Ind. Eng. Chem. Res. 2009, 48, 1603–1607
1603
Reaction Chemistry Between Aqueous Sulfuric Acid and Solid Sodium Borohydride Qinglin Zhang*,† and Richard M. Mohring Millennium Cell Incorporated, One Industrial Way West, Eatontown, New Jersey 07724
Chemical hydrides represent a class of promising hydrogen storage materials. Solid sodium borohydride in particular offers superior fuel stability and its reaction with acid provides a new approach for hydrogen generation. In this study, the chemistry involved in a hydrogen storage system based on solid NaBH4 and aqueous H2SO4 was studied experimentally. Factors influencing hydrogen generation dynamics, sodium borohydride conversion, and borax byproduct distributions were studied. A fuel-only energy density of 2310 Wh/kg (on the basis of lower heating value of hydrogen contained) is projected for hydrogen storage using pure sodium borohydride and 31 wt % H2SO4. 1. Introduction The hydrolysis of sodium borohydride has been investigated as a source of hydrogen for fuel cell applications1-3 because of its high energy density. In particular, catalytic hydrolysis of aqueous sodium borohydride solutions have been exploited to make hydrogen generators, particularly for portable fuel cell applications. The catalytic reaction with aqueous sodium borohydride proceeds as follows NaBH4 + 4H2O f NaB(OH)4 + 4H2v + ∼300 kJ (1) The metaborate product can be recycled to produce NaBH4 through Schlesinger process,4 for example, at a centralized processing site. One issue with aqueous sodium borohydride as a hydrogen storage medium is a limit to the fuel concentration that can be processed efficiently. Often, the reactor has to be operated at a reduced sodium borohydride concentration to avoid metaborate product precipitation in the reactor. Furthermore, the hydrogen storage capacity of aqueous fuel slowly degrades over time. In contrast, the direct reaction between acid and solid chemical hydride eliminates the fuel concentration limitation and provides excellent fuel stability and higher energy storage density, making it a potentially attractive alternative. Understanding the reaction stoichiometry, hydrogen generation dynamics, and product distribution is essential for the design of a solid chemical hydride-based hydrogen generator. However, there is little information about reaction chemistry between acid and solid sodium borohydride in the literature. Although the reaction between aqueous sodium borohydride solution and acid is known to generate hydrogen, most prior studies5-7 had been merely dealing with hydrogen generation from highly diluted aqueous sodium borohydride solutions; such approaches are of little practical value for hydrogen storage. It was the objective of this study to define the chemical reactions involved in acid-solid sodium borohydride system and to establish the fundamental reactions and stoichiometry for hydrogen generation. In this study, a stabilized fuel blend consisting of sodium borohydride and sodium hydroxide was selected for study in combination with sulfuric acid. The solid fuel blend of 87 wt % sodium borohydride and 13 wt % NaOH, also referred to herein as (87/13), was selected for experimenta* Corresponding author. Tel.: (732) 205-7106. E-mail: qinglin.zhang@ basf.com. † Current address: BASF Catalysts LLC, 25 Middlesex/Essex Turnpike, Iselin, NJ 08830-2721.
