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Progress in Natural Science: Materials International (xxxx) xxxx–xxxx

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Hydrogen - A sustainable energy carrier☆ ⁎

Kasper T. Møllera, Torben R. Jensena, , Etsuo Akibab,c,d, Hai-Wen Lib,c a

Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, Aarhus University, DK-8000 Aarhus, Denmark International Research Center for Hydrogen Energy, Kyushu University, Fukuoka 819-0395, Japan WPI International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan d Department of Mechanical Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan b c

A BS T RAC T Hydrogen may play a key role in a future sustainable energy system as a carrier of renewable energy to replace hydrocarbons. This review describes the fundamental physical and chemical properties of hydrogen and basic theories of hydrogen sorption reactions, followed by the emphasis on state-of-the-art of the hydrogen storage properties of selected interstitial metallic hydrides and magnesium hydride, especially for stationary energy storage related utilizations. Finally, new perspectives for utilization of metal hydrides in other applications will be reviewed.

1. Energy storage Energy storage is essential to improve the utilization of the intermittent harvested renewable energy, to level out the fluctuations in production and consumption, and to distribute and transport the energy [1,2]. Energy can be stored by several means with increasing potential for large-scale storage capacities: mechanical < thermal < electrochemical < chemical energy. Each approach has advantages and disadvantages e.g. related to energy density, capacity, price and potential for scale-up. This is illustrated in Fig. 1 as discharge time as a function of system size. Electrochemical and electrical energy storage e.g. batteries and supercapacitors, respectively, cover the mid-time range, minutes to hours and allow scale-up to MW-size. Potential mechanical energy as pumped-hydro and compressed air energy storage may reach GW size. However, the latter largely depends on the geographical conditions e.g. lakes in mountain areas or underground salt caverns. Chemical energy storage, as hydrogen, has the largest potential for large-scale energy storage, which is far out of the scale shown in Fig. 1. This may be achieved simply by storage of compressed hydrogen gas in large stationary tanks or underground cavities, liquid hydrogen, or liquid hydrogen carrier e.g. ammonia and liquid organic hydrogen carriers [3,4]. 1.1. Hydrogen, the number one element The physical and chemical properties of hydrogen are in many cases unique despite the fact that hydrogen appears to be the most simple

element with Z=1, which will be discussed in the following. Hydrogen, H, is the most abundant element and accounts for ~15 mol% on the surface of earth, e.g. in water, fossil fuel, and biomass. An advantage is that hydrogen is a non-poisonous, tasteless, colourless, and odourless gas. Molecular hydrogen, H2, is a gas at ambient conditions and the lightest molecule of all substances. Therefore, hydrogen is difficult to condense to a liquid due to the very low critical point, Tc=−240.01 °C and Pc=12.96 bar. Hydrogen is the substance with the fastest diffusion speed in air, ~20 m/s at RT. Therefore, hydrogen will quickly be dispersed uniformly from a leak, which contributes to safety, although the explosion range in air is broad, 4–75 vol%. However, hydrogen and oxygen form kinetically stable mixtures, similar to natural gas and O2, due to the high stability of the hydrogen molecule, i.e. the large bond dissociation enthalpy of ΔHD(H−H) =436 kJ/mol [6–8]. Importantly, hydrogen has the highest gravimetric energy density of all known substances, i.e. a lower heating value (LHV) of ~120 kJ/g. Molecules can be adsorbed on interfaces due to weak interactions denoted van der Waals, dispersion, or London-forces and the strength of this physisorption depends on the electron density of the interacting molecules and surfaces. The hydrogen molecule has the lowest number of electrons (two) and therefore the weakest dispersion interaction of all molecules with typical bond dissociation enthalpies, ΔH, in the range 1–10 kJ/(mol H2) and the storage media may have to be cooled, e.g. by liquid nitrogen, to keep the storage capacity at a reasonable level [9–11]. A significant advantage is that the physisorption process provides fast adsorption/desorption kinetics and almost instantaneous

