Applied Science and Convergence Technology 2019; 28(5): 169-172
Published online September 30, 2019
Copyright © The Korean Vacuum Society.
Department of Physics, Dankook University, Cheonan 31116, Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-CommercialLicense (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution,and reproduction in any medium, provided the original work is properly cited.
As ionic conductivities are related to the intrinsic structural features of materials, they are useful for elucidating structural changes. Herein, the ionic conductivities of crystals of NaBH4, KBH4, and their mixtures are determined for the first time. The melting and decomposition temperatures of various compositions are also measured. Thermal studies indicate that both the hydrides are well-mixed chemically, which is accomplished through a solid-state reaction using a simple ball milling process. The melting temperatures of Na1-
Keywords: NaBH4, KBH4, Ionic conductivity, Melting transition, Decomposition
Metal borohydrides have high gravimetric and volumetric hydrogen capacities. The dehydrogenation and rehydrogenation characteristics of these hydrides have attracted significant interest, owing to their possible applicability in reversible hydrogen storage materials for on-board vehicular applications. Many studies on the hydrogen desorption properties of lithium borohydrides have been reported. However, the thermodynamics and kinetics of hydrogen exchange in lithium borohydrides are not favorable for reversible hydrogen storage [1,2].
Sodium and potassium borohydrides have not been extensively studied because of their high decomposition temperatures. The melting point of NaBH4 was reported as 498 or 505 °C in the two different studies in the literature [3,4], and the decomposition in the molten state was reported to start at 565 or 577 °C, respectively. KBH4 also exhibits an identical decomposition pathway, but the melting and decomposition temperatures are much higher, i.e., by approximately 100 °C. The melting and decomposition temperatures in complex borohydrides are key parameters for the material design of potential hydrogen-storage materials. Therefore, recent studies on alkali metal borohydrides have focused on lowering or tuning the decomposition temperatures. Recently, the thermodynamic properties and phase diagrams of NaBH4–KBH4 and LiBH4–NaBH4–KBH4 systems have been reported [5,6].
In addition to hydrogen storage, a current challenge in the metal borohydride system involves the achievement of fast ion conduction before the occurrence of decomposition. Crystalline LiBH4 and related compounds are fast Li-ion conductors with promising applications such as electrolytes in all solid-state lithium batteries [7,8]. Ionic conductivities are related to the intrinsic structural features and reflect the structural changes. However, the ionic conductivities of NaBH4 and KBH4 crystals have not been measured till date. The aim of the present work is to study the melting and decomposition characteristics as well as the ionic conductivities of a solid solution of Na1-
NaBH4 and KBH4 with nominal purities of 99 and 97 %, respectively, were purchased from Sigma-Aldrich (St. Louis, Missouri, United States) and used directly without pretreatment. Powder mixtures with nominal compositions of (1-
For ionic conductivity measurements, compressed pellets (thickness of 1.5 mm and area of 78 mm2) were prepared with evaporated gold contacts. The electrical conductivity measurements were carried out in the temperature range of 200–600 °C with average intervals of 0.5 °C. The impedances were determined using an HP model 4284A impedance analyzer (Hewlett-Packard, Palo Alto, California, United States) measuring a frequency range from 100 Hz to 1 MHz, which afforded the bulk conductivity measurements through a typical impedance analysis.
Thermal analysis was performed using differential scanning calorimetry (DSC; DSC-50, Shimadzu Corporation, Kyoto, Japan). The sample chamber was purged with purified nitrogen and measurements were carried out at a heating rate of 5 ° min−1 from 30 to 700 °C.
NaBH4 and KBH4 are tetragonal at low temperatures and transform into cubic phases at approximately −83 and −203 °C, respectively . Recently, bialkali metal borohydrides, LiK(BH4)2 and NaK(BH4)2, were synthesized and reported to exhibit new structural phases . Unfortunately, these borohydrides were not found to be promising for hydrogen-storage applications as the former is very stable to allow hydrogen release and the crystal structure of the latter is metastable .
For NaBH4, two DSC peaks are observed upon heating the sample. The first endothermic peak is due to melting and the second endothermic peak is due to the decomposition in the molten state. After melting, NaBH4 decomposes according to the following reaction.
This reaction is known to occur partially during melting and continue at high temperatures with hydrogen release at the decomposition temperature. Theoretically, a total of 10.4 wt(%) of H2 is released upon the completion of the decomposition reaction. The decomposition of NaBH4 can also be described by the reaction shown below , particularly for mixed compounds such as NaBH4/MgH2  and NaBH4/Al , where NaH dissociates into Na and H2 at high temperatures.
To investigate the melting and decomposition characteristics of mixed Na1-
As observed in the DSC data, the melting peaks are slightly broad and the melting starts from approximately 10 °C below the peak temperature. Therefore, to measure the starting point of this transition more precisely, the dielectric constants were measured for the pellet-type samples coated with gold electrodes. When the temperature increases, partial dehydrogenation first occurs on the surface of the pellet samples. Thus, the dielectric constants increase abruptly with electrode damage at the beginning of the melting transition as shown in Fig. 2. The melting transition of NaBH4 is sharper than that of KBH4, as observed in both thermal and electrical data. This indicates that the hydrogen desorption kinetics of NaBH4 are faster than those of KBH4. The peak temperatures observed in the DSC data are higher than those obtained from the dielectric constant measurements. However, the starting temperatures of DSC melting peaks are consistent with those observed in the dielectric constant analysis.
The melting temperatures of Na1-
The ionic conductivities of Na1-
Bulk conductivity values, extracted from the complex plane plots, were analyzed using the Arrhenius equation:
For both NaBH4 and KBH4, a curvature is observed in the conductivity plots, which increases with an increase in temperature, indicating the dominance of one transport mechanism at low temperature and another at high temperature. The upward bend can be caused by many different factors such as defect interactions, new defect mechanisms, or an intrinsic temperature dependence of the migration or formation parameters of the defects . If the type of defect is assumed to be identical for all temperatures, the migration enthalpy is larger for the high-temperature transport mechanism than for the low-temperature transport. The temperatures at which the low-temperature regime changes to the high-temperature regime is approximately 340 °C for NaBH4 and 440 °C for KBH4. The activation energies for NaBH4 are 0.60 eV in the low-temperature region and 1.40 eV in the high-temperature region. The activation energy in the low-temperature region is comparable to that of LiBH4 (0.53 eV), which is measured just above the structural transition temperature at 117 °C . However, the activation energies for KBH4, i.e., 1.08 eV in the low-temperature region and 1.95 eV in the high temperature region, are higher than those for NaBH4.
For alkali borohydrides, when the low-temperature tetragonal phase transforms to a high-temperature cubic phase, the orientationally disordered BH4 groups are observed [9,22]. The upward bend in the ionic conductivities is attributed to the onset of the reorientational disorder of the tetrahedral groups at high temperatures [23,24]. The reorientation motions of the borohydride ions can serve as an additional degree of freedom to enhance the cationic mobility, affording an increased conductivity at high temperatures. However, for the mixtures, no significant bend is observed in the Arrhenius plots and the activation energies are in the range of 1.40–1.80 eV, which belong to the high-temperature regime. This can be ascribed to the rotational disorder of the hydride ions at low temperature due to the highly disordered structure of the mixed system.
For all Na1-
This work was supported by the National Research Foundation of Korea (NRF 2010-0022383). We would like to thank Editage (