Chemical combination to alter dehydrogenation path

Alkali-metal amides have been successfully synthesized since the early 1900s and used as reagents in organic synthesis. The thermal decomposition of amides alone usually gives off ammonia rather than hydrogen. Amides had not been considered as candidates for hydrogen storage until the LiNH₂-LiH composite was reported in 2002 to reversibly store a large amount of hydrogen.^{Reference Chen, Xiong, Luo, Li and Tan5}

The chemical interactions between LiNH₂ and LiH can be expressed by Equation 2, where LiNH₂ reacts with LiH to release one equiv. H₂ and form Li₂NH. The latter can further react with LiH to release the second equiv. H₂ and form Li₃N. In total, ∼10.5 wt% H₂ can be desorbed/absorbed in the composite LiNH₂–2LiH.^{Reference Chen, Xiong, Luo, Li and Tan5}

(2)$${\rm{LiNH}}_{\rm{2}} + {\rm{2LiH}} \leftrightarrow {\rm{Li}}_{\rm{2}} {\rm{NH}} + {\rm{LiH}} + {\rm{H}}_{\rm{2}} \leftrightarrow {\rm{Li}}_{\rm{3}} {\rm{N}} + {\rm{2H}}_{\rm{2}}$$

The enthalpy diagram of LiNH₂-LiH is demonstrated in Figure 1. LiH, a highly stable hydride with a formation enthalpy of –91 kJ/mol, desorbs H₂ at temperatures higher than 600°C. The decomposition of LiNH₂ takes place at temperatures above 300°C. However, upon forming the composite, LiNH₂ interacts with LiH and releases H₂ at relatively low temperatures (starting at temperatures <180°C). Distinguished from other hydrogen storage materials reported previously, the LiNH₂-LiH composite system has three distinct features:

1. 1. It contains non-metallic elemental N;

2. 2. Both protic hydrogen (H^δ+ in LiNH₂) and hydridic hydrogen (H^δ– in LiH) co-exist;

3. 3. H₂ generation relies on the chemical interaction of two reactants. The high chemical potential of the combination of H^δ+ and H^δ– to molecular hydrogen (Equation 3) can be regarded as one of the driving forces for dehydrogenation and results in improved thermodynamics.

(3)$${\rm{H}}^ + + {\rm{H}}^ - \to {\rm{H}}_{\rm{2}} \,\Delta {\rm{H}} = - {\rm{17}}{\rm{.37 eV}}^{\rm{6}}$$

Figure 1.

Enthalpy diagrams of dehydrogenation of LiH,^{Reference Veleckis7} LiNH₂-LiH,^{Reference Chen, Xiong, Luo, Li and Tan5} and Mg(NH₂)₂-2LiH^{Reference Xiong, Hu, Wu, Chen, Luo, Gross and Wang20} composites. T ^a is the calculated temperature corresponding to 1 bar equilibrium H₂. T ^b is the dehydrogenation onset temperature observed experimentally.

Based on this understanding, a variety of composites and complexes containing both H^δ+ and H^δ– components have been investigated in the past decade (see Figure 2). Figure 2b shows the chronosequence of the development of composite and complex systems, in which the hydrogen content, temperature for dehydrogenation, and reversibility are indicated. The hydrogen desorption enthalpies of these systems vary from highly endothermic (e.g., Li₂NH-LiH^{Reference Chen, Xiong, Luo, Li and Tan5}) to exothermic (e.g., 2LiNH₂-LiBH₄¹¹), showing an effective approach to tune the thermodynamics of dehydrogenation via compositional change. The hydrogen content varies from ca. 2 wt% for CaNH-CaH₂^{Reference Chen, Xiong, Luo, Li and Tan5} to ca. 17.8 wt% for LiBH₄·4/3NH₃.^{Reference Zheng, Wu, Li, Xiong, He, Guo, Chen and Chen18} Among them, most of the amide-elemental metal hydride composite systems exhibit endothermic hydrogen desorption with full or partial reversibility. In particular, the Mg(NH₂)₂-2LiH composite possesses both a relatively high reversible hydrogen capacity of 5.6 wt% and suitable thermodynamic properties (∆H _des = 39 kJ/mol H₂, ∆S _des = 112 J/[mol H₂ • K], see Figure 1), which allow hydrogen desorption at 1 bar H₂ pressure below 90°C, thus showing considerable promise for onboard hydrogen storage application.^{Reference Xiong, Hu, Wu, Chen, Luo, Gross and Wang20} However, hydrogen desorption from those composites or complexes containing Al or B is generally exothermic or mildly endothermic in nature, which is to a large extent due to the formation of stable Al–N or B–N bonds, and thus suffers from irreversibility under normal conditions.