Peer review under responsibility of Chinese Materials Research Society. ⁎ Corresponding author. E-mail addresses: [email protected] (T.R. Jensen), [email protected] (H.-W. Li). http://dx.doi.org/10.1016/j.pnsc.2016.12.014 Received 30 October 2016; Accepted 30 November 2016 1002-0071/ © 2017 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

Please cite this article as: Møller, K.T., Progress in Natural Science: Materials International (2017), http://dx.doi.org/10.1016/j.pnsc.2016.12.014

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Fig. 1. Comparison of key-type energy storage technologies in sense of storage capacity and discharge power duration (modified from reference [5]).

equilibrium between adsorbed hydrogen and hydrogen in the gas phase [11–13]. Physisorption is reviewed elsewhere and is not further discussed in this text. Hydrogen has very diverse chemistry despite the strong single bond in the H2 molecule, because this bond may be replaced by two similarly strong bonds to other elements. Hydrogen forms compounds with most of other elements in the periodic table and with a variety of chemical bonds. This is also reflected by the extreme variation in charge and size of, H+–H0–H−, by adding or removing one electron. The chemistry of the hydride ion, H−, resembles that of analogue fluorides, e.g. many ionic alkali metal hydrides and fluorides share the rock salt structure (NaCl). Considering the covalent chemistry the hydride ion, H−, may possibly be the most flexible ligand of all, which is illustrated by the fact that metals can be stabilised both in their lowest and highest oxidation states, e.g. Ni in oxidation state zero (d10) in Mg2NiH4 and rhenium as Re7+(d0) in K2ReH9 [14]. Hydrogen in the solid state has the potential for very high energy densities relevant for mobile applications [15–17]. However, there are multiple requirements (~20) for a material in mobile applications to become the successor for gasoline [4,9,18]. There is plenty of diversity in the chemistry of hydrogen, which provides great potential for discovering new compounds with novel structures and properties with a variety of possible energy-related applications. Complex metal hydrides are further discussed in another review [19]. Combining light elements with hydrogen in the solid state is advantageous since high gravimetric hydrogen densities can be achieved, as seen in Fig. 2. Additionally, very high volumetric densities ( > 80 kg H2/L) exceeding that of liquid H2 are commonly reached. Solid-state materials based on interstitial metallic hydrides and magnesium hydride are highlighted in this review.

Fig. 2. Overview of selected materials and their volumetric and gravimetric hydrogen density. The U.S. Department of Energy targets for the hydrogen storage system are also shown for comparison.

[20]. However, hydrogen compression to 700 bar consumes an amount of energy comparable to 13–18% of the lower heating value [21]. Additionally, when hydrogen is compressed to 700 bar the volumetric energy density becomes 5.6 MJ/L (ρV(H2)=40 g H2/L), which is far less than 32.0 MJ/L for gasoline [22]. Thus, solid-state hydrogen storage is considered. By combination of a high-pressure cylinder and a solidstate material containing hydrogen, the driving range of a hydrogenfuelled vehicle could be extended significantly or the tank volume could be reduced whilst maintaining the same driving range [23]. However, the solid-state material needs to operate in a suitable temperaturepressure range (−40 < T < 85 °C, 1 < p < 700 bar). Finally, management of heat transfer in the metal hydride tank is important and can be improved using multi-tubular tank geometries or a phase-change material [24,25]. However, these improvements will also increase the weight of the system. Thus, optimizations are often a compromise between heat transfer and hydrogen content [26]. To investigate materials properties at these conditions calls for a new research direction: solid-state hydrogen storage at elevated pressure of p(H2)

2. Basic hydrogen sorption theory and practice Hydrogen, H2, is a gas at ambient conditions with very low density i.e. 0.0813 g/L (at 25 °C and 1 bar) [8], hence it is difficult to store H2 in an efficient and compact way. On-board compressed hydrogen-gas storage (700 bar, 1 bar=105 Pa) is today introduced in hydrogenfuelled vehicles, mainly due to comparable refuelling time ( < 3 min) and driving range (≥500 km) to conventional gasoline-fuelled vehicles 2

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=300–700 bar. In this section, new sample environments and Sieverts’ techniques specifically designed for high-pressure studies up to 700 bar are presented together with the general pressure-composition-temperature measurement technique for determining thermodynamic properties of a system.

