Review on Ammonia Absorption Materials: Metal Hydrides, Halides

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Review

A Review on Ammonia Absorption Materials: Metal Hydrides, Halides, and Borohydrides Tengfei ZHANG, Hikaru Miyaoka, Hiroki Miyaoka, Takayuki Ichikawa, and Yoshitsugu Kojima ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00111 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 21, 2018

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ACS Applied Energy Materials

A Review on Ammonia Absorption Materials: Metal Hydrides, Halides, and Borohydrides

Tengfei Zhanga, Hikaru Miyaokaa, Hiroki Miyaoka*a, Takayuki Ichikawaa,b, Yoshitsugu Kojimaa

a

Natural Science Center for Basic Research and Development, Hiroshima University, Higashi-Hiroshima,

739-8530, Japan b

Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima, 739-8530, Japan

Abstract Ammonia is well known as a hydrogen carrier owing to its high hydrogen capacity (17.8 wt. %). However, the toxicity and the high storage pressure limit the application of ammonia. Consequently, storing ammonia in solid state has become the promising method to utilize ammonia for practical applications. In this review, ammonia absorption properties of metal hydrides, halides, and borohydrides to form metal amides and metal ammine complexes with various coordination numbers have been systematically summarized. Through these research, we found the correlation between the reactivity with ammonia and the Pauling electronegativity of neutral atoms according to different systems. Metal hydrides with small electronegativity value of the neutral atom of the cations can react with ammonia to form metal amides, which can be used as hydrogen storage material. For metal halides or borohydrides, the lower plateau pressure of ammonia absorption can be obtained in the material with larger electronegativity value of the neutral atom of cations. This useful tendency can be used in the materials design for the potential applications of ammonia-fed fuel cells.

Keywords Ammonia, metal hydrides, metal halides, metal borohydrides, Pauling electronegativity

Corresponding Author *Email: [email protected] (H. Miyaoka).

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Introduction Nowadays, energy consumption is mainly dependent on fossil fuels, which is dramatically increased year by year. In 2015, the total electricity production in the world was found to be about 23,950 TW h. Over 65 % of the world’s electrical energy is generated from burning fossil fuels, which are non-renewable resources whose supply will ultimately be exhausted1. The environmental issues include air quality and global warming become seriously due to rapid urbanization and industrialization followed by the large amount consumption of fossil fuels. For example, in 2015, CO2 emissions from fuel combustion is 31,760 Mt, largest among the last 25 years1. Drastic changes are required if the uneven distribution of energy sources is to be reformed. From the significant viewpoint on sustainable alternative energy sources, the increasing energy demand can not be satisfied with the existing energy production based on fossil fuels. To prevent greenhouse gas emissions from fossil fuels, a clean and sustainable energy system should be widely investigated as an attractive technology in modern society. Several alternative sources of natural energy have been proposed including solar, wind, water and geothermal energy, which have the potential to substitute the traditional route of energy generation. In general, researches on the investigation of sustainable energy conversion, high efficiency technology, storage and transportation have increased worldwide. Hydrogen as an alternative energy medium have been expected to solve these problems under the above circumstances, especially since the Fukushima nuclear accident in 2011. Hence, hydrogen storage materials as a significant energy carrier has been received particular attention2– 6

. The goal in 2017 is to provide adequate hydrogen storage to meet the U.S. Department of Energy (DOE)

hydrogen storage targets for practical applications by 2025. Specific system targets include the following: high reversible storage capacity (5.5 wt. % hydrogen), low cost (9 $/kWh for system), operating ambient temperature (-40 to 60 °C), good reversibility (1500 cycles), Fuel quality (99.97 % dry basis), and environmental health and safety (applicable standards)7. Research efforts have been focused on developing and verifying onboard automotive hydrogen storage materials to meet these targets. Many kinds of complex materials have been synthesized and studied for storing high capacity of hydrogen, such as amide/imide systems like LiNH2/Li2NH8–13, complex metal hydrides like LiAlH414–20 and NaAlH421–23, borohydrides like LiBH424–27, NaBH428–33, and Mg(BH4)234–40. However, these materials require improvements of the properties for development as practical materials. For instance, the amide/imide systems form by-product gas during dehydrogenation, complex metal hydrides are not easily reversible,


