Double-cation borohydrides MLim−n(BH₄)m (M = Zn, n = 2; M = Al, n = 3; M = Zr, n = 4; n≤ m) were expected to be synthesized and their thermodynamical stabilities were also examined experimentally. The samples with the compositions of ZnLi(BH₄)₃ and AlLi(BH₄)₄ disproportionate into Zn(BH₄)₂- (or Al(BH₄)₃-) and LiBH₄-based phases upon heating, respectively. However, no disproportionation reaction is observed in ZrLim−4(BH₄)m (m = 5 and 6). It should be emphasized that hydrogen desorption temperature Td of ZrLim−4(BH₄)m continuously increases from 440 to 650 K as the composition m increases from 4 to 6, and approaches to 740 K (Td of LiBH₄). The experimental results indicate that the combination of appropriate cations is an effective method to adjust the thermodynamical stabilities of metal borohydrides, similar to the conventional “alloying” method for hydrogen storage alloys

Li, H.-W.

Miwa, K.

Nakamori, Y.

Ohba, N.

Orimo, S.

Towata, S.

Züttel, Andreas

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http://doc.rero.chMaterials designing of metal borohydrides: Viewpointsfrom thermodynamical stabilitiesH.-W. Li a, S. Orimoa,∗, Y. Nakamori a, K. Miwab, N. Ohbab, S. Towata b, A. Zu¨ttel ca Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-Ku, Sendai 980-8577, Japanb Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japanc Physics Department, University of Fribourg, Pe´rolles, CH-1700, Fribourg, SwitzerlandAbstractDouble-cation borohydrides MLim−n(BH4)m (M=Zn, n= 2; M=Al, n= 3; M=Zr, n= 4; n≤m) were expected to be synthesized and theirthermodynamical stabilities were also examined experimentally. The samples with the compositions of ZnLi(BH4)3 and AlLi(BH4)4 dispropor-tionate into Zn(BH4)2- (or Al(BH4)3-) and LiBH4-based phases upon heating, respectively. However, no disproportionation reaction is observed inZrLim−4(BH4)m (m= 5 and 6). It should be emphasized that hydrogen desorption temperature Td of ZrLim−4(BH4)m continuously increases from 440to 650K as the composition m increases from 4 to 6, and approaches to 740K (Td of LiBH4). The experimental results indicate that the combinationof appropriate cations is an effective method to adjust the thermodynamical stabilities of metal borohydrides, similar to the conventional “alloying”method for hydrogen storage alloys.Keywords: Hydrogen absorbing materials; Mechanical alloying; Thermodynamic properties; Thermal analysis1. IntroductionMetal borohydrides (M(BH4)n), such as LiBH4 andMg(BH4)2 [1–19] have been attracting great interest as one ofthe potential candidates of advanced hydrogen storagematerials,because of their high gravimetric hydrogen densities.Recently, we have systematically investigated the thermody-namical stabilities of M(BH4)n (M=Li, Na, K, Cu, Mg, Zn, Sc,Zr and Hf; n= 1–4) with a single cation by both ﬁrst-principlesstudies and thermal desorption measurements [16]. The ﬁrst-principles calculations indicated that charge transfer from Mn+to [BH4]− is a key factor for the stability of M(BH4)n [5,8,16]and also there exists a linear relationship between the calculatedformation enthalpyH ofM(BH4)n and the Pauling electroneg-ativity χP of M [16]. Experimentally, the thermal desorptiontemperature Td of M(BH4)n examined by gas chromatographywas also closely correlated with χP [16,17], as shown in Fig. 1.That is, Td (closed circles) decreases with increasing the value∗ Corresponding author. Tel.: +81 22 215 2093; fax: +81 22 215 2091.E-mail address: orimo@imr.tohoku.ac.jp (S. Orimo).of χP. Therefore, the value of χP of M is concluded to be animportant indicator for estimating the thermodynamical stabil-ity of M(BH4)n, i.e. Td of M(BH4)n might be roughly adjustedby selecting M with the appropriate value of χP.However, in the case of M(BH4)n with a single cation Mn+,precise adjustment of Td might be difﬁcult due to the inherentand discrete value of χP for each metal. Thus, development ofMM′(BH4)n with double-cation (or multi-cation) is expectedto be quite useful to precisely