2.1. Metal hydride-hydrogen theory Considering hydrogen uptake in a solid-gas reaction: M(s)+H2(g)→MH2(s) the thermodynamic considerations on a general metal hydride system like this, establishes a correlation between the temperature of hydrogen release and uptake at a given equilibrium pressure and the enthalpy change for the reaction through the van't Hoff equation: Θ



ln(P(H2)/P )=(RT) ΔHr−(R) ΔSr

Fig. 4. Schematic representation of the gas manifold connected to the in-situ sample environment [35].

2.2. High-pressure Sieverts’ equipment


The Sieverts’ measurement technique utilizes pressure changes in a known volume to estimate the amount of gas absorbed/desorbed by a compound, e.g. a metal hydride. The technique requires measurements of temperature and pressure to calculate the amount of gas in a known volume [29,30]. A schematic representation of a high-pressure gas manifold is presented in Fig. 4. The gas manifold can be connected to an in-situ sample environment, which will be elaborated later. The gas manifold is constructed from commercially available valves/fittings rated to a minimum of 1000 bar. A reference volume is used to ensure accurate pressure measurements to determine hydrogen uptake and release. The amount of absorbed or desorbed gas is determined by calculating the amount of gas in the current volume before and after the change of pressure [29,31]. A metal hydride hydrogen compressor (here denoted hydrogen reservoir) utilizes a reversible heat-driven interaction, converting energy from heat into compressed hydrogen gas, from a hydrogen-containing metal, alloy or intermetallic compound [32,33]. The metal hydride hydrogen reservoir has to be made from an alloy with sufficiently high predicted ultimate tensile strength (UTS) at room temperature and also with a low diffusion of hydrogen. The reservoir may be loaded with a commercial AB5 alloy, e.g. with composition MmNi4.35Co0.8Al0.05 (Mm=Mischmetal, primarily La and Ce) which has an equilibrium pressure of about 70 bar at 293 K and 1370 bar at 473 K. The reservoir is used to generate the high hydrogen pressure. To quantitatively determine hydrogen uptake and release, a real gas model needs to be considered at elevated pressures, i.e. including the compressibility factor, Z, of hydrogen, pV=nRTZ, where Z(H2) equals 1.452 at 700 bar and 298 K [34]. By applying pressure and reaching equilibrium in steps, it is possible to determine the total amount of hydrogen absorbed by a sample. The requirements are that the volume

where P(H2) and PΘ denote an equilibrium pressure and a reference pressure of 1 bar, R is the gas constant, and T is the temperature. ΔHr and ΔSr are the enthalpy and entropy of the reaction. A material's hydrogen release temperature is often described as T(1 bar) given for an equilibrium pressure of Peq(H2)=1.0 bar. In this case, the van’t Hoff equation is reduced to: T(1 bar)=ΔHr/ΔSr


Most metal hydride systems have ΔSr≈130 J/(K mol) since the major contribution to the reaction entropy change ΔSr is from the change in state from molecular hydrogen gas, SΘ(H2(g))=130.7 J/ (K mol), to the solid state in which the entropy is assumed to be close to zero, SΘ(H2(s))≈0 J/(K mol) [8]. To reach an equilibrium pressure of P(H2)=1 bar at a moderate temperature of 25 °C the decomposition enthalpy should be ΔHr ~40 kJ/mol. To experimentally determine the reaction enthalpy and entropy, a pressure-composition-temperature (PCT) plot is created by performing pressure-composition-isotherm (PCI) experiments [27]. Initially, a solid solution is formed between the metal and hydrogen (the α-phase) before nucleation and growth of the metal hydride initiates (the βphase), see Fig. 3, left [28]. As the two phases coexist, an equilibrium pressure will be reached which is dependent on temperature. As the hydrogen content in the β-phase increases, it will at some level be saturated and pressure will start to increase. Performing several PCI experiments at different temperatures enables the construction of a PCT plot from which the equilibrium pressure as a function of temperature can be determined. This leads to creation of the van’t Hoff plot from which ΔH and ΔS can be extracted from the slope and the intersection of the line, respectively, see Fig. 3, right.

