Enhanced Hydrogen Storage Properties of Magnesium Nanotrees with Nanoleaves.

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1 Enhnced Hydrogen Storge Properties of Mgnesium Nnotrees with Nnoleves. Journl: 29 MRS Fll Meeting Mnuscript ID: Drft Symposium: Symposium W Dte Sumitted y the Author: Complete List of Authors: Cnsizoglu, Mehmet; University of Arknss t Little Rock, Applied Science Krck, Tnsel; University of Arknss t Little Rock, Applied Science Keywords: sorption, Mg, nnostructure

2 Pge 1 of 6 Enhnced Hydrogen Storge Properties of Mgnesium Nnotrees with Nnoleves Mehmet F. Cnsizoglu nd Tnsel Krck Deprtment of Applied Science, University of Arknss t Little Rock, Little Rock, AR 7224 ABSTRACT Hydrogen storge in dvnced solid stte mterils hs een n intense re of reserch due to mny drwcks in conventionl high pressure or cryogenic liquid hydrogen storge methods. A prcticl hydrogen storing mteril is required to hve high storge cpcity nd fst dehydrogention kinetics. Among mny solid stte mterils for hydrogen storge, mgnesium hydride (MgH 2 ) comines hydrogen cpcity of 7.6 wt % with the enefit of the low cost of production nd undnce. The min difficulties for implementing MgH 2 re slow sorption/desorption kinetics nd high rectivity towrds ir nd oxygen, which re lso common issues in most lightweight metl hydrides. Previously, improvements in hydrogen storge nd relese properties hve een reported y using nnostructured mgnesium tht cn e otined through vrious friction methods including ll-milling, mechnicl lloying, nd vpor trnsport. In this study, we investigte the hydrogen sorption nd desorption properties of mgnesium nnotrees fricted y glncing ngle deposition (GLAD) technique, nd lso conventionl Mg thin films deposited t norml incidence. Mg nnotrees re out 15 µm long, 1 µm wide, nd incorporte nnoleves of out 2 nm in thickness nd 1,2 µm in lterl width. A qurtz crystl microlnce (QCM) gs sorption/desorption mesurement system hs een used for our hydrogen storge studies. Nnostructured nd thin film Mg hve een deposited directly on the surfce of the gold coted unpolished qurtz crystl smples. QCM hydrogen storge experiments hve een performed t tempertures rnging etween 1-3 o C, nd t H 2 pressures of 1 nd 3 rs. Our QCM mesurements reveled tht Mg nnotrees cn sor hydrogen t lower tempertures nd lso t fster rte compred to Mg thin film. In ddition, Mg nnotrees cn rech hydrogen storge vlues of out 4.8 wt% t 1 o C, nd up to out 6.71 wt% (which is close to the theoreticl mximum storge vlue of Mg) t tempertures lower thn 15 o C. The significnt enhncement in hydrogen sorption properties of our Mg nnotrees is elieved to originte from novel physicl properties of their nnoleves. These nnoleves re very thin (~2 nm) nd oth surfces re exposed to hydrogen enhncing the diffusion rte of hydrogen together with decresed diffusion length. Bsed on X-ry diffrction mesurements, individul nnoleves hve non-closepcked crystl plnes tht cn further enhnce the hydrogen sorption kinetics. In ddition, our nnostructured Mg hve een oserved to quite resistnt to surfce oxidtion, which is elieved to due to the single crystl property of the Mg nnoleves, which further improves the sorption kinetics of hydrogen. INTRODUCTION Incresing concerns out the dependence of world economy on current fossil sed energy sources in the world hve further stimulted reserch towrds environmentlly friendly nd sustinle energy technologies. Among vrious cndidtes, hydrogen hs een considered n idel element for storge, trnsport, nd

