- Email: [email protected]
Eutectic melting in x(2LiBH4-MgH2) hydrogen storage system by the addition of KH
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Eutectic melting in x(2LiBH4-MgH2) hydrogen storage system by the addition of KH Pratibha Pal a, Ankur Jain b,*, Hiroki Miyaoka b, Yoshitsugu Kojima b, Takayuki Ichikawa a a
Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima, 739-8527, Japan Natural Science Centre for Basic Research and Development, Hiroshima University, Higashi-Hiroshima, 739-8530, Japan
b
article info
abstract
Article history:
The suitable thermodynamics and sorption properties of 2LiBH4-MgH2 system inspired us
Received 5 February 2019
to explore the effect of KH on this system. The addition of 5 mol% KH to this system didn't
Received in revised form
show any significant effect on desorption temperature, however, an interesting peak shift
9 April 2019
to lower temperature was observed that corresponds to the melting of 2LiBH4-MgH2.
Accepted 11 April 2019
This gave us a sign of eutectic phenomenon that has been observed in 2LiBH4-KBH4 system
Available online xxx
recently. To explore it in more detail, a series of (2LiBH4-MgH2)x-(KH)1-x system was examined and a pseudo-binary phase diagram for this system has been plotted. Using this
Keywords:
phase diagram, the eutectic composition has been identified for x ¼ 0.45 with the lowest
Lithium borohydride
melting temperature at 79 C.
Potassium hydride
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Catalyst Desorption Eutectic melting
Introduction Energy is the driving force directing the country's economic and social development. Increasing population and demand of energy is a serious problem, getting worse due to continuously dwindling conventional energy sources such as fossil fuels carbon, coal, petroleum, and natural gas. Renewable energy sources such as wind, solar, tidal, hydro, and geothermal are the energy sources that are cost-effective and reduces our dependency on fossil fuels. While it was a good switch from petrol and diesel to LPG (liquid petroleum gas) and then to CNG (compressed natural gas) to reduce CO2 emissions, there still
exists a need to move away from oil and gas to reduce our dependency on fossil fuels. Two technologies, namely hydrogen and Li-ion batteries are promising energy carriers that can replace the IC engines efficiently and would prove to be a great help in transportation of fuels. Due to limitations like the high cost of lithium and the safety concerns, Li-ion batteries are currently difficult to be used in cars. On the other hand, hydrogen has high gravimetric energy density, however it suffers from the low volumetric density. This has led the research community to develop efficient storage facilities for hydrogen [1e3]. High-pressure hydrogen storage and cryostorage are not considered good techniques due to their low energy density and safety concerns for onboard applications.
* Corresponding author. E-mail addresses: [email protected] (A. Jain), [email protected] (T. Ichikawa). https://doi.org/10.1016/j.ijhydene.2019.04.095 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Pal P et al., Eutectic melting in x(2LiBH4-MgH2) hydrogen storage system by the addition of KH, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.095
2
international journal of hydrogen energy xxx (xxxx) xxx
The hydrogen storage in solid state form in materials has been proven as the most promising and safe way to store hydrogen. Among all solid-state materials, complex metal hydrides such as alanates, borohydrides and amides-imides are considered as good candidates due to their light weight and high energy density [1,4]. In comparison with complex hydrides, metal borohydrides have higher hydrogen storage capacity suitable for stationary as well as transportation applications [5]. LiBH4 is one of the appropriate lightweight complex hydride because it has high gravimetric (18.5 wt% H2) and volumetric (121 g H2/ L) densities. However, due to its high thermodynamic stability and slow kinetics, it releases hydrogen completely above 900 C which makes it incompatible with proton-exchange membrane (PEM) fuel cells [2,6]. Researchers have proposed many solutions to lower the desorption temperature of LiBH4 and to enhance its kinetics, which led to the discovery of 2LiBH4eMgH2 system [7,8]. This new complex hydrogen storage system has displayed better reversibility than LiBH4 itself, however serious kinetics problem upon hydrogen release is still a barrier [9,10]. Shao et al. [11] mentioned the poor kinetics as a critical challenge for onboard applications. Many efforts have been made by researchers to solve the problem of kinetics but are still unable to thrive on the demand. Over past years, many catalysts and additives have been proposed to improve the kinetics of many different hydrogen storage systems [11]. With respect to kinetic enhancement, potassium-based intermediates are one of the prospective candidates to be considered a great achievement. For the first time, Santoru et al. studied the decomposition properties of K-Mg-N-H system and proposed several reaction pathways. Considering the mechanism for the reaction of KH with Mg(NH2)2 they concluded that potassium based additives are beneficial to improve the desorption properties of the overall system [12]. Potassium hydride has been recognized as one of the extraordinary catalyst, which was first introduced for the improvement of Mg(NH2)2 e LiH system by Wang et al. [13]. They explored the magical characteristics of potassium due to which hydrogen desorption temperature could be reduced from 186 C (pristine system) to 107 C (with potassium hydride). It was suggested that potassium diffuses into the imide and amide and combines with nitrogen, and as a result the amide N-H bonds and imide Li-N bonds are weakened which greatly enhances the dehydrogenation [13]. Superior catalytic effect of KH has been reported by Teng et al. also where they concluded that even a small amount of KH (5 mol%) was enough to improve the hydrogen desorption performance of the LiH-NH3 at 100 C [14]. Recently, Lin et al. [15] reported the improvement in dehydrogenation kinetics and cyclability by the addition of KH to Mg(NH2)2-LiNH2-LiH system. According to this report, only 3 wt% of KH was enough to reduce decomposition temperature of the composite Mg(NH2)2-LiNH2LiH to below 90 C without emission of NH3. Magical catalytic activities of potassium hydride evoked us to give a trial on the well-known complex hydrogen storage 2LiBH4-MgH2 system. Thus, the effect of KH addition is studied using thermogravimetry coupled with differential thermal analysis (TG-DTA) and differential scanning calorimetry (DSC) in this work. An interesting phenomenon of eutectic melting was also observed during the study, which will also be elaborated herein.