Fig. 3. Illustration of a pressure-composition-temperature (PCT) diagram (left) for a hypothetical metal hydride (MH) and the van’t Hoff plot (right) from which thermodynamic data can be extracted.


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Fig. 5. Hydrogen occupies octahedral or tetrahedral sites in interstitial hydrides. Interstitial sites are marked by brown dots.

temperature [39]. The metallic hydrides are mainly comprised of heavy transition metals which limits their gravimetrical hydrogen capacities. However, the intermetallic hydrides may have useful thermodynamic properties and can out-compete similar lead-battery based mobile applications on performance, e.g. fork lifts and vehicles for indoor use. The advantage is that e.g. forklifts do not loose lifting power when the metal hydride is not fully charged with hydrogen as compared to lead-battery technology. The driving range may also be longer and the recharging time much shorter. Metallic hydrides with appropriate hydrogen uptake and release properties at near room temperature are also good candidates for stationary energy storage, which is a promising solution to efficiently use renewable energies, as demonstrated by the H2One™ system in the Toshiba Corporation [40].

of the reference and the sample side is known together with exact measurements of temperature and pressure. The procedure has been further elaborated elsewhere [34]. The gas manifold can further be connected to an in-situ sample environment. This gives the opportunity to observe the hydrogen absorption and continuous changes in the sample, e.g. change in lattice parameters or formation of intermediates, when hydrogen pressure is applied. A special sample environment rated to P < 1000 bar has been designed for this purpose, utilizing single-ended sapphire tubes as sample holders [35]. 3. Metallic hydrides Metallic hydrides are interstitial hydrides, in which hydrogen occupies the octahedral and/or tetrahedral sites in the metal structure, as shown in Fig. 5. Formation of metallic hydrides will result in an expansion of the metal lattice by up to 20–30 vol%. Several types of metallic hydrides have been developed with the purpose of increasing hydrogen storage capacity and improving thermodynamic and kinetic properties. Table 1 shows several types of metallic hydrides. AB-type alloys are formed from an element, A, which forms a stable elemental hydride and another, B, which is a non-hydride forming element. LaNi5, a representative example of an AB5 type alloy, was found to have the ability to uptake and release hydrogen at room temperature in 1970 [36]. The chemical composition was later modified and used as negative electrodes for Ni-metal hydride batteries that were commercialized in 1990 [37,38]. Metals and alloys with body-centred cubic (BCC) structure have less close-packed structures as compared to those of face-centred cubic (FCC) and hexagonal close- packed (HCP) structures. Among the known metallic hydrides, BCC alloys reach the largest reversible hydrogen capacity, i.e. up to ~3 wt% at room

4. Magnesium hydride Magnesium hydride, MgH2, with 7.6 wt% of hydrogen is one of the promising hydrogen storage materials, especially due to the high abundance (ranked as 8 among all the elements) in the earth's crust and low cost of magnesium. MgH2 can be synthesized by hydrogenation of Mg, which proceeds in an exothermic reaction with ΔH=−75 kJ/ mol H2. However, dehydrogenation requires T > 300 °C while the kinetics are sluggish mainly due to formation of oxide layers on the surface and the slow diffusion of hydrogen in MgH2 [43]. MgH2 can also be synthesized by a homogenous catalysis process proposed by Bogdanović in 1984 [44]. The mass production of MgH2 (purity > 93%) can be achieved by hydrogenation of Mg powders and tablets compressed with mechanically ground Mg ribbons [45]. Magnesium is today utilized for stationary hydrogen storage [46]. Magnesium hydride has a rutile-type structure with partly covalent Mg–H bonds (expressed as Mg1.91+H0.26) [47]. Several approaches

