[Hydrogen Storage Methods]

[ENMI-20 Assignment 1]
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Liquefied hydrogen
As well as storing gaseous hydrogen under pressure, it is also possible to store cryo-
genic hydrogen in the liquid state. Liquid hydrogen (LH2) is in demand today in
applications requiring high levels of purity, such as in the chip industry for example.
As an energy carrier, LH2 has a higher energy density than gaseous hydrogen, but it
requires liquefaction at –253 °C, which involves a complex technical plant and an
extra economic cost. When storing liquid hydrogen, the tanks and storage facilities
have to be insulated in order to keep in check the evaporation that occurs if heat is
carried over into the stored content, due to conduction, radiation or convection.
Tanks for LH2 are used today primarily in space travel.
Cold- and cryo-compressed hydrogen
In addition to separate compression or cooling, the two storage methods can be
combined. The cooled hydrogen is then compressed, which results in a further
development of hydrogen storage for mobility purposes. The first field installations
are already in operation. The advantage of cold or cryogenic compression is a higher
energy density in comparison to compressed hydrogen. However, cooling requires an
additional energy input.
Currently it takes in the region of 9 to 12 % of the final energy made available in the
form of H2 to compress hydrogen from 1 to 350 or 700 bar. By contrast, the energy
input for liquefaction (cooling) is much higher, currently around 30 %. The energy
input is subject to large spreads, depending on the method, quantity and external
conditions. Work is currently in progress to find more economic methods with a
significantly lower energy input.
Materials-based H2 storage
An alternative to physical storage methods is provided by hydrogen storage in solids
and liquids and on surfaces. Most of these storage methods are still in development,
however. Moreover, the storage densities that have been achieved are still not
adequate, the cost and time involved in charging and discharging hydrogen are too
high, and/or the process costs are too expensive. Materials-based hydrogen storage
media can be divided into three classes: first, hydride storage systems; second, liquid
hydrogen carriers; and third, surface storage systems, which take up hydrogen by
adsorption, i.e. attachment to the surface.

Chemical Hydrogen Storage Materials :

Hydride storage systems
In metal hydride storage systems the hydrogen forms interstitial compounds with
metals. Here molecular hydrogen is first adsorbed on the metal surface and then
incorporated in elemental form (H) into the metallic lattice with heat output and
released again with heat input. Metal hydrides are based on elemental metals such
as palladium, magnesium and lanthanum, intermetallic compounds, light metals such
as aluminium, or certain alloys. Palladium, for example, can absorb a hydrogen gas
volume up to 900 times its own volume.
Liquid organic hydrogen carriers
Liquid organic hydrogen carriers represent another option for binding hydrogen
chemically. They are chemical compounds with high hydrogen absorption capacities.
They currently include, in particular, the carbazole derivative N-ethylcarbazole, but
also toluene
Cycloalkanes

Research on LOHC was concentrated on cycloalkanes at an early stage, with its

relatively high hydrogen capacity (6-8 wt %) and production of COx-free hydrogen.
[22] Heterocyclic aromatic compounds (or N-Heterocycles) are also appropriate for
this task. A compound featuring in LOHC research is N-Ethylcarbazole (NEC)[23] but
many others do exist.[24] Dibenzyltoluene, which is already used as a heat transfer
fluid in industry, was identified as potential LOHC. With a wide liquid range between
-39 °C (melting point) and 390 °C (boiling point) and a hydrogen storage density of
6.2 wt% dibenzyltoluene is ideally suited as LOHC material.[25] Formic acid has been
suggested as a promising hydrogen storage material with a 4.4wt% hydrogen
capacity.

N-Heterocycles

The temperature required for hydrogenation and dehydrogenation drops

significantly for heterocycles vs simple carbocycles.[33] Among all the N-
heterocycles, the saturated-unsaturated pair of dodecahydro-N-ethylcarbazole (12H-
NEC) and NEC has been considered as a promising candidate for hydrogen storage
with a fairly large hydrogen content (5.8wt%).[34] The figure on the top right shows
dehydrogenation and hydrogenation of the 12H-NEC and NEC pair. The standard
catalyst for NEC to 12H-NEC is Ru and Rh based. The selectivity of hydrogenation can
reach 97% at 7 MPa and 130 °C-150 °C.[22] Although N-Heterocyles can optimize the
unfavorable thermodynamic properties of cycloalkanes, a lot of issues remain
unsolved, such as high cost, high toxicity and kinetic barriers etc.