Experimental The starting material LiBH4 (95%, Sigma Aldrich) was used after heat treatment at 200 C for 24 h under dynamic vacuum to remove the solvent impurity. KH was also washed several times by hexane and dried under vacuum for several hours. MgH2 (98%, Alfa Aesar) was used as purchased without any treatment. All the materials were handled solely in a glove box filled with argon and coupled with a circulation purifier to maintain O2 and H2O level <1 ppm. The samples were prepared by mechanochemical treatment in a planetary ball mill under inert conditions (argon atmosphere). Batches of 500 mg samples were prepared with 20 ZrO2 balls (8 mm in diameter) in a milling pot of volume about 30 cm3. The milling was conducted for a total of 2 h with a 30 min pause after 1 h milling. The details of all the studied samples with compositions are given in Table 1. The TG-DTA data was obtained by using Rigaku, TG8120 connected to a mass spectrometer (MS, Anelva, M-QA200TS) under an argon gas flow (300 ml/min) at a heating rate of 5 C/min. To confirm the obtained TG-DTA results, thermal analysis was performed using differential scanning calorimetry (DSC). Power X-ray diffraction (PXD) measurements were carried out using Rigaku RINT-2500, lab X-ray diffractometer configured with a Cu Ka source (l ¼ 1.541 A) after ball milling and thermal analysis to identify the produced compounds. To prepare XRD samples a polyimide sheet (Kapton, Du PontToray Co. Ltd.) was used to cover the samples to avoid oxidation through the contact with air.
Results and discussion TG-DTA results of the composite 2LiBH4-MgH2 with and without 5% KH are shown in Fig. 1. Three endothermic peaks are found in the DTA profile for the pure 2LiBH4-MgH2 (upper panel, dashed line). The first two peaks at 115 C and 285 C belongs to the phase transition and melting of LiBH4 respectively. The weight loss is started around 340 C as observed
Table 1 e Sample composition and molar ratio of components. Sample S0 (0% KH) S1 (5% KH) S2 (10% KH) S3 (20% KH) S4 (30% KH) S5 (40% KH) S6 (50% KH) S7 (55% KH) S8 (60% KH) S9 (65% KH) S10 (70% KH) S11 (80% KH) S12 (90% KH)
x(2LiBH4-MgH2)
(1-x)KH
Molar ratio (LiBH4-MgH2-KH)
1.0 0.95 0.90 0.80 0.70 0.60 0.50 0.45 0.40 0.35 0.30 0.20 0.10
0.0 0.05 0.10 0.20 0.30 0.40 0.50 0.55 0.60 0.65 0.70 0.80 0.90
2.00:1.00:0.0 1.90:0.95:0.05 1.80:0.90:0.10 1.60:0.80:0.20 1.40:0.70:0.30 1.20:0.60:0.40 1.00:0.50:0.50 0.90:0.45:0.55 0.80:0.40:0.60 0.70:0.35:0.65 0.60:0.30:0.70 0.40:0.20:0.80 0.20:0.10:0.90
The eutectic composition is represented in bold.
Please cite this article as: Pal P et al., Eutectic melting in x(2LiBH4-MgH2) hydrogen storage system by the addition of KH, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.095
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 1 e DTA (upper panel) and TG analysis (lower panel) of 2LiBH4-MgH2 (dashed lines) and 2LiBH4-MgH2- 5%KH (solid lines).
