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Calcium Borohydride Ca(BH4)2: Fundamentals, Prediction and Probing for High-Capacity Energy Storage Applications, Organic Synthesis and Catalysis

National Institute of Materials Physics, Atomistilor 405A, 077125 Magurele, Romania
Energies 2023, 16(11), 4536;
Received: 1 May 2023 / Revised: 31 May 2023 / Accepted: 2 June 2023 / Published: 5 June 2023
(This article belongs to the Special Issue Metal Hydrides Hydrogen Storage, Thermal Management, and Applications)


Calcium borohydride (Ca(BH4)2) is a complex hydride that has been less investigated compared to its lighter counterpart, magnesium borohydride. While offering slightly lower hydrogen storage capacity (11.5 wt% theoretical maximum, 9.6 wt% under actual dehydrogenation conditions), there are many improvement avenues for maximizing the reversible hydrogen storage that have been explored recently, from DFT calculations and polymorph investigations to reactive hydride composites (RHCs) and catalytic and nanosizing effects. The stability of Ca(BH4)2, the possibility of regeneration from spent products, and the relatively mild dehydrogenation conditions make calcium borohydride an attractive compound for hydrogen storage purposes. The ionic conductivity enhancements brought about by the rich speciation of borohydride anions can extend the use of Ca(BH4)2 to battery applications, considering the abundance of Ca relative to alkali metal borohydrides typically used for this purpose. The current work aims to review the synthetic strategies, structural considerations of various polymorphs and adducts, and hydrogen storage capacity of composites based on calcium borohydrides and related complex hydrides (mixed anions, mixed cations, additives, catalysts, etc.). Additional applications related to batteries, organic and organometallic chemistry, and catalysis have been briefly described.

Graphical Abstract

1. Introduction

The road to sustainable energy has an ultimate endpoint: the hydrogen economy. Hydrogen remains the only energy source producing a high amount of clean energy with no carbon-containing by-products [1,2,3,4,5,6,7,8,9,10,11,12]. However, vehicular and stationary applications relying on hydrogen technology still face major drawbacks and important technological challenges, along with safety risks associated with H2 storage tanks where pressure spikes to 700 bar H2 [1].
In this context, solid-state hydrogen storage represents a safer alternative to classical high pressure H2 tanks, although the strict requirements of the US DOE have yet to be met [2]. The current target for 2025 is a material capable of storing 5.5 wt% hydrogen (1.8 kWh/kg, 1.3 kWh/L, with an estimated cost of $9/kWh). Perhaps the most important aspect of solid-state hydrogen storage is the material’s capability to reversibly store H2 without continual loss of this storage capacity. To this end, most experimental tanks have been fueled by various intermetallics (AB2-type or AB5-type, such as LaNi5H6 with 1.38 wt%), which offer a storage capacity not exceeding, at best, 2 wt% hydrogen. With a storage capacity of up to 20 wt% (Be(BH4)2) and high gravimetric densities (110–150 g H2/L), complex hydrides are the most promising solid-state hydrogen storage materials, and the amount of research in this area has been staggering. Still, the kinetic and thermodynamic factors of absorption/desorption processes hinder their wider use, while the potentially toxic boron-containing gases (diborane or higher homologues) are of concern for the environment.
Light metal borohydrides M(BH4)n (M = Li, Na, Mg, Ca) and alanates have received the most attention not only for their increased theoretical hydrogen storage capacity, but also due to promising results pointing to the possibility of reversibility during successive a/d cycles. With a high storage capacity of about 10 wt% (actually attainable under experimental conditions) and a relatively low dehydrogenation temperature, Ca(BH4)2 holds promise for a viable and reversible hydrogen storage material under suitable conditions. While dehydrogenation of Ca(BH4)2 has been reported in the range 350–500 °C, this is still lower than other light metal borohydrides. Driven by the high hydrogen storage capacity of calcium borohydride needed for energy storage applications, particularly in the field of hydrogen storage and fuel cells, researchers have investigated various methods to enhance the hydrogen release kinetics and lower the dehydrogenation temperature. Ca(BH4)2 has the potential to offer high energy density, enabling the development of compact and efficient energy storage systems. Calcium borohydride can serve as both a source of energy and a hydrogen reservoir within energy storage devices, such as lithium-ion batteries and sodium-ion batteries.
However, despite its potential, there are several challenges that need to be addressed before the wide acceptance of Ca(BH4)2 as an energy storage material. The kinetics of hydrogen release from calcium borohydride are relatively slow, limiting its practical application. Enhancing hydrogen release kinetics is a key research focus. The dehydrogenation reaction mechanisms of calcium borohydride are not yet fully understood, and further investigation is required to gain detailed insights into the underlying processes. Another obstacle comes from an economic point of view. The cost of Ca(BH4)2 is high; therefore, the development of cost-effective and scalable synthesis routes for calcium borohydride is another area that requires more research results.
The current review aims to describe the most significant aspects concerning Ca(BH4)2, from the synthesis routes, crystal structure and polymorphs, structural variability and modifications (DFT computations, nanoconfinement, destabilization, RHCs, solvates/adducts), characterization methods, hydrogen storage properties, and further applications in battery technology, organic synthesis, and organometallic chemistry. Optimizing the overall performance, stability, and efficiency of energy storage devices utilizing calcium borohydride is a critical research challenge.

2. Synthesis Routes

There are various methods reported for the synthesis of borohydrides [5,13] and, in particular, of Ca(BH4)2. For instance, Barkhordarian et al. reported the hydrogenation of the RHC: CaH2 + MgB2 at 350 °C and 140 bar H2 to produce Ca(BH4)2 + MgH2 (Equation (1)) [13].
CaH 2 + MgB 2 + 4   H 2   250   ° C , 140   bar   H 2 Ca ( BH 4 ) 2 + MgH 2     Δ H = 27.5   kg / mol
However, the layered structure of B in MgB2 was essential for Equation (1) to occur, as no reactivity was observed when substituting MgB2 for B. The reasoning seemed to lie in the structural features of MgB2, where layers of B are located between layers of Mg, a structure bearing similarities to graphite. The authors also reported enhanced kinetics of de-/rehydrogenation when using Ti(OiPr)4 as an additive [13].
Dodecahydroborates have been obtained by desolvation at ~120 °C of the MB12H12 solvates, or in their pure form, by the following Equation (2) [14], The mixture of M(BH4)2 and B10H14 is typically milled in a planetary ball mill (10 steel balls, 7 mm diameter, 0.1 MPa Ar, 5 h milling), followed by sintering in stainless steel crucibles at 380 °C for 2 h in the case of CaB12H12 [14].
M ( BH 4 ) 2 + B 10 H 14   MB 12 H 12 + 5   H 2   ( M = Mg ,   Ca )
Nanostructuring has long been utilized to achieve significantly better yields for reactions where classical synthetic routes have failed [6], while stabilization of reactive species has often been attained via complex formation, i.e., through ligand coordination, as is the case for ammine solvates of M(BH4)2 (M = Ca, Sr) [15]. Jepsen et al. reported the synthesis of four such solvates in the case of calcium-based borohydrides: Ca(NH3)n(BH4)2 (n = 1, 2, 4, and 6) (Equation (3)) [15]. The pure Ca(BH4)2 absorbs gaseous ammonia, forming the unstable hexa-ammine calcium borohydride Ca(NH3)6(BH4)2, which undergoes a rapid decomposition equilibrium to form the tetra-ammine calcium borohydride Ca(NH3)4(BH4)2.
Ca ( BH 4 ) 2 + 6   NH 3   ( g )   Ca ( NH 3 ) 6 ( BH 4 ) 2     Ca ( NH 3 ) 4 ( BH 4 ) 2 + 2   NH 3   ( g )    
Karabulut et al. synthesized Ca(BH4)2 in a solid-state reaction starting from anhydrous, synthetic colemanite, a calcium borate mineral of formula Ca2B6O11.5H2O [16]. By milling Ca2B6O11 and CaH2 in a 1:12 molar ratio in a spex-mill, calcium borohydride was obtained with a sole by-product in the form of CaO (Equation (4)). Using colemanite as the raw material has a clear economic advantage, as the cost of 1 g Ca(BH4)2 would be ~$4, compared to the commercial version’s (~$200).
Ca 2 B 6 O 11   + 12   CaH 2 Ca ( BH 4 ) 2 + 11   CaO
It is also interesting to note that the hydrogen content of Ca(BH4)2 could effectively double when subjected to hydrolysis, as Ca(BO2)2 is the main product along with H2 (Equation (5)).
Ca ( BH 4 ) 2 + 4   H 2 O Ca ( BO 2 ) 2 + 8   H 2
Given the reactivity of metal borohydrides towards water, it follows that the formation of a bimetallic borohydride in the form of LiCa3(BH4)(BO3)2, as isolated and characterized by Lee et al., is not that surprising after all [17]. The mixed borohydride was isolated from hydrogenation studies carried out on RHC based on 0.75 LiBH4–0.25 Ca(BH4)2 embedded in a mesoporous carbon matrix (Vp = 1.15 cm3/g), and most likely originates from oxygen-containing impurities (oxygen or water) [17]. The formation of LiCa3(BH4)(BO3)2 was tracked by XRD and was found to proceed through the intermediacy of mixed anion calcium borohydride, Ca3(BH4)3(BO3) (Equation (6)).
LiBH 4 + 2   Ca 3 ( BH 4 ) 3 ( BO 3 ) LiCa 3 ( BH 4 ) ( BO 3 ) 2 + 3   Ca ( BH 4 ) 2
The general route to prepare Ca(BH4)2 starts from CaH2 or alkoxide Ca(OR)2 and B2H6, or more conveniently by wet chemistry in a metathesis reaction, where CaCl2 and NaBH4 are ball milled in THF (tetrahydrofuran), producing the commercially available THF adduct of calcium borohydride, namely, Ca(BH4)2.2THF [18,19] (Equation (7)).
2   NaBH 4 +   CaCl 2 THF   Ca ( BH 4 ) 2 · 2 THF + 2   NaCl
Other adducts of Ca(BH4)2 are also reported; for instance, desolvation of Ca(BH4)2.2THF at 80 °C led to the formation of Ca(BH4)2·THF, as reported by Richter et al. [20].
Møller et al. reported the synthesis of M(BH4)n from the systems AlB2–MHx (M = Li, Na, Ca, but not Mg), where AlB2 served as a boron source, and H2 was introduced at a pressure in the range of 100–600 bar [21] (Equation (8)).
AlB 2 + 2 x MH x + 3   H 2   2 x   M ( BH 4 ) x + Al
The inclusion of Al in the case of Ca(BH4)2 can produce 6.3 wt% with a reaction enthalpy of 64.9 kJ/mol H2 and an entropy of 156.5 J/k mol H2, requiring a temperature of 140 °C at an equilibrium H2 pressure of 1 bar (Equation (9)). This alone is a marked improvement compared to neat Ca(BH4)2 (87.2 kJ/mol H2, 158.3 J/K mol H2, 280 °C) [21].
Ca ( BH 4 ) 2 +   Al   AlB 2 + CaH 2 + 3   H 2
Nakagawa et al. used ball milling of CaH2–CaB6 mixtures to obtain Ca(BH4)2 near room temperature, noting the important role of crystalline CaB6 in order to achieve reasonably high rates for hydrogenation [22].
Several experimental aspects have to be considered when synthesizing Ca(BH4)2, along with strong safety protocols. For instance, the influence of chlorides was illustrated in the mixture CaCl2:LiBH4, when the reactant ratio needs to be carefully optimized, otherwise solid solutions Li(BH4)1-xClx/Ca(BH4)2-yCly (milling performed under an Ar atmosphere) are obtained, and CaHCl forms instead of Ca(BH4)2 [23]. Ca(BH4)2 was also produced by milling CaB6 with CaH2 in a 1:2 molar ratio under 700 bar H2 at 400–440 °C, as evidenced by Ronnebro and Majzoub [11].
Ca(BH4)2 can serve as as borohydride source for other metathesis reactions, for instance with M2(SO4)x metal sulfates, with the driving force for obtaining M(BH4)x being the formation of the precipitate of CaSO4 [18]. Other authors have reported a mixed adduct Ca(BH4)2·Al(BH4)3 resulted by mixing the unsolvated starting borohydrides, decomposing into starting borohydrides upon heating, and having a postulated structure [Ca(BH4)]+[Al(BH4)4] supported by IR spectrum analysis [24].

