Work within the MHCoE on complex anionic hydrides focused on the synthesis and characterization of high weight capacity materials that contain well-defined chemical moieties, in particular, alanates (i.e., AlH₄^–, AlH₆^3–), amides/imides (NH₂^–/NH^–2) and borohydrides (BH₄^–). Early on in the MHCoE, work was performed on the alanates (i.e., NaAlH₄, Na₂LiAlH₆, K₂LiAlH₆, etc.) which showed appreciable reversible gravimetric hydrogen capacities, albeit at decomposition temperatures and pressures outside of the operational window for a PEM FC [11]. Also, since engineering assessments within the MHCoE suggested that the hydrogen storage system balance-of-plant could account for at least 50% of the mass and volume of the system, efforts shifted towards higher weight percent materials, namely the borohydrides.

Complex borohydrides have the potential to deliver some of the highest hydrogen storage capacities of many of the H₂ storage compounds as shown in Table 1. Magnesium borohydride, in particular, can deliver an exceptional 14.9 wt.% H₂ as demonstrated initially by Konopolev, who reported on the synthesis and desorption properties in a 1980 publication [12]. Mg(BH₄)₂ was synthesized in 80% yield from the combination of NaBH₄ and MgCl₂ in boiling ethanol and observed hydrogen desorption above 590 K with a ΔH of ~53 kJ/mol H₂, however, relatively little was known about the desorption pathway nor the reaction reversibility. Through the MHCoE, Zhao et al. [13,14,15], studied the correlation between crystal structure and hydrogen desorption dynamics. Figure 3 shows a combined in-situ XRD and RGA spectra of the desorption of Mg(BH₄)₂. In doing so, they were able to identify a stepwise dehydrogenation of Mg(BH₄)₂ to MgB₂ [Table 1, reactions 8a–c] that delivered 12 wt.% H₂ at a very reasonable ΔH of ~40 kJ/mol H₂ and additionally determined the formation of an intermediate magnesium polyborane (MgB₁₂H₁₂) species. Identification of the intermediate MgB₁₂H₁₂ closo-structure proved to be very significant towards discovering potential pathways for a reversible Mg(BH₄)₂ material system as this polyborane structure was suggested as to be both a kinetic and thermodynamic barrier to reversibility.

Reversibility of Mg(BH₄)₂ was further evaluated by Jensen et al. as they demonstrated conditions under which MgB₂ could be hydrogenated to Mg(BH₄)₂ in ~75% yield [17,24]. MgB₂ was hydrogenated to Mg(BH₄)₂ at 950 bar at 673 K and subsequent desorption of the product at 800 K yielded 11.4 wt.% H₂ with <5% converted to MgB₁₂H₁₂ as confirmed by the peak at −15 ppm using magic angle spinning (MAS) ¹¹B nuclear magnetic resonance (NMR) spectroscopy (Figure 4). Subsequent reports by Jensen et al. have demonstrated cycling of 12 wt.% at 803 K [38]. Attempts to lower the hydrogenation temperature through addition of oxidative additives (i.e., NiCl₂, FeCl₃, TiCl₃, etc.) had no effect. However, these results demonstrated the plausibility of a reversible high capacity borohydride material that has the potential to meet the stringent performance demands for automotive applications, as well as, being adaptable for emerging fuel cell applications.

Calcium borohydride, Ca(BH₄)₂, also received considerable attention within the MHCoE as it has the potential to reversibly release 9.5 wt.% H₂ through reactions 9a-b in Table 1. Investigations into the H₂ cycling behavior of Ca(BH₄)₂ showed a continual decrease in Ca(BH₄)₂ starting material (Figure 5) which suggested the formation of a thermally stable intermediate compound as in the decomposition reaction theoretically predicted by Ozolins et al. [25]. ¹¹B MAS NMR studies of cycled Ca(BH₄)₂ (Figure 6) verified the formation a CaB₁₂H₁₂ intermediate after comparison with other metal salt dodecahydro-closo-dodecaborate NMR spectra [39,40,41]. Further efforts focused on understanding the stability and the possibility of lowering the ΔH_dec for hydrogen release from CaB₁₂H₁₂ through destabilization experiments [41].