from TG (lower panel, dashed line), which reflects the starting temperature of hydrogen desorption and the corresponding DTA peak is detected at 365 C. The second step decomposition is observed at more than 400 C, which is in line with the literature [6]. On the addition of 5 mol% KH, a shift in desorption temperature is observed for the first step with a peak temperature of 268 C (solid line), while no significant change is observed for the second step decomposition. In addition to it, an unusual but quite interesting phenomenon of shift in melting peak (285 C) is observed in DTA curve for KH added 2LiBH4-MgH2 system. It is well known that the melting cannot be affected by the use of additive, unless it forms a eutectic mixture. Another possibility can be associated with the occurrence of some unknown irreversible reaction. This observation prompted us to investigate the effect of KH addition to this system in more detail. To investigate it more precisely, same samples were examined using DSC and the results are depicted in Fig. 2. The reversible nature of peak around 285 C confirms the melting (during heating) and solidification (during cooling), whereas a clear shift to the lower temperature is the testament of eutectic melting. In addition, a small endothermic is also observed with peak temperature of 110 C, just before the phase transition temperature of LiBH4. This must be associated with the eutectic melting of eutectic composition. Although the phenomenon of eutectic melting is complicated, we can describe it simply as follows: the eutectic melting happens if two or more components sustain their liquid state below the melting points of their pure components. These eutectic melts can be utilized as media in which substances can dissolve with good solubility. Melting of complex systems like borohydrides and mixture of metal borohydride, hydride and halides can also increase reactivity and can improve kinetics for the release of hydrogen [16,18]. Lower melting point metal borohydrides and complex systems which show eutectic melting behavior, act as ionic liquids and can be utilized for refueling of vehicles [17]. Deep eutectics Mg(BH4)2/LiBH4 systems in complex borohydrides might behave as liquid in PEM fuel cell which is advantageous in hydrogen reversibility [16]. Many researchers have focused
3
Fig. 2 e DSC profile of pure LiBH4-MgH2 (green color) and on the addition of 5 mol% KH in LiBH4-MgH2. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
on lowering the eutectic melting points by using the combination of different complex hydride systems due to its excellent onboard application. Bimetallic mixtures have been found to have much lower melting points in comparison with monometallic borohydrides M(BH4)x, where M ¼ Li, Na, K, Mg or Ca [16e19]. For example, the bimetallic mixture of lithium and potassium borohydrides 0.725LiBH4-0.275KBH4 have shown melting point Tm ¼ 105 C, which is much lower than the individual borohydride [6]. Similarly, 0.62LiBH4-0.38NaBH4 system displayed the melting at 210e220 C [16e19], lower than the individual components. In case of composite LiBH4Ca(BH4) system, it exhibits eutectic melting at 200 C and a partial reversibility has been confirmed for the first time [20]. The xLiBH4-(1-x)Mg(BH4)2, x ¼ 0.5e0.6 system exhibited a reduced initial temperature of hydrogen release i.e. 180 C (Tm ¼ 180 C) [21]. Thus, to investigate the anticipated similar phenomenon, a series of 2LiBH4-MgH2 with increasing mole percentage of potassium hydride was prepared and analyzed using differential scanning calorimetry (DSC) in a temperature range of room temperature to 500 C. However, Fig. 3 shows the DSC profile of samples only up to 150 C in order to make them more comprehensive. The onset temperature has been considered as melting temperature. The compositional details of the samples are given in Table 1. Sample S1 e S4 showed two very close peaks around 110e120 C and one peak in the temperature range 160e235 C. These could be assigned to the eutectic melting, phase transition and melting of the composite mixture respectively. With the increasing content of KH, the eutectic melting peak height is found to be increasing and shifting slightly to the lower temperature, whereas the peak corresponding to the phase transition is getting modest. The melting peak corresponding to the composite melting is also observed to be shifted to lower temperature. Finally, sample S5 shows only one peak that corresponds to the complete melting thus eliminating the peak corresponding to
Please cite this article as: Pal P et al., Eutectic melting in x(2LiBH4-MgH2) hydrogen storage system by the addition of KH, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.095
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international journal of hydrogen energy xxx (xxxx) xxx
Fig. 3 e DSC curves for pure 2LiBH4-MgH2 (first from upwards) and remaining with the increasing percentage of potassium hydride.
phase transition of LiBH4, as it is irrelevant in the melted state. Fig. 4 shows DSC profiles of some selected samples to determine the lowest melting temperature. The lowest onset temperature is found as 79 C for sample S7 which has a composition of 0.45(2LiBH4-MgH2)-0.55(KH). All the data corresponding to melting of composite mixtures is plotted in Fig. 5, which could serve as a phase diagram of x(2LiBH4-MgH2) e (1-x)KH system. Since the melting was not observed for samples S10 e S12, the curve is connected by dashed line, whereas the other part connected by solid line, clearly shows the eutectic composition as 0.45(2LiBH4-MgH2) e 0.55KH. To confirm the above melting and non-occurrence of any other reaction, X-ray diffraction (XRD) was performed for all the samples after milling as well as after DSC experiment up to 290 C. The XRD profiles of selected samples are shown in Fig. 6. The XRD profile after milling (Fig. 6a) suggest the existence of MgH2 and LiBH4 phases, however, no peaks corresponding to KH are visible for any samples up to the sample
Fig. 4 e DSC profiles of selected samples.
