Which of the following reasons cause the limited use of H2 as a fuel?
I) Its calorific value is low.
II) Its availability in free state is less.
III) It is difficult to store.
IV) It is not a clean fuel.
II and III
As a fuel, hydrogen is very difficult to store. It is extremely reactive and it combines with most of the metals and non-metals forming stable compounds. Thus it is not available in the free state.
On the other hand, the calorific value of H2 is very high - 150 KJ/g, and so can be very useful as a fuel.
Similarly, it is an extremely clean fuel that releases only water upon combustion. Thus, cleanliness doesn't hinder its usefullness either.