What type of gas is used to pressurize fire extinguishers?

Anybody ever top-up an extingusher using say CO2. This would be a
secondary (actually a third), back up domestic extingusher in addition
to the up to date two we have permanently mounted in a) Our kitchen b)
The workshop.

But we have an older one too good to throw away. Taking it in for
formal recharge which includes pressure test etc. will cost more than
buying a new, but of lesser quality, extinguisher. Which seems ironic!