Tailoring of Metal Borohydrides for Hydrogen Storage Applications
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0971-Z02-01
Tailoring of Metal Borohydrides for Hydrogen Storage Applications Yuko Nakamori1, Kazutoshi Miwa2, Hai Wen Li1, Nobuko Ohba2, Shin-ichi Towata2, and Shinichi Orimo1 1 Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai, 980-8577, Japan 2 Toyota Central R&D Labs, Nagakute, 480-1192, Japan
ABSTRACT Recent investigations on thermodynamical stabilities of metal borohydrides were reviewed. The first-principles calculations indicated that the heat of formation normalized by the number of BH4 complexs, ∆Hboro, show a good correlation with the Pauling electronegativitiese of M, χP, which is represented by the liner relation, ∆Hboro = 252.8χP – 396.4 in the unit of kJ/mol BH4. In order to clarify the correlation between the stability of borohydrides and the electronegativity χP of M, M(BH4)n (M = Mg, Ca, Sc, Ti, V, Cr, Mn, Zn, Zr and Al; n = 2-4) were systematically synthesized by mechanical milling. The thermal desorption analyses indicated that Td correlate with χP of M; Td decrease with increasing the values of χP, in M(BH4)n. Furthermore, the correlation can be reasonably extended to double cation ones, ZrLin-4(BH4)n. For single cation, M(BH4)n (M = Mn, Zn and Al; χP ≧ 1.5) desorb borane besides hydrogen, and M(BH4)n (M = Ti, V and Cr; χP ≧ 1.5) desorb small amount of hydrogen provably due to desorption reaction during milling. Therefore χP is an indicator to approximately estimate the stability of M(BH4)n, and appropriate χP in M(BH4)n is expected to be smaller than 1.5. The enthalpy change for the desorption reaction, ∆Hdes, is estimated using our predicted ∆Hboro and the reported data for decomposition product, ∆Hhyd/boride, which shows a good correlation with the observed Td. These results are useful for exploring M(BH4)n with appropriate stability for hydrogen storage applications
INTRODUCTION Metal borohydrides M(BH4)n are potential candidate for hydrogen storage applications because of their high gravimetric hydrogen densities. Among borohyrides, alkaline borohydrides such as LiBH4, NaBH4 and KBH4 are well known and their properties have been investigated [17]. Since these borohydrides are thermodynamically too stable and desorb hydrogen at relative high temperature, to decrease their hydrogen desorption temperature are required for practical applications. Recently, we have investigated the thermodynamical stabilities of borohydrides theoretically and experimentally by considering electronagativity of M.[8-14] In this paper, the systematic understanding of the stabilities of M(BH4)n will be reviewed. Moreover, the stability of double-cation borohydrides MM’(BH4)n will be discussed in order to extend our predicted correlation and to adjust the hydrogen desorption temperature precisely [16].
METHODS Computational method The present calculation have been performed using the ultrasoft pseudopotential method [17] based on density functional theory [18]. The generalized gradient approximation (GGA) [19] is adopted for the exchange-correlation energy. The cutoff energies used in t
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