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Cycling and engineering properties of highly compacted sodium alanate pellets
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 5 1 8 7 e1 5 1 9 5
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Cycling and engineering properties of highly compacted sodium alanate pellets M. Sulic a,*, M. Cai b, S. Kumar b a b
Optimal CAE, Inc., 14492 Sheldon Road, Plymouth Township, MI 48170, USA1 General Motors Research and Development, 30500 Mound Road, Warren, MI 48090, USA
article info
abstract
Article history:
Sodium alanate powder comprised of NaH and Al was doped with 3 mol% titanium chlo-
Received 18 May 2012
ride (TiCl3) and pelletized into highly compacted cylindrical pellets. The pelletization
Received in revised form
process was performed to improve thermal conductivity and volumetric hydrogen capacity
24 July 2012
of the metal hydride, compared to loose or tapped powder, which are vital requirements
Accepted 25 July 2012
for on-board hydrogen storage applications. The pelletization process was performed over
Available online 24 August 2012
a range of 69e345 MPa (10e50 kPSI) with a 95% increase in density and improvement in thermal conductivity 18 times greater compared to powder at the maximum pelletization
Keywords:
pressure (1.60 g/cm3 and 0.82 g/cm3; 9.09 W/m K and 0.50 W/m K, respectively). Hydrogen
Sodium alanate
cycling capacities and kinetics were not adversely affected by the pelletization process
Hydrogen storage
although 10 cycles are required to obtain full hydrogen capacity. Pellet cycling capacity
Thermal conductivity
maintained a stable 4 wt% H2 over 50 cycles. Ti-doped NaH þ Al pellets exhibited similar
Pelletization
thermal cycling expansion as with the loose powder; within 30 cycles there was a 50% loss
Expansion
in pellet density and by 50 cycles the loss in pellet structural integrity made handling
Density loss
problematic. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
A cost effective, robust and economical vehicular hydrogen storage system is paramount for the implementation of fuel cell technology by the automobile industry. For more than a decade extensive research has been conducted on solidstate complex metal hydrides as one of the potential solidstate storage material candidates in order to facilitate this shift in paradigm. Theoretically, solid-state complexes have greater per unit volume storage than compressed and liquefied hydrogen. This property translates to smaller vehicle storage tank volume and greater driving distances between refueling [1], attractive aspects not only for automotive companies, but for consumers as well.
Sodium aluminum tetrahydride (NaAlH4) is a complex metal hydride that has been known for decades and has long been utilized as a chemical reagent due to its strong reducing agent properties, similar to that of lithium aluminum tetrahydride. While in its pure form sodium alanate does not have favorable reversible hydrogen cycling kinetics (desorption 200e300 C; absorption 270 C, 17.5 MPa and 2e3 h); however, when doped with few mole percent of a titanium compound (e.g. TiCl3, TiF3, or Ti(OBu)4) cycling conditions become much more favorable (desorption 150 C; absorption 170 C, 15.2 MPa and 5 h) [2,3]. These findings were first reported in 1997 by Bogdanovic et al. and have since inspired the renewed interest in complex metal hydrides as a realistic means of hydrogen storage. The reversible decomposition
* Corresponding author. Tel.: þ1 586 986 5279; fax: þ1 586 986 2471. E-mail addresses: [email protected], [email protected] (M. Sulic). 1 Contracted to General Motors Research and Development. 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.07.113
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reaction of sodium alanate occurs in a two-step process (Equations (1) and (2)) with a theoretical capacity of 5.5 wt% hydrogen:
NaAlH4 / 1/3 Na3AlH6 þ 2/3 Al þ H2 (3.7 wt%)
(1)
Na3AlH6 / 3 NaH þ Al þ 3/2 H2 (1.8 wt%)
(2)
However, system gravimetric capacity targets established by the US Department of Energy for on-board vehicle hydrogen storage [4] surpass the theoretical 5.5 wt% of hydrogen absorption capacity of sodium alanate. While these targets exclude NaAlH4 as a potential candidate for on-board hydrogen storage, extensive research reported on the material makes this complex metal hydride an excellent candidate for engineering studies of a practical hydrogen storage system. In 2009, the US DOE established the Hydrogen Storage Engineering Center of Excellence (HSECoE) to address the hydrogen storage system issues for vehicular application [5]. The Center, a consortium of national labs, academic institutes, industrial laboratories and automobile companies, was formed as a five year project aimed at demonstrating viable technologies for on-board hydrogen storage that meet vehicle requirements in a cost-effective way. One of the technologies that the Center is evaluating is metal hydride based hydrogen storage. Successful demonstration will help the automobile industry commercialize fuel cell vehicles and help the nation move one step closer to a hydrogen economy. Reported herein is our study on the first complex metal hydride chosen by the Center, sodium alanate, and our examination of the material for practical automotive applications. As most hydrogen storage materials have poor heat transfer properties, our goal in this study was to improve upon such properties whilst examining the metal hydride’s mechanical stability and durability and to establish a foundation for which other solid-state hydrogen storage materials can be investigated.
2.
Experimental methods
All experimental procedures were carried out in an argon atmosphere unless otherwise noted.
2.1.
Material synthesis
The titanium doped sodium alanate used in this study was previously prepared and available in bulk. The mixture is comprised of the desorbed constituents of NaAlH4, sodium hydride (NaH) and aluminum powder (Al), and titanium trichloride (TiCl3). It was prepared in this form of sodium alanate to ensure long term shelf-life. Stoichiometric amounts of NaH and Al along with 3 mol% TiCl3 and 18 wt% excess Al were ball milled together with a planetary mill. The hydride mixture was prepared with excess aluminum in order to maximize the formation of NaAlH4 during rehydrogenation, thus compensating for the free Al taken up by Ti through cycling.
2.2.
Pelletization of sodium alanate
Pelletization of sodium alanate was carried out with a Carver Model C Laboratory Press with 11 metric tons maximum force. Pellet dies were custom made from hardened steel with diameters of 3.17, 6.35 and 9.50 mm (1/8, 1/4 and 3/8 inch, respectively); maximum pellet thickness/length of approximately 15 mm. All pellets were pressed at 345 MPa (50 kPSI) unless otherwise noted. No additional binder(s) were required to facilitate rigid pellets from alanate mixture.
2.3.
Hydrogen sorption/desorption measurements
Hydrogen cycling kinetics and capacity of the pelletized sodium alanate were measured using a manual Sieverts’ apparatus custom designed and made by HyEnergy LLC. Volumetric free space was determined at room temperature with helium gas. Ultra-high purity (99.999%) hydrogen and helium obtained from Airgas Inc. were used for all measurements. Desorption half cycles were performed under vacuum at 180 C; absorption half cycles were performed under 120 bar of hydrogen at 150 C. Heat was applied by cylindrical Watlow electrical heaters controlled by the apparatus software. The sample vessel was air cooled to room temperature after each half cycle. Temperature was regulated by two k-type thermocouples, one inserted into the exterior body of the vessel and one inserted internally in contact with the bottom of the pellet bed.
2.4.
Pellet packing within vessel
Pellets were packed within the vessel with 1e3 mm distance from the vessel wall to ensure direct contact did not influence pellet bed heating as to best simulate a typical large-scale tank bed that would not have direct heating to all the pellet bodies throughout the bed. Small aluminum cups were made to hold pellets stationary. The 6.35 and 9.50 mm diameter pellets were packed in a cylindrical stack for hydrogen cycling capacity measurements. The 3.17 mm diameter pellets were packed side by side to form a single layer with a minimum of two layers in a stack. The total in height for each pellet stack varied by 2 mm between the three different diameter pellets.
2.5.
