A pressurized-gas storage assembly has a liner defining a gas storage chamber and an end portion with a planar surface, with an access opening on the planar surface that is in fluid communication with the gas storage chamber. A raised circular ring extends from the planar surface and surrounds the access opening. The assembly also includes a polar boss having a longitudinally extending part that has a top surface with a recessed opening, and a planar bottom surface, with a channel extending through the polar boss to serve as a passageway through the polar boss. A circular groove is provided on the bottom flat surface of the polar boss, with the raised ring received inside the circular groove. An outer shell is provided around the entirety of the liner, with the outer shell having an edge-defining aperture that receives the polar boss.

Disclosed is a breathing apparatus comprising a pressure vessel. The pressure vessel may comprise a composite shell formed from a plurality of fibres and a matrix comprising a resin material and graphene particles. The pressure vessel may comprise at least one permeation barrier layer comprising graphene microparticles or graphene nanoparticles provided on an outer surface of the structural shell or on an outer surface of the liner. Methods of manufacture for pressure vessels are also disclosed.