Synthesis and Engineering Materials Properties of Fluid-Phase

Sep 1, 2015 - Chemical Hydrogen Storage Materials for Automotive Applications ... To utilize the existing infrastructure, a fluid-phase hydrogen stora...
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Synthesis and Engineering Materials Properties of Fluid-Phase Chemical Hydrogen Storage Materials for Automotive Applications Young Joon Choi,†,‡ Matthew Westman,† Abhi Karkamkar,† Jaehun Chun,† and Ewa C. E. Rönnebro*,† †

Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352, United States Globalfoundries, 400 Stone Break Road Extention, Malta, New York 12020, United States

‡

ABSTRACT: Among candidates for chemical hydrogen storage in PEM fuel cell automotive applications, ammonia borane (AB, NH3BH3) is considered to be one of the most promising materials due to its high hydrogen content of 14−16 wt % and high volumetric density of ∼146 g H2/liter. To utilize the existing infrastructure, a fluid-phase hydrogen storage material is attractive, and thus, we investigated the materials’ properties of AB in liquid carriers for a chemical hydrogen storage slurry system. Slurries composed of AB and high-boiling-point liquids were prepared by mechanical milling and sonication in order to obtain stable and fluidic properties. Kinetics of the H2 release reactions of the AB slurry and neat AB was studied using a volumetric gas buret. Viscometry and microscopy were employed to further characterize the engineering properties of the slurries. Using a tipsonication method, we have produced AB/silicone fluid slurries at solid loadings up to 40 wt % (6.5 wt % H2) with viscosities less than 500 cP at 25 °C. The fluidity of slurry as a medium to carry CHSs has advantages as alternative techniques in terms of pumping, transporting, and easier adaptation to existing infrastructure. The liquid spent fuel can be relatively easily collected and returned to a regeneration station as well. Among the class of candidates for chemical hydrogen storage in PEM fuel cell applications, ammonia borane (AB) is considered as one of the most promising materials due to its theoretically high hydrogen contents with 19.6 wt % capacity. Available hydrogen content has been reported to be 14−16 wt % below 200 °C.13 The gravimetric density is 196 g H2 kg−1 and the volumetric density is ca. 146 g H2 liter−1. We have chosen to use AB as a role model for testing our chemical hydrogen storage system. Solid AB meets a number of important technological U.S. Department of Energy (DOE) targets such as high volumetric and gravimetric density of hydrogen, fast kinetics at moderate temperatures, good thermal stability, facile synthesis at large scale, and safe handling under atmospheric conditions. Slurries provide an attractive option without significant drop in a gravimetric and volumetric density of hydrogen if the weight percent of the CHS is kept high. Additionally, the slurries potentially have the advantage of efficient heat transfer and heat management. As an effort to develop an advanced hydrogen storage technology, PNNL started to investigate a slurry system composed of AB/high-temperature liquids in 2011. When hydrogen is needed for the PEM fuel cell, high purity hydrogen will be produced based on exothermic chemical reactions when AB decomposed in the liquid carrier. This paper will specifically address the issues of preparation and the development of CHS slurry. There is little insight in the literature on hydrogen storage slurries. The practical feasibility of slurry hydride reactors was

1. INTRODUCTION A major challenge for using hydrogen as an energy carrier today is to develop efficient methods for hydrogen storage that can not only store hydrogen safely but also supply it where and when it is needed. Thus, the development of hydrogen storage materials that will operate at appropriate temperatures near 80−100 °C has been recognized as a key area of focus to enable hydrogen storage for fuel cell applications.1,2 Although substantial progress has been made in the past few years in the discovery of new materials, none of them have demonstrated both reversible storage capacity and gravimetric and volumetric contents of hydrogen required for practical application. Metal hydrides,2 chemical hydrogen storage (CHS) materials,3 and high surface area adsorbents4 including carbon and metal−organic frameworks (MOF)5 have been investigated as hydrogen storage media. Most complex metal hydrides dehydrogenate and hydrogenate at high temperatures (>250 °C) because of slow kinetics at low temperatures caused by the high energy barrier of the dehydrogenation reactions.6,7 In contrast, when high surface area adsorbents such as carbon nanoframeworks and MOFs are used, hydrogen adsorption requires low temperature ranges (∼77−200 K) due to weak interactions between the hydrogen molecules and the adsorbent materials. Most of the intermetallic alloys, such as LaNi58 (AB5 type), can absorb hydrogen at room temperature; however, these materials have low gravimetrical hydrogen storage capacities (250