3. Structural Variability, Polymorphs, Crystal Structure, DFT Predictions, and Phase Transitions

The structural stability aspects of many representative simple and complex hydrides (alanates, borohydrides) have been reviewed in the recent past [25], along with Ca-based RHCs ([Ca(BH4)2 + MgH2 and Ca(BH4)2 + MgH2 + 0.1NbF5]) [26]. Karimi et al. have highlighted the catalytic role of NbF5 on the hydrogen sorption kinetics of Ca(BH4)2 + MgH2 (10.5 wt% H2), when ~10 nm diameter NbB2 was evidenced to form and to remain stable during a/d cycles, while also being responsible for producing ~50% finer Ca-RHC + 0.1NbF5 nanocomposites and preventing agglomeration of calcium-RHCs [26].
The structural features of Ca(BH4)2 have been described by Llamas-Jansa et al., who reported the different behavior between α, β, and γ polymorphs of Ca(BH4)2 during hydrogenation studies [27]. Vibrational spectroscopy data was employed to correlate the increase in wavenumber to the increased decomposition temperature (15 °C between the α and γ polymorphs). The most effective H2 release was observed when ramping at 10 °C/min a sample of pure α-Ca(BH4)2 [27]. Moreover, Sharma et al. shed light on the B-H bond breaking/formation, using isotope exchange reactions with a D2 pressure of 20 bar, concluding an activation energy Ea = 82.1 ± 2.7 kJ mol−1 for the transformation of Ca(BH4)2 into Ca(BD4)2, and proposing a mechanistic scheme involving the formation of an activated complex: Ca ( ( BH 4 ) 2 +   D 2   Activated   complex   Ca ( BD x H 4 x ) 2 [28]. Considering all the literature data, it seems that the dehydrogenation behavior of Ca(BH4)2 depends greatly on the polymorph composition and their respective ratio in the starting sample material, especially considering the typical co-existence of α and β phases during hydrogenation experiments [29]. Soloninin et al. studied the reorientation motion of BH4 tetrahedra in M(BH4)2 (M = Mg, Ca) by proton and 11B spin-lattice relaxation rates and found that at low temperatures, the reorientation in the β phase is faster (Ea = 100–116(5) meV for β-, Ea = 286(7) meV for α-Ca(BH4)2), and depends on the changes in the local environment of BH4 groups [29].

3.1. Crystal Structure, Polymorphs, and Phase Transitions

The two most common phases of calcium borohydride are α-Ca(BH4)2 (lattice parameters: a = 8.7782(2) Å, b = 13.129(1) Å, c = 7.4887(9) Å; 300 K) and β-Ca(BH4)2 (a = 6.9509(5) Å, c = 4.3688(3) Å, 433 K). The alpha phase is stable from 0 K to 440 K, while from 440 K, the high temperature phase, β-Ca(BH4)2 becomes dominant. The metastable γ-Ca(BD4)2 crystal structure was reported by combined X-ray and neutron measurements, and was refined in the orthorhombic space group Pbca (a = 7.525(1) Å, b = 13.109(2) Å, c = 8.403(1) Å) [30,31]. The γ phase is less stable than the α phase, and its stability decreases with temperature; the expected phase transitions are expected to follow the trend α-Ca(BH4)2 → γ-Ca(BH4)2 → β-Ca(BH4)2 [30]. There have been some debates regarding the space group of both α and β phases; for instance, DFT computations conducted by Le et al. [32] over the hydrostatic pressure range 0–40 GPa showed that the F2dd space group seems to be the preferred structure of α-Ca(BH4)2 at low temperatures [32]. Crystal structures and total energy have been calculated based on the general gradient approximation (GGA) method for density functional theory (DFT), implemented by the ABINIT simulation package, while Hartwigsen–Goedecker–Hutter pseudopotentials have been used for calculations, and the exchange–correlation interactions were described by the Perdew–Burke–Ernzerhof (PBE) GGA functional [32]. The transition to the beta phase at higher temperatures was supported by calculations performed by Lee et al. [33], who pointed out a vibrational entropy excess of β-Ca(BH4)2 over α-Ca(BH4)2 of 16 J/(mol K). A total of nine possible crystal structures have been proposed in the literature for calcium borohydride: three α phases, one α’ phase, four β phases, and one γ phase [34]. The most agreed-upon structures are, however, the α and β phases [35] (Table 1).
Other reports have dealt with phase variation occurring via desolvation from the adduct Ca(BH4)2.2THF when a mixture of α-Ca(BH4)2 and γ-Ca(BH4)2 polymorphs was observed experimentally [35]. Filinchuk et al. have also reported a second-order transition, α-Ca(BH4)2 (F2dd) → α’-Ca(BH4)2 (I-42d), occurring at ~495 K [35]. The various polymorphs of α-Ca(BH4)2 (prepared by desolvation of commercially available Ca(BH4)2.2THF at 433 K for 1 h) have also been investigated under high pressure (10.4 GPa, room temperature) by Liu et al. [38] by combined Raman and IR spectroscopies, and it was found that the transformations are reversible throughout the pressure region investigated [38]. The authors also pointed out the need to start the compression from phase-pure Ca(BH4)2, otherwise H2 release and interaction in mixtures α-Ca(BH4)2β-Ca(BH4)2 (which are the typical starting point) would further complicate the analysis of lattice, bending, and stretching modes [38]. In order to avoid the α-Ca(BH4)2 → β-Ca(BH4)2 transition, a more conservative approach was proposed by Paolone et al., who heated Ca(BH4)2∙2THF at 125 °C for 20 h (incomplete conversion of THF adduct to α-Ca(BH4)2), followed by the α → α’ structural phase transition at 180–220 °C, and finally the α’→β transformation at 320–330 °C [39]. Monitoring the vibration frequency of Ca(BH4)2 via anelastic spectroscopy measurements, the authors also noted that upon further cooling the sample from 320 °C to room temperature, the borohydride did not undergo any phase transitions, consistent with irreversible transformations occurring during heating (α → α’ → β). Although not detected by DSC, evaluation of the relative variation/decrease of Young’s modulus upon heating can indirectly point out these transitions [39]. Some debate still exists in the literature, as some authors claim another phase in the C2/c space group for Ca(BH4)2 near ambient conditions [40].
Stoichiometric Equations (1)–(7) can describe several possible decomposition pathways for Ca(BH4)2 (Figure 1).
Judging by how widespread dodecahydro-closo-dodecaborates (B12H122−) are during the dehydrogenation of metal borohydrides M(BH4)n, it seemed reasonable to further study the implications of these intermediate species on hydrogen storage properties as well as on the unexpected increase in ionic conductivity of related alkali and alkali-metal compounds [41]. Some representative examples of polymorphs of Ca(BH4)2, mixed anion borohydrides, and dodecaborate anion are depicted in Figure 2.
The thermal stability of MB12H12 is rather high, but a tendency towards amorphization (perhaps of the tentative formula B12H12x2−) has been recorded upon heating to 400–450 °C and further, when anion polymerization may occur (CaByHz)n, but elemental B was not observed even at 750 °C [41].
Jepsen et al. have recently described the DFT-optimized and experimental crystal structures (synchrotron radiation powder XRD data) of Ca(NH3)n(BH4)2 (n = 1, 2, 4, and 6) (Figure 3) [15]. DFT calculations were performed to optimize the structures from the SR-PXD data by using the Vienna Ab-initio Simulation Package (VASP) in the Perdew–Burke–Ernzerhof generalized-gradient approximation [15]. Ca(NH3)4(BH4)2 (P21/c) has a molecular structure connected by dihydrogen bonds, and all ammine complexes release ammonia upon gentle heating (at a low partial pressure of NH3), or H2-NH3 (high partial pressure of NH3). These findings suggest metal ammine borohydrides can be used to store ammonia, or to release H2 by catalytic ammonia splitting [15].
Ca2+ is octahedrally coordinated to six equivalent [BH4] tetrahedra, featuring Ca-B bond distances of 2.82–2.97 Å. The divalent cations in α-Ca(BH4)2 form a close-packed, diamond-type structure where the tetrahydroborate groups exhibit T-shape coordination.
Another interesting intermediate that can be observed during thermal decomposition of Ca(11BD4)2; after initially being described as belonging to “δ-Ca(BH4)2”, it has been properly assigned to Ca3(11BD4)3(11BO3) and its structure was deduced by Riktor et al. based on synchrotron radiation powder XRD and supported by IR measurements [42].
With a decomposition enthalpy in the range 40.6–87 kJ/(mol H2) and an onset decomposition temperature of 278 °C under 1 bar equilibrium pressure, bulk Ca(BH4)2 requires considerable modifications in order to conform to DOE’s restrictions. Overcoming sluggish kinetics, problematic mass transfer, and high reaction enthalpies remain key steps needing further improvement.

