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Over the past few years, significant progress has been made in hydrogen-powered vehicles. Most of the development work focused on the powertrain and its integration into the vehicle. Currently, one of the key technologies that determines the development of the automotive industry are on-board hydrogen storage systems. Without efficient storage systems, the using of hydrogen to drive motor vehicles will be difficult to achieve. The physical storage density limits of compressed and liquid hydrogen have been more or less reached, whilst there is still potential in the development of various hydrogen storage materials. This chapter presents methods and problems related to hydrogen storage. Some of discussed technologies are immature, however this does not rule out for them future use therefore, their opportunities and foreseen potential were also presented.
Silesian University of Technology, Gliwice, Poland
Grzegorz Kubica
Silesian University of Technology, Gliwice, Poland
*Address all correspondence to: marek.flekiewicz@polsl.pl
1. Introduction
Hydrogen has the potential to become the sustainable fuel of the future, reduce global dependence on fossil resources and reduce pollutant emissions from transport. The spread of hydrogen as a fuel for vehicle propulsion poses several challenges regarding the way we use the energy contained in hydrogen, storing hydrogen on the vehicle and ensuring its availability. Currently, there are two ways to feed a motor vehicle using hydrogen. These are hydrogen internal combustion engines (Hydrogen ICE) and hydrogen fuel cells (FC). The first uses hydrogen to power an internal combustion engine. The second uses a fuel cell in combination with electric motors and a battery. Hydrogen-based internal combustion engines have recently attracted a lot of interest, but several practical barriers have prevented the rapid development of this technology. Therefore, the use of hydrogen as an additive to hydrocarbon fuels has been considered at this stage in order to achieve higher performance than purely hydrogen internal combustion engines. The use of a dual-fuel strategy increases combustion stability and thermal efficiency whilst reducing CO2 and unburnt hydrocarbon emissions and fuel consumption. The use of hydrogen in internal combustion engines achieves only 20–26% efficiency and low power output compared to internal combustion engines powered by fossil fuels [1]. However, its alternative use in fuel cells allows to achieve an efficiency of up to 60% [2]. Both ways of converting hydrogen energy require storing hydrogen in an amount that ensures a satisfactory vehicle mileage.
Hydrogen has the highest calorific value per mass of all chemical fuels. In addition, hydrogen is regenerative and environmentally friendly. Unfortunately, this element occurs in nature in the form of water and hydrocarbons. These means that its acquisition and storage require a significant amount of energy, its volumetric density is of crucial importance, which determines the weight and volume of the storage system. This problem is explained in Figure 1, which compares the values of hydrogen’s volumetric and gravimetric density with the values characteristic of conventional fuels and selected alternative fuels. The low energy density per unit volume of hydrogen makes storing and transporting gas a significant research and technical challenge. Consequently, storing hydrogen on a motor vehicle is a key technology enabling the development of hydrogen and fuel cell technologies [3, 4].
Figure 1.
Gravimetric and volumetric energy density of selected motor fuels [3].
Amongst the most important hydrogen storage methods that have been tried and tested over a long period of time is the physical method of storage based on compression or cooling, or a combination of both. Currently, many other new hydrogen storage technologies are being sought or investigated. These technologies can be grouped together under the name material-based storage technologies. These can include solids, liquids, or surfaces.
The hydrogen storage methods listed above should be characterised by technological simplicity and low price and should ensure operational safety. The current state of hydrogen storage technology in motor vehicles and the likely directions of their development are shown in Figure 2.
Figure 2.
Hydrogen storage technologies and directions of their development [5].
The weight and volume of a storage system based on each of the technologies listed above will depend on the expected mileage of the vehicle. To ensure a mileage of 400 km, the mass of hydrogen stored should be:
for a passenger car powered by an internal combustion engine, 8 kg of hydrogen, and 4 kg when a fuel cell is used,
for a truck, 32 kg, and respectively 16 kg,
and for the bus, respectively 41 kg and 20 kg [6].
