effect of graphite as a co-dopant on the dehydrogenation and hydrogenation kinetics of ti-doped...

Journal of Alloys and Compounds 395 (2005) 252–262

Effect of graphite as a co-dopant on the dehydrogenation andhydrogenation kinetics of Ti-doped sodium aluminum hydride

Jun Wanga, Armin D. Ebnera, Tanya Prozorova, Ragaiy Zidanb, James A. Rittera,∗a Department of Chemical Engineering, Swearingen Engineering Center, University of South Carolina, Columbia, SC 29208, USA

b Savannah River National Laboratory, Aiken, SC 29804, USA

Received 18 October 2004; accepted 8 November 2004Available online 7 January 2005

Abstract

The synergistic effect of graphite as a co-dopant on the dehydrogenation and hydrogenation kinetics of Ti-doped NaAlH4 has beenobserved for the first time. According to temperature programmed desorption curves obtained at 2◦C/min, the dehydrogenation temperaturein the 90–150◦C range decreased by as much as 15◦C for NaAlH4 co-doped with 10 wt.% graphite (G) and up to 4 mol% TiCl3 comparedto similarly doped and ball milled samples without graphite. Constant temperature desorption curves at 90 and 110◦C obtained for NaAlHc .0 timest The effectso as addedtA rms of someo ubricationp bide species,b ence mored lectronicc s somee th graphiteaP

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o-doped with 2 mol% TiCl3 and 10 wt.% G also, respectively, revealed improvements in the dehydrogenation kinetics of 6.5 and 3hat of a similarly prepared sample without graphite. In contrast, graphite as a single dopant was essentially inactive as a catalyst.f graphite persisted through dehydrogenation/hydrogenation cycling, and through the addition of aluminum (Al) powder, which w

o mitigate irreversible kinetic and capacity losses during cycling. A sample of NaAlH4 co-doped with 2.0 mol% TiCl3, 10 wt.% G and 5 wt.%l exhibited perhaps the best dehydrogenation and hydrogenation rates to date. The observed phenomena were interpreted in tef the unique properties of graphite: graphite might be playing a dual role by serving as a mixing agent manifested through lhenomena (i.e., graphene layer slippage and breakage), and as a micro-grinding agent manifested through the formation of caroth during high energy ball milling. In these capacities, graphite may have caused the Ti particles to be more finely ground and hispersed over the surfaces of the NaAlH4 particles and also the graphite particles themselves. Graphite might also be imparting an eontribution through the interaction of its facile�-electrons with Ti through a hydrogen spillover mechanism, whereby it back donatelectrons to Ti, which further facilitates hydrogen bond formation and cleavage through this Ti species. Research is continuing wis a co-dopant.ublished by Elsevier B.V.

eywords: Hydrogen storage; Complex hydrides; Sodium aluminum hydride; Titanium chloride; Graphite; Aluminum; Hydrogen spillover; Lubicro-grinding; SEM; XPS

. Introduction

Ti-doped NaAlH4 [1–44], and to a lesser extent, otheretal-doped complex hydrides[45–63], are being studiedith earnest to determine their potential as a new class of re-ersible hydrogen storage materials. To date, however, onlyhe metal-catalyzed NaAlH4 system has been shown to beeversible, with Ti-doped NaAlH4 providing around 3 wt.%ydrogen at around 100◦C [16,32]. Although this reversible

∗ Corresponding author. Tel.: +1 803 777 3590; fax: +1 803 777 8265.E-mail address:[email protected] (J.A. Ritter).

hydrogen storage capacity is too low for most transportaapplications, there are at least two key reasons to continexplore metal-doped NaAlH4. Firstly, since NaAlH4 is con-sidered to be a model system with mechanistic undersing still being sought, it is felt that once this knowledgeacquired, it can be used to foster the development and ptial reversibility of other, higher hydrogen capacity, comphydrides. Secondly, the fact that Ti-doped NaAlH4 exhibitsabout 3 wt.% reversible hydrogen storage at around 10◦Cmakes it one of the best hydrogen storage materials k(even better than most metal hydrides), with stationary acations clearly imminent. Hence, any improvement in the

925-8388/$ – see front matter Published by Elsevier B.V.oi:10.1016/j.jallcom.2004.11.053

J. Wang et al. / Journal of Alloys and Compounds 395 (2005) 252–262 253

hydrogenation (discharge) kinetics and more importantly inthe hydrogenation (charge) kinetics of metal-doped NaAlH4would move this hydrogen storage material that much closerto commercial viability, because an improvement in kineticsusually translates into a lower reversible operating tempera-ture, with reasonable charge rates at 80◦C still being sought,but not attainable yet with this system.

In recent studies, the dehydrogenation and hydrogenationkinetics of the metal-doped NaAlH4 system have been en-hanced by using high energy ball milling[2,3,8,10,12,14],by co-doping with other transition metals (such as Ti andFe [5,43]), and by using novel forms of Ti (such as Ti nan-oclusters[18,21,40]). The net result of these efforts towardcommercial feasibility of NaAlH4 is a typical discharge rateof 3 wt.% hydrogen in 60 min at 125◦C and 1 atm, and a typi-cal charge rate of 3.0 wt.% hydrogen in 25 min at 125◦C and84 atm using, for example, 2 mol% Ti as the catalyst[11].Although these rates have been shown to increase with tem-perature[11], the goal has always been to achieve higher ratesat lower temperatures, like shown recently with the Ti–Fe co-doped NaAlH4 system[43]. With this in mind, the objectiveof this study is to show that graphite, as an inexpensive ad-ditive, significantly enhances both the dehydrogenation andthe hydrogenation kinetics of Ti-catalyzed NaAlH4.

