(151f) Modeling of Self-Hydrolysis of Concentrated Sodium Borohydride Solution
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In spite of the US DOE recommended no-go for sodium borohydride for on-board vehicular hydrogen storage, a great deal of interest remains particularly with view to portable applications. Sodium borohydride hydrolysis, being well within the 2015 targets, regarding specific energy and energy density, falls short in practice due to the low efficiency of the water-based system. The excess water necessary to drive the reaction, the stabilization of sodium borohydride by the production of basic species in the course of the reaction, as well as water capture by the by-products, are factors known to decrease the gravimetric efficiency and which limits the use of concentrated solutions. In this work we report on experimental and modeling studies of the kinetics of self-hydrolysis of concentrated NaBH4 solutions for temperatures varying between 298 - 353 K, taking as a base an NMR study of the self-hydrolysis, where the metaborate is the by-product. Experiments were performed using 11B NMR measurements. NaBH4 solutions varying from 10 to 20 wt% were studied for a reaction time span of 25 hrs. The results indicated that hydrated metaborates or precipitates did not occur under the studied experimental conditions, furthermore it is suggested that alkalinization due to the production of borates is directly related to the arrest in the rate of self-hydrolysis.
In the current study the hydrolysis reaction is assumed to occur under ideal stoichiometric conditions, i.e. with a hydration factor of zero,
NaBH4+H2O → NaB2+4H2 (1) and kinetics to be described by a power-law model in borohydride concentration (equation 2). -d[NaBH4]/dt=k[NaBH4]n (2)
Where, k is the rate constant, and n is the order of reaction with respect to sodium borohydride concentration. On the basis of the work of Davis et al. , Gonçalves et al.  described the reaction, for 10 wt% borohydride solutions at temperatures between 300 and 363K, by an empirical correlation with two terms, expressing separately the contributions of the acidic and the alkaline conditions. A good agreement between experimental and simulated data was obtained, with the term including the proton predominating at low pH and the water term at high pH. The assumption now made that water concentration is constant and that the acidic term can be neglected, as reflected in equation 1, seems justified given the large excess water and alkalinity in all tests undertaken. The estimation of the kinetic parameters was undertaken by conventional means and also in gPROMS  by a reaction model, which incorporated the density of the solution estimated as a function of the wt% of NaBO2 and NaOH. These results which will be reported were always found to be statistically highly significant, while the match between predicted and experimental data was better achieved with the latter.
References  Marrero-Alfonso EY, Beaird AM, Davis TA, Matthews MA. Hydrogen generation from chemical hydrides. Ind Engg Chem Res 2009;48(3):3703-3712.  Pinto AMFR, Falcao DS, Silva RA, Rangel CM. Hydrogen generation and storage from hydrolysis of sodium borohydride in batch reactors. Int J Hydrogen Energy 2006; 31:1341-1347.  Wee JH, Lee KW, Kim SH. Sodium borohydride as the hydrogen supplier for proton exchange membrane fuel cell systems. Fuel Process Technol 2006;87:811-819.  Liu BH, Li ZP. A Review: Hydrogen generation from borohydride hydrolysis reaction. J Power Sources 2009;187:527-534.  Demirci UB, Akdim O, Miele P. Ten-year efforts for sodium borohydride for on-board automotive hydrogen storage, Int J Hydrogen Energy 2009;34:2638-2645.  Schlesinger HI, Brown HC, Finholt AE, Gilbreath JR, Hoekstra HR, Hyde EK. Sodium Borohydride, Its Hydrolysis and its Use as a Reducing Agent and in the Generation of Hydrogen, J. Am. Chem. Soc 1953; 75:215-219.  Davis R. E.,E. Bromels, C. L. Kibby, 1962, Boron Hydrides. III. Hydrolysis of Sodium Borohydride in Aqueous Solution, J. Am. Chem. Soc. 84, 885-892.  Gonçalves, A., Castro, P., Novais, A.Q., Fernandes, V., Rangel, C. and Matos, H., Dynamic Modeling of Hydrogen Generation via Hydrolysis of Sodium Borohydride, In Proceedings of PRES'07, 24-28 June, Ischia, Italy, (In Chemical Engineering Transactions, Vol. 12, 2007. Editor: Jiri Klemes, AIDIC, Italy, 243.)  gPROMS Advanced User Guide, 2004, Process Systems Engineering Ltd., London.
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