Formic acid

Formic acid is a highly effective hydrogen storage material, although its H2 density is
low. Carbon monoxide free hydrogen has been generated in a very wide pressure
range (1–600 bar). A homogeneous catalytic system based on water-soluble
ruthenium catalysts selectively decompose HCOOH into H2 and CO2 in aqueous
solution. This catalytic system overcomes the limitations of other catalysts (e.g. poor
stability, limited catalytic lifetimes, formation of CO) for the decomposition of formic
acid making it a viable hydrogen storage material. And the co-product of this
decomposition, carbon dioxide, can be used as hydrogen vector by hydrogenating it
back to formic acid in a second step. The catalytic hydrogenation of CO2 has long
been studied and efficient procedures have been developed. Formic acid contains 53
g L−1 hydrogen at room temperature and atmospheric pressure. By weight, pure
formic acid stores 4.3 wt% hydrogen. Pure formic acid is a liquid with a flash point
69 °C (cf. gasoline −40 °C, ethanol 13 °C). 85% formic acid is not flammable.

Carbohydrates

Carbohydrates (polymeric C6H10O5) releases H2 in a bioreformer mediated by the

enzyme cocktail—cell-free synthetic pathway biotransformation. Carbohydrate
provides high hydrogen storage densities as a liquid with mild pressurization and
cryogenic constraints: It can also be stored as a solid powder. Carbohydrate is the
most abundant renewable bioresource in the world.

Ammonia

Ammonia (NH3) releases H2 in an appropriate catalytic reformer. Ammonia provides

high hydrogen storage densities as a liquid with mild pressurization and cryogenic
constraints: It can also be stored as a liquid at room temperature and pressure when
mixed with water. Ammonia is the second most commonly produced chemical in the
world and a large infrastructure for making, transporting, and distributing ammonia
exists. Ammonia can be reformed to produce hydrogen with no harmful waste, or
can mix with existing fuels and under the right conditions burn efficiently. Since there
is no carbon in ammonia, no carbon by-products are produced; thereby making this
possibility a "carbon neutral" option for the future. Pure ammonia burns poorly at
the atmospheric pressures found in natural gas fired water heaters and stoves. Under
compression in an automobile engine it is a suitable fuel for slightly modified gasoline
engines. Ammonia is the suitable alternative fuel because it has 18.6 MJ/kg energy
density at NTP and carbon-free combustion byproducts.

Surface storage systems (sorbents)

Finally, hydrogen can be stored as a sorbate by attachment (adsorption) on materials
with high specific surface areas. Such sorption materials include, among others,
microporous organometallic framework compounds (metal-organic frameworks
(MOFs)), microporous crystalline aluminosilicates (zeolites) or microscopically small
carbon nanotubes. Adsorption materials in powder form can achieve high volumetric
storage densities.
Underground Storage
When it comes to the industrial storage of hydrogen, salt caverns, exhausted oil and
gas fields or aquifers can be used as underground stores. Although being more
expensive, cavern storage facilities are most suitable for hydrogen storage.
Underground stores have been used for many years for natural gas and crude oil/oil
products, which are stored in bulk to balance seasonal supply/demand fluctuations
or for crisis preparedness.
To date, operational experience of hydrogen storage caverns exists only on a in a few
locations in the USA and Europe. In particular, the underground natural gas stores in
Europe and North America could potentially be used as large reservoirs for hydrogen
generated from surplus renewable energies. However, only a relatively small
proportion of these are storage caverns; the most prominent and common form of
underground storage consists of depleted gas reservoirs. In addition, the natural gas
stores are unevenly distributed at a regional level.
Gas Grid
Another possibility for storing surplus renewable energy in the form of hydrogen is to
feed it into the public natural gas network (Hydrogen Enriched Natural Gas or HENG).
Until well into the 20th century, hydrogen-rich town gas or coke-oven gas with a
hydrogen content above 50 vol% was distributed to households in Germany, the USA
and England, for example, via gas pipelines – although not over long distances, for
which as yet no experience is available.
Infrastructure elements that were installed at the time, such as pipelines, gas
installations, seals, gas appliances etc., were designed for the hydrogen-rich gas and
were later modified with the switch to natural gas. Many countries have looked at
adding hydrogen into the existing natural gas networks. For the USA, it would be
possible to introduce amounts from 5 vol% to 15 vol% hydrogen without substantial
negative impact on end users or the pipeline infrastructure. At the same time, the
larger additions of hydrogen would in some cases require expensive conversions of
appliances. In Germany this limit has been set somewhat lower, at up to 10 vol%.  In
principle, gas at concentrations of up to 10 vol% hydrogen can be transported in the
existing natural gas network without the risk of damage to gas installations,
distribution infrastructure, etc. However, a number of components have been listed
that are still considered to be critical and to be generally unsuitable for operation
with these hydrogen concentrations. For CNG vehicles, the currently authorized limit
value for the proportion of hydrogen used is only 2 vol%, depending on the materials
built in (UNECE 2013).
It can be assumed that many of the gas transport networks, distribution lines and
storage facilities that were operated in the past are still in use today. In Leeds (UK),
for instance, the possibility has been explored of converting the existing natural gas
network in the region (used primarily for municipal heating supply) entirely to
hydrogen. Given their length, the large gas networks in many industrial countries
could store considerable amounts of hydrogen.
The Fuel Cell Technologies Office (FCTO) is developing onboard automotive hydrogen
storage systems that allow for a driving range of more than 300 miles while meeting
cost, safety, and performance requirements.
Research and Development Goals
FCTO conducts research and development activities to advance hydrogen storage
systems technology and develop novel hydrogen storage materials. The goal is to
provide adequate hydrogen storage to meet the U.S. Department of Energy (DOE)
hydrogen storage targets for onboard light-duty vehicle, material-handling
equipment, and portable power applications. By 2020, FCTO aims to develop and
verify onboard automotive hydrogen storage systems achieving targets that will
allow hydrogen-fueled vehicle platforms to meet customer performance
expectations for range, passenger and cargo space, refueling time, and overall vehicle
performance. Specific system targets include the following:
 1.5 kWh/kg system (4.5 wt.% hydrogen)
 1.0 kWh/L system (0.030 kg hydrogen/L)
 $10/kWh ($333/kg stored hydrogen capacity).

