Hiroshima University, Japan.
By Prof. Takayuki Ichikawa, Hiroshima University, Japan.
For hydrogen storage, heat storage and anode of Li ion batteries, various metal hydrides were focused as our group activities. By using the hydrogen absorption/desorption reactions of hydrogen storage alloys, hydrogen compression system can be realized based on non-mechanical techniques. Some of typical hydrogen storage alloys, ex. Ti-Cr-Mn, are capable of releasing hydrogen at more than 80 MPa at a relatively low temperature (less than 200°C), in which the standard enthalpy difference for hydrogenation is ca. 22 kJ/mol H2. In order to understand the fundamental information for this chemical compression, isobar properties of the hydrogen storage alloy were examined to determine the temperature required to achieve a dissociation pressure of 80 MPa [1].
For the heat storage system, a hydride material, TiH2, is considered to be suitable for practical applications due to its quite high enthalpy change about 144 kJ/mol H2. However, Ti itself is limited for hydrogen ab/desorption reactions because the thermal activation is necessary for first hydrogenation due to surface oxide layer of TiOx. In order to overcome this disadvantage, the surface chemical state and those hydrogen absorbing properties of Ti milled with an organic solvent were investigated. Although pristine Ti requires 300°C to absorb hydrogen, the titanium treated with acetone as one of the best candidate by ball milling can absorb hydrogen at “room” temperature. Of course, we tried various kind of solid catalysts for Ti. But superior catalytic effect for hydrogen absorption was not able to be observed. Therefore, the mechanism of surface modification would be quite interesting [2].
For the electrochemical applications, which are expected as anode of Li-ion batteries, various metal hydrides are considered to be a potential material for lithium-ion batteries. A hydride conversion reaction, MHx + xLi+ + xe− ↔ M + xLiH, is considered here owing to its high theoretical Li storage capacity, relatively low volume expansion, and suitable working potential with very small polarization. Although MgH2, in particular, as the most promising anode material for LIBs among all MHs, possesses a high theoretical Li storage capacity of 2038 [mAhg−1] and a suitable working potential of around 0.5 [V (vs. Li+/Li)] with extremely small polarization, it shows slow kinetics, poor reversibility, and unfavorable cyclability in conventional organic liquid electrolyte systems [3]. Our group focuses on all-solid-state Li-ion batteries, which are composed of a novel solid-state Li-ion conductor LiBH4, various hydride working electrode, and a Li metal counter electrode, in order to systematically investigate the relationship between stability of metal hydrides, and their electrochemical properties as anode for all-solid-state [4-6]. As a result, MgH2, TiH2, and VH2 revealed quite good relationship, possessing relatively low polarization.
References
[1] N. Tsurui, , K. Goshome, S. Hino, N. Endo, T. Maeda, H. Miyaoka, T. Ichikawa., Mater. Trans. 59 (2018) 855-857.
[2] K. Shinzato, S. Hamamoto, H. Miyaoka, T. Ichikawa, J. Phys. Chem. C, in print.
[3] Y. Oumellal, A. Rougier, G.A. Nazri, J.M. Tarascon, L. Aymard, Nat. Mater., 7 (2008) 916-921.
[4] L. Zeng, K. Kawahito, S. Ikeda, T. Ichikawa, H. Miyaoka, Y. Kojima, Chem. Commun. 51 (2015) 9773-9776.
[5] K. Kawahito, L. Zeng, T. Ichikawa, H. Miyaoka, Y. Kojima, Mater. Trans. 57 (2016) 755-757.
[6] Y. Matsumura, K. Takagishi, H. Miyaoka, T. Ichikawa, Mater. Trans. in print.
Biography
Dr. Takayuki Ichikawa was born in Okayama prefecture, Japan. He obtained Ph. D from Hiroshima University in 2002. He has focused mainly on inorganic hydrogen storage materials, such as amide-imide systems, magnesium hydride, ammonia, titanium hydride, ammonia borane and its derivative materials, M-Li alloys (M=C, Si, Ge, Sn) for hydrogen storage systems, and carbon materials. With respect to MgH2, hydride anode properties of Li-ion batteries are also investigated by using all-solid-state techniques. He has consistently focused on the reaction mechanism of these hydrogen storage materials and clarified it using various experimental methods, such as NMR, Raman spectroscopy, XAS, XPS, DSC, TG-DTA-MS, FT-IR, and PC isothermal measurement.