tion as it has practical advantages in terms of transportation regulations for hazardous materials.8 Sulfuric acid was selected because of its wide concentration range and its low volatility. Influences of acid concentration and feed ratio on hydrogen generation dynamics and extent of reaction were studied using a simple start-stop acid feeding as approach for controlled hydrogen generation. Reaction products were characterized using NMR and XRD to establish product distributions. Fuel-based energy stored densities were also projected for both pure and stabilized sodium borohydride fuels. 2. Experimental Section 2.1 Chemicals. Solid NaBH4/NaOH (87/13 wt/wt) fuel blend was supplied by Rohm and Haas. Concentrated sulfuric acid (96 wt %) was obtained from Aldrich. When the concentrated acid mixes with water, large amounts of heat are released; enough heat can be released at once to boil the water and spatter the acid. To dilute the acid to the desired acid concentration, the desired volume of the concentrated sulfuric acid was added slowly to cold water with constant stirring to limit the buildup of heat. The concentration of the diluted acid is determined by titration with a standard NaOH solution. 2.2. Experimental Setup. Acid catalyzed hydrolysis of a solid NaBH4/NaOH (87/13 wt/wt) fuel blend was carried out in a Pyrex glass reactor (250 mL). A schematic diagram of experimental setup is shown in Figure 1. The reactor was preloaded with solid NaBH4/NaOH powder (about 5.75 g) then purged with H2. A sulfuric acid solution with the desired concentration was fed into contact with solid NaBH4/NaOH powder in the reactor using a syringe pump. Hydrogen gas generated by reacting acid solution with NaBH4, passes through a trap/heat exchanger to cool to room temperature prior to contact with a silica drier for moisture removal. The flow rate of hydrogen after cooling and moisture removal was monitored using an online mass flowmeter. The reaction temperature was monitored with a thermocouple embedded in the NaBH4/NaOH fuel bed and a second thermocouple was used to measure the exterior wall temperature of the reactor. Hydrogen gas temperatures and the system pressure were also measured. All data were recorded using an online computer. Hydrogen generation rate in response to acid concentrations and acid feed rates is an important parameter for the design of a hydrogen generator based on the approach of reacting acid with solid NaBH4. In this study, dynamic responses of the
10.1021/ie800470d CCC: $40.75 2009 American Chemical Society Published on Web 01/12/2009
1604 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009
Figure 1. Schematic diagram of experimental setup.
reaction system to acid feeding rates under different acid concentrations were recorded to establish overall rate of reaction and to evaluate “start-stop” dynamics for hydrogen generation. 2.2. Analytical Techniques. The yield of NaBH4 converted into hydrogen (or simply, the fuel “conversion”) was established based on NMR assay of B-containing reaction products. After each run, B-containing samples were mixed with 5% NaOH solution to prepare them for NMR analysis. The flow rate of hydrogen generated during each run was measured using a mass flowmeter and was numerically integrated to give the total amount of hydrogen generation. The overall reaction stoichiometry for the “acid-H2O-solid NaBH4” hydrogen storage fuel system was thus established from the total amount of H2 generation and from NaBH4 conversion data determined using NMR. A water trap was installed in the system that serves as a heat exchanger to cool hydrogen generated and a trap to collect potential carryover of acid or other impurities in the gas stream. Typically, about 25 mL of deionized water was added into the trap prior to experimental runs and pH of the water before and after the run was measured. Additionally, gas samples prior to contact with the water trap were collected for GC/MS analysis. Solid reaction products and intermediates were analyzed using XRD. 3. Results and Discussion Controllable generation of hydrogen can be realized simply by starting or stopping the acid feed. Noting that the net system energy density of a fuel cell power system depends on energy density of the fuel, efficiency of hydrogen fuel cell, as well as the balance of plant (BOP) for hydrogen generation and conversion to power, this simple start-stop mechanism is advantageous as it allows for a simplified system BOP design. With a focus on the fuel properties and reactions involved in the fuel processing, the following terms were defined to aid our discussion. Fuel-only energy density (Wh/kg): defined as the ratio of lower heating value (LHV) of hydrogen generated by reacting acid with solid NaBH4 blend to the total weight of acid and solid NaBH4 blend. Off-hydrogen volume (mL): defined as the volume of hydrogen generated after stopping the acid feed. The “offhydrogen volume” is a critical parameter for the design of
Figure 2. Dynamic H2 generation profiles with 20 wt % H2SO4 at acid feed rate of 10 mL/h. Fuel, 5.75 g of 87 wt % NaBH4-13 wt % NaOH; acid feed, 10 mL; off H2 volume, 297 mL.
