2C, (k) N2 sorption curves and BET plot of V2C.

Fig.2.(a) Non-isothermal hydrogen desorption curves of MgH2, MgH2+10 wt% V2AlC, MgH2+x wt% V2C (x=5, 7.5, 10) with a heating rate of 2°C min-1,(b)DSC curves of V2C,MgH2 and MgH2+10 wt%V2C,(c)isothermal hydrogen desorption curves of MgH2,MgH2+10 wt%V2AlC,and MgH2+10 wt% V2C at 300 °C, (d) cycling isothermal hydrogen desorption curves of MgH2+10 wt% V2C at 300°C.

To gain an insight into the hydrogen desorption kinetics of the samples, the Johnson-Mehl-Avrami (JMA) equation [74,75]was employed to study the isothermal hydrogen desorption curves in Fig.3a-c (left).The JMA equation can be written as:

whereαis the degree of a reaction;tis the reaction duration;nis the Avrami exponent which reflect the nucleation and growth mechanisms;kis the rate constant.The activation energy (Ea) can be estimated by Arrhenius equation using the data obtained from Eq.(2).The Arrhenius equation can be written as:

whereTis the absolute temperature at which the hydrogen desorption is carried out;Eais the activation energy for the hydrogen desorption reaction;Ris the gas constant;Ais the pre-exponential factor.

The isothermal hydrogen desorption kinetic curves in Fig.3a-b (left) were firs transformed to JMA plots of ln[-ln(1-α)]vs.lntin Fig.3a-b (middle) in theαrange 0.2-0.8.From the intercepts of the JMA plots,kvalues were obtained.Then the Arrhenius's plots of lnkvs.1/Tcan be made to obtainEafrom the slopes of the plots in Fig.3a-b(right).The activation energies for the hydrogen desorption reactions of pure MgH2, MgH2+10 wt% V2AlC composite,and MgH2+10 wt% V2C composite were finall calculatedto be 134, 148, and 112kJ mol-1H2, respectively.Therefore,V2C addition can reduce the activation energy for the hydrogen desorption of MgH2, which directly contributes to the improvement of the hydrogen desorption kinetics of MgH2.

Fig.3.Isothermal hydrogen desorption curves (left), JMA plots (middle), and Arrhenius's plots (right) of MgH2 (a), MgH2+10 wt% V2AlC (b), and MgH2+10 wt% V2C (c).

From the slopes of the plots in Fig.3a-c (middle),nvalues that reflec the nucleation and growth mechanism of a solid phase reaction can be obtained.nvalue for pure MgH2falls in the range 2 to 4, which suggests an interfacecontrolled transformation [76].This is consistent with previously reported mechanism of the hydrogen desorption of MgH2[77].In contrast,nvalue for MgH2with V2C addition is close to 1, which suggests a diffusion-controlled transformation [76].Therefore, V2C addition has changed the ratecontrolled step of the decomposition of MgH2.According to the theory of transformations in metals and alloys, a transformation process generally includes many steps, with the slowest step as the rate-controlled step.As for the decomposition of MgH2, many steps could influenc the reaction process, such as: (1) diffusion of H atoms in MgH2crystal;(2) diffusion of H atoms in Mg crystal; (3) diffusion of H atoms through MgH2/Mg grain interfaces; (4) combination of H atoms to form H2molecular on a surface or at an interface;(5) dissociation of H2molecular; (6) nucleation of Mg phase;(7) movement of the MgH2/Mg interfaces; (8) other steps.There is a general trend suggesting that the hydrogen desorption of MgH2may begin with an instantaneous nucleation followed by an interface-controlled step in between MgH2and Mg phases.Therefore, it is the interface-controlled step that restrain the hydrogen desorption kinetics of MgH2.However,V2C addition may accelerate this step and change the rate-controlled step to the diffusion of H atoms in MgH2since the diffusion of H atoms in MgH2is also very slow [78].This may also contribute to the reduction of the activation energy of MgH2by V2C addition.

Fig.4.DSC curves of MgH2 (a) and MgH2+10 wt% V2C (b) with various heating rates, and the Kissinger's plots of the two samples (c).