Physical hydrogen storage methods are based primarily on the compression and cooling/liquification of hydrogen. The difference between these methods is explained in Figure 3.
Figure 3.
Hydrogen density dependence on pressure and temperature [7].
This figure shows areas characteristic of liquified hydrogen (LH2), liquified hydrogen at high-pressure (Cryo-Compressed Hydrogen) and compressed hydrogen (CGH2).
2.1 Compressed hydrogen storage
By storing compressed hydrogen-CGH2 (Compress Gaseous Hydrogen), we compress it to pressures ranging from 25 to 90 MPa and store it in pressure tanks. It is the conceptually simplest method of hydrogen storage, which allows the use of modern high pressure tanks type 3 and 4, based on lightweight composite materials (in which the material transferring the loads caused by huge pressures is a composite of carbon and glass fibres and the synthetic resins, secured from the inside with a metal or polymer liner). These designs allow hydrogen to be stored under a pressure exceeding 100 MPa. Today, the tanks are filled with hydrogen to a pressure of 35 or 70 MPa (Toyota Mirai and Hyundai Ix35 and Honda Clarity) [8, 9]. In practise, the compression of hydrogen to a pressure of 70 MPa requires energy approximately equal to 18 MJ/kg, which is about 15.5% of the energy obtained from hydrogen combustion. In addition to the significant advantages of this form of storage, such as a relatively simple construction of the tank, high rate of hydrogen release and refuelling, one of the most important disadvantages of these tanks is the low density of stored energy (Figure 4).
Figure 4.
Hydrogen storage tank under 70 MPa pressure for the Toyota Mirai car and a hydrogen storage system in the Honda FCX Clarity car [10, 11].
Storing compressed hydrogen-CGH2 (Compress Gaseous Hydrogen) is the conceptually most straightforward method of hydrogen storage, which allows the use of modern high pressure tanks type 3 and 4, based on lightweight composite materials (in which material transferring the loads caused by the high pressures is a composite of carbon fibre and synthetic resins, secured from the inside with a metal or polymer liner) [10, 11]. Today, the tanks are filled with hydrogen to a pressure of 35 or 70 MPa (Toyota Mirai, Hyundai Ix35, and Honda Clarity). In practise, hydrogen compression to a pressure of 70 MPa requires energy of approximately 18 MJ/kg, about 15.5% of the energy obtained from hydrogen combustion. In addition to the significant advantages of this form of storage, such as a relatively simple construction of the tank’s high rate of hydrogen release and refuelling, one of the most important disadvantages of these tanks is the low density of stored energy.
Vehicle manufacturers impose several requirements on tanks intended for hydrogen storage, not only regarding strength parameters defined in standards and regulations but also expect the tanks to be characterised by favourable performance parameters, i.e., satisfactory gravimetric and volumetric energy density of the energy storage system.
The mass of hydrogen per unit volume (kgH2/m3) at 25°C can be calculated using the simple relation 0.0807*p. This relationship originates in the ideal gas equations, and p is the hydrogen storage pressure in bars. For example, for a pressure of p = 35 MPa, the mass of stored hydrogen equals 28 kgH2/m3 [12]. Considering that 5 kg of hydrogen is necessary to ensure the light vehicle’s mileage in the 400–600 km range, the tank should have a capacity of 0.18 m3 [12]. The efficiency of energy storage in compressed hydrogen is about 94% and can be compared with the efficiency of energy storage in batteries, which is 75% [13].
It should be noted that increasing the hydrogen pressure increases the volumetric storage density (kgH2/m3) but reduces the overall energy efficiency. Moreover, doubling the pressure only increases the stored energy by 40–50%.
Type 1 steel tanks have been commonly used to store compressed hydrogen for many years, but steel is not desirable. The penetration and accumulation of hydrogen atoms inside the metal contribute to the so-called “Hydrogen embrittlement”, a group of corrosive phenomena that negatively affect the operation process. Considering that steel tanks are too heavy, tanks of types 3 and 4 are mainly used to store compressed hydrogen in modern motor vehicles.
