Kinetic models of concentrated NaBH 4 hydrolysis Lin Yu a , Perry Pellechia a,b , Michael A. Matthews a, * a Department of Chemical Engineering, Swearingen Engineering Center, University of South Carolina, 301 Main Street, Columbia, SC 29208, USA b Department of Chemistry, University of South Carolina, Columbia, SC 29208, USA article info Article history: Received 20 July 2013 Received in revised form 16 October 2013 Accepted 18 October 2013 Available online 17 November 2013 Keywords: Kinetic models Sodium borohydride Self-hydrolysis Concentrated solution abstract The hydrolysis of NaBH 4 in liquid solution has been extensively studied in the past few years; however, data on the kinetics of self-hydrolysis in concentrated solution are few. This work reports the kinetic modeling of self-hydrolysis of 10e20 wt.% NaBH 4 at 25e80  C. Also, pH data were obtained independently of the reaction kinetics data. The data obtained from Boron-11 NMR measurements and pH are used to determine kinetic parameters. An empirical power law model is evaluated over a wide pH range. The effects of temperature, pH and initial sodium borohyride concentration are reported. The power law model reproduced the trends of the kinetics of the hydrolysis reaction. In addition, a pseudo first order model derived from a proposed reaction mechanism is evaluated. The behavior of the pseudo first order rate constant k is interpreted in terms of the effect of pH. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 1. Introduction Hydrogen storage with chemical hydrides is tantalizing in part because the hydrides can be stored at ambient temperature and pressure. Chemical methods for hydrogen storage include adsorption in carbon nanotubes, metal hydrideecarbon composites, irreversible hydrides and borohydrides [1e4]. Sodium borohydride (NaBH 4 ) has attracted attention as a po- tential hydrogen storage material because it has a high theo- retical energy density of 10.8 wt.% based on aqueous hydrolysis, and is relatively inexpensive and easy to handle [5]. Storing NaBH 4 in the form of aqueous solutions has been proposed for safety and controllability, because excess water effectively acts as a thermal buffer, absorbing the exothermic heat of reaction and preventing thermal runaway [6]. How- ever, the gravimetric capacity of real storage system based on solutions will invariably be lower than 10.8 wt.% because excess water is required to dissolve the NaBH 4 and its byproduct, NaBO 2 , not to mention the mass of the reaction and storage vessels [6,7]. Therefore, the most likely near-term applications for hydrogen storage via sodium borohydride will be for small-volume portable devices and backup power, using dry NaBH 4 that is contacted with water at the time of demand. The ideal stoichiometry of aqueous hydrolysis by NaBH 4 is shown in Equation (1). The ideal hydrolysis has a gravimetric efficiency of 10.8 wt.% hydrogen, which can only be realized by minimizing water utilization and finding the most effective route to engineer low-volume, lightweight delivery systems. Detailed kinetic studies of the reaction in Equation (1), under a variety of conditions, are necessary to model and control the release of H 2 in an engineered system. NaBH 4 (s) þ 4H 2 O(l) / 4H 2 (g) þ NaB(OH) 4 (aq) þ heat (1) Research on the kinetics of hydrolysis of NaBH 4 has recently focused on catalyzed systems at low concentrations of NaBH 4 . Catalysts help overcome the well-known tendency of the self- * Corresponding author. Tel.: þ1 803 777 0556; fax: þ1 803 777 9597. E-mail address: mamatthe@mailbox.sc.edu (M.A. Matthews). Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 39 (2014) 442 e448 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.10.105