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Magnetic Measurements as a Sensitive Tool for Studying Dehydrogenation Processes in Hydrogen Storage Materials Enric Mene´ndez,*,†,+ Sebastiano Garroni,‡,+ Alberto Lo´pez-Ortega,§,+ Marta Estrader,§,+ Maciej O. Liedke,| Ju¨rgen Fassbender,| Pau Solsona,‡ Santiago Surin˜ach,‡ Maria D. Baro´,‡ and Josep Nogue´s⊥ Instituut Voor Kern- en Stralingsfysica and INPAC, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200 D, BE-3001 LeuVen, Belgium, Departament de Fı´sica, UniVersitat Auto`noma de Barcelona, E-08193 Bellaterra, Spain, Centre d’InVestigacio´ en Nanocie`ncia i Nanotecnologia (ICN-CSIC), Campus UniVersitat Auto`noma de Barcelona, E-08193 Bellaterra, Spain, Institute of Ion Beam Physics and Materials Research, Forschungszentrum Dresden-Rossendorf, P.O. Box 510119, D-01314 Dresden, Germany, and Institucio´ Catalana de Recerca i Estudis AVanc¸ats (ICREA) and Centre d’InVestigacio´ en Nanocie`ncia i Nanotecnologia (ICN-CSIC), Campus UniVersitat Auto`noma de Barcelona, E-08193 Bellaterra, Spain ReceiVed: June 18, 2010; ReVised Manuscript ReceiVed: July 28, 2010

Magnetic characterization is shown to be a highly effective, nondestructive, and commonly available method to accurately assess dehydrogenation temperatures and further clarify the reaction mechanisms during dehydrogenation in systems with superconducting or ferromagnetic constituents. As examples, the dehydrogenation temperature of NaBH4 in a nanostructured NaBH4/MgH2 system and the dehydrogenation process of nanostructured Mg2CoH5, based on the superconducting and ferromagnetic properties of MgB2 and Co, respectively, are determined. Introduction Solid-state hydrogen storage is currently being considered as an alternative energy carrier to fossil fuels. Intense research is being pursued in the search for low-cost, efficient methods for hydrogen production, storage, and delivery.1-3 For instance, complex hydrides of alkaline metals and transition metals are emerging as a group of suitable compounds to be utilized for hydrogen storage applications (e.g., for vehicular purposes).1 Essentially, it is due to the capability to theoretically desorb large quantities of hydrogen at temperatures below 300 °C, which enables these compounds to be compatible with polymer electrolyte membranes, used as proton-exchange membranes, in fuel cells devices.2 Particularly, NaBH4 has been one of the most widely studied boron-based complex hydrides due to both its high hydrogen storage capacity, of around 10.8 mass %, and its relative stability to air exposure.4,5 Actually, these characteristics fulfill some of the main requirements, concerning hydrogen storage targets, established by the U.S. Department of Energy.6 However, the high-temperature decomposition, which is fully reached at around 500 °C, and the lack of reversibility constitute the main drawbacks of this system for potential vehicular applications.7 Because particle size reduction in hydride species can both decrease the hydrogen desorption temperature and improve the hydrogen ab/desorption kinetics, high-energy ball milling has * To whom correspondence should be addressed. E-mail: [email protected]. † Katholieke Universiteit Leuven. ‡ Departament de Fı´sica, Universitat Auto`noma de Barcelona. § Centre d’Investigacio´ en Nanocie`ncia i Nanotecnologia (ICN-CSIC), Campus Universitat Auto`noma de Barcelona. | Forschungszentrum Dresden-Rossendorf. ⊥ Institucio´ Catalana de Recerca i Estudis Avanc¸ats (ICREA) and Centre d’Investigacio´ en Nanocie`ncia i Nanotecnologia (ICN-CSIC), Campus Universitat Auto`noma de Barcelona. + These authors contributed equally to this work.

