Hydrogen is like electricity and steam, it carries energy, but is hard to store.

Hydrogen storage under pressure

Hydrogen is difficult to store because it is difficult to liquefy. It's critical temperature is -240 ⁰C at 13 bar (13 atmos.). This means it will not liquefy above -240 ⁰C (at 13 bar), no matter what pressure is applied. Even when it is liquefied, the density is only 0.03. (water is 1.0). Petrol has nearly twice the hydrogen in the same volume of hydrogen.

The ways of storing hydrogen are:

Cryogenic liquid - must be kept below  -240 ⁰C at 13 bar 

Pressurised gas - Car makers Honda and Nissan are working with hydrogen at 350 and 700 bar (5,000 and 10,000 psi). The energy of compression needs to be recovered as the hydrogen is consumed.  If the energy is recovered during decompression only  2% of the energy for compression is lost.. The mass of the tank is a disadvantage. It weighs twice as much as the hydrogen.

Underground pressurised storage. ICI has for many years stored hydrogen underground in old salt domes. Some researchers have investigated underground hydrogen storage to provide grid energy storage for intermittent energy sources, like wind power.

Carbon nanotubes will store hydrogen, but only about 1% by mass

Hydrogen storage in a chemical

Hydrogen can be stored chemically as hydrides such as  MgH₂, NaAlH₄, LiAlH₄, LiH, LaNi₅H₆, TiFeH₂ and PdH_x (x<1)

The challenge is to find a hydride that releases the hydrogen at 100^oC, and combines or recharges at 700 bar or less.

Carbohydrate such as starch and sugar are very efficient carriers of hydrogen. Some researchers claim that sugar can be the most efficient fuel for cars. The sugar is converted to hydrogen by an on-board reformer, then used in a fuel cell. After a biomass charged battery vehicle, it was the most fuel efficient vehicle. More

Ammonia NH3 is a useful way to store hydrogen at room temperature. It can burn in an internal combustion engine.

Ammonia borane H₃NBH₃  Wikipedia  ​University of Oregon chemists have developed a boron-nitrogen-based liquid-phase storage material for hydrogen that works safely at room temperature and is both air- and moisture-stable More

Metal organic frameworks. Very porous substances will absorb up to 7% hydrogen by weight at 77^oK and one bar of pressure.

Clathrate hydrates, glass cailliaries and glass microspheres, are being researched.

In Toluene

Japan's largest petroleum wholesaler, JX Holdings, has devised a technology to transport large volumes of hydrogen safely in liquid form By dissolving hydrogen in toluene, a liquid derived from crude oil, the firm will be able to transport the liquid at ordinary temperatures and pressure. Using a proprietary catalyst, hydrogen will then be revaporized at the pump. Source

Research

A New storage medium

A team of engineers from the University of New South Wales has made an exciting discovery that it hopes will help advance the cause of hydrogen fuel as a viable alternative energy source – particularly for use in hydrogen fuel cell vehicles.

The researchers from UNSW’s Materials Energy Research Laboratory in nanoscale (MERLin) have demonstrated, for the first time, that hydrogen can be released and reabsorbed from a promising storage material, overcoming a major development hurdle.

The team synthesised nanoparticles of a commonly overlooked chemical compound called sodium borohydride and encased them inside nickel shells, with the resulting unique “core-shell” nanostructure demonstrating what they describe as “remarkable” hydrogen storage properties.

Ref

Magnesium Hydride

Queensland company Hydrexia has patented a process of storing hydrogen on magnesium hydride doped with nickel or similar transition metals. These metals act as catalysts.

It stores 6.5 - 7% of it's mass as hydrogen. The magnesium hydride has an energy density of 2.332 kWh/Kg.

It seems to work at atmospheric pressure, but the patent language is difficult to follow.

Now, a new process that effectively improves the kinetics of MgH.sub.2 --Mg hydrogen-storer systems has, surprisingly, been discovered.

The process in accordance with the invention consists of doping a finely divided form of the magnesium hydride or metallic magnesium by exposing it to (a) a solution of an appropriate transition-metal complex, (b) to a solution of an appropriate transition-metal organic compound or (c) transition metal per se in finely divided form. An extremely fine distribution of the particular transition metal precipitates over the surface of the particles of magnesium hydride or magnesium and assumes the function of catalyst in the dehydrogenation and hydrogenation cycles.

The elements of Subgroups IV-VII of the periodic table--titanium, vanadium, chromium, molybdenum, tungsten, iron, ruthenium, cobalt, rhodium, iridium, nickel, palladium, and platinum--are all appropriate transition metals.

Patent

Cella hydrogen storage

The Cella technology is based around the encapsulation and nano-structuring of chemical hydrides in plastic. This means that they can be handled in air, and allows the hydrogen to be released quickly and cleanly upon heating. We make them into plastic like pellets; heating one gram of Cella’s pellets will produce one litre of hydrogen (at normal pressures and temperatures).

Cella materials have achieved 9wt% weight percentage of hydrogen.

Source: http://www.cellaenergy.com/index.php?page=technology

Carbon nanotubes from chicken feathers

If bird feathers are heated without air between 350-450^oC,, they form hollow carbon nanotubes. These tubes can store hydrogen because they have such a large surface area and hydrogen bonds strongly to any carbon exposed on the surface.

Source

Hydrogen for storing electricity

The processes for storing electricity via hydrogen production is not very efficient, about 20-25%. You lose 75-80% of the energy during the AC  to hydrogen and back to AC again.

To make a profit the electricity sell price would need to be more than 5X the buy price. No doubt, this will improve with time.

Ref:

 Wikipedia