**Abstract**

We have investigated the complex metal hydrides involving light weight elements or compounds for the reversible hydrogen storage. The complex hydrides are prepared via an inexpensive solid state mechanochemical process under reactive atmosphere at ambient temperatures. The complex metal hydride, LiBH4 with different mole concentrations of ZnCl2 were characterized for the new phase formation and hydrogen decomposition characteristics of Zn(BH4)2. Furthermore, the complex metal hydride is destabilized using the addition of nano MgH2 for the reversible hydrogen storage characteristics. The structural, microstructural, surface, and other physicochemical behaviors of these lightweight complex metal hydrides have been studied via various metrological tools such as x-ray diffraction, Fourier transform infrared spectroscopy, thermal programed desorption, and PCT hydrogen absorption methods.

**Keywords:** hydrogen storage, mechanochemistry, complex metal hydrides, hydrogen sorption, thermal decomposition

## **1. Introduction**

Complex metal hydrides are basically composed of various light weight elements and compounds that bonded with the hydrogen atom in binary, ternary, or quaternary structures [1–3]. The binary hydrides are made of light-weight elements and compounds, for example, LiH and MgH2 often releases the absorbed hydrogen at very high temperatures (>400°C) and are irreversible in nature [4, 5]. The reversibility of hydrogen absorption and desorption can be improved by bringing either the cationic or anionic substitution to form a ternary compound for example, LiBH4, Mg2FeH6, or Zn(BH4)2 [6, 7]. Further to decrease the temperature of hydrogen sorption can be facilitated by destabilizing via introducing alkaline metal hydrides, to form a quaternary structure, for example, LaMg2NiH7 [8] or LiMg2RuH7 [9]. There are consistent efforts underway towards the development of a *holy-grail* light weight reversible hydrogen storage materials to meet the 2025 DOE technical targets [10].

Lithium and Magnesium are considered to be lightest and highly reactive elements due to their placement next to hydrogen in the periodic table. The ternary hydride of Lithium, namely, LiBH4 possesses very high storage density of hydrogen up to ~20 wt.% and ~ 125 kgH2/m3 in terms of gravimetric and volumetric measurements [11–14]. However, the significant hydrogen decomposition occurs at temperatures >400°C [15]. The dehydriding and reversible hydriding of LiBH4 follows the typical metal-hydrogen bonding reactions per the equations below.

$$LiBH\_4 \leftrightarrow LiH + B + \frac{3}{2}H\_2 \tag{1}$$

The reversibility enhancements and destabilization of LiBH4 at lower temperature was demonstrated by admixing of SiO2 [14], and MgH2 [16]. Particularly, the addition of half a mole of MgH2 destabilizes the LiBH4 structure and enables the formation of intermediate meta-stable MgB2 alloy phase during the hydrogen release and absorption phases per the equation below [16].

$$\rm LiBH\_4 + \frac{1}{2}MgH\_2 \leftrightarrow LiH + \frac{1}{2}MgB\_2 + H\_2 \tag{2}$$

MgH2 on the other hand, has a comparative hydrogen storage capacity of ~7.6 wt.%, at temperatures >325°C, however, the slow kinetics of reaction makes this metal hydride not usable for potential applications [5]. The role of different 3d transition metal catalysts for example, Ti, V, Mn, Fe, Co, Ni [17–21] and transition metal oxide, namely, Nb2O5 [22, 23], has demonstrated greater reversibility of hydrogen from the magnesium lattice with faster kinetics hydrogen enabled applications.

A new ternary complex metal hydride Zn(BH4)2, was formulated for the low temperature hydrogen decomposition by reacting either NaBH4 or LiBH4 with ZnCl2 salt in a mechanochemical process at room temperature according to the Equation [24, 25].

$$2\text{LiBH}\_4 + \text{ZnCl}\_2 \rightarrow \text{Zn(BH}\_4\text{)}\_2 + 2\text{LiCl} \tag{3}$$

Keeping the aforementioned metal hydrides and its salient characteristics, we have successfully synthesized new complex hydrides with combinations of LiBH4 and MgH2 for the formation of Zn substituted systems and catalysts assisted complex metal hydrides for reversible hydrogen storage and for vehicular on-board applications.

### **2. Experimental details**

The various chemical compounds with purities (in parentheses) such as, ZnCl2 (99.999%), TiF3 (99.999%), nano-Ni (99.9%), nano-Zn (99 + %) are procured from Sigma-Aldrich. The binary and ternary metal hydrides such as LiBH4 (95%), MgH2 (98%) are purchased from Alfa Aesar. The high purity nano-Ni (99.999%) was purchased from QuantumSphere Inc. All the chemicals have been stored in a nitrogen filled glove box, used readily without further purification. A mechanochemical milling of the mixtures have been carried out in a Fritsch Pulversitte planetary mono mill P6 using 80 ml stainless steel bowl sealed with a specially designed lid with two scharder valves for inert or reactive gas purging. Various experimental parameters such as ball to powder weight ratio (20:1), milling speed (300 rpm), milling time (20 min. to 2 h) and milling medium (hydrogen purging, 1 atm for every 30 minutes of milling) were optimized. All the sample manufacturing and manipulation for both synthesis and characterization were done in a nitrogen filled glove box (Innovation Technology).

*Light Weight Complex Metal Hydrides for Reversible Hydrogen Storage DOI: http://dx.doi.org/10.5772/intechopen.95808*

The thermogravimetric and the differential scanning calorimetric analyses were performed using the TA Instrument's SDT-600 with alumina crucibles heated at 5°C/min in flow of nitrogen or argon ambient. The Universal Analysis software V4.0C was deployed to analyze the results obtained from the SDT. The reversible hydrogen absorption and desorption measurements have been carried out using Setaram's PCTPro Sievert's type instrument with pre-calibrated volumes with an accuracy of ∓1°C. A Lab View software program was used for data monitoring and recording, and the measurement analysis was executed using Hy-Analysis macros in the Igor program.

X-ray diffraction characterization was carried out using a Philips X'pert diffractometer with CuKα radiation of λ = 5.4060 Ǻ. The x-ray beam from the cathode ray tube was incident on the sample, via incident slit of 1o , a 10 mm beam mask and soller slit of 0.04 rad. The diffracted x-ray beam was received by the detector via the receiving slit, a 2o anti scatter slit and a monochromator. The collected XRD patterns were analyzed using the PANalytical X'pert Highscore software version 1.0e for phase identification and crystallite size distribution. A polyethylene clear plastic wrap (thin foil) was used to protect the samples from air and moisture by wrapping the sample holder completely with the thin foil which shows diffraction peaks in the 2θ range of 21–28o . The chemical environment of the complex metal hydrides, such as B-H stretches and BH2 deformation bands were explored using a Perkin Elmer's FTIR spectrometer and the samples were specially prepared with KBr in nitrogen filled glove box.