The idea to use graphite or carbon to enhance the hy-drogen storage properties of metal[64–66] or complex hy-d ents cili-t f ballm sa ertieso itha gistice co-d ofc

andhg anceoa s ofT steda des-o TD)c ctronm opy(b ,a d asa f Ti-d cesoc dehy-d hy-d andb e

compared to each other to reveal the effect of graphite onhydrogenation kinetics.

2. Experimental

TiCl3 (Aldrich, 99.99%, anhydrous), aluminum pow-der (Alfa Aesar, 99.97%) and SFG 75 graphite powder(TimrexTM), were used as received. NaAlH4 powder (Fluka,99.5%) was recrystallized from a 3 M THF (Aldrich, 99.9%,anhydrous) solution, filtered through 0.7�m filter paper, andvacuum-dried. In 10 ml of THF, 1 g of NaAlH4 was mixedwith the catalyst precursor (TiCl3) to produce a doped sam-ple containing up to 4 mol% metal (wet doping procedure).The THF was evaporated while the NaAlH4 and the cata-lyst were manually mixed using a mortar and pestle for about30 min, or until the sample appeared dry. The sample was thenball milled for 2 h using a SPEX 8000 high-energy ball millloaded with a 65 cm3 SS vial containing 0.5–1 g of powderand a single SS ball (8.2 g) with a diameter of 1.3 cm. Af-ter ball-milling, graphite and aluminum powders were addeddirectly to the doped and ball milled sample (dry doping pro-cedure), and ball milling was continued for an additional 1 h(unless otherwise specified). The moles of Ti added to eachsample was based on the Na or Al content in NaAlH4; thegrams of graphite or Al added to each sample was based ont oped( ).A geng

rkinE Thed sam-p re infl orTr Dr sam-p thenh tely1

res-s luatet iousd -t on-i peda seli ectedt f de-h firstdh e,t ia to as oc-c en, a

rides[67] is relatively new. For example, some very rectudies collectively show that the addition of graphite faates the hydrogen absorption and desorption kinetics oilled Mg and Mg2Ni metal hydrides[64–66]. Carbon halso been reported to improve the dehydrogenation propf NaAlH4 [67], but it has never been used in concert wny metal-based catalyst to determine whether a synerffect persists, similar to that found when using Fe as aopant with Ti[5,43]. It is also worth noting, that the typearbon used in that study was not disclosed.

In this work, the first study on the dehydrogenationydrogenation kinetics of NaAlH4 co-doped with TiCl3 andraphite (G) is reported. The dehydrogenation performf freshly doped and ball milled samples of NaAlH4 cat-lyzed with Ti alone, G alone, various concentrationi–G, and various concentrations of Ti–G–Al are contrand compared in terms of temperature programmedrption (TPD) and constant temperature desorption (Curves. Based on evidence obtained from scanning eleicroscopy (SEM) and X-ray photoelectron spectrosc

XPS) studies of graphite, virgin NaAlH4, and/or NaAlH4all milled with either TiCl3 or both TiCl3 and graphiten explanation is offered as to why graphite, when useco-dopant, significantly improves the performance o

oped NaAlH4. Then, TPD dehydrogenation performanf similarly doped and ball milled samples of NaAlH4 areontrasted against each other after carrying out severalrogenation/hydrogenation cycles. Finally, qualitative derogenation and hydrogenation rates of similarly dopedall milled samples of NaAlH4 obtained during cycling ar

he total mass of the sample after being completely di.e., including the NaAlH4, TiCl3, and graphite and/or Alll sample handling procedures were performed in a nitrolove box.

Thermogravimetric analysis was carried out with a Pelmer, TGA 7 Series thermogravimetric analyzer (TGA).ehydrogenation rates of various doped and ball milledles of NaAlH4 were measured at atmospheric pressuowing helium (∼60 cm3/min) in TPD and CTD modes. FPD runs, the samples were heated to 250◦C at a rampingate of 2◦C/min after purging with He for 1 min. For CTuns, a similar procedure was followed except that theles were heated rapidly to the desired temperature andeld at that temperature for the desired time. Approxima0 mg of sample were used in each TPD or CTD run.

A 3000 psia Parr reactor, installed in an automated pure and temperature cycling system, was used to evahe dehydrogenation and hydrogenation kinetics of varoped and ball milled samples of NaAlH4 during and af

er cycling. The reactor conditions were continuously mtored and controlled with a computer. A sample of dond ball milled NaAlH4 was loaded into the reactor ves

n the glove box. The reactor was then sealed and conno the automated cycling system. A typical sequence oydrogenation/hydrogenation cycles was carried out byischarging a sample by heating the reactor to 125◦C andolding it at this temperature for 120 min. During this tim

he pressure in the sealed vessel increased from 15 pslightly higher pressure, indicating dehydrogenation hadurred. This is referred to as the 0th cycle discharge. Th

254 J. Wang et al. / Journal of Alloys and Compounds 395 (2005) 252–262

sample was hydrogenated (charged) by pressurizing the reac-tor with hydrogen (National Welders, HP Grade) to 1250 psiawhile maintaining the temperature at 125◦C and holding itat this temperature for 75 min. During this time, the pressurein the sealed vessel decreased from 1250 psia to a slightlylower pressure, indicating hydrogenation had occurred. Thisis referred to as the 1st cycle charge. The corresponding 1stcycle discharge was carried out by depressurizing the reactorto about 20 psia while maintaining the temperature at 125◦Cand holding it this temperature for 120 min. The cycling pro-cedure was repeated several times in this manner. After com-pleting the fifth charge cycle, the sample was maintained at1250 psia to prevent dehydrogenation while the temperaturewas reduced to room temperature (∼25◦C). Then, the pres-sure was decreased to atmospheric pressure (∼15 psia) andthe sealed reactor was transferred to the glove box to removethe sample for subsequent TPD and CTD studies.

