A hydrogen economy is needed, in order to resolve current environmental and energy-related problems. For the introduction of hydrogen as an important energy vector, sophisticated materials are required. This paper provides a brief overview of the subject, with a focus on hydrogen storage technologies for mobile applications. The unique properties of hydrogen are addressed, from which its advantages and challenges can be derived. Different hydrogen storage technologies are described and evaluated, including compression, liquefaction, and metal hydrides, as well as porous materials. This latter class of materials is outlined in more detail, explaining the physisorption interaction which leads to the adsorption of hydrogen molecules and discussing the material characteristics which are required for hydrogen storage application. Finally, a short survey of different porous materials is given which are currently investigated for hydrogen storage, including zeolites, metal organic frameworks (MOFs), covalent organic frameworks (COFs), porous polymers, aerogels, boron nitride materials, and activated carbon materials.
Today’s energy sector is accompanied by a number of environmental inconveniences. In order to overcome those problems, future energy concepts have to be put into practice. In particular, renewable energies are needed, because (i) fossil energy promotes global warming and environmental contamination, (ii) the supply of nonrenewable energy sources is finite, and (iii) nuclear energy presents a serious danger due to its radioactive waste products. Among renewable energies, technologies for hydrogen storage will be an important piece of the jigsaw.
The world’s current energy supplies are mainly based on fossil energy resources. These resources have their origin in organic (and therefore carbon-containing) compounds, which have been converted throughout millions of years. By burning them today, these resources are reintroduced into the natural carbon cycle and increase the CO2 content of the atmosphere. CO2 gas increases the world’s greenhouse effect, leading to global warming [
Global temperature (red) and CO2 emissions (blue) over the past decades [
Another problem with natural and fossil energy resources is their limited availability. In the case of oil, this fact can be exemplified by the so-called peak oil phenomenon. The term “peak oil” was introduced by Shell-company scientist Hubbert more than 50 years ago, who correctly predicted that the oil production of the USA would have a maximum in the 1970s [
Discovery of conventional oil reserves (blue) and oil consumption (red) [
Based on the aforementioned facts, it can be concluded that alternatives need to be found for the future energy market and that, in the long run, only renewable energy sources can provide sustainability. However, the usage of renewable energy sources leads to a number of new problems. On one hand, there is the temporal misfit between energy generation and consumption. For example, electricity can be produced by solar energy at daytime, but not during the night when it is needed for illumination. On the other hand, primary renewable energy sources are not efficient enough to be used in mobile applications, where space is limited and weight has to be low. Both of these problems can be solved by means of the so-called “hydrogen economy”.
Fundamental research on hydrogen was firstly made in the middle of the 18th century by Henry Cavendish, who discovered the element in 1766, and by Antoine Lavoisier, who gave it its name in 1783 [
However, the term “hydrogen economy” was not introduced until 1972 by Bockris [
Hydrogen is the lightest of all elements and the most abundant one in the universe [
In its gaseous state, hydrogen forms the diatomic molecule H2 (dihydrogen). Some basic properties of hydrogen are listed in Table
Hydrogen properties.
Atomic weight | 1.0079 | g mol−1 |
Van der Waals radius | 120 | pm |
Covalent radius | 31 | pm |
ΔH (H2 |
436 | kJ mol−1 |
Density at 273 K and 0.1 MPa | 0.0899 | g l−1 |
Liquid density (20.25 K) | 70.8 | g l−1 |
Boiling point | 20.25 | K |
Critical temperature | 32.97 | K |
Critical pressure | 1.276 | MPa |
Self-ignition temperature | 747 | K |
Flammability limit in air | 4.1–74.2 | Vol.% |
Explosive limits in air | 18.3–59 | Vol.% |
Diffusion coefficient in air | 0.634 | cm2 s−1 |
Hydrogen burns with invisible flame and over a wide range of concentrations in air. These points call critics into action and provoke concerns about hydrogen safety. The Hindenburg Zeppelin disaster from 1937 is a popular example which is often cited by critics against hydrogen technology. However, the subject is controversially discussed, and some studies of the accident have revealed that, instead of its hydrogen charge, a highly flammable skin material of the airship may have caused the fire [
Comparison of fuel safety properties in air [