hydrogen generators. In the case that hydrogen consumption ceases, the off-hydrogen generated after stopping the acid feed has to be contained within the hydrogen generating system. The designed volume of the system therefore needs to account for additional “ballast” to safely handle any resulting increase in system pressure, which ultimately reduces the overall system energy storage density. Hence, fast dynamic response of a hydrogen generator to variable loads with low off-hydrogen volume is one of the key performance metrics of a hydrogen storage and generation system. 3.1. Hydrogen Generation Dynamics. Figure 2 shows a typical dynamic profile of H2 generation from 5.75 g of solid fuel mixture (87/13). Instantaneous hydrogen generation was observed once the acid was fed into reactor and came into contact with the solid sodium borohydride. The hydrogen flow rate increased from 0 to 140 mL/min in less than 5 s when 20 wt % H2SO4 solution was fed at 10 mL/h into contact with NaBH4 fuel blend. The hydrogen generation rate remained fairly constant except that a small peak of hydrogen flow was observed after 37 min. It was further observed that the reaction products changed to a slurry with foaming formation when the acid feed volume reached a total of 9 mL with various acid concentrations ranging from 20 to 30 wt %. The observed peak in the hydrogen generation rate profile was apparently related to changes in the physical state of fuel from solid to slurry, mostly at the interface between acid and fuel.
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1605
Figure 3. Influence of sulfuric acid concentrations on off H2 volume at a constant acid feeding rate of 10 mL/h.
As shown in Figure 2, a sharp drop in hydrogen flow rate was observed after stopping acid feed. These data indicate that hydrogen generation can be started by bringing acid into contact with solid NaBH4 fuel when there is a hydrogen demand, and can be stopped by terminating the acid feed when there is no hydrogen demand. This fast and simple mechanism for hydrogen generation on demand reduces hydrogen storage system complexity and cost and offers the potential for achieving high overall system energy density. The observed fast start-stop characteristics of the hydrogen generation reaction provide an effective means to meet variable power loads for hydrogen fuel cell applications. Total offhydrogen volume was found to be 297 mL for this run after stopping acid feed. An overall fuel conversion of 56% was achieved in this run. As will be discussed further below, (a) complete fuel conversion can be achieved by increasing the acid to solid NaBH4 fuel ratio, and (b) the acid concentration was one of the most important parameters influencing the hydrogen generation profile. 3.2. “Start-Stop” Dynamics. The start-stop behavior for hydrogen generation is a key consideration for the design of hydrogen generators. Instantaneous generation of hydrogen was observed once acid was brought in contact with solid sodium borohydride. To illustrate the hydrogen generation characteristics in response to variable load conditions, we measured dynamic profiles of H2 output after acid feed was stopped. Two acid feed volumes were used to probe the effect of the fuel life cycle on hydrogen generation dynamics. The stage of fuel cartridge life or fuel conversion could influence the local environment of acid distribution and compositions of fuel and discharged fuel mixture. With a total acid feed of 5 mL, the total amount of off-hydrogen volume was found to be closely related to acid concentration (Figure 3). Off-hydrogen volume increased with acid concentration, particularly in the acid concentration range of 20-30 wt %. A similar trend was also observed for a total acid feed volume of 10 mL. It is worth noting that the amount of off-hydrogen depends on not only acid concentration and NaBH4 conversion but also is influenced by acid distribution and the overall rate of hydrogen generation. Acid distribution can be improved by tuning product properties and using a multiple-point acid feed. 3.3. Efficiency of Fuel Conversion. Acid concentration not only influences the start-stop dynamics of hydrogen generation, but also the efficiency of fuel conversion. Relative ratios of acid and water to sodium borohydride fuel are directly related to fuel conversion and system hydrogen storage density. To maximize energy density, it is highly desirable to have water
Figure 4. Influence of H2SO4 acid concentrations on fuel conversion at a constant acid feeding rate of 10 mL/h.