Lithiated amide/imide hydrides are another class of hydrogen storage materials with the potential to deliver high capacity at reasonable pressures; however, as observed for many complex materials, the temperature required to release hydrogen at reasonable pressures is too high for practical applications. Interest in amide/imide materials was stimulated by the report from Chen et al. [27] that >10 wt.% H₂ could be reversibly desorbed from Li₃N in two steps [Table 1, reaction 10]. However, the plateau pressure for the first step in the reaction is only 0.01 bar at 528 K which is below the operational pressure required for a PEM FC (i.e., 5 bar), thus future studies primarily focused on the lithium imide (i.e., Li₂NH) reaction. At least fifteen different amide/imide related materials were investigated during the course of the MHCoE; however, for brevity, the two most prominent systems (i.e., 2 LiNH₂ + MgH₂ and LiMgN) will be described.

Initial studies within the MHCoE were focused on lowering the temperature of the Li₂NH desorption reaction through destabilization methodologies. Luo et al. [42,43] used a partial substitution of Li with Mg by ball milling a 2:1 molar ratio of LiNH₂ and MgH₂ [Table 1, reaction 11] which reversibly desorbed ~5.2 wt.% H₂ at 493 K and 30 bar (ΔH = 39 kJ/mol H₂) as shown in the PCT (Figure 7), which is a substantial increase in the plateau pressure compared to [27]. Cyclic stability testing of this material indicated ~23% H₂ capacity loss after 264 cycles; presumably due to loss of N through ammonia release. Moreover, the addition of catalytic additives, such as KH, significantly increased the H₂ release kinetics of this system (Figure 8), allowing the desorption temperature to be lowered closer to the operational temperature range for a PEM FC (i.e., 353 K–393 K) which is thought to occur through the formation of K-N species after diffusion of K⁺ into the imide and amide phase boundaries; consequently weakening the amide and imide bonds [44].

Another prominent amide/imide reaction was discovered empirically by Fang et al. [45] by simply adjusting the molar ratio of reactants in the LiNH₂/MgH₂ system. First principle calculations predicted that a 1:1 LiNH₂:MgH₂ molar ratio would react [Table 1, reaction 12] to evolve 2 moles of H₂(8.2 wt.% H₂) and produce LiMgN which could be reversibly cycled with a ΔH ≈ 32 kJ/mol H₂ [46,47]. Initial experiments to rehydrogenate the LiMgN product indicated only ~5 wt.% H₂ uptake at 513 K and 138 bar as shown by TGA analysis (Figure 9). The thermal gravimetric analysis (TGA) curve also suggested that the reaction was a two-step process in contrast to what was originally predicted in reaction 14. XRD and TGA analysis of the hydrogenated species of reaction 12 indicated partial hydrogenation, the researchers presumed the incomplete hydrogenation was kinetically limited and attempted to adjust the adsorption/desorption kinetics by addition of a known catalyst for hydrogen storage reactions, TiCl₃. The addition of 4 wt.% of TiCl₃ to LiMgN allows almost complete rehydrogenation to 8.0 wt.% H₂ as determined by the TGA curve in Figure 10 of the subsequent dehydrogenation of Ti-catalyzed LiMgN at 513 K, which is a considerable improvement over the non-catalyzed case. Further XRD and FTIR analysis of the adsorption/desorption reaction products supported a two-step reversible reaction shown in reactions 13a-b of Table 1. However, long term cycling of the reaction still needs to be examined.

An issue related to the use of all amide/imide materials is the release of NH₃ during the desorption reaction as this represents both a loss of material during reaction that cannot be recovered and an impurity with sizable detrimental effects on PEM FC membrane materials even at trace concentrations (i.e., <1 ppm/v). As such, NH₃ impurities in the desorbed H₂ gas from the Li-Mg-N-H systems were measured and estimated by Luo et al. [48]. The measurements showed that NH₃ concentrations increased with the desorption temperature, from 180 ppm/v at 453 K to 720 ppm/v at 513 K. Additionally, after 270 cycles, the authors determined that NH₃-formation during desorption accounted for ~7% of the capacity loss due to the loss of nitrogen during cycling. Thus, before these materials can be used in practical hydrogen storage systems, more effort must be focused on reducing or mitigating NH₃ formation in the dehydrogenation reactions.