Scanning electron microscope (SEM) images of varioussamples of doped and un-doped NaAlH4, as well as samplesof NaAlH4 and graphite before and after ball milling wereobtained using an ESEM FEI Quanta 200 SEM (at the USCMicroscopy Center), and a Hitachi S-4700 Field EmissionSEM (at the University of Illinois, Center for Microanaly-sis of Materials). The following procedure was developed tohandle the air- and moisture-sensitive samples of NaAlH4.In a nitrogen-filled glove box, samples were loaded on two-s withs ode-c rbonl con-t void-a mpleh Au-s thep rob-l d.

ysi-c eter,mw plesw mpleh wast

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no other single metal catalyst has been shown to be moreactive, based essentially on one study that examined 31 dif-ferent single metal chloride species[20]. However, when Feis used as a co-dopant with Ti[5,43](e.g., 3 mol% Ti–1 mol%Fe[43]) a synergistic effect materializes, wherein the dehy-drogenation kinetics of NaAlH4 become faster than thoseexhibited by NaAlH4 doped alone with either 4 mol% Ti or4 mol% Fe under similar preparation and processing condi-tions. This fundamentally interesting, as well as practical,finding clearly opens up numerous possibilities for investi-gating the synergistic effects that may arise upon co-dopingwith Ti, as well as with other primary metals like Zr[43]. Theresults inFig. 1, which reveal a very pronounced synergisticeffect from the addition of a small amount of graphite to asample of NaAlH4 doped with Ti, prove this point very well.

Fig. 1 displays several TPD curves obtained at 2◦C/minfor recrystallized NaAlH4, NaAlH4 ball milled for 2 h,NaAlH4 ball milled for 2 h with 5 wt.% graphite (G), NaAlH4doped with 2 mol% Ti, and NaAlH4 doped with 2 mol% Tiand 5 wt.% G and ball milled for 30, 60 and 90 min, respec-tively (as explained in Section2). First and foremost, it isclear that graphite alone is not a very effective catalyst forthe dehydrogenation kinetics of NaAlH4. In fact, the slightlypositive effect observed below 190◦C for the sample dopedwith 5 wt.% G as a lone dopant, turns into an obvious deleteri-ous effect on the dehydrogenation kinetics of NaAlHabovet nifi-c na t.%h l ef-f lHc re-p nt,1 em-

Fjt G( h),a

ided carbon tape, mounted on sample holders, coatedeveral drops of a 50:50 (v/v) mixture of pentane and dane to ensure formation of a protective liquid hydrocaayer, and sealed in double-walled individual screw-topainers. Despite all precautions, these samples were unably exposed to ambient air for a few seconds as the saolders were removed from the containers, mounted in aputtering device, and pumped down. However, due toresence of the protective hydrocarbon oil layer, any p

ems associated with air-exposure were surely minimizeX-ray photoelectron spectra were collected on a Ph

al Electronics PHI 5400 X-ray Photoelectron Spectromaintaining the pressure below 2.5×10−8 Torr. All samplesere handled in the nitrogen filled glove box. The samere loaded on two-sided copper tape, mounted on a saolder, and sealed in an air-tight transfer chamber, which

hen loaded into the spectrophotometer.

. Results and discussion

Since the initial discovery that Ti catalyzes the dehyenation and more importantly the hydrogenation reacf NaAlH4, which can be written as[1]:

NaAlH4Ti←→Na3AlH6+ 2Al + 3H2 (1)

Na3AlH6Ti←→6NaH+ 2Al + 3H2 (2)

4his temperature. In either case, without Ti present, sigant dehydrogenation does not begin until 190◦C, and evet this elevated temperature only a little more than 1.0 wydrogen is released. It is noteworthy that these minima

ects of graphite on the dehydrogenation kinetics of NaA4ontradict the effects of some unidentified form of carbonorted by Zaluska et al.[67]. In contrast, when Ti is prese.0 wt.% hydrogen is released at a significantly lower t

ig. 1. Temperature programmed desorption (TPD) curves at 2◦C/min forust doped or un-doped and ball milled (0th cycle) samples of NaAlH4 con-aining 2 mol% Ti (ball milled 2 h), 2 mol% Ti (ball milled 2 h) and 5 wt.%balled milled an additional 30, 60 and 90 min), 5 wt.% G (ball milled 2nd 0 mol% Ti (ball milled 2 h).


perature (∼130◦C), and upon co-doping with graphite, thistemperature is lowered even further. In fact, the addition of5 wt.% G to the NaAlH4 sample already doped and balledmilled with 2 mol% Ti produces quite striking effects on thedehydrogenation kinetics of not only the first reaction depictin Eq.(1) but also the second reaction depict in Eq.(2). Also,the samples containing 2 mol% Ti and 5 wt.% G successivelyexhibit better dehydrogenation kinetics when ball milled foran additional 30 and 60 min, compared to the sample dopedwith only 2 mol% Ti. Ball milling for an additional 30 min,up to a total of 90 min, does not seem to improve the perfor-mance any further, however. A possible explanation for thisresult is offered later. It must be emphasized that althoughthe effect of graphite (as displayed in this figure) may notappear to be that substantial, on the contrary, the TPD curvefor the sample co-doped with 5 wt.% G and ball milled foran additional 60 min shifts to lower temperatures by about15◦C over the entire temperature range governed by both thefirst and second reactions, i.e., up to about 170◦C. This resultis rather marked when considering that the temperature scanrate is fairly high at 2◦C/min.