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and borohydrides have sluggish kinetics. In early 1900s, the Haber-Bosch process for the synthesis of ammonia from nitrogen and hydrogen has produced millions of tons of ammonia annually for more than 80 years41. The synthesis process is performed under 20-35 MPa pressure by using an activated iron catalyst at temperature range from 300 to 550 °C. Hydrogen and nitrogen react to form ammonia with a conversion efficiency of 15 %41. Ammonia-based fertilizers made from this process is estimated to be responsible for maintaining one third of the Earth’s population. It is roughly calculated that half of nitrogen in human body is achieved by this method to the initial fixation while the rest of the nitrogen is produced by azotobacterial. Aside from the Haber-Bosch process, many efforts have been made to synthesis ammonia under mild conditions during past twenty years. Previous advanced surface science and comprehensive theoretical calculations by Nørskov et al. have presented that the active surface and structure of transition metals (TM = Fe, Cu, Mo, Ru, and Pd)42,43 strongly affected the dissociation of N2 and H2 to give adsorbed N and H followed by the continuing hydrogenation of N to give NHx (x = 0, 1, 2 and 3)44–47. The transition state and adsorption energies of the reaction species has been determined over TM during this process. However, further dissociation of N2 on these nano-scale surface would be difficult owing to the N covering. Most recently, to intervene the aggregation of N, a second catalytic site, LiH, has been introduced in this process by Chen’s group48. The negatively charged hydrogen from LiH, which acts as an immediate source of hydrogen, help remove activated nitrogen from the surface of transition metal. This cooperation of TM and LiH creates a high efficient pathway to achieve low-temperature ammonia synthesis. Recently, ammonia (NH3) is recognized as a one of promising hydrogen carriers due to its several desirable characteristics. First, it contains 17.8 wt.% high hydrogen capacity. Second, ammonia is easily liquefied due to the strong hydrogen bonding between molecules at room temperature by compression up to 0.85 MPa, with a volumetric hydrogen density about 45 % higher than that if liquid hydrogen. This means that ammonia can be stored in a simple, inexpensive pressure tank. Additionally, ammonia can be cracked over a catalyst to produce the attractive hydrogen along with nitrogen a non-toxic, non-greenhouse gas. As an excellent transition fuel, ammonia can be burned directly in an internal combustion engine (ICE), cracked to provide hydrogen in a proton exchange membrane fuel cell (PEM FC), and generated to electricity directly in an alkaline fuel cell. Central production of ammonia from hydrocarbon fuels (e.g., natural gas, coke, and heavy crude oil) would remove CO2 either by absorption in aqueous ethanolamine solutions or by adsorption in pressure swing adsorbers (PSA) using proprietary solid adsorption media;



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transportation and distribution would be simpler and cheaper than hydrogen delivery due to worldwide infrastructure of transportation system49; and final use in an internal combustion engine or fuel cell would produce no carbon dioxide. Therefore, ammonia could be a possible alternative to hydrogen. Accordingly, Japanese government has promoted the research on NH3 as energy carrier in a cross-ministerial Strategic Innovation Promotion Program (SIP) since 2014 according to Japan’s domestic energy resources structure. In this program, hydrogen is generated from water by a new electrolysis technology of full utilization of solar light and heat. The ammonia synthesis process is designed to operate at low temperature. Furthermore, ammonia could be utilized directly in NH3 internal combustion engine or cracked to hydrogen for fuel cell. In this cycle, this work was focused on a summary of the system for transportation, dissociation, and purification of ammonia, which has been re-designed by using different ammonia absorption materials. In the whole manuscript, metal borohydrides materials were designed to transport ammonia owing to its large amount of ammonia absorption without high pressure condition; metal hydrides materials were used to crack ammonia to hydrogen with easier condition than Ru based catalyst; metal halides materials were applied to remove remaining ammonia from hydrogen obtained by cracking ammonia.50.

Table 1. Vapor pressures, relative toxicities, flash point, and explosion limits in air of different energy carriers 51–55

Gasoline Hydrogen Diesel ethanol Liquid ammonia Mg(NH3)6Cl2 Li(NH3)BH4

Vapor pressure (MPa, at 293 K)

IDLH* (ppm)

Flash point (°C)

Explosion (%)

5.06 x 10-2 N/A 2.66 x 10-4 5.86 x 10-3 8.5 x 10-1 1.40 x 10-4 2 x 10-3

750 N/A 770 3300 300 300 300

-43 -240 (Critical temp.) >52 16.6 N/A N/A N/A

1.1-3.3 4-76 0.6-6.5 3.3-19 15-28 N/A N/A

limits

*IDLH: Immediately Dangerous to Life or Health concentration.

The high toxicity and reactivity of NH3 have limited its application, such as PEM fuel cells. When ammonia is used as hydrogen carrier, N2 and NH3 concentration in the generated H2 from NH3 should be reduced to below 100 ppm and 0.1 ppm56, respectively. Therefore, the high purity hydrogen production from NH3 is key issue. Meanwhile, before the materials can be used as energy carriers, the risk of flammability and explosion is always an essential safety question. Vapor pressure of different high capacity


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carriers, their relative toxicities and explosion limits in air are compared in Table 1. It is obvious that ammonia is not flammable in air compared with very flammable gas, such as hydrogen, gasoline vapor, and ethanol. Moreover, ammonia requires higher concentrations of explosion limits in air than other energy carriers. On the other hand, the toxicity of ammonia is also clearly known as an important safety issue. Ammonia is a toxic gas or liquid due to the corrosion to tissues upon contact. When the concentration of ammonia is up to 300 ppm (IDLH: immediately dangerous to life or health), exposure to ammonia can be fatal. At room temperature, the vapor pressure is relative to the toxicity. The toxicity of liquid ammonia is obviously two orders of magnitude higher comparing with that of gasoline and ethanol. The flash point of liquid ammonia is a little lower than that of diesel and ethanol. Furthermore, recent researches illustrate that the storage of ammonia in solid state could obtain a much lower vapor pressure, while maintain a 10 wt.% hydrogen reversible capacity [41]. For ammonia stored in metal halides and metal borohydrides, such as Mg(NH3)6Cl2 and Li(NH3)BH4, a significant lower vapor pressure can be obtained at room temperature. In this case, the apparent toxicity and vapor pressure falls below those of gasoline, ethanol, and liquid ammonia. In order to store ammonia with high efficiency at moderate temperature and pressure, current research efforts are investigating new methods for storage ammonia in solid-state materials without cooling system. In this circumstance, the concentration of NH3 could be kept lower than 0.1 ppm before supplying hydrogen into fuel cells because NH3 easily damage the catalyst and membrane. For example, some of the halides or complex hydrides are used to absorb a large amount of NH3 to generate ammine complex in solid-state57. Compared with the volumetric hydrogen density of liquid NH3, that of ammine complexes are suitable for practical applications. Previous reviews have focused on NH3 desorption properties of ammine complex of metal halides57–59. These systematical studies have measured ammonia vapor pressure of ammine complex by using a manometer in a narrow range of pressure. The plateau pressure of ammonia absorption in alkaline earth halides has been reported up to 80 kPa. The hydrogen desorption properties of ammine complex, such as alkaline earth amides, metal borohydrides, have been reported as hydrogen storage materials, However, these chemicals can also be used for absorption of NH3 to form their ammine complexes. In recent years, important progress has been made on ammonia-based materials for easy handling and transportation. In this paper, we systematically review the ammonia absorption properties of various kinds of ammine complexes, the correlation between the thermodynamic properties and electronegativity of the neutral atom