3 Pge 2 of 6 conversion of energy. Hydrogen storge in solid stte mterils hs een n intense re of reserch during the recent yers. Solid stte storge offers mny dvntges such s reduced gs pressure requirements llowing lower continer weights, sfe hndling nd reduced mteril costs. For solid stte storge, metl hydrides hve een promising mterils nd mong these, mgnesium dihydride (MgH 2 ) comines H 2 cpcity of 7.6 weight percentge (wt %) with the enefit of the low cost of production nd undnce. The min difficulties for direct usge of pure MgH 2 re slow sorption/desorption kinetics, high thermodynmic stility, nd high rectivity towrd ir nd oxygen, which re lso common issues in most lightweight metl hydrides. 1-2 There hs een intensive reserch on nnostructured mterils including mgnesium through vrious friction methods, especilly derivtives of ll-milling nd mechnicl lloying techniques. 3,4,5 Nnostructured mgnesium cn e eneficil in mny spects: It is proposed tht oth mgnesium nd mgnesium hydride ecome less stle with decresing cluster size nd thus the hydrogen desorption energy decreses significntly when the crystl grin size ecomes smller. Thus, nno-sized fetures cn positively ffect the hydrogen sorption nd desorption properties of metl hydride mterils. 1,2 Nnostructuring of mgnesium my lso further enhnce hydrogen sorption-desorption rtes y incresed surfce re of interction nd decresed diffusion lengths of hydrogen into Mg crystl. 1,2 As recently developed nnostructure deposition method, glncing ngle deposition (GLAD) hs ecome n effective method to produce rrys of 3D nnostructures, in the form of nnosprings, nnoeds nd nnorods. 6 During GLAD, the incident em of toms is sent t n high olique ngle (typiclly lrger thn 7 o ) s mesured from the surfce norml of rotting or sttionry sustrte under vcuum conditions. Due to geometricl shdowing effect, the incident vpor is preferentilly deposited onto higher surfce fetures, which re etter exposed to incident em. This preferentil growth dynmic gives rise to the formtion of well-seprted nnostructures. The dimension of these nnostructures typiclly rnges from tens to hundreds of nnometers, depending on the deposition conditions nd mteril studied. Moreover, GLAD technique hs een shown to e cple of producing nnostructured columnr films with controllle crystl orienttions tht cn e quite different from conventionl flt thin films otined from the norml incidence deposition. 7 For exmple, thermlly evported mgnesium using GLAD hs een oserved to produce nnolde structures with unique crystl properties. 8,9 On the other hnd, mesurement of hydrogen uptke, nd desorption rtes, s well s the precise mesurement of the storge rte of the nnostructured or thin film storge mteril is chllenging tsk. Volumetric methods such s Sievert s pprtus suffer from the need for reltively lrge mounts of smple, inevitle H 2 lekge under high pressures, nd the temperture vritions during ditic gs exchnge process. 1 Grvimetric methods require smple cell, nd lnce to e in direct contct, which is trde off from ccurcy. 11 As n lterntive method qurtz crystl microlnce (QCM) hs ttrcted nd found usge in hydrogen storge reserch s n effective mesurement method 12 due to its very high mss sensitivity (~1 ng/cm2). Thus, QCM my provide

4 Pge 3 of 6 very ccurte mss determintion of hydrogen uptke in nnostructured or thin film storge mterils with miniml smple mount nd reltively low cost. In this study we hve produced mgnesium nnotrees with nnoleves on gold coted rough QCM crystls nd exmined their hydrogen storge rtios under vrying tempertures nd pressures. EXPERIMENT In our experiments, sets of Mg nnotrees nd thin films were deposited onto unpolished, gold coted QCM crystls (AT cut, 6 MHz) using custom designed vcuum deposition unit tht is cple of oth therml evportion GLAD. During deposition, the se pressure ws 7.* 1-7 mr. Mg ws evported using n Alumin crucile inside tungsten sket. Thin films were deposited t n ngle of θ = nd nnotrees were deposited t deposition ngle of θ = 85 (s mesured from the sustrte surfce norml). In ddition, oth types of depositions were ccompnied with n zimuthl sustrte rottion round the centrl sustrte surfce norml xis with speed of 1 RPM. HYDROGEN STORAGE EXPERIMENTS USING QCM SYSTEM We developed custom mde QCM system for the investigtion of sorption, desorption, nd mximum storge of hydrogen in nnostructured nd thin film coting mterils. The chmer is cple of housing two QCM smple holders which cn e used for two different smples t time. It is cple of pressures from high vcuum to 5 r. The heting is supplied vi temperture controlled heting jcket plced inside n outer chmer which lso supplies temperture isoltion nd stility. Temperture of the system cn e set from room temperture to up to out 5 o C. The Hydrogen storge mesurement procedure: Since QCM method is extremely sensitive to environmentl conditions such s temperture, pressure, nd surfce roughness, we needed to identify numer of test procedures for the hydrogen storge chrcteriztion of our Mg smples. First method utilized ws using single QCM crystl (Procedure-1). In this method we used single crystl to tke the se frequency mesurements of the QCM crystl efore the storge experiment under exct conditions to e used for the storge process. Then Mg nnostructures were deposited onto the crystl followed y the QCM storge process. By mesuring the chnge in the QCM frequency response, we clculted the hydrogen storge rtes. Second method (Procedure-2) utilized two QCM crystls t once, one eing the reference smple. Here reference holder contined re crystl, where ctul smple ws plced t the second. Using this dul-smple procedure we clculted the ctul storge rtes y normlizing the frequencies ccording to the reference signls. RESULTS AND DISCUSSION Cross-section nd top view scnning electron microscopy (SEM) imges of Mg nnotrees with nnoleves grown on qurtz crystl sustrtes re shown in Figure 2.

