Enhanced hydrogen uptake/release in 2LiH–MgB 2 composite with titanium additives

Enhanced hydrogen uptake/release in 2LiH–MgB 2 composite with titanium additives

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  Enhanced hydrogen uptake/release in 2LiH e MgB 2  compositewith titanium additives Ivan Saldan a,b, *, Renato Campesi c , Olena Zavorotynska d , Giuseppe Spoto d ,Marcello Baricco d , Anna Arendarska a , Klaus Taube a , Martin Dornheim a a Institute of Materials Research, Helmholtz Zentrum Geesthacht, 1 Max-Planck Str., D-21502 Geesthacht, Germany b Institute for Energy Technology, Physics Department, Instituttveien 18, Kjeller 2027, Norway c Institute for Energy, Joint Research Centre, NL-1755 ZG Petten, The Netherlands d NIS Centre of Excellence, Department of Chemistry, University of Torino, Via P. Giuria 7, I-10125 Torino, Italy a r t i c l e i n f o Article history: Received 12 August 2011Received in revised form13 October 2011Accepted 16 October 2011Available online 9 November 2011 Keywords: Hydrogen uptake/release reaction2LiH e MgB 2  compositeTitanium additive a b s t r a c t The influence of different titanium additives on hydrogen sorption in LiH e MgB 2  systemhas been investigated. For all the composites LiH e MgB 2 e X  ( X  ¼  TiF 4 , TiO 2 , TiN, and TiC),prepared by ball-milling in molar ratios 2:1:0.1, five hydrogen uptake/release cycles wereperformed. In-situ synchrotron radiation powder X-ray diffraction (SR-PXD) and attenu-ated total reflection infrared spectroscopy (ATR-IR) have been used to characterize crystalphases developed during the hydrogen absorption e desorption cycles.All the composites with the titanium additives displayed an improvement of reactionkinetics, especially during hydrogen desorption. The LiH e MgB 2 e TiO 2  system reacheda storage of about 7.6 wt % H 2  in w 1.8 h for absorption and w 2.7 h for desorption. Using in-situ SR-PXD measurements, magnesium was detected as an intermediate phase during hydrogen desorption for all composites. In the composite with TiF 4  addition the formationof new phases (TiB 2  and LiF) were observed. Characteristic diffraction peaks of TiO 2 , TiNand TiC additives were always present during hydrogen absorption e desorption. For all as-milled composites, ATR-IR spectra did not show any signals for borohydrides, while for allhydrogenated composites B e H stretching (2450 e 2150 cm  1 ) and B e H bending (1350 e 1000 cm  1 ) bands were exactly the same as for commercial LiBH 4 .Copyright  ª  2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rightsreserved. 1. Introduction Hydrogen can be one of the alternative energy carriers, whichshould replace the traditional fossil fuels in the near future.One of the promising materials for hydrogen mobile applica-tion which has been studied approximately for 10 years isLiBH 4  [1]. Having high gravimetric and volumetric hydrogendensity, this material, though, exhibits unfavorable kineticsand thermodynamics for real application in fuel cells.Recently, it was found that LiBH 4  can be destabilized by theaddition of MgH 2 , showing better decomposition kinetics withrespect to the pure compound [2]. A detailed analysis of thereversible interaction between LiBH 4  and MgH 2  was made in[3] and can be summarized as follow:2LiBH 4 þ MgH 2 4 2LiBH 4 þ Mg  þ H 2 4 2LiH þ MgB 2 þ 4H 2 (1) *  Corresponding author.  Institute for Energy Technology, Physics Department, IFE, Instituttveien 18, Kjeller 2027, Norway. Tel.: þ 47 63806079; fax:  þ 47 6381 0920.E-mail address: ivan_saldan@yahoo.com (I. Saldan).  Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 37 (2012) 1604 e 1612 0360-3199/$  e  see front matter Copyright  ª  2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2011.10.047  The direct reactions (1) take place at  w 400   C. Oppositereactions, with simultaneous formation of LiBH 4  and MgH 2 under 50 bar of H 2 , was confirmed at the temperatures250 e 300   C [3]. It was observed that suitable additives mightdecreasereactiontemperaturesandimprovekineticsofEq.