FOOF or Dioxygen Difluoride is an extremely dangerous chemical. It is not in itself flammable but makes the substances around it combust. How could a biological organism (say a dragon) use this as a breath weapon without itself dying in the process? I am not exactly concerned with the synthesis of the chemical, but how it could use the chemical (breath weapon sprayed from mouth) without bursting into flames and exploding itself.
The Dragon does not store FOOF in its body. It stores Fluorine and oxygen separately in PTFE organs. It has “standard” dragon fire breath capability allowing it to preheat an internal chamber with an opening at one end to 700 degrees C. It then squirts a stream of oxygen gas around the walls of the chamber and a stream of fluorine through the centre.
Some of the oxygen and fluorine react inside the chamber to form FOOF which is then immediately vented to the outside on to the intended victim. The chamber itself is protected by the oxygen stream that passes along the walls of the chamber.
So the FOOF is created on the hoof so to speak and does not come into contact with the dragons body.
If you search about FOOF you will find:
you run a mixture of oxygen and fluorine through a 700-degree-heating block. “Oh, no you don’t,” is the common reaction of most chemists to that proposal, “. . .not unless I’m at least a mile away, two miles if I’m downwind.” This, folks, is the bracingly direct route to preparing dioxygen difluoride, often referred to in the literature by its evocative formula of FOOF.
FOOF is only stable at low temperatures; you’ll never get close to RT with the stuff without it tearing itself to pieces. I’ve seen one reference to storing it as a solid at 90 Kelvin for later use, but that paper, a 1962 effort from A. G. Streng of Temple University, is deeply alarming in several ways.
“Being a high energy oxidizer, dioxygen difluoride reacted vigorously with organic compounds, even at temperatures close to its melting point. It reacted instantaneously with solid ethyl alcohol, producing a blue flame and an explosion. When a drop of liquid 02F2 was added to liquid methane, cooled at 90°K., a white flame was produced instantaneously, which turned green upon further burning. When 0.2 (mL) of liquid O2F2 was added to 0.5 (mL) of liquid CH4 at 90°K., a violent explosion occurred.”
And he’s just getting warmed up, if that’s the right phrase to use for something that detonates things at -180C (that’s -300 Fahrenheit, if you only have a kitchen thermometer). The great majority of Streng’s reactions have surely never been run again. The paper goes on to react FOOF with everything else you wouldn’t react it with: ammonia (“vigorous”, this at 100K), water ice (explosion, natch), chlorine (“violent explosion”, so he added it more slowly the second time), red phosphorus (not good), bromine fluoride, chlorine trifluoride (say what?), perchloryl fluoride (!), tetrafluorohydrazine (how on Earth. . .), and on, and on.
Darwinism would take care of any organism which would attempt using FOOF for any practical use, by reducing its DNA and all its body to a cloud of gases.
Unless this dragon has a magnetic AND cryogenic confinement unit in its guts, allowing it to keep FOOF cooled down and away from any substance until it is sprayed out.
Terminally fluorinated stuff.
FOOF will add its fluorine to anything which can accommodate fluorine addition. That includes some things, like water, which are already terminally oxidized and so impervious to oxygen radicals. As I understand it the fluorine displaces the oxygen to form hydrofluoric acid.
One would therefore contain FOOF in a terminally fluorinated compound. In this related question I proposed fluorspar, or calcium fluoride.
Fluorospar is fine for a fantasy; it is crystalline and beautiful. It should be impervious to oxidation by either fluorine or oxygen radicals.
Short Answer: Magic
The only way that FOOF can be stored is at cryogenic temperatures - it even decomposes to fluorine and oxygen at those elements' liquefaction temperatures. Unless your dragon has no metabolism to speak of, there's no way it radiates enough heat away to biologically chill something to less than a hundred degrees Kelvin.
reality-check notwithstanding, the way you store it is by having a magical refrigeration unit inside the dragon. A chamber whose very walls are imbued with magic that drain the heat out of anything inside them. (This, incidentally, would be very valuable for dragon-hunters. Imagine a perpetual icebox in a fantasy setting.)
Outside of this magical Frigidaire, there's no way. Notwithstanding synthesis of the chemical, it's so ridiculously unstable that even if you had an unreactive biological structure in which to store it (you don't, there isn't any carbon- or silicon-based biology that wouldn't burn in contact with the material), it would break down on its own.
On the plus side, you then have a quadruply-awful breath weapon. First, the dragon spews out something so cold it freezes everything it touches, which then rapidly oxidizes, setting fire to everything in its path, which results in some free elemental fluorine, poisoning everything it touches, and if there's any water around, you also get hydrofluoric acid. So cold, fire, poison, and corrosion, otherwise known as the Elemental Sampler.