Characteristics of the Dendrite Growth in the Electrochemical Alane Production Process

The electrochemical alane production process was proposed for a feasible production of alane. The operation of process was difficult because of short circuit by a dendrite growth in the reactor. Therefore, characteristics of the dendrite growth in the process were investigated. We conducted the electrochemical alane production process using Teflon block for inhibition of the dendrite growth. The obtained dendrite was characterized by XRD, SEM and ICP-AES. It was concluded that the dendrite growth was attributed to a melting and agglomeration of Al fine particles existed in the solution.


Introduction
Alane (aluminum hydride, AlH 3 ) is a metastable, crystalline solid with a very high hydrogen capacity (10 wt% H 2 of a gravimetric hydrogen density and 149 kg H 2 /m 3 of a volumetric hydrogen density).The energy density (1200 Wh/L) of the AlH 3 -fuel cell system was higher four times than that of lithium battery system.The AlH 3 -fuel cell system has expected to apply the future energy storage system including the hydrogen fuel cell vehicle [1].Therefore, AlH 3 is one of the promising candidate for development of the future hydrogen storage system [2,3].
The production of AlH 3 by a direct hydrogenation of aluminum (Al) is impracticable because it theoretically requires hydrogen pressure over 10 5 bar at room temperature.Zidan et al. recently proposed the AlH 3 production process including electrochemical method, which is close to a reversible cycle (Fig. 1) [4].This process consists of two processes: (1) electrochemical process which produces AlH 3 and MH (M=Li or Na) by electrochemical decomposition of MAlH 4 and (2) chemical process which regenerate the MAlH 4 by reacting hydrogen with Al and MH using catalysts.
The continuous operation of reaction was difficult due to short circuit by a dendrite growth during the electrochemical AlH 3 production [5].The problem of dendrite growth should be resolved to operate continuously the electrochemical AlH 3 production process.The studies for the mechanism of dendrite growth and the physicochemical method to inhibit the dendrite growth were not yet reported.Therefore, we identified the mechanism of dendrite growth to resolve the problem of dendrite growth.The electrochemical AlH 3 production was performed by using Al, Pd electrodes and LiAlH 4 -THF solution.

Experimental
Experimental procedure was performed in an argon-filled glove box.The electrochemical AlH 3 production was conducted in the glass reactor with 400 mL in volume.The Al electrode was polished using sandpaper and rinsed with acetone to remove the oxide layer.The Al plate (13.25 cm 2 ) and Pd foil (13.25 cm 2 ) were used as the anode and cathode in the reactor, respectively.A 150 mL of 1 M LiAlH 4 -THF (Sigma Aldrich) was fed to the reactor.The DC power supply (E3615A, Agilent) was connected to the Al and Pd electrodes.The reaction proceeded by applying a constant voltage for 12 h.

Results and Discussion
The Teflon block was installed in the reactor to inhibit the dendrite growth (Fig. 2(a)).The applied voltage was controlled at 5 V.The initial current density was measured at 11.32 mA/cm 2  reaction (Fig. 2(b)).Fig. 3 shows the variation of current density during the reaction.The current density increased after 500 min.This result was because the length between the dendrite formed at the cathode and the Al electrode decreased gradually.The final current density was achieved 24.91 mA/cm 2 .
The XRD and SEM results of the dendrite were shown in Figs. 4 and 5, respectively.From the XRD result, the peaks of the dendrite were well agreed with that of aluminum.was considered that the irregular particles of the dendrite were agglomerated each other at a microscopic level from the SEM images.We measured that the content of Li was 22.75 wt% in the dendrite by ICP-AES.On the other hand, we used the reactor of an H-cell type to inhibit the short circuit due to the contact between Al particles and dendrite.The applied voltage was controlled at 5 V.During the reaction for 6 h, it was not observed that the dendrite growth at the cathode and the gas emission at the anode, indicating that the reaction was difficult to proceed.This result was attributed to an increase in resistance due to decrease in the amount of LiAlH 4 -THF solution and the space between the electrodes.
Martínez-Rodríguez et al. reported that crystalline structure of dendrite consisted of Na 3 AlH 6 and Al in the electrochemical AlH 3 production using NaAlH 4 -THF solution [6].They explained that the dendrite consisting of Na 3 AlH 6 and Al was formed by reacting with Na + and NaAlH 4 at the cathode when H + ions were not present (Eq.( 1)).In addition, the dendrite could be formed with a similar procedure when LiAlH 4 was used instead of NaAlH 4 .3NaAlH 4 + 3Na + + 3e -ĺ 2Na 3 AlH 6 + Al (1) Although we did not observed the evidence of Li 3 AlH 6 component in the XRD result, it was considered that the dendrite growth proceeded by the Li plating due to reduction of Li + at the surface of Pd electrode or by the Li 3 AlH 6 component.The dendrite grew up rapidly by melting and agglomeration of Al fine particles that existed in the solution.From these results, it was found that the consumption of Al electrode (Eq.( 2)) and the formation of Al fine particles could not be avoided in the electrochemical AlH 3 production process.We also concluded that the dendrite growth was attributed to mainly the Al fine particles.

Conclusions
In this study, characteristics of the dendrite growth were investigated in the electrochemical AlH 3 production process.A series of experiments were performed in the reactor installed the Teflon block for inhibition of the dendrite growth.Major results were summarized as follows.
(1) A large amount of dendrite was obtained from the electrochemical AlH 3 production process.The XRD peaks of the dendrite were well agreed with that of aluminum.From the SEM images, it was considered that the irregular particles of the dendrite were agglomerated each other at a microscopic level.We measured that the content of Li was 22.75 wt% in the dendrite by ICP-AES.From these result, it was concluded that the structure of dendrite consisted of Al and Li components.
(2) The reactor of an H-cell type was not adequate for the electrochemical AlH 3 production process due to a high resistance.It was considered that the dendrite grew up by the Li plating due to reduction of Li + at the surface of Pd electrode or by the Li 3 AlH 6 component.The consumption of Al electrode and the formation of Al fine particles could not be avoided.The dendrite growth was attributed to mainly the Al fine particles.

Fig. 2 .
Fig. 2. (a) The reactor installed Teflon block in 1 M LiAlH 4 -THF solution and (b) dendrite growth in the reactor; electrodes: Al plate and Pd foil.

Fig. 5 .
Fig. 5. SEM images of dendrite obtained from electrochemical AlH 3 production . A large amount of dendrite grew up toward the Al electrode at 740 min of