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A Unitized Regenerative Fuel Cell (URFC) is a regenerative fuel cell (RFC) with both processes done in the same fuel stack. It is a Proton-Exchange Membrane Fuel Cell (PEM) style using usually hydrogen and oxygen. Reasons for development of this compact unitized RFC is less weight and mass.
A URFC combines an electrolyzer that separates the water molecules to make and store hydrogen and oxygen and reverse the process to combine the two elements and make electricity and H2O (water).
Direct Fuel CellRegenerative Fuel Cell Unitized Regenerative Fuel Cell

Livermore physicist Fred Mitlitsky studied the possibilities of reversible technology. In the mid 1990's Mitlitsky received some funding from NASA for development of Helios and from the Department of Energy for leveling peak and intermittent power usage with sources such as solar cells or wind turbines. By 1996 he produced a 50-watt prototype single proton-exchange membrane cell which operated for 1,700 ten-minute charge-discharge cycles, and degradation was less than a few percent at the highest current densities. A rated power of 18.5 kW URFC (regenerative fuel cell system (RFCS)) was installed in the Helios and was onboard during test flights in 2003. The aircraft unfortunately crashed on its second URFC test flight June 26, 2003.Helios in Flight
Helios URFC Hydrogen Tank

Another example is NASA Mars future surface missions. During the day the URFC would use the solar panel electricity to produce oxygen and hydrogen from water using either feedstock brought from Earth or directly from the surface of Mars. At night electrical energy is produced from the fuel cell using the stored fuel and oxygen. A huge amount of energy could be stored for times when it might be needed, as in: leaving the planet. To match that energy volume would require a impossible amount of batteries. Past Mars missions like the Viking Landers used RTGs which are radioisotope thermoelectric generators containing plutonium 238. RTGs are somewhat controversial because of the nuclear exposure part and they cannot be varied with demand or shut off when not needed.

URFC Schematic NASA Plans
The NASA Glenn Research Center Energetics Research Program is funding the development of a URFC system development that will use a URFC as the main component of a lightweight, compact energy storage system. The goal of this program is to demonstrate the feasibility of a URFC energy storage system that can achieve an energy density of >400 watt-hr per kg of mass. While the program does not have the funding to produce actual flight weight hardware, enough development and testing will be completed such that the >400 watt-hr per kg goal can be confidently projected.
NASA applications include high altitude airships, lunar or Mars-based outposts, and other secondary battery applications where the discharge period is 1 to 2 hours long or longer.

Electric-electric efficiency of a URFC is probably not yet as energy efficient as a RFC. The heavier RFC has separate cell plates for converting the chemical energy to electrical and another set of plates for the electrolyzer to convert the electrical energy back to chemical energy. Although the fuel to electrical part is efficient the electrolyzer part is not so much. Efficiency estimates of electrical to chemical back to electrical range from 30% to 65%. URFCs and even RFCs cannot match the high efficiencies offered by other energy storage technologies such as lithium batteries. But where they are a big advantage is in mass storage of energy and energy per kg. The Viking's RTGs are weight efficient producing 48kwh per kgram compared to 1kwh per kgram for a regenerative fuel cell (RFC).
The following chart shows how efficient compared to battery energy storage a URTC is.

As an energy storage system, the URFC system "charges" and "discharges" like a rechargeable battery. While charging, the URFC operates the electrolysis process, which splits water into hydrogen and oxygen. While discharging, the URFC operates the fuel cell process, which combines hydrogen and
oxygen and produces electricity.
URFC and Batttery Comparison Chart
The gases produced during electrolysis are expelled from the cell stack by the production of still more gas inside the cell stack. The continued production of gases by the cell stack pushes the gases into the reactant storage tanks, gradually pumping the gases to higher and higher pressure where they
are stored. In addition to the oxygen and hydrogen, a certain level of water vapor also accompanies these gases when they are expelled from the cell stack.
During the URFC fuel cell process, as gases are consumed inside the cell stack, more gas is delivered to the cell stack by the pressurized reactant storage tanks. The water formed inside of the URFC is removed by either the capillary action of wicking material that is in close proximity to the active
electrode sites or pressure differentials inside the cell stack. The water is pushed out of the cell by a pressure difference between the water pressure inside the cell stack and the water pressure inside an external water storage tank. For a URFC to act without circulation pumps require that the reactants not be circulated through the cell stack, but instead, be dead-ended into the cell stack.

For automotive use Livermore National Laboratory studies believe URFC machines could replace ICE engines and give a comparable driving range. It would be lighter than battery powered electric cars and have more range. Hydrogen would not have to be purchased but made on-board the vehicle when not in use and when plugged up.
Electrical power from the grid could power the electrolysis part of the fuel cell generating the hydrogen and oxygen which would be stored in separate on-board tanks. The oxygen tank being a small football size and hydrogen of a large carbon fiber or newer technology. While braking regenerative system would generate more hydrogen and oxygen.
In the discharge mode the stored hydrogen and atmosphere oxygen would power the fuel cell creating electricity for the electric motor. The URFC could be supercharged by using the oxygen tank for momentary peak power demands. A 50 kilowatt fuel cell would be required to power a small typical road vehicle with extra power when utilizing the concentrated oxygen.

Lynntech, Hydrogenics Corporation and Proton Energy Systems are leaders in developing URFCs.