Thermal conductivity measurements
Thermal conductivity of uncycled and cycled sodium alanate pellets (with and without graphite additives) was measured using a LFA 447 Nanoflash light flash system by Netzsch equipped with a high-performance Xenon flash lamp. All measurements were performed at 25 C. All uncycled pellets measured 1.5e2.5 mm in thickness and approximately 9.45 mm in diameter. Cycled pellet dimensions were greater than uncycled pellet dimensions and varied according to number of cycles. LFA 447 instrument sits on bench top and is not housed within an inert atmosphere. To minimize oxidation of samples only one pellet was measured at a time with the instrument’s filter wheel (% light) set to 50%. In addition, the laser flash was retrofitted with an adjustable flow of argon through sample bed. Trial measurements conducted periodically over 8 h period determined level of oxidation and effect
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on thermal conductivity to be acceptable (<5% variance). A typical measurement took under twenty minutes. Instrument software automatically adjusted for main amp gain and measurement duration for each sample. Analysis was performed with Proteus LFA Analysis software and Cowan model [15] was used for calculation of temperature versus time profiles. Graphite standards of known specific heat capacity (Cp) were used to determine Cp of each sodium alanate pellet. Thermal conductivity of each pellet was calculated from the Cp, bulk density and thermal diffusivity measured by instrument.
2.6.
Thermal conductivity enhancers
Expanded graphite and graphite flakes were added to sodium alanate, separately, and milled together using a SPEX 8000D mixer mill equipped with a variable speed motor. Powders were placed into a steel milling vial and sealed inside an argon atmosphere glove box. High-energy mixing was achieved by use of ten 10 mm diameter stainless steel ball bearings with a 70:1 ball to powder ratio (bpr). Low-energy mixing was achieved with no additional medium within milling vials other than powders. All samples were milled for 20 min at 1100 rpm.
2.7.
Pellet durability
Durability of pelletized sodium alanate was determined by subjecting pellets to rigorous vibration using a Retsch Micro Mixer Mill type MM2. A series of 6.35 mm diameter pellets, averaging 3 mm in length, were stacked inside a cylindrical glass tube housed in a vessel that was sealed inside an argon atmosphere glove box. The pellet stack was vibrated for 30 min at 800 rpm (maximum instrument vibration setting). Glass tube was 0.7 mm wider and 1.5 mm longer than pellet stack allowing for 33.4% free space and substantial pellet movement. Pellet dimensions and mass were measured before and after test.
2.8.
Pellet density
Pellet bulk density was determined through measurement of weight and external pellet dimensions using an electronic caliper precise to 0.01 mm. Pellet skeletal density was determined with a Micromeritics AccuPycª II 1340 series pycnometer. The Accupycª II was controlled remotely by proprietary interface software and uses a gas displacement technique to measure sample volume. Gas pressures are observed upon filling the sample chamber and discharge to a secondary empty chamber allowing for high-precision determination of the sample volume. Helium gas was used to ensure maximum void space occupancy enabling the true solid phase volume of the pellet measured.
2.9.
Scanning electron microscopy (SEM)
Pellets were mounted to SEM sample plates inside an argon gloved box with carbon tape or epoxy and sealed inside a glass vial for transportation. Micrographs were obtained with a Carl Zeiss NVision 40 CrossBeam Workstation SEM equipped with a high resolution field emission Gemini and high
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performance SIINT zeta FIB column. Pellets were momentarily exposed to atmosphere when transferred from glass vial to SEM sample chamber, no visible oxidation occurred. Vacuum was initiated immediately upon chamber door closure.
3.
Results and discussion
3.1.
Uncycled pellets
3.1.1.
Powder pelletization
The pelletization process of sodium alanate mixture used in this study (NaH þ Al þ TiCl3) was minimal. No additional workup before and after pressing (i.e. binders, thermal treatment, solvents and drying) was required, unlike carbon based pellets [6]. Once pelletized, the pellet surface had a metallic sheen due to the excessive aluminum. Pellets could withstand substantial mistreatment without fracturing; no damage was observed even when dropped from several meters. For comparison, pure NaAlH4 was also pelletized. Again, additional work-up was not required; however, some decomposition of the alanate occurred during pelletization as the pellet surface had a noticeable gray appearance (free aluminum) compared to loose powder. Titanium trichloride doped NaAlH4 was also pelletized. Once again, the powder compacted easily into a pellet; however, unlike the titanium doped dehydrided alanate (NaH þ Al) the Ti-doped hydrided alanate (NaAlH4) pellets lost structural integrity over time inside the glove box due to partial disassociation into Na3AlH6 and NaH resulting from evolving hydrogen. Cracks in the surface were also visible along with deformation of the Ti-doped NaAlH4 pellet, and the pellet fell apart during handling. During same time period Ti-doped NaH þ Al pellets did not change shape, remained structurally intact and no fracturing was observed during handling, thus pelletization of NaH þ Al þ TiCl3 (referred to as sodium alanate here on out) has the distinct advantage for mass production. In addition, pelletized sodium alanate also does not react as vigorously with water as does the powder. With far less surface area the pellet fizzes until consumed when immersed in or brought in contact with water. Sodium alanate powder was pelletized in a series of successive pressures, 69e345 MPa (10e50 kPSI). Each step-up in pressure resulted in greater densification of the powder with smaller incremental densification per step. Due to safety concerns and equipment limitations 345 MPa was chosen as the maximum pressure. At an average pellet density of 1.60 g/cm3 (345 MPa), compaction of sodium alanate was improved by 95% compared to the tap density of the loose powder, 0.82 g/cm3, Table 1. As a comparison, maximum pellet density of Ti-doped NaAlH4 averaged 15.3% less (1.35 g/cm3). This divergence is further discussed in the proceeding section. Percentage of void volume per pelletization pressure was calculated based on the skeletal density averaged over 20 pellets measured with a helium pycnometer (davg ¼ 2.23 g/cm3, STDV of 0.137).
3.1.2.
Thermal conductivity
Analogous to pellet density, thermal conductivity also increased with each subsequent increase in applied
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Table 1 e Summary of 9.50 diameter pellet density, thermal conductivity and void volume percentage pelletized over a range of pressures (loose alanate powder included). Pellet die pressure (MPa/kPSI) Powder 69/10 138/20 276/40 345/50
Density (g/cm3)
Thermal conductivity (W/m K)
Pellet void volume %
0.82 1.29 1.41 1.53 1.60
z0.50 2.48 4.50 7.41 9.09
63.3% 42.5% 38.4% 31.0% 29.1%
pelletization pressure. Over an increase in pellet density of 1.29e1.60 g/cm3 there was a 230% improvement in thermal conductivity (2.75e9.09 W/m K). In contrast, over the same pelletization pressure range Ti-doped NaAlH4 pellet density had a lesser increase (1.10e1.35 g/cm3) while thermal conductivity only improved by 115% (0.636e1.37 W/m K). Fig. 1 shows the exponential relationship between pellet density and thermal conductivity of Ti-doped NaH þ Al and the linear relationship for Ti-doped NaAlH4. The superior thermal conductivity observed results from the high levels of free aluminum (kAl ¼ 250 W/m K at 25 C) and greater compaction of the bulk powder. Compaction into denser pellets is attributed to particle size, which is influenced by starting materials and milling conditions. Interestingly, pure NaAlH4 had slightly higher thermal conductivity compared to Ti-doped NaAlH4 at 345 MPa pelletization pressure (2.30 and 1.37 W/m K, respectively). As mentioned above, pelletization of pure NaAlH4 resulted in an obvious gray appearance on the surface that is most likely due to free aluminum resulting from slight decomposition of the alanate during the pelletization process. If the same holds true for Ti-doped NaAlH4 greater decomposition should occur during pelletization resulting in more free aluminum; however, there must also be more of the intermediate phase, Na3AlH6, as well. Lower thermal conductivity of the Ti-doped NaAlH4, in comparison to pure alanate, can be accredited to
Fig. 1 e Thermal conductivity of pelletized Ti-doped NaH D Al ( ) and Ti-doped NaAlH4 ( ) compared with density (loose powder data from Table 1 included). Inset shows exponential fit of NaH D Al data up to 50 W/mK (correlating pellet density z2 g/cm3).
