Hydrogen is a great energy medium, though it has been touted as unsafe the reality is that it has been proven to be no more dangerous than the tank of gas in your car. Though hydrogen is a dense energy source as it is so light its has a rather large volume at stp (standard temperature and pressure) To increase the density of the system you compress the hydrogen. This is all well and good but you cant help having a little hydrogen boil off no mater how tight you build your tank. You can also increase hydrogen's density by cooling the gas. At very low temperatures the hydrogen will eventually condense into liquid hydrogen. To keep a large amount of hydrogen at these temperature would be prohibitively costly thus the problem of large scale hydrogen storage. More research is required to discover means of condensing hydrogen economically.