- formic acid destined to be decomposed into H2 and CO2,
- liquid CO2.
A moving piston separates the two liquids.
During use, formic acid decreases and CO2 increases.
At the filling station, a pistol with two ports simultaneously allows the reverse operation: fill in formic acid and evacuate liquid CO2.
An expandable envelope surrounds the tank and fills in it of CO2 in case of excessive pressure of it.