(1).Experimental evidence of kinetic improvement for reversiblemiddle-temperature Na, Li and Al based complex hydridesdoped by titanium additives appeared in 1997 [4]. It has also been reported that the kinetic improvement of the reaction (1)was reached by addition of 1 mol% of TiF 3  [5]. The propertyenhancement arising upon this additive persists well in thesubsequent hydrogen uptake/release cycles. Another prom-inent example of the additives effect was the compositeLiBH 4 e MgH 2 e Ti{OCH(CH 3 ) 2 } 4  mixed in molar ratio 2:1:0.1 [6].After ball-milling TiO 2  anatase was found and during 1-sthydrogen desorption Ti 2 O 3  and TiB 2  appeared to be stableaftercycling.XPSanalysisshowedthatthereductionofTi(IV)toTi(III) wascoupled withthemigrationoftitaniumspecies fromthesurfaceintothebulkofthecomposite.Theroleofadditivesand microstructure refinement in LiBH 4 e MgH 2  system werestudied in [7,8], revealing that two main factors, proposed as potentialdrivingforceforkineticimprovement,wererelatedto(i) favoring heterogeneous nucleation of MgB 2  and (ii)increasing of interfacial area trough grain refinement. Tita-nium diboride (TiB 2 ) has the same hexagonal lattice structureas MgB 2  with very small (1.85%) directional and interplanarmisfit. This fact is a necessary condition for heterogeneousnucleation of MgB 2  because of the interfacial energy lowering.The appropriate concentration of the additive and its homo-geneous distribution were found to be the main conditions forthe efficient heterogeneous nucleation of MgB 2 . Howeverbecause of no change of the limiting rate neither for hydrogenabsorption (contracting volume model) nor for desorption(interfaced-controlledone-dimensionalgrowth)[8],inducedbythe additives, the latter do not show catalytic behavior. Thetheoreticalwork[9]hasshownthatthermodynamicstabilityof point defects in complex hydrides defines the ground or inter-mediate states or the driving force for atomic motion. Incor-poration of Ti cations in LiBH 4  is energetically unfavorablesuggesting that only surface effect takes place.In order to understand thoroughly the effect of titaniumadditives, where metallic part is Ti and non-metal is theelementof2-ndperiodofthePeriodicTablefromFtoC,wehavestartedasystematicinvestigation.Inthisworkthestudyontheinfluenceofseveraltitaniumadditives(TiF 4 ,TiO 2 ,TiNandTiC)on reversible hydrogen reactions in 2LiH e MgB 2  system during five hydrogen uptake/release cycles is presented. 2. Experimental details Commercial LiH (  95%, Sigma Aldrich) and MgB 2  ( > 96%, AlfaAesar)powderswereusedtopreparecompositewithtitaniumadditives in molar ratios 2:1:0.1, respectively. TiF 4  (98%, AlfaAesar), TiO 2  (rutile, 99.7%, Sigma Aldrich), TiN (97.7%, AlfaAesar) and TiC (99.5%, Alfa Aesar) were chosen as additives.The composites of powders were high-energy milled for 5 husing Spex 8000 M Mixer Mill in argon atmosphere. Stainlesssteel balls 10 mm in size with 10:1 ratio balls to powders wereused.Hydrogen sorption measurements were carried out ina commercial Sievert’s type apparatus (PCTpro 2000). Themilled composites were hydrogenated under 50 bar of hydrogen pressure at 330 or 350   C in a special high pressure e temperature sample holder. Hydrogen desorption was per-formed under 5 bar back pressure of hydrogen at 380   C, afterprevious absorption. Five complete hydrogen uptake/releasecycles were performed.In-situ SR-PXD was performed in D3 beamline at DESYHamburg (Germany). The samples were airtight encapsulatedin sapphire capillaries to be installed in a special in-situ SR-PXD cell; further details are described in [10]. Samples after complete 1-st hydrogen absorption were heated at 5   C/minfrom room temperature up to 380   C and kept in isothermalconditionsfor2hand thencooleddowntoroomtemperature.All handling and preparation of materials took place ina glove-box with continuously purified argon atmosphere andoxygen and moisture values were less than 1 ppm.ATR-IR (Attenuated total reflection infrared) spectra weretaken with a Bru ¨ ker-ALPHA Platinum spectrometer with ATRdiamond crystal accessory. The spectra were recorded in4000 e 375 cm  1 range with 2 cm  1 resolution. Sixty four scanswere averaged for background and sample