"Backwards Runs the Reaction," Feature Article, April 2008

backwards
runs the reaction

Instead of carrying compressed oxygen or hydrogen, some advanced vehicles are generating critical gases directly from water.

by Robert Roy

It was a moment that bordered on catastrophe: A fire aboard the Mir space station in 1997 almost caused the loss of the orbiting platform and the deaths of its crew. With their route to an escape capsule blocked by the flames, the crew could only watch as the fire licked at the aluminum hull before dying out after 14 minutes.

That fire was traced to an on-board oxygen generator. The generator relied upon an exothermic chemical reaction to create a breathable atmosphere for the crew of the space station. But as even a junior high school student knows, combining heat and oxygen can lead to fire.

Oxygen for the crew of the International Space Station is supplied through an on-board electrolysis system.

The successor to Mir is the International Space Station, and it too has an on-board oxygen generator, though of a radically different design. Installed in 2006 and successfully used since last June, the generator operates on a simple principle— electrolysis, the dissociation of oxygen and hydrogen from water through the application of electricity. What makes the generator safe? The device uses a proton-exchange membrane, similar to that used in a PEM fuel cell, to separate the generated hydrogen and oxygen.

The fuel cell has been touted for decades as a device that can generate electricity from benign chemical reactions, such as reacting oxygen and methane to make water and carbon dioxide. Although fuel cells have successfully competed in many niche applications, they haven't exactly lived up to their potential—or the hype.

Quietly, however, PEM electrolyzers have found their way into many high-profile vehicles over the past three decades. My colleagues at Hamilton Sundstrand and I believe that this technology soon may find its way into satellite thrusters or energy storage systems.

An Old Idea Made New

A PEM-based water electrolyzer works in much the same way as the archetypal junior high school chemistry experiment—the one with two wires attached to a lantern battery that are stuck into a beaker of dilute salt water— except that it uses a solid acid or membrane rather than a liquid solution. Water is oxidized at the oxygen electrode, or cell anode, to produce oxygen gas, releasing hydrogen ions, or protons, and electrons.

The hydrogen ions migrate from the cell anode to the cell cathode, or hydrogen electrode, under the effect of the electric field imposed across the cell, while the electrons are transferred by a dc power source. The protons and electrons recombine at the cell cathode to produce hydrogen. Liquid water is also released at the cathode due to a process called electro-osmotic drag. Oxygen and hydrogen are generated in a stoichiometric ratio—two volume units of hydrogen for every one of oxygen—at a rate proportional to the applied cell current.

The water to be electrolyzed can be either liquid or vapor. Indeed, in a liquid-phase setup, water is introduced on the cathode side and soaks through the membrane to the anode, where it is oxidized to make oxygen gas. As a result, the oxygen gas stream is produced without any additional liquid. Water can also be used to carry away any excess heat.

The pressure of the produced gas can be ramped up as needed, as high as 3,000 pounds per square inch. For every factor of 10 increase in pressure, the equilibrium cell voltage need be increased by only 44 mV. And thanks to its solid membrane, a PEM electrolyzer is able to withstand large differentials in pressure.

Because the membrane is non-porous, a PEM cell can safely separate product gases at superatmospheric pressures.

Most of the design elements used to produce the water electrolysis cell stack were originally developed during the 1980s for submarine-based life support systems. The oxygen generating plant, for instance, was built for the U.S. Navy's Seawolf-class submarine, to produce oxygen and hydrogen for storage in high-pressure banks. At the time, however, the cell stack could operate only at relatively low pressure. The plastic frames that contained the fluids at their operating pressure within the cell cavities and the metal screens used to distribute water within the cell and provide support and electrical conductivity to the electrolysis cell membrane could withstand less than 200 psig. The risk for operating above that pressure was that hydrogen could leak into the submarine's atmosphere, or that because of a membrane breach, hydrogen and oxygen could mix.

The design solution involved placing the cell stack within a pressurized vessel to balance the hoop stresses across the plastic frames, and providing pressure regulation between the oxygen and hydrogen fluid circuits to ensure that the proper cross-cell pressure differential is maintained during all aspects of system operation. Nitrogen provided by the shipboard's system served as a reference gas for the pressure control system. That limited the number of single barrier interfaces between oxygen and hydrogen to just the cell stack. It also served as the pressurized blanket for the cell stack housed in the pressure vessel.