The main parameters considered when considering hydrogen storage in automotive pressure tanks are type of vehicle, tank capacity, pressure of the stored hydrogen, and tank price.
Compressed hydrogen is stored in a closed system (Figure 5), which means that hydrogen can be stored without loss for a long time, provided that the materials used prevent hydrogen diffusion. The cylindrical shape given to the currently used pressure tanks results primarily from the favourable distribution of stress and the recommended and proven in-operation starting materials for their production. Designers strive to develop tanks whose form will be adapted to the spatial possibilities created by the structure of the chassis or body of the vehicle.
Figure 5.
Components of a pressurised hydrogen storage tank.
New materials and technologies create the possibility of producing nanocomposite layers, which are characterised by very low permeability of hydrogen (0.6cm3μm/m2dayatm). These ultra-high-performance nanocomposite barrier layers are achieved, amongst other things, by spray coating liquid crystal nano silicates mixed with polyvinyl alcohol (PVA) on a PET substrate film. The technology of improving the surface of the liner by spray coating allows for the creation of a barrier limiting the permeability of hydrogen in irregularly shaped tanks [14].
2.2 Equipment of the hydrogen storage tank
The equipment of the compressed hydrogen storage tank should consist of at least the following: manual and automatic shut-off valves, pressure relief devices and non-return valves on the gas supply and discharge lines. A diagram explaining the flow of hydrogen through the channels of the multi-function valve is shown in Figure 6. Selected solutions of these valves and their technical parameters are shown in Figure 7.
Figure 6.
A diagram explaining the hydrogen flow through the operational valve [15].
Figure 7.
Sketch of the HTV 350 OMB Saleri operation valve for composite valve and its primary technical data.
Following the requirements of the Regulations of the European Economic Commission as well as the relevant technical standards, the tank equipment is selected by the tank manufacturer. Considering the needs of motor vehicle manufacturers, the tanks are mounted in containers in the so-called gaseous fuel storage systems. These systems, in addition to the previously mentioned elements, also contain additional devices, such as, for example, a preliminary hydrogen pressure regulator, a ventilation system, etc. (Figure 8A and B). An example of a hydrogen storage system for an electric bus with a fuel cell is shown in Figure 9.
Figure 8.
Examples of compressed hydrogen storage systems, A-35 MPa and B-70 MPa.
Figure 9.
Compressed hydrogen storage system for a bus.
Following the requirements of the Regulations of the European Economic Commission as well as the relevant technical standards, the tank equipment is selected by the tank manufacturer. Considering the needs of motor vehicle manufacturers, the tanks are mounted in containers in the so-called hydrogen storage systems (HSS).
These systems, in addition to the previously mentioned elements, also contain additional devices, such as, for example, a preliminary hydrogen pressure regulator, a ventilation system, etc. (Figure 8A and B). An example of a hydrogen storage system for an electric bus with a fuel cell is shown in Figure 9.
In the lower part of Figure 10, there are pipes constituting the gas main serving all storage tanks. The upper line, with a diameter of 10 mm, supplies gas to the tanks during their filling, whilst the lower pipe, with a diameter of 8 mm, allows the collection of the stored gas.
Figure 10.
Operation valve of one of the tanks, its essential devices, and its connection to the other tanks.
Figure 11 shows the other two devices protecting the tank, i.e., in its central part and the end plug located at the other end of the tank. Each of these devices is connected by pipes to the inside of the tank, i.e., when opened it ensures the emptying of the tank.
Figure 11.
Thermic fusible (TPRD) mounted in the middle of the tanks and on the opposite side of the operational valve (red arrows indicate the direction of the gas outlet after opening the fuse).
