Reversible fuel cells: electricity storage with hydrogen

Reversible fuel cells can generate electricity or hydrogen - as needed. German researchers have now set an efficiency record. An important step towards market maturity.

Scientists at Forschungszentrum Jülich have developed a fuel cell system that not only generates electricity from hydrogen - it also takes on the role of the electrolyzer in splitting hydrogen from water.
This reversible high-temperature fuel cell (English: reversible Solid Oxide Cell, short rSOC) still needs some development work before it can be used on the market. However, the scientists have already succeeded in achieving success: the electrical efficiency in hydrogen operation is 62 percent. That's a world record.
This has been achieved by the team led by Professor Ludger Blum at Forschungszentrum Jülich using a more powerful conversion unit, the so-called stack (see title photo). "Our stack can boast five kilowatts of power, which could be used to cover two households' electricity needs, so far, you've always had to combine several units on a kilowatt scale to achieve comparable performance," says Blum.
The researcher hopes that the cost of manufacturing the high-temperature fuel cell, which operates at around 800 degrees Celsius, will also decrease if fewer stacks are needed. That would be another big step towards market readiness.

Fuel cell promising battery challenger

The advantage of the fuel cell compared to the battery is the large amount of storage and the high energy density. For lithium-polymer batteries, it is 140-180 watt-hours per kilogram of mass (Wh / kg), with hydrogen it is about 33,000 Wh / kg. This makes the volatile gas so interesting for storing large amounts of electricity. Even though batteries currently offer significantly more weight in a small space - hydrogen must be compressed under high pressure to compensate for this.
In the fuel cell, energy converters and hydrogen are clearly separated from each other. So can be fed again and again hydrogen or derived. The size of the storable amount of energy are therefore virtually unlimited. " As a stationary storage facility for electricity from wind farms, hydrogen as an energy storage device can bridge dark skies," explains Bernd Emonts, deputy director of the Institute for Electrochemical Process Engineering in Jülich.
Equally interesting is the use of the "rSOC" in residential buildings in order to save the PV power for self-consumption. "This could make users self-sufficient," explains Emonts the potential.
The currently used high-pressure tanks, in which hydrogen gas is stored at up to 700 bar, the deputy director of the institute considers to be less attractive. A much better option is metal hydrides or organic hydrogen carriers such as LOHC. The gas is stored in an oil. As a result, the hydrogen is liquid and can be stored without pressure and can be handled as easily as gasoline.
LOHC, which stands for Liquid Organic Hydrogen Carrier, is considered a promising German innovation . The development of Daniel Teichmann and his company Hydrogenious Technologies was even nominated for the German Future Prize. It makes it possible to store large amounts of hydrogen - "enough to cover the energy needs of a single-family home for a week," says Emonts.

Fuel cell for aircraft, ships, but no cars

As a mobile application, "rSOC" can provide on-board power in aircraft, ships or trains, such as lighting and air conditioning. So far, conventional fuel with its air polluting consequences is still used. In Hamburg, shipping accounts for around 39 percent of all nitrogen oxides. The senate even expects a rise of 6.2 percent by 2025.
However, a high-temperature fuel cell is not suitable for driving trains, trucks or even cars. A quick change between acceleration and deceleration would destroy the cell.
And all other applications for the "rSOC" are still in the future: "Building a system that can handle both the electrolysis and hydrogen storage as well as the power generation is not trivial," Emonts makes it clear. Above all, the overall efficiency is far from satisfactory.
In the electrolysis mode, the efficiency is 70 percent. As a result, the pilot plant is already operating more efficiently than alkaline and polymer electrolyte electrolyzers, which account for 60 to 65 percent and are now standard. The fuel cell stands out with 62 percent compared to low-temperature systems with 50 percent. 
The overall balance, however, is meager. Measured from electricity to electricity, the efficiency is only 43 percent. A competition with batteries with an efficiency of 90 percent is the "rSOC" so far not. The scientists want to increase this so-called "round trip" to over 50 percent.
The service life is still too low. The high-temperature fuel cell is ready for the market with 100,000 hours. However, the electrolysis creates only 20,000 hours, which is about two and a half years and is therefore still insufficient. Emonts: "That would be enough to make the system ready for the market." That takes time. From the point of view of the deputy director of the institute, the developments will take seven years before a prototype of the "rSOC" is created, which is competitive.