spectra. All themeasurements were carried out in the nitrogen filled glove-box with oxygen and moisture levels less that 0.1 ppm. 3. Results and discussion 3.1. Hydrogen uptake/release cycling As a reference, a complete hydrogen uptake/release cycle forthe LiH e MgB 2  system without any additive has been per-formed. In Fig. 1, the results of volumetric analysis of hydrogenabsorption at 350   C and 50 bar H 2  and desorption at380   C and 5 bar H 2  for LiH e MgB 2  in molar ratio 2:1 arepresented.The reaction rate of hydrogen absorption and desorption isapproximately six times different:  w 20 h are required for 020406080100120-10-8-6-4-20246810    H  y   d  r  o  g  e  n  s  o  r  p   t   i  o  n   (  w   t .   %    H    2    ) Time (h)  absdes Fig. 1 e Hydrogen sorption for LiH e MgB 2  in molar ratio 2:1.Conditions for absorption and desorption were 350   C at 50 bar H 2  and 380   C at 5 bar H 2 , respectively. international journal of hydrogen energy 37 (2012) 1604 e 1612  1605  hydrogenation( w 8.7wt%H 2 )andmorethan120hforcompletedehydrogenation. A two-step hydrogen release was observed,displaying:approximately w 2.3wt%H 2 fromMgH 2 and w 6.4wt% H 2  from LiBH 4  decomposition. Absorption curve is verysimilartothatobtainedin[11]atthesameconditions.Probably,due to a slightly smaller desorption temperature (400   C in [11]and 380   C in present work) the process in our case was muchslower although it showed the same two-step reaction.The 1-st hydrogen absorption e desorption cycle in theLiH e MgB 2 e TiF 4  system (Fig. 2) showed similar hydrogenabsorption time ( w 20 h) but lower gravimetric capacity( w 7.5 wt% H 2 ) compared to that of the unmodified LiH e MgB 2 (Fig. 1). Because of slower reactions at the 1-st cycle (1-sthydrogen absorption and 1-st desorption), an activationprocess could take place. It might be explained by grainrefinement in solid material under repeating of hydrogensorption reactions. After that, the system displays three-stepreversible reaction ( w 2.3 wt % H 2  in w 0.7 h; w 6.6 wt% H 2  in w 2.2 h; and further to be complete) and ( w 2.3 wt% H 2  in w 0.6 h; w 7.4 wt% H 2  in w 5.4 h; and further to be complete) onhydrogen absorption and desorption, respectively. It can beconcluded that the rate of hydrogen absorption and desorp-tion at various steps is increased because of the addition of TiF 4  to the LiH e MgB 2  system, though hydrogen storagecapacity was lowered by 1.2 wt% H 2 .For the LiH e MgB 2 e TiO 2  system, at least two cycles werenecessary to stabilize hydrogen absorption e desorptionproperties (Fig. 3). After the 2-nd cycle, the system showed w 8.1 wt% H 2  hydrogen storage capacity after  w 10 h of hydrogenation. Beginning from 3-rd cycle two reaction stepswere very well distinguished and resulted to w 7.6 wt% H 2  in w 1.8 h and 2.7 h for hydrogen absorption and desorption,respectively. In this case, rates for hydrogen absorption anddesorption are rather similar. During the 1-st cycle,LiH e MgB 2 e TiO 2  showed the same value of hydrogen gravi-metric density as the unmodified LiH e MgB 2  but with fasterkinetics.The results of hydrogenation/dehydrogenation reactionsintheLiH e MgB 2 e TiNsystemareshowninFig.4.Afterthe1-stcycle, the process is stable and requires w 20 h to reach themaximum gravimetric capacity of  w 8.0 wt% H 2 . The hydro-genation curve does not show distinctive steps, howeverduring the hydrogen desorption three separate steps areclearly visible. The 4-th desorption cycle displays only twosteps ( w 2.6 wt% H 2  in w 0.5 h; w 7.9 wt% H 2  in w 9.2 h); and theprocess is not completed. The LiH e MgB 2 e TiN system, indeed,showed 12 times faster desorption rate than the unmodifiedLiH e MgB 2 , though hydrogen storage capacity was reduced by w 0.7 wt% H 2 .For the LiH e MgB 2 e TiC system at least four cycles wereneeded in order to have a stable hydrogen uptake/releasereaction. Hydrogen storage capacity of   w 7.0 wt% H 2  within w 20hwasachievedatthe5-thcycle(Fig.5).Onlyforhydrogendesorptionitwaspossibletodistinguishthefirststep( w 2.0wt 048121620240246810  1 abs 2 abs 3 abs 4 abs 5 abs    A   b  s  o  r  p   t   i  o  n   (   H  w   t   %   ) 050100150200250300350400  Temperature ( o C)    T  e  m  p  e  r  a   t  u  r  e   (  o   C   ) a 04812162024-10-8-6-4-20  1 des 2 des 3 des 4 des 5 des    D  e  s  o  r  p   t   i  o  n   (   H  w   t   %   ) Time (h) 050100150200250300350400 b  Temperature ( o C)    T  e  m  p  e  r  a   t  u  r  e   (  o   C   ) Fig. 2  e  Hydrogen sorption for LiH e MgB 2 e TiF 4   in molarratio 2:1:0.1 during 5 cycles. Absorption (  a  ) at 350   C and50 bar H 2 ; desorption (  b  ) at 380   C and 5 bar H 2 . 