Table 1 Selected interstitial hydrides and related properties. Type of Hydride




ρm wt% H2b

T (1 bar) °C


Elemental AB5 AB2 A2B AB BCC

Pd LaNi5 TiMn1.5 Mg2Ni FeTi TiV2

PdH0.6 LaNi5H6 TiMn1.5H2.5 Mg2NiH4 FeTiH2 TiV2H4

Fm3m P6/mm P6/mm P6mm Pm3m BCC

0.57 1.40 1.90 3.62 1.90 2.6

– 12 −21 255 −8 –

[41] [42] [42] [42] [42] [41]


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competitive with both mechanical and newly developed electrochemical and ionic liquid pistons compression. Main advantages of MHx compression are the absence of moving parts, compactness, safety, and reliability whereas waste heat may be used for heating the MHx for the hydrogen release process [32,33]. Commonly, AB5 compounds e.g. LaNi5 are used in the hydrogen compressors as they are easily activated, often possess fast hydrogen desorption-absorption properties, and are tolerant towards impurities in the H2 gas [32]. These compounds have variable plateau pressures at RT depending on composition [42]. The hydrogen pressure can be increased by heating of the solid metal hydride and pressures of up to 5 kbar have been achieved [85,86]. Additionally, many studies have been performed on LaNi5 partially substituting La with Ce or Ni with Co, Al or Mn resulting in variations of the thermodynamic stability and enhancement of the durability during extensive absorption/desorption cycling, respectively [32,87–90].

have been designed to facilitate dehydrogenation at lower temperatures [48–50], including: (a) forming an alloy with a non-hydride forming transition metal like Fe [51], Co [52], Ni [53,54] or In [55], (b) changing the reaction pathway by making reactive hydride composites like the typical MgH2/Si system [56–58], and (c) reducing the crystal grain size < 2 nm as predicted by theoretical calculation [59]. Alloying effects may also improve hydrogen release and uptake kinetics and possibly the thermodynamics, e.g. with Al, Cu, or Pd [60–63]. Dehydrogenation/rehydrogenation kinetics can be promoted by nanoengineering and utilization of catalysts. Ball milling is a convenient way to produce nanostructured MgH2, which helps to break the oxide layers on the surface of Mg, introduce a large number of defects, and decrease the diffusion length of hydrogen [64,65]. The dehydrogenation kinetics can also be accelerated by the nanoconfinement of MgH2 into mesoporous scaffold materials [66,67]. Furthermore, a large amount of catalysts including transition metals, metal oxides, and carbon materials, have been proven to be effective for dehydrogenation/rehydrogenation of MgH2 [48,68]. Among them, Nb2O5, is well known to show the best catalytic performance, due to formation of Mg1−xNbO surface layers, which may break the dense MgO layers and facilitate hydrogen diffusion [69,70]. Even the addition of 0.05 mol% Nb2O5 to MgH2 enables the full hydrogen absorption of ~7 wt% within 60 s [71,72].