Also, because the Navy required the system to be as small as possible, the oxygen generating plant uses a liquid anode feed system so it can operate with a high current density. While the plant's electrolysis cell stack operates at up to 1,400 amps per square foot, there is a trade-off between volume and electrical efficiency. Typical efficiency for the cell stack is only about 65 percent at beginning of life; however, the submarine is not limited in either power or cooling resources.

The oxygen generating plant is in service aboard the three Seawolf-class submarines—the USS Seawolf, the USS Connecticut, and the USS Jimmy Carter.

However, the complex nature of the system and the need for high-pressure pumps and other components has resulted in high lifecycle costs for the plants. Additionally, nitrogen consumption is high, necessitating frequent recharges of the shipboard's nitrogen system. An alternate solution for providing respirable oxygen for submarine life support was needed.

Making Oxygen in Mid-Air

An advanced cell design was developed by Hamilton Sundstrand in the 1990s to permit high-pressure gas generation without the need for a pressure vessel or complex pressure control system. The new cell was fashioned from patterned foil layers that were bonded together using a fluoroelastomer coating. A sintered powder metal porous plate provided structural support to the electrolysis cell membrane while operating at high differential pressure, simultaneously allowing fluid transport both to and from the bulk water stream and the electrode surface.

The first application for this new cell design was for an oxygen recharge system for commercial aircraft. Currently, aircraft are required to maintain the emergency oxygen system at 1,400 psia or higher prior to dispatch. The on-board oxygen generating system, or OBOGS, can recharge the high-pressure emergency oxygen system on commercial aircraft without the need for any oxygen servicing by ground personnel. The cell stack uses water from the galley to generate oxygen at up to 2,010 psia, while the hydrogen and water loop are maintained at essentially ambient pressure.

The result of this advanced high differential pressure cell design is a greatly simplified fluid system, with only the oxygen fluid circuit maintained at elevated pressure. This has proved to be a robust design: Although the requirements called for 6,000 hours of operation, a single cell assembly operating at nominal recharge conditions operated in the laboratory for over 54,000 hours.

The USS Jimmy Carter relies on an electrolyzer to generate oxygen.

The high differential pressure cell design may also produce high-pressure hydrogen with oxygen at ambient pressure. A 65-cell stack currently in production for the U.S. Navy's latest fast-attack submarine class is capable of providing oxygen at ambient pressure and hydrogen at up to 800 psig. In this setup, the oxygen is vented directly to the shipboard ventilation system, while the hydrogen is at sufficient pressure to dump overboard. The Royal Navy has recently adopted this new cell technology for its new submarine class.

Because of the limited availability of power aboard the International Space Station, energy efficiency is a high priority for the oxygen generating system that was launched in 2006 and first successfully operated in July 2007. The system employs a 28-cell liquid cathode feed PEM electrolyzer operating at a current density of only 200 amps per square foot to produce oxygen at ambient pressure with 80 percent efficiency. What's more, because the power from the photovoltaic cells rises and falls with the station's day-night cycle—53 minutes of daylight followed by 37 minutes of darkness—the cell stack can switch rapidly into a standby mode where its electrical draw drops by 97 percent.

To ensure the safety of the space station crew, the oxygen generating system's electrolyzer as well as any hydrogen-containing components are enclosed in an evacuated dome. Any gas leaks are vented directly into space. Because it's critical to get as much use as possible from the water, the water-hydrogen mixture exiting the cell stack will be spun in a rotary device that separates the two components. The water will recirculate through the loop to a heat exchanger to reject waste heat from the process before returning to the cell stack. The hydrogen will be dumped overboard. (Eventually, the hydrogen will be sent to a Sabatier system, where it will combine with carbon dioxide to produce water and waste methane.)

The water processing system is yet to be delivered, so for now the station will rely on a Russian-built system to replenish the atmosphere. In time, though, the generating system will be able to supply enough oxygen for up to seven astronauts.

