Mercurium isochromate

From Memory Alpha, the free Star Trek reference.

Mercurium isochromate is a substance used by some species in the Delta Quadrant as an energy source for starship propulsion.

In 2375, Captain Kathryn Janeway offered Torat twenty centiliters of mercurium isochromate, enough to power his ship for a year, in exchange for his help in locating a wormhole out of Devore space. (VOY: "Counterpoint")