Hydrogen storage tanks delivered to vehicle manufacturers must meet several requirements, including the assurance of the purity so-called “hydrogen purity”, essential for protecting the fuel cell from damage. The required purity of hydrogen and the level of its impurities for ensuring the homogeneity of hydrogen intended for use in road vehicle systems with proton cells (PEM) are specified in the EC 79/2009 and ISO/TS 14687-2 standards. These requirements mean that after the process of manufacture, the container is subjected to leak tests. Then the tank is subjected to rinsing, first with nitrogen, to obtain an atmosphere below the hydrogen explosion level, and is flushed with hydrogen to achieve a minimum concentration of 99.97% in the entire volume. For this purpose, pure hydrogen, 99.99% is used. Completing achieving the required purity, a certain amount of gas is left in the storage tanks, with pressure not lower than 0.2 MPa, which protects the storage tank against air entering its interior.
2.3 Storage of liquefied hydrogen
Liquid hydrogen (LH2) is a very light liquid (density 70 g/dm3 at −253° C). LH2 evaporates quickly at standard temperature, producing approximately 845 litres of hydrogen gas from 1 litre of LH2. Immediately after evaporation, gaseous hydrogen is still very cold and has a weight comparable to that of air, which makes it spread practically horizontally. It does not heat up quickly; its density gradually decreases and rises to the top.
Hydrogen can be stored in liquid form in tanks specially adapted for this purpose [16]. Changing the state of hydrogen from gaseous to liquid (liquefaction) allows us to increase the energy density of hydrogen significantly. However, hydrogen liquefaction is a process that requires much more energy than its compression. The energy needed to liquefy hydrogen is estimated to be 30–40% higher than that required to compress hydrogen. Thus, the energy efficiency of this process is relatively low. However, storing liquid hydrogen allows us to obtain a much higher density by mass and volume than other hydrogen storage methods. In addition, the storage of liquefied hydrogen in cryogenic tanks ensures much lower losses due to gas “escape” than in pressurised tanks for compressed hydrogen. These losses are even 25 times lower.
In order to keep hydrogen in a liquid state, it is necessary to use very low temperatures (of the order of 20 K, i.e. −253.15°C), for this reason, the tanks must be constantly cooled and thermally insulated from the environment.
The liquefied hydrogen storage tank developed by MAGNA STEYR Fahrzeugtechnik AG & Co KG is shown in Figure 12. This tank, consisting of an inner and an outer tank, stores approximately 10 kg of hydrogen. The materials used to make this tank are stainless steel or aluminium alloy. Both materials are very resistant to hydrogen embrittlement and exhibit negligible hydrogen permeation. In addition, their low specific weight, high strength, and high coefficient of thermal expansion, combined with excellent thermal conductivity properties, make the total weight of the tank with accessories around 150 kg.
Figure 12.
Liquefied hydrogen storage tank [17].
The space between the inner and outer tanks is filled with a highly insulating material and a vacuum, i.e., about 40 layers of foil limited the heat transfer through thermal radiation with a weight of 1.5–3.0 kg/m2, composed of polished aluminium or aluminized and polymer foils, which are separated from each other by fibreglass spacers. The vacuum pressure at 20 K is about 10−3 Pa, reducing thermal convection to a minimum. This insulation ensures that, when the vehicle is not used for more than 3 days, the heat input contributes to the evaporation of liquefied gas in an amount not exceeding 1–3% per day.
The cryogenic filling valve (7) and cryogenic check valve (8) are open during filling. Liquid hydrogen flows from the filling station through the Johnston-Cox connector (3) and the cryogenic filling valve into the inner vessel (1). The evaporated hydrogen leaves the internal tank through a cryogenic non-return valve and flows back to the filling station to maintain low pressure. When filling is complete, both cryogenic valves close. The evaporated hydrogen flows from the internal tank to the cooling water heat exchanger (5). The hydrogen heats to ambient temperature and flows further to the pressure control valve (9). If the inlet pressure is higher than the set regulation pressure, the flow will be closed, and hydrogen will not be able to flow through the tank heater (4). The pressure will drop because no heat will be delivered to the internal tank heater. In standby mode, both cryogenic valves are closed. When the vehicle is parked for a long time, the hydrogen pressure in the inner tank increases to the valve opening pressure (11). Overpressure in the inner tank cannot open the cryogenic valves. If the valve (11) is damaged, the pressure in the internal tank increases until the safety valve (12) opens. The last device that prevents the tank from exploding is the plate valve (PRV) (16).