Pge 4 of 6 Figure 2: Cross-sectionl view SEM imges of Mg nnotrees with nnoldes fricted y GLAD evportion on QCM

Figure 3: Top-view SEM imges of Mg nnotrees with nnoldes fricted y GLAD evportion on QCM sustrte.

However, nnotrees re composed of thinner, stcked formtions tht hd individul nnolef thicknesses of less thn 2 nm

The conventionl Mg thin films deposited t norml incidence were out 1 µm in thickness (not shown).

5 Pge 4 of 6 Figure 2: Cross-sectionl view SEM imges of Mg nnotrees with nnoldes fricted y GLAD evportion on QCM sustrte. The scle rs re 1 µm. Figure 3: Top-view SEM imges of Mg nnotrees with nnoldes fricted y GLAD evportion on QCM sustrte. The scle r is 1 nm on the left nd 1 µm on the right. The length of the Mg nnotrees is mesured to e out 7 µm. However, nnotrees re composed of thinner, stcked formtions tht hd individul nnolef thicknesses of less thn 2 nm on the verge. (Figure 3). The conventionl Mg thin films deposited t norml incidence were out 1 µm in thickness (not shown). Percentge chnge in the mss (% wt) % wt 15 2 Figure 3: Results of QCM hydrogen sorption nd desorption mesurements for Mg thin film t T=1 o C (), nd sorption mesurement t T=3 o C () re shown. The mesurements re mde in QCM set-up t hydrogen pressure P = 3 rs during sorption nd t 1-2 rs of vcuum during desorption.

6 Pge 5 of 6 We hve performed storge experiments ccording to the procedure-1 descried ove for thin films nd nnostructured smples of Mg t T=1 C temperture nd P=3 rs of pressure in our QCM system. The results shown in Fig. 3 indicte the mximum storge cpcity chieved for thin film smples ws ~.35 wt%. The storge cpcity is mesured upon the stiliztion of QCM oscilltion frequency within 1 Hz in time frme of 5 minutes. On the other hnd, under the sme pressure nd temperture conditions, Mg nnotrees hve een oserved to store up to out ~4.8 wt% of hydrogen (Fig. 4), which is significntly higher thn tht of thin film of Mg. In ddition, we hve performed 3-15 C vrile temperture storge experiment for the Mg nnotrees using procedure-1. As shown in Fig. 4, the storge performnce of nnotrees ws improved drsticlly nd storge vlue s high s out 6.71 wt% ws reched within short time of 1 seconds (~17 minutes). Percentge chnge in the mss (% wt) Vcuum Hydrogen introduction t 3 r ~ 4.8 % Heting Temperture ( o C) Figure 4: Results of hydrogen sorption for Mg nnotreess t T=1 o C () nd sorption nd desorption curve for t T=15 o C () smples re shown.. The mesurements re mde in QCM set-up t sorption pressure P= 3 rs nd desorption t 1-2 rs For Mg nnotree smples, we lso hve further nlyzed the 6 C -1 C intervl of QCM results shown ove this time using the procedure-2 descried ove under P=3 rs of hydrogen. Initilly, the chmer ws kept t strting temperture of 6 C/ Then H 2 ws pumped in t 3 r nd system ws heted up to 1 C. At this temperture we wited till the QCM oscilltion stilized, nd then the chmer ws cooled down to 6 C where the pressure ws lso reduced to 1 r. The comprison of initil nd finl mss vlues show storge vlue of 4.6 wt%. This is consistent with our result shown in Fig. 4, where 4.8 wt% storge ws mesured t 3 r hydrogen pressure nd t T = 1 C using procedure-1. As second step, the sme nnostructured smple, which lredy hd 4.6 wt% hydrogen, ws tken trough similr process ut this time strting from 6 o C up to 3 o C, under P= 3 rs of hydrogen. During this dditionl tretment, the mss incresed n dditionl 2.75 wt% tht mde the finl totl storge vlue s 7.25 wt%. (Figures 5 nd ). This totl storge vlue is close to the ~7.6 wt% theoreticl mximum hydrogen cpcity of Mg. We elieve tht the enhnced storge chrcteristics of mgnesium nnotrees re cused y the extremely thin mteril structure oserved in Mg ldes, thus decresed cluster size nd diffusion length. Other fctors such s crystl orienttion nd reduced oxidtion of nnotrees might lso hve helped for ese of diffusion long the mteril.