adjust Td for hydrogen storageapplications [20]. This proposition is similar to the conventional“alloying” method for hydrogen storage alloys [21,22].In the present study, we select some metals with differentχP, whose basic information is summarized in Table 1. Amongthem, borohydrides composed of metals with larger χP, suchas Zn (χP = 1.6), Al (χP = 1.5) and Zr (χP = 1.4), are reportedto have lower Td, i.e. 398 [17,18], 334 [19] and 440K [17],respectively. However, Zn(BH4)2 and Al(BH4)3 are so unstablethat diborane as an impurity gas is released along with hydrogendesorption on heating [18,19]; while Zr(BH4)4 evaporates easilydue to its low melting point. On the other hand, LiBH4 with thesmaller χP of Li is so stable that its Td is approximately as highas 740K. Therefore, their thermal stabilities are expected to be1Published in "Journal of Alloys and Compounds 446-447: 315-318, 2007"which should be cited to refer to this work. http://doc.rero.chFig. 1. Thermal desorption temperature Td (main peak) as a function of thePauling electronegativity χP. Closed and open circles indicate the series ofsingle-cation borohydrides M(BH4)n (M=Li, Na, Mg, Sc, Zr, Zn; n= 1–4) anddouble-cation ones ZrLim−4(BH4)m (m= 4–6), examined by gas chromatog-raphy and quadrupole mass spectroscopy, respectively. There is a differencebetween Td determined by gas chromatography and by quadrupole mass spec-troscopy, owing to a longer distance between the detector and sample and alsoto a lower gas ﬂow rate in gas chromatography.adjusted for hydrogen storage application by the combination oftwo cations, i.e. the double-cation borohydridesMLim−n(BH4)m(M=Zn, n= 2; M=Al, n= 3; M=Zr, n= 4; n≤m) [23], whichare expected to be synthesized according to the following reac-tion:MCln +mLiBH4 → MLim−n(BH4)m + nLiCl. (1)The enthalpy changes of this reaction are calculated by usingthe reported values of the formation enthalpies forMCln [24] andLiBH4 [5] and the values for MLim−n(BH4)m formed from M,Li, B and H2, can be theoretically predicted by the equation;248.7χP − 390.8 (χP is the averaged values of Pauling elec-tronegativity described later) [16]. Any of the samples have theirown negative values of Hmilling (not shown) depending on m,indicating that the reactions can occur “exothermically” in thewide composition ranges of m [20].The purpose is to clarify whether the method to preciselyadjust the hydrogen desorption temperature of metal boro-hydrides by the combination of two metals with differentelectronegativities is feasible or not. This study provides theTable 1Basic information of M and their borohydrides M(BH4)nMZn Al Zr LiχP 1.6 1.5 1.4 1.0Td (K) of M(BH4)n 398a [17,18] 334a [19] 440b [17] 740a Diborane as a impurity gas is released along with hydrogen desorption onheating.b Zr(BH4)4 has low melting point, i.e. 301.7K.fundamental information on the development of newmetal boro-hydrides with appropriate thermal desorption temperature forhydrogen storage applications.2. ExperimentalAnhydrous MCln (M=Zn, Al and Zr) with 99.9–99.999% purities andLiBH4 with 95% purity, were purchased from Aldrich Co. Ltd. The mixtureof MCln and mLiBH4 was premixed manually using an agate mortar and pestleand then mechanically milled by planetary ball milling (Fritsch P-7) in 0.1MPaAr for 5 h to synthesize MLim−n(BH4)m (M=Zn, n= 2; M=Al, n= 3; M=Zr,n= 4; n≤m) according to equation (1). The milling process was paused every15min to avoid an increase in temperature of the sample. The samples, thusprepared were subjected to powder X-ray diffraction measurement (Cu K radi-ation), Raman spectroscopy (532 nm-laser with back scattering geometry) andthermal desorption spectroscopy detected by quadrupole mass spectroscopy (Heﬂow of 150ml/min and heating rate of 5K/min). All of the samples were alwayshandled in a glove box ﬁlled with puriﬁed Ar/He (dew point below 183K) gasto avoid (hydro-)oxidation.3. Results and discussion3.1. Syntheses of MLim−n(BH4)mFig. 2 shows the powder X-ray diffraction proﬁles of the sam-ples after mechanical milling of (a) ZnCl2 +mLiBH4 (m= 2 and3), (b) AlCl3 +mLiBH4 (m= 3 and 4) and (c) ZrCl4 +mLiBH4(m= 4–6). The proﬁles of startingmaterials ZnCl2, AlCl3, ZrCl4Fig. 2. Powder X-ray diffraction proﬁles of the samples after mechanicalmilling of (a) ZnCl2 +mLiBH4 (m= 2, 3), (b) AlCl3 +mLiBH4 (m= 3, 4), (c)ZrCl4 +mLiBH4 (m= 4–6) and (d) the purchased LiBH4. The proﬁles of startingmaterials ZnCl2, AlCl3 and ZrCl4 are also shown for references.2http://doc.rero.chFig. 