Aluminum hydride, AlH₃, is a well characterized hydrogen storage material with a gravimetric capacity of 10 wt.% H₂ and volumetric capacity of 149 g-H₂/L through an endothermic dehydrogenation reaction [Table 1, reaction 5]. It is a thermodynamically unstable but kinetically stabilized material that can deliver more than 0.01 g-H₂-s⁻¹kg⁻¹ above 385 K. However, direct hydrogenation of aluminum to alane requires over 10⁵ bars of hydrogen pressure at room temperature and traditional organometallic routes are economically impractical for large scale implementation of this material [49], thus the focus of the MHCoE with respect to alane was towards developing a cost and energy effective regeneration method. The approaches considered within the MHCoE were (1) an organometallic adduct-assisted process and (2) a non-aqueous electrochemical process.

Graetz et al. investigated a method for forming alane through a multi-step organometallic adduct-assisted process [19,50]. In general, the process involves (1) the formation of an adduct-intermediate hydride phase by direct hydrogenation and (2) then the decomposition of the intermediate phase to recover the pure hydride. In preliminary work on adduct-assisted process, a Ti-doped triethyleneamine (TEDA)–AlH₃ adduct regeneration scheme was demonstrated to be low-temperature, low-pressure and reversible; however, the TEDA-AlH₃ adduct was particularly stable so that attempts to recover the AlH₃ product thermally resulted in large quantities of H₂ being desorbed as well, thus lowering the efficiency of the overall process. Consequently, formation of the complex from a less stable adduct (i.e., lower boiling point) proved unsuccessful as well. An alternative approach to circumvent this problem was to form the adduct-complex with a moderately stable adduct (dimethylethylamine, DMEA) and then convert (i.e., transaminate) the adduct-complex to a form that was easily recoverable by thermal means. This approach was successfully demonstrated experimentally in three steps as shown in Equations 12–14 and followed via vibrational spectroscopy, shown in Figure 11. The Ti-catalyzed DMEA-AlH₃ adduct is formed [Equation 2] under mild pressure (65–70 bar) at a 50% yield followed by transamination [Equation 3] with triethylamine (TEA) at 328 K and 1 bar and finally the TEA is desorbed at 343 K and partial vacuum [Equation 4] to give the AlH₃ product in <80% yield from the TEA. The FTIR spectra in Figure 11 show the distinct shift in peaks of the Al-H stretch during the transamination from DMEA to TEA. All reagents except Al and H₂ are expected to be recovered and recycled. However, a significant drawback of the method was the energy required to execute the chemical processing steps as Ahluwalia et al. found the well-to-tank energy efficiency (WTTE) to be only 24% [4,51], although the WTTE increases to 42% if low temperature waste heat can be used. Yet, this still falls short of the 70% WTTE needed for alane to meet the DOE well-to-powerplant efficiency target of 60% for off-board regenerable materials [5] after factoring in the predicted onboard efficiency of alane (i.e., 85%) [51].

Adduct Formation:

Transamination:

Separation/Recovery:

Another approach to regenerate alane was explored by Zidan et al. [52,53]. This process uses electrolytic potential to replace the high hydrogen pressure potential and increase the hydrogen activity to drive the chemical reactions and yield high purity AlH₃. The idea is described by Faraday’s Equation [Equation 5] where E is the electrical potential, F is Faraday’s constant, R is the gas constant, T is temperature and P is the hydrogen pressure.

Since the electrolytic potential scales as the natural log of pressure, the thermodynamic requirements can be met with a reasonably low voltage. The electrolysis reaction is carried out in an electrolytic solution of a complex alanate (i.e., NaAlH₄) dissolved in a polar, aprotic solvent such as THF. A constant potential is used to drive the formation of alane and an alkali halide, NaH, from the alanate electrolyte. The starting alanate is regenerated by direct hydrogenation of the spent Al metal with the byproduct NaH. Assuming high yield of the alane, the process should notionally cost the heat of formation of NaAlH₄, which is −115 kJ/mol. However, empirically, recovery of pure alane at 100% yield has not been demonstrated with this reaction and also energy losses through processes of adduct separation have been observed. More recently, the same group demonstrated yield and efficiency improvements with the use of lithium alanate in DME as the electrolyte (demonstrated by Jensen et al. [54]) and the introduction of LiCl as an electro-catalytic additive [53].