the higher percent of hexahydride phase its crystal structure. With greater inter-atomic distances than the tetrahydride phase [7], Na3AlH6 has a detrimental effect on pellet thermal conductivity [8]. The thermal conductivity of complex metal hydrides plays a significant role in the heat transfer rates within the tank bed and efficient removal of heat of adsorption, therefore, any increase in thermal conductivity is important for system design. While increased densification of pelletized alanate dramatically increased thermal conductivity (Fig. 1), we explored additional thermal enhancement additives. It has been reported that expanded graphite improves poor thermal conductivity of metal hydride powder beds by addition to the hydride followed by compaction [9]. In addition to combining expanded graphite with sodium alanate, graphite flakes was also examined as a potential thermal conductivity enhancer. Thermal conductivity of graphite enhanced pellet samples are shown in Fig. 2. Neither high- or low-energy milled graphite enhancers had a beneficial effect on thermal conductivity of the alanate pellets. In fact, high-energy milling had a significant negative effect on thermal conductivity. By subjecting the graphite additives to high-energy milling, particle reduction most likely occurred, thus damaging the high thermally conductive carbon-array of graphite (119e165 W/m K at 25 C) [10,13,14]. The graphite additives exhibited thermal conductivity properties comparable to amorphous carbon, which has a very low thermal conductivity of 1.70 W/m K at 25 C [10]. Pellets prepared from low-energy milling of alanate and graphite exhibited similar thermal conductivity as pellets without graphite, but were still noticeably lower in comparison even though pellet densities were comparable. The anisotropic nature of graphite combined with the random orientation of the enhancers after milling contributed to either no improvement or slight decrease in thermal conductivity. In addition, the high pressures used for pelletization may have also caused some degradation of the graphitic structure. This is seen in the thermal conductivity measurements conducted on pellets with and without graphite enhancers. At the lower pressure, pellets with added graphite had 10% higher k values than pellets of alanate alone whereas at the higher pressure graphite enhanced pellets were 10% lower.
Fig. 2 e Thermal conductivity of Ti-doped sodium alanate with 5 mol% graphite enhancers (Exp Gr [ expanded graphite, Gr Fl [ graphite flake) and without. Pellets pressed at 345 MPa unless otherwise noted.
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3.1.3.
Pellet durability
To examine mechanical durability in an automotive on-board storage system, a series of pellets were confined in a fixed volume and subjected to constant vibrations over a thirty minute period. The pellet stack occupied 1.73 cm3 of the test cell’s 2.60 cm3, leaving just over 33% free space for pellet movement during simulation. The pellet stack lost 0.36% in volume and 1.23% in overall density through the duration of the test. None of the pellets were observed to be fractured, chipped or damaged in any way; all pellets maintained their original shape. The loss observed in volume and density occurred due to friction between pellets and test cell wall. A few milligrams of loose powder were recovered from the bottom and walls of the test cell. While the observed loss of sodium alanate pellet volume over the 30 min period was negligible, prolonged or repetitive movement may result in greater deterioration of pelletized sodium alanate; therefore, it would be crucial to minimize movement of pellets inside the storage tank as well as all interior tank components.
3.2.
Thermally cycled pellets
3.2.1.
Kinetics and capacity
The motivation for implementing pelletized complex metal hydrides into potential tank designs is two-fold; increased system volumetric capacity and prevention of loose powder contaminating the fuel system components. While it is clear that thermal conductivity and bulk density of sodium alanate is improved through pelletization, it is important to study the effect of pelletization on hydrogen absorption capacity and kinetics. Fig. 3 shows a plot of hydrogen desorption of pellets and powder in comparable sample beds with heating rates of 5 C/min and 2 C/min. While a lengthy onset for hydrogen release was anticipated for both pellets and powder with the slow heating rate, pelletized sodium alanate does not exhibit the dramatic difference in kinetics and capacity as seen with the powder. The long-term shelf life of the sodium alanate mixture used in this study (NaH þ Al þ TiCl3) is a distinct advantage over the more commonly used TiCl3 doped NaAlH4. However, the consequence of manually combining these constituents is that an intimate mixture of sodium hydride and aluminum cannot be achieved through milling alone, which does occurs when Ti-doped NaAlH4 is thermally dehydrided. As a result,
Fig. 3 e Kinetic profiles of Ti-doped NaH D Al powder (solid line) and pellet (dashed line) at heating rates of (a) 5 C/min and (b) 2 C/min.
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the powder mixture can require up to 3 absorption/desorption cycles to reach full hydrogen capacity, not shown. In addition, compaction of the powder into pelletized form further prolongs the activation process to achieve optimum hydrogen capacity and kinetics. Fig. 4 shows 9.50 mm diameter pellet kinetic profiles from cycles 1, 5 and 10. There is a clear progression towards maximum capacity, as well as kinetics. Beyond 10 cycles there is no further improvement in hydrogen cycling properties of the sodium alanate pellets. This observation in the behavior of sodium alanate pellets is independent of pellet diameter. The slowed down activation in alanate pellets to achieve maximum capacity and kinetics is directly related to powder compaction. The highly compacted pellets do not allow for hydrogen gas to thoroughly permeate into the bulk of the body; therefore, the pellet cannot reach full absorption. As the pellets are cycled they expand, thereby allowing hydrogen gas to pass through the entire body causing the progression in capacity and kinetics seen in Fig. 4. In subsequent sections expansion and the effect it has pellet properties is described in greater detail. While activation is independent of pellet dimensions, cycling kinetics is quite dependent on pellet diameter and thickness. Fig. 5a and b shows the effect of pellet diameter and thickness, respectively, on cycling kinetics and capacity. As seen in Fig. 5a, the smallest diameter pellets (3.17 mm) had the slowest kinetics while 6.35 and 9.50 mm diameter pellets had faster kinetics, counter to expected outcome. This is a direct result of the stacking arrangement of the pellets within the reactor vessel. Pellets with 6.35 and 9.50 mm diameters could be easily stacked face to face allowing for greater surface area contact and better heat transfer between pellets. On the other hand, for the 3.17 mm diameter case, stacking pellets proved to be difficult, therefore, the pellets were placed side by side to create a single layer. This arrangement resulted in less surface to surface contact and therefore inferior heat transfer between pellets due to the cylindrical shape of the pellets. A study of different pellet shapes (i.e. rectangular, triangular or
Fig. 4 e Cycling activation of 9.50 mm diameter sodium alanate pellets over the first 10 cycles (cycle 1: black, cycle 5: red, cycle 10: blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5 e a) Sodium alanate pellet kinetic dependence on pellet diameter (3.17 mm: black, 6.35 mm: red, 9.50 mm: blue), b) pellet kinetic dependence on pellet thickness (1.00 mm: black solid, 1.70 mm: red, 3.50 mm: black dashed, 6.00 mm: blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
square) may shed more light on the difference observed in kinetics regarding pellet diameter, but was outside the scope of this study. In Fig. 5b the kinetics and capacity of 9.50 mm diameter pellets of varying thicknesses are illustrated. As before, the results were contrary to the expected outcome. The thinnest of the four thicknesses, 1.00 mm, in fact had slower kinetics than pellets almost four times thicker, 3.50 mm. While it might be expected that with radial heating of the sample vessel heat transfer through the pellets in an axial direction should not be a significant factor; however, Fig. 5b shows otherwise. Axial heating of highly compacted alanate pellets, especially when surface contact resistance between neighboring bodies of a stack of pellets is a factor, plays a significant role. With thinner pellets more pellets were required to match the mass, or stack height, of thicker pellets; therefore, thermal conductivity decreases due to the increased number of pellets and contact resistance seen at each face. The decrease in overall thermal conductivity slows the heat transfer through the reactor bed resulting in the slower kinetics observed. An upper limit to pellet thickness for faster kinetics was also observed (Fig. 5b). It is clear, that random pellet dimensions and therefore random pellet arrangement within a storage tank would not be the favorable approach. As mentioned previously, once sufficient pellet expansion has occurred and hydrogen permeability during desorption and absorption was not impeded capacities averaging 4 wt% H2 were observed. It has been reported that over 100 cycles the hydrogen capacity of titanium doped sodium alanate powder peaks in the first half of cycles then slowly levels off during the second half [11]. Fig. 6 shows alanate pellet capacity over 50 cycles. The alanate pellets exhibit similar cycling capacity characteristic as reported by Sun et al. [11].
