A method and apparatus for compressing gaseous hydrogen or other gases utilize a liquid compressor to achieve high pressure while controlling vapor content. Gaseous hydrogen from a source at an inlet pressure is compressed in a liquid compression chamber using an incompressible liquid, preferably water or a water-based liquid, to a predetermined pressure at which vapor in the compressed gas is below the required concentration level for applications. The compressed gas flows into a high-pressure gas chamber, where it is isolated from the compression liquid once the predetermined pressure is reached, ensuring the vapor content remains below a specified threshold. The compressed gas is then transferred to a storage tank at a lower storage pressure. Embodiments include cooling the high-pressure chamber, adding freezing-point-lowering additives to the liquid. The invention enhances compression efficiency, reduces costs, and meets stringent purity requirements for applications such as fuel cells.

A device for isothermal expansion and compression of a gas, ensuring the compression of the gas by consuming mechanical energy and the restitution of the mechanical energy by the expansion of the gas. The device includes at least two liquid pistons movable in at least two chambers each including a gas that is able to be compressed or expanded by movement of the liquid pistons. A mechanical actuator, including at least one solid piston, ensures the movement of the liquid pistons in the chambers. Each chamber includes an insert, through which the liquid and the gas can circulate, the insert including through-cells, which extend in a direction parallel to the direction of movement of the liquid piston in the insert. The device includes at least one first phase separator connected to an outlet of each of the chambers, and to a tank for storing pressurized gas.