Ï¡ÓнðÊô(Ó¢ÎÄ°æ) 2017,36(07),574-580

Improving dehydrogenation properties of Mg/Nb composite films via tuning Nb distributions

Wen-Cheng Huang Jun Yuan Jin-Guo Zhang Jiang-Wen Liu Hui Wang Liu-Zhang Ouyang Mei-Qin Zeng Min Zhu

Key Laboratory of Advanced Energy Storage Materials of Guangdong Province,School of Materials Science and Engineering,South China University of Technology

ÊÕ¸åÈÕÆÚ£º28 February 2017

»ù½ð£ºsupported by the National Natural Science Foundation of China(Nos.51621001, 51571091,and 51471070);Guangdong Natural Science Foundation(Nos.2016A030312011 and 2014A030313222);

Improving dehydrogenation properties of Mg/Nb composite films via tuning Nb distributions

Wen-Cheng Huang Jun Yuan Jin-Guo Zhang Jiang-Wen Liu Hui Wang Liu-Zhang Ouyang Mei-Qin Zeng Min Zhu

Key Laboratory of Advanced Energy Storage Materials of Guangdong Province,School of Materials Science and Engineering,South China University of Technology

Abstract£º

To investigate the effect of Nb on the dehydrogenation properties of Mg-Nb composite films,Mg/Nb eightlayer film and Mg-10 at% Nb alloy film with the similar Mgto-Nb atomic ratio were prepared by magnetron sputtering.Results show that both Mg/Nb eight-layer film and Mg-10 at% Nb alloy film exhibit excellent de/hydrogenation properties.Mg-10 at% Nb alloy film starts to release hydrogen at 108 ¡æ,and its desorption peak temperature is lower to 146 ¡æ,which is much better than that of pure MgH₂ under the same condition.Scanning electron microscopy(SEM) results demonstrate that the dispersive Nb nanoparticles in Mg/Nb eightlayer film may serve as nucleation sites for Mgú±MgH₂ reactions,which can provide channels for H diffusion.For Mg-10 at% Nb alloy film,the uniform distributions of Nb can accelerate the hydrogen diffusion and effectively improve the dehydrogenation kinetics for MgH₂.This study provides an enlightening way for designing and preparing Mg-based composite films with excellent dehydrogenation properties.

Keyword£º

Hydrogen storage; Mg-Nb; Sputtering; Thin film; Kinetics;

Author£º Jiang-Wen Liu e-mail:mejwliu@scut.edu.cn;

Received£º 28 February 2017

1 Introduction

Magnesium hydride (MgH2) is considered as a promising and ideal hydrogen carrier because of its high theoretical hydrogen capacity,low cost,reversibility,and environmental benefits [ 1, 2, 3] .However,the high thermodynamic stability and poor kinetics of MgH₂ at moderate temperatures impede its practical applications.In the past decades,much effort has been devoted to improving the kinetics of Mg MgH₂ reactions and to lowering the de/hydrogenation temperatures,including forming nanocrystallines,alloying with other elements,catalyzing,and forming composites [ 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14] .With regard to the catalysts,the effective ones are transition metals (TM) and their oxides.Compared with pure MgH₂,TM additives can accelerate the hydrogen desorption with reduced activation energy,which was explained by the presences of extended interfaces between MgH₂ and TM nanoclusters [ 15] .Among them,the additions of Nb in Mg/MgH₂ systems were widely explored [ 16, 17, 18, 19] .In the mixed powders of Mg and Nb,bcc-(Nb,Mg) solid solution formed and bcc-magnesium niobium hydride (Mg_0.75Nb_0.25)H₂ had a reversible capacity of 4 wt% [ 20, 21] .The catalytic mechanisms of Nb were generally explained by spillover,d-electrons,and electronegativity effects [ 22] .Besides,more attention has been paid to the thin film techniques due to their exact control effects of the compositions,interfaces,and grain sizes of Mg.Pd layer can prevent magnesium from oxidation,leading to its significant improvement of the kinetics;thus,the Pd-capped Mg composite thin films were widely studied as a type of novel Mg-based storage alloys in recent years [ 23, 24, 25, 26, 27] .Moreover,Mg/TM composite films combining nanostructuring and catalyzing dramatically accelerated the de/hydrogenation kinetics and destabilized the thermodynamics of Mg/MgH₂ [ 12, 13, 26, 27, 28, 29] ,which showed better performances than that of pure MgH₂ [ 30, 31, 32] .

