The holy grail is to store energy as hydrogen. Hydrogen can store 142 mega joules of energy per kilo gram. Our favorate energy source is gasoline(some hate it) at 46 mega joules per kilo gram. Better yet, If a hydrogen fuel cell is used to power a car it is 60 percent energy effecient versus an internal combustion energy at 16%. The Honda Clarity will travel 100 miles on a kilo gram of hydrogen while a Toyota Prius will tavel 7 miles on a kilo gram of gasoline.Theoretically the ICE is a Rube Goldberd design compared to the fuel cell. However; The hydrogen solution has been just as ellusive as the holy grail.
Here is good link for comparing energy storage densities. You will see that NG is a good energy storage "device".
PS: The exhaust is water vapor. No pollution from the fuel cell!