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Effect of metal type and loading on hydrogen storage on NaAlH4

Although hydrogen has a great potential as clean energy, safe practical storage of hydrogen for applications such as fuel cells has been a major challenge. NaAlH4 is one of the metal hydrides, which are candidates for hydrogen storage in vehicles. However, the rather slow absorption/desorption kinetics is still a significant drawback. To alleviate this problem, purified NaAlH4 was ground with TiCl3, ZrCl4, or HfCl4. Desorption kinetics and capacities were observed under TPD-like operation. Absorption efficiency was determined by raising the temperature up to 125 8C. Of the three doped metals investigated for the positive effect on facilitating NaAlH4 decomposition, TiCl3 assists the best on the first reaction while ZrCl4 and HfCl4 do for the second one. Despite the kinetics enhancement directly involves with the ZrCl4 amount, there is a threshold of ZrCl4-content which affects. 6% ZrCl4 is considered as an appropriate amount to improve the hydrogen release because it simultaneously decreases the desorption temperature and gives the outstanding rate. In hydrogen desorption, ZrCl4 provides the most amount of released hydrogen, but for hydrogen absorption TiCl3-doped NaAlH4 possesses the highest capacity. It is believed that the metal size is one of the key factors resulting in such the behavior.


Introduction
Hydrogen is a promising fuel to replace the existing fossil fuels. It can be cleanly converted to electricity in fuel cell. However, storing hydrogen effectively and economically is still a challenge since, hydrogen is the lightest gas. For application in vehicles, high-pressure storage tank is impractical due to the weight and safety concerns, while the cryogenic hydrogen storage has volumetric constraints and economic cost. Hydrogen storage in metal hydrides is one of the potential means to meet the requirements of onboard hydrogen storage.
For decades, many scientific and engineering studies involving chemical absorption/desorption of hydrogen from metal hydrides and development of such storage devices have been carried out. Unfortunately, no material has been identified that has the combination of a high gravimetric hydrogen density, adequate hydrogen dissociation energetic, and low cost required for commercial vehicular application [1].
Among several potential hydrides, sodium aluminum hydride, NaAlH 4 , seems to be a candidate for practical onboard hydrogen storage material. It can store high weight percent hydrogen with the low price per unit of stored hydrogen. Unlike the interstitial intermetallic hydrides, NaAlH 4 releases hydrogen through a series of decomposition reactions e.g.: Theoretically, the first and second steps consist of 3.7 and 1.9 wt% of H 2 released, respectively, for a net reaction of 5.6 wt% gravimetric H-storage. In 2001, Jensen and Gross [2] reported that, the first decomposition occurs at 210-220 8C for a relative short time (%3 h) but the decomposition reactions of NaAlH 4  reversed under severe conditions and these reaction kinetics are very slow at low temperature. In 1997, Bogdanović and Schwickardi [3] studied the wet doping of b-TiCl 3 or Ti(OBu) 4 on NaAlH 4 . They concluded that a few mole percents of titanium compounds show superior kinetics for hydrogen absorption and desorption. Both alkoxide and chloride compounds have been used by different methods (wet doping, semi-doping and dry doping) to improve hydrogen storage of NaAlH 4 [4,5].
However, chlorides such as TiCl 3 , ZrCl 4 , VCl 5 or other transition metals perform better on dehydrogenation reaction than alkoxides since, they do not leave any hydrocarbon impurities during H 2 discharge that can cause fuel cell less efficiency [4,6,7]. In addition, in 1999, Zidan et al. [8] found some effects of two doped metals on different steps of decomposition by using temperature-programmed measurements. They showed that zirconium is inferior to titanium as a catalyst for the dehydriding of NaAlH 4 to Na 3 AlH and Al but a superior one for the dehydring of Na 3 AlH to NaH and Al. The significant keys for improvement of absorption/ desorption kinetics are the amount of doping [9], milling time [4,7,10] and particle size [6,7].
In this work, effects of ZrCl 4 , TiCl 3 , and HfCl 4 on the NaAlH 4 hydrogen absorption/desorption kinetics and capacity were investigated.
To study the effect of added catalyst, the amount of metals chloride (TiCl 3 , ZrCl 4 , HfCl 4 ) was varied between 0 and 9 mol%. After grinding, about 1.3 g of powder mixture was put under evacuum (1.013 kPa) for 2 h and was then immediately placed into stainless steel sealed vessel (20 cm 3 ). To observe the released amount of hydrogen in the desorption process, the known volume vessel was subsequently pressurized to 356.368 kPa (37 psig) in He and heated up to achieve desired desorption temperature (250 8C) with the heating rate of 5 8C/min and hold at this temperature about an hour. The reactor temperature was controlled by a PID temperature controller. A K-type thermocouple with thermo well was placed in the reactor. The pressure in the system was monitored by high precision transducers (Setra Co., Ltd., model 206) transmitting the signal to the computer every 2 s.
For hydrogen absorption, the procedure was the same as that of the previous study [11]. The reactor was pressurized with H 2 to approximately 13887.327 kPa (2000 psig) and heated with the 20 8C/min heating rate until 125 8C. Hereafter, the hydrogen uptake began and further extended for the next 12 h. Hydrogen capacities were calculated by the equation of state with the compressibility factor at different temperature. The above experimental set-up was shown schematically in Fig. 1.

