Cup-stacked Carbon NanofiberAs our planet is running out of petroleum resources, a new energy source must be found. Hydrogen is regarded as an ideal alternative to fossil fuel because of its abundance in environment, renewability, and zero emission. However, the most challenging and important aspect for the successful transition to a hydrogen economy is the problem of hydrogen storage.

Two-thirds of U.S. oil consumption is used to meet transportation energy needs, so unless there is a way to safely and efficiently store hydrogen on board a vehicle, it will be impossible to make a step forward to a hydrogen based economy.

Hydrogen Storage Methods

Presently there are three major directions in research and development of hydrogen storage systems:

  • Gaseous Hydrogen – storing hydrogen as a gas in tanks under high pressure. Recently 10,000-psi tanks made of lightweight carbon-fiber reinforced composites have been demonstrated and certified; however, the energy content of hydrogen at even at 10,000 psi is only 1/8th of gasoline at the same volume.
  • Liquid Hydrogen – storage of liquid hydrogen in cryogenic containers offers a significant advantage: more hydrogen can be stored in a given volume as a liquid than can be stored in gaseous form. A major drawback of liquid storage is a lot of energy required for liquefaction, plus the loss of hydrogen due to evaporation.
  • Solid-state – storing hydrogen in metal hydrides, in complex hydride materials, and in nanostructured materials. Researchers believe that metal hydrides may represent ideal storage systems. An example of metal hydride would be sodium borohydride (NaBH4) that by produces hydrogen while reacting with water:

  • NaBH4 + 2 H2O –> 4 H2 + NaBO2

    Although there are more than 2,000 elements, compounds, and alloys that form hydrides, none of these materials has yet been demonstrated to meet all of the FreedomCAR targets set by the U.S. Department of Energy (see a table below).

Nanostructured Materials
A novel promising method for hydrogen storage.Nanotubes with Hydrogen Molecules

Since 1997, when it was reported that carbon nanotubes (CNTs) can store hydrogen, numerous experimental and theoretical works have been performed in order to investigate the hydrogen adsorption in CNTs and improve the storage capacity of the tubes by doping them with alkali metals such as lithium and potassium. But most of these efforts failed to reach the year 2010 target of 6 wt % as set by the FreedomCAR.

One novel material that could be promising for H2 storage is the carbon nanoscroll (CNS). This carbon material shows a spiral form and can be obtained by a twisting of a graphite sheet. The researches from Greece implemented a multiscale theoretical approach to investigate the hydrogen storage in CNSs (DOI:10.1021/nl070530u).

Initially the researches failed to show any positive results because the intralayer distance of CNSs was too small to insert molecular hydrogen. Then they tried to open the spirals by inserting various alkali metals – Li, Na, K, and Rb, and only in case of Rubidium that opened the structure to 6.2 Å there was some adsorption of hydrogen.

hydrogen adsorption on doped CNS structures

Froudakis and colleagues determined that by opening the spiral structure even more to approximately 7 Å followed by alkali doping can make CNSs very promising materials for hydrogen storage application, reaching 3 wt % at ambient temperature and pressure.

FreedomCAR Hydrogen Storage System Targets

Targeted Factor200520102015
Specific energy (MJ/kg)5.47.210.8
Hydrogen (wt%)4.56.09.0
Energy density (MJ/L)4.35.49.72
System cost ($/kg/system)963
Operating temperature (°C)-20/50-20/50-20/50
Cycle life-time (absorption/desorption cycles)50010001500
Flow rate (g/s)345
Delivery pressure (bar)2.52.52.5
Transient response (s)0.50.50.5
Refueling rate (kg H2/min)0.51.52.0

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