As above mentioned,it is known that suitable additives can play not only as effective catalysts but also as components of nanocomposites.Nb has shown noble effect on the enhanced kinetics for hydrogen absorption/desorption [ 11, 20, 21, 22, 30, 31, 32, 33, 34] .Superior kinetics and thermodynamic enhancement of such Mg/Nb composites were attributed to the catalytic effect of the niobium-containing phases.However,the exact catalytic mechanisms of the niobium-containing phases have not been thoroughly verified;hence,intensive studies are required to provide detailed insights and discussions to illustrate these arguments.Moreover,different preparation methods of Mg-Nb composites may cause different existing forms of Nb,such as the sizes,distributions,and relative phase structures,which may affect the hydrogen storage performances.Therefore,in this work,Mg/Nb multilayers and Mg-Nb alloy films were prepared by magnetron sputtering,and their dehydrogenation properties and microstructural evolutions were investigated to provide insights into the effect of Nb with different existing forms on Mg MgH₂reactions and their catalytic mechanisms.

2 Experimental

Pd-capped Mg (100 nm)/Nb (10 nm) eight-layer films and Pd-capped Mg-10 at%Nb alloy films were prepared by magnetron sputtering.For simplicity,hereafter Pd-capped Mg (100 nm)/Nb (10 nm) eight-layer films and Pd-capped Mg-10 at%Nb alloy films are denoted as Mg/Nb eightlayer films and Mg-10 at%Nb alloy films,respectively.Both the two types of films were deposited in an ultrahigh vacuum magnetron sputtering system with a background pressure of 6¡Á10^-4 Pa under Ar atmosphere.The deposition processes were carried out under a pressure of 0.5 Pa with an Ar flow rate of 16 ml¡¤min^-1.When the vacuum of sputtering chamber reached 6.0¡Á10^-4 Pa,pre-sputtering targets were firstly performed for 5 min before the substrate shutter opened.The sputtering processes of Mg/Nb eight-layer films were as follows:10-nm Nb layers were firstly deposited on Si (001) wafers and glass substrates using a Nb (99.99%) target by radio frequency (RF) sputtering,and then,100-nm Mg layers were deposited on the Nb layers using a Mg (99.99%) target by direct current(DC) sputtering.The designed Mg/Nb eight-layer films were prepared via alternate and repeated deposition processes of Nb layers and Mg layers.Finally,10-nm Pd layers were deposited on the top of Nb layers using a Pd(99.99%) target by DC sputtering.The co-sputtering processes of Mg-10 at%Nb alloy films were as follows:10-nm Nb layers were firstly deposited onto Si (001)wafers and glass substrates using a Nb (99.99%) target by RF sputtering,and the Mg-Nb alloy layers were co-deposited on the Nb layers.The co-sputtering powers of Nb target and Mg target were DC 100 mA and RF 150 W,respectively.Then,10-nm Nb layers were deposited on the Mg-Nb alloy layers.Finally,10-nm Pd layers were deposited on the top of Nb layers using a Pd (99.99%)target by DC sputtering.The composition of Mg-10 at%Nb alloy film was examined by energy-dispersive spectroscopy (EDS).The Mg-to-Nb atomic ratio is 0.90:0.10.For the Mg/Nb eight-layer film,the Mg-to-Nb atomic ratio is calculated to be 0.89:0.11.Therefore,the compositions of atomic ratio for Mg/Nb eight-layer film and Mg-10 at%Nb alloy film are similar.Based on the controlled depositions and film thickness measurements,the total thicknesses of Mg/Nb eight-layer film and Mg-10 at%Nb alloy film are approximately 350.0 nm and 1.1¦Ìm,respectively.