Another concept that was explored within the MHCoE to improve dehydrogenation thermodynamics involved incorporating a second species into the reaction of a thermodynamically stable complex hydride (Figure 12) also called “destabilization”. It is worth pointing out that the individual compounds are not destabilized but rather the overall reaction is destabilized by decomposing to a more stable alloy thus requiring less energy for decomposition. During the MHCoE, many new destabilized hydride material systems were explored both computationally [46,47,55,56,57,58,59] and experimentally (e.g., LiBH₄/MgF₂, LiBH₄/MgCl₂, LiNH₂/MgH₂, LiBH₄/Mg₂NiH₄, etc.) [18,45,57,60,61,62,63,64,65,66,67,68,69,70,71,72,73]; however, we will briefly highlight an example and discuss implications for further research.

Vajo et al. [16,67] demonstrated a remarkable “destabilized” material system in LiBH₄/Mg₂NiH₄. The reaction revealed many new features contradictory to the individual complex compounds such as full reversibility, low enthalpy and entropy and reaction through a unique low-temperature kinetic pathway. The dehydrogenation of the destabilized mixture is shown in Figure 13 and the complete dehydrogenation reaction is shown in Table 1, reaction 2. This reaction releases 6.5 wt.% H₂ on a total material basis and as Figure 13 shows, the dehydrogenation occurs at temperatures lower than the individual pure components (i.e., LiBH₄ or Mg₂NiH4) begin to desorb significant quantities of H₂ (i.e., ~598 K) and indicates a new hydrogen release pathway that likely occurs through reaction of the individual components rather than sequential release and reaction of the components as observed in other destabilized material systems [16]. The enhanced kinetics of the system allowed it’s equilibrium to be characterized over a wider temperature range as shown in Figure 14. This system demonstrated the lowest thermodynamic properties of any reversible complex hydride with a ΔH = 15 kJ/mol H₂ and ΔS = 62 J/K-mol H₂, however the capacity for the low temperature step (i.e., 2.6 wt.% H₂) is too low for practical use on-board vehicles.

Although this approach has proven to be effective in tuning the thermodynamic properties of complex hydrides, the weight penalty of adding a second species can reduce the gravimetric capacity of these systems unless all species added contain H₂ (i.e., MgH₂/LiBH₄). Also, as these systems involve two or more hydrides with multiple potential reaction pathways, judicious choice of the hydride combinations is needed that allows extremely fast kinetics which facilitates almost simultaneous decomposition and avoids thermodynamic “sinks”.

Computational studies were an integrated function of each of the experimental endeavors within the MHCoE. First-principles methods such as Prototype Electrostatic Ground State [74] were developed and used to predict new crystalline materials and their thermodynamic properties, as well as provide validation of experimental results. It is estimated that computational methods allowed such rapid screening that over twenty million reactions were assessed for favorable hydrogen release properties over the duration of the Center [23,25,46,47,55,56,57,59,74,75,76]. For example, during the course of the Center interest had turned to Ca(BH₄)₂ as a potential high capacity material; however the true structure of the complex hydride was unknown. The traditional approach was lengthy to implement using a database method to iteratively search for all possible materials with the same general formula and reduce that set down to possible matches based on lowest ground state crystal structure alone, an approach that did not account for the complex electrostatic interactions that define the crystal structure of these compounds. A more logical rapid screening approach was implemented by Majzoub et al. [74] that incorporated their understanding that the complex anionic materials are dominated by electrostatic interactions. By treating the complex materials as rigid units and performing a global minimization of electrostatic interactions over the crystal unit, an optimized set of crystal structures based on total energy calculations is provided that can guide or validate experimental determinations. This method was applied to many different materials screened and developed within the MHCoE. For further information on the details of the computational efforts within the MHCoE, the reader is directed to the annual progress reports published by the DOE FCT program [77].

Significant progress was made over the five year tenure of the MHCoE to improve thermodynamics, adsorption/desorption kinetics and reversibility of metal hydride materials. Materials with exceptional gravimetric and volumetric capacities were demonstrated that make these materials attractive candidates across all hydrogen fuel cell applications. Tremendous efforts were focused on developing materials that can meet the thermodynamic requirements for automotive applications in particular; however, kinetic barriers and cyclic stability still limit their incorporation into practical applications.

