Article pubs.acs.org/JPCC
Effect of Ti Intermetallic Catalysts on Hydrogen Storage Properties of Magnesium Hydride Chengshang Zhou,† Zhigang Zak Fang,*,† Chai Ren,† Jingzhu Li,† and Jun Lu‡ †
Department of Metallurgical Engineering, The University of Utah, 135 South 1460 East, Room 412, Salt Lake City, Utah 84112-0114, United States ‡ Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ABSTRACT: Magnesium hydride is a promising candidate for solid-state hydrogen storage and thermal energy storage applications. A series of Tibased intermetallic alloy (TiAl, Ti3Al, TiNi, TiFe, TiNb, TiMn2, and TiVMn)doped MgH2 materials were systematically investigated in this study to improve its hydrogen storage properties. The dehydrogenation and hydrogenation properties were studied by using both thermogravimetric analysis and pressure−composition−temperature (PCT) isothermal to characterize the temperature of dehydrogenation and the kinetics of both desorption and absorption of hydrogen by these doped MgH2. Results show significant improvements of both dehydrogenation and hydrogenation kinetics as a result of adding the Ti intermetallic alloys as catalysts. In particular, the TiMn2-doped Mg demonstrated extraordinary hydrogen absorption capability at room temperature and 1 bar hydrogen pressure. The PCT experiments also show that the hydrogen equilibrium pressures of MgH2 were not affected by these additives.
1. INTRODUCTION Magnesium hydride (MgH2) is considered a promising candidate material for hydrogen storage owing to its high theoretical H capacity (7.6 wt %), reversibility, and low cost. However, despite the fact that MgH2 meets many criteria for a hydrogen storage material, there are two major limitations preventing it from being used in practical applications: (1) MgH2 is thermodynamically too stable. The dehydrogenation reaction requires a high temperature of 350−400 °C.1 A temperature of 288 °C is required to reach a dissociation hydrogen pressure of 1 bar.2 (2) The Mg−H2 reaction has poor kinetics. Hydrogenation of Mg without additives is usually very sluggish and typically requires a temperature higher than 300 °C and pressure more than 20 bar. In the past two decades, many efforts were directed to investigate how to improve the hydrogen storage properties of Mg. A very effective technique is to use mechanical ball milling to create fresh surface areas and reduce crystallite size to nanoscales.3 Recently, a quantum chemistry study predicted that, for example, if the crystalline size can be reduced to 0.9 nm, the enthalpy of decomposition of MgH2 can be decreased to 63 kJ/K mol·H2, and the corresponding theoretical equilibrium temperature at 1 bar hydrogen pressure is reduced to 200 °C.4 However, it is very difficult, if not impossible, to make nanosized powder to be smaller than a few nanometers using current ball-milling techniques, and it is even more challenging to maintain such fine crystalline sizes during thermal cycling. © XXXX American Chemical Society
Another widely used pathway to enhance the kinetics is to use catalysts or additives that could enable magnesium to absorb and release hydrogen at relatively low temperatures. Effective additives reported in literatures can be categorized in three different groups: transition metals (TMs),5−7 transition metal oxides,8−11 and intermetallic compounds.12−17 These additives can also be classified as catalytic additives or alloying additives depending on their interactions with Mg. Although the effects of different materials on MgH2 are not all well understood, we can generally refer to those as “catalysts” that are effective in increasing the rate but do not participate in the de/hydrogenation reactions. Furthermore, usually only a small amount is added as “catalysts”. In contrast, those that are added to MgH2 in relatively large percentages are regarded as alloying additives, which may form alloys with Mg or participate in the reactions. As examples of catalytic additive, Liang et al.5 reported that the hydrogen storage properties of MgH2 can be improved by adding TMs (Ti, V, Mn, Fe, and Ni). Among these elements, it was found that V and Ti have superior catalytic effects on desorption and absorption properties, respectively. A widely studied catalyst is Nb2O5. It was shown that adding just 1 mol % Nb2O5 can lead to a rapid roomtemperature absorption of hydrogen and desorption at temperature as low as 200 °C.18 Another catalyst for MgH2 is Received: March 20, 2013 Revised: May 25, 2013
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dx.doi.org/10.1021/jp402770p | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
Table 1. Raw Materials for This Study materials
formula
composition
supplier, part no.
