3.1. History

As for hydrogen storage technologies history, there are different starting points. In the case of compressed hydrogen storage technology, the use of this technique is not new. In Teeside, England, 5 MPa compressed hydrogen was stored in caverns and, in France, between 1956–1972, an aquifer was used to store a synthetic gas of 50–60% hydrogen [59]. However, as subway hydrogen storage systems were too limited to be implemented on a large scale [60], coupled with a growing interest in hydrogen in the 1970s as a result of the oil crisis [61], led to the research and development of hydrogen storage tanks, whose vessels were initially made of aluminium; however, as they were not strong enough, vessel materials were changed to a carbon-fibre composite [62].

As for the liquid hydrogen histort, although hydrogen began to be stored in liquid form in the 1940s, it was not until the 1980s and 1990s that liquid hydrogen began to be studied seriously because of its high potential [62].

On the other hand, the origins of metal hydrides storage technology date back to 1810, when Luigi Sementini, a professor of chemistry in Naples, published a report of a new method for extracting potassium and sodium. Many years later, Thomas Graham became interested in metal hydrides because he saw in this technology a new aspect of his studies on gas diffusion and extended the research with metals from platinum to palladium to observe the high adsorption of hydrogen. However, it was not until the laboratory work of Winkler (whose work led to the discovery of a larger number of metal hydrides), Moissan and others, that industrial developments became possible. Thus, in 1905, Bitterfeld claimed in a patent that molten calcium in an iron vessel rapidly absorbed hydrogen, and a 1943 patent established the idea of hydrogen as a fuel cell in a chemical energy package thanks to metal hydrides [63].

In the case of complex hydrides storage technology history, although they are known since 19th century (there is a report [64] on metal amides from 1809), it was not until the 1960s when they were started to be studied as potential hydrogen storage materials [64]. Many years later [65], in the mid-nineties, Bogdanovic discovered hydrogen uptake and release for sodium alanate, NaAlH₄, at moderate conditions. Later, in 2002 [65], P. Chen discovered reversible nitrogen-based complex hydrides and, in 2003, A. Züttel, and co-workers were the first to start investigating tetrahydridoboranates, such as LiBH₄, [65].

Finally, regarding alkalimetal+H₂O technology, it is far from be implemented, as it is still in the development phase. It was in 2003 that Li et al. generated hydrogen from a reaction between sodium borohydride (NaBH₄), with a gravimetric density of 10.6 wt. %, and water at an operating temperature of 60 °C [66].

3.2. Influence. Fundamental Principles

3.2.1. Compressed Hydrogen Storage

Compressed hydrogen storage is currently the most common method of hydrogen storage. Typically, hydrogen gas is pressurized in a metal-composite tank at a given pressure, which can vary widely depending on the tank and its use, from 200 bar to 700 bar [67]. Higher pressures have been used for the storage of gaseous hydrogen in order to achieve volumetric densities close to that of liquid hydrogen, 70.8 g/L (about 1000 times the volumetric density of hydrogen gas at ambient pressure and temperature).

3.2.2. Liquid Hydrogen Storage

To obtain liquid hydrogen, it is necessary to reach the hydrogen critical temperature, 33 K, i.e., almost to absolute zero. In this sense, liquid hydrogen storage tanks are at ambient pressure and a temperature of 21.2 K; in addition, the process to obtain it is a Joule-Thomson cycle in which gaseous hydrogen is first compressed, then cooled in a heat exchanger and finally expanded in a valve isenthalpically to form liquid. Although theoretically only 3.23 kWh/kg of energy are needed to liquefy hydrogen, in the practice, this values are as high as 15.2 kWh/kg, i.e., half of the Lower Heating Value (LHV) of hydrogen (33.36 kWh/kg) [68].

3.2.3. Metal Hydrides Storage

Metal hydrides (MHx) are the most technologically relevant class of hydrogen storage materials, as they can be used in a variety of applications at ambient pressure and temperature. In this case, gaseous hydrogen molecules are absorbed by the material in the solid state to form metallic hydrogen compounds and the hydrogen is distributed compactly throughout the metal lattice.

The reaction is reversible and exists as an equilibrium state under certain conditions of pressure and temperature. In other words, by changing the conditions, the reaction can move in the forward or backward direction. The heat on the right-hand side indicates that heat or energy is released when the MHx is formed (during the charging process), and therefore heat must be put into the system to release the hydrogen from the metal hydride (during the discharging process).