Fig. 5 e Phase diagram of x(2LiBH4-MgH2)-(1-x)KH system. The eutectic composition and melting temperature were determined by DSC, onset temperature was considered as the melting point.
having 70% KH (S10). In addition, some peaks could be indexed for KBH4 phase for the samples having 60% KH or more (S8-S12). The formation of KBH4 revealed that the reaction between LiBH4 and KH and anion exchange takes place during milling. The formation of amorphous KBH4 can also be expected for other samples with less KH content. Another possibility of the absence of KBH4 peaks in XRD of sample S1-S7 can be attributed to the presence of KBH4 in very small amount, which is not in the detection limit of XRD. This speculation is confirmed from the XRD profile of sample S11 (80%KH), where the peaks corresponding to both phases i.e. KH as well as KBH4 are present due to the excess amount of KH. On the other hand for the samples after melting, KBH4 peaks are clearly visible for the S1 sample with only 5% KH. It suggests the crystallization of KBH4 phase during solidification. The peaks corresponding to MgH2 along with KBH4 are visible in all the samples after heating at 290 C. In addition, the peaks corresponding to potassium hydride are also detected in samples S11 similar to the milled sample. On combining the DSC and XRD data, it can be summarized that the melting peak shift to the lower temperature is purely due to melting which arose probably due to the eutectic composition of LiBH4-KBH4 and no other side reaction took place during the heating range. Eutectic melting in LiBH4- KBH4 has already been investigated by L.B. Morten group in 2014 [17] and the lowest eutectic temperature was reported as 105 C. In our case we observed the lowest temperature as 79 C, which is different from the above, however, it might be associated with the different starting materials and/or different measuring method (TPPA in the above report, while DSC in this work). The effect of KH as a catalyst for the first decomposition step of the above system can also be understood on the basis of this eutectic melting. Since KH reacted with LiBH4 during the milling process, thus changing to KBH4 which formed a eutectic composition with LiBH4, thus reducing the melting temperature of the above system. The ionic mobility is enhanced due to the weakening of Li-B-H bonding in the molten state of LiBH4, thus enhancing the sorption kinetics
Please cite this article as: Pal P et al., Eutectic melting in x(2LiBH4-MgH2) hydrogen storage system by the addition of KH, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.095
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 6 e XRD patterns of selected samples (a) after milling (b) after heating at 290 C.
and reducing the decomposition temperature of first step reaction which corresponds to the LiBH4 decomposition.
Conclusion The addition of KH to well-known 2LiBH4-MgH2 reduced the decomposition temperature of 1st step reaction, however, no significant effect could be observed for the 2nd major step of decomposition. An interesting phenomenon of eutectic melting through the formation of KBH4 as an intermediate was observed and a phase diagram was reported herein. The eutectic composition for x(2LiBH4-MgH2) e (1-x)KH was assured to be x~ 0.45 using the combination of differential scanning calorimetry and X-ray diffraction. The eutectic composition was found to melt at ~79 C (Tmin) which is the
lowest melting temperature reported for a complex borohydride-hydride mixture so far. This work suggests the new possibilities to develop more complex hydride/borohydride systems to improve kinetic issues associated with the hydrogen storage applications. Advancement in techniques to decrease the decomposition temperature for on-board applications, while maintaining high hydrogen capacity will be the future road map.
references
[1] Jain IP, Jain P, Jain A. Novel hydrogen storage materials: a review of light weight complex hydrides. J Alloy Comp 2010;503:303e39.
Please cite this article as: Pal P et al., Eutectic melting in x(2LiBH4-MgH2) hydrogen storage system by the addition of KH, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.095
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[2] Schlapbach L, Zuttel A. Hydrogen-storage materials for mobile applications. Nature 2001;414:353e8. [3] David WIF. Effective hydrogen storage: a strategic chemistry challenge. Faraday Discuss 2011;151:399e414. [4] Huang Z-Q, Chen W-C, Chuang F-C, Majzoub E-H, Ozolins V. First-principles calculated decomposition pathways for LiBH4 nanoclusters. Sci Rep 2016;6:26056. [5] Kumar S, Jain U, Jain A, Miyaoka H, Ichikawa T, Kojima Y, Dey GK. Development of Mg-Li-B based advanced material for onboard hydrogen storage solution. Int J Hydrogen Energy 2017;42:3963e70. [6] Zeng L, Miyaoka H, Ichikawa T, Kojima Y. Superior hydrogen exchange effect in the MgH2-LiBH4 system. J Phys Chem C 2010;114:13132e5. [7] Cova F, Ronnebro Ewa CE, Choi YJ, Gennari FC, Larochette PA. New insights into the thermodynamic behavior of 2LiBH4MgH2 composite for hydrogen storage. J Phys Chem C 2015;119:15816e22. [8] Yu XB, Grant DM, Walker GS. A new dehydrogenation mechanism for reversible multicomponent borohydride systems e the role of Li-Mg alloys. Chem Commun 2006;37:3906e8. [9] Pinkerton FE, Meyer MS, Meisner GP, Balogh MP, Vajo JJ. Phase boundaries and reversibility of LiBH4/MgH2 hydrogen storage material. J Phys Chem C Lett 2007;111:12881e5. [10] Nakagawa T, Ichikawa T, Hanada N, Kojima Y, Fujii H. Thermal analysis on the LieMgeBeH systems. J Alloy Comp 2007;446e447:306e9. [11] Shao H, He L, Lin H, Li H-W. Progress and trends in magnesium-based materials for energy-storage research: a review. Energy Technol 2018;6:445e58. [12] Santoru A, Pistidda C, H Sorby M, Chierotti M R, Garroni S, Pinatel E, Karimi F, Cao H, Bergemann N, Le T T, Puszkiel J, Gobetto R, Baricco M, Hauback B C, Klassen T, Dornheim M. KNH2eKH: a metal amideehydride solid solution. Chem Commun 2016;52:11760.