Thus, current research in metal hydride material systems is focused predominantly on four distinct areas: (1) exploring new ways to alter reaction pathways to reduce thermodynamic “sinks”; (2) exploring novel destabilization combinations to improve thermodynamic and kinetic properties while also using lightweight materials to improve capacities; (3) understanding the influence and correlation of additives and catalysts on crystal structures and reaction steps; and (4) discovering and applying new nanostructural approaches to improve kinetics and thermodynamics by improving diffusion pathways and limiting formation of unwanted material phases.

Much of the research within the CHSCoE focused on AB (nNH₃BH₃) and its derivatives as they demonstrated high capacities and rapid kinetics of hydrogen release. Thermolysis of solid AB is believed to proceed through a series of step-wise reactions [Table 1, reactions 16a–c] with each delivering one equivalent of hydrogen per mole of AB to deliver ~17 wt.% H₂ at 423 K. The decomposition is characterized by an induction, nucleation then growth mechanistic pathway, whereas the solid-state AB reactions have demonstrated 180 min induction period and required up to 360 min to release only ~0.8 H₂-equivalents at 358 K [79,80]. Autrey et al. used ¹¹B MAS-NMR to gain fundamental insight into the hydrogen release pathways from solid AB and observed the formation of new species, such as the diammoniate of diborane (DADB), during different isothermal desorptions, including the induction phase [81]. This information gave insight into approaches to reduce the induction period for hydrogen release at lower temperatures. For example, addition of 5 wt.% DADB to AB was demonstrated to reduce the induction period by >75% compared to neat AB heated at 353 K. However, during dehydrogenation of solid AB, the material is inclined to foam and evolves significant quantities of gas phase impurities (e.g., ammonia, borazine and diborane) and research within the center was directed to resolve these issues, identify approaches to minimize foaming and additives to minimize impurities in solid AB [34,82]. The CHSCoE also investigated solubilized AB materials that involved liquid based compositions that could effectively utilize catalysts to reduce the temperature of hydrogen release while maintaining hydrogen capacity above 2 equivalents of H₂.

Alternative approaches to thermal dehydrogenation of solid AB are solution-based catalysis and ionic liquid enhancements. Numerous examples of catalytically amended AB were demonstrated over the course of the CHSCoE. Homogenous catalysis efforts included Lewis acids [83], non-nucleophilic bases (i.e., Proton Sponge) [84], and various transition metals [83,85,86,87,88]. One of the most effective homogenous catalysts was demonstrated by Baker et al. [89] using nickel N-heterocyclic carbene (Ni-NHC) complex generated through an in situ reaction with bis(cyclooctadiene)Ni(0). Addition of a 25 wt.% solution of AB in diglyme to the active catalyst (Ni-NHC) rapidly evolved 3 equivalents of H₂ at 333 K in 150 min which is a 20-fold increase in activity when compared to the uncatalyzed reaction. While homogenous catalysts have demonstrated moderate success in changing the thermodynamics and kinetics of AB thermolysis, they could be rather problematic in an engineered system due to the complicated separation and recovery from spent fuel. Therefore, efforts turned towards discovering a heterogeneous catalyst for the dehydrogenation of AB.

Burrell et al. investigated the use of transition metal heterogeneous catalysts for the dehydrogenation of AB in non-aqueous media at temperatures below the standard PEM FC operating conditions (i.e., <343 K) [30,32,90]. Platinum, palladium and ruthenium demonstrated catalytic activity, with the strongest activity observed for platinum [90]. At 343 K, only approximately half of the 3 equivalents of H₂ were extracted after 120 min, but more than 2 equivalents were extracted when the temperature was increased to 383 K. Additionally, non-precious metal base catalysts were screened that showed high release rates and extracted greater than 9 wt.% H₂ at 343 K in Figure 15 [30]. However, in both cases the solvents reduced the capacity of useable H₂ to less than 7 wt.% and generated insoluble solid products that deposited on the active sites of the catalysts that led to these activities being discontinued within the center.