3.2. Cation Substitution

Cation substitution has been pursued in dual-cation borohydrides in order to decrease the energy barriers in hydrogenation studies of calcium borohydride [43,44,45,46,47]. For instance, Fang et al. studied the dual-cation (Li, Ca) borohydride LiCa(BH4)3 [43]. The mixed borohydride was synthesized by ball milling a 1:1 mixture of LiBH4:Ca(BH4)2, when an intermediate phase Li0.9Ca(BH4)2.9 was observed to transform into the final stoichiometric mixed-cation borohydride LiCa(BH4)3 upon heating. The dual cation Li0.9Ca(BH4)2.9 was produced most likely by incorporation of dissolved LiBH4 into β-Ca(BH4)2 (Equation (10)).
LiBH 4 + Ca ( BH 4 ) 2   ball   milling   Li 0.9 Ca ( BH 4 ) 2.9 + 0.1   LiBH 4   Δ   LiCa ( BH 4 ) 3
Contrary to the desorption of individual borohydrides, the mixed-phase LiCa(BH4)3 started to desorb H2 at 200 °C, considerably lower than the onset of LiBH4 (400 °C) or Ca(BH4)2 (300 °C), and lost ~9.6 wt% after heating to 500 °C [43]. The Li-Ca-B-H system also bypassed the formation of the [B12H12]2− anion, following a two-step reaction that yielded 1.2 wt% and 8.1 wt% H2, respectively (Equation (11)). The reversibility of the second dehydrogenation step was, however, only moderate, affording 5.3 wt% H2 recharging [43].
LiCa ( BH 4 ) 3   Δ LiCaB 3 H 11 + 1 2 H 2   Δ   LiCa ( BH ) 3 + 9 2 H 2
Jiang et al. observed a new mixed-borohydride adduct when mixing 3 LiBH4:1 CaCl2 in THF, namely, LiBH4·Ca(BH4)2·2THF, which released a mixture of H2-B2H6 upon heating in a complex, 4-step process involving the detection of yet-unknown Li-Ca-B-H phases [44]. Although typically mixing metal halides such as MCln with LiBH4 can yield mixed alkali metal-transition metal borohydrides (Equation (12)), a different reaction pathway was revealed when tuning the CaCl2:LiBH4 molar ration, while also using THF as reaction media.
MCl n   + m   LiBH 4   MLi m n ( BH 4 ) m + n   LiCl
Additionally, LiBH4·Ca(BH4)2·2THF was shown to store reversibly up to 5.63 wt.% H2 in the second a/d cycle [44]. Switching LiBH4 with NaBH4, Mattox et al. have reported the influence of the borohydride starting material on the additive used (40% CaCl2), concluding that NaBH4 might interpenetrate the CaCl2 lattice, expanding it [46]. The hypothesized product was marked as CaNa(BH4)1-xClx, akin to the expected product based on Equation (12).
Lang et al. reported the decomposition of another substituted borohydride, the perovskite-type NH4Ca(BH4)3 (15.7 wt.% hydrogen capacity, Equation (13)) [45]. The origin of the low desorption temperature (65 °C) was found to reside in the destabilization of H+ in NH4+ and H in BH4 [45].
LiBH 4 + Ca ( BH 4 ) 2 + NH 4 Cl   NH 4 Ca ( BH 4 ) 3 + LiCl
The low onset correlates well with the reaction between NH4+ and BH4, which takes place at 85 °C according to Equation (14).
NH 4 + +   BH 4   NH 3 BH 3 + H 2
Upon heating, NH4Ca(BH4)3 releases H2 with the formation of Ca(BH4)2NH3BH3, an adduct that further desorbs ammonia borane AB: NH3BH3 (which undergoes a complex decomposition process also releasing H2), and Ca(BH4)2 that can further react with LiCl to form anion-substituted borohydrides (Equation (15)) [45].
NH 4 Ca ( BH 4 ) 3   H 2   Ca ( BH 4 ) 3 · NH 3 BH 3   + LiCl , AB   Ca ( BH 4 ) x Cl 2 x +   Li ( BH 4 ) y Cl 1 y
It appears that ~3.8 wt% H2 can be released upon heating to 140 °C for 25 min, with very little other impurity gases such as NH3 or B2H6 (Figure 4).
The previous study on NH4Ca(BH4)3 in relation to the proposed destabilization borohydride-ammonium system, was reported by Schouwink et al. [47]. In addition to Equation (13), Schouwink used a novel approach: soft milling Ca(BH4)2 (30 min, 500 rpm) with temperature-stabilized NH4BH4 (cooling at 243 K) (Equation (16)).
NH 4 BH 4 + Ca ( BH 4 ) 2   ether NH 4 Ca ( BH 4 ) 3
The crystal structure of NH4Ca(BH4)3 was also reported with a structural model in space group P-43m [47] (Figure 5).

3.3. Anion Substitution

Grove et al. have studied halide substitution in Ca(BH4)2 by ball milling the mixtures Ca(BH4)2:x CaX2 (X = F, Cl, Br) with various molar ratios (x = 0.5, 1, 2) [48]. The compositions from x = 0–0.6 yield solid solutions of composition β-Ca((BH4)1-xClx)2 as revealed by Rietveld analysis of diffraction data, but there was no substitution of X for BH4 occurring for X = F or Br, possibly due to the positive enthalpy of mixing (X = F) or the lack of orthorhombic-to-tetragonal phase transition (X = Br) (Equation (17)) [48].
x   CaCl 2   + β Ca ( BH 4 ) 2   ( 1 + x )   β Ca ( ( BH 4 ) 1 x Cl x ) 2
Even though DFT calculations predicted Br substitution to be possible, this possibility was experimentally discarded. Additionally, DSC of β-Ca((BH4)0.5Cl0.5)2 showed a higher decomposition temperature compared to pure Ca(BH4)2. Interestingly, no substitution was observed at temperatures below 250 °C or in the case of α-Ca(BH4)2, the only polymorph undergoing this substitution being β-Ca(BH4)2 (Figure 6) [48]. The calculations were performed applying the periodic quantum-mechanical software CRYSTAL09 within the density functional theory, PBE functional [48].
Extending the range of calcium halides used for anion substitution in calcium borohydride, Rude et al. have employed CaI2 in the system Ca(BH4)2–CaI2 and found three new components formed by halide substitution: a solid solution Ca((BH4)1-xIx)2 with x~0.3, the trigonal tri-Ca((BH4)0.70I0.30)2, and the orthorhombic ort-Ca((BH4)0.64I0.36)2 [49]. The hydrogen release occurs from tetragonal Ca((BH4)1–xIx)2 via CaHI; however, the anion substitution strategy affords borohydrides with a decomposition temperature similar to that of Ca(BH4)2 [49].

3.4. DFT Computation and Predictions

Several DFT computation studies have surfaced related to calcium borohydride, from crystal structure under high pressure [50], predictive evaluation of mixed-anion Ca(BH4)(NH2) [51], simulation of the nanosizing effect of Ca(BH4)2 [52], to the inclusion of metal borohydrides in a combined multidisciplinary approach aimed to further adjust hydride materials for vehicular applications [1]. Aidhy et al. have used a combination of density functional theory (DFT) calculations and a Monte Carlo (MC)-based crystal structure prediction tool, the Prototype Electrostatic Ground State (PEGS) method [51]. Albanese et al. have utilized for the theoretical investigation of Ca (BH4)2 the periodic density functional theory (DFT) calculations employing the PBE functionals as implemented in the CRYSTAL program, with the following all-electron basis set used for all the atoms, namely, 865-11G (2d) for Ca, 6-21G (d) for B, and 31G (p) for H [52]. Other studies have investigated the metal electronegativity role in the M(BH4)x–LiNH2 system as a means to enhance dehydrogenation thermodynamics and kinetics in the RHC [53]. Blanchard et al. have combined quasielastic neutron scattering (QENS) and DFT calculations to elucidate the role of hydrogen rotational and translational diffusion in calcium borohydride as an important factor of hydrogen dynamics in crystalline Ca(BH4)2 [54]. The calculations were performed by using the atomic simulation environment (ASE) package, and the DACAPO plane-wave basis-set implementation was used to solve the electronic structure problem within the DFT formalism; the ion cores were described by ultrasoft pseudopotentials, and the exchange and correlation effects were described by the PW91 functional [54].
Alkali-earth metal borohydride stability M(BH4)2 (M=Be, Mg, Ca) has been studied since the 1990s by Bonaccorsi et al. by systematic nonempirical calculations performed on M(BH4)2, HMBH4, and MBH4+, resulting in optimized geometries [3]. Interestingly, the authors employed the effect of electron correlation in the study of the multi-step decomposition leading to borane elimination, an experimental detail observed in the case of many TM-based borohydrides (Equation (18)).
M ( BH 4 ) 2 HMBH 4 + BH 3   MH 2 + 2   BH 3   M + H 2 + 2   BH 3
However, a critical point regarding DFT computations is that the agreement with experimental data must be reasonably good; with this goal in mind, Franco et al. used Quantum ESPRESSO to perform DFT computations on seven simple and complex hydrides (including Ca(BH4)2), and found that coupling the GIPAW (gauge-including projected augmented-wave) ab initio method with solid-state NMR experiments led to a good agreement among all investigated samples [55]. For example, 11B MAS (Hahn echo) spectra of Ca(BH4)2 as a mixture of α and β phases showed corresponding signals at −32.5 pp (β phase) and −29.9 ppm (α phase), and 1H SSNMR data also showed good correlation with prediction shifts (PAW: 0.48 ppm error, QE: 0.35 ppm error) (Figure 7) [55].
There is, however, an important need to develop new systems for energy storage, and Wolverton et al. employed an atomic scale computational approach using Perdew–Wang GGA (generalized gradient approximation) in this regard, with three-fold advances being recorded: the prediction of hydriding enthalpies and free energies, the prediction of favorable decomposition pathways, and the prediction of low-energy crystal structures of complex hydrides [12]. The results suggest that the DFT approach is useful in evaluating potential decomposition pathways, as was exemplified for Li4BN3H10, a metastable phase with three potential decomposition pathways for T < 300 K and thermodynamically possible with ΔG < 0 [12].

4. Characterization and Stability

As previously mentioned, there are several early reports related to the structure and stability of calcium borohydrides [3,35,56]. For instance, Filinchuk et al. [35] and Bosenberg et al. [57] reported the characterization of metal hydride using in situ synchrotron radiation powder X-ray diffraction (SR-PXD). SR-PXD is a powerful tool to track solid-gas reactions, and hydrogenation studies made use of a cell that allows control over wide pressure and temperature ranges (up to 200 bar, 550 °C) [57,58,59]. The authors reported that β-Ca(BH4)2 and MgH2 were observed when a mixture of CaH2-MgB2 was heated, as well as the β → α phase transition of calcium borohydride upon cooling [57]. Dematteis et al. deduced the heat capacities and thermodynamic properties of complex hydrides, which will further ease the thermodynamic data computations (ΔS, ΔH and ΔG) [60].
Other physical characterization methods include inelastic neutron scattering [61], high-resolution laser excitation spectroscopy (albeit for hypothetical, gas phase CaBH4 molecule) [62], vibrational spectra (IR and Raman, for α-, β-, and mixed (β,γ)-Ca(BH4)2) [38,63,64], solid-state NMR (1H, 11B MAS for α-, β-Ca(BH4)2 [29,55,65], confirmation of [B12H12]2− species during borohydride dehydrogenation [66], 11B spin-lattice relaxation [67], analysis of RHCs based on calcium borohydride like LiBH4–Ca(BH4)2 [68]), and various DFT computations (excluding CaB2H2 as dehydrogenation intermediate, while favoring CaB12H12 as an intermediate, as argued by Frankcombe, Equation (19) [69], or evaluating [Ca(BH4)2]n=1–4 clusters for hydrogen storage by Han et al. [70]).
Ca ( BH 4 ) 2 1 6 CaB 12 H 12 + 5 6   CaH 2 + 13 6   H 2     1 3   CaB 6 + 2 3   CaH 2 + 10 3   H 2
Hattrick-Simpers et al. described the use of high-throughput backscattering Raman spectroscopic measurements for combinatorial in situ Raman spectroscopy of Ca(BH4)2 and other RHCs (up to 10 Mpa and 823 K) [71].
The stability of Ca(BH4)2 has been investigated in many reports, including Miwa et al. [72], Nakamori et al. [73], Riktor et al. [58], and others. Since the decomposition of complex hydrides is related to their hydrogen storage capacity, these results will be discussed in Section 5.

4.1. Neat vs. Nanoconfined

Several aspects of nanosizing are regarded as beneficial for enhancing de-/rehydrogenation of complex hydrides and related RHCs. For instance, when impregnating LiBH4 into ferrite-catalyzed-graphene, a synergistic effect was observed and an important reduction in activation energy was recorded [74].