7 Pge 6 of 6 Percentge chnge in the mss (wt%) 4.6% Figure 5: Results of hydrogen sorption for Mg nnoldes t T=1 o C t the first cycle () nd sorption fter cooling down to RT nd heting up to T=3 o C () smples re shown. The mesurements re mde in QCM set-up t sorption pressure P= 3 rs. CONCLUSION We hve utilized glncing ngle deposition (GLAD) technique for the growth of nnostructured mgnesium rrys in the shpe of verticl nnotrees with nnoleves with thicknesses elow 2 nm. Hydrogen sorption chrcteristics of these nnotrees were studied using new custom developed qurtz crystl microlnce (QCM) system with optimized procedures for H 2 storge. The QCM results were lso compred to those of conventionl Mg thin films. We hve oserved superior hydrogen storge chrcteristics of Mg nnotrees such s significnt enhncement in low temperture H 2 storge vlues (e.g. ~4.8 wt% t 1 o C, nd ~6.71 wt% elow 15 o C) nd compred to Mg thin films (e.g. ~.35 1 o C). 1 A. Zlusk, L. Zluski nd J. O. Ström Olsen, Nnocrystlline mgnesium for hydrogen storge, J. of Alloys nd Compounds 288, 217 (1999). 2 P. K. Prnzs, M. Dornheim, D. Bellmnn, K.-F. Aguey-Zinsou, T. Klssen, nd A. Schreyer, SANS/USANS investigtions of nnocrystlline MgH 2 for reversile storge of hydrogen, Physic B 385, 63 (26). 3 A. Zlusk, L. Zluski nd J. O. Ström-Olsen, Synergy of hydrogen sorption in ll-milled hydrides of Mg nd Mg 2 Ni, J. of Alloys nd Compounds 289, 197 (1999). 4 J. Huot, G. Ling, S. Boily, A. Vn Neste nd R. Schulz, Structurl study nd hydrogen sorption kinetics of ll-milled mgnesium hydride, J. of Alloys nd Compounds , 495 (1999). 5 M. Au, Hydrogen storge properties of mgnesium sed nnostructured composite mterils, Mt. Scie. nd Eng. B 117, Issue 1, 37 (25). 6 T. Krck, G.-C. Wng, nd T.-M. Lu, Physicl self-ssemly nd the nucletion of 3D nnostructures y olique ngle deposition, J. Vc. Sci. Technol. A 22, 1778 (24). 7 F. Tng, T. Krck, P. Morrow, C. Gire, G.-C. Wng nd T.-M. Lu, Texture of Ru columns grown y olique ngle sputter deposition, Phys. Rev. B 72, (25). 11 T. Krck, A. Mllikrjunn, J.P. Singh, D.-X. Ye, G.-C. Wng, nd T.-M. Lu, β-phse W nnorod formtion y olique-ngle sputter deposition, Appl. Phys. Lett. 83, 396 (23). 8 F. Tng, T. Prker, H.F. Li, G.C. Wng, T.M. Lu, J. Nnosci. Nnotechnol. 7 (27) Y.P. He, Y.P. Zho, J.S.Wu, Appl. Phys. Lett. 92 (28) Ihor Kulchytskyy, Mrtin G. Kocnd, nd To Xu, Applied Physics Letters 91, _27_ 11 6X. S. Li, H. W. Xu, C. L. Xu, Z. Q. Mo, nd D. H. Wu, Int. J. Hydrogen Energy 28, 1251 _23_. E. Henneerg, B. Bernhrdt, nd K. Bohmhmmel, Thermochim. Act 415, 43 _24_. 12 D. A. Buttry nd M. D. Wrd, Chem. Rev. _Wshington, D.C._ 92, 1355 _1992_. J. Arik, A. Aidl, nd K. Kukli, Appl. Surf. Sci. 75, 18 _1994,.J. W. Elm nd M. J. Pellin, Anl. Chem. 77, 3531 _25_. Percentge chnge in the mss (wt%) %

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