Figure 5. Determination of mole ratio of H2O/NaBH4 for complete fuel conversion with 27 wt % H2SO4.
and acid to sodium borohydride ratios close to their stoichiometric ratios while maintaining fast startup and shutdown dynamics with minimal off-hydrogen generation. For example, at an acid feed volume of 5 mL to 5.75 g of sodium borohydride fuel blend (87/13), the fuel conversion increased with acid concentration and reached a plateau at an acid concentration of about 30 wt % (Figure 4). The optimal acid concentration was found to be in the range of 23-30 wt % for high fuel conversion with low off-hydrogen volume. It was also found that high acid concentrations led to formation of solid products that had different properties from those obtained with acids of lower concentrations. The change in the properties influenced acid distribution and led to different hydrogen generation rate and start-stop dynamics. To further illustrate the importance of acid concentration, data for 27wt% H2SO4 were plotted versus the molar ratio of H2O: NaBH4. A nearly linear dependence of fuel conversion on H2O: NaBH4 molar ratio was observed (Figure 5). The linear dependence of fuel conversion on the amount of water in the acid indicates a high efficiency of acid utilization at a fuel conversion up to 90%. H+ in the acid served as the catalyst for NaBH4 hydrolysis though acid can also directly react with NaBH4. Further increase in the acid feed beyond 90% conversion did not result in a proportional increase in fuel conversion. This is presumably a mass-transport effect, as the increased formation of borax byproducts reduces the availability of acid to react with NaBH4 fuel. With more borax formation at high fuel conversion, the diffusion path for acid to reach NaBH4 increases, which not only results in low conversion per unit acid feed, but also lowers the rate of hydrogen generation
1606 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009
Figure 6. X-ray diffractograms of acid hydrolysis products under various conditions 9, H3BO3;b, Na2B4O7 · 5H2O; ×, Na3H(SO4)2; +, Na2SO4.
relative to the rate generated from fresh fuel. The linear dependence of fuel conversion to acid feed is extrapolated to project the H2O:NaBH4 ratio required for complete fuel conversion. As shown in Figure 5, a molar ratio of H2O to NaBH4 of roughly 4.5 is projected for complete fuel conversion with 27 wt % H2SO4. In contrast, a fuel system consisting of a solid catalyst and 20 wt % aqueous NaBH4 solution has a H2O:NaBH4 ratio of 8.7. Therefore an acid hydrolysis system has significant upside in fuel energy density relative to a typical aqueous fueled system. GC/MS analysis of the gas samples indicated no CO or sulfur containing species in the hydrogen stream (detection limits 1 g/mL for discharged fuel were observed. The fuel bulk density increase is advantageous for achieving high system energy storage density; a decrease would force the system design to be larger to account for expansion of the products relative to the starting materials. From the XRD analysis, it can be seen that reactions between sulfuric acid and NaBH4/NaOH fuel blends resulted in the formation of sodium sulfate (Na2SO4), sodium hydrogen sulfate (Na3H(SO4)2), and sodium tetraborate pentahydrate (Na2B4O5 · 5H2O) as the main products (Figure 6). Sodium tetraborate pentahydrate is a preferred reaction product to sodium tetraborate decahydrate (Na2B4O5 · 10H2O) because the pentahydrate has less water associated with the product and as well as better
thermal stability. As the result, partially spent fuel has good chemical and thermal stability. The stable reaction intermediate is important for safety and for achieving a long shelf life of partially used fuel cartridges. In the acid-NaBH4 system, reactions between H+ and NaBH4 and between H2O and NaBH4 both take place. The hydrogen generation rate significantly depends on acid concentration. Furthermore, the formation of different product compositions also influences overall mass transfer rates of acid to NaBH4 fuel. As a result, acid concentration affects the intrinsic reaction rates as well as acid distribution and mass transfer rates. Acid concentration proved to be one of the most important parameters for control of product distribution and hydrogen generation dynamics. 3.5. Overall Stoichiometric Ratio of H2/NaBH4. Accurate measurements of NaBH4 conversion and the total amount of H2 generation are necessary to establish the overall stoichiometries of acid-catalyzed reactions. In this study, the total hydrogen generation was measured by integrating readings from a mass flowmeter, and fuel conversion was established based on NMR analysis of B-containing products in the resultant solid products. A stoichiometric ratio of H2:NaBH4 ≈ 4 was established over various acid concentrations (Figure 7).
Figure 7. Determination of stoichiometric ratio of H2/NaBH4 for solid NaBH4-H2SO4 reaction system.