Liquid hydrogen storage reaches the highest gravimetric and volumetric storage densities and, about adequate energy availability, is the most suitable fuel storage solution for future hydrogen vehicles. Despite the use of highly high-performance super-insulation designs, it has not yet been possible to find a solution for the problem of the boil-off losses occurring in liquid hydrogen storage tanks induced by heat input during relatively long periods of idleness or in unfavourable driving cycles. In particular, the limited down-scalability of liquid hydrogen storage tanks defines the optimum field of application in the segments of large passenger cars, buses, and trucks.
2.4 Cryo-compressed hydrogen storage tanks
The storage of hydrogen in cryo-compressed tanks offers many more advantages, opportunities and potential as compared with pressurised hydrogen and liquid hydrogen storage technology [18]:
lower requirements for expensive carbon fibres due to designing for a maximum tank pressure of 350 bar compared with 700 bar for state-of-the-art CGH2 storage tanks could lead to lower material and even production costs for CcH2 compared with CGH2 tanks,
long hydrogen loss-free dormancy time minimise the risk of hydrogen boil-off losses during long parking periods or low vehicle use compared to LH2 tanks.
energy and heat management requirements are compatible with internal combustion engines and fuel cell systems. In particular, low-temperature PEM fuel cells might benefit from the cooling power of cryogenic hydrogen that is warmed up by waste heat from the fuel cell.
As a possible solution for the problem of boil-off losses and the high requirements regarding insulation quality, BMW has developed the concept of supercritical cryo-compressed hydrogen storage (CcH2 Cryo-compressed Hydrogen), which promises more straightforward and more cost-efficient insulation whilst enabling loss-free operation of the storage tank in under all typical vehicle operating conditions [19]. This innovative concept reduced the weight of the finished tank to one-third of the importance of a conventional cylindrical steel tank (Figure 13). Freedom in shaping its form ensures a high degree of flexibility and allows for significant energy savings in the manufacturing process. As all auxiliary systems have been integrated into the tank housing, it takes up less car space and is much easier to handle. The internal tank is designed modularly, simplifying the production process compared to existing hydrogen tanks. This tank, filled with 10 kg of hydrogen, could allow a range of over 500 kilometres.
Figure 13.
A tank developed by the BMW group for a hydrogen-powered car [20].
A prototype solution for a tank storing cryo-compressed hydrogen at a pressure of 30 MPa is shown in Figure 14. It is a tank also developed by BMW, where it is possible to store both liquefied and compressed gas. The essential technical parameters of this tank are listed in Table 1.
Figure 14.
Prototype solution of a tank for storing compressed and liquefied hydrogen [7].
Maximum usable capacity
CcH2–7,8 kg (260 kWh) CGH2–2,5 kg (83 kWh)
Working pressure
≤ 35 MPa
Emergency opening pressure
≥ 35 MPa
Filling pressure
CcH2–30 MPa CGH2–32 MPa
Filling time
< 5 minutes
Storage system capacity
~235 dm3
Mass of the storage system plus mass of hydrogen
~145 kg
Hydrogen losses
<< 3 g/day 3 ÷ 7 g/day (CcH2) < 1% per year
Additional features
Active pressure control inside the tank
Integration with the car body
Using the waste heat of an internal combustion engine or fuel cell
Table 1.
Characteristics of the CGH2, and CcH2 storage system.
Considering the advantages and disadvantages of the above methods, compressed hydrogen (CGH2) is the best system and is a state-of-art technology [21]. Hydrogen storage systems with 70 MPa tanks are sufficient to ensure desired mileage of vehicle. Tanks can be refuelled with hydrogen in 3–5minutes. The storage of liquefied hydrogen is also a relatively mature technology, but it is associated with a very significant problem of hydrogen losses whilst shutting down the vehicle from traffic. These losses and the related losses during refuelling are very substantial. Cryo-compress hydrogen storage helps to improve volumetric hydrogen density and safety over compressed hydrogen or cryogenic LH2 alone. However, the availability and cost of infrastructure are still the main obstacles to developing this storage method.