After deposition,all samples were transferred into the glove box with the protection of high-purity Ar atmosphere.The dehydrogenation temperatures were measured by temperature program desorption-mass spectrum (TPD-MS,Hiden QIC-20).The hydrogenation of both films for TPD-MS test was carried out under a hydrogen pressure of3 MPa at room temperature.The de/hydrogenation kinetics was measured by a commercial Sieverts-type apparatus(Advanced Materials Corporation gas reaction controller)at 200,220,and 250¡æ,respectively.

The phase identifications of all samples were carried out on an X-ray diffractometer (XRD,PANalytical Empyrean)with Cu K¦Áradiation and grazing incidence method using grazing angle of 2¡ã.The microstructures of all samples were characterized by using a scanning electron microscope (SEM,Zeiss Supra 40) equipped with EDS (Bruker5010) and a transmission electron microscope (TEM,JEM-2100) operating at 200 kV.

3 Results and discussion

3.1 Dehydrogenation temperatures

The dehydrogenation temperatures of Mg/Nb eight-layer film and Mg-10 at%Nb alloy film after hydrogenated at room temperature were measured by TPD-MS,and the results are shown in Fig.1.For the Mg-10 at%Nb alloy film,the hydrogenated sample starts to release hydrogen at108¡æand the peak temperature of hydrogen desorption is146¡æ.For the hydrogenated Mg/Nb eight-layer film,the hydrogen evolution starts at 113¡æand the peak temperature of hydrogen desorption is 158¡æ.The dehydrogenation temperatures of hydrogenated Mg/Nb eight-layer film and Mg-10 at%Nb alloy film are much lower than that of pure MgH₂.For Mg/MgH₂,both the absorption and desorption of hydrogen require a temperature at least 350and 400¡æ [ 1, 2, 3] .The hydrogen absorption of a conventional and non-catalyzed Mg powder is almost negligible under 300¡æ.A slow absorption of hydrogen is observed even when the powder was heated to 400¡æ [ 14] .Simultaneously,the dehydrogenation of un-milled MgH₂ is negligible at 300¡æ,and it exhibits a slow desorption of hydrogen even at 350¡æ [ 4] .Compared with pure Mg and Mg/Nb eight-layer film,the Mg-10 at%Nb alloy film shows superior dehydrogenation property.

Fig.1 TPD-MS profiles of Mg/Nb eight-layer film and Mg-10 at%Nb alloy film

3.2 Microstructure analysis

Figure 2a shows XRD patterns of Mg/Nb eight-layer film in various states.For the as-deposited Mg/Nb eight-layer film,the Mg,Nb,and Pd phases can be detected.Mg fully transforms into MgH₂ after hydrogenation and completely recovers after dehydrogenation.And there is no detectable variation of Nb phase in the hydrogenated and dehydrogenated states.Figure 2b shows XRD patterns of Mg-10 at%Nb alloy film in various states.In the as-deposited state,hcp-Mg(Nb) solid solution phase increasingly forms,and a small amount of dissociative Mg phases can be traced.Compared with pure Mg phase,Mg(Nb) 002,Mg(Nb) 103,and Mg(Nb) 112 diffraction peaks shift toward higher angle,because the effective atomic radius of Nb (r=0.1429 nm) is smaller than that of Mg(r=0.1605 nm).No evidence of Nb Bragg peaks shows up in the as-deposited state,indicating that Nb dissolves in supersaturated solid solution of Mg(Nb).After hydrogenation,the most prominent peaks are MgH₂ and Mg_xNb_1-xH solid solution.Besides,previously dissolved Nb partly segregates out of Mg/MgH₂ lattices during hydrogenation;hence,a small amount of NbH_0.95 can be detected.For the dehydrogenated sample,the Mg phase fundamentally recovers other than traced MgH₂ and NbH_0.95 because of incomplete dehydrogenation.

