Destabilization of LiH by Li Insertion into Ge Ankur Jain,* ,† Erika Kawasako, ‡ Hiroki Miyaoka, § Tao Ma, ∥ Shigehito Isobe, ∥ Takayuki Ichikawa,* ,†,‡ and Yoshitsugu Kojima †,‡ † Institute for Advanced Materials Research, ‡ Graduate school of Advanced Sciences of Matter, and § Institute for Sustainable Sciences and Development, Hiroshima University, Higashi-Hiroshima 739-8530, Japan ∥ Graduate School of Engineering, Hokkaido University, N-13, W-8, Sapporo 060-8278, Japan ABSTRACT: Lithium hydride has high hydrogen capacity (12.7 mass %), but could not be considered as practical hydrogen storage media because of being very stable (required 900 °C for 0.1 MPa desorption pressure). Recently, C and Si have been found suitable to reduce the stability of LiH. This motivates us to investigate the properties of other alloys of Li, formed with the other elements. In the present work, Li 3.75 Ge (Li 15 Ge 4 ) alloy was synthesized by mechanical milling, which transformed into Li 4.2 Ge (Li 21.1875 Ge 5 ) and Li 3.5 Ge (Li 7 Ge 2 ) phases during the vacuum heating at 400 °C. Hydrogenation of thus formed alloys at 400 °C under 3 MPa hydrogen pressure during PCI experiment transforms this mixed phase into Li 2 GeH 0.5 (Li 4 Ge 2 H) and LiH phase. A remarkable decrease in the desorption temperature (∼300−450 °C) is observed by preparing the above alloy with Ge as observed from TG-DTA-MS experiment. The enthalpy of the reaction has also been calculated using the van’t Hoff plot. The present work concluded with the establishment of a direct relationship between hydrogen storage parameters and electrochemical parameters using the Nernst equation and van’t Hoff equation. A good agreement is found between the values of required potential for lithiation/delithiation as obtained by two methods. 1. INTRODUCTION Hydrogen energy is considered as one of the solutions of future energy needs. The main obstacle to establish hydrogen economy worldwide is its efficient storage. 1 Hydrogen storage in the form of solid-state metal hydride has been proposed as one of the interesting media. Especially light metal hydrides having a high content of hydrogen such as LiH, MgH 2 are attractive contenders to fulfill the projected storage capacity of US-DOE and other energy agencies worldwide. 2 However, their high stability forms a bottleneck for them to be used as a practical storage media. LiH (12.7 mass % H 2 ) requires 900 °C for 0.1 MPa desorption pressure. This high stability of LiH is due to the presence of a strong ionic bond in contrast to the delocalized metallic bonding presented in transition metal hydride. 3,4 The high directionality of this ionic bond generates a high activation barrier for atomic motion, and thus lowers the sorption kinetics and generates many thermodynamic con- straints. 5 These thermodynamic constraints can be removed by using a third element that forms a new compound with Li upon dehydrogenation of LiH. 6 This new compound should have more stability than elemental Li, for example, as in the case of LiC 6 . 7 With this proposal applied on LiH/MgH 2 −Si, Vajo et al. suggested interesting possibilities of destabilization using other elements such as B, C, N, P, and S. 6 Since then, there are many reports to destabilize these systems by addition of a third element. In the previous report from our group, it was confirmed that Li intercalated graphite is capable of absorbing/ desorbing hydrogen as per the following reaction 8 in the temperature range 200−500 °C: + ↔ + x x C Li /2H 6C LiH x 6 2 (1) More recently, our group studied the hydrogen storage properties of destabilized lithium silicon system and proposed the following reaction 9 with a lower enthalpy of reaction, that is, −117 and −99 kJ/mol-H 2 , for the first and second steps of the reaction, respectively: + ↔ + x x Li Si /2H LiH Si x 2 (2) Received: January 4, 2013 Revised: March 1, 2013 Published: March 1, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 5650 dx.doi.org/10.1021/jp400133t | J. Phys. Chem. C 2013, 117, 5650−5657