124 (BP: 257)

≈30

≈20

≈22.5

≈20

≈50

≈3.7

viscosity (cP, 25 °C) a

160 (BP:280) ≈3.9

The flash point of a volatile liquid is the lowest temperature at which it can vaporize to form an ignitable mixture in air.

were conducted in a nitrogen atmosphere with a flow rate of 100 mL/ min.

3. RESULTS AND DISCUSSION 3.1. Optimization of Carrier Fluid for AB Slurry. In order to obtain stable and fluidic AB slurry, the optimization of several variables is critical. The most important variables include: liquid media/stabilizing agents, particle size, and slurry homogenization systems/slurry stability.27−31 More specifically, when preparing slurries, it is important to achieve a distribution of the solid in the liquid in such a way that the solid does not settle or flocculate. In addition, the liquid media and any dispersants need to be chemically stable not only by themselves but also with AB through AB’s decomposition. To evaluate promising liquid carriers for stable and fluidic AB slurry, each candidate mixture of liquid carrier and a small amount of AB was mixed and categorized based on their wet-ability and suspension state for the first 24 h. We have explored hightemperature liquids that are chemically compatible with AB and its dehydrogenation products. They are P alkyl aromatics (HT), synthetics hydrocarbons (HTR), MK1 - a mixture of biphenyl and diphenyl ether, and silicone fluids, many of which are stable up to 350 °C. Table 1 lists candidate liquid carriers and information including physical properties. To select a liquid carrier, we mixed the same amount of AB in the same amount of liquid carrier in a glass vial and observed if a suspension was formed. The result is summarized in Table 1. First of all, in the test of the liquid carrier MK1, it has been found to cause an unacceptable odor, which may result in environmental hazard. Thus, this candidate was eliminated. The other liquid media including HT, HTR and silicone fluids did not cause an unpleasant odor or observable side reaction during the test. To further investigate the wet-ability, suspension state, flowability and chemical stability of AB in the liquid media at 25 °C, 10−40 wt % of AB was added to HT, HTR, and silicone fluids. The results indicated that AB powder slowly wetted in the liquids and would separate relatively rapidly due to the difference of densities between AB (0.78 g/mL at 25 °C) and candidate carriers (0.89−1.075 g/mL at 25 °C). For instance, Sample 1 in Figure 1 represents the mixtures of 10 wt % AB/ silicone oil, which clearly shows phase separation between AB and silicone oil. In addition, the maximum loading capacity of neat AB to form slurry which flows readily in the respective liquid carriers has been found to be up to 30 wt %. Further increase of the AB loading over 40 wt % did not show any degree of flowability, and most of the liquid carriers were absorbed by the AB powder, as shown in Sample 6 in Figure 1, a 40 wt % (ca. 6.5 wt % H2) AB/AR20 slurry. The slurry should be pumpable and stable enough that it will not settle or float

Figure 1. AB/silicone oil slurries; Samples 1−7 corresponding to compositions in Table 2.

within the time that it is to be used. A readily flowing slurry will flow to the pump inlet and it will be easier to pump thus lowering the energy required to distribute the slurry to the engine. Slurries of 20−30 wt % (ca. 3.2−4.8 wt % H2) were then produced by mixing solutions on a stir plate. The slurries were qualitatively evaluated for their ability to flow through a 12.7 mm tube. Although all the fluids produced reasonable slurries, the HT fluid was eliminated because of quite slow flow through the tube and appeared dense and nearly saturated. Thermogravimetric analysis was then performed on the remaining fluid carriers to determine if any mass loss occurred during a typical decomposition reaction. As seen in Figure 2 all but the AR20 and HTR undergo significant mass losses greater than 2 wt % over the target temperature range. Based on the results of this test, only the AR20 silicone fluid and the HTR synthetic hydrocarbon fluid were selected as potential fluid candidates.