3.2.2.
Fig. 7. Through 30 cycles, pellet density decreased by an average of 50%, from 1.35 g/cm3 to 0.70 g/cm3 for pellets pressed at 69 MPa (10 kPSI) and from 1.60 g/cm3 to 0.80 g/cm3 for pellets pressed at 345 MPa (50 kPSI). Pellets were cycled beyond 30 cycles to determine if a plateau for expansion could be reached, but cycling was ceased after 50 cycles due to the pellets’ increasing fragility, making handling difficult, and measurement of pellet density data impossible. In general, if the pellet expansion was not impeded by limited volume within sample reactor, pellets did not deform or lose their original shape. Hydrogen cycling and resulting pellet expansion also had a detrimental effect on thermal conductivity as shown in Fig. 8. Over the first 10 cycles, pellet thermal conductivity decreased by 79% from 9 W/m K to 1.83 W/m K (dehydrided alanate). Interestingly, each half cycle had slightly different k values. The fully hydrided half cycle (NaAlH4) was consistently lower than fully dehydrided half cycle (NaH þ Al). This difference in thermal conductivity can be contributed to the state in which aluminum finds itself at each end of the reaction; when NaAlH4 is fully dehydrided, aluminum exists in
Pellet expansion
Titanium doped NaAlH4 powder is well known to expand during hydrogen cycling and can sinter if free space for expansion is limited [12]. Unfortunately, our pellets were not exempt to this characteristic. Expansion is independent of pellet dimensions and pelletization pressure, as shown in
Fig. 6 e Pellet hydrogen capacity over 50 cycles (black dashed: theoretical capacity of 5.5 wt%). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 7 e Density loss of sodium alanate pellets (a) 3.17, (b) 6.35 and (c) 9.50 mm in diameter pressed at 69 MPa (blue) and 345 MPa (red) over 30 cycles. Lone points at 10 cycles indicate pellet density after first half-cycle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) a free state as opposed to the tetrahedral structure, [AlH4], for a fully hydride sample. After 10 cycles the average thermal conductivity was comparable to that of densest uncycled Ti-doped NaAlH4 pellets (Fig. 1).
3.2.3.
Thermal enhancers
Graphite enhanced pellets were investigated as a possible method to counter loss in pellet thermal conductivity due to cycling (Fig. 9). As with the non-graphite enhanced pellets a significant decrease in thermal conductivity, up to 70%, was observed within the first 10 cycles. While overall pellet density loss for all samples (with or without graphite) was similar, z30%, the difference in overall loss in thermal conductivity, z10%, is mostly attributed to the 11% difference in k values corresponding to uncycled non-graphite and graphite enhanced pellets. However, at each cycle evaluated (1, 5 and 10) the graphite enhanced pellets had an average thermal
Fig. 8 e Thermal conductivity of 9.50 mm diameter pellets at each half-cycle (blue: fully absorbed, red: fully desorbed) after 1, 5 and 10 cycles. Pellet density at each cycle measured (black). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
conductivity 18.3% greater than pellets without graphite. As the densities of pellets with and without graphite at each cycle were comparable, it can be concluded that graphite enhancers do aid in countering loss of thermal conductivity; however, not to a degree significant enough to justify the loss of wt% H2 by the overall system. Addition of graphite beyond 5 mol% may further prevent loss in pellet thermal conductivity; however, it would cause significant reduction in hydrogen uptake. The loss in thermal conductivity (with or without graphite additives) is a direct result of expansion sodium alanate pellets experience through hydrogen cycling. The key factor contributed to the observed pellet expansion and loss in thermal conductivity is an increase in void space within the interior of each pellet (Fig. 10). Inter-atomic forces associated
Fig. 9 e Thermal conductivity of 9.50 mm diameter alanate pellets with expanded graphite (black dotted) and graphite flakes (blue dashed) at 1, 5 and 10 cycles. Pellet density at each cycle (black solid: expanded graphite, blue solid: graphite flakes). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 10 e SEM micrographs (33k magnification, 1 mm scale) showing the internal side of alanate pellets: a) 0 cycles, b) 5 cycles, c) 20 cycles and d) 50 cycles.
with continuous formation and reformation of the sodium alanate absorption/desorption reaction products, NaAlH4 and NaH (Equations (1) and (2)), combined with pathways formed as hydrogen gas permeates in and out of the bulk material contribute to the noted pellet expansion. This unavoidable formation of void space within each pellet affords inferior thermal conduction through individual pellets and throughout the overall tank bed over continuous cycling.
4.
Conclusions
Sodium alanate pellets were investigated for their practical application for on-board hydrogen storage in a fuel cell vehicle. Compared to the tapped powder, material density was increased by 100% upon pelletization. It was also observed that while activation is required for optimum capacity and kinetics, pelletization of sodium alanate does not hinder kinetics or weight percent of hydrogen absorbed or released. In fact, for poor heating rate conditions the pelletized alanate outperformed the powder. Although the complex metal hydride’s kinetics and capacity were not adversely affected through 50 hydrogen cycles, pellet structural integrity and thermal conductivity degraded due to expansion. A key aspect of compacting sodium alanate into pellet form is to improve system volumetric capacity. However, since the outcome of cycling pellets is a regression back to powder form, pelletization does not offer any benefits for on-board storage. The solution may lie with the implementation of a complex
metal hydride that has minimal crystal structure difference between the fully hydrided and dehydrided phases. Another possibility is to devise a mechanism to restrain pellets that will prevent expansion while allowing sufficient flow of hydrogen in and out of the bulk.
Acknowledgments This work was performed under the DOE contract DE-FC3609GO19003 as GM’s contribution to the DOE Hydrogen Storage Engineering Center of Excellence (HSECoE). The authors would like to thank Sandia National Laboratory for providing the complex metal hydride studied in this work. The authors would also like to thank the HSECoE members for their valuable comments and suggestions.
references
[1] Schlapbach L, Zuttel A. Hydrogen storage materials for mobile applications. Nature 2001;414:353e8. B, Schwickardi M. Ti-doped alkali metal [2] Bogdanovic aluminum hydrides as potential novel reversible hydrogen storage materials. J Alloys Comp 1997;253:1e9. [3] Orimo S, Nakamori Y, Eliseo JR, Zu¨ttel A, Jensen CM. Complex hydrides for hydrogen storage. Chem Rev 2007;107: 4111e32. [4] http://www1.eere.energy.gov/hydrogenandfuelcells/storage/ pdfs/targets_onboard_hydro_storage.pdf.
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[5] http://apps1.eere.energy.gov/news/news_detail.cfm/news_ id¼12014. [6] Bose TK, Chahine R, Starnaud J-M. High-density adsorbent and method of producing same. US Patent 004999330. B. [7] Ro¨nnebro E, Nore´us D, Kadir K, Reiser A, Bogdanovic Investigation of the perovskite related structures of NaMgH3, NaMgF3 and Na3AlH6. J Alloys Comp 2005;299:101e6. [8] Gaume´ R, Viana B, Vivien D, Roger J-P, Fournier D. A simple model for the prediction of thermal conductivity in pure and doped insulating crystals. Appl Phys Lett 2003;83:1355e7. [9] Kim KJ, Montoya B, Razani A, Lee K-H. Metal hydride compacts of improved thermal conductivity. Int J Hydrogen Energy 2001;26:609e13. [10] Touloukian YS, Powell RW, Ho CY, Klemens PG. Thermophysical properties of matter e the TPRC data series. In: Thermal conductivity e nonmetallic solids, vol. 2. New York: IFI/Plenum; 1971.