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of cation and anion is discussed to obtain the guidelines for designing NH3 storage materials as renewable sources for practical applications.

Why choose solid-state materials for ammonia storage? The volumetric hydrogen density in the metal ammines have been compared to that of liquid ammonia and relative metal hydrides as shown in Table 2. Such solid-state materials exhibit comparable volumetric hydrogen densities to that of liquid ammonia, for instance, 109 g/L for Mg(NH3)6Cl2 compared to 108 g/L for liquid ammonia. The mass of storage hydrogen in different form is also compared. Without considering of the weight of storage tank, the mass of ammine complexes is heavier than that of gas or liquid phase. However, onboard hydrogen tanks (40 L, 41.5 kg) are often constructed of a high molecular weight polymer internal liner with an external carbon fiber reinforced composite, which is much heavier than the lightweight aluminum tank with polymer liners for ammonia storage. The amount of insulation also adds to the mass and volume in the whole system due to the safety issues of high pressure (70 MPa) hydrogen gas. Thus, the solid-state ammonia absorption materials are attractive for practical use and better alternatives from safety point of view.

Table 2. Mass and volumetric density of 10 kg hydrogen stored reversibility in different form without considering the storage tank. 49,57 H2 (liquid) LiAlH4 LaNi5H6 Mg2NiH4 H2 (liquid) NH3 (liquid) Li(NH3)BH4 Mg(NH3)6Cl2

Volumetric density (g/L)

Mass (kg)

71 26 36 40 71 108 53 109

10 95 730 392 10 96 291 109

Metal ammine complexes (M(NH3)nXm) have been investigated for more than one century as ammonia-storage-materials in solid state, where M is a metal cation such as Li, Na, Mg, and Ca, and X is an anion like H, Cl, and BH4. Ammine complexes, such as Mg(NH3)6Cl2 and Li(NH3)BH4, are formed easily by introducing ammonia into metal halides, or metal borohydrides at moderate temperature. For practical applications in ammonia system, metal amide (NaNH2) can crack NH3 to H2 and N2 with high


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efficiency compared with traditional catalysts, which promotes the utilization of NH3 into fuel cell systems60. Metal halides can purify ammonia even at room temperature with a very low vapor pressure (NiI, 10-5 MPa) before supplying hydrogen into PEM FC. Metal borohydrides can absorb a large amount of hydrogen with a comparable volumetric hydrogen density (LiBH4, 53 g/L) to that of liquid ammonia (108 g/L). There are good prospects for utilization NH3 by using solid-state materials in near future (Figure 1). The absorption/desorption of ammonia is completely reversible in these solid-state complexes. Once ammonia is released from these complexes it can be easily converted into hydrogen by an ammonia decomposition catalyst. And then hydrogen can be purified and supplied to fuel cell.

Figure 1. A prospect for utilization NH3 by using solid-state materials.

Metal Hydrides-NH3 Some elements, especially belong to groups of I-IV, can form metal hydrides. These materials can easily react ammonia to form a series of their nitrides and amides/imides. In 1933, Bergstrom and Fernelius systematically summarized the reaction between alkali-metal and ammonia for synthesis of the extensive amides61. These alkali amides have generally used in industry and in synthetic organic chemistry. Gay-Lussac and Thenard first prepared sodium and potassium amides after Davy obtained sodium and potassium in 180762. After that, the amide of lithium, rubidium and cesium have also been prepared from the molten alkali-metals and gaseous ammonia. LiNH2 was synthesized by Titherley in 189463, which were traditionally used as reagents in organic synthesis. However, after more than 100 years, many researchers have given great effort to study amide as a potential candidate for reversible hydrogen storage materials. Ammonia requires more than 673 K to decompose into hydrogen and nitrogen, resulting in the limitation of practical application. To utilize ammonia as a hydrogen carrier, hydrogen should be generated around an ambient temperature from



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ammonia. Then, NH3 and MeH (Me=Li, Na, K, Mg, and Ca) systems have been designed for hydrogen generation based on the background. The reaction is expressed in the following. MeH + NH3 → MeNH2 + H2

(1)

For instance, Leng et al. synthesized LiNH2, NaNH2, KNH2, Mg(NH2)2 and Ca(NH2)2 by ball-milling related metal hydrides (MeH) only in a pure ammonia gas atmosphere of 0.5 MPa at room temperature64,65. The decomposition behaviors of these materials have been investigated (Table 3). At the same time, Kojima et al. has investigated the recyclability of these amides. LiNH2, NaNH2, and KNH2 were put into a hydrogen flow atmosphere (0.5 MPa) for 4, 4, and 2 hrs, respectively. After that, 96 %, 100 %, and 92 % of samples have been transformed to LiH, NaH, and KH, respectively66–68. By using the above reactions of MH with NH3, pure hydrogen can be generated and be supplied into fuel cell after purification.