4.2. DFT Simulation of the Nanosizing Effect

Albanese et al. have studied the nanostructuring of β-Ca(BH4)2 by atomistic thin film models to mimic nanosizing effects (DFT, using CRYSTAL software) in order to evaluate the reduction of dehydrogenation enthalpy of calcium borohydride, showing that for thickness in the range 5–20 Å, the reduction on enthalpy reaches 30–35 kJ/mol H2, making calcium borohydride an appealing candidate for hydrogen storage under nanosizing conditions [52]. The decomposition pathways for Ca(BH4)2 have been investigated by Zhang et al. by means of DFT calculations of free energy, predicting a new CaB2H6 compound as a decomposition intermediate [75]. Other groups have evaluated the effectiveness of DFT computations in establishing metastable reaction pathways [76].
Dual-cation borohydrides of form M1M2(BH4)3(NH3)2–6 have been investigated by Emdadi et al., finding several potentially promising new solvates and stable alloys, including M2=Ca [77]. Nanosizing was invoked in the case of LiBH4–Ca(BH4)2 RHC when catalyzed by LaMg3, with an improvement registered in hydrogenation behavior due to nanoparticulate alloy addition affording a dehydrogenation onset at ~200 °C, 100° lower than pristine LiBH4–Ca(BH4)2 and rehydrogenation proceeding at 150 °C [78].
Guo et al. performed first-principles calculations on the system KBH4/Ca(BH4)2 and proposed three new reactions in the mentioned RHC (20), (21) and (22), two of them involving the formation of the [B12H12]2− anion [79,80].
KBH 4 + 5 2   Ca ( BH 4 ) 2   1 2   K 2 B 12 H 12   + 5   NaH + 13 2   H 2 ;   6.0   wt %   t c = 64   ° C
KBH 4 + 1 10   Ca ( BH 4 ) 2   1 10   CaB 12 H 12   + KH + 13 10   H 2 ;   4.3   wt %   t c = 58   ° C
KBH 4 + 1 4   Ca ( BH 4 ) 2   1 4   CaB 6 + KH + 5 2   H 2 ;   7.0   wt %   t c = 105   ° C
Moreover, Kulkarni et al. described by first principles the dodecaborane, amorphous phase CaB12H12 as a result of an overlap of structurally distinct crystallites [81].
Huang et al. performed first-principles calculations involving Cr-doping of α-Ca(BH4)2 and found half-metallic behavior (0.98 eV for Ca doped sites, 0.63 eV for B) useful for further use in spintronic devices [82]. Computational thermodynamics coupled with CALPHAD (CALculation of PHAse Diagram) modeling revealed the formation of Ca(BH4)2 as a result of Ca addition on LiBH4, along with CaB6, which significantly reduces the H2 releasing temperature of the system [83].
The utilization of Ca(BH4)2 in electrolyte solution for Ca-ion batteries has been shown feasible by a computational approach called AIMD (ab initio molecular dynamics) performed on ether- and ester-based electrolytes on a calcium anode, revealing an aspect that was confirmed experimentally, namely, that the best combination for an electrolyte solution is that of Ca(BH4)2 and the polar, aprotic THF solvent [84]. Calcium borohydride (Ca(BH4)2) has shown potential for use in both lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) due to its high hydrogen storage capacity and favorable electrochemical properties. Regarding LIBs, it must be noted that Ca(BH4)2 can release H2 through a reversible electrochemical process, enabling its use as an energy and H2 reservoir within the battery. The high capacity of Ca(BH4)2 for Li storage (by theoretical calculations, one mole of Ca(BH4)2 can potentially store four moles of lithium, leading to a high energy density) makes it an attractive solution for LIB applications. However, compared to Mg(BH4)2, which afforded 540 mAh/g (first discharge)/250 mAh/g (recharge), the reported values for Ca(BH4)2 were lower. With good chemical stability, calcium borohydride showed good stability during lithium insertion/extraction, therefore showing good potential for long term cycling stability. Several downsides should be addressed, such as developing the proper electrolyte system that would afford better overall battery performance and stability. As with LIBs, SIBs show similar advantages and downsides for Ca(BH4)2 use in battery applications.
Mikhailin et al. employed potential energy surfaces along minimal energy pathways to study the decomposition of M(BH4)2(NH3)1–2 and the [M(BH4)2(NH3)2]2 dimers (M=Be, Mg, Ca, and Zn) using the B3LYP method (DFT). The dimeric structures have almost no effect on the energy barriers, while coordination of the ammonia molecule to the metal center reduced these barriers considerably, affording dehydrogenation at temperatures much lower than those of their corresponding pristine counterparts [85]. The potential decomposition pathways of calcium ammine borohydride complexes would lead to Ca(BH2NH)(BHN), Ca(BH4)(BHN) or Ca2(BH4)2(NH3)2(BH3NH2)2 in the case of the dimer [85]. The case of decomposition of Ca(BH4)2·NH3 was further investigated by Zhang et al. by means of DFT and showed a reasonable agreement with decomposition in the range 190–250 °C, releasing 11.3 wt% hydrogen [86]. Notably, the interaction N-H…H-B was found to play an essential role in favoring H2 release, recognizing the unique role of dihydrogen bonding in H2 release from solid compounds such as complex hydrides and their adducts [86]. Various complementary predictions have been made on the basis of thermodynamic computations by Siegel et al. [87], and Vajeeston et al. [88,89].

4.3. Destabilization Strategies

Destabilization of boron-based materials can be achieved by using additives (various compounds based on Mg, B, Al, C, S, Ti, intermetallics, etc.), chemical modifications, and nanosizing effects [90,91,92,93,94,95,96,97,98]. Various RHCs have been employed, and most showed improved desorption temperatures compared to pristine components; for instance, Mg(BH4)2/Ca(BH4)2, Mg(BH4)2/CaH2, or Mg(BH4)2/CaH2/3NaH (Equations (23)–(25)) (Figure 8) [92,94].
5   Mg ( BH 4 ) 2 +   Ca ( BH 4 ) 2     CaB 12 H 12 + 5   MgH 2 + 13   H 2
3   Mg ( BH 4 ) 2 +   CaH 2   3   MgH 2 + CaB 5 + 10   H 2
3   Mg ( BH 4 ) 2 +   CaH 2 + 3   NaH   3   NaMgH 3 + CaB 5 + 10   H 2
Ibikunle et al. also studied the RHC Mg(BH4)2/Ca(BH4)2 and found the H2 release to be diffusion- and phase boundary-controlled (Figure 9) [94,95].
An extension of the destabilization strategy proposed by Durojaiye in 2010 [92] was employed by Huang et al. in 2016, who used Mg(AlH4)2 in order to destabilize Ca(BH4)2 instead of magnesium borohydride [93]. The destabilization effect of magnesium alanate Mg(AlH4)2 in the 1 Mg(AlH4)2:1 Ca(BH4)2 RHC was quantified and shown to exhibit a 10-fold faster desorption at 300 °C (0.337 wt.% H2/min) compared to pristine Ca(BH4)2 (2.6 wt% H2 at the same temperature), releasing 8.4 wt% H2 at 330 °C in a three-step reaction (with release peaks at 130, 280, and 310 °C) [93]. A key step in the enhancement observed was attributed to the formation of the Al (Mg) solid solution, effectively destabilizing Ca(BH4)2 [93].
Destabilization in the system NaBH4/Ca(BH4)2 has also been reported (Equation (26)), and the TPD release curves were altered by a decomposition product of Ca(BH4)2; the system behaved reversibly under ~5.5 Mpa H2 at 400 °C when recharged for 10 h [97].
4   NaBH 4 +   Ca ( BH 4 ) 2   4   Na +   CaB 6   + 12   H 2 ;   10.9   wt %   H 2
An important aspect of the reversibility reaction of many metal borohydrides is the regeneration of B–H bonds present in the [BH4] anion [90]. This issue was tackled by Cai et al., who proposed a general route potentially applicable to M(BH4)2 as well (M=Mg, Ca), based on the reaction of SiB4 with MHn=1–2 to form M(BH4)n=1–2 under mild conditions in the case of LiBH4 (10 Mpa, 250 °C) (Equation (27)). The chemical bonding of boron in the reagent (SiB4) was found essential for the best hydrogenation performance; for comparison, the synthesis of LiBH4 from the elements requires 700 °C, while the mildest conditions are required by the solid-gas reaction between LiH and B2H6, yielding LiBH4 at 120 °C.
SiB 4 + 4   LiH + 6 H 2   4   LiBH 4 + Si   ;   Δ H = 65 kJ molH 2

5. Hydrogen Storage Properties

Various aspects related to hydrogen desorption behavior have been investigated in recent years, from the detailed decomposition of pristine calcium borohydride by Kim et al. [99,100], to the kinetic and thermodynamic investigation of Ca(BH4)2 decomposition [101], to the further decomposition behavior of the CaB12H12 intermediate [66] or the debated formation of the CaB2Hx intermediate reported by Riktor et al. [102]. Other reports shed light on the H2 backpressure effect [100] or the dehydrogenation temperature used (350 °C vs. 400 °C), which can allow different pathways for the decomposition reaction (via CaB2H6 at 350 °C, or via B and CaH2, leading to CaB6 when heated at 400 °C) [103,104]. Further experimental details related to the H2 desorption potential of Ca(BH4)2 will be discussed in the following sections (Section 5.1,Section 5.2,Section 5.3,Section 5.4,Section 5.5 and Section 5.6), as well as the applicability of calcium borohydride to ion conductivity studies and applications (Section 5.7).

5.1. Bulk Ca(BH4)2

As early as 2008, Aoki et al. investigated the hydrogen release properties of Ca(BH4)2 and reported the phase transition (LT, low temperature, to HT, high temperature), while also identifying an unassigned intermediate compound and CaH2 as the final phase identified by XRD data analysis [105]. Decomposition of pristine calcium borohydride has been thought and further demonstrated to proceed through the formation of CaB12H12 [14,66] or another intermediate type of the general formula CaB2Hx (DFT computations) [34]. A pressure investigation performed on a mixture α- and β-Ca(BH4)2 led George et al. to identify a novel phase in which the β– phase transforms when reaching 10.2 Gpa, but no α-to-β transition was observed around 5.3 Gpa, as theoretically predicted [106]. Other DFT computations were run on [Ca(BH4)2]n=1–4 clusters and found the smallest dissociation energy to belong to the tetramer [Ca(BH4)2]4, indicative of it being the better hydrogen storage candidate among the investigated clusters [70].
The dehydrogenation of Ca(BH4)2 and that of the destabilized system Ca(BH4)2–MgH2 were investigated by Kim et al., highlighting the essential role of the CaB6 product in achieving system reversibility [107], as corroborated by the findings of Sahle et al. a few years later [108]. It was also suggested that CaB6 forms as the final product of the reaction from intermediate phases likely containing B and CaH2 [108].
As for the decomposition of CaB12H12, the decomposition pathway differs from that of Ca(BH4)2, producing no CaB6, and can be described by Equation (28) [14]:
CaB 12 H 12   x 2 H 2   CaB 12 H 12 x z n H 2 1 n   ( CaB y H z ) n
Moreover, although there are conflicting reports regarding the role of CaB12H12 in the dehydrogenation of Ca(BH4)2, it seems unlikely that the dodecaborate is a stable dehydrogenation intermediate [14,37].

5.2. Nanoconfined

Nanoconfinement remains a viable route for tuning the thermodynamic parameters of hydrogen storage materials and of Ca(BH4)2 in particular, with several reports describing marked improvements over the pristine compounds’ thermal behavior [68,74,96,109,110,111,112,113,114,115,116,117,118]. Some of the most important characteristics of these systems are summarized in Table 2. A porous borohydride, CaB2H7, was obtained by heating Ca(BH4)2 with Ti(Oet)4 at 160 °C for 1 h and showed improved release and uptake of hydrogen (Equation (29)) [112].
Ca ( BH 4 ) 2 + 0.1   Ti ( OEt ) 4   160   ° C ,   1   h   CaB 2 H 7   0.1 TiO 2 + 0.2   C 2 H 6 + 0.2   C 2 H 6 O + 0.2   H 2
The desorption of the porous borohydride occurred in the 300–420 °C interval, following a two-step dehydrogenation pathway (Equation (30)) [112].
CaB 2 H 7 300   ° C ,   1   h 0.6   CaB 2 H 4 + 2 15 CaB 6 + 4 15 CaH 2 + 3.8   H 420   ° C ,   1   h 1 3 CaB 6 + 2 3 CaH 2 + 5.7   H
When LiBH4 is used in the eutectic mixture 0.7 Li(BH4)2-0.3 Ca(BH4)2 displaying eutectic melting, the reversible coupled RHC system obeys the reversible pathway described by Equation (31).
4   LiBH 4 +   Ca ( BH 4 ) 2     CaB 6 + 4   LiH + 10   H 2
Other studies have focused on the infiltration and interaction between the eutectic LiBH4–Ca(BH4)2 and mesoporous scaffolds (CMK–3, MCM–41) by means of NMR investigation, including the lack of surface modification of the inert carbon scaffold CMK-3, as revealed by 13C MAS and 1H-13C CPMAS (cross-polarization MAS) spectra [68]. Other studies have used mesoporous silica as the host (MCM–41 and SBA–15) for LiBH4–Ca(BH4)2 infiltration and found that the mesoporous structure of the scaffold was altered even with low-energy ball milling [115] or reported on the infiltration of the same RHC into mesoporous carbon [116]. One should also note the affinity of the borohydride for the SiO2 and surface silanol groups, which may lead to another side reaction [115].