The effect of the amount of graphite (G) on the dehydro-genation kinetics of Ti-catalyzed NaAlH4 is shown inFig. 2.Fig. 2a displays TPD curves for samples of NaAlH4 dopedwith 2 mol% Ti and 5, 10 and 20 wt.% G.Fig. 2b displays sim-ilar TPD curves for samples of NaAlHdoped with 4 mol% Tia on ki-n phiteu rvedf TPDco ert c-t rvedi per-a tent,c thes

tt Thiss pen-dc dw1 ingd5 0t reaterf t.%G in-c 10a f thes uablyma ,t cs for

Fig. 2. Temperature programmed desorption (TPD) curves at 2◦C/min forjust doped and ball milled (0th cycle) samples of NaAlH4 containing (a)2 mol% Ti, and 5, 10 and 20 wt.% graphite, and (b) 4 mol% Ti, and 10 and20 wt.% graphite.

NaAlH4 doped with only 2 mol% Ti (and of course co-dopedwith graphite) to date.

It is important to point out, however, that under these con-ditions the hydrogen released corresponds principally (if notonly) to that resulting from the first dehydrogenation reac-tion (i.e., Eq.(1)). Fig. 3b shows that the sample co-dopedwith 10 wt.% G releases about 2.5 wt.% hydrogen in a verylinear fashion in just under 40 min, which clearly indicatesthat hydrogen is desorbed mainly from the first reaction. Thisamount of hydrogen is close to the 2.9 wt.% theoretical limitof the first reaction, with this limit accounting for the reac-tion between TiCl3 and NaAlH4 [11], as well as for the factthat the wt.% basis in this figure is given for the NaAlH4plus the dopants. The incremental linear release of hydrogenover the next 60 min at a markedly lower rate does not ex-ceed 0.5 wt.%, and most likely corresponds to that comingfrom the much slower second reaction (Eq.(2)). However,it is quite clear that the rate of the second reaction is alsosomewhat influenced by the presence of graphite, as gleanedfrom a comparison of the two CTD curves inFig. 3b for theNaAlH4 samples co-doped with 5 and 10 wt.% G.

4nd 10 and 20 wt.% G. In both cases, the dehydrogenatietics of both reactions increases with an increase in grap to 10 wt.%. However, no further improvement is obse

or samples co-doped with 20 wt.% G. For example, theurve for the sample doped with either 2 mol% Ti (Fig. 2a)r 4 mol% Ti (Fig. 2b) and 10 wt.% graphite shifts to low

emperatures, again by about 15◦C, and again for both reaions. The overall losses in the hydrogen capacity (obsen the horizontal regions of the TPD curves at high temtures), clearly increasing with increasing graphite conan be attributed to the additional weight of graphite inample.

The TPD curves inFigs. 1 and 2prove without a doubhe existence of a synergism between Ti and graphite.ynergistic effect is further substantiated by the indeently obtained results shown inFig. 3. Fig. 3displays CTDurves for samples of NaAlH4 doped with 2 mol% Ti, anith 2 mol% Ti and 5 and 10 wt.% G at 90◦C (Fig. 3a) and10◦C (Fig. 3b), respectively. The successively increasehydrogenation kinetics of NaAlH4 upon co-doping withand 10 wt.% G is clearly observed. For example, at 9◦C

he hydrogen desorption rates are 2.9 and 6.5 times gor the samples respectively co-doped with 5 and 10 w

than for the sample doped with 2 mol% Ti alone. Therease in the desorption rates for the same systems at 1◦Cre almost as impressive, being 1.7 and 3.0 times that oample without graphite. These desorption rates are arguch faster than those reported elsewhere for NaAlH4 dopednd ball milled in a similar fashion at 125◦C [11]; hence

hey perhaps represent the best dehydrogenation kineti


Fig. 3. Constant temperature desorption (CTD) curves at (a) 90◦C and (b)110◦C for just doped and ball milled (0th cycle) samples of NaAlH4 con-taining 2 mol% Ti, and 0, 5 and 10 wt.% graphite.

Fig. 4. Constant temperature desorption (CTD) curves at 90◦C for justdoped and ball milled (0th cycle) samples of NaAlH4 containing 2 mol%Ti, 4 mol% Ti, 2 mol% Ti and 10 wt.% graphite, and 4 mol% Ti and 10 wt.%graphite.

