Implantable medical devices (IMDs) and their various components, including flat electrolytic capacitors for same, and methods of making and using same and providing for outgassing of gases released during capacitor charge and discharge cycles are disclosed. A gas vent and liquid electrolyte barrier into the electrolyte fill tube lumen that is used to fill the interior case chamber with electrolyte and then needs to be closed to prevent leakage of electrolyte. The fill port is shaped to comprise a fill port tube having interior and exterior tube ends and a fill port ferrule intermediate the ends of the fill port tube and comprising a fill port ferrule flange extending transversely to and away from the fill port tube. The fill port ferrule is mounted in an opening disposed in one of the case wall and the cover wall with the ferrule flange in sealing engagement therewith to locate the exterior tube end extending outwardly away from the fill port ferrule flange and the interior tube end within the interior case chamber. A microporous plug is injected into and fills the fill port lumen, the plug formed of a microporous material allowing the escape of gas released from the liquid electrolyte during capacitor charging while preventing escape of liquid or vaporized electrolyte.