[13] Wang J, Liu T, Wu G, Li W, Liu Y, Araujo CM, Scheicher RH, Blomqvist A, Ahuja R, Xiong Z, Yang P, Gao M, Pan H, Chen P. Potassium-Modified Mg(NH 2)2/2LiH System for hydrogen storage. Angew Chem Int Ed 2009;48:5828e32. [14] Teng Y-L, Ichikawa T, Miyaoka H, Kojima Y. Improvement of hydrogen desorption kinetics in the LiHeNH3 system by addition of KH. Chem Commun 2011;47:12227e9. [15] Lin H-J, Li H-W, Paik B, Wang J, Akiba E. Improvement of hydrogen storage property of three-component Mg(NH2)2eLiNH2eLiH composites by additives. Dalton Trans 2016;45:15374. [16] Paskevicius M, Ley MB, Sheppard DA, Jensen TR, Buckleya CE. Eutectic melting in metal borohydrides. Phys Chem Chem Phys 2013;15:19774e89. [17] Ley MB, Roedern E, Jensen TR. Eutectic melting of LiBH4eKBH4. Phys Chem Chem Phys 2014;16:24194e9. [18] Roedern E, Hansen Bjarne RS, Ley MB, Jensen Torben R. Effect of eutectic melting, reactive hydride composites, and nanoconfinement on decomposition and reversibility of LiBH4KBH4. J Phys Chem C 2015;119:25818e25. [19] Zavorotynska O, Deledda S, Li G, Matsuo M, Orimo Shin-ichi, Hauback BC. Isotopic exchange in porous and dense magnesium borohydride. Angew Chem Int Ed 2015;54:10592e5. [20] Lee JY, Ravnsbaek D, Lee YS, Kim Y, Cerenius Y, Shim JH, Jensen TR, Hur NH, Cho YW. Decomposition reactions and reversibility of the LiBH4-Ca(BH4)2 composite. J Phys Chem C 2009;113:15080e6. [21] Bardaji EG, Zhao-Karger Z, Boucharat N, Nale A, Setten MJV, Lohstroh W, Rohm E, Catti M, Fichtner M. LiBH4-Mg(BH4)2: a physical mixture of metal borohydrides as hydrogen storage material. J Phys Chem C 2011;115:6095e101.
Please cite this article as: Pal P et al., Eutectic melting in x(2LiBH4-MgH2) hydrogen storage system by the addition of KH, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.095
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Eutectic melting in x(2LiBH4-MgH2) hydrogen storage system by the addition of KH Pratibha Pal a, Ankur Jain b,*, Hiroki Miyaoka b, Yoshitsugu Kojima b, Takayuki Ichikawa a a
Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima, 739-8527, Japan Natural Science Centre for Basic Research and Development, Hiroshima University, Higashi-Hiroshima, 739-8530, Japan
b
article info
abstract
Article history:
The suitable thermodynamics and sorption properties of 2LiBH4-MgH2 system inspired us
Received 5 February 2019
to explore the effect of KH on this system. The addition of 5 mol% KH to this system didn't
Received in revised form
show any significant effect on desorption temperature, however, an interesting peak shift
9 April 2019
to lower temperature was observed that corresponds to the melting of 2LiBH4-MgH2.
Accepted 11 April 2019
This gave us a sign of eutectic phenomenon that has been observed in 2LiBH4-KBH4 system
Available online xxx
recently. To explore it in more detail, a series of (2LiBH4-MgH2)x-(KH)1-x system was examined and a pseudo-binary phase diagram for this system has been plotted. Using this
Keywords:
phase diagram, the eutectic composition has been identified for x ¼ 0.45 with the lowest
Lithium borohydride
melting temperature at 79 C.
Potassium hydride
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Catalyst Desorption Eutectic melting
Introduction Energy is the driving force directing the country's economic and social development. Increasing population and demand of energy is a serious problem, getting worse due to continuously dwindling conventional energy sources such as fossil fuels carbon, coal, petroleum, and natural gas. Renewable energy sources such as wind, solar, tidal, hydro, and geothermal are the energy sources that are cost-effective and reduces our dependency on fossil fuels. While it was a good switch from petrol and diesel to LPG (liquid petroleum gas) and then to CNG (compressed natural gas) to reduce CO2 emissions, there still
exists a need to move away from oil and gas to reduce our dependency on fossil fuels. Two technologies, namely hydrogen and Li-ion batteries are promising energy carriers that can replace the IC engines efficiently and would prove to be a great help in transportation of fuels. Due to limitations like the high cost of lithium and the safety concerns, Li-ion batteries are currently difficult to be used in cars. On the other hand, hydrogen has high gravimetric energy density, however it suffers from the low volumetric density. This has led the research community to develop efficient storage facilities for hydrogen [1e3]. High-pressure hydrogen storage and cryostorage are not considered good techniques due to their low energy density and safety concerns for onboard applications.