Ionic liquids (ILs) were found to be promising solvents and activating media to promote the rate and extent of H₂ extraction from AB. Sneddon et al. demonstrated the unique attributes of ILs for activating AB by reacting 1-butyl-3-methylimidazolium chloride (bmimCl) with AB (50:50 wt.%) at 358 K [31,91]. The AB-IL mixture exhibited no induction period and released 1.0 H₂-equivalent in 67 min and 2.2 H₂-equivalents total over 330 min which compares very favorably to thermolysis of AB at the same temperature releasing ~0.8 H₂-equivalents after 360 min and an 180 min induction period. It was also shown that moderate increases in temperature significantly enhanced the H₂ release rate. For example, the 50:50 wt.% AB:IL solution held at 383 K release 1.0 H₂-equivalents in 5 min and 2.2 H₂-equivalents in just 20 min Solid-state and solution ¹¹B NMR studies indicate that the IL promotes conversion of AB into its more reactive ionic DADB form which dramatically improves the H₂ release rate. It was also noted that reducing the IL concentration to 20 wt.% still released 2.0 H₂-equivalents in 52 min at 393 K, which equates to 11.4 wt.% H₂ on a material basis at steady-state. Moreover, Burrell et al. reported that H₂ produced from the AB/IL mixtures contained only trace levels of borazine with no detectable levels of diborane as determined by IR spectroscopy [30].

Additional increases in the activity of AB/IL systems were demonstrated using catalysts and additives such as transition metals [88] and non-nucleophilic bases [84]. For example, Baker et al. investigated the influence of various homo- and heterogeneous transition metal catalysts on the dehydrogenation selectivity and rate. As shown in Figure 16, all catalyst systems tested showed marked improvement over the baseline system (i.e., 50:50 wt.% AB:IL). The study also observed a relationship between the pairing of IL and metal catalyst as preferential coordination of the IL to the catalyst metal centers can lead to different decomposition pathways that can influence the final reaction products.

Balancing the need to realize a reasonable dehydrogenation rate at low temperatures with the need for a stable material is a challenge for this and most hydrogen release systems. While the combinations of AB/IL/catalysts and additives that were demonstrated through the CHSCoE are promising, challenges remain in maintaining soluble reaction products in IL systems, especially when more than 1 H₂-equivalent is released at high AB concentrations. Aardahl et al. [92] estimated that in order to reach a 6 wt.% H₂-system basis, a minimum of 2.0 H₂-equivalents must be released from a solution containing a 50:50 wt.% AB/IL solution. Since 50:50 wt.% AB/IL solutions are approaching the solubility limit for AB in IL, maximizing H₂ release from these becomes extremely important. Furthermore, challenges with system reactor design when using heterogeneous catalysts or separation of catalyst from spent fuel with homogeneous catalysts will require careful optimization of fuel form and catalyst. It is useful to note that the engineering challenges of chemical hydrogen storage materials-based systems are being addressed within the Hydrogen Storage Engineering Center of Excellence and detailed information regarding the status of these systems have been reported [93].

While there was significant progress towards dehydrogenation of AB within the CHSCoE, regeneration of the spent fuel is arguably the most important aspect for AB to become a practical solution for PEMFC applications. Several iterations of AB regeneration reactions have been at least partially demonstrated on the bench starting from the spent fuel polyborazylene (PB) [29,32,87, ,94,95,96]. Perhaps the most promising, especially from an overall processing perspective, was reported by Sutton et al. [95]. A simple one-pot regeneration scheme (Figure 17) was demonstrated that reacts hydrazine (H₂NNH₂) with spent fuel (i.e., PB) in liquid ammonia at 313 K to give a quantitative conversion to AB (99%). However, cost and efficiency analyses showed that although the hydrazine one-pot method reduces processing costs, it is dominated by the cost and efficiency of producing hydrazine to such an extent that for this method to be practical, the energy intensity of the most efficient hydrazine production process would need to be reduced by 80% [4,97].