been proposed as an effective route to make the properties of complex hydrides more adequate for applications.8-11 Nevertheless, some hydrides, such as NaBH4, are resistant to being refined up to the nanocrystalline range, even for long-term milling.12 Alternatively, the combination of complex alkaline metal hydrides with simple alkaline metal hydrides, such as MgH2, has been demonstrated to improve the thermodynamic properties of the constituents.13,14 For example, in the 2NaBH4 + MgH2 T 2NaH + MgB2 + 4H2 system, the endothermic desorption process proceeds with the exothermic formation of MgB2, which contributes to reducing the reaction enthalpy in comparison with single NaBH415 and lowering the NaBH4 desorption temperature.8 Remarkably, even though the thermodynamic properties of the aforementioned system are, to some extent, understood, the corresponding desorption mechanism has not yet been fully elucidated. Namely, the desorption process has been proposed to take place in three different steps: (I) the desorption of MgH2, which leads to the formation of Mg; (II) the decomposition process of NaBH4 in NaH and the formation of a possible intermediate compound, such as Na2B12H12; and (III) the reaction of the intermediate compound with the free magnesium to result in MgB2, NaH, and H2. Consequently, if the temperature onset of the MgB2 formation is evaluated, the dehydrogenation temperature of NaBH4 can be, to some extent, determined and the possible origins of intermediate compounds (e.g., Na2B12H12) further elucidated. Similarly, light metal hydrides based on Mg are another class of compounds widely studied for hydrogen storage applications due to their high storage capacities both by volume and by mass.16-18 In particular, Mg-based transition metal, TM, complex hydrides (Mg2TMHx) are of great interest owing to their low dehydrogenation temperatures as compared with pure MgH2.19,20 In fact, the TM element, present in these ternary hydrides, changes the thermodynamics of the system, which can be tuned to favor the hydrogen sorption properties. Most of these systems

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have been synthesized by direct hydrogenation of MgTM alloys, although, in the case of Mg2CoH5 (4.5 mass % H2), this route is not possible.21-23 However, Deledda et al. have recently obtained a yield close to 100% by means of milling Mg and Co metal powders under 50 bar hydrogen.24 Whereas in this system, the desorption pathway is well-understood, the desorption temperature onset is not well-established. Importantly, it should be noted that using X-ray diffraction (XRD) to estimate the formation of the desorption products is somewhat inaccurate, not only due to the limited power of XRD to differentiate between phases (usually taken to be around 2-5 vol %) but also due to the reduced size (and the concomitant broadening of the XRD peaks) of materials at the early stages of the transformation. In this article, we demonstrate that magnetic measurements can be used to accurately determine the dehydrogenation process of a broad range of different hydrogen storage materials. As examples, we have determined the dehydrogenation temperature of NaBH4 in a nanostructured NaBH4/MgH2 system and we have investigated the dehydrogenation process of nanostructured Mg2CoH5, based on the superconducting and ferromagnetic properties of MgB2 and Co, respectively. The high sensitivity of magnetic measurements and the large number of hydrogen storage systems based on superconducting or ferromagnetic elements (or alloys) demonstrate the versatility and potential of this approach. Experimental Section As-received NaBH4 (Aldrich, 98%) and MgH2 (Tego Magnan, 95%) powders were ball-milled in order to obtain 2NaBH4/ MgH2 nanostructured composites. The milling of the NaBH4/ MgH2 (2:1 molar ratio) mixture was carried out for 20 h in a planetary ball mill (Fritsch Pulverisette 5, P5), under an Ar atmosphere, using hardened stainless steel vials and balls (each vial of 80 mL in volume and 14 balls of 12 mm in diameter), with a ball-to-powder mass ratio of 10:1. The disk angular frequency and the vial frequency were kept at 230 and 287.5 rpm, respectively. Importantly, the powder mixtures were always manipulated inside a glovebox (MBraun-20-G) under a highpurity Ar atmosphere with O2 and H2O levels below 0.1 ppm. The as-milled mixtures were annealed at different temperatures in the range of 345-400 °C (with no holding time) and at 400 °C for diverse times of isothermal holding (30 min, 2 and 15 h), in a tubular furnace under vacuum (air pressure < 10-5 mbar). The heating and cooling rates were chosen to be 5 °C min-1 in order to avoid any quenching related phenomena. Commercial metal powders of Mg (99.5%) and Co (99.8%) were purchased from Alfa-Aesar. A 5 g portion of a 2:1 molar ratio mixture of Mg and Co powders was sealed in a stainless steel vial and subsequently ball-milled in a planetary ball mill (Fritsch Pulverisette 5, P5) for 40 h with a ball-to-powder mass ratio of 20:1. The disk angular frequency and the vial frequency were kept at 360 and 450 rpm, respectively. The milling was performed under a hydrogen atmosphere (50 bar) while monitoring the pressure and temperature by means of a special designed vial (Evico Magnetics). The as-milled powders were annealed at different temperatures by heating them to 270, 350, and 500 °C at 2 °C min-1, under an Ar flow of 120 sccm. The structural parameters, such as phase percentages and crystallite sizes (average coherently diffracting domain sizes), were evaluated by fitting the full X-ray diffraction (XRD) patterns, recorded with Cu KR radiation (NaBH4/MgH2) or Co KR1 radiation (Mg2CoH5), using the “Materials Analysis Using Diffraction” (MAUD) Rietveld refinement program. Specifically,