The microstructures of Mg/Nb eight-layer film and Mg-10 at%Nb alloy film in the as-deposited,hydrogenated,and dehydrogenated states were also observed by SEM.Figure 3a-c presents the surface morphologies of Mg/Nb eightlayer film in various states.For the as-deposited film,it shows a typical hexagonal shape of Mg particles with a size less than 100 nm.After hydrogenation and dehydrogenation,both their surface morphologies present progressive distinction.Figure 3d-f shows cross-sectional SEM images of Mg/Nb eight-layer film in various states.For the as-deposited sample,Nb layers are too thin to be distinguished from Mg/Nb eight-layer film.While the interfaces between Mg and Nb layers are clearly distinguished in the hydrogenated state,which may be due to the volume expansion and stress constraint during hydrogenation.After dehydrogenation,some cracks and disintegration form because of repeated phase transformations and volume strains.

Figure 4a-c shows surface SEM images of Mg-10 at%Nb alloy film in various states.The typical morphology of regular hexagonal Mg particles can be observed in the asdeposited state.The regular hexagonal Mg particles disappear after hydrogenation and partly recover after dehydrogenation.Compared with those of Mg/Nb eightlayer film,the surface morphologies of Mg-10 at%Nb alloy film in various states are much more smooth and uniform.Figure 4d-f shows cross-sectional SEM images of Mg-10 at%Nb alloy film in various states.For the Mg-10 at%Nb alloy film,the columnar morphology of Mg(Nb) solid solution can be observed in the as-deposited state,which indicates that the solid solution of Nb does not change the growth orientation of Mg(Nb) alloy film.However,the columnar crystal completely disappears due to the hydrogenation and recrystallization of Mg hydrides.After dehydrogenation,the columnar crystal mainly recovers along with some cracks and delamination with substrate because of phase transformations and volume contractions.

Fig.2 XRD patterns of a Mg/Nb eight-layer film and b Mg-10 at%Nb alloy film in various states

Fig.3 SEM images of a-c surface and d-f cross-sectional morphologies of Mg/Nb eight-layer film in various states:a,d as deposited,b,e hydrogenated,and c,f dehydrogenated

Fig.4 SEM images of a-c surface and d-f cross-sectional morphologies of Mg-10 at%Nb alloy film in various states:a,d as deposited,b,e hydrogenated,and c,f dehydrogenated

Considering the superior dehydrogenation property of Mg-10 at%Nb alloy film,the fine structures of the asdeposited Mg-10 at%Nb alloy film were further observed by TEM.The powder samples were scraped from the Mg-10 at%Nb alloy films for TEM observations.Figure 5a shows a typical micros truc ture of Mg(Nb) solid solution and the selected area diffraction patterns (SADPs) on the left bottom corresponding to the red circle area.The electron diffraction analysis indicates that the main strong diffraction cycles are caused by abound Mg(Nb) solid solution and the weak Mg(101) diffraction cycle is caused by dissociative Mg phase,which is consistent with XRD result.The weak MgO diffraction may be attributed to the slight oxidization in scraping powders.Figure 5b presents the high-resolution transmission electron microscopy(HRTEM) image corresponding to the red circle area in Fig.5 a.HRTEM result indicates that the micros truc ture of Mg-10 at%Nb alloy film contains nanosized Mg(Nb) solid solution phase and dissociative Mg phase.