5.3. Additives Used to Improve H2 Storage Features of Calcium Borohydride

While still holding the highest theoretical hydrogen storage capacity of all known materials, complex hydrides still face many shortcomings related to sluggish kinetic and/or thermodynamic barriers that are hard to bypass. To tackle this issue, several additives have been used, with important improvements recorded: MgB2 yielded Ca(BH4)2 + MgB2 composite with 8.3 wt% gravimetric hydrogen capacity [119]; Co ion-loaded poly(acrylamide-co-acrylic acid) (p(Aam-co-Aac)-Co) hydrogel when a decrease in conversion rate from 98.5 to 83.5% was reported over ten a/d cycles [120,121]; urea yielded a new complex hydride Ca(BH4)2·4CO(NH2)2, releasing 5.2 wt% hydrogen below 250 °C [122]; 5 wt% CoCl2 doping in Ca(BH4)2-4LiNH2 afforded catalyzed RHC (Co NPs formed during ball milling as active catalytic species), releasing 7 wt% hydrogen at 178 °C [123]; CoCl2-doped Ca(BH4)2·2NH3 desorbed at 200 °C, releasing 7.6 wt% H2 with high purity [124]; Mg(BH4)2 yielded corresponding RHCs Mg(BH4)2/Ca(BH4)2 [92], MgF2 in Ca(BH4)2-MgF2 reversible system [125]; in situ generated TiO2 to afford novel CaB2H7/0.1TiO2 catalyzed hydride system [112]; particulate LaMg3 additive in the RHC Ca(BH4)2 + LiBH4 [78]; DFT investigation on Cr-doped α-Ca(BH4)2 [82]; TiF3 introduced via ball milling in Ca(BH4)2 [126]; NbF5 doping in Ca(BH4)2 + MgH2 [127]; Nb and Ti doping on Ca(BH4)2 investigated by DFT [128]; Co-Ni-Fe-P alloy as hydrolysis catalyst [129]; TiCl3 doping of Ca(BH4)2 yielded 2 wt% H2 reversible at 300 °C and 9 Mpa, while also suppressing borohydride melting [103]; 10 mol% addition of CaX2 (X=F, Cl) produced Ca(BH4)2–CaX2 mixtures, where CaX2 changed the dehydrogenation pathway [130].
Other reports deal with the additive role of calcium borohydride in the Mg(NH2)2–2LiH–0.1Ca(BH4)2 system, yielding CaH2 and LiBH4 during ball milling to account for 4.5 wt% storage capacity [131]; TiF4 and NbF5 doped in Ca(BH4)2-MgH2 RHC, where a slight improvement was seen in the case of NbF5 doping, and there was evidence of CaB12H12 formation along with the formation of active metal boride nanoparticulate species TiB2 and NbB2 (XANES data) with partial reversibility reported [132,133,134]; Ti(OiPr)4 and CaF2 [134]; TiF3 additive in the 2NaAlH4 + Ca(BH4)2 system generating Al3Ti and CaF2 with a synergistic catalytic role and affording improved a/d kinetics [135]; investigation of the boron effect in the Ca(BH4)2 + B system, showing only minor differences in the maximum rate of H2 evolution [136]; CaF2 replacement of CaH2 in CaH2 + MgB2 yielding the nonstoichiometric CaF2–xHx solid solution with a direct influence over a/d behavior of the system [137]; TiX3 (X=F, Cl) showing a superior effect of TiF3 on reversibility (57% yield) but not of TiCl3, despite their electronic similarities [138].
The catalytic role of additives in the synthesis of calcium borohydride from caB6 and CaH2 was confirmed by Ronnenbro et al. utilizing RuCl3 or, even better, mixtures of TiCl3 + Pd [11]. Another synthesis route of calcium borohydride starts from the CaH2 + MgB2 system, where the beneficial role of the boride additive AlB2 was reported by Schiavo et al. [139], or the role of fluoride-based additives in the same CaH2 + MgB2 system, as shown by Suarez-Alcantara et al., who investigated the systems 9CaH2 + CaF2 + 10MgB2, 10Ca(BH4)2 + 9MgH2 + MgF2, and 9Ca(BH4)2 + Ca(BF4)2 + 10MgH2 [59].

5.4. RHCs—Reactive Hydride Composites Containing Ca(BH4)2 or Its Precursors

There have been many attempts to catalyze Ca(BH4)2 and its RHCs; however, not all of them showed palpable results. For instance, while MgH2 was considerably improved using Ni (5 wt%) and Ni5Zr2 (5 wt%) catalysts, these catalysts showed very little influence over Ca(BH4)2 [140]. On the other hand, the system Ca(BH4)2–MgH2 was shown to produce the species Ca[B12H12] [141]. The same Ca(BH4)2–MgH2 RHC (10.5 wt% H2 theoretical, 6.4 wt% H2 desorbed within 3 h) has been characterized when catalyzed by 0.05 mol TiF4 and NbF5, yet the catalyst employed did not suppress dodecaborane anion formation, and only some minor improvements were recorded in the case of the NbF5 catalyst’s usage of TM fluorides, as shown by XANES data to form TM boride nanoparticles acting as active catalysts [132]. Minella et al. have also extended the study of Ca(BH4)2–MgH2 RHC by using Mg, which behaves as an adjuvant for heterogeneous nucleation of CaB6 formed during desorption [142]. CaB6 formation was also reported in the RHC 6LiBH4 + CaH2 (11.7 wt% theoretical), which afforded 9.1 wt% reversible storage capacity when catalyzed by 0.25 mol TiCl3 [143,144]. Other authors have investigated the role of the boron source (SiB4, FeB, and TiB2) on the production of Ca(BH4)2 from the hydrogenation of SiB4 and CaH2 [90].
The hydridic–protic interaction has been exploited in a series of RHCs containing borohydride–amide mixtures [145,146]. When the 1:4 molar ratio mixture Ca(BH4)2-4LiNH2 was ball milled, Equation (32) occurred, leading to the formation of Ca(NH2)2, LiBH4, and Li3(NH2)2BH4, affording a 26.2% reduction in EA compared to LiBH4-2LiNH2 chosen as reference [53].
Ca ( BH 4 ) 2 + 4   LiNH 2   Li 3 ( NH 2 ) 2 BH 4 + LiBH 4 + Ca ( NH 2 ) 2
Other reports describe a slightly different reaction pathway for the system Ca(BH4)2-4LiNH2, corresponding to Equation (33) [147]. The formation of mixed anion Ca(BH4)(NH2)2-x was also found by Morelle et al. in the RHC system Ca(BH4)2–xNaNH2 (x = 1, 2, and 3), showing release of NH3 (300 °C) and H2 (above 350 °C), and a decreasing decomposition temperature as the amide fraction of the composite increased, which advocates for the destabilization of starting Ca(BH4)2 by the amide compound (NaNH2) [148].
Ca ( BH 4 ) 2 + 4 LiNH 2 Li 4 BN 3 H 10   + [ Ca ( BH 4 ) ( NH 2 ) ] 1 4 LiCa 4 ( BN 2 ) 3 + 5 4   Li 3 BN 2 + 8 H 2
Other molar ratios of Ca(BH4)2-xLiNH2 have also been investigated, for instance, for x = 3 [149]. The intramolecular destabilization in the systems Ca(BH4)2-MNH2 (M=Li, Na) has also been evidenced by Poonyayant et al., when a metathetical reaction led to the formation of cation-mixed, anion-mixed complex hydrides of type mCa(BH4)2(NH2) with a release profile starting at ca. 150 °C through hydridic-protic interactions in a two-step release accounting for 9.3 wt% [98]. The Mn(BH4)2–CaH2 mixture system (1:1 molar, manual grinding) led to the more stable Ca(BH4)2 through a double exchange reaction, which effectively suppressed the formation of diborane in the utilized system [150].
Investigation of the Ca(BH4)2-2.5 Mg2NiH4 RHC was shown to proceed through three different pathways, depending on the hydrogen backpressure, which again highlights the aspects of chemical equilibrium (Equations (34) - 2.3 wt%, (35) - 4.2 wt%, (36) - 4.6 wt%) [151]. At 1 bar, the reaction proceeds via Equation (36), whereas at 20 or 50 bar, all three reactions occur simultaneously.
Ca ( BH 4 ) 2 + 2.5   Mg 2 NiH 4   0.25   Ca 4 Mg 3 H 14 + MgNi 2.5 B 2 + 3.25   MgH 2 + 4 H 2
Ca ( BH 4 ) 2 + 2.5   Mg 2 NiH 4   0.25   Ca 4 Mg 3 H 14 + MgNi 2.5 B 2 + 3.25   Mg + 7.25   H 2
Ca ( BH 4 ) 2 + 2.5   Mg 2 NiH 4     CaH 2 +   MgNi 2.5 B 2 + 4   Mg   + 8   H 2
The influence of alkali metal amides on the desorption properties of calcium borohydride was studies by Chu et al. [152], concluding that the driving force of reactions (37) and (38) must lie in the interaction B–H and N–H present in the milled RHC M(NH2)2–Ca(BH4)2 (M=Mg, Ca).
Ca ( BH 4 ) 2 + 2   Mg ( NH 2 ) 2   1 3   [ Ca 3 Mg 6 ( BN 2 ) 6 ] + 8   H 2
Ca ( BH 4 ) 2 + 2   Ca ( NH 2 ) 2   1 3   Ca 9 ( BN 2 ) 6 + 8   H 2
Binary, ternary and quaternary mixtures in the LiBH4-NaBH4-KBH4-Mg(BH4)2-Ca(BH4)2 system have been investigated by Dematteis et al. showing that new phases occurring might originate from the interaction Mg(BH4)2-Ca(BH4)2 [153,154,155]. This interaction in the Mg(BH4)2-Ca(BH4)2 was studied in the 5:1 molar ratio showing a destabilization effect of Ca(BH4)2 over Mg(BH4)2 [94,95].
Another improvement strategy regarding H2 storage performance is utilization of eutectic mixture, and 0.68 LiBH4-0.32 Ca(BH4)2 is a known and tested model system [156,157]. Interestingly, the eutectic mixture of borohydrides can release H2 below each individual components, and even a release onset below the melting of pristine complex hydride [157]. When used as an additive, Ca(BH4)2 can produce very notable improvements; Mg(NH2)2–2LiH–0.1Ca(BH4)2 for instance can release 4.5 wt% H2 with onset at 90 °C, and a rehydrogenation temperature of 60 °C [131]. The synergistic effect of the formed CaH2 and LiBH4 probably played a role in the improvement recorded [131]. Li et al. have studied the system 2LiNH2–MgH2xCa(BH4)2 with a very low desorption onset of 80 °C and a 8.2 wt% hydrogen storage content (x = 0.3); the system 2LiNH2–MgH2–0.1Ca(BH4)2 was also studied. [158]. It’s worth noting that a tentative metathesis reaction of NaBH4 and CaCl3 did not yield the expected Ca(BH4)2 product, and il led to CaNa(BH4)1-xClx instead, which forms as a result of a diffusional process of one reagent into the other [46].
Other additives potentially leading to calcium borohydride have also been employed: 2LiBH4–CaH5 (featuring a lower 1.1 wt% storage capacity at a lower temperature of 270 °C) [159]. A combination of different complex hydrides (alanate–borohydride) was proposed by Moller et al., namely, NaAlH4 + Ca(BH4)2 [160]. This approach led to NaBH4 and Ca(AlH4)2 through a metathesis reaction, releasing ca. 6 wt% H2 below 400 °C [160]. The system NaAlH4 + Ca(BH4)2 has been investigated mechanistically by Mustafa et al., who identified various decomposition intermediates in the multi-step desorption of the RHC, like CaAlH5, Al, CaH2, Al4Ca, and Al2Ca (Figure 10) [135,161].
The ternary system Ca(BH4)2–LiBH4–MgH2 was reported to exhibit long-term cycling stability [162], the precursors CaH2–MgB2 thin films allowed identification of individual steps leading to the formation of Ca(BH4)2 [163], the effect of MgF2 additive was studied in the system Ca(BH4)2-MgF2 with up to 5.8 wt% H2 uptake at 330 °C after 2.5 h during the first three a/d cycles [125], the formation of β–Ca(BH4)2 in the γ-Mg(BH4)2-CaH2 RHC system [164], LaMg3-catalyzed Ca(BH4)2–LiBH4 with H2 release onset at 150 °C and good capacity retention of 70% after five a/d cycles [78], or DFT investigations in the KBH4–Ca(BH4)2 system [79].
When MgH2 is used, destabilization occurs, and a system with a total H2 capacity of 9.1 wt% is generated (Equation (39)) [165]. This type of reactivity was also proven by Kim et al., who demonstrated the reversibility of the Ca(BH4)2+MgH2 RHC, where the formation of CaB2 boride seems to be essential while the formation of a-B is a major downside [107].
Ca ( BH 4 ) 2 + MgH 2   2 3   CaH 2 + 1 3   CaB 6 + Mg + 13 3   H 2 ;   9.1   wt %   H 2
Other RHCs like LiBH4-Ca(BH4)2 were investigated over a wide compositional domain by Lee et al. in xLiBH4+(1-x)Ca(BH4)2 (0 < x < 1) [166], by Yan et al. in the eutectic composite LiBH4+Ca(BH4)2 [167], or when nanoconfined in mesoporous scaffolds of type CMK-3 [68,116,118] or SBA-15 as support, with enhanced interface interaction [115]. In fact, combining nanocatalysts (NbF5) and nanoconfinement (CMK-3 ordered mesoporous carbon) afforded a total capacity of 13.3 wt% for the system LiBH4–Ca(BH4)2 in the resulting nanocomposite 0.68LiBH4–0.32Ca(BH4)2–0.05NbF5@CMK-3 [118].
xNaBH4-(1-x)Ca(BH4)2 composite, however, unlike the Mg(BH4)2 counterpart, yielded no eutectic melting, hence there was no improvement over H2 release temperature [168]. Employing organic borohydrides, such as guanidinium borohydride (GBH), led to a marked improvement in the Ca(BH4)2/C(NH2)3]+[BH4] coupled system, which suppressed ammonia release and afforded ca. 10 wt% H2 evolution between 60 and 300 °C [169].
There have been attempts to increase the stability of Al(BH4)3 (volatile), and compounding it with Ca(BH4)2 by Titov et al. was one of them, leading to the formation of the complex Ca(BH4)2·Al(BH4)3 (slow reaction: 3–4 days) with a postulated structure [Ca(BH4)]+[Al(BH4)4], in line with experimental IR data [24]. Some representative RHCs and their hydrogen storage performance are summarized in Table 3.