Figure 2. TGA thermograms of fluid carriers showing that only AR20 and HTR do not undergo significant mass losses below 140°C and therefore are suitable. 6697

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Energy & Fuels Table 2. Comparison of AB/Silicone Oil Slurries and Their Properties Sample 1

Sample 2

Sample 3

Sample 4

Sample 5

Sample 6

Sample 7

composition wt % AB in silicone oil milling conditions

10 wt %

20 wt %

20 wt %

20 wt %

30 wt %

40 wt %

40 wt %

N/A

N/A

jar-roll milling 16 h

jar-roll milling 72 h

N/A

planetary milling 24 h

flowabilitya slurry stability

4.5 phase separation (AB and silicone oil)

4.5 phase separation (AB and silicone oil)

planetary milling 4h 3.5 forming soft packb , phase separation

3.5 forming soft packb , phase separation

3 forming soft packb , phase separation

1.5 N/A

2 forming soft∼hard packc , phase separation

a

In scale: 1, Very poor; 5, Very good. bSoft pack: diffuse and easily reincorporated. cHard pack: dense and difficult to reincorporate back into the slurry.

3.2. Development of Mixing Method of AB/Carrier Fluid and Optimization of AB Slurry. In order to obtain satisfactory results of stable and fluidic AB slurry, the optimization of several variables is critical. The most important variables include the following:

As a next step to reduce particle size, low-energy ball-milling systems, i.e., jar-roll and planetary mill, were applied to AB/ silicone fluid slurries. A variety of milling times were investigated between 5 min and 72 h. The container and grinding medium for milling were made of tungsten carbide, and the ball-to-AB slurry weight ratio was 10:1. Milling was performed at atmospheric pressure, and the milling speed was 200 rpm. Under these conditions, we obtained a promising 20− 30 wt % (ca. 3.2−4.8 wt % H2) AB/silicone oil composition that showed acceptable flowability after certain milling time, as shown in Samples 3, 4, and 5 in Figure 1. Each composition and experimental detail is explained in Table 2. To compare flowability of the samples, we ranked the samples on a scale from 1 to 5, where 5 is the best flowability. As can be seen in Table 2, slurry stability was qualitatively described, providing observations of phase separation and how soft/hard the pack of the solid in the carrier appeared. A soft pack is diffuse and easily reincorporated, whereas a hard pack is dense and difficult to reincorporate back into the slurry. The impurities of the milling tools probably cause the change of color in Sample 3. Again, we have relied on visual inspection to evaluate the fluidity of the slurry during the development of an effective method for AB particle size reduction and mixing process of AB/liquid carrier. The photos in Figure 1 indicate that AB powder floats due to the difference of densities between AB and silicone oil regardless of the loading amount of AB or the use of ballmilling. In addition, the samples show a considerably reduced thickness of the separated AB layer compared to that of AB slurries without ball-milling treatment, implying that the wet milling technique successfully reduced the particle size of AB. Finally, as seen in Samples 4 and 5, jar-rolling from 16 to 72 h results in additional particle size reduction as observed by comparing the thickness of AB layer. However, in Sample 5, the 72 h mill time resulted in a denser surface layer, which is not easily reincorporated back into the suspension state. This is possibly due to the fact that AB particles became too small during milling and formed a tighter cohesive matrix upon settling out of the suspension. Samples 6 and 7 compare the samples before and after 24 of hours milling via a planetary mill. Each sample contains the same amount of AB and silicone oil. This result indicates that despite the lack of flowability due to the dense AB layer, the wet milling technique is beneficial for reducing the particle size of AB. Sample 8, shown in Figure 3, shows 40 wt % (ca. 6.5 wt % H2) AB/silicone fluid slurry after 1 h milling by SPEX mill. Silicone oil was completely absorbed by AB and formed agglomerated gray flakes due to the high intensity from SPEX milling. Thus, it can be concluded that under these experimental conditions, the morphology of the AB powder has been significantly deformed upon high-energy milling.