Kissinger's equation [79]can also be employed to estimate the activation energy for a reaction.The Kissinger's equation can be written as:

whereβis the heating rate employed in the DSC test;TPis the peak temperature of the relevant reaction;Ris the gas constant;Ais a constant.Experimentally, the DSC curves at various heating rates (β=5, 7.5, 10, 12.5°C min-1) of pure MgH2and MgH2+10 wt% V2C composite were firs collected to obtain the peak temperatures of the hydrogen desorption reaction at various heating rates (Fig.4a and b).Then the Kissinger's plots of ln(β/TP2) vs.1000/TPwere made to obtain the slopes of the plot lines (Fig.4c).Finally, the activation energies for the hydrogen desorption reactions of pure MgH2and MgH2+10 wt% V2C were calculated from the slopes to be 126.3 and 87.6kJ mol-1H2, respectively(Fig.4c).Such results also mean that V2C addition signifi cantly lowers the activation energy for the hydrogen desorption reaction of MgH2.

It should be noted that the activation energy of pure MgH2estimated by Arrhenius's method (134kJ mol-1H2) is similar with that by Kissinger's method (126kJ mol-1H2).However,the activation energy of MgH2+10 wt% V2C estimated by Arrhenius's method (112kJ mol-1H2) is larger than that by Kissinger's method (87kJ mol-1H2).This is because that the DSC curve of MgH2+10 wt% V2C is composed of two peaks, with the lower-temperature peak corresponding to the desorption of the activated MgH2and the higher-temperature peak corresponding to the desorption of the non-activated MgH2.For MgH2+10 wt%V2C composite,the activation energy by Arrhenius's method covers the two desorption peaks,while the activation energy by Kissinger's method covers only the lower-temperature peak.Therefore,different activation energy values were obtained by the two methods.

To investigate the hydrogen desorption thermodynamics of MgH2and MgH2+10 wt% V2C, the PCT curves at various temperatures were collected, which are shown in Fig.5a and b.The hydrogen desorption reaction enthalpy can be estimated by the van't Hoff equation written as:

wherepis the equilibrium pressure of material at temperatureT; p0is the normal atmospheric pressure (equals to 0.1MPa);ΔHis the hydrogen desorption reaction enthalpy; ΔSis the hydrogen desorption reaction entropy which equals approximately to 130kJ mol-1H2-1K-1for many metal hydrides;Ris the universal gas constant.The van't Hoff plots of ln(p/p0)vs.1000/RTare displayed in Fig.5c and d.The hydrogen desorption reaction enthalpy of MgH2+10 wt% V2C was calculated to be 73.6kJ mol-1H2(Fig.5c), which is slightly lower than that of pure MgH2(77.9kJ mol-1H2(Fig.5d)).

The above investigations demonstrate that V2C addition not only significantl improves the hydrogen desorption kinetics of MgH2, but also slightly reduces the thermal stability of MgH2.For comparisons, Table 1 summarizes the hydrogen storage performances of various MgH2-based hydrogen storage materials that were tailored by MXenes and their derivatives, together with other classical metal halides or oxides.

Table 1Hydrogen storage performances of various MgH2-based hydrogen storage materials.

Table 2Bond length and binding energy of Mg-H in pure MgH2 and Mg2/V2C.

The XRD measurements were carried out to study the phase evolutions of MgH2+10 wt% V2C composite during hydrogen sorption process.Fig.6a-f display the XRD patterns of as-milled MgH2+10 wt% V2C, dehydrided MgH2+10 wt% V2C at 300°C, and rehydrided MgH2+10 wt% V2C at 300 °C and 6MPa H2for 3h.From the XRD patterns of the as-milled MgH2+10 wt% V2C in Fig.6a,MgH2and a small amount of V2AlC are observed.However,V2C is not observed, which may due to the low content or low crystallinity.After hydrogen desorption, MgH2decomposes to form Mg (Fig.6b).However, some trace of MgH2is observed in the dehydrided MgH2+10 wt% V2C sample,which suggests that the decomposition of MgH2does not proceed fully.When subject to hydrogen absorption,MgH2in the MgH2+10 wt% V2C was fully recovered (Fig.6c), which indicates that the MgH2+10 wt% V2C composite shows a feature of full reversibility.It is observed from Fig.6a-c that V2AlC does not change obviously during the hydrogen sorption process.Fig.6d-f display the XRD patterns for thethree samples at 2Theta range 27°-37°.It is shown that the diffraction peaks of MgH2for the MgH2+10 wt% V2C composite in hydrided state shift to lower angle, which suggests an expansion of the crystal structure of MgH2in hydrided state according to the Bragg's equation (Eq.(6)).The expansion of the MgH2crystal structure indicates that the Mg-H bonds may have been lengthened after the addition of V2C,which contributes to the destabilization of MgH2.