3.2.4. Physisorption

The physisorption of gas molecules, Figure 10, onto the surface of a solid, is the result of resonant fluctuations of charge distributions (i.e., Van der Waals interactions, composed of two terms: an attractive and a repulsive one, which decrease with distance, , in a ratio of  and , respectively). For this reason, physisorption is weak, and significant physisorption can only be observed at low temperatures (<273 K) [68]. Porous materials are a potentially promising storage technology for absorbing hydrogen, since they can reach a high capacity and can release the gas reversibly [69,70]. Among all porous materials, porous carbon materials and Metal Organic Frameworks (MOFs) are known to be promising. This technology has advantages such as low cost of materials, high surface area, faster charging and discharging processes or the possibility of mitigating thermal management issues. However, the need for low pressure and temperature, the weight of carrier materials and low gravimetric and volumetric density make the application of this technology difficult. These disadvantages have meant that experiments with this technology have been unsatisfactory; and it is far from be widely used, being only possible to find applications in small-scale experiments [70].

Figure 10. Physisorption process.

3.2.5. Complex Hydrides Storage

Complex hydrides are generally solid ions composed of cations bonded to complex anion groups (centered on Al, B or N, among others), such as AlH4−, NH2− or NH2−, through a covalent bond, in which hydrogen participates. Due to slow kinetics reactions, decomposition of complex metal hydrides take place at high temperatures (in the case of LiBH4, it takes place at 500 °C), while hydriding reaction take place at high pressures (up to 200 MPa) because of a faster reaction rate [71]. Figure 11 shows physical process of complex hydrides.

Figure 11. Complex hydrides process.

3.2.6. Alkalimetal+H₂O

This technology combines a particular case in complex hydrides (which it is made, for this hydrogen storage technique, from alkali or alkaline earth cations bonded to the complex anions introduced above) with water to cause a simple reaction in which hydrogen is obtained as a product [72]. Figure 12 shows the physical process of this technology.

Figure 12. Alkalimetal+H₂O physical process.

3.3. Technical Comparison

From a technical point of view, for compressed hydrogen storage technologies there are currently different types of compressed hydrogen tanks: type I are all-metal tanks in which hydrogen is usually stored between 200 and 300 bar (this type of tank has a low gravimetric density of about 1 wt. % due to the materials from which it is made); type II are metal tanks with their cylindrical part reinforced by Carbon Fibre Reinforced Polymer (CFRP) composite material, which allows this kind of tank to reach a slightly higher storage capacity than type I tanks (both type I and type II tanks are used in stationary applications); type III are tanks that have a fully CFRP-wrapped metal liner; type IV tanks are similar to type III tanks except that their liner is polymeric and not metallic (type IV tanks have reached a gravimetric density of 4.2 wt. %, because these tanks are made of lighter materials, and a volumetric density of 24 g/L at 700 bar); and, finally, type V tanks are in the development phase and are completely made of composite materials (unlined structures). Among these tanks, the most common and widely used are type III and type IV tanks due to their light weight and fatigue resistance, as well as the fact that their nominal working pressure is usually 350 to 700 bar (considerably higher than type I and type II tanks), which explains the use of these type of tanks in the automotive sector or for industrial purposes [73–75]. In addition, the industry has set a target of storing compressed hydrogen in 700 bar tanks, whereby a 6 wt. % gravimetric density and a 30 g/L volumetric density can be achieved [69]; in comparison, a 350 bar hydrogen tank has a gravimetric density of 5.5 wt. % and a volumetric density of 17.6 g/L [76]. Furthermore, this technique has the advantage that it can be used to repeatedly (and frequently) charge and discharge the hydrogen in the tank up to a lifetime of 20 years [77]. In fact, in the case of type III tanks, they can be charged and discharged up to 5122 times before their lifetime comes to an end [78]. Although this technique consumes 2.21 kWh/kg for an isothermal compression from 0.1 to 80 MPa, this value is much lower than that of other techniques such as hydrogen liquefaction [69].

Regarding liquid hydrogen storage, this technique has the advantage of a high volumetric density, 71 g/L, compared to compressed gaseous hydrogen storage tanks [79], but the disadvantage of having to deal with boiloff ratio, which can be 0.1 to 0.2% per day of the total liquid hydrogen depending on the ratio of area to volume, so in order to reduce these losses, it is necessary to reduce the area, in proportion, more than the volume [80]. Furthermore, according to the U.S. Department of Energy (DOE), liquid hydrogen storage tanks lifetime are set at 30 years by 2025 [81].