Results and discussion
3.1. Hydrogen desorption 3.1.1. Purified NaAlH 4 The amount of desorbed hydrogen in term of weight percent of as-received and purified NaAlH 4 are plotted as a function of temperature as shown in Fig. 2, comparing with the previous study [5]. The purified NaAlH 4 has higher hydrogen storage capacity than the as-received one, but the hydrogen desorption occurs at temperature higher than the unpurified one, probably because the purified NaAlH 4 composes crystalline phase, which is more difficult to decompose. However, the hydrogen storage capacity is less than the theoretical value (5.6 wt%) [2,4,5] and that reported by Jensen et al. [5]. In this experiment, the technique is similar to that of Jensen et al. [5] but less moisture and oxygen control. Thus, the contamination of water and oxygen on NaAlH 4 , may cause surface damages of NaAlH 4 and obstruct the desorption/absorption of hydrogen.

Effect of metal loading types
Even though the doped metallic compounds belong to the same group in the periodic table, the metals act differently to the hydrogen absorption/desorption as shown in Fig. 3. Doping TiCl 3 , ZrCl 4 and HfCl 4 on NaAlH 4 can lower the first hydrogen desorption temperature to around 170 8C. The second decomposition reaction takes place at around 190 8C for TiCl 4 , around 210 8C for ZrCl 4 and HfCl 4 . Moreover, among the three doping catalysts, ZrCl 4 shows the best kinetic performance and the highest amount of released hydrogen. Fig. 4 shows the hydrogen desorption capacity in different amounts of ZrCl 4 doped in NaAlH 4 . Both desorption temperature and rate were dependent on the amount of doped ZrCl 4 on NaAlH 4 (Fig. 5). It was found that 6 mol% ZrCl 4 doped NaAlH 4 results in hydrogen desorption at the lowest temperature among the samples. The maximum rate of hydrogen released corresponds to ZrCl 4 level, which starts to converse beyond 6 mol%. It can be hypothesized that this may be caused by the lattice distortion (atomic substitution in formation of Zr 2C and Zr 3C instead of Zr 4C ) [12]. In the low metal adding, the distortion causes the increase in the lattice of NaAlH 4 therefore hydrogen can desorb easily.

Hydrogen absorption
The dependency of amount of absorbed hydrogen in doped NaAlH 4 with various halide compounds is shown in Fig. 6. TiCl 3 doped NaAlH 4 has the highest hydrogen absorption rate in the first hour. This is due to the ionic radius of Ti ion [10]. It was expected that the low rate of hydrogen    Temperature, °C Hydrogen capacity, wt%   discharge be observed for doped metals with radii significantly larger or smaller than 7.4 nm, the midpoint between Na C (9.7 nm) and Al 3C (5.1 nm). Likewise, there is a range of ionic radius (7.3-8 nm) that holds the highest potential to enhance hydrogen charge rate. Because Ti 3C (7.6 nm) has smaller ionic radius than that of Zr 4C (8.6 nm) and Hf 4C (8.5 nm) [10,13], this Ti 3C can easily substitute into the lattice on the surface of material. A reason why the hydrogen uptake rate of HfCl 4 -doped material is lower than that of the ZrCl 4 -doped one is possibly concerned with symmetry of orbital. Among the three species, Hf 4C has an electron configuration in the f orbital whereas those of others are in the p orbital, synchronizing with Al 3C . With this configuration, it is unlikely to cause the atomic overlap between Al 3C and Hf 4C . The sample rendered by ZrCl 4 sustains hydrogen at higher capacity so it can be interpreted that the activity of HfCl 4 to facilitate absorption reaction is considerably lower than that of ZrCl 4 .
In the first cycle, H 2 contained in the metal hydride is desorbed for use, and then H 2 is absorbed for storage. Because of irreversibility of the desorption-absorption cycle, the desorbed amount is significantly larger than the absorbed amount. In the second and third cycle, the desorption and absorption capacity are found to become nearly constant and essentially equal the first cycle absorption capacity. Hence, the degree of rehydrogenation [11] or the cyclic ability of the doped materials is calculated as the ratio of absorption to desorption capacity in the first cycle and summarized in Table 1. Obviously, all doped samples are not entirely reversible and show significantly lower absorption capacity than the first desorption capacity. A major reason of the incomplete absorption may be the formation of large metallic aluminum particles from NaAlH 4 decomposition in Eq. (1), thereby hampering the diffusion mechanism necessary for recombination [14,15].

Conclusions
Doping TiCl 3 , ZrCl 4 , and HfCl 4 on NaAlH 4 enhances the kinetics and decreases the reaction temperature in both hydrogen absorption and desorption. TiCl 3 renders the best on the first decomposition reaction, while ZrCl 4 and HfCl 4 do for the 4 one. A small amount of ZrCl 4 on NaAlH 4 is sufficient for catalyzing the discharge of hydrogen. However, 6 mol% ZrCl 4 was found to be an appropriate amount because it simultaneously decreases the desorption temperature and gives the outstanding rate. Of all the three doped metals, TiCl 3 provides the highest absorption capacity. It is believed that the ionic radius of the doped metal is one of the key factors resulting in such the behavior.