* Corresponding author. E-mail addresses: [email protected] (A. Jain), [email protected] (T. Ichikawa). https://doi.org/10.1016/j.ijhydene.2019.04.095 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Pal P et al., Eutectic melting in x(2LiBH4-MgH2) hydrogen storage system by the addition of KH, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.095
2
international journal of hydrogen energy xxx (xxxx) xxx
The hydrogen storage in solid state form in materials has been proven as the most promising and safe way to store hydrogen. Among all solid-state materials, complex metal hydrides such as alanates, borohydrides and amides-imides are considered as good candidates due to their light weight and high energy density [1,4]. In comparison with complex hydrides, metal borohydrides have higher hydrogen storage capacity suitable for stationary as well as transportation applications [5]. LiBH4 is one of the appropriate lightweight complex hydride because it has high gravimetric (18.5 wt% H2) and volumetric (121 g H2/ L) densities. However, due to its high thermodynamic stability and slow kinetics, it releases hydrogen completely above 900 C which makes it incompatible with proton-exchange membrane (PEM) fuel cells [2,6]. Researchers have proposed many solutions to lower the desorption temperature of LiBH4 and to enhance its kinetics, which led to the discovery of 2LiBH4eMgH2 system [7,8]. This new complex hydrogen storage system has displayed better reversibility than LiBH4 itself, however serious kinetics problem upon hydrogen release is still a barrier [9,10]. Shao et al. [11] mentioned the poor kinetics as a critical challenge for onboard applications. Many efforts have been made by researchers to solve the problem of kinetics but are still unable to thrive on the demand. Over past years, many catalysts and additives have been proposed to improve the kinetics of many different hydrogen storage systems [11]. With respect to kinetic enhancement, potassium-based intermediates are one of the prospective candidates to be considered a great achievement. For the first time, Santoru et al. studied the decomposition properties of K-Mg-N-H system and proposed several reaction pathways. Considering the mechanism for the reaction of KH with Mg(NH2)2 they concluded that potassium based additives are beneficial to improve the desorption properties of the overall system [12]. Potassium hydride has been recognized as one of the extraordinary catalyst, which was first introduced for the improvement of Mg(NH2)2 e LiH system by Wang et al. [13]. They explored the magical characteristics of potassium due to which hydrogen desorption temperature could be reduced from 186 C (pristine system) to 107 C (with potassium hydride). It was suggested that potassium diffuses into the imide and amide and combines with nitrogen, and as a result the amide N-H bonds and imide Li-N bonds are weakened which greatly enhances the dehydrogenation [13]. Superior catalytic effect of KH has been reported by Teng et al. also where they concluded that even a small amount of KH (5 mol%) was enough to improve the hydrogen desorption performance of the LiH-NH3 at 100 C [14]. Recently, Lin et al. [15] reported the improvement in dehydrogenation kinetics and cyclability by the addition of KH to Mg(NH2)2-LiNH2-LiH system. According to this report, only 3 wt% of KH was enough to reduce decomposition temperature of the composite Mg(NH2)2-LiNH2LiH to below 90 C without emission of NH3. Magical catalytic activities of potassium hydride evoked us to give a trial on the well-known complex hydrogen storage 2LiBH4-MgH2 system. Thus, the effect of KH addition is studied using thermogravimetry coupled with differential thermal analysis (TG-DTA) and differential scanning calorimetry (DSC) in this work. An interesting phenomenon of eutectic melting was also observed during the study, which will also be elaborated herein.
Experimental The starting material LiBH4 (95%, Sigma Aldrich) was used after heat treatment at 200 C for 24 h under dynamic vacuum to remove the solvent impurity. KH was also washed several times by hexane and dried under vacuum for several hours. MgH2 (98%, Alfa Aesar) was used as purchased without any treatment. All the materials were handled solely in a glove box filled with argon and coupled with a circulation purifier to maintain O2 and H2O level <1 ppm. The samples were prepared by mechanochemical treatment in a planetary ball mill under inert conditions (argon atmosphere). Batches of 500 mg samples were prepared with 20 ZrO2 balls (8 mm in diameter) in a milling pot of volume about 30 cm3. The milling was conducted for a total of 2 h with a 30 min pause after 1 h milling. The details of all the studied samples with compositions are given in Table 1. The TG-DTA data was obtained by using Rigaku, TG8120 connected to a mass spectrometer (MS, Anelva, M-QA200TS) under an argon gas flow (300 ml/min) at a heating rate of 5 C/min. To confirm the obtained TG-DTA results, thermal analysis was performed using differential scanning calorimetry (DSC). Power X-ray diffraction (PXD) measurements were carried out using Rigaku RINT-2500, lab X-ray diffractometer configured with a Cu Ka source (l ¼ 1.541 A) after ball milling and thermal analysis to identify the produced compounds. To prepare XRD samples a polyimide sheet (Kapton, Du PontToray Co. Ltd.) was used to cover the samples to avoid oxidation through the contact with air.