As for metal hydrides storage technology, current metal hydride tanks can reach an operating charge pressure of more than 100 bar. However, for practical and economic reasons, less than 30 bar is preferred; on the other hand, the discharge process must be carried out at a pressure of about 2 bar and at a temperature below 100 °C. Although this technology provides a high volumetric density, 100 g H2/L, its maximum gravimetric density is below 7 wt. %, but in practice, it is usually around 3 wt. %. Although this storage method has efficiency of 88% [82], its main disadvantage is the lifetime (up to 1500 cycles or 10 years) [83].

The storage materials to be combined with hydrogen in this technology include Ni, Co, Al, Mn, Sn, Cr, Fe, V, Mg, among others [82,84]. 

On the other hand, for the complex hydrides storage technology, high volumetric and gravimetric densities can be found (which will be different based on the material used). For example, for ammonia borane, NH3BH3, a 19.6 wt. % and a 150 g/L gravimetric and volumetric densities, respectively, are found [85], while for Mg2FeH6, a 5.5 wt. % and a 150 g/L gravimetric and volumetric densities, respectively, are found [86]. Furthermore, this technology has a long lifetime, with more than 5000 adsorption/desorption cycles [65]. Although these densities made complex hydrides storage a serious option to store large amounts of hydrogen (up to 700 kg [65]), this technology has to cope with issues such as thermal management (heat removal at several hundred of degrees) during refuelling [85].

Finally, for Alkalimetal+H₂O technology, the materials that can be used have a wide range for both volumetric and gravimetric densities, varying from 25.86 g/L for CsH to 138.08 g/L for BeH2 in the case of volumetric density, and from 0.75 wt. % for CsH to 18.39 wt. % for LiBH4 in the case of gravimetric density [72]. Furthermore, an advantage of this technology over complex hydrides is a lower operating temperature, for example, the decomposition temperature of NaBH4 to obtain hydrogen is 565 °C [72], while for the reaction of this material with water, the operating temperature is just 60 °C [66]. Due to their higher gravimetric and volumetric densities, together with a higher stability of borohydrides, some of the most promising materials in this technology (still under research), LiBH4, NaBH4 or Ca(BH4)2, among others [72], can be found.

Once the different hydrogen storage techniques have been analysed, a comparison has been made between gravimetric and volumetric densities for the different hydrogen storage alternatives, Figure 13 (where alkali metals are treated as a particular case of complex hydrides, but in the case of a reaction with water, the operating temperature is much lower). In addition, Table 10 gives a technical comparison between the different commercial hydrogen storage technologies discussed in the paper.

Figure 13. Alternatives for hydrogen storage: technical comparative.

Table 10. Summary of technical parameters of commercial hydrogen storage technologies.

4. Battery and hydrogen storage technologies applications

Regarding the possible applications of the different energy storage systems that have been previously studied, different applications in a wide range of power can be found.

First, in the case of batteries, different applications depending on the technology that is studied can be found. For example, regarding conventional batteries applications, for lead-acid batteries applications such as household Uninterruptible Power Supply (UPS) of the order of few Wh or such as submarine power or load-levelling of the order of several MWh can be found [87], while for Li-ion batteries applications such as portable electronic devices or electric vehicles (EVs) can be found [88] and finally for NiCd batteries applications such as aviation safety, telecommunication network or off-grid PV can be found [47].

As for the applications of molten salt batteries, ZEBRA batteries can be used in electric and hybrid electric vehicles or energy storage applications, while NAS batteries can be used for wind power integration or high-value grid services [46].

On the other hand, regarding the applications of RFBs, different examples  such as UPS, interseasonal storage, load levelling function or electric and hybrid vehicles, especially those of large dimensions (due to the low specific energy) [50] can be found.

Regarding the hydrogen storage technologies, compressed hydrogen storage technique can be used in different applications, such as small-scale storage, so it can be used in different vehicles like cars and buses [82], as a storage system in a microgrid [89] or even stored in a 700 bar tank in a hydrogen-powered snow groomer [90].

On the other hand, although it is currently a problem to implement liquid hydrogen storage in hydrogen-powered automotive applications, it is used in aerospace engineering [91] and in marine applications as temperature sensors [92]. One future direction for this technology is to combine with compressed hydrogen storage technology to obtain cryocompressed hydrogen storage tanks, which are capable of increasing volumetric density from 70 g/L at 1 atm to 87 g/L at 237 atm [62].

As for metal hydrides storage, this technology can be found to be used in different applications such as stationary and mobile, heat storage, automotive, railroads or a high pressure metal hydride tank (>875 bar) for refuelling fuel cell vehicles [84,93].

Finally, complex hydrides storage technology can be used in different applications such as on-board hydrogen storage, stationary storage, or portable power [71].