Figure 1. (a) X-ray diffraction (XRD) patterns of the 2NaBH4/MgH2 composites: as-milled and annealed at 375 °C, 400 °C, 400 °C for 30 min, 400 °C for 2 h, and 400 °C for 15 h. (b) Enlarged XRD patterns of the 2NaBH4/MgH2 composites: as-milled and annealed at 345 °C, 350 °C, 375 °C, 400 °C for 30 min, and 400 °C for 15 h. The main XRD peaks corresponding to the NaBH4 (#), MgH2 (§), Mg (+), and MgB2 (/) phases are indicated. Note that these patterns were recorded using Cu KR radiation.

MAUD fits the XRD patterns deconvoluting the “pure” material profile from the instrumental broadening and, then, by describing the peaks using pseudo-Voigt analytical functions having two components. From the integral width of each of the components corresponding to the “pure” profile, the crystallite size and the microstrains are obtained.25-29 The magnetic properties were measured by means of a superconducting quantum interference device (SQUID, Quantum Design MPMS XL 7T). The temperature dependence of the magnetization was measured in the T ) 2-50 K range and under an applied magnetic field of 10 Oe after zero-field cooling. Hysteresis loops were recorded in the (70 kOe range at 10 K after zero-field cooling from room temperature. Results and Discussion As can be seen from the XRD diffractograms of the as-milled NaBH4/MgH2 sample (Figure 1), there is no formation of new phases after the mechanical treatment. From the X-ray line profile analysis, the amount of the NaBH4 and MgH2 phases is found to be 76 and 24 mass %, respectively, in agreement with the initial mole ratio mixture. Moreover, a pronounced broadening of the MgH2 and NaBH4 XRD peaks is induced due to crystallite size refinement. Namely, the mechanical milling of

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Figure 3. XRD patterns of the Mg2CoH5 system: as-milled and annealed at 270, 350, and 500 °C. The main XRD peaks corresponding to the Mg2CoH5 (O), MgO (b), MgCo ([), Mg (0), and Co (/) phases are indicated. Note that these patterns were recorded using Co KR1 radiation.

Figure 2. Superconducting quantum interference device (SQUID) measurements, under an applied magnetic field of 10 Oe, of the 2NaBH4/MgH2 composites: (a) as-milled and annealed at 400 °C, 400 °C for 30 min, 400 °C for 2 h, and 400 °C for 15 h and (b) annealed at 345, 350, and 375 °C. The solid lines are guides to the eye. M stands for magnetization.