Figure 2.

(a) Representative chemicals containing protic or hydridic hydrogen for making composites or complexes. (b) The chronosequence of the development of amide-hydride materials.

It is worth highlighting that this approach of constructing composites or complexes is not restricted to hydrides having H^δ+ and H^δ–, but has been employed as a general guide in the design and development of other non-amide hydrogen storage materials. A few successful examples show that elements^{Reference Vajo, Mertens, Ahn, Bowman and Fultz21} or halides^{Reference Gosalawit-Utke, Colbe, Gornheim, Jensen, Cerenius, Bonatto, Peschke and Bormann22} can also be mixed with hydrides and alter the de-/hydriding thermodynamics by either interacting with dehydrogenated or hydrogenated products or both. However, in this case, it leads to a significant loss of the hydrogen capacity.

Structural features hint reaction mechanism

Clearly, the combination of hydrides comprising H^δ+ and H^δ– offers an effective approach to develop new hydrogen storage materials with tunable thermodynamic properties. However, hydrogen desorption from these composite materials usually undergoes multistep reactions, involves complicated intermediates, and possibly co-exists with competing reaction pathways. Interestingly, most of the amide-elemental hydride composites, such as LiNH₂-LiH, CaNH-CaH₂, Mg(NH₂)₂-LiH, and Ca(NH₂)₂-LiH, have notably good reversibility, which is likely due to the significant structural similarities between amides and their corresponding dehydrogenated products (i.e., imides). As examples, structural similarities were found for LiNH₂ versus Li₂NH, Mg(NH₂)₂ versus MgNH, CaNH versus Ca₂NH, Mg(NH₂)₂ versus Li₂Mg(NH)₂, Ca(NH₂)₂ versus Li₂Ca(NH)₂ and Ca(NH₂)₂ versus MgCa(NH)₂. Considerable investigations have clarified the structures of LiNH₂, Li₂NH, Mg(NH₂)₂, and Li₂Mg(NH)₂.^{Reference Sørby, Nakamura, Brinks, Ichikawa, Hino, Fujii and Hauback23–Reference Noritake, Aoki, Matsumoto, Miwa, Towata, Li and Orimo27} As shown in Figure 3, NH₂^– and NH^2– anions in the structures of LiNH₂, Li₂NH, Mg(NH₂)₂, and Li₂Mg(NH)₂ are all arranged in a close-packed configuration. The metal cations (Li⁺ and/or Mg²⁺) are tetrahedrally coordinated by the anions. The difference between the amides and imides, from a structural viewpoint, can be regarded as different extents of cation occupation in the tetrahedral interstices in the nitrogen framework (i.e., more interstices are occupied by cations when in imide form than in amide form). The apparent structural similarity between the reactant and product implies that the transport and exchange of small cations of Li⁺ and H⁺ in the N framework of amide/imide phases may play an important role in the de-/hydrogenation of LiNH₂-LiH and Mg(NH₂)₂-2LiH systems.^{Reference Rijssenbeek, Gao, Hanson, Hunag, Jones and Toby25,Reference Noritake, Aoki, Matsumoto, Miwa, Towata, Li and Orimo27,Reference David, Jones, Gregory, Jewell, Johnson, Walton and Edwards28} Such a mechanistic understanding of hydrogen storage in these composites may shed light on the design and prediction of new hydrogen storage materials as well as subsequent kinetic improvements.

Figure 3.

Crystal structures of (a) LiNH₂ (I-4, 8 cells), (b) Li₂NH (disorder Fm-3m, 16 cells, H not shown), (c) Mg(NH₂)₂ (I4₁/abc), and (d) Li₂Mg(NH)₂ (Iba2, 8 cells). The origin is set at an N atom. The crystal data used are from References 23–25.