If a summary study for the application field of each and every storage technique (for both hydrogen and battery storage techniques) is done, Figure 14, it can be found that none of the technologies that have been analyzed can be used in the order of bulk power (greater than 100 MW), what explains why large scale energy storage is not currently available; however, they can be used in other applications such as UPS Power Quality (for example, lead acid batteries, ZEBRA batteries or redox flow batteries), in vehicles (such as ion lithium batteries, hydrogen composite cylinders or potentially metal-air batteries) or as a grid support (for example, compressed hydrogen composite cylinders or lead acid batteries, among others).

Figure 14. Application field for each technology

5. Why and when choose one energy storage system over another

Currently, there are two major green energy storage alternatives that people are considering for small and medium-sized applications: batteries and hydrogen storage. It is obvious that batteries are the most widespread (computers, mobile phones, vehicles), while hydrogen storage is used in lesser-known applications like heavy transport (trucks, buses, railways, submarines or spy-planes), or renewable sources-based microgrids.

In order to select the best choice, the first comparison to be made is the lifetime of the storage systems. For example, for a hydrogen storage system, a lifetime of 5122 cycles can be found for a type III compressed hydrogen tank [78], which is a longer lifetime than most of the batteries (only redox flow batteries and some lithium-ion batteries can present better behaviour in terms of lifetime), while for a metal hydride tank, the lifetime is up to 1500 cycles [83], which is only competitive against some battery technologies such as metal-air batteries or lead-acid batteries. On the other hand, regarding storage systems round trip efficiency, for metal hydride tanks, efficiencies of 88% can be found [82]. Although some batteries can be found with higher round-trip efficiency, this storage system is competitive against battery ones. However, contrary to battery storage systems (which convert electrical energy into chemical energy to store energy in the charging process and, in the discharging process, batteries convert chemical energy into electrical energy), a hydrogen storage system does not convert the electrical energy into chemical energy in the charging process by itself (as well as it does not convert chemical energy into electrical energy in the discharging process), since a device is needed to produce hydrogen from electrical energy. If green hydrogen (the one obtained via renewable powered electrolysis) is required to be produced, it is necessary to use an electrolyser to produce hydrogen. For an alkaline electrolyser (the most developed electrolysis technology) this process has efficiencies between 43–66% [94]. Furthermore, once the hydrogen is stored in the tank, to obtain electrical energy from it, another device is needed; if that device is a polymeric electrolyte membrane fuel cell (such as the one used to discharge the metal hydride tank in the article), its efficiency is around 45%. In this sense, for the whole process (production, storage and conversion into electricity), even if the energy losses associated with storage process (which in the case of metal hydride tanks, are about 12% of the stored energy), the overall efficiency is 20–31%, while for battery storage systems, the round-trip efficiency varies from 60% to 95%, i.e., between 2 and 4.75 times the hydrogen process efficiency (even if the losses associated with storage process are ignored). In addition, since the fuel cell efficiency is 45% (i.e., much lower than the whole round-trip efficiency of battery storage systems), the equivalent energy in a hydrogen storage tank needs to be considerably higher than the energy stored in a battery in order to obtain the same electrical energy.

Finally, although hydrogen systems (i.e., apart from the hydrogen storage, an electrolyser to produce hydrogen and a fuel cell to convert chemical energy from hydrogen into electric energy are needed) hardly can compete with batteries in terms of overall efficiency, in terms of specific energy density, hydrogen has no rival. Hydrogen has a lower heating value of 33.36 kWh/kg, i.e., more than a hundred times batteries specific energy. Therefore, hydrogen storage systems can complement batteries and be used in renewable sources-based plants as a long-term storage system, while batteries would act as short and medium-term storage system.

Author Contributions: Conceptualization, J.R. and F.S.; methodology, F.S. and J.R.; software, F.J.V.; validation, F.J.V.; formal analysis, J.R. and F.S.; investigation, J.R. and F.J.V.; resources, J.M.A.; data curation, F.J.V.; writing—original draft preparation, J.M.A., F.S. and J.R; writing—review and editing, J.R. and F.S.; visualization, F.J.V.; supervision, F.S. and J.M.A.; project administration, J.M.A.; funding acquisition, J.M.A. All authors have read and agreed to the published version of the manuscript.

Funding: This research was funded by 1) Spanish Government, grant Ref: PID2020-116616RB-C31, 2) Andalusian Regional Program of R+D+i, grant Ref: P20-00730, and 3) FEDER-University of Huelva 2018, grant Ref: UHU-1259316.

Institutional Review Board Statement: Not applicable

Informed Consent Statement: Not applicable

Data Availability Statement: Not applicable.

Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

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