5.5. Adduct Formation and Dihydrogen Bonding in Solvates

From molecular insights into the dihydrogen bonding in borohydride ammoniates [174] to DFT modeling of chemical bonding [175,176] or the dehydrogenation mechanism in Ca(BH4)2·2NH3 [177,178], as well as experimental confirmation of the theoretical aspects [179], various adducts of Ca(BH4)2 have been described and investigated [39,179,180], including calcium tetradecahydroundecaborate Ca(B11H14)2·4Dg (Dg = diglyme) [181]. The first ammoniate of calcium borohydride was synthesized and characterized by Chu et al., who isolated it by direct synthesis from components Ca(BH4)2·2NH3 (space group Pbcn) featuring longer B–H and N–H bond lengths due to dihydrogen bonding, which facilitates the release of 11.3 wt% H2 at 250 °C [182]. DFT studies performed on Ca(BH4)2·2NH3 reveal that the Ea of 1.41 eV for thermal decomposition can be tuned by using metal dopants (Fe, Co, and Ni) that can modify the Fermi level [183]. While the adduct Ca(BH4)2·NH3 typically releases NH3 upon heating at 162–250 °C, when a multi-cation strategy was employed by Tang et al., the RHC Ca(BH4)2·NH3/LiBH4 released 12 wt% H2 of high purity (>99%) at 250 °C [184]. The reaction leading to such a behavior was a two-step process described by Equation (40) [184].
Ca ( BH 4 ) 2 · NH 3 + LiBH 4   3 H 2 [ LiCaNB 3 H 9 ] 7 2 H 2 1 4 [ Li 4 B 10 H 6 ] + 1 4 Ca 3 B 2 N 4 + 1 4 CaH 2
Furthermore, the addition of Mg(BH4)2 over Ca(BH4)2.nNH3 (n = 1, 2, 4) revealed an increase in H2 purity and storage capacity in the system Ca(BH4)2.4NH3–Mg(BH4)2, a synergy that inhibited ammonia release to yield >99% pure H2 upon thermal treatment at 500 °C [179].
Extending the scope of borohydride adducts of calcium borohydride, Li et al. studied the hydrogen release properties of calcium borohydride hydrazinates Ca(BH4)2.nN2H4 (n = 1, 4/3, 2, 3). Among these compounds, the monohydrazinate Ca(BH4)2.4/3 N2H4 showed 10.8 wt% H2 release with a dehydrogenation temperature of 140 °C when utilizing an Fe-based catalyst (2 wt% FeCl3) [185].
Other complexes have also been reported, such as the AB (ammonia borane) adducts of complex hydrides, mainly Ca(BH4)2(NH3BH3)2 and its lithium counterpart [186]. The AB adduct of calcium borohydride showed partially reversible behavior (2.4 wt%) under moderate conditions (82 bar H2, 400 °C), with a release of 11 wt% H2. Guadinate adducts of metal borohydrides have been reported by Wu et al. in the case of Li, Mg, and Ca complex hydrides MBH4·nCN3H5 [187]. The low contamination of gaseous decomposition products with ammonia and diborane was based on the C–N bonds of guanidine and H anion of borohydrides. Ca(BH4)2·2CN3H5 released about 10 wt% H2 under modest conditions [187].

5.6. Reversibility Assessment

As a key element in advancing hydrogen storage materials, reversibility remains one of the toughest problems in solid-state energy storage [11,104,188,189,190]. The influence that metal fluorides exert on Ca(BH4)2 dehydrogenation/rehydrogenation was studied in the case of VF4, TiF4, and NbF5 [133]. Several systems were investigated and showed various degrees of reversibility, such as pristine [191] and TiF4- and NbF5–catalyzed Ca(BH4)2–MgH2 [132], Ca(BH4)2–Mg2NiH4 [151,192], LiBH4–Ca(BH4)2 [166], ternary RHCs with higher reversibility Ca(BH4)2–LiBH4–MgH2 [162], Ca(BH4)2–MgF2 [125], the implications of CaB12H12 on cycling capacity of calcium-based RHCs [41,193,194,195], eutectic mixture LiBH4–Ca(BH4)2 [156], the use of catalyst (TiCl3 [196]; TiF3, NbF5, NbCl5 [197]) on the reversibility of Ca(BH4)2 (rehydrogenation under 90 bar H2, 623 K, 3.8 wt% H2) [196].
The catalytic effect of Ca(BH4)2 was explored in a RHC Mg(NH2)2–2LiH–0.1Ca(BH4)2 composite (4.5 wt% H2 reversible, onset at 90 °C and release at 140 °C, and rehydrogenation with onset at 60 °C) [131]. Other RHCs serve as precursors for calcium borohydride, and these systems already showed promising results regarding cycling behavior, like LiBH4–Ca(AlH4)2 (4.5 wt% reversible, 450 °C) [198]. TiX3 (X=F, Cl) has been successfully used to lower the thermodynamic barriers of Ca(BH4)2 [138] or in RHCs 6LiBH4–CaH2 systems [143]. A critical step for the reversibility of Ca(BH4)2 is the formation of CaB6, presumably from a CaB2H6 precursor that can only be produced between 320 and 350 °C; otherwise, amorphous B would result, which is a clear bottleneck in the cycling pathway of calcium borohydride [104,199].

5.7. Ionic Conductivity

Ionic conductivity [173,200,201,202,203,204,205,206,207] and superconductivity [208,209] have also been investigated recently, with some surprising results and direct implications in the applicability of Ca(BH4)2 in electrolytes for batteries or even as high-temperature superconductors when properly doped, as DFT computations have revealed.
It has been hypothesized that Ca2+ might be too big for migration in Ca-based electrolytes, preferring an octahedral coordination by binding six ligands, but recent reports imply that using weaker coordinating ligands, such as B12H122− might increase Ca2+ mobility.

6. Ca(BH4)2 in Organic Synthesis and Organometallic Chemistry

6.1. Catalyst/Initiator

Calcium borohydride extends its utility in organic synthesis as well. For instance, the THF adduct [Ca(BH4)2(THF)2] was prepared from Ca(oMe)2 and BH3.THF reacting in THF solvent and used for the polymerization ε-caprolactone and L-lactide in a combined DFT and experimental investigation [210]. The active catalysts employed were produced by the treatment of [Ca(BH4)2(THF)2] with KCp* (Cp* = (η5-C5Me5)) and K{(Me3SiNPPh2)2CH}, namely producing dimeric heteroleptic mono-borohydride derivatives [Cp*Ca(BH4)(THF)n]2 and [{(Me3SiNPPh2)2CH}Ca(BH4)(THF)2] (Figure 11). These heteroleptic borohydrides were used as initiators for the ring-opening polymerization (ROP) of ε-caprolactone and L-lactide in good yields [210].