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[11] Srinivasan SS, Brinks HW, Hauback BC, Sun D, Jensen CM. Long term cycling behavior of titanium doped NaAlH4 prepared through solvent mediated milling of NaH and Al with titanium dopant precursors. J Alloys Comp 2004;377: 283e9. [12] Sandrock G, Gross K, Thomas G, Jensen C, Meeker D, Takara S. Engineering considerations in the use of catalyzed sodium alanates for hydrogen storage. J Alloys Comp 2002; 330:696e701. [13] Dedrick DE, Kanouff MP, Replogle BC, Gross KJ. Thermal properties characterization of sodium alanates. J Alloys Comp 2005;389:299e305. [14] Dedrick DE, Kanouff MP, Larson RS, Johnson TA, Jorgensen SW. 2009 ASME summer heat transfer conference (HT2009e88246), San Francisco, California; July 2009. [15] Cowan RD. Pulse method of measuring thermal diffusivity at high temperatures. J Appl Phys 1963;34:929e1027.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Cycling and engineering properties of highly compacted sodium alanate pellets M. Sulic a,*, M. Cai b, S. Kumar b a b
Optimal CAE, Inc., 14492 Sheldon Road, Plymouth Township, MI 48170, USA1 General Motors Research and Development, 30500 Mound Road, Warren, MI 48090, USA
article info
abstract
Article history:
Sodium alanate powder comprised of NaH and Al was doped with 3 mol% titanium chlo-
Received 18 May 2012
ride (TiCl3) and pelletized into highly compacted cylindrical pellets. The pelletization
Received in revised form
process was performed to improve thermal conductivity and volumetric hydrogen capacity
24 July 2012
of the metal hydride, compared to loose or tapped powder, which are vital requirements
Accepted 25 July 2012
for on-board hydrogen storage applications. The pelletization process was performed over
Available online 24 August 2012
a range of 69e345 MPa (10e50 kPSI) with a 95% increase in density and improvement in thermal conductivity 18 times greater compared to powder at the maximum pelletization
Keywords:
pressure (1.60 g/cm3 and 0.82 g/cm3; 9.09 W/m K and 0.50 W/m K, respectively). Hydrogen
Sodium alanate
cycling capacities and kinetics were not adversely affected by the pelletization process
Hydrogen storage
although 10 cycles are required to obtain full hydrogen capacity. Pellet cycling capacity
Thermal conductivity
maintained a stable 4 wt% H2 over 50 cycles. Ti-doped NaH þ Al pellets exhibited similar
Pelletization
thermal cycling expansion as with the loose powder; within 30 cycles there was a 50% loss
Expansion
in pellet density and by 50 cycles the loss in pellet structural integrity made handling
Density loss
problematic. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
A cost effective, robust and economical vehicular hydrogen storage system is paramount for the implementation of fuel cell technology by the automobile industry. For more than a decade extensive research has been conducted on solidstate complex metal hydrides as one of the potential solidstate storage material candidates in order to facilitate this shift in paradigm. Theoretically, solid-state complexes have greater per unit volume storage than compressed and liquefied hydrogen. This property translates to smaller vehicle storage tank volume and greater driving distances between refueling [1], attractive aspects not only for automotive companies, but for consumers as well.
Sodium aluminum tetrahydride (NaAlH4) is a complex metal hydride that has been known for decades and has long been utilized as a chemical reagent due to its strong reducing agent properties, similar to that of lithium aluminum tetrahydride. While in its pure form sodium alanate does not have favorable reversible hydrogen cycling kinetics (desorption 200e300 C; absorption 270 C, 17.5 MPa and 2e3 h); however, when doped with few mole percent of a titanium compound (e.g. TiCl3, TiF3, or Ti(OBu)4) cycling conditions become much more favorable (desorption 150 C; absorption 170 C, 15.2 MPa and 5 h) [2,3]. These findings were first reported in 1997 by Bogdanovic et al. and have since inspired the renewed interest in complex metal hydrides as a realistic means of hydrogen storage. The reversible decomposition
* Corresponding author. Tel.: þ1 586 986 5279; fax: þ1 586 986 2471. E-mail addresses: [email protected], [email protected] (M. Sulic). 1 Contracted to General Motors Research and Development. 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.07.113
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reaction of sodium alanate occurs in a two-step process (Equations (1) and (2)) with a theoretical capacity of 5.5 wt% hydrogen:
NaAlH4 / 1/3 Na3AlH6 þ 2/3 Al þ H2 (3.7 wt%)
(1)
Na3AlH6 / 3 NaH þ Al þ 3/2 H2 (1.8 wt%)
(2)
However, system gravimetric capacity targets established by the US Department of Energy for on-board vehicle hydrogen storage [4] surpass the theoretical 5.5 wt% of hydrogen absorption capacity of sodium alanate. While these targets exclude NaAlH4 as a potential candidate for on-board hydrogen storage, extensive research reported on the material makes this complex metal hydride an excellent candidate for engineering studies of a practical hydrogen storage system. In 2009, the US DOE established the Hydrogen Storage Engineering Center of Excellence (HSECoE) to address the hydrogen storage system issues for vehicular application [5]. The Center, a consortium of national labs, academic institutes, industrial laboratories and automobile companies, was formed as a five year project aimed at demonstrating viable technologies for on-board hydrogen storage that meet vehicle requirements in a cost-effective way. One of the technologies that the Center is evaluating is metal hydride based hydrogen storage. Successful demonstration will help the automobile industry commercialize fuel cell vehicles and help the nation move one step closer to a hydrogen economy. Reported herein is our study on the first complex metal hydride chosen by the Center, sodium alanate, and our examination of the material for practical automotive applications. As most hydrogen storage materials have poor heat transfer properties, our goal in this study was to improve upon such properties whilst examining the metal hydride’s mechanical stability and durability and to establish a foundation for which other solid-state hydrogen storage materials can be investigated.
2.
Experimental methods
All experimental procedures were carried out in an argon atmosphere unless otherwise noted.
2.1.
Material synthesis
The titanium doped sodium alanate used in this study was previously prepared and available in bulk. The mixture is comprised of the desorbed constituents of NaAlH4, sodium hydride (NaH) and aluminum powder (Al), and titanium trichloride (TiCl3). It was prepared in this form of sodium alanate to ensure long term shelf-life. Stoichiometric amounts of NaH and Al along with 3 mol% TiCl3 and 18 wt% excess Al were ball milled together with a planetary mill. The hydride mixture was prepared with excess aluminum in order to maximize the formation of NaAlH4 during rehydrogenation, thus compensating for the free Al taken up by Ti through cycling.
2.2.
Pelletization of sodium alanate
Pelletization of sodium alanate was carried out with a Carver Model C Laboratory Press with 11 metric tons maximum force. Pellet dies were custom made from hardened steel with diameters of 3.17, 6.35 and 9.50 mm (1/8, 1/4 and 3/8 inch, respectively); maximum pellet thickness/length of approximately 15 mm. All pellets were pressed at 345 MPa (50 kPSI) unless otherwise noted. No additional binder(s) were required to facilitate rigid pellets from alanate mixture.
2.3.