• liquid media/stabilizing agents • particle size • slurry homogenization systems/slurry stability When preparing slurry, it is important to achieve a distribution of the solid in the liquid in such a way that the solid does not settle down or float. In addition, the liquid media and dispersant need to be chemically stable not only by themselves but also with AB up to the peak operating target temperature. Our first step was to explore mixing methods to create stable, flowable slurries with as high weight percent AB as possible. To achieve higher loading of 40−50 wt % (ca. 6.5−8 wt % H2) AB solids, and above, it is necessary to have smaller AB particles in the AB/silicone fluid composition because a velocity associated with buoyancy (i.e., settling or floating velocity) is proportional to the square of particle size under the same density difference between solid and liquid media. Therefore, it is crucial to adopt an efficient tool to reduce particle size. As a result of the inability to dry mill the AB an approach to obtain smaller particle sizes of the solid hydride while in the fluid phase has been explored. In this study, we have decided to compare two processes: (1) a wet milling/mixing method and (2) a sonication process. Starting at 30 wt % (ca. 4.8 wt % H2) AB/silicone fluid, we strived to reach 50 wt % (ca. 8 wt % H2) while maintaining its fluidity. 3.2.1. Mechanical Milling. Mechanical milling can involve both high-energy and low-energy processes. Each applied technique and relevant process parameter is subdivided in the Tables 2, 4, and 5. As seen in Samples 1 and 2 in Figure 1, slurries with 10 and 20 wt % (ca. 1.6 and 3.2 wt % H2) AB/ silicone fluid formed large conglomerations which floated to the surface. After 24 h, all the AB would phase separate and form a dense layer difficult to break up. In order to reduce the AB particle size in the fluid phases, a high sheer mixer with a disintegrating head was used to prepare 10−40 wt % (ca. 1.6−6.5 wt % H2) AB/silicone slurries. The slurries were mixed at 1500 rpm for 10 min. To minimize the temperature increase due to the high sheer mixing, the slurries were immersed in ice water. The results of this initial experiment indicated that the maximum loading of AB slurry is up to 30 wt % due to the limitation of particle size reduction when using this method. Also, above 30 wt %, it became quite difficult to control the rise in temperature caused by the mixing. Loadings over 40 wt % could not be effectively achieved. 6698

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Figure 4. Microscope analysis of solid AB (A) Aviabor, (B) SigmaAldrich, (C) PNNL-made.

different particle sizes and shapes for the three different batches. In addition, their tap-densities vary as well. Because the solid AB from Sigma-Aldrich has relatively smaller particles of 0.1−1 mm and the highest bulk densities, 0.32 g/mL, as seen in Figure 4 and Table 3, all slurries were prepared from this

Figure 3. AB/silicone oil slurries, Samples 8−11 corresponding to compositions in Table 4.

Sample 9 represents 50 wt % (ca. 8 wt % H2) AB/silicone oil milled by planetary mill for 24 h. We found that by mixing AB with methyl cellulose (MC), the degree of foaming was significantly reduced. We therefore prepared sample 10, which is 30 wt % AB/MC/silicone oil composition, to explore the effect of adding MC on the slurry stability. Neither sample 9 or 10 showed any degree of flowability, and most of the liquid carrier was absorbed by AB powder, which is similar to Sample 6 in Figure 1. Sample 11 shows approximately 3 g of 30 wt % (ca. 4.8 wt % H2) AB/silicone fluid slurry after dehydrogenation at 100 °C for 12 h under flowing nitrogen. During the dehydrogenation process, foaming was observed. The flowability of the sample after the process has been slightly decreased possibly due to the change of morphology. Overall, the experimental results indicate that the maximum loading of AB slurry is up to 40 wt % (ca. 6.4 wt % H2) due to the limitation of particle size reduction and the lack of flowability when using ball-milling methods. More specifically, most AB particles floated in the silicone oil after milling and formed a dense AB layer, which prevents the slurry from flowing readily. Loadings over 50 wt % (ca. 8 wt % H2) could not be effectively achieved under these conditions. In conclusion, the SPEX vibratory mill was not a useful method to prepare AB slurries because of its high energy impact and instead caused the AB particles to agglomerate and turn into a flake shape within a minute due to the ductility of AB. Therefore, we tried to mill AB with the liquid carrier, but without much improvement. None of the employed milling techniques could effectively prepare AB slurries with desirable properties. 3.2.2. Sonication. Sonication is commonly used in the preparation of evenly dispersed nanoparticles in liquids.27−31 The primary advantage of a sonication process such as ultrasonic bath or an ultrasonic probe is that it yields materials with high level of homogeneity at the atomic level by breaking intermolecular interactions. Thus, the application of this technique could not only decrease the particle size of AB in the fluid phase but also achieve a homogeneous and stable slurry. In this study, we found that sonication is highly beneficial for obtaining well-dispersed, significantly reduced particles and slurries with acceptable flowability with up to 40 wt % (ca. 6.5 wt % H2) solids. We found that the morphology of the starting particles has proved critical in the development of quality AB slurries. Smaller, more regular, and uniform particles would generally lead to slurries with greater flowability. We obtained powder AB from three different sources which include the following: Sigma-Aldrich, Aviabor, and AB synthesized at PNNL.23 The micrograph photos shown in Figure 4 reveal significantly