Strategies for kinetic improvement

If a hydrogen storage material is to be used in a FCV, good de-/hydriding kinetics are required in order to deliver hydrogen sufficiently fast. Due to the complexity of the heterogeneous solid-state reaction, hydrogen desorption from the amide-hydride composite materials generally encounters severe kinetic barriers that may originate from H₂ dissociation, interface reactions, nucleation/nuclei growth, or/and diffusion processes. We consider the Mg(NH₂)₂-2LiH system as an example. Thermodynamic calculations show that the temperature for hydrogen release at 1 bar pressure is below 90°C, but an appropriate rate of dehydrogenation was only observed at temperatures above 180°C.^{Reference Xiong, Hu, Wu, Chen, Luo, Gross and Wang20} Despite controversial interpretations of the reaction mechanism of the amide-hydride system,^{Reference David, Jones, Gregory, Jewell, Johnson, Walton and Edwards28–Reference Aguey-Zinsou, Yao and Guo33} various strategies for adequate mixing and an introduction of catalytic additives have proven to be effective in improving the de-/hydriding kinetics.

Catalytic additives

Transition metals or their compounds have been recognized as proper catalysts for NaAlH₄ and MgH₂.^{Reference Bogdanović and Schwickardi39,Reference Barkhordarian, Klassen and Bormann40} Significant catalytic effects of transition metals, such as Fe, Co, Ni, and Pt, were also observed in the amide-complex hydride system (e.g., 2LiNH₂-LiBH₄⁴¹) and complex hydride ammoniates (e.g., LiBH₄•NH₃¹⁸) or hydrazinates (e.g., LiBH₄•N₂H₄¹⁹) (see Figure 2). But the roles of transition metals in the amide-elemental hydride system have not yet been well identified.^{Reference Wang, Liu, Wu, Li, Liu, Araujo, Scheicher, Blomqvist, Ahuja, Xiong, Yang, Gao, Pan and Chen16,Reference Ichikawa, Isobe, Hanada and Fujii42–Reference Yao, Shang, Aguey-Zinsou and Guo44} Efforts to improve the Li⁺ conductivity of materials by introducing BN,^{Reference Aguey-Zinsou, Yao and Guo33} LiBH₄,^{Reference Lewis, Sachtler, Low, Lesch, Faheem, Dosek, Knight, Halloran, Jensen, Yang, Sudik, Siegel, Wolverton, Ozolins and Zhang45,Reference Hu, Liu, Wu, Xiong, Chua and Chen46} and halides^{Reference Anderson, Chater, Hewett and Slater47} yielded significant kinetic enhancement of hydrogen sorption as well as suppression of ammonia generation, however, little reduction in dehydrogenation onset temperature was obtained. A notable kinetic enhancement was achieved by introducing ∼3 mol% KH to the Mg(NH₂)₂-2LiH system.^{Reference Wang, Liu, Wu, Li, Liu, Araujo, Scheicher, Blomqvist, Ahuja, Xiong, Yang, Gao, Pan and Chen16} Compared to the K-free Mg(NH₂)₂-2LiH sample, the dehydrogenation onset and peak temperatures were significantly reduced by more than 70°C and 50°C for the post-milled and pre-treated K-doped Mg(NH₂)₂-2LiH samples, respectively, together with undetectable NH₃ formation in the dehydrogenation processes (see Figure 4). The prior interaction between KH and Mg(NH₂)₂ likely plays a key role in reducing the onset temperature. A mechanistic investigation showed that in the dehydrogenation, a kinetically stable intermediate of K₂Mg(NH₂)₄, whose N–H bonds are considerably weakened compared to that of Mg(NH₂)₂, was formed that facilitates its subsequent interaction with LiH.^{Reference Wang, Wu, Shen, Guo, Xiong, Zhang, Gao, Pan and Chen48} This result indicates that severe kinetic barriers exist for the interface reaction of Mg(NH₂)₂ and LiH, which has received less attention in the past. Therefore, the introduction of multifunctional additives should be necessary in order to obtain high performance of hydrogen storage materials. There is thus room for further materials development in this area.

Figure 4.

Temperature dependence of H₂ and NH₃ desorption from KH-doped and KH-free Mg(NH₂)₂-2LiH samples and a KH-doped Mg(NH₂)₂ sample (adapted from Reference 16).

