it is said that when a mass of matter is completely converted into energy it is as follows:
E=mc^2
c = 299,792,458 m/s ~ 300,000 km/s
m is mass (kg)
E energy (Joule)
==> 1kg ~ 9 x 10^16 Joules
But to completely convert matter into energy, nothing is as easy as exploiting the nuclear fuel Uranium 235 and some similar elements...
Actually, from my understanding, all matter can react chemically to change the substance. Simply, the electrochemical reaction in a battery of just a few grams can produce millions of kWh, Uranium is just because the reaction density is so low that they produce a lot of energy.
This is just a way of understanding the difficulty of converting matter into energy, which I attributed to... just having a chemical reaction converts matter into energy...
Well, today's research is here, and how to do it, there will be longer research... :D