Hydrogen has great potential as a future clean energy source. The challenge is that hydrogen gas is difficult to store and carry safely. The intensity with which it burns may have made liquid hydrogen the fuel-source of choice for the space industry, but it can only be liquified at extremely low temperatures. Now a Japanese research team has invented a compact, flexible polymer that could be used to create a plastic container for hydrogen that's safe to carry in your pocket.

Researchers around the world have been searching for some time for a way to better store hydrogen so it can be more widely harnessed as a fuel source. At this stage, the focus is on developing compact hydrogen storage for the car industry and for other small mobile applications. Current hydrogen fuel cell vehicles like the Honda Clarity Fuel Cell don't burn hydrogen, but chemically fuse hydrogen gas with oxygen from the air to generate electricity, with the hydrogen stored in expensive, highly pressurized tanks that are built to withstand extreme impacts.

This is where the new polymer shows promise. The Waseda University researchers have so far proven their new ketone/alcohol polymer is safe to handle and carry even when filled with hydrogen gas.

The discovery was inspired by recent studies into hydrogen-absorbing organic compounds, which showed potential as a practical way of storing hydrogen because the compounds can safely and reversibly store hydrogen by forming chemical bonds with it. Until now, these compounds have still had to be carried in special high-pressure or temperature-controlled tanks, and scientists had yet to discover a safe and convenient way to release the hydrogen from the compound.

The solution from Waseda University was created by developing a polymer from a ketone called fluorene – an organic compound that is currently used to make antimalarial drugs.

The polymer can be molded as a plastic sheet, and can fix hydrogen via a simple electrolytic hydrogenation at -1.5V (versus Ag/AgCl) in water at room temperature. When heated to 80°C (176° F) with an aqueous iridium catalyst, the flourenol polymer then releases the hydrogen. These are, as the researcher's state in a paper on the discovery, "mild conditions", using (basically) water, heat and iridium, which is widely used in electronics and the automotive, industrial and medical industries.

The researchers wrote: "The theoretical hydrogen storage capacity of fluorenol is 1.1 wt% (0.29 wt% as the fluorenol polymer system); however, we anticipated its improvement by the use of, for example, piperazine tetranol (2.8 wt%) as the hydrogen-fixing unit."

At this stage, these look like fairly low energy returns, but it's worth noting that as the researchers are comparing a gas with a solid by weight, we're talking about quite a large volume of hydrogen gas compared with the polymer.

So there are reasons to be optimistic – the resulting polymer is safe and easy to handle, moldable, robust, non-flammable and possesses low toxicity. It can be put through repeated cycles of fixing and releasing hydrogen under mild conditions without significant deterioration.

This means that future forms of the polymer could be used to create a plastic container for hydrogen that can be carried around in your pocket. The future potential uses are inspiring – for example, it could be used to create hydrogen carrying and releasing cartridges as a fuel source for vehicles and other devices, and to create on-site hydrogen supply systems for homes, businesses and remote research bases on the Moon or Mars…

Want to come back down to Earth? You can read a paper about the research in Nature Communications.

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