Complicated multistep dehydrogenation and possible reversibility

We consider the widely studied Mg(BH₄)₂ as an example to depict the dehydrogenation. Mg(BH₄)₂ releases approximately 14.9 mass% of hydrogen through several endothermic events together with the formation of intermediate compounds in the temperature range of 230∼530°C^51–54 (see Figure 5a). One of the intermediate compounds has been theoretically predicted and experimentally confirmed as being MgB₁₂H₁₂ under certain reaction conditions,^{Reference Li, Kikuchi, Nakamori, Ohba, Miwa, Towata and Orimo52,Reference Soloveichik, Gao, Rijssenbeek, Andrus, Kniajanski, Bowman, Hwang and Zhao54,Reference Li, Miwa, Ohba, Fujita, Sato, Yan, Towata, Chen and Orimo64–Reference Ozolin, Majzoub and Wolverton66} similar to reaction mechanisms of LiBH₄^{Reference Hwang, Bowman, Reiter, Rijssenbeek, Soloverchik, Zhao, Kabbour and Ahn65–Reference Friedrichs, Remhof, Hwang and Züttel68} and Ca(BH₄)₂.^{Reference Ozolin, Majzoub and Wolverton66,Reference Wang, Graham, Robertson and Johnson69} Formation of B₁₂H₁₂^2– was suggested as a sequential BH condensation pathway involving a buildup of the increasing higher B_nH_n+x species,^{Reference Chong, Karkamkar, Autrey, Orimo, Jalisatgi and Jensen70} as shown in Figure 5b.

Figure 5.

(a) Summary of multistep dehydrogenation processes of Mg(BH₄)₂. The question mark indicates the possible formation of an intermediate compound during the dehydrogenation. Reprinted with permission from Reference 62. © 2011 MDPI. (b) Schematic illustration of a BH condensation pathway under certain reaction conditions. Reprinted with permission from Reference 70. © 2011 RSC Publishing.

The complicated dehydrogenation pathway results in several common issues for most M(BH₄)_n:

1. 1. De-/rehydrogenation temperature higher than 230°C and sluggish reaction rates;

2. 2. Accumulation of intermediate compounds containing B₁₂H₁₂^2–, which degrades the reversibility;

3. 3. Difficulties in obtaining exact enthalpy for each reaction step and identifying the rate-determining step;

4. 4. Formation of different B_nH_n+x species that may affect overall entropy.

Reversibility is of great importance for onboard hydrogen storage, therefore, a major effort was initiated in the United States, Europe, and Asia for exploring reversibility of lithium, calcium, and magnesium borohydrides during the last decade. Reversibility of LiBH₄ could be achieved at a very high temperature of 600°C,^{Reference Orimo, Nakamori, Kitahara, Miwa, Ohba, Towata and Züttel50} which is impractical for use with PEMFC applications, but may be feasible for TES applications. Therefore, focus was shifted to Ca and Mg borohydrides. Reversibility of Ca(BH₄)₂ was, for the first time, shown by using a high-pressure, high-temperature route, starting with its presumed end-products—CaB₆ and CaH₂.^{Reference Rönnebro and Majzoub55} This promising result has led to numerous studies of crystal structures, reaction mechanisms, and attempts to improve performance. Reversibility of Mg(BH₄)₂ was shown using a similar route from hydrogenation of MgB₂.^{Reference Severa, Rönnebro and Jensen71}

Electronegativity’s role in tuning enthalpy

The Pauling electronegativity χ_P of M may be adopted as an indicator to roughly estimate and tailor the formation enthalpy ∆H _boro of M(BH₄)_n. Theoretical prediction suggests a good linear relationship between ∆H _boro of M(BH₄)_n and χ_P of M (i.e., the larger χ_P of M suggests a smaller ∆H _boro of M(BH₄)_n⁷²) (see Figure 6a). Experimental results indicate the dehydrogenation temperature T _d of M(BH₄)_n decreases with increasing χ_P of M ^{Reference Nakamori, Miwa, Ninoyiya, Li, Ohba, Towata, Züttel and Orimo72} (see Figure 6b). Assuming that borohydrides, except for Zn(BH₄)₂, decompose into elemental hydrides MH_m in the first dehydrogenation as follows,

(4)$$M({\rm{BH}}_4 )_n \to M{\rm{H}}_m + n\,{\rm{B}} + (4n - m){\rm{/}}2\,{\rm{H}}_2 ,$$

Figure 6.

