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Deliquescence in the Hydrolysis of Sodium Borohydride by Water Vapor Amy M. Beaird, Thomas A. Davis, and Michael A. Matthews* UniVersity of South Carolina, Department of Chemical Engineering, 301 Main Street, Columbia, South Carolina 29208
The interaction between sodium borohydride (NaBH4) and water vapor leading to hydrolysis and hydrogen generation has been investigated by a visual technique. In situ video monitoring confirms that reaction is preceded by deliquescence of NaBH4 upon exposure to water vapor, forming a viscous liquid solution that releases hydrogen. A regime of temperature and relative humidity under which the deliquescence is favorable has been determined. A relative humidity threshold exists below which NaBH4 powder does not absorb water vapor into the bulk to form a solution and thereby does not undergo reaction to form hydrogen. The deliquescence behavior of NaBH4 in water vapor provides an alternative reaction pathway that has potential to improve hydrogen storage density by reducing excess water and other additives. 1. Introduction This paper investigates the physical phenomena inherent in the water vapor hydrolysis of sodium borohydride (NaBH4), a material of interest as a lightweight hydrogen storage medium. NaBH4 is the primary focus of this work because of its relative safety, low cost, and high energy density and because it is the most widely available and well-studied chemical hydride. Other hygroscopic hydrides (e.g., LiBH4, NaAlH4, etc.) are expected to undergo similar reaction pathways with analogous chemistry.1 Hydrolysis of NaBH4 by water vapor is a nascent concept, so exploitation for commercial purposes has not been attempted. The hydrolysis of sodium borohydride by liquid water has been thoroughly investigated in recent years. In the conventional aqueous reaction pathway (eq 1), NaBH4 is dissolved in excess liquid water at room temperature. NaBH4(aq) + 4H2O(l) f 4H2(g) + NaB(OH)4(aq) + ∆Hr (1) Equation 1 represents a simplified overall stoichiometry; however, it is well-known that NaBH4 undergoes stepwise hydrolysis. The reaction mechanism is pH-dependent with acidic conditions favoring reaction. Under neutral and basic conditions the hydrolysis reaction is self-stabilizing.2 As the basic byproduct NaB(OH)4 is formed, the reaction slows, resulting in low hydrogen yields. Therefore, acidification or a catalyst is required to achieve stoichiometric hydrogen yields in the room-temperature aqueous reaction.2,3 In addition, a large excess of water is necessary to predissolve the NaBH4 (solubility of 55 g/(100 g of H2O)),4 and still more water is required in order for the less soluble NaB(OH)4 product (solubility of 28 g/(100 g of H2O))5 to remain in solution after reaction. If aqueous NaBH4 is to be stored for any length of time, NaOH or other simple hydroxides are also added to increase the pH, thereby inhibiting hydrogen release. Although the dilute aqueous NaBH4 solution approach is desirable from a materials handling perspective, the excess water and other additives significantly decrease the hydrogen storage capacity, rendering it impractical for many applications.6 In fact, the United States Department of Energy (DOE) made a “no-go” decision on the viability of aqueous NaBH4 for its FreedomCar program, citing the solubility/ crystallization issue as a major concern.7 An overview of * To whom correspondence should be addressed. E-mail: matthews@ cec.sc.edu.