The collaborative Hydrogen Storage Engineering Center of Excellence conducts

analysis activities to determine the current status of materials-based storage system
technologies.
The Hydrogen Materials—Advanced Research Consortium (HyMARC) conducts
foundational research to understand the interaction of hydrogen with materials in
relation to the formation and release of hydrogen from hydrogen storage materials.
Challenges

Right side picture : Comparison of specific energy (energy per mass or gravimetric density) and
energy density (energy per volume or volumetric density) for several fuels based on lower heating
values.
Left side picture : The 2010 U.S. light-duty vehicle sales distribution by driving range.
High density hydrogen storage is a challenge for stationary and portable applications
and remains a significant challenge for transportation applications. Presently
available storage options typically require large-volume systems that store hydrogen
in gaseous form. This is less of an issue for stationary applications, where the
footprint of compressed gas tanks may be less critical.
However, fuel-cell-powered vehicles require enough hydrogen to provide a driving
range of more than 300 miles with the ability to quickly and easily refuel the vehicle.
While some light-duty hydrogen fuel cell electric vehicles (FCEVs) that are capable of
this range have emerged onto the market, these vehicles will rely on compressed gas
onboard storage using large-volume, high-pressure composite vessels. The required
large storage volumes may have less impact for larger vehicles, but providing
sufficient hydrogen storage across all light-duty platforms remains a challenge. The
importance of the 300-mile-range goal can be appreciated by looking at the sales
distribution by range chart on this page, which shows that most vehicles sold today
are capable of exceeding this minimum.
On a mass basis, hydrogen has nearly three times the energy content of gasoline—
120 MJ/kg for hydrogen versus 44 MJ/kg for gasoline. On a volume basis, however,
the situation is reversed; liquid hydrogen has a density of 8 MJ/L whereas gasoline
has a density of 32 MJ/L, as shown in the figure comparing energy densities of fuels
based on lower heating values. Onboard hydrogen storage capacities of 5–13 kg
hydrogen will be required to meet the driving range for the full range of light-duty
vehicle platforms.
To overcome these challenges FCTO is pursuing two strategic pathways, targeting
both near-term and long-term solutions. The near-term pathway focuses on
compressed gas storage, using advanced pressure vessels made of fiber reinforced
composites that are capable of reaching 700 bar pressure, with a major emphasis on
system cost reduction. The long-term pathway focuses on both (1) cold or cryo-
compressed hydrogen storage, where increased hydrogen density and insulated
pressure vessels may allow for DOE targets to be met and (2) materials-based
hydrogen storage technologies, including sorbents, chemical hydrogen storage
materials, and metal hydrides, with properties having potential to meet DOE
hydrogen storage targets.
Remarks

References :
https://hydrogeneurope.eu/hydrogen-storage
https://www.sciencedirect.com/science/article/abs/pii/S1364032115004694
https://en.wikipedia.org/wiki/Hydrogen_storage#:~:text=Compressed%20hydrogen
%20is%20a%20storage,type%20IV%20carbon%2Dcomposite%20technology.