(a) Relation between formation enthalpy ∆H _boro of M(BH₄)_n and Pauling electronegativity χ_P of M. The zero-point energy contributions to ∆H _boro are approximately taken into consideration. (b) Dehydrogenation temperature T _d of M(BH₄)_n as a function of Pauling electronegativity χ_P of M. The inset shows the relation between T _d and estimated ∆H _des of dehydrogenation of Equation 4. Reprinted with permission from Reference 72. © 2006 The American Physical Society.

the dehydrogenation enthalpy ∆H _des can be estimated from the predicted ΔH _boro and the experimental data ∆H _hyd of MH_m. The good correlation between ∆H _des and T _d (see Figure 6b) supports the theoretical prediction.^{Reference Nakamori, Miwa, Ninoyiya, Li, Ohba, Towata, Züttel and Orimo72} It should be noted that T _d obtained from temperature programmed desorption spectra, including both thermodynamic and kinetic parameters, and the chemical state of dehydrogenated products will largely affect the ∆H _des.

Inspired by this finding, mixing two kinds of metals having different electronegativities to produce bimetallic borohydrides MM’(BH₄)_n, has been proposed to be an approach to tune thermodynamic stability.^{Reference Li, Orimo, Nakamori, Miwa, Ohba, Towata and Züttel73} Guided by this approach, several novel MM’(BH₄)_n were synthesized, including LiK(BH₄)₂,^{Reference Nickels, Jones, David, Johnson, Lowton, Sommariva and Edwards74} LiSc(BH₄)₄,^{Reference Hagemann, Longhini, Kaminski, Wesolowski, Černý, Penin, Sørby, Hauback, Severa and Jensen75} Mg_1–xMn_x(BH₄)₂,^{Reference Černý, Penin, d’Anna and Ruziska76} and NaY(BH₄)Cl₂.^{Reference Rude, Nielsen, Ravnsbæk, Bösenberg, Ley, Richter, Arnbjerg, Dornheim, Filinchuk, Besenbacher and Jensen63} Most MM’(BH₄)_n, except some forming a large [M’(BH₄)_n]^m− anion unit such as [Sc(BH₄)₄]⁻, exhibit moderate T _d between those of M(BH₄)_n and M’(BH₄)_n. ^{Reference Rude, Nielsen, Ravnsbæk, Bösenberg, Ley, Richter, Arnbjerg, Dornheim, Filinchuk, Besenbacher and Jensen63}

Reactive hydride composites to tune enthalpy

Combining M(BH₄)_n with elements, metal hydrides, or complex hydrides (also called reactive hydride composite) is another approach to reduce ∆H _des through the formation of more stable dehydrogenated products. Figure 7 shows a typical example of the 2LiBH₄-MgX (X indicates halogen) system for this approach.^{Reference Vajo and Olson77} Take the most studied 2LiBH₄-MgH₂ system as an example. Pure LiBH₄ releases approximately 13.8 mass% of hydrogen according to the following reaction,(5)

(5)$${\rm{LiBH}}_{\rm{4}} \leftrightarrow {\rm{LiH}} + {\rm{B}} + {\rm{3/2}}\,{\rm{H}}_{\rm{2}} .$$

Figure 7.

Enthalpy diagram for destabilization of LiBH₄ by the MgX (X indicates halogen) system. Reprinted with permission from Reference 77. © 2007 Elsevier.

∆H _des for pure LiBH₄, predicted to be 67 kJ/mol H₂, can be considerably reduced to 46 kJ/mol H₂ by the formation of MgB₂ according to the following reaction,^{Reference Vajo, Skeith and Mertens78}

(6)$${\rm{2LiBH}}_{\rm{4}} + {\rm{MgH}}_{\rm{2}} \leftrightarrow {\rm{2LiH}} + {\rm{MgB}}_{\rm{2}} + {\rm{4H}}_{\rm{2}} {\rm{.}}$$