hydrogen storage based on chemical hydrides and other technologies is available in our recent review26 on the topic. Alternative reaction approaches are necessary if improvement of hydrogen storage capacity utilizing NaBH4 will be achieved.6 Since the DOE no-go decision in 2007, many researchers have shifted focus toward approaches involving solid NaBH4, thus eliminating the solubility limitation. Some of the emerging concepts include premixing an acid or catalyst with a liquid water feed prior to contact with NaBH4 powder,8-11 physically mixing a catalyst powder with NaBH4 prior to contact with liquid water,12-14 combusting fine metal powders to propagate hydrolysis,15 or passing steam/water vapor over solid NaBH4 at elevated temperatures.16,17 Each of these concepts has new fundamental mechanistic aspects to consider. Sodium borohydride powder is known to be very hygroscopic, absorbing copious amounts of water when exposed to humid air. Below 37.4 °C, NaBH4 readily forms a dihydrate, which is slightly more hydrophobic than the anhydrous solid.18,19 During prolonged exposure to humidity, sodium borohydride readily dissolves in sorbed water, a phenomenon known as deliquescence. Despite the well-known hygroscopic nature, the investigation of hydrolysis by water vapor sorption is limited to just a few studies at room temperature where no appreciable reaction occurs18,20 and our previous studies at elevated temperature (110-150 °C) described by Aiello16 and Marrero-Alfonso.17,21 Furthermore, except for one study,18 the deliquescence description traditionally applied to food,22 pharmaceutical,23 and aerosol24 particles has not formally been extended to NaBH4 powders until now. In our reaction scheme (eq 2), steam or water vapor is contacted with solid NaBH4 powder at elevated temperatures. Hydrogen is generated as well as sodium metaborate (NaBO2 · xH2O) with a temperature-dependent hydration state (x). NaBH4(s) + (2 + x)H2O(g) f 4H2(g) + NaBO2 · xH2O(s) + ∆Hr
(2)
No catalyst is required to obtain hydrogen yields in excess of 90% while feeding pure steam at 110 °C.16,17 Gas chromatography (GC) analysis of the outlet gas confirmed that hydrogen was the only gaseous product.16,17 We have previously demonstrated that, at 110 °C, the byproduct exists as a dihydrate (NaBO2 · 2H2O or NaB(OH)4),21 the same product determined by others25 for aqueous hydrolysis at room temperature.
10.1021/ie100244v 2010 American Chemical Society Published on Web 09/17/2010
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Figure 1. Apparatus for in situ visualization of NaBH4 hydrolysis by water vapor.
Although excess water of solution is not required to dissolve the NaBH4 in this approach, the two bound waters remaining in the byproduct still reduce the volumetric and gravimetric energy density.26 Minimizing the water of hydration (x) is crucial for a practical system design based on the hydrolysis of NaBH4. Other hydration states, such as a 1/3 hydrate, are known to exist when NaBO2 precipitates from saturated aqueous solutions at higher temperatures (115 °C for x ) 0.33).27,28 Hydrolysis by water vapor affords an opportunity to reduce the hydration state and ultimately improve energy densities, if conditions can be optimized for reaction at elevated temperatures. The mechanism for the release of hydrogen by water vapor/ NaBH4 contact has not been well understood. Aiello16 and Marrero-Alfonso17 both observed decreasing reaction rates and hydrogen yields via steam hydrolysis when temperature was increased from 110 to 140 °C. From a kinetic standpoint, the lower rate at higher temperatures is unexpected. MarreroAlfonso observed that the solid hydrolysis products were hard and dry at 140 °C but soft and moist at 110 °C.17 It was originally postulated that the formation of an impermeable metaborate “shell” at higher temperatures limited the mass transfer of steam into the solid resulting in lower yields.16 However, subsequent experimentation did not support this hypothesis. Therefore, the purpose of this study was to investigate the physical phenomena associated with water vapor/ solid hydride reaction pathway and to establish the conditions under which hydrolysis by this approach is favorable. As will subsequently be demonstrated, it is now apparent that solid, vapor, and liquid are all present during the course of NaBH4 hydrolysis by water vapor. The initial step in the pathway is deliquescence of NaBH4. A basic understanding of the phase behavior during the water vapor hydrolysis reaction is necessary should the reaction pathway be exploited in future engineering designs. 2. Experimental Section 2.1. Reagents. Anhydrous sodium borohydride (98%, Alfa Aesar) was used as received. It was stored and handled in an inert glovebox environment to prevent unwanted interactions with ambient humidity. Water was deionized to 17 ΜΩ · cm resistivity in a Barnstead system. Ultrahigh-purity nitrogen (National Welders) was employed as a carrier gas. 2.2. Apparatus and Methods. The hydrolysis of sodium borohydride by water vapor was performed in an isothermal semibatch visualization reactor system illustrated in Figure 1. A 0.25 in. diameter glass tube (the reactor) was filled with 0.1-0.3 g of NaBH4 powder that was constrained with glass wool. A Hawkeye Classic Precision Borescope video camera was positioned above the reactor through an opening in the top oven wall. The camera enabled continuous in situ visualization of the reactor contents. To prevent premature absorption of water
Figure 2. Stages of NaBH4 hydrolysis reaction at 110 °C and 0.51 water mole fraction (38.4% RH).