Table 3. Measured Densities of Solid AB (g/mL at 25 °C) tap density

Aviabor

Sigma-Aldrich

PNNL-made

0.30

0.32

0.25

material source. Again, we rely on visual inspection to evaluate the flowability of the slurry during the AB particle size reduction and mixing process of AB/fluid carrier. The preliminary experiments revealed that there is no beneficial effect to obtain better homogeneity using an ultrasonic bath regardless of the experimental variables/ parameters. On the other hand, the samples prepared by the ultrasonic probe (referred to as “tip-sonication” hereafter) showed significantly improved properties in terms of homogeneity and flowability. A typical AB slurry was prepared using the following procedure: approximately 10 g of AB was added to a flask containing stirring silicone fluid. The flask was then submerged in an ice bath and the sonicator tip was lowered into the flask just below the orifice of the vortex. During the sonication process the flask was kept under a positive flow of nitrogen. The ice bath was changed every half hour or as needed. Based on these conditions, we achieved 10− 40 wt % AB slurries, as shown in Samples 12, 13, 14, and 15 in Figure 5, with composition and experimental details in Table 5. All samples were sonicated for 1 h and the flowability is as expected highest for the lowest solid loading of 10 and 20 wt % and lowest for the highest solid loading of 40 wt %. As an additional effort to decrease the particle size of the neat AB, we used an US 35 (500 μm) screen with the assistance of a marble pestle. Optical microscopy analysis was conducted to

Figure 5. AB/silicone oil slurries, Samples 12−15 corresponding to compositions in Table 5. 6699

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Energy & Fuels Table 4. Comparison of AB/Silicone Oil Slurries and Their Properties composition wt % AB in silicone oil milling conditions color flowabilitya slurry stability

a

Sample 8

Sample 9

Sample 10

Sample 11

40 wt %

50 wt %

30 wt % AB/MC

desorbed 30 wt % AB

SPEX milling 5/10/30/60 min opaque gray and oil 1 forming hard packb

planetary milling 24 h opaque gray and oil 1 forming hard packb

N/A opaque white and oil 1 forming hard packb

AB agglomerated and formed flake, AB absorbed silicone oil completely

AB absorbed silicone oil completely

AB absorbed silicone oil completely

N/A opaque white and oil 2 slight foaming during desorption (100 °C, 6 h, N2) porous and solid spent AB

In scale: 1, Very poor; 5, Very good. bHard pack: dense and difficult to reincorporate back into the slurry.

Table 5. Comparison of AB/Silicone Oil Slurries and Their Properties composition wt % AB in silicone oil sonicating conditions flowabilitya slurry stability

Sample 12

Sample 13

Sample 14

Sample 15

10 wt %

20 wt %

30 wt %

40 wt %

1h 4.5 phase separation (AB and silicone oil)

1h 4.5 phase separation (AB and silicone oil)

1h 3.5 forming soft packb, no phase separation

1h 2.5 forming soft∼hard packc, no phase separation

a

In scale: 1, Very poor; 5, Very good. bSoft pack: diffuse and easily reincorporated. cHard pack: dense and difficult to reincorporate back into the slurry.

reincorporated back into the suspension state. In contrast, Samples 14 and 15, which represent 30 and 40 wt % AB slurries after tip-sonication, displayed an increased duration of suspension and only formed a thin hardened layer on the surface, implying enhanced stability and improved fluidic properties. In conclusion, based on the indication from visual observations, we found that tip-sonication is highly beneficial for obtaining well-dispersed slurry with acceptable flowability at loadings up to 40 wt % (ca. 6.5 wt % H2). In Figure 7, we see

examine the morphology and particle size of AB before and after sieving, as shown in Figure 6. The particles before grinding

Figure 6. Microscope photos of ammonia borane (A) AB pre sieving and (B) AB post sieving.