Table 3. Synthesis of metal amides through the reaction between metal hydrides and gaseous NH3 by ball-milling Reaction

BM Time (h)

LiH+NH3→LiNH2+H2 NaH+NH3→NaNH2+H2 KH+NH3→KNH2+H2 MgH2+2NH3→Mg(NH2)2+2H2 CaH2+2NH3→Ca(NH2)2+2H2

2 1 1 13 8

NH3 Pressure (MPa)

Decomposition Temperature (°C)

Χp of cation

0.4

230-500 240-500 600-700 180-500 70-500

0.98 0.93 0.82 1.31 1.0

In LiNH2, two H atoms covalently bond with N atom, which forms the amide ion [NH2]-, and the amide ion attach with Li ion by ionic bonding. Thus, the formation enthalpy of LiNH2 is low compared to that of the hydrides (LiH) formed by the ionic bond. The decomposition of LiNH2 occurs at a relatively high temperature to form final product, Li3N. The similar tendency was found in NaNH2. It is noticeable that the decomposition behaviors of Mg(NH2)2 and Ca(NH2)2 are more unstable than that of LiNH2 in the thermodynamic and kinetic point of view. Hence, the electronegativity of the neutral atom of cation and anion can affect the decomposition properties of the related amide. In general, since the electronegativity of Mg is larger than that of Li, or Na, the ionic bond between Mg2+ and [NH2]- would be weaker than that between Li+ and [NH2]-. This well explains the faster synthesis of LiNH2 but higher temperature decomposition compared with other amides in this work. Additionally, it is summarized that these metal hydrides reacted with ammonia easier along with the electronegativity value of the neutral atom of cation


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decreasing. However, these related metal amides with larger electronegativity value of the neutral atom of cation could easily release ammonia. On the other hand, David et al. have developed NaNH2 as an effective ammonia cracking catalyst, which promotes the utilization of ammonia through involving the stoichiometric decomposition and formation of NaNH2 from Na metal 60. NaNH2→Na + 1/2N2 + H2 Na + NH3→NaNH2 +1/2 H2

(2) (3)

In this way, these two reactions should affect the decomposition of NH3 by concurrently run between sodium amide and sodium metal. 2NH3→N2+3H2

(4)

99.2 % decomposition efficiency at 530 °C (0.5g NaNH2, 60 sccm NH3 flow) indicates that the performance of as-received NaNH2 for continuous stoichiometric NH3 decomposition is as effective as a ruthenium catalyst. Then, Yamaguchi et al. have suggested that the partial pressure of gaseous species strongly affect the decomposition pathway of sodium amide

69

. An imide-like intermediate might form

during decomposition. More recently, we have reported the decomposition pathway of sodium amide first time by using Mass spectroscopy as well as in-situ Transmission Electron Microscopy (TEM) 70. It is also reported NH3 suppression by adding different metal hydrides as additives. To date, the following systems have been investigated as hydrogen storage materials with an impressive volumetric and gravimetric hydrogen densities, such as Li-N-H[6, 12, 58, 66], Na-N-H60, Li/Mg-N-H13,72–75, Ca-N-H6, etc.

Metal Halides-NH3 While hydrogen is generated from cracking NH3 and metal-amides system (e.g. NaNH2), a small amount of ammonia is thermodynamically accompanying with it. The gas mixture should be purified due to the toxicity and corrosion of NH3. Therefore, another approach to lower the risks of utilization ammonia is using metal halides as a media to remove ammonia and purify atmosphere. From the previous reports, suitable metal halides can absorb large amount of ammonia to generate ammine complexes, and the volumetric hydrogen densities of ammine complexes are almost comparable with that of liquid ammonia (Table 2). Hence, the vapor pressure of these materials can be controlled safely and effectively. Particularly, ammonia concentration in feed gas could reduce as well during hydrogen utilization to PEM fuel cell. The reactions to form the ammine complexes after ammonia absorption are shown in the following equation.



MeXm + nNH3 ⇄ Me(NH3)nXm

(5)

So far, NH3 absorption properties of MgCl2 have been well studied due to its high gravimetric and volumetric hydrogen density (9.19 wt.% and 109 g/L) and relatively low vapor pressure (5 x 10-4 MPa) compared with liquid ammonia57,76,77. Furthermore, NH3 desorption properties of Mg(NH3)6Cl2 proceeds by the following steps: Mg(NH3)6Cl2 → Mg(NH3)2Cl2 + 4NH3

(6)

→ Mg(NH3)Cl2 + 5NH3

(7)

→ MgCl2 + 6NH3

(8)