Fig.5.PCT curves at various temperatures and van't Hoff plots of MgH2 (a, c) and MgH2+10 wt% V2C (b, d).

Fig.6.XRD patterns of as-milled MgH2+10 wt% V2C (a, d), dehydrided MgH2+10 wt% V2C (b, e), and rehydrided MgH2+10 wt% V2C (c, f).XRD patterns of as-synthesized V2C (g), as-milled MgH2+50 wt% V2C (h), and dehydrided MgH2+50 wt% V2C (i).

Fig.7.XPS spectra of V 2p and C 1s of as-synthesized V2C (a, b) and MgH2 +10 wt% V2C before desorption (c, f), after desorption (d, g), and after re-absorption (e, h).

The Scherrer's equation [91]was employed to estimate the grain size of MgH2.Scherrer's equation is written as:

whereDis the grain size;Kis a constant depending on the shape of the grains and is assumed to be 0.89 in general;λis the wavelength of the incident X-ray used;Bis the full width at half maximum of a diffraction peak;θis the Bragg angle of a diffraction peak.By Eq.(7), the grain size of MgH2in the as-milled MgH2+10 wt% V2C composite was calculated to be 47.9nm (Fig.6d).After subject to hydrogen sorption, the grain size of MgH2increases to 95.7nm.It should be noted that, the hydrogen desorption kinetics does not decrease though the grain size of MgH2increases.Therefore, it is believed that the grain size of MgH2is not the predominant factor impacting the hydrogen sorption kinetics of MgH2+10 wt% V2C composite.

To reveal the states of V2C during the hydrogen sorption process of the MgH2-V2C composite, the content of V2C addition was increased and a MgH2+50 wt% V2C composite was prepared.The XRD patterns of as-synthesized V2C,as-milled MgH2+50 wt% V2C and dehydrided MgH2+50 wt%V2C are displayed in Fig.6g-i.The as-synthesized V2C is a mixture of V2C, V2AlC, and a small amount of Al2O3(Fig.6g).After ball milling MgH2with the as-synthesized V2C, the peaks of V2C disappear and V2AlC still exists in the as-milled MgH2+50 wt% V2C composite (Fig.6h).No other new phases were detected in the XRD patterns of asmilled MgH2+50 wt% V2C.This means that V2C may have changed to amorphous state or reacted with MgH2to form other amorphous phases.After hydrogen desorption, MgH2in the MgH2+50 wt% V2C composite has decomposed to Mg(Fig.6i), with V2AlC still existing in the composite.However, no V2C-relevant phases were detected in the dehydrided MgH2+50 wt% V2C composite.Therefore, the manner of V2C in the MgH2-V2C composite is difficul to be clarifie by XRD method.

The XPS measurements were performed to study the chemical states of V2C.Fig.7 displays the XPS spectra of V 2p and C 1s of the as-prepared V2C and the MgH2+10 wt% V2C composite at different stages.According to the previous published work[73], V 2p3/2at 517.1eV and V 2p1/2at 524.4eV correspond to V4+of V2C and VO2, while C-C,V-C, and O-C=O locating at 284.8, 286.6, and 288.9eV are related to the several C atomic environments of V2C.Fig.7c-h demonstrate that V2C does not change during the hydrogen sorption process of MgH2+10 wt% V2C composite, which suggests that V2C may act as a catalyst for MgH2.Since V2C in the composite cannot be observed by XRD method and was detected with a low intensity by XPS measurements,it is supposed that V2C may exist in an amorphous state in the composite.However, more advanced characterization methods should be utilized to study the real state of V2C during the hydrogen sorption process of MgH2-V2C composite.