6.2. Reducing Agent

Borohydride compounds can typically be used for reduction reactions, given their [BH4] anion-reducing characteristics. For example, Ca(BH4)2 afforded the reduction of the corresponding organic precursor to β-vanillyl-γ-butyrolactone [211] (Figure 12).
The reduction of β–keto esters was also shown to be effective when using Ca(BH4)2, yielding corresponding hydroxy-acids and, further on, water elimination to yield α, β—unsaturated acids [212]. Aliphatic and aromatic esters R(Ar)COOEt were fully reduced to alcohols E(Ar)CH2OH by Ca(BH4)2 in the presence of alkene catalysts such as 1-decene, cyclohexene [213,214], or cyclooctadiene (COD) [215].
A sulfurated modified calcium borohydride complex, Ca(BH2S3)2, was synthesized and used for the reduction of aryl azides and aryl nitro derivatives to their corresponding Ar-NH2 amino compounds when refluxed in THF, resulting in high yields [216]. The synthesis of Ca(BH2S3)2 proceeds via a metathetical reaction of NaBH2S3 and CaCl2 in THF, the driving force being the higher stability of the calcium-sulfurated borohydride compared to the sodium counterpart, which undergoes decomposition when exposed to air and moisture (Equation (41)) [216]. Additionally, Ca(BH2S3)2 can easily reduce carbonyl compounds (aldehydes, ketones, acyloins, α-diketones, α, β—unsaturated carbonyl compounds, azides, and carboxylic acid chlorides) to their corresponding alcohols in high yields [217].
NaBH 4 + 3 S   H 2   NaBH 2 S 3   1 2   CaCl 2 1 2   Ca ( BH 2 S 3 ) 2 + NaCl
With a lower reactivity towards esters compared to lithium borohydride, Ca(BH4)2 was used together with Grignard reagents in a molar ratio of 0.25 Ca(BH4)2:4 EtMgBr to produce the reduction of methyl esters RCOOMe in THF at room temperature to RCH2OH alcohol (6%), RCH(OH)Et (83%), and RC(OH)Et2 (11%) [218]. Both Ca(BH4)2 and Zn(BH4)2 were investigated in this reduction and showed similar results.
Another modified borohydride complex, calcium amidoborane Ca(NH2BH3)2 was used successfully to reduce α, β–unsaturated aldehydes and ketones (carbonyl compounds) to allylic alcohols [219]. Its synthesis involves the reaction of CaH2 and AB (ammonia borane) in THF (Equation (42)) [219].
CaH 2 + 2   NH 3 BH 3   THF   Ca ( NH 2 BH 3 ) 2 + 2 H 2

6.3. Promoter of Cyclization in Various Reactions

Calcium borohydride was also involved as an active catalyst in σ-bond metathesis, a fundamental reaction in organic chemistry [220]. Notably, Bellham et al. reacted amine-borane t-BuNH2·BH3 with a β-diketiminate-supported silylamido calcium complex with elimination of HN(SiMe3)2 while also isolating the active catalyst, characterized by XRD and shown to belong to a polymeric infinite chain of calcium borohydride, in soluble in organic media, namely, [Ca(BH4)2·THF] [221]. Access to polylactide macrocycles by cyclo-polymerization of L-lactide was reported to proceed in good yield (up to 77%, 20 min) when catalyzed by Ca(BH4)2 by intramolecular transesterification occurring during ROP [222,223,224].

6.4. Reaction Inhibitor and Miscellaneous Reactivity

There are reports of other derivatives of calcium borohydride, like the cyclopentadienyl complex Cp2Ca(THF)2 (obtained by mixing Ca(BH4)2 and CpNa in THF) and the methyl-substituted analog, (MeCp)2Ca(THF)2 (MeCp = ηS-CH3C5H4) [225]. Similarly, [BmMeBenz]2Ca(THF)2 was also obtained and characterized by the reaction of Ca(BH4)2·2THF with 1-methyl-1,3-dihydro-2H-benzimidazole-2-thione, where Ca is eight-coordinate and features two Ca…H-B interactions [226].
Ca(BH4)2 has served as the starting point for the synthesis of thorium and uranium metallocene borohydride complexes, allowing the isolation and structure determination of (C5Me5)2Th(η3-H3BH)2 [227].
Alternatively, calcium borohydride found its use as an inhibitor for the synthesis of fluorine-modified polysilazanes, leading to a solid, soluble fluorinated polysilazane suitable for metal substrates coating [228], or as a catalyst for regioselective hydroboration of terminal alkenes [214].

7. Remaining Challenges and Future Prospects

While bearing some similarities to its lighter counterpart Mg(BH4)2, calcium borohydride features some unique features like the relative ease of desolvation from synthesis adducts, different de-/rehydrogenation enthalpies, a lower and thus more feasible activation energy Ea, and different decomposition pathways. This decomposition could be further tuned, allowing the generation of high-purity hydrogen with small amounts of boranes.
The use of calcium borohydride as an energy storage material has the potential to impact the environment in several ways. It is essential to consider both the potential benefits and the hazards associated with the production, handling, and disposal of this complex hydride and its related composites. Regarding the production route, hazardous chemicals are involved in the synthesis of calcium borohydride as well as energy-intensive processes such as ball milling. These processes may involve the use of solvents, reagents, and catalysts that could have adverse effects on human health and the environment if not properly handled and managed. The production of calcium borohydride requires significant energy inputs, which, depending on the energy source, could contribute to greenhouse gas emissions. Handling complex hydrides involves some strict safety precautions needed to prevent accidental exposure and ensure safety. It is important to follow established safety protocols to minimize risks during transportation, storage, and usage of Ca(BH4)2. Being reactive towards moisture with the subsequent release of hydrogen gas, calcium borohydride can be flammable and potentially lead to fire or explosion hazards; therefore, adequate measures must be taken to prevent accidental ignition and control the release of hydrogen gas.
An important aspect linked to the circular economy and a sustainable hydrogen economy is the disposal of end-of-life hydrogen storage materials based on Ca(BH4)2. Since a reaction with water would lead to the release of hydrogen, proper disposal methods should be followed to ensure the safe handling of any H2 generated during disposal processes. The impact associated with the disposal of calcium borohydride waste, if not properly managed, could be linked to adverse environmental effects. It is important to adhere to local regulations and guidelines for the disposal of hazardous waste to prevent contamination of soil, water, or air. Furthermore, the overall environmental impact of calcium borohydride as an energy storage material can be evaluated by life cycle assessment studies that consider the potential environmental impact throughout the entire life cycle of the compound, including production, usage, and disposal, and could guide the development of more sustainable processes.
Reversibility perhaps remains the highest barrier to adopting Ca(BH4)2 as a mainstream hydrogen storage fuel, along with the currently rather cumbersome synthesis procedure. The formation of various rock-stable boron by-products (CaB6, for instance) or other very stable compounds still represents important research avenues to explore. The multistep decomposition can be altered by using catalysts/additives or by conducting the reaction in a suitable solvent capable of decreasing reaction enthalpies by forming borohydride adducts. Among TM-based catalysts, Nb and Ti showed the best results, lowering rehydrogenation conditions (350 °C, 24 h, 90 bar H2). These results add to other reports where nanoconfined Ca(BH4)2 in various nanosized supports, such as carbonaceous hosts [229], achieved a reliable H2 storage capacity, although much lower than the theoretical one.
Additionally, research efforts should focus on developing environmentally friendly synthesis routes, optimizing energy consumption during production, and exploring recycling methods for calcium borohydride waste. Adherence to regulations, responsible waste management, and continuous improvement in production and disposal practices are essential to minimizing the environmental footprint of calcium borohydride and ensuring its safe and sustainable use.

8. Conclusions

With a high production cost, calcium borohydride sits in an awkward place among solid-state hydrogen storage materials; it offers nearly 10 wt% H2 storage but with restricted reversibility and sluggish kinetics, which require the use of catalysts and potentially novel additives. Nanostructuring may be another avenue for researchers to follow in order to achieve improved behavior of Ca(BH4)2 during hydrogenation studies. Calcium borohydride Ca(BH4)2 remains one of the most promising tetrahydridoborates due to its high hydrogen storage capacity, the relative abundance of starting raw materials, and its pivotal role in catalysis for many decades.
The current study has highlighted the synthesis routes for Ca(BH4)2, the identified polymorphs, and various adducts that could potentially yield high-capacity hydrogen storage systems while also significantly reducing production costs. Catalyst/additive compounding is another route to improving a/d kinetics, together with nanosizing and/or nanoconfinement. The wide range of applications of Ca(BH4)2, from energy storage systems to batteries, organic synthesis, organometallic chemistry, and catalysis, have been reviewed.
Understanding the key steps and intermediates of the decomposition pathways of Ca(BH4)2 under different experimental conditions could allow further advances by tuning the thermodynamics and kinetics of hydrogen release/uptake. Other strategies focused on using RHCs containing Ca(BH4)2, while species containing B12H122− anion might be suitable in ion conductivity studies. Addressing the challenges related to synthesis methods, reaction mechanisms, hydrogen release kinetics, and overall device performance is essential for realizing its full potential. Further research is needed to bridge the existing knowledge gaps and unlock the practical applications of calcium borohydride in energy storage systems. Additionally, several reports of using Ca(BH4)2 in catalysis and organic synthesis as a reductant reaffirm the advantages of the plurivalent calcium borohydride.


This work was supported by the Romanian Ministry of Research and Innovation through Project No. PN-III-P1-1.1-TE-2021-1657 (TE 84/2022) and by the Core Program of the National Institute of Materials Physics, granted by the Romanian Ministry of Research, Innovation and Digitization through the Project PC3-PN23080303.

Data Availability Statement

Not applicable.


This work was supported by the Romanian Ministry of Research and Innovation through Project No. PN-III-P1-1.1-TE-2021-1657 (TE 84/2022) and PN19-03 (contract no. PN21N/2019).

Conflicts of Interest

The author declares no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.