Fuel | Autoignition temperature | Flammable limits | Explosive limits | Buoyant velocity | Ignition energya |
---|---|---|---|---|---|
K | Vol.% | Vol.% | m s−1 | mJ | |
Hydrogen | 673–858 | 4.1−74.2 | 18.3−59.0 | 1.2–9.0 | 10 |
Methane | 923 | 5.3−15.0 | 6.3−13.5 | 0.8–6.0 | 20 |
Ethane | 788 | 3.0−12.5 | — | —b | — |
Propane | 723 | 2.3−9.5 | 3.1−7.0 | —b | — |
Butane | 678 | 1.9−8.5 | — | —b | — |
Gasoline | 553–729 | 1.4−7.6 | 1.1−3.3 | —b | — |
Ethanol | 696 | 3.3−19.0 | — | —b | — |
Methanol | 743 | 6.0−36.5 | — | —b | — |
Due to its high conversion efficiency and its zero emission (referring to emissions of toxic pollution or greenhouse gases like, e.g., CO2), hydrogen is regarded as an ideal secondary energy carrier. It can be produced using renewable energy and water, and, by means of storing it, the hydrogen can be retransformed to energy and water in another time and space. During those conversions no contamination is produced. For energy conversion, well-established technologies like internal combustion engines (ICEs) may be operated with hydrogen fuel if they are slightly modified [
Aside from high costs of FCs, there are two other main obstacles which impede the introduction of hydrogen as an important energy vector today: hydrogen production and its storage. Thus, inexpensive methods for hydrogen mass production need to be found, and efficient, economic, and safe hydrogen storage technologies need to be developed in order to compete with less expensive fossil fuels and for introducing a hydrogen economy [
As an energy carrier, hydrogen has to compete against other fuels. For the use in electric vehicles, hydrogen could outperform electric batteries which are relatively bulky and heavy [
In Figures
Heat of combustion on a gravimetric basis, shown for various gaseous, liquid, and solid fuels under standard conditions. The ranges of values were obtained by utilizing a multitude of sources [
Heat of combustion on a volumetric basis, shown for various gaseous, liquid, and solid fuels under standard conditions. The ranges of values were obtained by utilizing a multitude of sources [
The aforementioned reveals that, for hydrogen storage application, the storage volume needs to be reduced. Putting it in another way, this means that, for a given storage volume, the distances between hydrogen molecules need to be decreased. However, such an increase of the volumetric storage density has to come along with maintaining the gravimetric storage density on a high level. An important aspect is that, even though systems for hydrogen onboard storage in vehicles may be bulkier and heavier than for other fuels, these constraints can be acceptable, taking into account the highly efficient conversion of hydrogen and the fact that a relatively small amount of fuel is needed to travel a given distance [
Acceptable compromises need to be found regarding hydrogen storage systems which guarantee a successful introduction of hydrogen technology. In order to provide a framework for the development of hydrogen storage technology, governmental institutions and other funding bodies established a number of targets and milestones. These targets set limits to, for example, the refilling time, the cycle lifetime, the costs, and the densities of the storage system [
DOE hydrogen storage targets for light-duty vehicles [
Storage parameter | Units | 2010 | 2015 | Ultimate target |
---|---|---|---|---|
Gravimetric system density | kWh kg−1 | 1.5 | 1.8 | 2.5 |
MJ kg−1 | 5.4 | 6.48 | 9 | |
wt.% | 4.5 | 5.5 | 7.5 | |
| ||||
Volumetric system density | kWh l−1 | 0.9 | 1.3 | 2.3 |
MJ m−3 | 3.24 | 4.68 | 8.28 | |
|
28 | 40 | 70 | |
| ||||
Fuel cost at pump | $ |
3–7 | 2–6 | 2-3 |
| ||||
System filling time for 5 kg H2 | min | 4.2 | 3.3 | 2.5 |
|
1.2 | 1.5 | 2 | |
| ||||
Maximum loss of useable H2 | g (h |
0.1 | 0.05 | 0.05 |
Hydrogen can be stored, either in an empty tank (e.g., in the case of high pressure hydrogen storage or liquefied hydrogen storage) or in a tank containing a solid (solid-state H2 storage). In the latter case, the storage density is often referred exclusively to the solid used, in order to facilitate the performance comparison of different materials.
Thus, it is extremely important to distinguish between storage densities on a material basis and those which are based on the whole storage system. It has to be pointed out that the technological requirements and targets, which were mentioned previously and which are listed in Table
In the previous section, it was shown that the main problem for hydrogen storage is its extremely low density under normal conditions. In order to reduce the occupied volume by hydrogen gas, a number of different technologies are being investigated.
Compressing hydrogen up to high pressures is a relatively simple and cost-efficient technology [ all metal cylinder, partly fiber-wrapped, load-bearing metal liner, fully fiber-wrapped, non-load-bearing metal liner, fully fiber-wrapped, non-load-bearing nonmetal liner.