Recently, Trudel et al. found a hysteresis behavior of MgCl2 during its absorption and desorption process78. One of possible reasons is that an activation barrier exists because of large volume expansion and atomic rearrangement during the formation of ammine complexes. The volume expansion can be estimated from the densities change of Mg(NH3)6Cl2 (1.24 g/cm3), Mg(NH3)2Cl2 (1.70 g/cm3), and MgCl2 (2.35 g/cm3), respectively. Additionally, the formation of the skeletal structure with narrow pores (12-15 nm) during ammonia desorption suggests the apparent volume expansion. Most recently, Aoki et al. reported that the plateau pressure corresponding to the formation of Mg(NH3)6Cl2 after NH3 treatment (1 cycle of NH3 ab/desorption) was clearly lower than that of pristine MgCl2, suggesting that the kinetic properties have a significant effect on NH3 absorption process77. In order to decrease the desorption temperature from ammine salts, Ca(NH3)8Cl8 has also been considered as a promising material for applications, because the release temperature of ammonia is lower than that of Mg(NH3)6Cl2 with comparable gravimetric hydrogen density (9.78 wt. %)57. However, the lower desorption temperature results in a higher ammonia vapor pressure at 293 K (7x104 Pa), which is only one order of magnitude lower than that of liquid ammonia. More recently, Liu et al. have studied NH3 absorption properties of five kinds of alkaline earth metal halides (MgCl2, CaCl2, CaBr2, SrCl2, and SrBr2) and their mixtures at low pressure(40-80 kPa)79. These results showed that MgCl(OH), CaCl2 and CaBr2 were applicable to the ammonia separation material between 298 and 473 K at 40 kPa. After that, Kubota et al. have reported NH3 absorption and desorption properties of metal chlorides (NiCl2, MgCl2, MnCl2, CaCl2, and SrCl2) by using a thermogravimetric analyzer at 303 K under 84 kPa of NH3 pressure80. Particularly, MnCl2 absorbed the highest amount of NH3 (0.79 kgNH3/kg MnCl2 at 600 s), following by NiCl2 with 0.70 kgNH3/kg NiCl2. On the other hand, they found that the ammonia absorption rate of CaCl2, SrCl2, and MgCl2 was relatively slow. At the same time, the reversible reaction between intermediate product Me(NH3)2Cl2 and


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final product Me(NH3)6Cl2 is found to be effective in releasing NH3 using low-temperature thermal energy below 473 K. However, each of these reports studied the ammonia ab/desorption properties among metal halides separately. The relationship of ab/desorption properties among these halides has not been explained clearly and systematically. Most recently, ammonia absorption properties of metal halides have been systematically studied by NH3 pressure-composition isothermal (PCI) measurements in our group. Ammonia absorption properties were measured in the pressure range from 0.001 to 0.8 MPa. The lower limitation is due to the measurement range of the pressure gauge, and the higher limitation is due to the vapor pressure of liquid ammonia at 293 K81. For lithium halides with different halogen anions, the plateau pressures of LiF, LiCl, and LiBr are more than 0.8 (unabsorbed), 0.178, and less than 0.001 MPa, respectively. Moreover, the same tendency is also found in the series of Na and Ca halides as follows, Peq NaCl (more than 0.8 MPa) > Peq NaI (0.055 MPa), and Peq CaF (more than 0.8 MPa) > Peq CaCl (0.03 MPa). Additionally, it is possible to further lower the ammonia vapor pressure over the hexa-ammine complex by substituting transition metal (Mn, Ni) for alkali/alkali earth metal that binds ammonia more strongly. Particularly, we have investigated the other metal halides. For instance, NiCl2, NiBr, and NiI absorbed 6 mol of ammonia below 0.001 MPa, whereas KBr and MgF2 could not absorbed ammonia as the same as those of LiF, NaCl, and CaF2. The results of ammonia absorption PCI experiments of different groups are summarized in the following Table 4.



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Table 4. Ammonia absorption amount (n) and equilibrium pressure at plateau (Peq) of different metal halides at 293 K 79–81 reaction

n (mol NH3/mol

Peq (MPa)

material)

Χp difference between the neutral atom of cation and anion

LiF+nNH3 ⇄ LiF(NH3)n

N/A

>0.800

3

LiCl+4NH3⇄ LiCl(NH3)4

4

0.178

2.18

LiBr + 2NH3 ⇆ LiBr(NH3)2

2

0.800

2.23

NaI + 5NH3 ⇆ NaI(NH3)5

5

0.055

1.73

KBr + nNH3⇆ KBr(NH3)n

N/A

>0.800

2.14

MgF2 + nNH3 ⇆MgF2(NH3)n

N/A

>0.800

2.67

MgCl2 + 6NH3 ⇆MgCl2(NH3)6

6

0.0005

1.85

CaF2 + nNH3 ⇆ CaF2(NH3)n

N/A

>0.800

2.98

CaCl2 + 4NH3 ⇆ CaCl2(NH3)4

4

0.022

2.16

CaBr2 + 8NH3 ⇆ CaBr2(NH3)8

8

0.007

1.96

SrCl2 + 8NH3 ⇆ SrCl2(NH3)8

8

0.024

2.21

BaCl2 + 8NH3 ⇆ BaCl2(NH3)8

8

0.127

2.27

MnCl2 +6NH3 ⇆ MnCl2(NH3)6

6

0.0027

1.61

NiCl2 + 6NH3 ⇆ NiCl2(NH3)6

6

0.00006

1.25

NiBr2 + 6NH3 ⇆ NiBr2(NH3)6

6

Na (0.93) > K (0.82), Mg (1.31) > Ca (1.00) > Sr (0.95), Ni (1.91) > Mn (1.55), and F (3.98) > Cl


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(3.16) > Br (2.96) > I (2.66). Among a series of lithium halides that are compounds of Li+ and different halogen anions, the plateau pressures of LiF, LiCl, and LiBr are more than 0.8, 0.178, and less than 0.001 MPa, respectively. Hence, the plateau pressure decreases with decreasing Xp value of the corresponding anion. Moreover, the same tendency is also found in other series of metal halides as follows, Peq (LiBr) < Peq (LiCl) < Peq (LiF), Peq (NaI) < Peq (NaCl), Peq (MgCl2) < Peq (MgF2), and Peq (CaBr2) < Peq (CaCl2) < Peq (CaF2). Significantly, it can be concluded that, with the same metal cation, the electronegativity difference reduces along with the decreasing of the anionic electronegativity. Therefore, lower equilibrium pressure could be obtained. Furthermore, in a series of metal chlorides that are compounds of different metal cations and chloride anion, the plateau pressures of LiCl, NaCl, MgCl2, CaCl2, and NiCl2 are 0.178, more than 0.8, around 0.001, 0,030, and less than 0.001 MPa, respectively. The plateau pressure decreases with increasing Xp value of the corresponding cation as follows, Peq (NaCl) < Peq (LiCl) < Peq (CaCl2) < Peq (MgCl2) < Peq (NiCl2). Particularly, it can be summarized that, with the same anion and different cations, the lower equilibrium pressure would be obtained from the smaller the electronegativity difference (Fig. 2).