Fig.5 TEM images of as-deposited Mg-10 at%Nb alloy film:a partial morphology of powdery films (inset being SADPs taken from to red cycle area) and b HRTEM micrograph

3.3 De/hydrogenation kinetics of Mg-10 at%Nb alloy film

Mg-10 at%Nb alloy film exhibits lower dehydrogenation temperature than Mg/Nb eight-layer film.Therefore,further studies were carried out for the de/hydrogenation kinetics of Mg-10 at%Nb alloy film.Figure 6a shows superior isothermal hydrogenation kinetic curves of Mg-10 at%Nb alloy film under a pressure of 3 MPa,which absorbs 5.6 wt%H₂ within 3.5 min at 220¡æand within8.0 min at 200¡æ,respectively.

In addition,the dehydrogenation kinetics of Mg-10 at%Nb alloy film was,respectively,measured at 250 and220¡æ,and the results are shown in Fig.6b.The Mg-10 at%Nb alloy film can release 4.6 wt%H₂ within100.0 min at 250¡æand 1.9 wt%H₂ within same time at220¡æ,which is much better than pure MgH₂ at the same condition [ 1, 2, 3, 11, 14] .

3.4 Catalytic mechanism for MgMgH2

Mg/Nb eight-layer film and Mg-10 at%Nb alloy film have similar compositions of Mg-to-Nb ratio but different micros truc tures with different Nb distributions,hence presenting different dehydrogenation properties.Here the mechanisms about different distributions and existing forms of Nb catalyzing Mg MgH₂ reactions are proposed.Figure 7 illustrates the different distributions of Nb in Mg/Nb eight-layer film and Mg-10 at%Nb alloy film.For the Mg/Nb eight-layer film,the Nb particles exist as dispersive ones rather than forming continuous layers.In the de/hydrogenated states of Mg/Nb eight-layer film,H₂/H dissociates/recombines on Nb particles.These Nb nanoparticles may conduct as nucleation sites for MgH₂/Mg and provide channels for H diffusion.However,for Mg-10 at%Nb alloy film,Nb mainly dissolves into hcpMg lattice in solid solution,which is responsible for lattice contraction.Results here are different from previous reports in which bcc-Nb(Mg) structure formed [ 20, 21] .On the basis of XRD results,the dissociative Mg phase is detected in as-deposited and dehydrogenated states.Therefore,in Mg-10 at%Nb alloy film,Nb exists as substitute solute atoms in Mg(Nb) solid solution,and the distributions of Nb in Mg-10 at%Nb alloy film are much more uniform than that in Mg/Nb eight-layer film.In addition,based on HRTEM result,Mg grains in Mg-10 at%Nb alloy film are smaller than those in Mg/Nb eight-layer film.Nb in solid solution or small clusters provides more nucleation sites for MgH₂/Mg and channels for H diffusion.These factors together provide better understanding for the superior dehydrogenation properties of Mg-10 at%Nb alloy film.

Fig.6 Isothermal a hydrogenation and b dehydrogenation kinetics of Mg-10 at%Nb alloy film

Fig.7 Sketches for illustrating different distributions of Nb in Mg/Nb eight-layer film and Mg-10 at%Nb alloy film

4 Conclusion

In this paper,the dehydrogenation properties and micro structural evolutions during de/hydrogenation of Mg/Nb eight-layer film and Mg-10 at%Nb alloy film were comparatively studied.Mg-10 at%Nb alloy film has better dehydrogenation properties due to the induced changes of the microstructures by the solid solution of Nb.The hydrogen release of Mg-10 at%Nb alloy film occurs at108¡æ,and the peak temperature of hydrogen desorption is 146¡æ,which indicates decreased thermodynamic stability of Mg-based hydride.In addition,this alloy film absorbs 5.6 wt%H₂ within 3.5 min at 220¡æand within8.0 min at 200¡æ,respectively,and it could release4.6 wt%H₂ within 1100 min at 250¡æ.This study provides a new insight for designing and fabricating composite films with different microstructures to improve the dehydrogenation properties of Mg.2014A030313222).

Acknowledgements This study was financially supported by the National Natural Science Foundation of China (Nos.51621001,51571091,and 51471070) and Guangdong Natural Science Foundation (Nos.2016A030312011 and 2014A030313222).

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