5.3. Switchable physical properties (smart windows and hydrogen sensors) In 1996, Y and La thin films were discovered to exhibit exotic optical properties upon hydrogenation [91]. The hydrogen concentration in e.g. YHx may be varied between 0≤x≤3 at RT and p(H2)≤1 bar. As x approaches 3, the YHx undergoes a change from a reflective metallic state to a transparent semiconducting state entirely dependent on hydrogen content [92,93]. The examination of YHx and LaHx has led to several other compounds with similar properties, e.g. a range of bimetallic compounds with Mg as one component: Mg-Ni, Mg-Ti, and Mg-Ca thin films with varying degree of transparency and colours [94]. This may allow windows in buildings to adapt to outdoor and indoor conditions which may reduce energy costs related to heating/cooling [95]. Currently, WO3 thin films are used in commercialized smart windows [96]. However, WO3 is only able to change between a transparent and a blue light-absorbing state which reemits some of the absorbed energy as heat. Hence, the reflective mirror state in the MHx is desired [94], and MHx has a big potential for advancement in smart windows. Development of cheap, reliable hydrogen sensors is another very important research field if hydrogen is to be introduced as a future energy carrier in both small and large-scale applications. The introduction of hydrogen-fuelled vehicles is a good example as such a car may contain multiple hydrogen detection sensors. However, many parameters of the sensor have to be optimized e.g. sensitivity, selectivity, size, and cost [97]. Introduction of thin films in optical hydrogen sensors was suggested in the 1980s as a Pd thin film which changes optical response as function of hydrogen uptake [98,99]. Later, a sensor was developed relying not only on the change in optical but also the electrical properties of PdHx with varying hydrogen content [100]. The drawback of using Pd is that cycling between Pd and PdHx but also the phase transition between α- and β-PdHx makes it susceptible to cracking i.e. mechanical damage. However, it has been shown that e.g. alloying with Ni, Au, or Ag can suppress this drawback [101–103]. As with the smart windows, the colour change observed in some metal hydrides may be exploited for hydrogen sensors [97].

5. New properties of metal hydrides in applications Besides acting as hydrogen carriers, metal hydrides also have other useful properties which will be highlighted in this section. Properties of complex metal hydrides is further discussed in another review in this issue of Progress in Natural Science: Materials International [19]. 5.1. Storage of solar thermal power Most commonly, the thermal energy storage in concentrated solar thermal power (CSP) applications is achieved by utilizing the specific heat of molten NaNO3/KNO3 salts, operating around 565 °C [73]. However, the efficiency of the CSP plants may be improved significantly, both in relation to cost and the amount of heat that is possible to store [74]. Many materials have thus been identified as promising for CSP purposes including metal hydrides [75]. The main advantage of metal hydrides is that the heat storage capacity may be 30 times higher than that available from current state-of-the-art molten nitrate salts [74,76], e.g. LiH has a theoretical heat storage capacity of 8397 kJ kg−1 and may operate at temperatures above 950 °C [77]. A CSP system based on a metal hydride operates through the highly endothermic and exothermic processes of hydrogen desorption and absorption, respectively. A metal hydride with high decomposition temperature (operating temperature) will be heated from solar energy during a day-cycle and will thus release hydrogen. The released hydrogen gas must then be stored either in a volumetric gas tank or in another metal hydride that operates at low temperature i.e. the low-temperature hydride has a much smaller enthalpy of reaction compared to the high-temperature hydride. As the system enters the night-cycle, hydrogen will be released from the low-temperature hydride (or gas tank) and reform the hightemperature hydride in an exothermic process developing heat. Optimization of the high-temperature metal hydride may be achieved through anion substitution with e.g. fluoride.[78,79] Studies show that fluorine substitution increases the thermal stability of the hydride and hence the operating temperature in a positive direction [80–83]. Indeed, optimizing the properties of the metal hydride itself is just as crucial as engineering of the metal hydride tank and the heat extraction system [84].

6. Conclusion Hydrogen, the lightest element of all, has a great potential as energy carrier for smoothing fluctuating renewable energies or storing the surplus electricity generated from renewable energies. As solid-state hydrogen storage materials, interstitial metallic hydrides with appropriate hydrogen uptake and release properties at near room temperature, have been attracting interest as promising candidates for stationary energy storage, in addition to be used as negative electrode materials for nickel metal hydride batteries. Magnesium hydride is also considered as a potential candidate for stationary energy storage due to

5.2. Metal hydride hydrogen compressor Utilization of a metal hydride hydrogen compression technology is 5

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