Investigation on LiBH4-CaH2 composite and its potential for thermal energy storage

The LiBH4/CaH2 composite are firstly studied as Concentrating Solar Power Thermal Storage Material. The LiBH4/CaH2 composite according to the stoichiometric ratio are synthesized by high-energy ball milling method. The kinetics, thermodynamics and cycling stability of LiBH4/CaH2 composite are investigated by XRD (X-ray diffraction), DSC (Differential scanning calorimeter) and TEM (Transmission electron microscope). The reaction enthalpy of LiBH4/CaH2 composite is almost 60 kJ/mol H2 and equilibrium pressure is 0.482 MPa at 450 °C. The thermal storage density of LiBH4/CaH2 composite is 3504.6 kJ/kg. XRD results show that the main phase after dehydrogenation is LiH and CaB6. The existence of TiCl3 and NbF5 can effectively enhance the cycling perfomance of LiBH4/CaH2 composite, with 6–7 wt% hydrogen capacity after 10 cycles. The high thermal storage density, high working temperature and low equilibrium pressure make LiBH4/CaH2 composite a potential thermal storage material.

Solar energy is the most plentiful renewable and clean alternative to fossil fuels 1 . International Energy Agency (IEA) points out that solar energy will make up 22 percent of the global electricity, and it is possible that solar photovoltaics (PV) and concentrating solar thermal (CST) power technology will play roughly equal, but complementary roles by 2050 2 . The CST power technology can store energy as heat that can be assessed on demand to generate electricity when PV technology is inefficient, such as at night or during rainy days.
As thermal storage (TS) material is the key element in the CST, improving the energy storage density and working temperature have great value on power generation efficiency and cutting back on the cost. There are three basic methods of thermal storage. Considering the condition of CST plants, sensible heat storage with molten salt is of low-efficiency and can be corrosive sometimes and latent heat storage using NaNO 3 is of high flammability and reactivity and is uncertain over its longevity. Thermochemical heat storage materials have quite high energy storage density, in which hydrides' exceed 1700-4000 kJ kg −1 (10~30 times more than molten salts' energy storage density and 4~10 times more than phase change materials' energy storage density) 3,4 . The characteristics of common and potential thermal storage materials are listed in Table 1.
Among all hydrides, complex hydrides such as LiBH 4 , NaAlH 4 and NaBH 4 possess a quite high forming enthalpy due to the transition to an ionic or covalent compound of metals upon hydrogen absorption 9 It seems that complex hydrides are promising thermal storage materials in CST plants.
In fact, LiBH 4 is mostly researched as a hydrogen storage material due to the second highest hydrogen content (18.4 wt.%) of all alanates and boranates [10][11][12][13] . The thermal hydrogen desorption of pure LiBH 4 starts at ~320 °C and proceeds mainly in the temperature region 400-600 °C, which is in accordance with the working temperature of thermal storage material in CST plant.
However, the sluggish kinetics and poor reversibility of LiBH 4 are the problems that limit its use in hydrogen storage [11][12][13] . The destabilization was proposed to change the reaction process by adding a reactive additive 14 . Thus, many new systems have been proposed based on DFT calculation of reaction enthalpies in multi-component systems 15,16 . Among all systems, LiBH 4 -CaH 2 is one of promising composites that are suitable for thermal storage due to its onset temperature. And this reaction can produce around 11.7 wt% hydrogen. But Yang reported that Scientific RepoRts | 7:41754 | DOI: 10.1038/srep41754 LiBH 4 -CaH 2 composite is irreversible under the condition tested (350 °C, 150 bar) 17 . The sluggish kinetics in LiBH 4 -CaH 2 composite is another problem that need to be solved 18 . In this research, the enthalpy Δ H and equilibrium pressure of desorption according to reaction 2 are determined by PCT (pressure, concentration, and temperature) measurements. Only LiBH 4 -CaH 2 without catalysts was measured due to the fact that influence the thermodynamics of the mixture as Ti-doped NaAlH 4 12 . The additives such as TiCl 3 and NbF 5 are investigated their effect on kinetics and cycling performance of LiBH 4 -CaH 2 composite.

