A cartridge connectable to a fuel cell is disclosed. The cartridge comprises an outer casing and an inner flexible liner containing fuel for the fuel cell. The inner flexible liner may have an insert disposed inside the inner liner to facilitate the transport of fuel from the cartridge to the fuel cell. The insert minimizes the fuel that is trapped within the cartridge. The inner flexible liner can be used without the outer casing. The outer casing can be substantially rigid or flexible. The cartridge is also adaptable to receive byproducts from the fuel cell. The cartridge can also be pressurized to push fuel to the fuel cell. Unidirectional relief valves are also disclosed to prevent internal pressure in the cartridge from becoming too high or too low.