The CTD runs inFig. 4 unquestionably reveal the syn-ergistic effect associated with adding 10 wt.% G to samplesof NaAlH4 doped with 2 and 4 mol% Ti, respectively. Thefaster dehydrogenation rate obtained with the higher concen-tration of Ti is expected and consistent with results reportedelsewhere[11]. However, there are two very intriguing trendsin this figure that may be providing additional clues to helpexplain the synergism associated with the use of graphite asa co-dopant. First, the incremental increases in the slopes ofthe CTD curves are approximately the same when increas-ing the Ti concentration from 2 to 4 mol%, whether graphiteis present or not. Second, the incremental increases in theslopes of the CTD curves are approximately the same afteradding 10 wt.% G to samples of NaAlH4 doped with either 2or 4 mol% Ti. These surprising similarities in the incrementalchanges in the slopes may simply be due to the graphite be-having as a micro-grinding agent that effectively causes theTi catalyst to be dispersed into more finely divided particleson the surfaces of the NaAlH4 particles. Hence, a sampledoped with 2 mol% Ti may be mimicking the behavior of asample doped with a greater amount of Ti after being betterdispersed by 10 wt.% G; and similarly, for the sample dopedwith 4 mol% Ti. These observations also support the notionthat graphite enhances the mixing of the reagents, leadingto a better distribution of the catalyst particles in the bulkof the alanate, and therefore facilitates the reactions takingp stingp t dis-c factt ly am lHb ni itst ballmt iallyb gess

cww edw -ps hitem pedN per-s ,n noww arti-c itionsw lyst,b oreg p-p iquem y en-

lace during the dehydrogenation process. These intereossibilities are addressed in more detail in subsequenussions. This supposition is also consistent with thehat graphite alone is an ineffective catalyst, having onarginal effect on the dehydrogenation kinetics of NaA4elow 190◦C and a deleterious effect above 190◦C, as show

n Fig. 1. It is also consistent with the apparent upper limo the effects of graphite as a co-dopant in terms of bothilling time (Fig. 1) and concentration (Fig. 2). Evidence

hat further supports this supposition of graphite potenteing a micro-grinding agent is provided by the SEM imahown inFig. 5and the XPS spectra shown inFig. 6.

Fig. 5a–d depict SEM images of virgin NaAlH4 re-rystallized from THF, graphite as received, NaAlH4 dopedith 2 mol% Ti via 2 h of ball milling, and NaAlH4 dopedith 2 mol% Ti via 2 h of ball milling and then further dopith 10 wt.% G via 1 h of ball milling, respectively. A comarison of the particle sizes of NaAlH4 in Fig. 5c and dtrongly suggests that during the ball milling process grapay be acting as a micro-grinding agent on the Ti-doaAlH4 particles. The net effect is perhaps a greater dision of the Ti particles over the NaAlH4 particle surfacesow with both potentially having more surface area, andith both potentially being dispersed on the graphite ples themselves. It is easy to understand why these condould necessarily improve the performance of the cataut only if graphite is somehow causing substantially mrinding to occur during ball milling. This interesting suosition can be understood in terms of some of the unechanical and chemical properties of graphite and b


Fig. 5. SEM images of (a) virgin NaAlH4 recrystallized from THF, (b) virgin graphite as received, (c) a sample of NaAlH4 doped with 2 mol% Ti (ball milled2 h), and (d) a sample of NaAlH4 doped with 2 mol% Ti (ball milled 2 h) and 10 wt.% G (ball milled an additional 1 h).

Fig. 6. XPS spectra of carbon 1s peaks for (a) virgin graphite as received,(b) a sample of NaAlH4 doped with 2 mol% Ti (ball milled 2 h) and 10 wt.%G (ball milled an additional 1 h), and (c) a sample of NaAlH4 doped with2 mol% Ti (ball milled 2 h) and 10 wt.% G (ball milled an additional 1 h),and after the 5th dehydrogenation/hydrogenation cycle: (A) graphitic carbon,287 eV; (B) metal carbide, 285 eV. The broad peak at∼292 eV is due to Na-Auger lines. No charge correction is made, which results in the shift of therelative position of the C 1s peaks.

visioning the follow events occurring during the high energyball milling process.

Since graphite is highly anisotropic with only weak vander Waals forces governing the bonding interaction betweenthe individual graphene layers, the layers tend to slide pastone another quite easily during the ball milling process. Thisslippage phenomenon gives rise to the well known lubricationproperties of graphite[68–71]. As these layers move apart,separate from each other, and form small particles of graphite(i.e., as exfoliation takes place), more graphite surface areais exposed. This event (i.e., graphite playing a role as a mix-ing agent through lubrication phenomena, i.e., graphene layerslippage and breakage) in itself may improve the performanceof NaAlH4 by providing graphitic surface sites for the smallparticles of NaAlH4 and Ti to reside on, which naturally im-proves the dispersion of these species throughout the entiresample. However, this event alone does not explain the role ofgraphite as a micro-grinding agent, because graphite is prob-ably much too soft of a material to impart the formation ofthe small particles observed inFig. 5c and d through grind-ing. However, this event is a necessary step toward the highlyprobable formation of carbides, which are some of the hard-est materials known. The first evidence for carbide formationin this system is provided inFig. 6 in terms of XPS spectrafor a sample of virgin graphite (a), a sample of doped andballed milled NaAlH4 containing 2 mol% Ti and 10 wt.% G(t (c).T

them ationo madeo ex-a cta e in-

b), and a sample of doped and balled milled NaAlH4 con-aining 2 mol% Ti and 10 wt.% G and cycled five timeshese results are explained below.