during startup, the reactor was purged with nitrogen while the oven was heated to the reaction temperature. The desired mole fraction of water (yH2O) was achieved by flowing nitrogen carrier gas at 0.1 SLPM through a heated, 500 mL jacketed vessel (saturator) containing deionized water. The saturated gas was directed through a coil housed inside the oven to equilibrate to the reaction temperature. The relative humidity of the saturated nitrogen stream entering the reactor was determined from the temperatures of the saturator and reactor. Initially, blank runs (i.e., no NaBH4) were performed to demonstrate water balance closure in the system; the water mass balance closed to within 5%. The visualization reactor allowed for control of water mole fraction between 0.03 and 0.9, over a range of reaction temperatures between 50 and 150 °C. The relative humidity was maintained below 40% to ensure there was no condensation in the reactor system. The system was allowed to equilibrate for 10 min at each set of conditions. If no water uptake was observed during the 10 min interval, the reaction temperature was lowered, or alternatively the relative humidity was raised incrementally and allowed to equilibrate again. This process was continued until water sorption by NaBH4 was visually observed. 3. Results and Discussion Previously unknown phase behavior during the reaction of NaBH4 with water vapor was directly observed in this study. A set of sequential images taken during the exposure of NaBH4 powder to a humid N2 stream (0.51 mol fraction of water) at 110 °C (38.4% RH) is shown in Figure 2. Prior to the introduction of water, discrete NaBH4 crystals were visible in the glass reactor bed (Figure 2a). Upon introduction of water vapor, the powder began absorbing water, particularly around the edges (Figure 2b), until the NaBH4 was completely dissolved (Figure 2c). During the deliquescence process, hydrogen bubbles were formed within the liquid phase (Figure 2c). Aiello and Marrero-Alfonso previously confirmed that hydrogen is the only gaseous byproduct by GC analysis.16,17 The production of hydrogen continued for several minutes (Figure 2d). After 22 min, the bubbling ceased and a clear viscous solution remained (Figure 2e). Finally, as the supply of water vapor was terminated and the reactor was purged with dry nitrogen, a white film of solid sodium metaborate byproduct coated the glass reactor (Figure 2f). Deliquescence and subsequent reaction were not observed under all conditions of temperature and water mole fraction. In
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Figure 3. NaBH4 deliquescence phase diagram ([, stable powder; b, deliquescence occurs; 9, Stockmayer data20).
Figure 4. Deliquescence relative humidity of NaBH4 powder as a function of temperature ([, stable powder; b, deliquescence occurs; 9, Stockmayer data20).
