‘Green hydrogen’ can be generated using renewable energy, providing a clean source of fuel for industry, homes and transport.
Mobility is also changing. Faced with deadly air pollution and untenable carbon emissions, cities around the world are increasingly populated by scooters and other small electric vehicles, replacing large, polluting cars with a more sustainable – and convenient – solution.
Electric urban mobility and hydrogen might seem like a perfect match then, but there is a catch – installation of high-pressure 700bar tanks on scooters is impractical, and the pressure surge during refilling would be too great.
The Fraunhofer Institute for Manufacturing Technology and Advanced Materials (Ifam) in Germany has an intriguing solution. Known as Powerpaste, the grey goo could be a safe way of storing hydrogen without an expensive network of filling stations.
Extreme energy density
“Powerpaste stores hydrogen in a chemical form at room temperature and atmospheric pressure, to be then released on demand,” says Dr Marcus Vogt, research associate at Ifam in Dresden.
To make the thick fluid, magnesium powder is combined with hydrogen at 350ºC and five to six times atmospheric pressure, forming magnesium hydride. An ester and a metal salt are then added to form the finished product.
Onboard a vehicle, Powerpaste would be released from a cartridge by a plunger, combining with water from an onboard tank to generate the specific amount of hydrogen required by the fuel cell.
Only half of the hydrogen comes from the Powerpaste, with the rest coming from the water, and Ifam says this gives the novel fuel a huge energy storage density. With conversion losses of 50% it has specific energy of 1.6kWh/kg and energy density of 1.9kWh/L – higher than a petrol engine with 17% efficiency, for example.
“It is substantially higher than that of a 700bar high-pressure tank. And, compared to batteries, it has 10 times the energy storage density,” says Vogt.
The goo reportedly only begins to decompose at 250ºC, making it safe for hours of use in the hot summer sun. Refuelling would be easy, with riders simply adding new cartridges and water from the tap. Such a system would avoid the need for expensive refuelling stations.
“I think it’s really promising technology,” says Dr Hu Li, an energy engineer from the University of Leeds.
Li, who is not involved in the project, says the paste could offer improved safety, convenience and costs compared to pressurised hydrogen. “High-pressure, 700bar storage is one of the biggest problems with a fuel-cell vehicle compared to electric, because of the cost.”
Ifam’s claimed energy density would be a significant achievement, he adds, with batteries and fuel cells well known for storing less energy per unit weight than liquid fossil fuels. “In terms of energy density, rarely can they compare to gasoline or diesel.”
The associate professor is positive, however, about Powerpaste’s application, and says it could have wider use than just in electric scooters and other small vehicles. Ifam also says it could eventually be used in larger vehicles. Other applications might include large drones, extending flight time from 20 minutes to several hours.
The Fraunhofer institute is building a production plant, scheduled to begin operation this year.
It remains to be seen whether hydrogen’s full potential will ever be realised in the race to ‘net zero’, but imaginative engineering like Ifam’s Powerpaste might give it a greater chance of success.
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