3. Raman spectra of the samples after mechanical milling of (a)ZnCl2 +mLiBH4 (m= 2, 3), (b) AlCl3 +mLiBH4 (m= 3, 4), (c) ZrCl4 +mLiBH4(m= 4–6) and (d) the purchased LiBH4.and LiBH4 are also shown for references. The broad diffractionpeaks at around 20◦ in all the proﬁles, result from the diffractionof tape covered samples to avoid exposing to air. LiCl, one ofthe products of Eq. (1), is identiﬁed in all the diffraction pro-ﬁles for the samples after mechanical milling. Furthermore, nodiffraction peaks of the starting materials of MCln and LiBH4are observed. Thus, the X-ray diffraction results suggest the pro-gression of reaction of Eq. (1), i.e. the possible formation ofMLim−n(BH4)m and LiCl.There is no traces of MLim−n(BH4)m in the X-ray diffrac-tion proﬁles, which is due to the absence of any long rangeordering in the structure, similar to other M(BH4)n synthesizedby mechanical milling [16,17]. Therefore, Raman spectra wereexamined to obtain information on the B H bonding to con-ﬁrm the existence of MLim−n(BH4)m in the samples preparedby mechanical milling. The Raman spectra of the samples aftermechanical milling are shown in Fig. 3. The spectrum of LiBH4is also shown as a reference, where the B H stretching (ν1)and bending (ν1 and also ν′2) modes in the [BH4]− complexanion are observed at around 2300 and 1300 cm−1, respec-tively. Since ZnLim−2(BH4)m (m= 2 and 3) is easily scorchedby laser irradiation for Raman spectroscopy, only a small andweak peak at about 2400 or 2300 cm−1 is observed, respec-tively, which are expected to correspond to the stretching modesof the B H bonding. There are two or three stretching modes ataround2200–2580 cm−1 in theRaman spectra of the synthesizedAl(BH4)3 and Zr(BH4)4, as shown in Fig. 3(b and c), respec-tively [25,26]. On the other hand, the similar vibration modesin the [BH4]− complex anion to that of LiBH4 are observedin the Raman spectra of the samples with double-cation, i.e.ZnLi(BH4)3, AlLi(BH4)4, ZrLi(BH4)5 and ZrLi2(BH4)6. Thevarieties in the Raman spectra depending on the sample com-positions, which are expected to be due to the change of thecondition surrounding the [BH4]− anion, indirectly indicate theprogression of Eq. (1).3.2. Thermal desorption properties of MLim−n(BH4)mFig. 4 shows the hydrogen desorption proﬁles of the mixtureof MCln +mLiBH4 after mechanical milling, detected byquadrupole mass spectroscopy. Though the samples synthe-sized by mechanical milling is mixtures of MLim−n(BH4)mand LiCl, the thermal desorption proﬁles originate only fromMLim−n(BH4)m, because LiCl decomposes at the temperaturehigher than 878K. The result of LiBH4 is also shown as areference. Both Zn(BH4)2 and Al(BH4)3 have lower Td asshown in Fig. 4(a and b), i.e. 393 and 334K, respectively;but impurity gases of diborane, were released along with thedesorption of hydrogen (not shown) [18,19]. On the otherhand, the thermal desorption proﬁles of the samples with thecompositions of ZnLi(BH4)3 and AlLi(BH4)4 indicate that bothFig. 