Regenerative fuel cell (RFC) systems produce power and electrolytically regenerate their reactants using stacks of electrochemical cells. Energy storage systems with extremely high specific energy (>400 Wh/kg) have been designed that use lightweight pressure vessels to contain the gases generated by reversible (unitized) regenerative fuel cells (URFCs), Progress is reported on the development, integration, and operation of rechargeable energy storage systems with such high specific energy, A primary fuel cell test rig with a single cell (46 cm(2) active area! has been modified and operated reversibly as a URFC (for up to 2010 cycles on a single cell,, This URFC uses bifunctional electrodes (oxidation and reduction electrodes reverse roles when snitching from charge to discharge, as with a rechargeable battery) and cathode feed electrolysis (water is fed from the hydrogen side of the cell), Lightweight pressure vessels with performance factors (burst pressure x internal volume/tank weight) > 50 km 12.0 million in.) have been designed, and a vessel with performance factor of 40 km (1.6 million in.) was fabricated, These vessels use lightweight bladder liners that act as inflatable mandrels for composite overwrap and provide the permeation barrier for gas storage, Bladders are fabricated using materials that are compatible with humidified, electrolyzed gases and are designed to be compatible with elevated temperatures that occur during fast fills or epoxy curing cycles, RFC systems are considered that use hydrogen/oxygen, hydrogen/air, or hydrogen/halogen chemistries, Hydrogen/halogen URFCs are capable of higher round-trip efficiency than hydrogen/oxygen URFCs but are significantly heavier, Therefore, hydrogen/halogen URFCs are advantageous for stationary applications, whereas hydrogen/oxygen URFCs are advantageous for mobile applications, Safety aspects of halogens may prohibit their use in densely populated areas and some commercial applications, so these niches can also benefit from hydrogen/oxygen URFCs.