The ball milling process provides energy not only forovement of the graphene layers, but also for the formf metastable materials that, in some cases, cannot ber are difficult to make by any other synthetic means, formple, carbides[72–75]. The high energy, localized impassociated with ball milling causes localized temperatur


creases of the materials being ball milled. These temperatureincreases can be high enough to cause the formation of chemi-cal bonds. In this case, it is suspected that some of the graphiteis undergoing carbidization during the ball milling process,especially in the presence of NaAlH4 doped with TiCl3. Toverify formation of metal carbide during ball milling, X-rayphotoelectron spectroscopy (XPS) was performed on someof the samples. It must be emphasized, however, that thisXPS analysis is used here primarily for monitoring the qual-itative changes in the surface composition of the material, asno charge corrections are made. Indeed, the XPS spectrumin Fig. 6b reveals the presence of carbidic species in NaAlH4co-doped with 2 mol% Ti and 10 wt.% G, possibly verifyingthat this high-energy ball milling process promotes the in situformation of carbidic species[72–75]. Notice the formationof a shoulder (labeled B) on the main carbon 1s peak (la-beled A) that is completely absent in the spectra inFig. 6a.It is suspected that this shoulder peak is due to the forma-tion of a metal carbide species[74]. It is also interesting thatthe spectra inFig. 6c show that the intensity of this peak in-creases after several dehydrogenation/hydrogenation cyclesare carried out. This increase can be attributed to the con-tinued growth of the newly formed metal carbide nanoparti-cles due to the elevated temperature (125◦C) used during thehydrogen discharge and charge processes. Whereas the for-mation of sodium carbide is highly improbable, it is possiblet ,T ngb heg

os-s ingr n-h f Fewc echa-n erys mostl ayb reb y d-o ceda thei verm aug-m herei hy-d thef Eqs.(

sd e0 ped,bT yner-g dro-

Fig. 7. Temperature programmed desorption (TPD) curves at 2◦C/min fordoped and ball milled samples of NaAlH4 after the 0th, 5th and 10th dehy-drogenation/hydrogenation cycles and containing (a) 2 mol% Ti and 10 wt.%G, and (b) with 4 mol% Ti and 10 wt.% G.

genation/hydrogenation cycles are carried out, and whetherthese synergistic properties are exhibited and sustained dur-ing hydrogenation.Figs. 7 and 8reveal the influence ofgraphite over dehydrogenation after several dehydrogena-tion/hydrogenation cycles are carried out, whileFig. 9 sim-ilarly reveals the influence of graphite over hydrogenationafter cycling. The TPD curves inFig. 7depict the effect of car-rying out 0, 5 and 10 dehydrogenation/hydrogenation cycleson samples of NaAlH4 doped with 2 mol% Ti and 10 wt.% G(Fig. 7a), and 4 mol% Ti and 10 wt.% G (Fig. 7b). For both Ticoncentrations, the hydrogen kinetic and capacity losses ex-hibited upon cycling are significant. Substantial, up to about10◦C shifts towards higher dehydrogenation temperaturesare observed for the first reaction and up to about 20◦C shiftsfor the second reaction. These shifts in temperature are ac-companied by substantial decreases in the hydrogen capacity,on the order of 0.75 and 1.0 wt.% for the samples containing2 and 4 mol% Ti, respectively, and for both reaction regimesafter five cycles. However, the kinetic and capacity lossesbeyond five cycles are minimal, with additional losses beingonly about 1◦C and 0.1 wt.% in going from 5 to 10 cycles.These minimal losses are especially noticeable for the sample

hat aluminum carbide, AlxCy [76–78]and titanium carbideiCx [78–80] (very hard materials) may be forming duriall milling in small but sufficient quantities to improve trinding process.

However, it is not possible at this time to rule out the pibility of an electronic effect associated with graphite beich in �-electrons that may facilitate the role of Ti in eancing the kinetics. This would be similar to the effect ohen it is used as a co-dopant with Ti[5,43]. However, in thisase, the event may actually be based on a spillover mism[81–83], especially when considering the fact that vmall, high surface area, exfoliated graphite particlesikely exist in the ball milled sample. For example, it me that some of the higher energy�-electrons of graphite aeing back-donated to the lower energy partially emptrbitals of Ti. This phenomenon ultimately leads to enhanctivation of the catalyst, which in turn further facilitates

nteraction of hydrogen with Ti, perhaps involving a spilloechanism with graphite. It is envisioned that graphiteents the electronic structure of graphite to the point w

t more easily dissociates molecular hydrogen to atomicrogen, and this event in turn further facilitates not only

orward reactions but also the reverse reactions depict in1) and (2), as shown below.

Except for the results presented inFig. 6c, all of the resultisplayed and discussed inFigs. 1–5, correspond only to thth cycle discharge (i.e., to the behavior of freshly doall milled and discharged samples of uncycled NaAlH4).he obvious questions that remain are, whether the sistic properties of graphite persist after several dehy


Fig. 8. Temperature programmed desorption (TPD) curves at 2◦C/min fordoped and ball milled samples of NaAlH4 after 0th and 5th dehydrogena-tion/hydrogenation cycles and containing (a) 2 mol% Ti, 2 mol% Ti and10 wt.% G, and 2 mol% Ti, 10 wt.% G and 5 wt.% Al, and (b) 2 mol% Ti and5 wt.% Al, and 2 mol% Ti, 10 wt.% G and 5 wt.% Al.

containing 2 mol% Ti, which exhibited essentially no changein kinetics or capacity between 5 and 10 cycles. These cy-cling losses are very similar to that reported elsewhere forsamples of NaAlH4 doped only with Ti[5], and they indicatethat graphite when used as a co-dopant does not impart anydeleterious effects upon cycling. Unfortunately, these resultsalso indicate that graphite as a co-dopant does not impart anyfurther improvement in the dehydrogenation kinetics duringcycling, at least not beyond that already observed inFigs. 1–4.These two facts may be providing additional evidence forgraphite playing two roles by serving as a mixing agent man-ifested through lubrication phenomena (i.e., graphene layerslippage and breakage) and as a micro-grinding agent mani-fested through metal carbide formation, both during the ballmilling process.