MgH2 Ti TiH2 TiFe TiNi TiAl Ti3Al TiVMn TiMn2 TiNb
MgH2 Ti TiH2 Ti0.44Fe0.56 Ti0.74Ni0.26 TiAl Ti3Al TiV0.62Mn1.5 Ti0.98Zr0.02V0.43Fe0.09 Cr0.05Mn1.5 TiNb0.2
MgH2 > 98% Ti > 99.7% TiH2 > 98% TiFe + TiFe2 intermetallics elemental Ti + elemental Ni mainly TiAl intermetallic mainly Ti3Al intermetallic AB2 type intermetallic AB2 type intermetallic solid solution alloy
Sigma-Aldrich, 683043 Sigma-Aldrich, 268496 Sigma-Aldrich, 209279 Alfa Aesar, 43500 Alfa Aesar, 88384 Alfa Aesar, 88393 Alfa Aesar, 43101 GfE, Hydralloy C Sigma-Aldrich, 685941 sintering, custom making
nickel. Borgdanivić et al19 reported a systematic study of Nidoped Mg powder on its kinetics, H capacity, and cycling stability, which demonstrated the effectiveness of Ni-doping. Lu20,21 and Choi22−24 reported the catalytic effects of titanium hydride. It was demonstrated that the nanosized Mg−Ti−H system prepared by using a custom-made ultrahigh-energy-high-pressure (UHEHP) planetary ball milling facilitated both the hydrogenation and the dehydrogenation reaction for MgH2. The results showed that the presence of TiH2 decreases the enthalpy ΔH and also the entropy ΔS21 of the dehydrogenation of MgH2. Moreover, the system can absorb a significant amount of hydrogen at room temperature.20 The effects of TiH2 are attributed to the presence of Ti element, which raises the issue of whether other Ti-based alloys and intermetallic compounds could be more effective than TiH2. When a large percentage of materials are added into MgH2 to boost the performance, the additives can no longer be considered solely as catalysts, rather as “alloying additives” that may participate in the reactions. As an example of using alloying additives to improve the hydrogen storage properties of MgH2, Liang et al.14 studied hydrogen absorption of Mg−50 wt % LaNi5 prepared by using mechanical milling. The system showed improved absorption kinetics. About 2.5 wt % hydrogen uptake was observed under 15 bar pressure and at room temperature (29 °C). Furthermore, Liang reported that adding 35 wt % intermetallic FeTi1.2 into Mg results in improvements of both desorption and absorption kinetics.12 Reule et al. reported a Mg−40 wt % (Fe0.8Mn0.2)Ti system with a dehydrogenation temperature of 297 °C compared with ∼447 °C of pure magnesium hydride.25 A Mg−30 wt % amorphous TiMn1.5 system was also reported to have a beneficial effect on the dehydrogenation kinetics of MgH2.26 Yu et al.27 reported that 20 wt % Ti0.4Cr0.15Mn0.15V0.3 alloy ball-milled with MgH2 could absorb more than 90% of its initial hydrogen capacity within 100 min at temperatures below 100 °C. These positive results demonstrate that the intermetallic compounds of TMs can be used effectively as additives to improve the performance of MgH2. The effects of these additives were likely attributable to the synergistic hydrogenation/dehydrogenation effects between the MgH2 and these added materials. However, the H capacity of MgH2 was significantly decreased when large percentages of these additives were used. Furthermore, it was not clear if the catalytic effects of these intermetallic compounds played a role, if any. It should also be noted that several ternary Mg−Ti TM (transition metal) systems prepared by thin film deposition have been studied and reported, adding to the promising prospects of TM additives. Zahiri et al.28 investigated the Mg−
Ti−Cr composites and found that it has a significant roomtemperature hydrogen uptake at a pressure as low as 2 bar. An Mg−Fe−Ti system reported by Kalisvaart et al.29 showed remarkably rapid hydrogen adsorption and desorption at low temperatures (200 °C) even after 500 cycles. Similarly, Mg layers with 2 nm AlTi layers could achieve 5.1 wt % H capacity at 473 K.30 A computational study using first-principle techniques showed that Mg−Ti−X (X = Al or Si) alloys could be designed for destabilizing the hydride material and tuning the energy of hydrogenation.31 These encouraging results further demonstrate the positive effects of Ti-based materials for improving the hydrogen storage properties of MgH2. The present research is designed to investigate the catalytic effects of titanium-based intermetallic materials on the hydrogen adsorption and desorption properties of MgH2. Selected Ti-based intermetallic alloys were used to dope MgH2 in small mole fractions to minimize the contributions to de/ hydrogenation reaction by the additives themselves. We focus on the effects of these materials on the hydrogen storage behavior of magnesium. MgH2−5 atom % M (M = Ti or Tibased compounds) were prepared using an ultra-high-energyhigh-pressure (UHEHP) planetary milling machine. The kinetics of hydrogen absorption and desorption were characterized. The apparent activation energies for both desorption and absorption reactions of these systems were calculated by using the Ozawa−Flynn−Wall (OFW) method and Johnson−Mehl−Avrami (JMA) model, respectively. The results demonstrate that all of the selected Ti-based intermetallic catalysts have significant catalytic effects on the hydrogen storage properties of magnesium hydride.
2. EXPERIMENTAL SECTION The information on raw materials for this work is provided in Table 1. Note that there are significant impurities in the hydride materials that were purchased for this work including MgH2 and TiH2. All of the material handling was carried out in a glovebox filled with circulating purified argon (99.999%), which contains