Hydrogen sorption/desorption measurements
Hydrogen cycling kinetics and capacity of the pelletized sodium alanate were measured using a manual Sieverts’ apparatus custom designed and made by HyEnergy LLC. Volumetric free space was determined at room temperature with helium gas. Ultra-high purity (99.999%) hydrogen and helium obtained from Airgas Inc. were used for all measurements. Desorption half cycles were performed under vacuum at 180 C; absorption half cycles were performed under 120 bar of hydrogen at 150 C. Heat was applied by cylindrical Watlow electrical heaters controlled by the apparatus software. The sample vessel was air cooled to room temperature after each half cycle. Temperature was regulated by two k-type thermocouples, one inserted into the exterior body of the vessel and one inserted internally in contact with the bottom of the pellet bed.
2.4.
Pellet packing within vessel
Pellets were packed within the vessel with 1e3 mm distance from the vessel wall to ensure direct contact did not influence pellet bed heating as to best simulate a typical large-scale tank bed that would not have direct heating to all the pellet bodies throughout the bed. Small aluminum cups were made to hold pellets stationary. The 6.35 and 9.50 mm diameter pellets were packed in a cylindrical stack for hydrogen cycling capacity measurements. The 3.17 mm diameter pellets were packed side by side to form a single layer with a minimum of two layers in a stack. The total in height for each pellet stack varied by 2 mm between the three different diameter pellets.
2.5.
Thermal conductivity measurements
Thermal conductivity of uncycled and cycled sodium alanate pellets (with and without graphite additives) was measured using a LFA 447 Nanoflash light flash system by Netzsch equipped with a high-performance Xenon flash lamp. All measurements were performed at 25 C. All uncycled pellets measured 1.5e2.5 mm in thickness and approximately 9.45 mm in diameter. Cycled pellet dimensions were greater than uncycled pellet dimensions and varied according to number of cycles. LFA 447 instrument sits on bench top and is not housed within an inert atmosphere. To minimize oxidation of samples only one pellet was measured at a time with the instrument’s filter wheel (% light) set to 50%. In addition, the laser flash was retrofitted with an adjustable flow of argon through sample bed. Trial measurements conducted periodically over 8 h period determined level of oxidation and effect
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on thermal conductivity to be acceptable (<5% variance). A typical measurement took under twenty minutes. Instrument software automatically adjusted for main amp gain and measurement duration for each sample. Analysis was performed with Proteus LFA Analysis software and Cowan model [15] was used for calculation of temperature versus time profiles. Graphite standards of known specific heat capacity (Cp) were used to determine Cp of each sodium alanate pellet. Thermal conductivity of each pellet was calculated from the Cp, bulk density and thermal diffusivity measured by instrument.
2.6.
Thermal conductivity enhancers
Expanded graphite and graphite flakes were added to sodium alanate, separately, and milled together using a SPEX 8000D mixer mill equipped with a variable speed motor. Powders were placed into a steel milling vial and sealed inside an argon atmosphere glove box. High-energy mixing was achieved by use of ten 10 mm diameter stainless steel ball bearings with a 70:1 ball to powder ratio (bpr). Low-energy mixing was achieved with no additional medium within milling vials other than powders. All samples were milled for 20 min at 1100 rpm.
2.7.
Pellet durability
Durability of pelletized sodium alanate was determined by subjecting pellets to rigorous vibration using a Retsch Micro Mixer Mill type MM2. A series of 6.35 mm diameter pellets, averaging 3 mm in length, were stacked inside a cylindrical glass tube housed in a vessel that was sealed inside an argon atmosphere glove box. The pellet stack was vibrated for 30 min at 800 rpm (maximum instrument vibration setting). Glass tube was 0.7 mm wider and 1.5 mm longer than pellet stack allowing for 33.4% free space and substantial pellet movement. Pellet dimensions and mass were measured before and after test.
2.8.
Pellet density
Pellet bulk density was determined through measurement of weight and external pellet dimensions using an electronic caliper precise to 0.01 mm. Pellet skeletal density was determined with a Micromeritics AccuPycª II 1340 series pycnometer. The Accupycª II was controlled remotely by proprietary interface software and uses a gas displacement technique to measure sample volume. Gas pressures are observed upon filling the sample chamber and discharge to a secondary empty chamber allowing for high-precision determination of the sample volume. Helium gas was used to ensure maximum void space occupancy enabling the true solid phase volume of the pellet measured.
2.9.
Scanning electron microscopy (SEM)
Pellets were mounted to SEM sample plates inside an argon gloved box with carbon tape or epoxy and sealed inside a glass vial for transportation. Micrographs were obtained with a Carl Zeiss NVision 40 CrossBeam Workstation SEM equipped with a high resolution field emission Gemini and high
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performance SIINT zeta FIB column. Pellets were momentarily exposed to atmosphere when transferred from glass vial to SEM sample chamber, no visible oxidation occurred. Vacuum was initiated immediately upon chamber door closure.
3.
Results and discussion
3.1.
Uncycled pellets
3.1.1.
Powder pelletization
The pelletization process of sodium alanate mixture used in this study (NaH þ Al þ TiCl3) was minimal. No additional workup before and after pressing (i.e. binders, thermal treatment, solvents and drying) was required, unlike carbon based pellets [6]. Once pelletized, the pellet surface had a metallic sheen due to the excessive aluminum. Pellets could withstand substantial mistreatment without fracturing; no damage was observed even when dropped from several meters. For comparison, pure NaAlH4 was also pelletized. Again, additional work-up was not required; however, some decomposition of the alanate occurred during pelletization as the pellet surface had a noticeable gray appearance (free aluminum) compared to loose powder. Titanium trichloride doped NaAlH4 was also pelletized. Once again, the powder compacted easily into a pellet; however, unlike the titanium doped dehydrided alanate (NaH þ Al) the Ti-doped hydrided alanate (NaAlH4) pellets lost structural integrity over time inside the glove box due to partial disassociation into Na3AlH6 and NaH resulting from evolving hydrogen. Cracks in the surface were also visible along with deformation of the Ti-doped NaAlH4 pellet, and the pellet fell apart during handling. During same time period Ti-doped NaH þ Al pellets did not change shape, remained structurally intact and no fracturing was observed during handling, thus pelletization of NaH þ Al þ TiCl3 (referred to as sodium alanate here on out) has the distinct advantage for mass production. In addition, pelletized sodium alanate also does not react as vigorously with water as does the powder. With far less surface area the pellet fizzes until consumed when immersed in or brought in contact with water. Sodium alanate powder was pelletized in a series of successive pressures, 69e345 MPa (10e50 kPSI). Each step-up in pressure resulted in greater densification of the powder with smaller incremental densification per step. Due to safety concerns and equipment limitations 345 MPa was chosen as the maximum pressure. At an average pellet density of 1.60 g/cm3 (345 MPa), compaction of sodium alanate was improved by 95% compared to the tap density of the loose powder, 0.82 g/cm3, Table 1. As a comparison, maximum pellet density of Ti-doped NaAlH4 averaged 15.3% less (1.35 g/cm3). This divergence is further discussed in the proceeding section. Percentage of void volume per pelletization pressure was calculated based on the skeletal density averaged over 20 pellets measured with a helium pycnometer (davg ¼ 2.23 g/cm3, STDV of 0.137).
3.1.2.