4. Thermal desorption proﬁles of the samples after mechanical millingof (a) ZnCl2 +mLiBH4 (m= 2, 3), (b) AlCl3 +mLiBH4 (m= 3, 4), (c)ZrCl4 +mLiBH4 (m= 4–6) and (d) the purchased LiBH4 detected by quadrupolemass spectroscopy. Td of ZrCl4 +mLiBH4 (m= 4–6) after mechanical milling issummarized in Fig. 1 with open circles.3http://doc.rero.chof them disproportionate into Zn(BH4)2- (or Al(BH4)3-) andLiBH4-based phases during heating process, respectively. Thepeaks below 400K for ZnLim−4(BH4)m and AlLim−4(BH4)mshift slightly to higher temperature side about 10K with theincreasing value of m, probably originating from the partialcombination effect of Zn or Al with Li, respectively.However, it is worth emphasizing that hydrogen desorptionproﬁles of ZrLi(BH4)5 and ZrLi2(BH4)6 do not show obviousdisproportionation reaction into Zr(BH4)4- and LiBH4-basedphases upon heating, although some additional reactions withsmall peaks are detected.Only hydrogenwas detected in the ther-mal desorption measurement of ZrLim−4(BH4)m. Moreover, Tdfor ZrLim−4(BH4)m, i.e. 440K (m= 4), 595K (m= 5) and 650K(m= 6), shifts gradually to higher temperature with increasingcomposition ratio ofLi/Zr and continuously approaches to 740Kwhich is Td for LiBH4.The relationship between Td and the averaged value of χP ofZrLim−4 is expressed in Fig. 1. Here, the averaged value of χPfor ZrLim−4 depending on m is simply calculated as follows:χP = 1.4 + 1.0(m − 4)1 + (m − 4) . (2)Apparently, Td of ZrLim−4(BH4)m, namely MM′(BH4)n withdouble-cation, is closely related to the averaged χP as what havebeen observed in the M(BH4)n with a single cation [16]. Thisexperimental feature highly suggests that the hydrogen desorp-tion temperature ofmetal borohydrides can be precisely adjustedby the appropriate combination of cations. The criteria for theselection of appropriate cations which can effectively and pre-cisely adjust hydrogen desorption temperature are expected tobe clariﬁed in near future.Some additional reactions with small peaks in Fig. 4 are pos-sibly resulted from small amounts of residual LiBH4 as thestarting material and Zr-rich borohydrides in the milled sam-ples. The origin of these peaks cannot be well clariﬁed due tothe absence of diffraction peaks of other phases except for LiCl,as shown in Fig. 2. New synthesis processes of well-crystallizedZrLin−4(BH4)n are now being developed, which seems to beindispensable for the further study.4. SummaryA series of double-cation borohydrides MLim−n(BH4)m(M=Zn, n= 2; M=Al, n= 3; M=Zr, n= 4; n≤m) wereexpected to be synthesized by mechanical milling of MClnand mLiBH4. Their thermodynamical stabilities were exam-ined by quadrupole mass spectroscopy. Thermal desorptionmeasurement results indicate that the samples with the com-positions of ZnLi(BH4)3 and AlLi(BH4)4 disproportionate intoZn(BH4)2- (or Al(BH4)3-) and LiBH4-based phases upon heat-ing, respectively. However, no disproportionation reaction intoZr(BH4)4- and LiBH4-based phases is conﬁrmed in the case ofZrLim−4(BH4)m. The hydrogen desorption temperature Td ofZrLim−4(BH4)m increases from 440K (m= 4) to 650K (m= 6)and continuously approaches to 740K (Td of LiBH4). That is,Td of ZrLim−4(BH4)m, namely MM′(BH4)n with double-cationis closely related to the averaged χP as what have been observedin the M(BH4)n with a single cation. Consequently, the above-mentioned results indicate that the appropriate combination ofcations is an effective method to adjust the thermodynami-cal stability of metal borohydrides, similar to the conventional“alloying” method for hydrogen storage alloys.AcknowledgementsThis study was partially supported by the New Energyand Industrial Technology Development Organization (NEDO),International Joint Research under the “Development for SafeUtilization and Infrastructure of hydrogen” Project (2005–2007)and the Grants-in-Aid of Japan Society for the Promotion ofScience (JSPS).References[1] A. Zu¨ttel, S. Rentsch, P. Fisher, P. Wenger, P. Sudan, Ph. Mauron, Ch.Emmenegger, J. Alloys Compd. 356–357 (2003) 515–520.[2] A. Zu¨ttel, P. Wenger, P. Sudan, Ph. Mauron, S. Orimo, Mater. Sci. Eng. B108 (2004) 9–18.[3] S. Orimo, Y. Nakamori, A. Zu¨ttel, Mater. Sci. Eng. B 108 (2004) 51–53.[4] H. Hagemann, S. Gomes, G. Renaudin, K. Yvon, J. Alloys Compd. 363(2004) 129–132.[5] K. Miwa, N. Ohba, S. 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Materials designing of metal borohydrides: Viewpoints from thermodynamical stabilities

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