(Figure 6A) have angular shapes and sizes in the range of 0.1− 1.5 mm by estimation on spherical diameters, whereas the primary particle size has been reduced between 50 and 500 μm after sieving (Figure 6B). The sieved AB powder was subsequently used for further optimization of slurry composition of the samples 13, 14, and 15. Samples 12, 13, 14, and 15 represent 10, 20, 30, and 40 wt % AB slurries after 1 h tip-sonication, respectively. These samples indicate that there is a substantial difference in the degree of homogeneity between the slurry with neat AB (Sample 12) and the slurries with ground AB (Samples 13, 14, 15). The samples show well-dispersed slurries. Although AB powder would float in the silicone oil due to the difference of densities in Sample 13 (20 wt % AB slurry), there is no indication of such phase separation in higher AB loading over 30 wt % in Samples 14 and 15. This implies a significant improvement over the suspension state compared to the slurries after ball milling or ultrasonic bath processing. Finally, the slurries after tipsonication show considerable improvement in terms of flowability compared to those after ball-milling. The AB powders in Samples 5 and 7 (ball-milled 30 and 40 wt % AB slurries) conglomerate shortly after processing and float to the surface creating a hard dense layer which is not easily

Figure 7. Microscope photos of ammonia borane slurries (A) AB particles in the 20 wt % slurry and (B) AB particles in the 40 wt % slurry.

that slurries of 20 and 40 wt % still consist of unconglomerated particles after tip sonication. Figure 8 indicates that by increasing the amount of AB from 10 to 40 wt % in the silicone oil the density of slurry has been decreased from 0.97 to 0.92 g/mL. This is to be expected as the high density of silicone oil, 1.05 g/mL is displaced by the lower density of AB in increased solids loadings. We performed a stability test of a 20 wt % AB slurry by letting it sit undisturbed on the shelf for four months at room temperature. According to nuclear magnetic resonance (NMR) analysis performed on a solution in THF, we did not detect any decomposition products which indicates that the slurry met the stability requirement. 6700

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under a positive nitrogen flow prior to testing. After subsequent purging, the flask was then submerged in an oil bath and heated to 80 °C with active stirring. Once thermal equilibrium was reached, the temperature was increased 5 °C every 30 min until a temperature of 120 °C was reached. After performing a 30 min isotherm at 120 °C, the temperature was then increased to 150 °C and held for 1 h. The reaction product was then cooled slowly under active mixing and a nitrogen blanket until it returned to room temperature. As seen in Figure 10, the post-reaction product is a white slurry that upon phase separation settles to the bottom of the vial. The loss of hydrogen must increase the density above that of the silicone fluid. In addition to the change in phase separation properties, the viscosity and yield stress dropped after the decomposition reaction. Viscosity decreased from 331 cP to 257 cP and yield stress decreased from 7.6 to 0.8, as summarized in Table 6. It is believed that during the

Figure 8. Measured densities (g/mL) of AB slurries.

3.3. Bench Scale AB Decomposition Reaction Tests. To better understand the fundamental properties of AB/ silicone fluid slurries, a series of bench-scale decomposition reactions were performed. The goal of these tests was to determine how the hydrogen release would affect the slurries’ ability to flow. In a typical reaction, approximately 10 g of slurry was added to a round-bottom flask equipped with a mechanical mixer shown in Figure 9. The flask was purged for 30 min

Table 6. Viscosity of AB Slurry (40 wt %) Measured before and after H-Release AB slurry before H- AB slurry after H- measured temp release release (oC) plastic viscosity (cP) yield stress (Pa)