Experimental Details
LiBH 4 (≥ 95% pure), purchased from Acros Organics, CaH 2 (≥ 98% pure) and NbF 5 ( ≥ 99% pure), purchased from Alfa Aesar, and TiCl 3 (≥ 95% pure), synthesized by the reaction of titanium tetrachloride with metallic titanium in molten CaCl 2 and the enrichment process with HCl gas 26 , were utilized directly without any further purification. The mole ratio of LiBH 4 -CaH 2 composite according to reaction 2 is 6:1. The pure LiBH 4 -CaH 2 composite and LiBH 4 -CaH 2 composite doped with different additives (1 mol% TiCl 3 , and 5 wt% NbF 5 ) was ball-milled under argon atmosphere by using a QM-2B high energy mill (Nanjing NanDa Instrument Plant) at a rotating speed of 1200 rpm for 1 h. Two kinds of stainless steel balls with 4 mm and 8 mm diameters were added with a ball-to powder weight ratio of 12.5:1. Typically, 4 g mixture was sealed in the stainless steel vessel within a high purity argon atmosphere during milling. To avoid excess heating of the stainless steel vessel, there were 10 min intervals between each 5 min milling process.
The isothermal desorption was measured by using the Sieverts-type pressure-composition-temperature (P-C-T) apparatus (General Research Institute for Nonferrous Metals, China). The maximum pressure, maximum vacuum degree and maximum temperature of this apparatus is 10 MPa, 10 −1 Pa and 800 °C, respectively. Typically, 60-100 mg sample was loaded into the vessel, and then heated up to 450 °C under 0.1 MPa hydrogen atmosphere. Following the dehydrogenation, the samples were subjected to rehydrogenation studies at 450 °C under 8 MPa hydrogen pressure for 16 h. It should be noted that the additional content was not taken into consideration when calculating the released hydrogen. The PCI (Pressure-composition isotherms) curves were measured at 405 °C, 420 °C, 435 °C, 450 °C and 465 °C, respectively.
The phase structure of the samples after milling and dehydrogenation was examined by an MXP21VAHF X-ray Diffractometer (XRD with Cu Kα radiation, 40 kV, 300 mA), with the 2θ angle ranged from 10° to 90° with a scanning rate of 10° min −1 . X-ray photoelectron spectroscopy (XPS) was performed with the PHI-5300 spectrometer. The morphology and phase constitution of all samples after ball milling and desorption were observed by and transmission electron microscopy (Tecnai G2 F30 S-TWIN, FEI, USA). Simultaneous differential scanning calorimetry (DSC) and Thermogravimetric Analysis (TGA) experiments were conducted under 50 mL min −1 argon flow in a NETZSCH STA 449F3 Jupiter instrument between 50 °C and 500 °C with a heating rate of 5 °C min −1 . The samples were transferred to Al 2 O 3 crucibles under argon atmosphere for the DSC-TGA measurements.
All samples handling was performed under strictly inert conditions (≥ 99.99% Ar atmosphere) in the glove box (Mikrouna, Super-750) equipped with oxygen/humidity sensors and recirculation system to avoid oxidation and moisture. Oxygen and H 2 O levels were kept below 0.1 ppm.

Results and Discussion
XPS characterization. The XPS results of three LiBH 4 -CaH 2 composites after milling are presented in Fig. 1(a-c), which confirms the existence of element Li, B and Ca in both composites. Element Nb, F and Cl are identified in the catalyst-doped composite, while Ti are not discovered due to the low amount addition. The XRD results are presented in Fig. S1. There are only two obvious peaks in both composites, which are characterized as CaH 2 . It can be inferred that the structure of LiBH 4 after milling becomes amorphous. No peaks of LiBH 4 , TiCl 3 or NbF 5 are detected. The XPS narrow spectra of ball-milled LiBH 4 -CaH 2 composite are showed in Fig. 1(d) Fig. 2(a). Due to the sluggish kinetics, the pressure value in each platform can only be read after 4-hours waiting. Even so, the platform inclination is quite a lot, especially in low temperature condition.
Considering the platform is a slope to some extent, the equilibrium pressures from 405 °C to 465 °C are calculated  What's more, the low equilibrium pressure makes LiBH 4 -CaH 2 composite possible to be operated at higher temperature. The higher working temperature can increase overall solar to electricity conversion efficiency and reduce the cost in CST plants 27 . The dehydrogenation capacity of pure LiBH 4 -CaH 2 composite is mostly ranging from 10.5 wt% to 11.6 wt%, which is close to their theoretical value. The sluggish kinetics resulting from the relatively low temperature (405 °C) may account for the lower capacity (9.5 wt%). Only LiBH 4 -CaH 2 without catalysts was measured due to the fact that influence the thermodynamics of the mixture as Ti-doped NaAlH 4 12 . Liu 22 reported that LiCl forms during ball milling of 6LiBH 4 /CaH 2 /xTiCl 3 . LiF and CaF 2 are observed after the ball milling reaction of NbF 5 and LiBH 4 or CaH 2 24 . Thermodynamics of pure LiBH 4 -CaH 2 composite might have changed due to the formation of LiCl or LiF and CaF 2 .
A plot of ln P against 1000/T in Fig. 2(b) results in a nearly straight line. Calculation of Δ H = R · (lnP 2 − lnP 1 )/ (1/T 2 − 1/T 1 ) from ln P and 1/T values at 405 °C and 465 °C provides a Δ H of 60.555 kJ mol −1 H 2 . According to the reaction 2, a thermal storage density value of 3504.6 kJ kg −1 is calculated. It shows a superior capacity to sensible and latent thermal storage materials, even to thermochemical thermal storage materials shown in Table 1.