Results and discussion TG-DTA results of the composite 2LiBH4-MgH2 with and without 5% KH are shown in Fig. 1. Three endothermic peaks are found in the DTA profile for the pure 2LiBH4-MgH2 (upper panel, dashed line). The first two peaks at 115 C and 285 C belongs to the phase transition and melting of LiBH4 respectively. The weight loss is started around 340 C as observed
Table 1 e Sample composition and molar ratio of components. Sample S0 (0% KH) S1 (5% KH) S2 (10% KH) S3 (20% KH) S4 (30% KH) S5 (40% KH) S6 (50% KH) S7 (55% KH) S8 (60% KH) S9 (65% KH) S10 (70% KH) S11 (80% KH) S12 (90% KH)
x(2LiBH4-MgH2)
(1-x)KH
Molar ratio (LiBH4-MgH2-KH)
1.0 0.95 0.90 0.80 0.70 0.60 0.50 0.45 0.40 0.35 0.30 0.20 0.10
0.0 0.05 0.10 0.20 0.30 0.40 0.50 0.55 0.60 0.65 0.70 0.80 0.90
2.00:1.00:0.0 1.90:0.95:0.05 1.80:0.90:0.10 1.60:0.80:0.20 1.40:0.70:0.30 1.20:0.60:0.40 1.00:0.50:0.50 0.90:0.45:0.55 0.80:0.40:0.60 0.70:0.35:0.65 0.60:0.30:0.70 0.40:0.20:0.80 0.20:0.10:0.90
The eutectic composition is represented in bold.
Please cite this article as: Pal P et al., Eutectic melting in x(2LiBH4-MgH2) hydrogen storage system by the addition of KH, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.095
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Fig. 1 e DTA (upper panel) and TG analysis (lower panel) of 2LiBH4-MgH2 (dashed lines) and 2LiBH4-MgH2- 5%KH (solid lines).
from TG (lower panel, dashed line), which reflects the starting temperature of hydrogen desorption and the corresponding DTA peak is detected at 365 C. The second step decomposition is observed at more than 400 C, which is in line with the literature [6]. On the addition of 5 mol% KH, a shift in desorption temperature is observed for the first step with a peak temperature of 268 C (solid line), while no significant change is observed for the second step decomposition. In addition to it, an unusual but quite interesting phenomenon of shift in melting peak (285 C) is observed in DTA curve for KH added 2LiBH4-MgH2 system. It is well known that the melting cannot be affected by the use of additive, unless it forms a eutectic mixture. Another possibility can be associated with the occurrence of some unknown irreversible reaction. This observation prompted us to investigate the effect of KH addition to this system in more detail. To investigate it more precisely, same samples were examined using DSC and the results are depicted in Fig. 2. The reversible nature of peak around 285 C confirms the melting (during heating) and solidification (during cooling), whereas a clear shift to the lower temperature is the testament of eutectic melting. In addition, a small endothermic is also observed with peak temperature of 110 C, just before the phase transition temperature of LiBH4. This must be associated with the eutectic melting of eutectic composition. Although the phenomenon of eutectic melting is complicated, we can describe it simply as follows: the eutectic melting happens if two or more components sustain their liquid state below the melting points of their pure components. These eutectic melts can be utilized as media in which substances can dissolve with good solubility. Melting of complex systems like borohydrides and mixture of metal borohydride, hydride and halides can also increase reactivity and can improve kinetics for the release of hydrogen [16,18]. Lower melting point metal borohydrides and complex systems which show eutectic melting behavior, act as ionic liquids and can be utilized for refueling of vehicles [17]. Deep eutectics Mg(BH4)2/LiBH4 systems in complex borohydrides might behave as liquid in PEM fuel cell which is advantageous in hydrogen reversibility [16]. Many researchers have focused
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Fig. 2 e DSC profile of pure LiBH4-MgH2 (green color) and on the addition of 5 mol% KH in LiBH4-MgH2. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
on lowering the eutectic melting points by using the combination of different complex hydride systems due to its excellent onboard application. Bimetallic mixtures have been found to have much lower melting points in comparison with monometallic borohydrides M(BH4)x, where M ¼ Li, Na, K, Mg or Ca [16e19]. For example, the bimetallic mixture of lithium and potassium borohydrides 0.725LiBH4-0.275KBH4 have shown melting point Tm ¼ 105 C, which is much lower than the individual borohydride [6]. Similarly, 0.62LiBH4-0.38NaBH4 system displayed the melting at 210e220 C [16e19], lower than the individual components. In case of composite LiBH4Ca(BH4) system, it exhibits eutectic melting at 200 C and a partial reversibility has been confirmed for the first time [20]. The xLiBH4-(1-x)Mg(BH4)2, x ¼ 0.5e0.6 system exhibited a reduced initial temperature of hydrogen release i.e. 180 C (Tm ¼ 180 C) [21]. Thus, to investigate the anticipated similar phenomenon, a series of 2LiBH4-MgH2 with increasing mole percentage of potassium hydride was prepared and analyzed using differential scanning calorimetry (DSC) in a temperature range of room temperature to 500 C. However, Fig. 3 shows the DSC profile of samples only up to 150 C in order to make them more comprehensive. The onset temperature has been considered as melting temperature. The compositional details of the samples are given in Table 1. Sample S1 e S4 showed two very close peaks around 110e120 C and one peak in the temperature range 160e235 C. These could be assigned to the eutectic melting, phase transition and melting of the composite mixture respectively. With the increasing content of KH, the eutectic melting peak height is found to be increasing and shifting slightly to the lower temperature, whereas the peak corresponding to the phase transition is getting modest. The melting peak corresponding to the composite melting is also observed to be shifted to lower temperature. Finally, sample S5 shows only one peak that corresponds to the complete melting thus eliminating the peak corresponding to
Please cite this article as: Pal P et al., Eutectic melting in x(2LiBH4-MgH2) hydrogen storage system by the addition of KH, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.095
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Fig. 3 e DSC curves for pure 2LiBH4-MgH2 (first from upwards) and remaining with the increasing percentage of potassium hydride.