048121620240246810    A   b  s  o  r  p   t   i  o  n   (   H  w   t   %   )  1 abs 2 abs 3 abs 4 abs 5 abs a 050100150200250300350400  Temperature (C)    T  e  m  p  e  r  a   t  u  r  e   (  o   C   ) 04812162024-10-8-6-4-20    D  e  s  o  r  p   t   i  o  n   (   H  w   t   %   ) Time (h)  1 des 2 des 3 des 4 des 5 des 050100150200250300350400  Temperature (C)    T  e  m  p  e  r  a   t  u  r  e   (  o   C   ) b Fig. 3  e  Hydrogen sorption for LiH e MgB 2 e TiO 2 (rutile) inmolar ratio 2:1:0.1 during 5 cycles. Absorption (  a  ) at 330   Cand 50 bar H 2 ; desorption (  b  ) at 380   C and 5 bar H 2 . international journal of hydrogen energy 37 (2012) 1604 e 1612 1606  % H 2  in w 0.5 h; and further to be complete). We conclude that,among all presented system with titanium additives, theLiH e MgB 2 e TiC one showed the major reduction in hydrogenstoragecapacity( w 1.7wt%H 2 ),thoughmuchhigherhydrogendesorption rates were observed with respect to the unmodi-fied LiH e MgB 2  system.In conclusion, all titanium additives in the LiH e MgB 2 e X systems ( X  ¼  TiF 4 , TiO 2 , TiN and TiC) demonstrated kineticimprovement, especially during hydrogen desorption. Thesystem with the TiO 2  additive showed faster rates for bothhydrogen uptake and release, together with the lowestdecrease in hydrogen storage capacity ( w 0.6 wt% H 2 ). Oneimportantnotethatshouldbementionedinthissubtitleisthevalue of hydrogen absorption and desorption capacity. Theo-retically, during hydrogen uptake/release cycling the value of hydrogen storage capacity must be exactly the same as undercomplete absorption or desorption, of course if the system isstable. In present results (Figs. 1 e 5) quite often small uncer-tainties were present where hydrogen capacity underdesorptionwas higher than that under absorption. It might beexplained by a shorter incubation period before the systemstart to react, especially in the next cycles. It is suggested thatduring manual switching from dehydrogenation to hydroge-nation some amount of hydrogen could be absorbed whendata acquisition had not been active yet. 3.2. In-situ SR-PXD for LiH e MgB 2 e X  (  X  ¼  TiF 4 , TiO 2 ,TiN, TiC) systems For all hydrogenated LiH e MgB 2 e X  ( X  ¼  TiF 4 , TiO 2 , TiN, TiC)systemsin-situSR-PXDanalysiswasperformed(Fig.6).Tracesof MgB 2  were found in all the samples after 1-st hydrogenabsorption, suggesting that the hydrogenation process wasnot completed. At the beginning of first hydrogen desorptionreaction, pure magnesium phase was clearly visible, con-firming the occurrence of a two-step reaction (1).In the LiH e MgB 2 e TiF 4  system (Fig. 6 a ), after the 1-sthydrogen absorption step, the expected products (LiBH 4  andMgH 2 ) were present as the main phases. In addition, newphases TiB 2  and LiF were detected in the sample, whereasthere was no evidence of present TiF 4 . Most probably, LiFformed during milling (similar to TiF 3  in [5]) by the following reaction:4LiH þ TiF 4 / 4LiF þ TiH 2 þ H 2  (2)And TiH 2  can easily react at higher temperatures withLiBH 4  to produce TiB 2  (estimated reaction enthalpy is w 6.5 kJ/mol H 2 ):2LiBH 4 þ TiH 2 / 2LiH þ TiB 2 þ 4H 2  (3) 048121620240246810  1 abs 2 abs 3 abs 4 abs    A   b  s  o  r  p   t   i  o  n   (   H  w   t   %   ) 050100150200250300350  Temperature ( o C)    T  e  m  p  e  r  a   t  u  r  e   (  o   C   ) a 04812162024-10-8-6-4-20  1 des 2 des 3 des 4 des    D  e  s  o  r  p   t   i  o  n   (   H  w   t   %   ) Time (h)    T  e  m  p  e  r  a   t  u  r  e   (  o   C   ) 050100150200250300350400 b  Temperature ( o C) Fig. 4  e  Hydrogen sorption for LiH e MgB 2 e TiN in molarratio 2:1:0.1 during 5 cycles. Absorption (  a  ) at 330   C and50 bar H 2 ; desorption (  b  ) at 380   C and 5 bar H 2 . 