The Water Rocket

The International Space Station is not the only orbital destination where this technology can be used. A high-pressure vapor feed electrolyzer was recently designed, built, and tested as an alternate propulsion system for advanced satellites. Developed as part of a DARPA program, the cell stack generated hydrogen and oxygen gases using similar design elements as the high differential pressure water electrolysis cell.

The system, named the Water Rocket, would electrolyze water using power from the vehicle's solar array to charge storage tanks to 2,000 psia. When required, the high-pressure gases would be delivered to a hydrogen-oxygen thruster array to provide orbital reboost and maneuverability to satellites to improve their useful life, extend their capability, and enhance their overall versatility. The logistics of refueling the vehicle would also be simplified, requiring only the transfer of liquid water from one vehicle to the other rather than the more hazardous propellant fluids currently in use.

Hamilton Sundstrand recently tested a five-cell Water Rocket stack. The breadboard demonstration generated oxygen and hydrogen at up to 2,000 psia.

Another application for this technology would service individual spacesuits. The spacesuits onboard the International Space Station provide oxygen to the astronauts from oxygen bottles located within the suit's primary life support system. These bottles are charged from high-pressure oxygen storage tanks attached externally to the station's airlock. Those tanks are recharged using a mechanical compressor that takes boil-off from the Space Shuttle's cryogenic oxygen tank and compresses it to 2,700 psia into the tanks. Of course, such a system requires a visit from the shuttle, and since NASA is retiring the shuttle fleet in 2010, the agency is evaluating a high-pressure oxygen generating system that could recharge the tanks.

Under contract to NASA, Hamilton Sundstrand has conducted system-level trade studies and is currently developing the cell stack to operate at a 3,000 pounds per square inch differential of oxygen over hydrogen. The oxygen would be dried prior to storage in the high-pressure tanks, while the hydrogen would be vented to space. Testing of a two-cell prototype stack assembly is scheduled for this spring. Oxygen compatibility testing of a mockup that simulates the water electrolysis cell stack is also planned early in the program to validate the system-level mitigation approach for managing catastrophic failures of the hardware.

Extraterrestrial Applications

Energy storage solutions using water electrolysis and fuel cell systems are being examined for applications ranging from backup power systems and lighter-than-air vehicles to extraterrestrial bases on the moon and Mars. The basic architecture of a regenerative fuel cell energy storage system includes a high-pressure water electrolysis system, a fuel cell, a fluid management and storage system, a thermal management system, and a power management system. Depending on whether the system has access to atmospheric oxygen, the oxygen from water electrolysis can either be stored as a pressurized gas or simply vented as a waste by-product.

For extraterrestrial applications, the system would be used in tandem with a photovoltaic array. During the day, excess electricity would power the electrolysis system, which would store the produced hydrogen and oxygen. At night, the oxygen and hydrogen would be delivered to a fuel cell to provide power to the base; the product water from the fuel cell would be stored and subsequently consumed by the water electrolysis system, thereby closing the fluid cycle. The water electrolysis system and the fuel cell would be thermally linked so that neither would freeze when not in use.

One day, electrolyzers may help fill space suits' air tanks during long missions.

Recent studies have focused on oxygen and hydrogen storage pressures of between 1,000 and 2,000 psi, requiring the development of a high, balanced-pressure water electrolysis cell stack and balance of plant to safely manage these fluids. The choice of technology for the water electrolysis and fuel cell systems depends on a number of factors, including electrical efficiency, fluid purity, and ease of fluid management.

Water electrolyzers may become more commonplace in the future as we move forward into a hydrogen-based economy. Fuel cell-powered vehicles hold the promise of reducing greenhouse gas emissions from the transportation sector, provided the hydrogen fuel is produced from a renewable energy source, such as a high-pressure water electrolyzer operating from wind, solar, or nuclear power. Backup power systems that currently depend upon lead-acid batteries may someday be supplanted by environmentally friendly regenerative fuel cell systems that are free of any lead content. Helium-buoyed airships may dot the skies, providing platforms for telecommunications payloads or surveillance equipment around the clock, thanks to an energy system that combines solar arrays and an onboard RFC power system.

All that—based on a concept that can be demonstrated with a beaker of liquid, two wires, and a battery.

Robert Roy is electrochemical programs engineering manager at Hamilton Sundstrand in Windsor Locks, Conn.