contrast to the results presented in Figure 2, at 0.31 mol fraction and 110 °C (21.7% RH) there was no change in appearance or phase of the sample over 3.5 h of exposure to water vapor. The powder remained loose and free-flowing when removed from the reactor and promptly began absorbing moisture when exposed to ambient humidity. The marked contrast between these results suggested a factorial design of experiments in which the deliquescence regime was determined as a function of temperature. The deliquescence phase diagram for NaBH4 is presented in Figure 3. The circles correspond to the observation of NaBH4 deliquescence, and diamonds represent conditions where the powder remained unchanged. A distinct threshold of conditions necessary for deliquescence of NaBH4 is exhibited. The slope of the deliquescence curve is similar to the dew point curve below yH2O ) 0.6, but it begins deviating at higher water content. The dew point and deliquescence curves bracket the region where deliquescence of NaBH4 occurs. Below the dew point curve, water condenses from vapor to liquid. Above the deliquescence curve, the NaBH4 is retained as a solid powder, thereby preventing reaction. The deliquescence relative humidity (DRH) of NaBH4 varies between 19 and 32% depending on reaction temperature, exhibiting a maximum near 110 °C, as shown in Figure 4. Variation of DRH with temperature is exhibited by many crystalline salt materials.29 This maximum is likely due to a change in the surface energy of NaBH4 with temperature, and further studies are planned to explore this hypothesis using in situ Raman spectroscopy.
Comparison of our results with two independent measurements reported in the literature indicates good agreement. Stockmayer et al.20 measured the activity of water above a roomtemperature saturated NaBH4 solution to be aw ) 0.262 ( 0.0524. It is common in the deliquescence literature to equate aw × 100% to deliquescence relative humidity for a single solid component.22,23,30 Therefore, the DRH for NaBH4 at 25 °C can be approximated as 26.2 ( 0.524%. As shown by the square marker in Figures 3 and 4, the Stockmayer20 activity measurement is both consistent with and complementary to the results of the current study. Murtomaa et al.18 used a gravimetric technique to determine water uptake for discrete relative humidity conditions at 25 °C. They observed no water uptake at 19%, but at 31% RH, NaBH4 absorbed appreciable moisture. This suggests that the deliquescence relative humidity is between 19 and 31%, which is consistent with the Stockmayer observation20 and our result of 25% RH at 50 °C. No evidence of reaction (i.e., weight loss due to hydrogen release) is apparent in the room-temperature water uptake study by Murtomaa.18 Another investigation by Kong et al.31 also exposed sodium borohydride to water vapor at room temperature and moisture sorption occurred. No hydrogen was detected until the NaBH4 formed a solution and flowed into a water reservoir. The lack of reaction at room temperature is likely because the concentrated aqueous solution formed remains kinetically stabilized at low temperatures. Murtomaa et al.18 also previously demonstrated that an increase in relative humidity results in an increase in both the rate and total amount of moisture uptake by NaBH4 at 25 °C. This was qualitatively observed in our experiments at higher temperatures as well. In terms of the variables in Figure 3, a reduction in relative humidity is achieved either by lowering the reaction temperature or by increasing the water content of the inlet gas. If we consider the previous work by Aiello16 and Marrero-Alfonso17,21 with steam (yH2O ) 1), we postulate that the reason for reduced yields at higher temperatures is a lower rate and overall quantity of moisture uptake by the solid at 140 °C than at 110 °C. We assert that the combination of favorable deliquescence conditions and elevated reaction temperature results in the nearly complete conversions observed at 110 °C. 4. Conclusions This paper establishes the region of temperature and water vapor content under which deliquescence of NaBH4 is favored. Between 25 and 160 °C the DRH varies between 19 and 32% with a maximum near 110 °C. Below the deliquescence relative humidity at a given temperature, the powder cannot spontaneously deliquesce to form a solution and reaction is thereby inhibited. It was observed that a higher relative humidity resulted in a faster deliquescence step; however more investigation is needed to quantify the rate, hydrogen yield, and the intermediate reaction steps. Further work into this is planned using in situ Raman spectroscopy to investigate the rate of deliquescence and hydrogen release. It is clear that a combination of elevated reaction temperatures and deliquescent conditions are necessary for hydrolysis by this approach. Understanding the deliquescence behavior of NaBH4 is a critical step toward the design of a hydrogen storage system based on the concept. It also provides insight into the conditions at which NaBH4 can be stored to prevent unwanted moisture uptake.