The effects of cycling on the dehydrogenation perfor-mance of NaAlH4 co-doped with Ti and G and mixed withan additional co-dopant, Al powder, are illustrated furtherwith the results shown inFig. 8. Fig. 8a displays the TPDcurves obtained at 2◦C/min for doped and ball milled sam-ples of NaAlH4 containing 2 mol% Ti alone and 2 mol% Ti

Fig. 9. Qualitative hydrogenation and dehydrogenation rates during fivecharge (Po = 1250 psia) and four discharge (Po = 20 psia) cycles carried outat 125◦C for doped and ball milled samples of NaAlH4 containing 2.0 mol%Ti and 5 wt.% Al, and 2 mol% Ti, 10 wt.% G and 5 wt.% Al. The filled sym-bols correspond to samples containing G; the empty symbols correspond tosamples not containing G.

and 10 wt.% G, and 2 mol% Ti, 10 wt.% G and 5 wt.% Al after0th and 5th dehydrogenation/hydrogenation cycles. A carefulexamination of these curves reveals that similar cycling lossesoccur after five cycles with or without graphite being presentand that the synergistic effects of graphite on the dehydro-genation kinetics of both the first and second reactions andfor both the 0th and 5th cycle dehydrogenations prevail andare again quite pronounced. What is most intriguing, how-ever, is that these cycling losses are essentially eradicated bythe addition of Al powder to the sample, as clearly observedin the sample doped with 2 mol% Ti, 10 wt.% G and 5 wt.%Al. Over the entire temperature range up to 230◦C, this triplydoped sample shows effectively no loss of hydrogen capacityupon cycling, with the dehydrogenation performance beingessentially identical to the uncycled sample of NaAlH4 dopedwith only 2 mol% Ti and 10 wt.% G up to 130◦C. The over-all losses in the capacities of the first and second reactionsexhibited by the triply doped sample, respectively, at around130 and 150◦C are simply due to the wt.% basis in this figurealso accounting for the G and Al. InFig. 8b, TPD curves fordoped and ball milled samples of NaAlH4 containing 2 mol%Ti and 5 wt.% Al, and 2 mol% Ti, 10 wt.% G and 5 wt.% Alafter 0th and 5th dehydrogenation/hydrogenation cycles, con-vincingly show that the effects of graphite and aluminum arealso independent of each other.

The independent nature of these two additives, namely,a ows.Ar id-aa it iss ium

luminum powder and graphite, can be explained as folls reported and explained elsewhere[22], during the forward

eaction in Eq.(1), Al becomes highly dispersed and unavobly inaccessible due to the formation of separate Na3AlH6nd Al phases and noticeable Al segregation. Hence,urmised that the addition of excess Al shits the equilibr


and hence enables the first reaction in Eq.(1) to be reversedand proceed nearly to completion. This becomes possible byproviding the necessary Al to initiate the transformation ofthe Na3AlH6 phase back to the NaAlH4 phase. In such acase, reversibility approaches the theoretical hydrogenationcapacity of the 0th cycle after every dehydrogenation cycle,with the net effect of Al being minimal capacity loss withcycling. The results inFig. 8b show this to be the case, evenafter five cycles. In contrast, the results inFig. 8b also showthat graphite, when added as a co-dopant, markedly improvesthe dehydrogenation performance of both the first and sec-ond reactions. The subtle losses in hydrogen capacity due tothe presence of 10 wt.% G are also very clearly seen in theseTPD curves.

The results just discussed above show that the very fa-vorable synergistic effect of graphite on the dehydrogena-tion performance of Ti-doped NaAlH4 persists even after cy-cling. The results inFig. 9 show a similarly positive effectof graphite on the hydrogenation performance of Ti-dopedNaAlH4 even after cycling. This figure displays qualitativehydrogenation and dehydrogenation rates obtained duringfive charge (Po = 1250 psia) and four discharge (Po = 20 psia)cycles carried out at 125◦C with doped and ball milled sam-ples of NaAlH4 containing 2.0 mol% Ti and 5 wt.% Al, and2 mol% Ti, 10 wt.% G and 5 wt.% Al. The qualitative kinet-ics of charge and discharge are inferred from the observedl e dist tion( rge)k andc cyclen vioro s Al.T rves,l ithg by af ultsm mpleo isd thep h islp st

w Hd ratea .T n: ist ieu ofm tioni hN

3

Consequently, using 6 mol% Ti limits the theoretical max-imum reversible hydrogen capacity of the first, and com-bined first and second reactions depicted in Eqs.(1) and (2)to 2.58 and 3.88 wt.% hydrogen, respectively. Whereas us-ing 2 mol% Ti and 10 wt.% G increases these limits to 2.99and 4.49 wt.%, simply because graphite does not react withNaAlH4 during the doping process. This corresponds to 16and 13% increases, respectively, in the reversible hydrogenstorage capacities of the first, and combined first and secondreactions simply by using graphite in place of some TiCl3,to provide the same (possibly even better) performance ofdoped and ball milled NaAlH4.