DSC Calculation.
The DSC and TGA curves of pure LiBH 4 -CaH 2 composite are shown in Fig. 3. There are mainly three endothermic peaks during the heating process. The endothermic effect at 108-112 °C is reversible and corresponds to polymorphic transformation of LiBH 4 . The second peak at 268-286 °C corresponds to the fusion of LiBH 4 . The third peak corresponds to the dehydrogenation behavior of LiBH 4 . The onset temperature is 392 °C and the peak temperature is 446 °C. According to TGA results, dehydrogenation reaction ends at 497 °C. The integration of DSC on temperature from 392 °C to 497 °C is calculated as enthalpy of reaction 2, with a value of 60.706 kJ mol −1 H 2 .
The DSC and TGA curves of LiBH 4 -CaH 2 composites with TiCl 3 and NbF 5 addition are shown in Fig. S3. There are both three endothermic peaks in these two composites. NbF 5 addition shows a more remarkable influence on the decrease of onset temperature than TiCl 3 . The onset temperature, dehydrogenation reaction enthalpy and thermal storage density of three composites and other potential TS system are shown in Table 2   Investigation on kinetics. The Fig. 4 shows the desorption behavior of three LiBH 4 -CaH 2 composites. The addition of TiCl 3 significantly improves the dehydrogenation kinetics of LiBH 4 -CaH 2 composite, while NbF 5 influence it in an opposite way. Both composites can release 9-10 wt% hydrogen in an hour. After 4 hours, the   The results are shown in Fig. 6. The initial hydrogen capacity of the pure composite and TiCl 3 doped composite shows a nearly theoretical hydrogen capacity (11.7 wt%), while NbF 5 doped composite only desorbs around 10 wt% hydrogen. The hydrogen capacity of both composites declines during cycling. It is worth mentioning that TiCl 3 doped composite can reversibly store 9 wt% hydrogen during first three cycles. After 10 cycles, the remaining hydrogen capacity of pure composite, NbF 5 doped composite and TiCl 3 doped composite is 3.8 wt%, 6.4 wt% and 7.1 wt%, respectively. TiCl 3 and NbF 5 seems effectively raise the cycling stability performance of LiBH 4 -CaH 2 composite.
The TEM images of pure LiBH 4 -CaH 2 composite after 10 cycles are shown in Fig. 7(a-d). The main phases are small particles with a diameter of 3-6 nm, separately scattering. Particle aggregation shown in Fig. 7(c), which may result from the sintering, is also found. The diffraction ring in Fig. 7(d) is very obvious, indicating that amorphous structure is formed. The particle aggregation and amorphous structure of products accounts for the dramatic loss of hydrogen capacity of pure LiBH 4 -CaH 2 composite during cycling. TEM images of TiCl 3 doped composite and NbF 5 doped composite after 10 cycles are shown in Fig. 7(e,f,g and h). The small particles with a diameter of 3-6 nm are both observed. However, the results of electron diffraction indicate that the TiCl 3 doped composite after 10 cycles is crystal structure, while NbF 5 doped composite after 10 cycles is amorphous structure. By analyzing the diffraction ring diameter, the crystal structure is assumed to be CaB 6 . The amorphous structure of B is not good for the reverse reaction to produce LiBH 4 , while the crystal structure of CaB 6 is in favor of the reverse reaction 29,31,32 . This explains why TiCl 3 plays a more effective role in raising the cycling stability performance of LiBH 4 -CaH 2 composite than NbF 5 . Moreover, it is noteworthy that a graphene-like lamellar structure are found in NbF 5 doped composite after 10 cycles. The value of interlamellar spacing (d) is 0.3364 nm, which is corresponding to NbF 5 . But the lamellar structure of NbF 5 is never reported. Thus, the d value of NbB 2 is 0.3321 nm, which is close to the 0.3364 nm. Minella reported that NbB 2 nanoparticles was observed after milling or upon sorption reactions of Nb-based Ca(BH 4 ) 2 doped composites 33 . It is reasonable to be assume that a small amount of NbB 2 can also be formed in Nb-based LiBH 4 -CaH 2 doped composites. It needs more research work to identify this graphene-like lamellar structure in NbF 5 doped composite.

Conclusion
The reaction enthalpy of LiBH 4 /CaH 2 composite is almost 60 kJ/mol H 2 and equilibrium pressure is 0.482 MPa at 450 °C. The thermal storage density of LiBH 4 /CaH 2 composite is 3504.6 kJ/kg. XRD results show that the main phase after dehydrogenation is LiH and CaB 6 . The exsience of TiCl 3 and NbF 5 can effectively enhance the cycling perfomance of LiBH 4 /CaH 2 composite, with 6-7 wt% hydrogen capacity after 10 cycles. The high thermal sotrage density, high working temperature and low equilibrium pressure make LiBH 4 /CaH 2 composite a potential thermal storage material.
Although the high price of starting materials, such as LiBH 4 , will limit its usage, the LiBH 4 /CaH 2 composite could serve as the additives for Magnesium-based alloys in TS. The research will be continued on the pair study of LiBH 4 /CaH 2 composite with another metal hydride working at lower temperature.