phase transition of LiBH4, as it is irrelevant in the melted state. Fig. 4 shows DSC profiles of some selected samples to determine the lowest melting temperature. The lowest onset temperature is found as 79 C for sample S7 which has a composition of 0.45(2LiBH4-MgH2)-0.55(KH). All the data corresponding to melting of composite mixtures is plotted in Fig. 5, which could serve as a phase diagram of x(2LiBH4-MgH2) e (1-x)KH system. Since the melting was not observed for samples S10 e S12, the curve is connected by dashed line, whereas the other part connected by solid line, clearly shows the eutectic composition as 0.45(2LiBH4-MgH2) e 0.55KH. To confirm the above melting and non-occurrence of any other reaction, X-ray diffraction (XRD) was performed for all the samples after milling as well as after DSC experiment up to 290 C. The XRD profiles of selected samples are shown in Fig. 6. The XRD profile after milling (Fig. 6a) suggest the existence of MgH2 and LiBH4 phases, however, no peaks corresponding to KH are visible for any samples up to the sample
Fig. 4 e DSC profiles of selected samples.
Fig. 5 e Phase diagram of x(2LiBH4-MgH2)-(1-x)KH system. The eutectic composition and melting temperature were determined by DSC, onset temperature was considered as the melting point.
having 70% KH (S10). In addition, some peaks could be indexed for KBH4 phase for the samples having 60% KH or more (S8-S12). The formation of KBH4 revealed that the reaction between LiBH4 and KH and anion exchange takes place during milling. The formation of amorphous KBH4 can also be expected for other samples with less KH content. Another possibility of the absence of KBH4 peaks in XRD of sample S1-S7 can be attributed to the presence of KBH4 in very small amount, which is not in the detection limit of XRD. This speculation is confirmed from the XRD profile of sample S11 (80%KH), where the peaks corresponding to both phases i.e. KH as well as KBH4 are present due to the excess amount of KH. On the other hand for the samples after melting, KBH4 peaks are clearly visible for the S1 sample with only 5% KH. It suggests the crystallization of KBH4 phase during solidification. The peaks corresponding to MgH2 along with KBH4 are visible in all the samples after heating at 290 C. In addition, the peaks corresponding to potassium hydride are also detected in samples S11 similar to the milled sample. On combining the DSC and XRD data, it can be summarized that the melting peak shift to the lower temperature is purely due to melting which arose probably due to the eutectic composition of LiBH4-KBH4 and no other side reaction took place during the heating range. Eutectic melting in LiBH4- KBH4 has already been investigated by L.B. Morten group in 2014 [17] and the lowest eutectic temperature was reported as 105 C. In our case we observed the lowest temperature as 79 C, which is different from the above, however, it might be associated with the different starting materials and/or different measuring method (TPPA in the above report, while DSC in this work). The effect of KH as a catalyst for the first decomposition step of the above system can also be understood on the basis of this eutectic melting. Since KH reacted with LiBH4 during the milling process, thus changing to KBH4 which formed a eutectic composition with LiBH4, thus reducing the melting temperature of the above system. The ionic mobility is enhanced due to the weakening of Li-B-H bonding in the molten state of LiBH4, thus enhancing the sorption kinetics
Please cite this article as: Pal P et al., Eutectic melting in x(2LiBH4-MgH2) hydrogen storage system by the addition of KH, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.095
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Fig. 6 e XRD patterns of selected samples (a) after milling (b) after heating at 290 C.
and reducing the decomposition temperature of first step reaction which corresponds to the LiBH4 decomposition.
Conclusion The addition of KH to well-known 2LiBH4-MgH2 reduced the decomposition temperature of 1st step reaction, however, no significant effect could be observed for the 2nd major step of decomposition. An interesting phenomenon of eutectic melting through the formation of KBH4 as an intermediate was observed and a phase diagram was reported herein. The eutectic composition for x(2LiBH4-MgH2) e (1-x)KH was assured to be x~ 0.45 using the combination of differential scanning calorimetry and X-ray diffraction. The eutectic composition was found to melt at ~79 C (Tmin) which is the
lowest melting temperature reported for a complex borohydride-hydride mixture so far. This work suggests the new possibilities to develop more complex hydride/borohydride systems to improve kinetic issues associated with the hydrogen storage applications. Advancement in techniques to decrease the decomposition temperature for on-board applications, while maintaining high hydrogen capacity will be the future road map.
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Please cite this article as: Pal P et al., Eutectic melting in x(2LiBH4-MgH2) hydrogen storage system by the addition of KH, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.095