From type to type, the weight of the vessel is successively reduced by substituting metals with other lighter materials. The two basic components of a state-of-the-art type 4 pressure vessel are (a) a liner which prevents hydrogen permeation and (b) a composite structure which accounts for the mechanical stability of the structure. In Figure
Schematic diagram of a type 4 pressure vessel and its components for gaseous hydrogen storage (Quantum Technologies, Inc., Irvine). Reprinted from [
An important concern regarding high pressure vessels is safety issues, especially in the case of mobile applications. Thus, pressure vessel has to be extensively tested regarding overpressure and accidental impacts. Another drawback of gaseous storage is the energy expenditure which is needed for the compression process. Thus, around 12% of the stored hydrogen energy is consumed for reaching 35 MPa and 15% for 70 MPa, respectively [
In order to determine gas densities at given pressure-temperature conditions, adequate models are needed. In general, the ideal gas equation (
A better approximation for describing the thermodynamic behavior of real gases can be made by the van der Waals equation (see (
In Table
Van der Waals constants for hydrogen.
Description | Symbol | Value | Unit | Reference |
---|---|---|---|---|
VdW correction |
|
0.2484 | (l2bar) mol−1 | [ |
VdW correction |
|
0.02651 | l mol−1 | [ |
Despite its descriptive character, the van der Waals equation turns out to be relatively imprecise if it is applied to hydrogen gas [
Another possibility is to store hydrogen in liquid form. However, hydrogen only condensates at very low temperature (20.4 K). The liquefaction process is based on the Joule-Thomson effect, which explains the temperature change of a pressurized real gas when it is throttled [
Taking into account the transformation from the ortho- to the para-state, the ideal work of liquefaction of hydrogen is 11.88 MJ kg−1 [
In Figure
Schematic diagram of a liquid hydrogen tank and its components (Magna Steyr, BMW). Reprinted from [
Among the car manufacturers, opinions regarding liquid hydrogen storage are contradictory. Thus, BMW equipped a number of hydrogen prototypes with such systems [
A third possibility is to store hydrogen through dissociative chemisorption, followed by the formation of a new compound. Among them, metallic hydrides are being widely studied. During chemisorption in metal hydrides, the hydrogen molecule is first dissociated on the surface, and then its atoms diffuse into the metal host [
Materials which have been investigated for storing chemically bonded hydrogen include numerous compounds. Although often more sophisticated alloys are investigated, some basic metal hydride systems are usually classified as
The fact that the intermediate compounds decompose at different temperatures
Due to the Westlake Criterion, the H-H distance in stable hydrides can reach values of 2.1 Å [
Comparison of volumetric fuel densities for different hydrogen storage technologies: compressed hydrogen (GH2) at room temperature for 0.1, 20, 70, and 100 MPa [
For hydrogen storage application, however, the hydride is usually available in powder form, in order to minimise diffusion paths and to increase reaction kinetics. Therefore, the value for the crystal density of the hydride should not be confused with the volumetric storage density, which is expected to be considerably lower. Furthermore, the material density changes in the course of the metal-hydride reaction. By continuously being loaded and discharged with hydrogen, the material matrix periodically expands and contracts for every hydrogen cycle, leading to a sintering of the material. Those density changes represent a major challenge for the practical implementation of this technique. In the case of
On a gravimetric basis, only very low values are reached with room temperature hydrides. Light metal hydrides perform much better. However, these materials involve high binding enthalpies which lead to a considerable amount of heat that needs to be transferred during hydrogen charging and discharging [
Physisorption (or
These relations among the different storage technologies are visualized in the thermophysical phase diagram in Figure
Density diagram for hydrogen, including the DOE targets for volumetric storage density. Thermo-physical data was obtained from NIST [
In general, two different strategies are considered for utilizing adsorbents in hydrogen storage tanks. In the first case, the tank is maintained at low temperatures (“cryoadsorption”) [
From an application point of view, it would be more interesting to operate the tank at room temperature. In this second scenario, costs for cooling could be avoided and no isolation would be needed, thus reducing weight and volume of the storage device. However, at increased temperature, the influence of the van der Waals forces decreases due to an increased thermal vibration of the adsorbate molecules. Due to the low adsorption potential under these conditions, high pressures are needed to reach sufficient storage capacities. However, also at room temperature it is possible to reduce the operating pressure for the storage of a determined amount of hydrogen by using an adsorbent, in comparison with an empty vessel [
Three different kinds of van der Waals forces can contribute to the interaction between adsorbent and adsorbate [ permanent dipole-permanent dipole forces (Keesom interaction), permanent dipole-induced dipole forces (Debye interaction), instantaneous induced dipole-induced dipole forces (London dispersion forces).
In the case of hydrogen adsorption on carbon surfaces, the first two contributions are usually not taken into account [
In Figure
Lenard-Jones constants for the carbon-hydrogen interaction.