The hydrogen storage technologies discussed above in gaseous and liquefied form are characterised by the structure’s maturity and many years of operational experience. However, searching for a technology that could increase the energy density of stored hydrogen has created an option that today engages the efforts of many research centres worldwide. This possibility is based on metal hydrides, carbon sorbents, and chemical hydrides (Figure 15).
Figure 15.
Simplified division of hydrides and an exemplary structure of selected compounds.
The main requirements for these modern materials are high gravimetric density (over 6.0% by weight), easy hydrogen absorption/desorption at moderate temperatures and pressures, low price of materials and their ecological safety [22].
Metal hydrides are solid materials that enable safe hydrogen storage at moderate temperatures and pressures. Many metal hydrides reach volumetric energy densities that approximate those of liquefied hydrogen. Metal hydrides consist of metal ions that form a lattice structure. Hydrogen adsorbs at a metal centre, dissociates to form atomic hydrogen, and is finally inserted into the metal lattice. The total process is exothermic [22]. Thus, when the carrier is loaded, heat must be removed.
In recent years, research into new hydrogen storage materials has focused on light metals such as Li, Be, Na, Mg, B and Al. These metals are of particular interest because they can contain a high percentage by weight of hydrogen. The US Department of Energy (DOE) has outlined several requirements for an onboard hydrogen storage system. For large-scale storage, the sensitivity of metal hydrides is problematic. Moreover, their solid nature, combined with the considerable enthalpy of dehydrogenation, creates challenges concerning heat transfer for fast hydrogen release. Whereas heat transfer onto liquids and gases is comparatively simple, uniform heat distribution into a large solid mass is possible only when complex and expensive devices are used. However, hydrogen storage in metal hydrides is undoubtedly more appropriate for small- and very-small-scale applications. Although metal hydride storage has a very high volumetric storage density (> 100 H2 kg/m3), the gravimetric storage density is very low due to the heavy metals or alloys. The gravimetric storage density of hydrides is between about 0.02–0.07 H2kg/kg. At ambient temperature and pressure gravimetric storage density is of about 0.03 H2 kg/kg [22].
In summary, the features that determine the suitability of hydrides as hydrogen storage include:
the ability to reversibly store hydrogen,
large capacity of stored hydrogen,
low pressure and temperature (up to ~90°C) of hydride dissociation,
high rate of absorption and desorption,
a small amount of energy required to release hydrogen,
low sensitivity to gaseous pollutants,
a large number of possible charging and discharging cycles,
safety (low pressure, non-flammability),
low price.
The prospective hydrogen storage may be highly porous carbon materials with a large specific surface area. The storage of molecular and atomic hydrogen in carbon materials takes place through electrochemical reactions and physical sorption (adsorption) on the surface of solids, primarily due to the interaction of Van der Waals forces. Desorption of hydrogen from carbonaceous materials occurs due to the supply of the appropriate amount of thermal energy to the system. The most considered carbon materials storing hydrogen include activated carbon, graphite, fullerenes and carbon nanotubes. Activated carbon is an interesting material for hydrogen storage because it has a very large specific surface resulting from its structure, with numerous pores and microcracks. Micropores have the greatest adsorption capacity because their sizes are comparable to potential particles that can be stored on activated carbon. Mesopores play a minor role in the adsorption process, whilst macropores are used mainly to transport substances. The hydrogen sorption capacity achieved at a temperature of 77 K by activated carbon, whose specific surface area is 1315 m2/g, is equal to 2% m/m (where m/m—the mass of hydrogen to the mass of the storage material). Modifying activated carbon with potassium hydroxide (KOH) allows for further development of such material’s porous structure, increasing the amount of stored hydrogen by 3.7 times.