Thermal conductivity
Analogous to pellet density, thermal conductivity also increased with each subsequent increase in applied
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Table 1 e Summary of 9.50 diameter pellet density, thermal conductivity and void volume percentage pelletized over a range of pressures (loose alanate powder included). Pellet die pressure (MPa/kPSI) Powder 69/10 138/20 276/40 345/50
Density (g/cm3)
Thermal conductivity (W/m K)
Pellet void volume %
0.82 1.29 1.41 1.53 1.60
z0.50 2.48 4.50 7.41 9.09
63.3% 42.5% 38.4% 31.0% 29.1%
pelletization pressure. Over an increase in pellet density of 1.29e1.60 g/cm3 there was a 230% improvement in thermal conductivity (2.75e9.09 W/m K). In contrast, over the same pelletization pressure range Ti-doped NaAlH4 pellet density had a lesser increase (1.10e1.35 g/cm3) while thermal conductivity only improved by 115% (0.636e1.37 W/m K). Fig. 1 shows the exponential relationship between pellet density and thermal conductivity of Ti-doped NaH þ Al and the linear relationship for Ti-doped NaAlH4. The superior thermal conductivity observed results from the high levels of free aluminum (kAl ¼ 250 W/m K at 25 C) and greater compaction of the bulk powder. Compaction into denser pellets is attributed to particle size, which is influenced by starting materials and milling conditions. Interestingly, pure NaAlH4 had slightly higher thermal conductivity compared to Ti-doped NaAlH4 at 345 MPa pelletization pressure (2.30 and 1.37 W/m K, respectively). As mentioned above, pelletization of pure NaAlH4 resulted in an obvious gray appearance on the surface that is most likely due to free aluminum resulting from slight decomposition of the alanate during the pelletization process. If the same holds true for Ti-doped NaAlH4 greater decomposition should occur during pelletization resulting in more free aluminum; however, there must also be more of the intermediate phase, Na3AlH6, as well. Lower thermal conductivity of the Ti-doped NaAlH4, in comparison to pure alanate, can be accredited to
Fig. 1 e Thermal conductivity of pelletized Ti-doped NaH D Al ( ) and Ti-doped NaAlH4 ( ) compared with density (loose powder data from Table 1 included). Inset shows exponential fit of NaH D Al data up to 50 W/mK (correlating pellet density z2 g/cm3).
the higher percent of hexahydride phase its crystal structure. With greater inter-atomic distances than the tetrahydride phase [7], Na3AlH6 has a detrimental effect on pellet thermal conductivity [8]. The thermal conductivity of complex metal hydrides plays a significant role in the heat transfer rates within the tank bed and efficient removal of heat of adsorption, therefore, any increase in thermal conductivity is important for system design. While increased densification of pelletized alanate dramatically increased thermal conductivity (Fig. 1), we explored additional thermal enhancement additives. It has been reported that expanded graphite improves poor thermal conductivity of metal hydride powder beds by addition to the hydride followed by compaction [9]. In addition to combining expanded graphite with sodium alanate, graphite flakes was also examined as a potential thermal conductivity enhancer. Thermal conductivity of graphite enhanced pellet samples are shown in Fig. 2. Neither high- or low-energy milled graphite enhancers had a beneficial effect on thermal conductivity of the alanate pellets. In fact, high-energy milling had a significant negative effect on thermal conductivity. By subjecting the graphite additives to high-energy milling, particle reduction most likely occurred, thus damaging the high thermally conductive carbon-array of graphite (119e165 W/m K at 25 C) [10,13,14]. The graphite additives exhibited thermal conductivity properties comparable to amorphous carbon, which has a very low thermal conductivity of 1.70 W/m K at 25 C [10]. Pellets prepared from low-energy milling of alanate and graphite exhibited similar thermal conductivity as pellets without graphite, but were still noticeably lower in comparison even though pellet densities were comparable. The anisotropic nature of graphite combined with the random orientation of the enhancers after milling contributed to either no improvement or slight decrease in thermal conductivity. In addition, the high pressures used for pelletization may have also caused some degradation of the graphitic structure. This is seen in the thermal conductivity measurements conducted on pellets with and without graphite enhancers. At the lower pressure, pellets with added graphite had 10% higher k values than pellets of alanate alone whereas at the higher pressure graphite enhanced pellets were 10% lower.
Fig. 2 e Thermal conductivity of Ti-doped sodium alanate with 5 mol% graphite enhancers (Exp Gr [ expanded graphite, Gr Fl [ graphite flake) and without. Pellets pressed at 345 MPa unless otherwise noted.
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3.1.3.
Pellet durability
To examine mechanical durability in an automotive on-board storage system, a series of pellets were confined in a fixed volume and subjected to constant vibrations over a thirty minute period. The pellet stack occupied 1.73 cm3 of the test cell’s 2.60 cm3, leaving just over 33% free space for pellet movement during simulation. The pellet stack lost 0.36% in volume and 1.23% in overall density through the duration of the test. None of the pellets were observed to be fractured, chipped or damaged in any way; all pellets maintained their original shape. The loss observed in volume and density occurred due to friction between pellets and test cell wall. A few milligrams of loose powder were recovered from the bottom and walls of the test cell. While the observed loss of sodium alanate pellet volume over the 30 min period was negligible, prolonged or repetitive movement may result in greater deterioration of pelletized sodium alanate; therefore, it would be crucial to minimize movement of pellets inside the storage tank as well as all interior tank components.
3.2.
Thermally cycled pellets
3.2.1.
Kinetics and capacity
The motivation for implementing pelletized complex metal hydrides into potential tank designs is two-fold; increased system volumetric capacity and prevention of loose powder contaminating the fuel system components. While it is clear that thermal conductivity and bulk density of sodium alanate is improved through pelletization, it is important to study the effect of pelletization on hydrogen absorption capacity and kinetics. Fig. 3 shows a plot of hydrogen desorption of pellets and powder in comparable sample beds with heating rates of 5 C/min and 2 C/min. While a lengthy onset for hydrogen release was anticipated for both pellets and powder with the slow heating rate, pelletized sodium alanate does not exhibit the dramatic difference in kinetics and capacity as seen with the powder. The long-term shelf life of the sodium alanate mixture used in this study (NaH þ Al þ TiCl3) is a distinct advantage over the more commonly used TiCl3 doped NaAlH4. However, the consequence of manually combining these constituents is that an intimate mixture of sodium hydride and aluminum cannot be achieved through milling alone, which does occurs when Ti-doped NaAlH4 is thermally dehydrided. As a result,
Fig. 3 e Kinetic profiles of Ti-doped NaH D Al powder (solid line) and pellet (dashed line) at heating rates of (a) 5 C/min and (b) 2 C/min.