Description | Symbol | Value | Unit | Reference |
---|---|---|---|---|
Lenard-Jones constant |
|
32.06 | K | [ |
Lenard-Jones constant |
|
0.318 | nm | [ |
Lennard-Jones potential of the carbon-hydrogen interaction. Values for
Designing an optimized adsorbent for hydrogen storage is a challenging task. In general, there are two basic requirements which need to be taken into account [
In order to increase the adsorption enthalpy of adsorbents, three possibilities are generally considered [
Also in the case of unsaturated adsorption centers, the chemical composition of the material host is changed by introducing other species like, for example, metals. However, their interaction with the hydrogen molecule is weaker and limited to increase the adsorption potential. This is accomplished by donation of charge between the molecules and the functionalized interaction sites [
The third method, decreasing the pore sizes of the material, manages without the introduction of additional species. Here, the concept is to build a very narrow porosity in which the hydrogen molecule can simultaneously interact with multiple pore walls. Due to their proximity, the adsorption potentials of opposed pore walls overlap, resulting in an amplified attraction.
At 77 K, a correlation of hydrogen adsorption with the BET surface area has been reported [
At room temperature, the optimum pore size was found to be around 0.7 nm [
Numerous kinds of materials have been investigated as adsorbents for hydrogen. These include carbon-based materials (activated carbons, carbon nanotubes, nanofibers, fullerenes, carbons from templates, etc.), zeolites, silica, alumina, metal organic frameworks (MOFs), covalent organic frameworks (COFs), porous polymers, aerogels, and boron nitride materials. In the following, a brief overview of the different classes of materials as well as their performance for hydrogen storage is given.
Zeolites are aluminosilicates which can accommodate a wide variety of cations [
Some natural or synthetic oxides like silica and alumina can also reveal high porosity which often has a high contribution of mesopores. Due to this characteristic, these materials find a lot of applications in catalysis, separations, sensors, drug delivery, optical devices, and molecular sieves [
By means of sol-gel reaction, aerogels with large specific surface areas of more than 3000
More recently, also noncarbon nanotubes have been proposed for hydrogen storage [
Another class of materials which recently were proposed for hydrogen storage application are porous polymers [
Coordination polymers like metal-organic frameworks (MOFs) and covalent organic frameworks (COVs) are a different kind of porous polymer. Similar to zeolites, these materials are of highly crystalline nature. However, different from zeolites, the structure of these frameworks is composed of lighter elements, giving rise to higher specific porosity. Thus, MOF structures are built by metal ion clusters which are connected by organic linking groups. In the case of COFs, aromatic rings are the primary structural element [
Activated carbon (AC) is an interesting and well-known adsorbent with a highly adaptable porosity. Activated carbon materials are a form of carbon which does not occur naturally and which has to be synthesized technically. The technical process is called “activation” and can be performed utilizing different methods, as well as a large number of carbon-containing precursors. The porosity in ACs can cover a wide range of pore sizes and can be controlled by adjusting the activation parameters. Furthermore, the characteristics of the adsorbent can be adjusted by manipulating its surface chemistry. Activated carbon materials can be used in numerous applications, for example, in liquid and gas phase treatments (e.g., removal of volatile organic compounds (VOC) and SO2 or for CO2 and CH4 separation and storage), energy storage (as electrical double layer capacitors), as adsorbents in cryocoolers, and so forth [
Together with zeolites, carbon materials were among the first adsorbents studied for hydrogen storage [
A lot of data can be found for commercial activated carbon materials [
However, fewer studies have been carried out regarding synthesis and analysis of activated carbon materials that are specially tailored for hydrogen storage. Measurements of metal-carbide-derived activated carbons revealed 3 wt.% at 77 K and 0.1 MPa [
Strong efforts have been made in order to reach the targets for hydrogen storage in mobile applications which were established by bodies like the DOE. Yet, none of the investigated technologies currently fulfills all of the requirements. Today, the lion’s share of hydrogen vehicles is based on pressure vessels with compressed hydrogen. However, this technology may not be appropriate for reaching the ambitious ultimate targets, and security concerns exist over the high pressures involved. The use of liquid hydrogen is handicapped by the energy intensive liquefaction process as well as hydrogen boil-off.
The storage of hydrogen inside materials could be advantageous in a number of aspects. Metal hydrides and other compounds with chemisorbed hydrogen can reach high capacities. However, one has to bear in mind that storage capacities of the materials alone have to be significantly higher according to the established targets which are system-based. In addition, materials with chemically bonded hydrogen often present other difficulties like heat management problems, low kinetics, and/or irreversibility. A promising alternative could be hydrogen storage by physisorption in porous materials. The challenges for designing suitable adsorbents are (1) increasing their porosity, (2) tuning of pore sizes, (3) optimization of adsorption potentials, and (4) enhancement of the bulk material density in order to reach high volumetric capacities.
The authors confirm that no conflict of interests exists and that they do not have any direct financial relation with any of the commercial identities mentioned in the paper.
The authors would like to thank the Generalitat Valenciana and FEDER (Project PROMETEO/2009/047) for financial support.