A novel class of materials identified by the CHSCoE for continued research is the metal amidoboranes [29]. Similar to the concept of destabilized complex hydrides, metal amidoboranes exchange one or more of the hydrogen atoms in AB with a metal to potentially decrease the enthalpy of decomposition, reduce the formation of volatile impurities and improve the rate of hydrogen release. For example, Burrell et al. reported the synthesis of Ca(NH₂BH₃)₂ from the reaction of calcium hydride and AB in THF [Table 1, reaction 17] [33]. Investigation of the desorption properties of the Ca-amidoborane revealed that the compound released up to 3.6 H₂-equivalents (7.2 wt.% H₂) at 443 K, in contrast to thermolysis of solid AB that releases ~2.0 H₂-equivalents at any temperature above 393 K. This was attributed to the endothermic release pathway of Ca(NH₂BH₃)₂ that was estimated to have a dehydrogenation enthalpy of 3.5 kJ/mol-H₂. In comparison, thermolytic release from lithium amidoborane [Table 1, reaction 18] was found to give 10.9 wt.% H₂ and the sodium analogue [Table 1, reaction 19] delivered 7.5 wt.% H₂ at 364 K [35]. A large range of compounds synthesized from lightweight metals including the alkali [35,98,99] and alkaline earth metals [33,98] have shown encouraging release rates and enthalpies compared to solid AB thermolysis. Unfortunately, all the metal amidoboranes investigated to date have been too exothermic and not able to uptake H₂ at reasonable pressures or temperatures for on-board regeneration. However, the results demonstrate that the thermodynamic and kinetic properties can be tuned from those of solid AB using this approach and provide promise for future research.

Another class of promising materials towards a reversible chemical hydrogen storage system are boron and nitrogen doped organic (C-B-N) compounds that couple the exothermic H₂ release from B-N bonds with the endothermic H₂ release from C-C bonds to create compounds that may be tailored to undergo dehydrogenation with ΔG ≈ 0 [Table 1, reaction 20] [36,37,100]. Liu et al. are investigating the use of BN-heterocyclic analogues of cyclohexane to reversibly uptake/release H₂ as shown in Figure 18. In fact, computational studies have suggested that the release of three H₂ molecules from the parent molecule (reaction 3 in Figure 18), 1,2-azaborine, exhibits thermodynamics that are conducive to a reversible system with ΔG ≈ 8.0 kJ/mol-H₂ (i.e., 1.9 kcal/mol-H₂) with the potential to release 7.1 wt.% H₂ [36]. Synthetically, the parent material is a challenge to prepare, however regeneration of a t-butyl analogue of the spent fuel, 1,2-dihydro-1,2-azaborine, was demonstrated under relatively mild conditions to give the charged fuel which has the potential to deliver 4.1 wt.% H₂. Additional research is needed to discover catalysts that will dehydrogenate both the B-N and C-C bonds to increase the practical capacity of these materials. However, initial results demonstrate that these materials do indeed show the propensity to release H₂ at near thermoneutral conditions.

As discussed in Section 2.1.5, a key attribute of the Center’s approach involved the integration and guidance of experimental efforts with computational chemistry. In particular, computational chemistry provided distinct direction in cases where there were potentially hundreds or thousands of chemical pathways, which considerably accelerated progress in the areas of catalyst development, thermodynamic tailoring and development of regeneration chemistries. To support engineering assessments of various material or regeneration choices, first principles approaches were used to predict physical properties of reagents, intermediates and products that were previously unknown [36,37,83,94,95,101]. Also, parameters such as heats of vaporization and heats of formation/dehydrogenation were determined computationally to determine process heat requirements. For more detailed discussions of specific examples of computational efforts, the readers are directed to the various project reports and presentations given through the US DOE FCT annual progress reports and annual merit review proceedings [29,101,102].

As ascertained from this very small “snapshot” of materials research within the CHSCoE, the Center made substantial progress in identifying and developing covalent chemical compounds with promising H₂ capacities, H₂ release properties and understanding the mechanistic pathways that control H₂ release. In addition, the CHSCoE made significant progress in exploring ways to regenerate the spent fuel. Although materials that meet many of the DOE system targets have been identified, a material that satisfies all the targets simultaneously has not been found; however the results from the Center do point toward promising approaches for future R&D on chemical hydrogen storage materials.

Accordingly, research in chemical hydrogen storage material systems continues to be focused around the following four topics; (1) investigations of new reversible materials that allow near thermoneutral H₂ release and regenerated by addition of H₂ under moderate conditions; (2) development of H₂ dense complete fluid systems that maintain liquid phase for the complete fuel cycle and can meet the needs for a range of PEM FC applications (i.e., man-portable to automotive); (3) additives or catalysts that can eliminate dehydrogenation impurities; and (4) inexpensive and energy efficient regeneration schemes for off-board renewable materials.