Ignis

Ignis (イグニズ) is a presumably fake explosive gas made up by Frenda Seivelun. According to her, the gas isn't harmful to humans when breathed in and quickly fills the space it is filled in. It is apparently a highly explosive gas as Frenda boasts that an entire sealed room can make said room a large bomb and explode violently even if a small spark is created.

Etymology
Ignis is a Latin word which means fire.

Sisters Arc
As Mikoto tries to take down the facility she is in that is involved with the Level 6 Shift, she comes upon Frenda who has set-up traps against her. Unable to contend against the sheer power Mikoto has, Frenda tries to level the playing field by using a bottle of explosive, to which the gas she states is Ignis and one small spark from Mikoto can kill them all.

In actuality, it was a bluff to stop Mikoto from using her powers. The gas filled the room quickly because Frenda mixed the air with nitrogen using the vents. Needless to say, after Frenda gets caught off guard and one of her explosive tapes accidentally explode, Mikoto soon discovers that Frenda is lying.

It is unknown if such an explosive is truly available in Academy City.