048121620240246810    A   b  s  o  r  p   t   i  o  n   (   H  w   t   %   )  1 abs 2 abs 3 abs 4 abs 5 abs    T  e  m  p  e  r  a   t  u  r  e   (  o   C   ) 050100150200250300350  Temperature ( o C) a 04812162024283236-10-8-6-4-20 b    T  e  m  p  e  r  a   t  u  r  e   (  o   C   )   D  e  s  o  r  p   t   i  o  n   (   H  w   t   %   ) Time (h)  1 des 2 des 3 des 4 des 5 des 050100150200250300350400  Temperature ( o C) Fig. 5  e  Hydrogen sorption for LiH e MgB 2 e TiC in molarratio 2:1:0.1 during 5 cycles. Absorption (  a  ) at 330   C and50 bar H 2 ; desorption (  b  ) at 380   C and 5 bar H 2 . international journal of hydrogen energy 37 (2012) 1604 e 1612  1607  Upon heating the hydrogenated LiH e MgB 2 e TiF 4  compositestarted hydrogen desorption through MgH 2  decompositionand Mg formation just before isothermal conditions. During the heat treatment two diffraction peaks characteristic of a cubic phase appeared. These peaks were quite broad, likelydue to the superposition of reflections due to LiH and LiFphases. Upon cooling, the peaks of o-LiBH 4  reappeared. Itshould be denoted that, after the isothermal treatment of 2 hat 380   C, no diffraction peaks due to MgH 2  or Mg phases weredetected, suggesting that after this period the first step of thereaction (1) was completed.The SR-PXD pattern of the LiH e MgB 2 e TiO 2  system (Fig. 6 b )after 1-st hydrogen absorption showed diffraction peaks dueto the products of the reversible reaction (1) (LiBH 4  and MgH 2 ),together with a small amount of residual MgB 2 . Most of TiO 2 peaks were present during whole SR-PXD experiment. Itmeans that this additive might be chemically inert toward thereagents. During heating of the hydrogenated mixture,hydrogen desorption started just before 380   C, as it wasalready observed for the previous composite, showing puremagnesium as intermediate. After cooling, no diffractionpeaks related to Mg or MgH 2  phases were observed.The pattern of the LiH e MgB 2 e TiN system (Fig. 6 c ) showeddiffraction peaks dueto the productsof reactions(1), similarlytothepreviouscases,togetherwithevidenceoftheparentTiNphase. The additive was present during the whole SR-PXDmeasurement, confirming that no chemical reactionsbetween the additive and the hydrides took place. Overall, thebehavior of LiH e MgB 2 e TiN sample under heating and cooling was similar to that observed for LiH e MgB 2 e TiO 2  mixture.The LiH e MgB 2 e TiC system (Fig. 6 d  ) also showed theproducts of hydrogen absorption e desorption and unreactedtitanium-based additive, similarly to the case of LiH e MgB 2 e TiO 2  and LiH e MgB 2 e TiN mixtures. Four diffrac-tion peaks of TiC phase, indeed was observed SR-PXDpatterns, confirming that the additive does not react withthe hydrides.Based on SR-PXD observations, it can be concluded thatonly TiF 4  additive was chemically reacting in the compositeduring ball-milling, so that the new LiF and TiB 2  phases wereformed. 3.3. ATR-IR spectroscopy of milled and hydrogenatedLiH e MgB 2 e X  (  X  ¼  TiF 4 , TiO 2 , TiN, TiC) systems Infrared spectroscopy is a suitable tool for characterization of metal borohydrides since the molecular vibrations of [BH 4 ]  group are readily distinguishable in the spectrum. Further-more, the normal modes of [BH 4 ]  group are very sensitive tothe surrounding so that the alterations in a borohydridechemical composition, lattice symmetry, and the bond typecan be identified in the spectrum (see reference [12] andreferences therein).Free [BH 4 ]  species belong to  T d  symmetrypoint group with four normal modes of vibration:,  ~ v 2 ,  ~ v 3 , and ~ v 4 , out of which the latter two triply degenerate modes are IR- Fig. 6  e  In-situ SR-PXD under 5 bar H 2  for LiH e MgB 2 e Ti  X (   X [ TiF 4   (  a  ); TiO 2  (  b  ); TiN (  c  ); TiC (  d  )) in molar ratio 2:1:0.1composites after complete 1-st hydrogen absorption. international journal of hydrogen energy 37 (2012) 1604 e 1612 1608
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