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1607
4. Reaction Chemistry and Theoretical Fuel-Only Energy Density The overall stoichiometry for hydrogen generation has thus been established to be 4 moles of H2 per mole of NaBH4. The hydrogen generation stoichiometry and products identification are in good agreement with the following reactions proposed for NaBH4/NaOH-H2SO4-H2O system
acid can be achieved by selecting an appropriate acid and its concentration as well as the desired acid to chemical hydride ratio. These products offer good fuel stability and a long shelf life of fuel cartridges. Further improvements in hydrogen generation dynamics can be achieved by optimizing reaction conditions and controlling product properties. Acknowledgment
4NaBH4+H2SO4+17H2O f Na2B4O7·10H2O + 16H2+ Na2SO4(1)
We thank Ibrahim M. Qureshi and Joseph Podsiadlik for their efforts in experimental measurements.
4NaBH4 + H2SO4 + 12H2O f Na2B4O7·5H2O + 16H2 + Na2SO4(2)
Literature Cited
3NaBH4 + 2H2SO4 + 9H2O f 3H3BO3 + 12H2 + Na3H(SO4)2(3) 2NaOH + H2SO4 f Na2SO4 + 2H2O
(4)
A fuel-only energy density of 1864 Wh/kg can thus be projected based on the reaction stoichimetry listed above for a fuel system consisting of 31 wt % H2SO4 and an 87 wt % NaBH4 /13 wt % NaOH fuel blend. Similarly, a fuel-only energy density of 2310 Wh/kg is projected for a fuel system containing 31 wt % H2SO4-pure NaBH4. These energy densities represent theoretical or stoichiometric fuel energy densities for a given fuel composition. Additional factors such as rate of hydrogen generation, “start-stop” dynamics, etc., have to be considered in the design of a hydrogen generator based on this reaction scheme. In this work, we have demonstrated a fuel-only energy density of 1618 Wh/kg experimentally with complete fuel conversion using 27 wt % H2SO4 and an 87 wt % NaBH4 /13 wt % NaOH fuel blend. No apparent acid vapor or other impurities were found in the hydrogen stream, and thus hydrogen generated from the reactions can be fed directly to a hydrogen fuel cell for conversion into electrical power. Formation of the desired reaction products of sodium tetraborate pentahydrate or boric
(1) Ritter, J. A.; Ebner, A. D.; Wang, J.; Zidan, R. Implanting a Hydrogen Economy. Mater. Today 2003, 18, 23. (2) Maccarley, C. A. Development of a sodium borohydride hydrogen fuel storage system for vehicular applications. - Ei1 77-12 04021 Symposium on AlternatiVe Fuel Resources, Santa Maria, CA, March 1976; Vol. 20, pp 315321. (3) Zhang, Q.; Smith, G. M.; Wu, Y.; Mohring, R. Catalytic Hydrolysis of Sodium Borohydride in an Auto-Thermal Fixed-Bed Reactor. Int. J. Hydrogen Energy 2006, 31, 961–965. (4) Schlesinger, H. I.; Brown, H. C.; Finholt, A. E. The Preparation of Sodium Borohydride by the High Temperature Reaction of Sodium Hydride with Borate Esters. J. Am. Chem. Soc. 1953, 75, 205–209. (5) Schlesinger, H. I.; Brown, H. C.; Finholt, A. E.; Gilbreath, J. R.; Hoekstra, H. R.; Hyde, E. K. Sodium Borohydride: Hydrolysis and Its Generation of Hydrogen. J. Am. Chem. Soc. 1953, 75, 215–219. (6) Mesmer, R. E.; Jolly, W. The Hydrolysis of Aqueous Hydroborate. Inorg. Chem. 1962, 1, 608–612. (7) 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. 1985, 307, 313. (8) Mohring, R. M. Hydrogen Battery Technology for Portable Applications. Proceedings of the 8th International Conference on Small Fuel Cells for Portable Applications, Washington, D.C., 2006.
ReceiVed for reView March 24, 2008 ReVised manuscript receiVed July 18, 2008 Accepted July 30, 2008 IE800470D