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the powder mixture can require up to 3 absorption/desorption cycles to reach full hydrogen capacity, not shown. In addition, compaction of the powder into pelletized form further prolongs the activation process to achieve optimum hydrogen capacity and kinetics. Fig. 4 shows 9.50 mm diameter pellet kinetic profiles from cycles 1, 5 and 10. There is a clear progression towards maximum capacity, as well as kinetics. Beyond 10 cycles there is no further improvement in hydrogen cycling properties of the sodium alanate pellets. This observation in the behavior of sodium alanate pellets is independent of pellet diameter. The slowed down activation in alanate pellets to achieve maximum capacity and kinetics is directly related to powder compaction. The highly compacted pellets do not allow for hydrogen gas to thoroughly permeate into the bulk of the body; therefore, the pellet cannot reach full absorption. As the pellets are cycled they expand, thereby allowing hydrogen gas to pass through the entire body causing the progression in capacity and kinetics seen in Fig. 4. In subsequent sections expansion and the effect it has pellet properties is described in greater detail. While activation is independent of pellet dimensions, cycling kinetics is quite dependent on pellet diameter and thickness. Fig. 5a and b shows the effect of pellet diameter and thickness, respectively, on cycling kinetics and capacity. As seen in Fig. 5a, the smallest diameter pellets (3.17 mm) had the slowest kinetics while 6.35 and 9.50 mm diameter pellets had faster kinetics, counter to expected outcome. This is a direct result of the stacking arrangement of the pellets within the reactor vessel. Pellets with 6.35 and 9.50 mm diameters could be easily stacked face to face allowing for greater surface area contact and better heat transfer between pellets. On the other hand, for the 3.17 mm diameter case, stacking pellets proved to be difficult, therefore, the pellets were placed side by side to create a single layer. This arrangement resulted in less surface to surface contact and therefore inferior heat transfer between pellets due to the cylindrical shape of the pellets. A study of different pellet shapes (i.e. rectangular, triangular or
Fig. 4 e Cycling activation of 9.50 mm diameter sodium alanate pellets over the first 10 cycles (cycle 1: black, cycle 5: red, cycle 10: blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5 e a) Sodium alanate pellet kinetic dependence on pellet diameter (3.17 mm: black, 6.35 mm: red, 9.50 mm: blue), b) pellet kinetic dependence on pellet thickness (1.00 mm: black solid, 1.70 mm: red, 3.50 mm: black dashed, 6.00 mm: blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
square) may shed more light on the difference observed in kinetics regarding pellet diameter, but was outside the scope of this study. In Fig. 5b the kinetics and capacity of 9.50 mm diameter pellets of varying thicknesses are illustrated. As before, the results were contrary to the expected outcome. The thinnest of the four thicknesses, 1.00 mm, in fact had slower kinetics than pellets almost four times thicker, 3.50 mm. While it might be expected that with radial heating of the sample vessel heat transfer through the pellets in an axial direction should not be a significant factor; however, Fig. 5b shows otherwise. Axial heating of highly compacted alanate pellets, especially when surface contact resistance between neighboring bodies of a stack of pellets is a factor, plays a significant role. With thinner pellets more pellets were required to match the mass, or stack height, of thicker pellets; therefore, thermal conductivity decreases due to the increased number of pellets and contact resistance seen at each face. The decrease in overall thermal conductivity slows the heat transfer through the reactor bed resulting in the slower kinetics observed. An upper limit to pellet thickness for faster kinetics was also observed (Fig. 5b). It is clear, that random pellet dimensions and therefore random pellet arrangement within a storage tank would not be the favorable approach. As mentioned previously, once sufficient pellet expansion has occurred and hydrogen permeability during desorption and absorption was not impeded capacities averaging 4 wt% H2 were observed. It has been reported that over 100 cycles the hydrogen capacity of titanium doped sodium alanate powder peaks in the first half of cycles then slowly levels off during the second half [11]. Fig. 6 shows alanate pellet capacity over 50 cycles. The alanate pellets exhibit similar cycling capacity characteristic as reported by Sun et al. [11].
3.2.2.
Fig. 7. Through 30 cycles, pellet density decreased by an average of 50%, from 1.35 g/cm3 to 0.70 g/cm3 for pellets pressed at 69 MPa (10 kPSI) and from 1.60 g/cm3 to 0.80 g/cm3 for pellets pressed at 345 MPa (50 kPSI). Pellets were cycled beyond 30 cycles to determine if a plateau for expansion could be reached, but cycling was ceased after 50 cycles due to the pellets’ increasing fragility, making handling difficult, and measurement of pellet density data impossible. In general, if the pellet expansion was not impeded by limited volume within sample reactor, pellets did not deform or lose their original shape. Hydrogen cycling and resulting pellet expansion also had a detrimental effect on thermal conductivity as shown in Fig. 8. Over the first 10 cycles, pellet thermal conductivity decreased by 79% from 9 W/m K to 1.83 W/m K (dehydrided alanate). Interestingly, each half cycle had slightly different k values. The fully hydrided half cycle (NaAlH4) was consistently lower than fully dehydrided half cycle (NaH þ Al). This difference in thermal conductivity can be contributed to the state in which aluminum finds itself at each end of the reaction; when NaAlH4 is fully dehydrided, aluminum exists in
Pellet expansion
Titanium doped NaAlH4 powder is well known to expand during hydrogen cycling and can sinter if free space for expansion is limited [12]. Unfortunately, our pellets were not exempt to this characteristic. Expansion is independent of pellet dimensions and pelletization pressure, as shown in
Fig. 6 e Pellet hydrogen capacity over 50 cycles (black dashed: theoretical capacity of 5.5 wt%). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 7 e Density loss of sodium alanate pellets (a) 3.17, (b) 6.35 and (c) 9.50 mm in diameter pressed at 69 MPa (blue) and 345 MPa (red) over 30 cycles. Lone points at 10 cycles indicate pellet density after first half-cycle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) a free state as opposed to the tetrahedral structure, [AlH4], for a fully hydride sample. After 10 cycles the average thermal conductivity was comparable to that of densest uncycled Ti-doped NaAlH4 pellets (Fig. 1).
3.2.3.
Thermal enhancers
Graphite enhanced pellets were investigated as a possible method to counter loss in pellet thermal conductivity due to cycling (Fig. 9). As with the non-graphite enhanced pellets a significant decrease in thermal conductivity, up to 70%, was observed within the first 10 cycles. While overall pellet density loss for all samples (with or without graphite) was similar, z30%, the difference in overall loss in thermal conductivity, z10%, is mostly attributed to the 11% difference in k values corresponding to uncycled non-graphite and graphite enhanced pellets. However, at each cycle evaluated (1, 5 and 10) the graphite enhanced pellets had an average thermal
Fig. 8 e Thermal conductivity of 9.50 mm diameter pellets at each half-cycle (blue: fully absorbed, red: fully desorbed) after 1, 5 and 10 cycles. Pellet density at each cycle measured (black). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
conductivity 18.3% greater than pellets without graphite. As the densities of pellets with and without graphite at each cycle were comparable, it can be concluded that graphite enhancers do aid in countering loss of thermal conductivity; however, not to a degree significant enough to justify the loss of wt% H2 by the overall system. Addition of graphite beyond 5 mol% may further prevent loss in pellet thermal conductivity; however, it would cause significant reduction in hydrogen uptake. The loss in thermal conductivity (with or without graphite additives) is a direct result of expansion sodium alanate pellets experience through hydrogen cycling. The key factor contributed to the observed pellet expansion and loss in thermal conductivity is an increase in void space within the interior of each pellet (Fig. 10). Inter-atomic forces associated
Fig. 9 e Thermal conductivity of 9.50 mm diameter alanate pellets with expanded graphite (black dotted) and graphite flakes (blue dashed) at 1, 5 and 10 cycles. Pellet density at each cycle (black solid: expanded graphite, blue solid: graphite flakes). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 10 e SEM micrographs (33k magnification, 1 mm scale) showing the internal side of alanate pellets: a) 0 cycles, b) 5 cycles, c) 20 cycles and d) 50 cycles.
with continuous formation and reformation of the sodium alanate absorption/desorption reaction products, NaAlH4 and NaH (Equations (1) and (2)), combined with pathways formed as hydrogen gas permeates in and out of the bulk material contribute to the noted pellet expansion. This unavoidable formation of void space within each pellet affords inferior thermal conduction through individual pellets and throughout the overall tank bed over continuous cycling.
4.
Conclusions
Sodium alanate pellets were investigated for their practical application for on-board hydrogen storage in a fuel cell vehicle. Compared to the tapped powder, material density was increased by 100% upon pelletization. It was also observed that while activation is required for optimum capacity and kinetics, pelletization of sodium alanate does not hinder kinetics or weight percent of hydrogen absorbed or released. In fact, for poor heating rate conditions the pelletized alanate outperformed the powder. Although the complex metal hydride’s kinetics and capacity were not adversely affected through 50 hydrogen cycles, pellet structural integrity and thermal conductivity degraded due to expansion. A key aspect of compacting sodium alanate into pellet form is to improve system volumetric capacity. However, since the outcome of cycling pellets is a regression back to powder form, pelletization does not offer any benefits for on-board storage. The solution may lie with the implementation of a complex
metal hydride that has minimal crystal structure difference between the fully hydrided and dehydrided phases. Another possibility is to devise a mechanism to restrain pellets that will prevent expansion while allowing sufficient flow of hydrogen in and out of the bulk.
Acknowledgments This work was performed under the DOE contract DE-FC3609GO19003 as GM’s contribution to the DOE Hydrogen Storage Engineering Center of Excellence (HSECoE). The authors would like to thank Sandia National Laboratory for providing the complex metal hydride studied in this work. The authors would also like to thank the HSECoE members for their valuable comments and suggestions.
references
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