The microstructures of MgH2+10 wt% V2C composite after re-absorption at 300°C and 4MPa for 3h were further studied by SEM and TEM measurements.Fig.8a and b display the SEM images of the sample, from which particles of several microns are observed.It should be noted that the particles are composed of many sub-particles with a much smaller size.It is believed that smaller particles may benefi the hydrogen sorption process of MgH2.Fig.8c displaysthe electron image and the elemental mapping of rehydrided MgH2+10 wt% V2C composite.The elements of Mg and V both distribute very uniformly in the material,which also benefit the interaction between MgH2with V2C.Fig.8d displays the TEM image and electron diffraction pattern of rehydrided MgH2+10 wt% V2C composite.The electron diffraction pattern shows the existence of MgH2, V2AlC, and Mg in the material, which agrees with the XRD results in Fig.6c.The existence of Mg is ascribed to that the sample used for TEM tests was subject to hydrogen absorption at a milder conditions of 300°C and 4MPa for 3h.Under such conditions,MgH2was not fully recovered and partial Mg was left.Fig.8e displays the HRTEM image of the rehydrided MgH2+50 wt% V2C, with a higher content of V2C.The d-spacings of 0.217,0.218 and 0.213nm belong to V2AlC(103)plane,while the d-spacings of 0.249, 0.255, 0.258nm are related to MgH2(101) plane.The d-spacing of MgH2(101) plane is generally 0.251nm according to the PDF#74-0934.However, it is observed that the d-spacing of MgH2(101)plane near V2AlC in Fig.8e is 0.255nm, which is slightly larger than the theoretical value of 0.251nm.This means that V2C/V2AlC addition could slightly expand the MgH2lattice and as a result, slightly destabilize the MgH2.This is consistent with the PCT results in Fig.5 and the XRD results in Figs.6d and f.Unfortunately, V2C is still not observed by HRTEM, even increasing the content of V2C addition in the composite.

Fig.8.SEM images (a, b), element mapping (c), TEM image and SAD pattern (d) of MgH2+10 wt% V2C after re-absorption at 300°C and 4MPa for 3h.HRTEM image of rehydrided MgH2+50 wt% V2C.

It is demonstrated in Fig.5 that V2C addition can slightly reduce the thermal stability of MgH2.However, the XPS results in Fig.7 indicates that V2C does not change during the hydrogen sorption process of the MgH2-V2C composite.The first-principle DFT calculations were performed to reveal how V2C impact the thermal stability of MgH2.The optimized crystal structures of the optimized 2×2 supercell of the V2C monolayer in top view and side view are shown in Figs.9a and b.The atoms are arranged in a triple-layer structure in the sequence of V(1)-C-V(2), which can be regarded as thatone C atom layer intersperses between two V atom layers.The bond lengths of V(1)-C and V(2)-C are calculated to be 2.001 °A, and the layer thicknesses of V(1)-C-V(2)atoms is 2.31 °A.These optimized structural data are agreement with the previous theoretical predictions [92].

Fig.9.Top (a) and side (b) views of the atomic structure of the optimized 2×2 supercell of V2C monolayer.(c) and (d) are the top view of MgH2 adsorbed on (110) surface of V2C before and after optimization.

In order to calculate the bond length and the binding energy of Mg-H.A supercell of 4×4×1 V2C (Fig.9c)was built.A three-layer close-packed V2C (110) plane and a vacuum space of 20 °A in thezdirection were used to simulate the V2C catalyst.Fig.9c and d show the top view and side view of MgH2adsorbed on the surface of V2C (110) plane before and after optimization.Table 2 displays the calculated bond length and binding energy of Mg-H in pure MgH2and MgH2/V2C.The bond length of Mg-H is increased from 1.71 °A for pure MgH2to 2.14 °A for MgH2/V2C.As a result, the Mg-H bond of MgH2with V2C addition is easier to be broken.Besides, the binding energy is reduced from -4.36eV for pure MgH2to -3.07eV for MgH2/V2C.In Fig.9d, it is observed that after structure optimization, V atoms will attract the H atoms and thus increasing the bond length of Mg-H and changing the thermodynamics of MgH2.These theoretical calculation results indicate that V2C addition can destabilize MgH2, thus improve the hydrogen desorption thermodynamics of MgH2.

4.Conclusions

Two-dimensional V2C MXene was synthesized and introduced to improve the hydrogen desorption properties of MgH2.V2C addition not only significantl improves the hydrogen desorption kinetics of MgH2, but also slightly reduces the thermal stability of MgH2.Low hydrogen desorption temperature and excellent cycling stability of MgH2were achieved for MgH2+10 wt% V2C composite.This study demonstrates that the thermodynamics and kinetics of MgH2can be simultaneously improved by addition of two-dimensional MXene materials.Though the mechanism of V2C was preliminarily discussed in this work, more advanced characterization methods are required to disclose the real role of V2C in enhancing the hydrogen storage performances of MgH2.

Acknowledgments

This work was financiall supported by National Natural Science Foundation of China (No.52001079), Education Department of Guangxi Zhuang Autonomous Region(No.2019KY0021), and the Natural Science Foundation of Guangxi Province (2019GXNSFBA185004, 2018GXNSFAA281308, 2019GXNSFAA245050).