Figure 2. Correlation between ammonia vapor pressure and the electronegativity of different cations elements in metal halides.

In summary, the plateau pressure of ammonia absorption for different materials are plotted as a function of the value Xp difference in Fig.3. Herein, the limitation of ammonia pressure was set from 0.84 to 1 x 10-5 MPa. The materials, such as LiF, which could not absorb ammonia in this pressure range, are not included



in this figure. It is obviously that the plateau pressure of absorption ammonia of metal halides can be regularity by Xp difference, besides other factors, such as crystal structure, kinetics of reaction. The materials could not absorb ammonia when the electronegativity difference between cation and anion elements is larger than 2.3. However, while the electronegativity difference of the sample is smaller than 2.3, the materials could absorb ammonia to form stable solid phase. Then, the plateau pressure decreases with the reduction of Xp difference. For Li(NH3)4Cl and Ba(NH3)8Cl2, the vapor pressure is larger than 101 kPa, which is not suitable for ammonia storage materials. On the other hand, materials with lower plateau pressure due to small Xp difference are available as ammonia removal materials to generate high purity hydrogen.

Figure 3. Correlation between ammonia vapor pressure and the electronegativity difference between cations and anions elements in metal halides. Red color means group IA elements. Black color means group IIA and transition metal elements. The blue dash circle means expected material for ammonia absorption with low vapor pressure in this work.

Metal Borohydrides-NH3 The study of a pure alkali metal borohydride dates back to the work of Schlesinger and Brown in 194082, who synthesized lithium borohydride (LiBH4) through lithium reacting with diborane (B2H6). The direct synthesis (eq. 9) using alkali metal hydride with diborane in ethereal solvents under moderate condition has been reported to produce high-yield borohydrides. 2MH + B2H6 → 2MBH4

(9)

Then, the reaction has been improved by reacting metal and boron under 3-15 MPa hydrogen pressure at


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high temperature in 195883. M + B +2H2 → MBH4

(10)

One year later, the first systematic study of coordination compound generated between LiBH4 and NH3 has been reported by Sullivan and Johnson84. The solid-state reaction of borohydrides to form ammine complexes under ammonia atmosphere is shown in the following eq. 11, where n = 1, 2, 3, and 4. Additionally, thermal decomposition of LiBH4 gradually releases ammonia stage by stage, giving ammonia capacity of approximately 44, 61, 70, and 76 wt % as n=1, 2, 3, and 4, respectively. MBH4 + nNH3 ⇆ M(NH3)nBH4

(11)

These complexes were subsequently prepared via substitution of [BH4]-1 for a halide ion. For instance, James and Wallbridge obtained the complex by an exchange reaction of magnesium chloride with sodium borohydride in liquid ammonia85. The use of an excess of NaBH4 is necessary to complete substitution totally. MgCl2 + 2NaBH4 → Mg(BH4)2 + 2NaCl

(12)

Recently, these complex metal borohydrides M(BH4)n, such as LiBH4, Mg(BH4)2, and Al(BH4)3, with high hydrogen capacity and potential to meet the DOE targets are investigated deeply as hydrogen storage materials86–90(Table 5).

Table 5. The properties of complex metal borohydrides as hydrogen storage materials Reactant

Product

Hydrogen capacity (wt. %)

-∆H, kJ/mol H2

Decomp. temp. (°C)

LiBH4 NaBH4 Mg(BH4)2 Ca(BH4)2 Al(BH4)3 Ti(BH4)3

LiH + B NaH + B Mg + 2B 2/3CaH2+ MgB2 Al + 3B TiB2 + B

13.9 7.9 14.9 9.7 16.9 13.1

75 90 40 75.5 6 N/A

470 595 323 360 150 25

Even though these borohydrides have a high capability without considering of the storage system, the hydrogen release temperature are too high to apply in practical hydrogen storage. Sometimes highly toxic by-product (B2H6) release from these systems. To overcome such kinds of drawbacks of metal borohydrides, the use of ammonia complexes of M(BH4)n has been investigated in a direction of ammonia absorption materials. Such solid-state materials could absorb ammonia, which volumetric density is



Page 16 of 41

comparable to that of liquid ammonia. Moreover, solid-state materials should present an acceptably low vapor pressure of ammonia at room temperature. The thermodynamics on ammonia absorption of metal borohydrides were investigated by an automatic vapor absorption isothermal measurement (Belsorp max). It can measure absorption isotherms from relative pressure as low as 1 x 10-8, using a 13.3 Pa pressure transducer. Compared previous study81, the absorption isotherm can be measured with high accuracy. The amount of ammonia absorption was evaluated by using ammonia density at each temperature and pressure, referring from NIST (National Institute of Standards and Technology) database91. The key parameters with those of solid-state ammoniates are compared in Table 6.