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Figure 1. Decomposition pathways possible for calcium borohydride (Equations (1)–(7)).
Figure 1. Decomposition pathways possible for calcium borohydride (Equations (1)–(7)).
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Figure 2. (a) Representation of the dodecahedral [B12H12]2− ion of the icosahedral frame. Boron atoms (green), hydrogen atoms (white). (b) Crystal structure of Ca(NH3)4(BH4)2. Ca, B, N, and H are represented by yellow, red, blue, and gray spheres, respectively; the molecular [Ca(NH3)4(BH4)2] octahedra form hexagonal patterns in the bc plane. (c) The layers are stacked in the order AAA along the a direction (reproduced with permission from Ref. [15]). (d) Crystal structures of α-Ca(BH4)2 in space group Fddd and (e) β-Ca(BH4)2 in space group P 4 ¯ . The color codes for atoms are green (calcium), pink (boron), and white (hydrogen). Each BH4 unit is shown by a tetrahedron. Reprinted with permission from Ref. [38].
Figure 2. (a) Representation of the dodecahedral [B12H12]2− ion of the icosahedral frame. Boron atoms (green), hydrogen atoms (white). (b) Crystal structure of Ca(NH3)4(BH4)2. Ca, B, N, and H are represented by yellow, red, blue, and gray spheres, respectively; the molecular [Ca(NH3)4(BH4)2] octahedra form hexagonal patterns in the bc plane. (c) The layers are stacked in the order AAA along the a direction (reproduced with permission from Ref. [15]). (d) Crystal structures of α-Ca(BH4)2 in space group Fddd and (e) β-Ca(BH4)2 in space group P 4 ¯ . The color codes for atoms are green (calcium), pink (boron), and white (hydrogen). Each BH4 unit is shown by a tetrahedron. Reprinted with permission from Ref. [38].
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Figure 3. (a) In situ SR-PXD pattern of Ca(NH3)6(BH4)2 heated from RT to 350 °C (5 °C min−1, p(Ar) = 1 bar, λ = 0.9941 Å). (b) Normalized integrated diffraction intensities plotted as a function of temperature. Reprinted with permission from Ref. [15]. (c) Decomposition temperatures for selected ammine metal borohydrides and the respective metal borohydrides under an Ar atmosphere, as a function of the metal M electronegativity. Zn(BH4)2 is not observed experimentally and is considered unstable. Reprinted with permission from Ref. [15].
Figure 3. (a) In situ SR-PXD pattern of Ca(NH3)6(BH4)2 heated from RT to 350 °C (5 °C min−1, p(Ar) = 1 bar, λ = 0.9941 Å). (b) Normalized integrated diffraction intensities plotted as a function of temperature. Reprinted with permission from Ref. [15]. (c) Decomposition temperatures for selected ammine metal borohydrides and the respective metal borohydrides under an Ar atmosphere, as a function of the metal M electronegativity. Zn(BH4)2 is not observed experimentally and is considered unstable. Reprinted with permission from Ref. [15].
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Figure 4. (a) XRD patterns of starting materials and as-milled samples; (b) MS profiles of NH4Ca(BH4)3; (c) isothermal hydrogen desorption curves of NH4Ca(BH4)3 at varied temperatures. Reprinted with permission from Ref. [45].
Figure 4. (a) XRD patterns of starting materials and as-milled samples; (b) MS profiles of NH4Ca(BH4)3; (c) isothermal hydrogen desorption curves of NH4Ca(BH4)3 at varied temperatures. Reprinted with permission from Ref. [45].
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Figure 5. Structural models for (a) NH4Ca(BH4)3 in space group P-43m; NH4+ (green), BH4 (red), Ca (blue0; and (b) Ca(BH4)2·AB; N (green), B (red), H (grey), Ca (blue). Reprinted with permission from reference [47].
Figure 5. Structural models for (a) NH4Ca(BH4)3 in space group P-43m; NH4+ (green), BH4 (red), Ca (blue0; and (b) Ca(BH4)2·AB; N (green), B (red), H (grey), Ca (blue). Reprinted with permission from reference [47].
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Figure 6. (a) In situ SR-PXD measured for Ca(BH4)2+CaCl2 in molar ratio 1:1, heating rate 3 K min−1. The temperature increases from 40 to 360 °C. (a) 3D plot of the selected 2θ area. Λ = 0.703511 Å; (b) DSC data for Ca(BH4)2 (dashed) and Ca(BH4)2–CaCl2 (1:1) (solid) with a heating rate of 10 K min−1. Reproduced with permission from Ref. [48].
Figure 6. (a) In situ SR-PXD measured for Ca(BH4)2+CaCl2 in molar ratio 1:1, heating rate 3 K min−1. The temperature increases from 40 to 360 °C. (a) 3D plot of the selected 2θ area. Λ = 0.703511 Å; (b) DSC data for Ca(BH4)2 (dashed) and Ca(BH4)2–CaCl2 (1:1) (solid) with a heating rate of 10 K min−1. Reproduced with permission from Ref. [48].
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Figure 7. (a) Experimental vs. calculated 1H chemical shifts (TMS as reference). (b) Experimental (black) and simulated (red) 11B MAS (Hahn echo) spectra of α-Ca(BH4)2, and Ca(BH4)2 Aldrich (α + β form) recorded with a spinning speed of 14 kHz. Reproduced with permission from Ref. [55].
Figure 7. (a) Experimental vs. calculated 1H chemical shifts (TMS as reference). (b) Experimental (black) and simulated (red) 11B MAS (Hahn echo) spectra of α-Ca(BH4)2, and Ca(BH4)2 Aldrich (α + β form) recorded with a spinning speed of 14 kHz. Reproduced with permission from Ref. [55].
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Figure 8. (a) TPD profiles comparing the desorption temperatures of several solid-state hydrogen storage materials. (b) Activation energy plots using the Kissinger equation for several mixtures. Reprinted with permission from Ref. [92].
Figure 8. (a) TPD profiles comparing the desorption temperatures of several solid-state hydrogen storage materials. (b) Activation energy plots using the Kissinger equation for several mixtures. Reprinted with permission from Ref. [92].
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Figure 9. (a) Plots of reacted fraction versus time for Mg(BH4)2, Ca(BH4)2, and the Mg(BH4)2/Ca(BH4)2 mixture at 450 °C. (b) Modeling curves for Mg(BH4)2/Ca(BH4)2. Reprinted with permission from reference [94].
Figure 9. (a) Plots of reacted fraction versus time for Mg(BH4)2, Ca(BH4)2, and the Mg(BH4)2/Ca(BH4)2 mixture at 450 °C. (b) Modeling curves for Mg(BH4)2/Ca(BH4)2. Reprinted with permission from reference [94].
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Figure 10. (a) TPD curves of the as-milled NaAlH4, as-milled Ca(BH4)2, as-milled NaBH4, and 2NaAlH4 + Ca(BH4)2; (b) XRD patterns of the 2NaAlH4 + Ca (BH4)2 composite after 6 h of ball milling and after dehydrogenation at 180 °C, 310 °C, 400 °C, and 550 °C. Reprinted with permission from Ref. [161].
Figure 10. (a) TPD curves of the as-milled NaAlH4, as-milled Ca(BH4)2, as-milled NaBH4, and 2NaAlH4 + Ca(BH4)2; (b) XRD patterns of the 2NaAlH4 + Ca (BH4)2 composite after 6 h of ball milling and after dehydrogenation at 180 °C, 310 °C, 400 °C, and 550 °C. Reprinted with permission from Ref. [161].
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Figure 11. Ca-based (M=Ca) borohydride complexes used for ROP reactions [210].
Figure 11. Ca-based (M=Ca) borohydride complexes used for ROP reactions [210].
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Figure 12. Chemical reduction of organic esters and acids performed by calcium borohydride [211].
Figure 12. Chemical reduction of organic esters and acids performed by calcium borohydride [211].
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Table 1. Crystal structure of calcium borohydride with corresponding space groups, decomposition temperatures (and rehydrogenation temperatures, where available) (°C), and crystal systems.
Table 1. Crystal structure of calcium borohydride with corresponding space groups, decomposition temperatures (and rehydrogenation temperatures, where available) (°C), and crystal systems.
Metal BorohydrideDecomposition
Temperature (°C)
Temperature (°C)
Space GroupCrystal
α-Ca(BH4)2347–387 °C, 397–497 °C (2-step)RT stableFddd or F2ddOrthorhombic[32,35,36]
β-Ca(BH4)2α - Ca ( BH 4 ) 2   167   ° C β-Ca(BH4)2; 330–400 °CRT, metastable, HT polymorphP42/m or P 4 ¯ Tetragonal[33,36]
α’-Ca(BH4)2--I 4 ¯ 2dTetragonal[35]
Table 2. Hydrogen storage characteristics of nanocomfined Ca(BH4)2 and its RHCs.
Table 2. Hydrogen storage characteristics of nanocomfined Ca(BH4)2 and its RHCs.
Ca-Based Storage MaterialNanoscaffold Usedwt% H2 Storage PerformanceObs. (Reversibility, Catalyst)Ref.
Ca(BH4)2 in CMK-3/Ca(BH4)2/TiCl3CMK-3 with SBET = 1320 m2/g, Vp = 1.48 cm3/g (wet impregnation from NH3 liq. solution to 70% pore filling)H2 release onset at 150 °CTiCl3 as catalyst (1:0.05 molar)[110]
Ca(BH4)2 in Ca.(BH4)2=MC-aMC-a (activated mesoporous carbon 1780 m2/g, 1.01 cm3/g); incipient wetness method 2.4 (reversible after 18 cycles)Desorption onset at ~100 °C[111]
Ca(BH4)2 in Ca.(BH4)2=MCM–41MCM–41 (2.4 nm) and fumed silica (7 nm); wet infiltration1H and 11B VT MAS NMR technique[113]
Ca(BH4)2 in Ca.(BH4)2=rGOGraphite and rGO (reduced graphene oxide); ball milling6.5% a 50 °C reduction in decomposition onset (220°, graphite; 170 °C, rGO)[117]
0.7 Li(BH4)2-0.3 Ca(BH4)2 (eutectic mixture); ball milling (120 min, BPR 1:18)CMK–3 mesoporous carbon (5 nm), ASM carbon (20–30 nm), and CD (non-porous carbon disks 20–50 nm thick, 0.8–3 µm diameter)3.96 (CMK-3), 4.88 (ASM), 6.07 (CD)Kinetic improvements when pore size ~5 nm, synergically coupled with a catalytic effect[109]
0.7 Li(BH4)2-0.3 Ca(BH4)2 (eutectic mixture)Two resorcinol-formaldehyde carbon aerogel scaffolds (pristine CA 689 m2/g, CO2-activated CA–6, 2660 m2/g); 60 vol.% pore filling; melt infiltration methodUp to 12.08 % pristine (7.71% CA–6, 3.36% CA)Reduced EA from 204bulk to 156CA and 130CA-6 kJ/mol[114]
CaB2H7/0.1TiO2,nano via Ca(BH4)2 + Ti(Oet)4Porous Ca-based hydride with in situ TiO2 catalyst6.2 % (300–420 °C, 1 h)Recharge at 350 °C, 90 bar H2, 1 h[112]
0.68LiBH4–0.32Ca(BH4)2 in LiBH4–Ca(BH4)2–NbF5@CMK-3 compositeCatalyzed mesoporous carbon host, NbF5@CMK-313.3% (250 min) vs. 10.4% (pristine RHC)Desorption onset reduced by 120 °C compared to pristine[37]
Table 3. Hydrogen storage characteristics of RHCs based on Ca(BH4)2 and its precursors.
Table 3. Hydrogen storage characteristics of RHCs based on Ca(BH4)2 and its precursors.
Ca-Based Storage MaterialSynthesis Methodwt% H2 Storage PerformanceObs. (Reversibility, Catalyst)Ref.
3CaH2 + 4MgB2 + CaF2Ball milling in a Spex mill (87 h in total)7.0 %Ca(BH4)2 and MgH2 formed after hydrogenation; EA = 116 75 kJ mol−1 H2 after cycling[170,171]
Ca(BH4)2 + MgH2Ball milling; TiF4/NbF5 catalysts10.5%60% reversibility (350 °C, 90 bar H2)[165]
Ca(BH4)2-3LiNH2Ball milling7.2%300 °C, 3 h; desorption onset at 200 °C[146,149]
Ca(BH4)2-4LiNH2Liquid ball milling8.86 %NH3 evolution is restrained[53]
Ca(BH4)2-4LiNH2Ball milling8% (288 °C)5 wt% CoCl2 additive lowers onset to 178 °C[147]
(x = 1,2,3)
Hand grinding7.5–9% (NH3 + H2)Metathesis at 100–150 °C to Ca(BH4)(NH2); NH3 release (t < 300 °C) and H2 (t > 350 °C)[148,161]
Ca(BH4)2–Mg2NiH4Fritsch Planetary P6 milling4.6%Mutual destabilization; formation of MgNi2.5B2[151]
Ca(BH4)2–6Mg2FeH6Ball milling-Desorption onsets at 310 °C (vs. 350 °C for pristine Ca(BH4)2)[172]
Mg2NiH4-LiBH4-Ca(BH4)2 Ball milling-MgNi2.5B2, Mg, and Mg2NiH0.3 as intermediates[173]
Ca(BH4)2–2Mg(NH2)2 and Ca(BH4)2–2Ca(NH2)2Ball milling (Retsch PM400 planetary ball mill, 200 rpm, 5 h, Ar)8.3%, resp. 6.8%Desorption onsets at 220 °C[152]
Mg(BH4)2–Ca(BH4)2 (1:2, 1:1, 2:1)Fritsch Pulverisette 6 planetary milling-Partial reversibility reported only for 2Mg(BH4)2–Ca(BH4)2[94,153]
NaAlH4–Ca(BH4)2Fritsch Pulverisette 6, WC vials and balls, BPR 406%Ca(AlH4)2 was partially stable (10 months), yielding Al and CaH2[160]
2NaAlH4 + Ca(BH4)2 (+5 wt% TiF3 catalyst)Ball milling (6 h)4.1 wt% (at 420 °C)TiF3 doping reduced release onset from 125 °C (pristine) to 60 °C; Al3Ti, CaF2 intermediates (also catalysts)[135,161]
[C(NH2)3]+[BH4] (GBH)
Ball milling10%Onset at 60 °C, full release below 300 °C (GBH = guanidinium borohydride)[169]
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Comanescu, C. Calcium Borohydride Ca(BH4)2: Fundamentals, Prediction and Probing for High-Capacity Energy Storage Applications, Organic Synthesis and Catalysis. Energies 2023, 16, 4536.

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Comanescu C. Calcium Borohydride Ca(BH4)2: Fundamentals, Prediction and Probing for High-Capacity Energy Storage Applications, Organic Synthesis and Catalysis. Energies. 2023; 16(11):4536.

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Comanescu, Cezar. 2023. "Calcium Borohydride Ca(BH4)2: Fundamentals, Prediction and Probing for High-Capacity Energy Storage Applications, Organic Synthesis and Catalysis" Energies 16, no. 11: 4536.

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