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Acknowledgment This work is supported by NSF CBET 0756089. We acknowledge helpful discussions with Professor J. Karl Johnson of the University of Pittsburgh and John Patton of Trulite. Literature Cited (1) Laversenne, L.; Goutaudier, C.; Chiriac, R.; Sigala, C.; Bonnetot, B. Hydrogen Storage in Borohydrides Comparison of Hydrolysis Conditions of LiBH4, NaBH4 and KBH4. J. Therm. Anal. Calorim. 2008, 94 (3), 785– 790. (2) Schlesinger, H. I.; Brown, H. C.; Finholt, A. E.; Gilbreath, J. R.; Hoekstra, H. R.; Hyde, E. K. 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. (3) Kaufman, C. M.; Sen, B. Hydrogen Generation by Hydrolysis of Sodium Tetrahydroborate: Effects of Acids and Transition Metals and Their Salts. J. Chem. Soc., Dalton Trans. (1972-1999) 1985, 1972-1999 (2), 307–13. (4) James, B. D.; Wallbridge, M. G. H. Metal Tetrahydroborates. Progress in Inorganic Chemistry; Stephen, J. L., Ed.; John Wiley and Sons: New York, 1970; Vol. 11, pp 99-231. (5) Kojima, Y.; Suzuki, K.; Fukumoto, K.; Sasaki, M.; Yamamoto, T.; Kawai, Y.; Hayashi, H. Hydrogen Generation Using Sodium Borohydride Solution and Metal Catalyst Coated on Metal Oxide. Int. J. Hydrogen Energy 2002, 27 (10), 1029–1034. (6) Demirci, U. B.; Akdim, O.; Miele, P. Ten-Year Efforts and a NoGo Recommendation for Sodium Borohydride for on-Board Automotive Hydrogen Storage. Int. J. Hydrogen Energy 2009, 34 (6), 2638–2645. (7) Go/No-Go Recommendation for Sodium Borohydride for on-Board Vehicular Hydrogen Storage, Independent ReView, NREL/MP-150-42220. November 2007; U.S. Department of Energy Hydrogen Program: Washington, DC, 2007. (8) Gislon, P.; Monteleone, G.; Prosini, P. P. Hydrogen Production from Solid Sodium Borohydride. Int. J. Hydrogen Energy 2009, 34 (2), 929– 937. (9) Javed, U.; Subramanian, V. Hydrogen Generation Using a Borohydride-Based Semi-Continuous Milli-Scale Reactor: Effects of Physicochemical Parameters on Hydrogen Yield. Energy Fuels 2009, 23, 408–413. (10) Murugesan, S.; Subramanian, V. Effects of Acid Accelerators on Hydrogen Generation from Solid Sodium Borohydride Using Small Scale Devices. J. Power Sources 2009, 187 (1), 216–223. (11) Zhang, Q.; Mohring, R. M. Reaction Chemistry between Aqueous Sulfuric Acid and Solid Sodium Borohydride. Ind. Eng. Chem. Res. 2009, 48, 1603–1607. (12) Andrieux, J.; Swierczynski, D.; Laversenne, L.; Garron, A.; Bennici, S.; Goutaudier, C.; Miele, P.; Auroux, A.; Bonnetot, B. A Multifactor Study of Catalyzed Hydrolysis of Solid NaBH4 on Cobalt Nanoparticles: Thermodynamics and Kinetics. Int. J. Hydrogen Energy 2009, 34 (2), 938– 951. (13) Cento, C.; Gislon, P.; Prosini, P. P. Hydrogen Generation by Hydrolysis of NaBH4. Int. J. Hydrogen Energy 2009, 34 (10), 4551–4554. (14) Liu, B. H.; Li, Z. P.; Suda, S. Solid Sodium Borohydride as a Hydrogen Source for Fuel Cells. J. Alloys Compd. 2009, 468 (1-2), 493– 498.