Table 6. Thermodynamics on ammonia absorption of alkali metal and alkali earth metal borohydrides

Sample

NH3 capacity (mol/mol)

Plateau pressure (MPa, 293 K)

Χp difference

Mg(BH4)2

1 2 5 1 2 5 1 2 3 2 0

0.000057 0.000096 0.002 (This work) 0.000037 0.0003 0.0065 (This work) 0.002 0.006 0.012 (This work) 0.096 (This work) N/A

1.31

Ca(BH4)2

LiBH4

NaBH4 KBH4

1

0.98

0.93 0.8281

LiBH4 could absorb 1 mol of NH3 below 0.002 MPa, 2 mol of NH3 at 0.006 MPa, and then formed Li(NH3)3BH4 with the fourth plateau pressure at 0.012 MPa (Fig. 4a). The insert figure shows the details of absorption properties at low pressure. The plateau pressure of NaBH4 was observed around 0.096 MPa, corresponding to generate Na(NH3)2BH4 (Fig. 4b). Additionally, solid NaBH4 ammine complex could be liquified during NH3 pressure increasing. However, KBH4 showed no ammonia absorption under 0.8 MPa ammonia pressure. In this study, Mg(BH4)2 and Ca(BH4)2 had a maximum amount of 5 mol ammonia absorption (Fig. 4c and 4d). The plateau pressure of Mg(BH4)2 absorbing 1, 2, and 5 mol ammonia was 0.0 00057, 0.000096 and 0.002 MPa, respectively. The plateau pressure of Ca(BH4)2 was 0.000037, 0.0003, and 0.0065 MPa, respectively. Overall, the tendency of the plateau pressure followed the order as Peq (Na(NH3)2BH4) > Peq (Li(NH3)2BH4) > Peq (Ca(NH3)2BH4) > Peq (Mg(NH3)2BH4) in a series of metal


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borohydrides which are composed of different cations and same anion (BH4-). Furthermore, the plateau pressure of ammonia absorption of KBH4 is higher than 0.8 MPa. Since the anion is same among these borohydrides, the plateau pressure of different metal borohydrides increases during the reduction of electronegativity of cationic elements at 293 K.

Figure 4. PC isotherms for NH3 absorption of LiBH4 (a), NaBH4 (b), Ca(BH4)2 (c), and Mg(BH4)2 (d) at 293 K.

The similar tendency as metal halides has been found in metal borohydrides. Figure 5 shows the relationship between ammonia pressure of M(BH4)x(NH3)y and Xp of different cations. The vapor pressure has decreased with the reduction electronegativity difference when the coordination number is 2 mol (NH3)/mol (sample). Mg(BH4)2 shows the lowest vapor pressure due to the smallest value of electronegativity difference between the neutral atom of Mg2+ and (BH4)-. For practical applications, the vapor pressure of metal borohydrides is lower than ammonia stored by liquefaction (850 kPa, 293 K). Most importantly, ammonia absorbed in solid-state materials can be stored and transported in normal tank other than high pressure tank such as liquid ammonia. In this case, the process of storage and



transportation can become easier, cheaper, and safer than traditional method. Above all, NaBH4 has moderate properties as NH3 absorption materials because the plateau pressure is close to ambient conditions (96 kPa, 293 K). Therefore, NaBH4 has been investigated as a representative system in the following.

Figure 5. Correlation between ammonia vapor pressure and the electronegativity of different cationic elements in metal borohydrides.

Materials Design As discussed above, the correlation between the plateau pressure and the electronegativity value between cationic and anionic elements in the metal amides, halides and borohydrides was clarified. In the case of amides, a hydrogen storage system composed of ammonia and metal hydrides system have been designed. A high capacity and an appropriate enthalpy change can be obtained for practical applications. It is remarkable that ammonia can react with metal hydrides to release hydrogen at even room temperature without any catalyst and heat activation. After that, the reaction product metal amides are completely recycled back to hydride and ammonia under 0.5 MPa of hydrogen flow. From different application point of view, the reaction of metal hydrides with NH3 can be used as NH3 removal media to purify the H2 produced by cracking NH3 as fuel for PECFC because the reaction of metal hydrides is exothermic and thereby the partial pressure of NH3 in feed gas can be decreased to low level. The NaNH2-based ammonia cracking system can promote the utilization of ammonia sustainable energy storage purposes as the innovative catalytic process. Taking metal halides and borohydrides for example, the properties of


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ammonia absorption can be controlled due to the electronegativity value of metal cationic elements. The materials with smaller electronegativity difference value between cationic and anionic elements exhibited a much lower equilibrium pressure on ammonia absorption. By use of this correlation, the ammine complexes with suitable vapor pressure according to requirements of different practical applications (transportation/purification) can be chosen.

Figure 6. (a) PCI curves for ammonia absorption of NiI2 at 293 K, (b) PCI curves for ammonia absorption of MgCl2 at 293 K (Reprinted with the permission from (J. Phys. Chem. C 2015, 119, 26296-26302). Copyright (2015) American Chemical Society).