2NaBH4/MgH2 mixtures is rather effective in reducing the crystallite size of both NaBH4 and MgH2 phases, which reaches 80 and 19 nm, respectively (compared with 140 and 151 nm of the as-received powders). From the XRD patterns in Figure 1a, it can be observed that, when annealing the milled samples at 375 °C, MgH2 has almost vanished and there is no hint of MgB2. In Figure 1b, an extended view of the XRD patterns is presented in order to highlight that no traces of MgB2 are detectable for the samples annealed at 345, 350, and 375 °C. Conversely, as the annealing temperature is increased to 400 °C, an incipient peak of MgB2 appears. These results are in qualitative agreement with dynamic synchrotron XRD studies of the same system.8 When the annealing process is carried out at 400 °C for different amounts of time, it can be observed that both the NaBH4 and the MgH2 peaks progressively vanish, forming Mg and MgB2, resulting eventually in pure MgB2 with a crystallite size of around 9 nm. As shown in Figure 2a, the superconducting features of MgB2 can be clearly observed in the magnetization measurements for the samples annealed at 400 °C for different times. Namely, the magnetization starts to decrease between 30 and 35 K for the samples annealed at 400 °C for 30 min, 400 °C for 2 h, and 400 °C for 15 h, that is, close to the bulk transition temperature of MgB2 (TC ) 39 K).30 The slightly reduced transition temperature could be due to a nonstoichiometry of the samples, the incorporation of Na in the structure, or the presence of magnetic impurities (e.g., debris from the magnetic medium),31-34 even though the rather small crystallite size of

the MgB2 phase probably plays a dominant role in the TC reduction.35,36 The sample annealed at 400 °C for the shortest time, for which the MgB2 XRD peak is hardly visible, still shows a rather strong magnetic signal, although with a reduced critical temperature (probably related to its very small crystallite size). Interestingly, as it can be observed in Figure 2b, the specimens annealed at either 350 or 375 °C exhibit an upturn of the magnetization when compared with the as-milled sample. This indicates the formation of intermediate phases with a paramagnetic character. Remarkably, for the samples annealed at either 350 or 375 °C, which have no signs of MgB2 in the XRD patterns (see Figure 2b), the magnetization curve exhibits a downward turn of the magnetization as a result of the existence of superconducting MgB2. In contrast, the sample annealed at 345 °C does not exhibit any decrease in the magnetization, evidencing that no MgB2 is present. Therefore, assuming that the dehydrogenation of NaBH4 starts when the MgB2 is formed, the temperature onset of the NaBH4 dehydrogenation is established to take place around 345 °C. Interestingly, the dehydrogenation process of many other complex hydrides proposed for hydrogen storage, such as LiBH4/MgH2,37 Ca(BH4)2/MgH2, or Mg(BH4)2, can lead to superconducting MgB2.38,39 Notably, many other hydrogen storage systems incorporate superconducting elements, such as V, Nb, or Ta (V2H, NbH, or TaH),40-42 which would be equally susceptible of being magnetically studied. Similar studies can be also carried out in systems where ferromagnetic elements or alloys are produced as a result of the dehydrogenation process. As an example, we have studied the Mg2CoH5 system. Shown in Figure 3 are the XRD patterns of the Mg2CoH5 powders prepared by reactive ball-milling (asmilled) and after different heat treatments. The pattern of the as-milled sample evidences that there is a complete formation of the tetragonal Mg2CoH5 phase. After heating the sample to 270 °C, only the growth of the Mg2CoH5 crystallites and the incipient formation of MgO can be detected. Upon heating to 350 °C, new peaks corresponding to the face-centered cubic Co phase appear, confirming the decomposition of Mg2CoH5, even though it is still far from complete. In addition, it should be noted that the broad bump centered around 50° is attributed to the amorphous Co formed during the desorption route. Finally, upon reaching 500 °C, the complete desorption process of the Mg2CoH5 phase takes place, giving rise to the formation of MgCo. From the XRD data, it can be inferred that the