The combination with MgH₂ not only decreases T _d of LiBH₄ but also kinetically improves rehydrogenation, so this significantly improves the reversibility. Formation of MgB₂ during dehydrogenation is crucial to maintain good reversibility, however, this largely depends on the hydrogen pressure. Hydrogen pressure at least higher than 0.3 MPa is required to suppress the formation of Li₂B₁₂H₁₂ from the individual decomposition of LiBH₄.^{Reference Bösenberg, Doppiu, Mosegaard, Barkhordarian, Eigen, Borgschulte, Jensen, Cerenius, Gutfleisch, Klassen, Dornheim and Bormann79,Reference Pinkerton, Meyer, Meisner, Balogh and Vajo80} In situ powder neutron and x-ray diffraction studies of the dehydrogenation pathway at low partial pressures of hydrogen suggest that Li-rich samples form a passivating hydride layer, hindering further hydrogenation.^{Reference Price, Grant, Weston, Hansen, Arnbjerg, Ravnsbæk, Jensen and Walker81} Similar hydrogen pressure dependence phenomena of the formation of borides such as YB₄ and CaB₆ are also observed in other systems (e.g., LiBH₄-YH₃^{Reference Shim, Lim, Rather, Lee, Reed, Kim, Book and Cho82} and LiBH₄-CaH₂⁸³). Another prominent combination is between borohydrides with amides (see Figure 2). In these systems, reaction between BH₄^– and NH₂^– anions forming a very stable B–N related compound results in exothermic dehydrogenation that makes hydrogenation irreversible. In addition, a “self-catalyzing” strategy for enhancing the kinetics was proposed in the ternary LiBH₄-2LiNH₂-MgH₂ system.^{Reference Yang, Sudik, Siegel, Halliday, Drews, Carter, Wolverton, Lewis, Sachtler, Low, Faheem, Lesch and Ozolins14}

Additives to enhance kinetics

Most borohydrides, similar to amides, are known to encounter sluggish kinetics. Stimulated by the pronounced improvement of the de-/rehydrogenation kinetics of the NaAlH₄ system with the addition of Ti-based compounds,^{Reference Bogdanović and Schwickardi39} hundreds of additives, including oxides, halides, metals, and carbon-based materials, have been extensively screened. Some of them, such as halides, exhibited considerable catalytic capability. With regard to the most studied LiBH₄ due to its early commercialization, a mixture of 0.2 MgCl_2–0.1 TiCl₃ displayed a superior catalytic effect compared to other additives, releasing 5 mass% of hydrogen from 60°C to 400°C; it can be rehydrogenated to 4.5 mass% at 600°C in 7 MPa H₂.^{Reference Au, Jurgensen and Zeigler84} ZrCl₄ substantially influences the dehydrogenation kinetics of 2LiBH₄-MgH₂ and significantly reduces the incubation period for the first dehydrogenation. ZrB₂ nanoparticles formed during ball milling are suggested to act as heterogeneous nucleation sites for MgB₂, thus avoiding the incubation period and refining the microstructures.^{Reference Bösenberg, Kim, Gosslar, Eigen, Jensen, Bellosta von Colbe, Zhou, Dahms, Kim, Günther, Cho, Oh, Klassen, Bormann and Dornheim85} Though the catalytic mechanism in most additives is still uncertain, the particle size and dispersion state of additives are important to the exertion of catalytic function. For this reason, halides that readily react with borohydrides in situ to form nano-sized active species display a superior catalytic effect relative to the pure elements.

Nanoconfinement to overcome kinetic barriers

Fabricating and maintaining nano-sized metal borohydrides is expected to be an effective way to overcome the kinetic barriers originating from, at least in part, the mass transport of M, B, and H accompanying de-/rehydrogenation of M(BH₄)_n. Nanoconfinement is an alternative approach that can improve kinetics and sometimes alter thermodynamics as well. For instance, dehydrogenation rates of LiBH₄ nanoconfined into carbon scaffolds with a 13-nm pore size can be increased up to 50 times at 300°C, and the capacity loss over three cycles was significantly reduced from 72% to 40% compared to that of bulk LiBH₄.^{Reference Gross, Vajo, van Atta and Olson86} Nanoporous carbon with a smaller pore size of 2 nm can further reduce the dehydrogenation temperature and significantly suppress the release of the impurity gas B₂H₆.^{Reference Liu, Peaslee, Jost, Baumann and Majzoub87} Detailed information regarding the improvement effects and mechanisms of nanoconfinement on various hydrides are discussed in the article by de Jongh et al. in this issue.