The risk of exposure limits the potential use of ammonia as energy carrier. From the viewpoint of safety and purification, Nickel iodide (NiI2) with a smallest electronegativity difference (0.75, Table 4) was chosen as a promising material for ammonia purification. This system exhibits multiple reactions, e.g., NiI2/NH3, NiI2(NH3)/(NH3)2, and NiI2(NH3)2/(NH3)6. Therefore, a fundamental study was carried out with the PC isotherms of NiI2 at 293 K. The thermodynamic properties of NiI2 were investigated as a typical system compared with previous sample MgCl2. The PC isotherm of NiI2 at 293 K are shown in Fig 6 (a). The plateau pressures were located on 30 and 40 Pa corresponding to the ammonia absorption amount of 2 mol/mol and 6 mol/mol NiI2, respectively. In Fig 6 (b), the results of activated-MgCl2 are compared77. The plateau region appeared from 200 to 500 Pa in agreement with the ammonia absorption amount of 2 mol/mol and 6mol/mol MgCl2. Since the PC isotherms were performed under the same condition of a certain waiting time for both samples, the plateau pressure should strongly be affected by the sample itself, such as the electronegativity difference. The plateau pressure of NiI2 was obviously lower than that of



MgCl2 as shown in the figure, suggesting that the smaller electronegativity difference could obtain a lower ammonia vapor pressure. Therefore, NiI2 can be used as an ammonia purification material before supplying hydrogen into PEM fuel cell.

Figure 7. PCI curves for ammonia absorption and desorption of NaBH4 at 293 K.

Since NaBH4 could absorb ammonia at room temperature, it is investigated as a candidate of ammonia absorption and transportation materials. Previous study in our group reported that ∆H for NH3 absorption process of NaBH4 was evaluated to be −29 kJ/mol, which is almost same as that of ammonia liquefaction process (-23 kJ/mol). The plateau pressure of NaBH4 absorption ammonia (0.09MPa) is close to ambient conditions at 293 K. It is also suggested that ammonia absorption and desorption processes could be controlled only by pressure at 293 K. After the pressure is larger than 0.09 MPa, the solid-state Na(NH3)2BH4 would change to liquid phase. Considering of the whole system including package or tank, liquid phase Na(NH3)xBH4 (x>2) could occupy a maximum volume of the whole system without high pressure. Herein, a reversible property of NaBH4 absorption ammonia was investigated by using the PC isotherm at 293 K (Fig. 7). The plateau pressure of NaBH4 was 0.09 MPa, which was lower than that of liquid ammonia (0.86 MPa). It is also indicated that the reaction is reversible and easy to absorb/desorb ammonia at 293 K.

Conclusion Growing environmental problems caused by fossil fuels have led to a worldwide revival of attention in


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sustainable clean energy. Considering of their many advantages, metal hydrides have been investigated as the hydrogen carrier. However, until now, only a few success examples could be used in practical applications. Most hydrides can be applied only at high temperature and have presented poor ab/desorption kinetics. This thought may now change. Many materials are considered as hydrogen carriers in the nature. Ammonia is an attractive indirect hydrogen storage material because it does not contain carbon and therefore would not release CO2 when used as fuel. At the same time, the worldwide infrastructure of production and transportation system is well established to help increase the utilization of ammonia. The major disadvantage of ammonia is its toxicity. However, storage of ammonia in solid-state is acceptable for transportation and to prevent the leakage of toxicity. Herein, ammine complex materials have received increasing interest due to the properties of absorption ammonia over the past decade with a wide range of new materials discovered. Based on the large number of experimental results, this review has outlined trends between reactivity with ammonia and the electronegativity difference between cationic and anionic elements of the materials. Ammonia can be released more easily when the electronegativity difference value is smaller between cation and anion in different amide system. The same tendency has been found in metal halides and borohydrides. The sample with smaller electronegativity difference value exhibited a much lower equilibrium pressure on the ammonia absorption. This correlation could be applied to design different utilization according to the suitable vapor pressure. Here, there should be other factors, such as crystal structure, to well understand the essential properties. These would be reason of the dispersion on relation between electronegativity and equilibrium pressure. Thus, further investigation about the other factors are necessary in future works. This review has presented the knowledge of the past decade of research within metal amide/imide, halides, and borohydrides. An overview of the current frontiers of research and new approaches for the practical application is also provided. This review also gives new perspectives that there is still plenty of room for investigating new metal ammine complex since there materials can be utilized as energy storage materials.

Acknowledgments This work was partially supported by Council for Science, Technology, and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (funding agency: JST).



The authors would like to thank Dr. Taihei Aoki for his previously work. And, this work is carried out as research in Research Center for Nitrogen Recycling Energy Carrier of Hiroshima University.

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Supporting Information Supporting Information Available: [Measurements and Characterization]



A prospect for utilization NH3 by using solid-state materials. 109x47mm (300 x 300 DPI)


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Correlation between ammonia vapor pressure and the electronegativity of different cations in metal halides. 201x140mm (300 x 300 DPI)



Correlation between ammonia vapor pressure and the electronegativity difference between cations and anions in metal halides. Red color means group IA elements. Black color means group IIA and transition metal elements. The blue dash circle means expected material for ammonia absorption with low vapor pressure in this work. 201x140mm (300 x 300 DPI)


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PC isotherms for NH3 absorption of LiBH4 (a), NaBH4 (b), Ca(BH4)2 (c), and Mg(BH4)2 (d) at 293 K. 231x185mm (300 x 300 DPI)





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Correlation between ammonia vapor pressure and the electronegativity of different cations in metal borohydrides. 201x140mm (300 x 300 DPI)



(a) PCI curves for ammonia absorption of NiI2 at 293 K 256x239mm (300 x 300 DPI)


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PCI curves for ammonia absorption of MgCl2 at 293 K (Reprinted with the permission from (J. Phys. Chem. C 2015, 119, 26296-26302). Copyright (2015) American Chemical Society). 194x174mm (300 x 300 DPI)



PCI curves for ammonia absorption and desorption of NaBH4 at 293 K. 258x244mm (300 x 300 DPI)


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Table of Contents graphic 38x24mm (300 x 300 DPI)


Review on Ammonia Absorption Materials: Metal Hydrides, Halides

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