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J. Phys. Chem. C, Vol. 114, No. 39, 2010 16821 systems. Importantly, many systems proposed for hydrogen storage applications incorporate a magnetic element (Ni, Co, and Fe and even rare-earths), such as Mg2NiH4, Mg2CoH5, Mg2FeH6, and LaNiHx, and, consequently, could be studied by magnetic means. Conclusions The temperature onsets of MgB2 and Co formation are assessed, and concomitantly, the starting dehydrogenation temperature of NaBH4 in the 2NaBH4/MgH2 system and the dehydrogenation process of Mg2CoH5 are evaluated by magnetic means (based on their superconducting and ferromagnetic properties, respectively). Therefore, magnetic characterization is established as a powerful tool to sensitively determine dehydrogenation temperatures and to further clarify the reaction mechanisms during dehydrogenation in systems with superconducting or ferromagnetic counterparts. Moreover, it is important to stress that magnetic measurements are capable of detecting extremely small signals (parts per million in many cases). In effect, this makes them a highly effective, simple, and nondestructive method that needs extremely small amounts of sample (less than 1 mg, in contrast to tens or hundreds of milligrams needed for many other chemical or physical characterization methods) and that could open new investigation pathways to ease the search for hydrogen storage materials.

Figure 4. (a) Hysteresis loops at 10 K, measured by SQUID magnetometry, of the Mg2CoH5 system: as-milled and annealed at 270, 350, and 500 °C. Shown in (b) is the dependence of the saturation magnetization, MS, on the annealing, where the formed phases are indicated. The solid lines in both panels are guides to the eye. M and Happlied stand for magnetization and applied magnetic field, respectively.

Acknowledgment. This work has been partially financed by the 2009-SGR-1292 and MAT2007-66302-C02 research projects. E.M. thanks the Fund for Scientific ResearchsFlanders (FWO) for financial support and V. Vanhooren, S. Enzo, and M. Baricco for fruitful discussions. S.G. is indebted to the European Commission for the support through the MRTNContract “Complex Solid State Reactions for Energy Efficient Hydrogen Storage” (MRTN-CT-2006-035366). A.L.-O. acknowledges his FPI fellowship from the Spanish MICINN, cofinanced by the ESF. M.D.B. was partially supported by an ICREA ACADEMIA award. References and Notes

desorption path corresponds to: Mg2CoH5 f 2Mg + Co + 5/2H2; Mg + Co f MgCo.23 To assess the dehydrogenation process of Mg2CoH5 from magnetic measurements, it was established that hysteresis loops, magnetization versus applied magnetic field (see Figure 4a), were more adequate in this case. The loop corresponding to the as-milled sample shows a saturation magnetization, MS ) 12.1 emu g-1, which we attribute to the weak ferromagnetic character of Mg2CoH5. As the sample is heated to 270 °C, the magnetic signal almost doubles to MS ) 21.8 emu g-1, implying the formation of a strong magnetic phase, that is, Co. However, XRD shows no obvious traces of Co (in analogy to the MgB2 case). Thus, magnetically, we can safely determine that the desorption temperature of Mg2CoH5 is below 270 °C. Upon further increasing the temperature to 350 °C, MS increases dramatically to MS ) 60.4 emu g-1, in agreement with the major generation of Co observed by XRD. Remarkably, MS decreases to MS ) 21.0 emu/g upon reaching 500 °C. This is in concordance with the formation of MgCo, as observed by XRD, because this alloy has a much smaller MS than pure Co (see Figure 4b). Hence, magnetic measurements allow us not only to accurately determine the onset of the desorption but to follow correctly the desorption pathway also in ferromagnet-based

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