Graphite, one of the allotropes of carbon, is also considered a potential hydrogen storage material. It has a multi-layer structure composed of single graphene layers. The individual layers of graphite are connected to each other by weak Van der Waals interactions, which makes it possible to manipulate them appropriately. The storage of hydrogen between graphite layers can be configured by adjusting the distance between adjacent layers. On the other hand, hydrogen desorption can occur when the storage system is heated to a temperature of about 450°C. In the case of a graphite layer with a specific surface area of 1315 m2/g, the sorption capacity of such a system is 3.3 m/m.
Fullerenes, like graphite, are one of the allotropes of carbon. Fullerene molecules are built of pentagonal or hexagonal rings forming a hollow, closed block, which is built of 28 ÷ 1500 carbon atoms. The most popular fullerene is C60 fullerene, built of 60 carbon atoms. Due to its susceptibility to hydrogen addition, it finds the most comprehensive application as hydrogen storage amongst other types of fullerenes (Figures 16 and 17).
Figure 16.
Schematic image of C60 peapods @ SWCNTs: (a) side view; (b) front view; (c) tilted view (Carbon Peapod is a hybrid nanomaterial and consists of spherical fullerenes enclosed in carbon nanotubes) [23].
Figure 17.
Scanning photo of carbon nanotubes [24].
Carbon nanotubes, due to their unique structure, are interesting material for storing hydrogen due to their unique structure. Nanotube structures are made of cylindrically wound graphene layers in the form of hollow cylinders. Nanotubes differ in length, diameter, and angle of rotation. Due to the number of graphene walls that build carbon nanotubes, the following division is used:
Single-Walled Carbon Nanotubes-SWCNT,
Double-Walled Carbon Nanotubes-DWCNT,
Multi-Walled Carbon Nanotubes-MWCNT.
Carbon materials are characterised by high hydrogen capacity, low desorption and allow hydrogen storage depending on the form of carbon in the following weight percentages: activated carbon-5.5%, graphite-4.48%, carbon fibres-6.5%. Single-walled carbon nanotubes-4.5%, multi-walled carbon nanotubes-6.3%.
In addition to a group of materials listed above, which includes coal and materials with a large surface area should be mentioned a technology for storing hydrogen in glass capillary arrays and glass microspheres [25]. Glass capillary arrays are placed in pressure-resistant vessels. Capillaries are closed by melting one end and closing the other with an alloy. Filling the vessel with hydrogen continues until the storage pressure is reached. Glass capillary arrays are used for the safe filling, storage, and controlled release of hydrogen in mobile applications [26, 27]. Flexible glass capillaries using the cryo-compression method of hydrogen storage can provide the expected gravimetric and volume density. However, this technology is accompanied by disadvantages that limit the durability of the storage system. These defects occur in glass structures and are caused by bubbles, cracks or grooves (Figure 18).
Figure 18.
Examples of hydrogen storage system solutions used by C.En the hydrogen storage solution [28].
An innovative storage device for compressed hydrogen is based on glass capillary arrays with a honeycomb-like structure and a diameter of approximately 100 μm. This structure is mechanically stable and can manage pressures over 1000 bar, even with thin capillary walls [29]. The experimental work reports that the volumetric and gravimetric capacities of the investigated structures can exceed the DOE target, at least in the investigated microscale device, which has a volume of 8.5 cm3.
Another method is to use glass microspheres that are first filled with hydrogen at high pressure of around 350–700 bar at 300°C, followed by rapid cooling at room temperature. The spheres are then transferred to the low pressured vehicle tank and are reheated again at 200–300°C for controlled release of hydrogen. The materials possess low volumetric density and require high pressure for filling [30].
Also, of interest are those technologies that would allow the use of the existing infrastructure and conventional fuel tanks. The method is described by researchers at the University of Oregon, who have developed a liquid-phase hydrogen storage material-based on boron and nitrogen that works safely at room temperature; it is resistant to both air and moisture; releases H2 in a controlled and clean manner below or on the proton membrane (PEM) at 80°C; moreover, it uses cheap and readily available catalysts for H2 desorption and has a relatively good gravimetric and volumetric energy density [31, 32] (Figure 19).