4. Conclusions

For the first time, the very positive, sustaining and syn-ergistic effects of graphite (G) as a co-dopant on both thedehydrogenation and hydrogenation kinetics of Ti-dopedNaAlH4 are reported. It was demonstrated that the co-dopingof 2 mol% Ti-doped NaAlH4 with 10 wt.% G, improvesthe kinetics of both the first and second reactions by low-ering the dehydrogenation temperature over the importantrange of 90–150◦C by as much as 15◦C, compared to thatwithout graphite. Moreover, the addition of graphite wasshown to improve the dehydrogenation kinetics at 90 and1 ast,g as ac

n thed oughd e ad-d thei ntly,t justa gena-t cles.I2 theb jectedt 1250a llyw ith asl

EMi ggestt yinga rica-t n ofv ballm d theT dis-p eg ithn thisd ver

inear decreases and increases in pressure over time. Thinct influence of graphite not only on the dehydrogenadischarge) kinetics, but also on the hydrogenation (chainetics, is clearly observed. The fact that the dischargeharge rates tend to decrease slightly with increasingumber, but not the final capacity, is typical of the behaf a sample cycled at constant temperature that containhese subtle changes are difficult to observe in TPD cu

ike those shown inFig. 8. Nevertheless, upon co-doping wraphite, the time for charging is markedly decreased

actor of 4, from about 60 min to about 15 min. These resay represent the best charge kinetics to date for a saf NaAlH4 doped with as little as 2 mol% Ti. Although itifficult to quantify this claim at this point, because ofaucity of hydrogenation kinetics in the literature, whic

imited to only seven studies[5,11,13,21,24,32,42], a com-arison with the work by Sandrock et al.[11] substantiate

his claim at least qualitatively.In fact, a qualitative comparison of the results inFig. 9

ith Sandrock’s work[11] suggests that a sample of NaAl4oped with 6 mol% Ti produces a similar hydrogenations a sample of NaAlH4 doped with 2 mol% Ti and 10 wt.% Ghis interesting observation raises an important questio

here an advantage to using graphite as a co-dopant in lore TiCl3 as the lone catalyst? The answer to this ques

s emphatically yes, because TiCl3 irreversibly reacts witaAlH4 during the doping process according to[11]:

NaAlH4+TiCl3→ 3NaCl+ Ti + 3Al + 6H2 (3)

-10◦C by factors of 6.5 and 3.0, respectively. In contrraphite alone was found to be essentially inactiveatalyst.

These significant effects of graphite (as a co-dopant) oehydrogenation performance were shown to persist threhydrogenation/hydrogenation cycling, and through thition of aluminum powder, which essentially mitigated

rreversible losses associated with cycling. Most importahe effect of graphite on the kinetics was shown to bes pronounced and as sustaining during several hydro

ion cycles, as it was during several dehydrogenation cyn fact, a doped and balled sample of NaAlH4 containing.0 mol% Ti, 10 wt.% G and 5 wt.% Al may have exhibitedest hydrogenation rates to date, even after being sub

o five dehydrogenation/hydrogenation cycles betweennd 20 psia at 125◦C. This is an exciting result, especiahen considering the fact that the sample was doped w

ittle as 2 mol% Ti.These TPD and CTD curves, in conjunction with S

mages and XPS spectra, provided some evidence to suhat the effect of graphite might perhaps be due to it pla

dual role and serving as a mixing agent through lubion and as a micro-grinding agent through the formatioery hard carbide species, both during the high energyilling process. In these capacities, it may have causei particles to be more finely ground and hence moreersed over the surfaces of the NaAlH4 particles and also thraphite particles. With this last comment in mind and wo direct or completely conclusive evidence to supportual-role supposition, the possibility of a hydrogen spillo


mechanism could not be ruled out. In this capacity, graphitemay also be imparting an electronic contribution throughthe interaction of its facile�-electrons with Ti, wherebygraphite back donates some electrons to Ti, which further fa-cilitates hydrogen bond formation and cleavage through thisTi species.

It must be emphasized that in the event that the roles ofgraphite are limited to those of being a mixing agent and amicro-grinding agent, it would still be far more beneficial touse graphite as a co-dopant over simply adding more TiCl3 toimprove the performance of NaAlH4: TiCl3 (the most com-monly used form of Ti as the catalyst) reacts irreversiblywith NaAlH4 during the doping process. In contrast, it wasshown in this work that graphite, when used as a co-dopant,does not exhibit any deleterious effects during the dopingprocess or during cycling. In fact, qualitatively similar hy-drogenation rates were obtained with a sample of NaAlH4doped and ball milled with just 2 mol% Ti and 10 wt.% G(this work) compared to 6 mol% Ti (literature result). Us-ing graphite as a co-dopant in this manner would possiblygive rise to a 13% increase in the overall reversible hydrogencapacity of NaAlH4, with a corresponding 16% increase inthe capacity of just the first dehydrogenation/hydrogenationreaction, which is the more important one because it oc-curs in a temperature range more amenable to engineeringapplications.

withg pos-s ena-t likeN te ncea ualr phe-n d asm tion,w tri-b it ofg andw t witho entlyb ies arf

A

F/W wasg mis-s ho-t nterf tedb wasa

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cknowledgements

Financial support provided, in part, by SCURESRC/DOE under contract WEST052, KG09725-O

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