Monday, November 5, 2007 - 1:10 PM
98c

Ab-Initio Calculations for Ni Additives in Zn(BH4)2

Pabitra Choudhury1, Venkat R. Bhethanabotla1, and Elias Stefanakos2. (1) CERC and SRL, Chemical Engineering, University of South Florida, 4202 E Fowler Ave, ENB 118, Tampa, FL 33620, (2) Clean Energy Reseach Center (CERC), University of South Florida, 4202 E Fowler Ave, ENB 118, Tampa, FL 33620

I.       Motivation/Background                                                                                                                                     

Hydrogen is considered to be the cleanest fuel amongst all other alternative fuels available for onboard vehicular application. The high operational cost of liquid hydrogen as well as the low explosive limit (4%) of hydrogen in the gas phase makes its storage an important aspect for potential vehicular application. The technical challenge is to find materials that can exhibit favorable thermodynamics and kinetics for hydrogen de-sorption and absorption, and have the ability to store a sufficient amount of hydrogen by weight and as well as by volume.  Zn(BH4)2 is considered to be a potential candidate for on-board application as it has very high theoretical hydrogen storage capacity of 8.4 wt% and also reasonably low decomposition temperature compared to complex alkali borohydrides.

Improvement of the hydrogen ab- and de-sorption kinetics in complex hydrides is essential for this material to be a good reversible hydrogen storage media in the transportation sector. It is known that the addition of titanium-based compounds have improved the kinetics significantly for NaAlH41-3. Experimental results found in our research group suggest that Ni doping in Zn(BH4)2 improves the de-hydrogenation but the actual physical understanding remains elusive. Hence, in this work, density functional theory (DFT) is utilized to evaluate the effect of Ni doping in Zn(BH4)2 complex. 

 

II.    Computational details and methodology                                                                                                

 

We utilized the DFT approach to model gas-metal interactions at the electronic scale to gain useful insights into the effect of catalysts / additives to the Zn(BH4)2 structural behavior. The Vienna Ab-initio Simulation Package (VASP)4-6 was used to calculate the electronic structure and mixing energy of doped Zn(BH4)2.

We started our calculations based on the possible two substitution reactions as follows:                                                                                                                 

Zn(BH4)2 + xNi = Zn1-xNix(BH4)2 + xZn                                                                         (1)

Zn(BH4)2 + xNi = ZnB2-xNixH8 + xB                                                                              (2)

Zn(BH4)2 = Zn + 2B + 4H2                                                                                            (3)                   

                                                                                                                                   

We calculated the cohesive energy of doped Zn(BH4)2 (i.e. mixed complex hydrides Zn1-xNix(BH4)2 and ZnB2-xNixH8, (x=1–4)) by considering the supercell containing 16 Zn(BH4)2 formula units (176 atoms), and substituted 1-4 of the Zn or B atoms with Ni atoms as shown in the Figures 2 & 3. The total energies of Zn(BH4)2, Zn, B, Ni and H2 were also calculated to evaluate the enthalpy changes for reactions (1-3) given above based on the following structures: orthorhombic for Zn(BH4)2, hcp for Zn (P63/mmc), trigonal for B (R-3m), fcc for Ni. For H2, we performed ab-initio MD simulation for the H2 dimmer as shown in Figure 1. The interatomic distance was varied from 0.65 Ǻ to 0.88 Ǻ in steps of 0.01 Ǻ and in each step the total energy of the system was calculated. All the structures were optimized during the total energy calculations.                                                                                                          

                                                                                                                                   

III. Results/Discussion

Using the above methods, we investigated the possibility of Ni dissolution into zinc borohydride from the electronic structure calculations and also from the enthalpy changes, i.e. the enthalpy difference between the mixture and the pure materials. We find that Ni substitutes two possible positions i.e. Zn and B in Zn(BH4)2 and their effect on crystal stability and lattice parameters also investigated in this work.                                                                                                                       

                                                                                                                       

References:

  1. Eung-Kyu Lee, Young Whan Cho, Jong Kyu Yoon, Journal of Alloys and Compounds 416 (2006) 245–249
  2. Tejs Vegge, Phys. Chem. Chem. Phys., (2006), 8, 4853–4861
  3. E. H. Majzoub and K. F. McCarty, Phy. Rev. B, 71, 024118 (2005)
  4. G. Kresse and J. Furthmüller, Phys. Rev. B 54, 11169 (1996).
  5. G. Kresse and J. Furthmüller, Comput. Mater. Sci. 6, 15 (1996).
  6. G. Kresse and J. Hafner, Phys. Rev. B 47, R558 (1993).

 

Figure 1. Bond legth vs total energy of H2 dimer

Figure 2. Cohesive energy of pure and Ni substitutional Zn(BH4)2

Figure 3. Three dimensional Ni doped Zn15NiB32H64 complex. (Blue: Zn, red: H and Green: B  and purple: Ni atom)

Figure 4. Bonding analysis of relaxed doped structure projected on y-z plane.


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