Figure 19.
An example of hydrogen adsorption and desorption using BN-methylcyclopentane and a catalyst [31].
Figure 20 explains the impact of using liquid hydrides in propulsion systems on the construction of a motor vehicle. Liquid fuel and low pressure during its storage allow using a tank of any shape. However, the tank design must allow for the collection of products generated during hydrogen desorption, and the fuel infrastructure should be adapted to receive these products during the next refuelling. This waste threatens the environment, and the condition for disseminating this storage method is to solve the problem of their disposal.
Figure 20.
A simplified diagram of a vehicle powered by hydrogen stored in liquid hydrides.
In summary, the features that determine the suitability of hydrides as hydrogen storage include:
the ability to reversibly store hydrogen,
large capacity of stored hydrogen,
low pressure and temperature (up to ~90°C) of hydride dissociation,
high rate of absorption and desorption,
a small amount of energy required to release hydrogen,
low sensitivity to gaseous pollutants,
a large number of possible charging and discharging cycles,
Hydrogen storage is a key technology enabling the development of hydrogen-powered vehicles. However, storing enough hydrogen on board to achieve a range of 500 km is a significant challenge. Due to hydrogen’s gravimetric and volumetric density, hydrogen storage systems today face challenges in cost, durability, operational safety, and infrastructure costs. Consequently, the widespread commercialization of hydrogen-powered vehicles may be limited if new innovative technologies are not implemented.
Methods and technologies of hydrogen storage are currently the subject of intensive research, and their objective assessment is challenging because each of the technologies presented above has advantages and disadvantages. However, specific criteria can be adopted that compromise determining which technology is recommended for today’s automotive industry.
To address all challenges of hydrogen storage systems, performance targets for light-duty vehicles were developed by the U.S. Department of Energy (DOE) assuming an estimated mileage of circa 500 km. The goals set by the DOE, which are presented in Table 2, determine the research directions of most research centres [33].
Storage Parameter
Units
2020
2025
Ultimate
System gravimetric capacity: Usable, specific energy from H2 (net useful energy/max system mass)
kWh/kg (kg H2/kg system)
1.5 (0.055)
1.5 (0.055)
2.2 (0.065)
System volumetric capacity: Usable energy density from H2 (net useful energy/max system volume)
kWh/L (kg H2/L system)
1.0 (0.030)
1.3 (0.040)
1.7 (0.050)
Storage system cost:
$/kWh net ($/kg H2)
10 333
9 300
8 266
Fuel cost
$/gge at pump
4
4
4
Durability/Operability: Min delivery pressure from storage system
bar (abs)
5
5
Charging/Discharging rates: System fill time
min
3–5
3–5
3–5
Table 2.
Technical system targets: Onboard hydrogen storage for light-duty fuel cell vehicles [1].
Useful constants: 0.2778 kWh/MJ; Lower heating value for H2 is 33.3 kWh/kg H2; 1 kg H2 ≈ 1 gal gasoline equivalent (gge) on energy basis.
It is found that the pressure vessel technology is favourable because it is easy to implement with high storage energy efficiency and at low cost. The main drawback is low volumetric storage density. The influence of this disadvantage can be lessened if the technology develops for safe operation at higher storage pressures. Hydrogen liquefaction is more suitable for space applications than automotive because of its high volumetric and gravimetric efficiency. The disadvantages are the high cost and low energy efficiency. Metal hydride and carbon nanotube adsorption are promising hydrogen storage technologies as the volumetric efficiency is very high, and the gravimetric efficiency is comparable with the high pressure gas compression method. Therefore, metal hydride and carbon nanotube adsorption should receive more research efforts to realise a sustainable hydrogen economy.
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Written By
Marek Flekiewicz and Grzegorz Kubica
Submitted: 23 August 2023Reviewed: 09 September 2023Published: 31 October 2023
Open access peer-reviewed chapter
