Metal borohydrides thermodynamical stability

Y. Nakamori, K. Miwa, A. Ninomiya, H. Li, N. Ohba, S.-l. Towata, A. Ziittel, S.-l. Orimo, Correlation between thermodynamical stabilities of metal borohydrides and cation electronegatives First principles calculations and experiments , Phys. Rev. B 74 (2006) 45126. 

K. Miwa, N. Ohba, S. Towata, Y. Nakamori, A. Ztittel, S. Orimo, First-principles study on thermodynamical stability of metal borohydrides Aluminum borohydride A1(BH )2 , J. Alloys Compd. 446-447 (2007) 310-314. 

The thermal desorption temperature Td of M(BH4) determined using gas chromatography was also closely correlated with Xp [20], as shown in Fig. 1. Td (closed circles) decreases with an increase in the value of Xp- Therefore, we conclude that the value of Xp of the cation (metal) is an indicator that assists in the estimation of the thermodynamical stabilities of single-cation borohydrides M(BH4) with the corresponding value of Td in various temperature ranges. 

In this study, we examine the thermodynamical stabilities of double-cation borohydrides MM (Q Td) , and then we verify whether the correlation between Td and Xp determined for single-cation borohydrides can be reasonably extended also to double-cation ones. (The possible extended correlation might lead to precise adjustments of the thermodynamical stabilities of borohydrides, which is considered to be difficult in case of single-cation borohydrides due to the discrete value of Xp of each cation (metal).) For the abovementioned purpose, we propose (xp = 1.4, Td of Zr(BH4)4 is approximately 440 K) and Li ... 

The thermodynamic stabilities for the series of metal borohydrides M(BH4) (M = Li, Na, K, Mg, Ca, Sc, Zr, Hf, Cu, Zn and Al n = -4) have been systematically investigated by first-principles calculations.The heat of formation of M(BH4) AT/boro were estimated from the difference of the total energies between the left- and right-hand sides of Eq. (15.23) ... 

Li H-W, Orimo S, Nakamori Y, Miwa K, Ohba N, Towata S and Ziittel A (2007), Materials designing of metal borohydrides viewpoints from thermodynamical stabilities, J. Alloys Compounds, 446-447, 315-318. 

Li, H.-W. Orimo, S. Nakamori, Y. Miwa, K. Ohba, N. Towata, S. ZtitteL A., Materials designing of metal borohydrides Viewpoints fiom thermodynamical stabilities, J. Alloys Compd. 2007,446-447,315-318. 

Sodium borohydride, or correctly sodium tetrahydroborate, is a white crystalline solid that is stable in dry air to 300°C. It is highly soluble in water and slowly decomposes until the solution becomes alkaline. Decomposition is more rapid in acidic solution, but NaBH4 is kinetically stable in alkaline solutions when the pH is above about 12.9. For this reason, 2%-3% sodium hydroxide is typically added to NaBH4 solutions to provide stability. Although kinetically stable, aqueous solutions of NaBH4 are thermodynamically unstable, and decomposition can be catalyzed by a number of transition metals. 

Factors that determine the stability of borohydride complexes have been discussed. 3 Thermodynamic considerations include the ease with which a coordinated [BH4] anion can reduce the metal center, Eq. 10.5 ... 

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