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(15) Shafirovich, E.; Diakov, V.; Varma, A. Combustion-Assisted Hydrolysis of Sodium Borohydride for Hydrogen Generation. Int. J. Hydrogen Energy 2007, 32 (2), 207–211. (16) Aiello, R.; Sharp, J. H.; Matthews, M. A. Production of Hydrogen from Chemical Hydrides via Hydrolysis with Steam. Int. J. Hydrogen Energy 1999, 24, 1123–1130. (17) Marrero-Alfonso, E. Y.; Gray, J. R.; Davis, T. A.; Matthews, M. A. Hydrolysis of Sodium Borohydride with Steam. Int. J. Hydrogen Energy 2007, 32 (18), 4717–4722. (18) Murtomaa, M.; Laine, E.; Salonen, J.; Kuusinen, O. On Effects of Ambient Humidity on Sodium Borohydride Powder. Powder Handling and Processing 1999, 11 (1), 87–90. (19) Filinchuk, Y.; Hagemann, H., Structure and Properties of NaBH4 · 2H2O and NaBH4. Eur. J. Inorg. Chem. 2008, 20, 3127-3133. (20) Stockmayer, W. H.; Rice, D. W.; Stephenson, C. C. Thermodynamic Properties of Sodium Borohydride and Aqueous Borohydride Ion. J. Am. Chem. Soc. 1955, 77, 1980–1983. (21) Marrero-Alfonso, E. Y.; Gray, J. R.; Davis, T. A.; Matthews, M. A. Minimizing Water Utilization in Hydrolysis of Sodium Borohydride: The Role of Sodium Metaborate Hydrates. Int. J. Hydrogen Energy 2007, 32 (18), 4723–4730. (22) Al-Muhtaseb, A. H.; McMinn, W. A. M.; Magee, T. R. A. Moisture Sorption Isotherm Characteristics of Food Products: A Review. Food Bioprod. Process. 2002, 80 (2), 118–128. (23) Kontny, M. J.; Conners, J. J., Water Sorption of Drugs and Dosage Forms. Encyclopedia of Pharmaceutical Technology, 3rd ed.; Informa Healthcare: New York, NY, 2006. (24) Chan, C. K.; Ha, Z.; Choi, M. Y. Study of Water Activities of Aerosols of Mixtures of Sodium and Magnesium Salts. Atmos. EnViron. 2000, 34 (28), 4795–4803. (25) Stepanov, N.; Uvarov, V.; Popov, I.; Sasson, Y. Study of By-Product of NaBH4 Hydrolysis and Its Behavior at a Room Temperature. Int. J. Hydrogen Energy 2008, 33 (24), 7378–7384. (26) Marrero-Alfonso, E. Y.; Beaird, A. M.; Davis, T. A.; Matthews, M. A. Hydrogen Generation from Chemical Hydrides. Ind. Eng. Chem. Res. 2009, 48, 3703–3712. (27) Krc, J. Crystallographic Data. 44. Sodium Metaborate Dihydrate. Anal. Chem. 1951, 23, 806–806. (28) Nies, N. P.; Hulbert, R. W. Solubility Isotherms in the System Sodium Oxide-Boric Oxide-Water. Revised Solubility-Temperature Curves of Boric Acid, Borax, Sodium Pentaborate, and Sodium Metaborate. J. Chem. Eng. Data 1967, 12, 303–313. (29) Greenspan, L. Humidity Fixed Points of Binary Saturated Aqueous Solutions. J. Res. Natl. Bur. Stand., Sect. A 1977, 81a (1), 89–96. (30) Salameh, A. K.; Taylor, L. S. Deliquescence in Binary Mixtures. Pharm. Res. 2005, 22 (2), 318–324. (31) Kong, V. C. Y.; Foulkes, F. R.; Kirk, D. W.; Hinatsu, J. T. Development of Hydrogen Storage for Fuel Cell Generators. I: Hydrogen Generation Using Hydrolysis of Hydrides. Int. J. Hydrogen Energy 1999, 24 (7), 665–675.
ReceiVed for reView February 1, 2010 ReVised manuscript receiVed May 7, 2010 Accepted June 29, 2010 IE100244V