∼331

∼257

25

∼7.6

∼0.8

25

decomposition reaction, the release of hydrogen causes the crystalline structure of AB to break up subsequently further reducing the particle size. 3.3. Dehydrogenation Kinetics. The dehydrogenation kinetics of AB slurry was investigated with PNNL’s buret system. Two experiments were performed at 100 °C and at 120 °C and compared to results for solid AB. Solid AB is known to have a 20−30 min induction period. It can be seen in Figure 11 that the slurry AB does not have any delay in hydrogen release, instead it occurs immediately. Moreover, a higher hydrogen content was released compared to solid AB. At one point, the temperature was increased to 160 °C to finish the reaction and up to 2.5 equiv of hydrogen, or about 16 wt %, was released. The reason for not observing a delay in on-set of hydrogen release in AB slurry may be because of better heat transfer in the liquid carrier.

Figure 9. Experimental setup for thermal decomposition of AB Slurry.

Figure 10. Photos of decomposed products settling: (a) just after decomposition, (b) after 1 h. 6701

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Figure 11. Measured dehydrogenation kinetics of slurry AB vs solid AB by volumetric gas buret.

3.4. Rheology Study. In addition to qualitative flowability observations, the viscosity of slurries was measured. Slurries with solid loadings ranging from 10 to 35 wt % were measured a minimum of 5 times. No indication of non-Newtonian behavior observed for sample slurries; Newtonian viscosity was sufficient to identify the flowability of slurries. Figure 12 depicts

homogeneous mixture of AB/liquid carrier. We found that sonication is highly beneficial for obtaining well-dispersed slurries, and acceptable flowability has so far been obtained up to 40 wt % loading. A test indicated that the slurry meets the stability requirements and do not decompose in several months. We are underway with further development of the method to produce slurries with increased loadings of 50 wt % or above (∼8 wt % hydrogen).

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (509) 372-6877. Fax: (509) 375-4448. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge support from the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy and the Fuel Cells Technology Office (FCTO). This work was performed as part of the Hydrogen Storage Engineering Center of Excellence. Jamie Holliday, Kriston Brooks and Tom Autrey at PNNL and Kevin Simmons (former PNNL) are acknowledged for fruitful discussions and valuable comments. Pacific Northwest National Laboratory is operated for U.S. DOE by Battelle Memorial Institute.

Figure 12. Measured viscosity of AB slurries.

a plot of viscosity versus solids loadings. As expected from typical suspension behavior, with increased loading, the viscosity increases. We have shown that tip sonication can produce quality slurries with low viscosities.

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REFERENCES

(1) Schlapbach, L.; Züttel, A. Hydrogen-storage materials for mobile applications. Nature 2001, 414, 353−358. (2) Rönnebro, E. C. E.; Majzoub, E. H. Metal hydrides for clean energy applications, Recent advances in metal hydrides for clean energy applications. MRS Bull. 2013, 38 (06), 452−458. (3) Huang, Z.; Autrey, T. Boron-nitrogen-hydrogen (BNH) compounds: recent developments in hydrogen storage, applications in hydrogenation and catalysis, and new syntheses. Energy Environ. Sci. 2012, 5 (11), 9257−9268. (4) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Storage of hydrogen in single-walled carbon nanotubes. Nature 1997, 386, 377−379. (5) Rosi, N. L.; Eddaoudi, M.; Vodak, D. T.; Eckert, J.; O’Keeffe, M.; Yaghi, O. M. Hydrogen Storage in Microporous Metal-Organic Frameworks. Science 2003, 300, 1127−1129.

4. CONCLUSIONS To optimize chemical hydrogen storage slurry for use as a liquid fuel to provide hydrogen for automotive fuel cell applications, we selected ammonia borane in silicone oil as a role model material. We investigated preparation methods and found that tip-sonication is the most beneficial method. The experimental results indicate that the maximum loading of AB slurry is up to 40 wt % (ca. 6.5 wt % H2) due to the limitation of particle size reduction. The primary advantage of the tipsonication process is that it yields materials with a high level of homogeneity at the atomic level by breaking intermolecular interactions. This technique was applied to decrease the particle size of AB in the fluid-phase composition and also to achieve a 6702

DOI: 10.1021/acs.energyfuels.5b01307 Energy Fuels 2015, 29, 6695−6703

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DOI: 10.1021/acs.energyfuels.5b01307 Energy Fuels 2015, 29, 6695−6703