A group of scientists has developed a system capable of capturing, storing, and releasing solar energy in liquid form, using accessible and commercially available materials.
The energy trapped in the fluid can later be released as hydrogen, without the need for external electricity. The finding, published in the journal Advanced Materials, represents a key step towards the conversion of solar energy into a portable and adaptable resource.
How the system works
The process separates into three stages: capture, storage, and release. It employs graphitic carbon nitride, a yellow powder that acts as a photocatalyst, and ammonium metatungstate, a compound of tungsten and oxygen that retains electrons as if it were a small rechargeable battery.
Both materials are mixed in water with a small proportion of methanol, which plays an essential role: absorbing the positive charges generated by the interaction of light with the carbon nitride, preventing the electrons from recombining too quickly and allowing their conservation.
When exposing the carbon nitride to blue light, pairs of electrons and holes are generated. The electrons migrate to the tungsten clusters of the metatungstate, where they are stored. This phenomenon is evidenced by the change in color of the liquid: from pale yellow to deep blue, indicating that the tungsten atoms are reduced from a +6 charge state to +5.
Hydrogen production in the dark
To release the accumulated energy in the form of hydrogen, researchers add a platinum on carbon catalyst to the solution in the absence of light. The platinum acts as a reaction site, where the stored electrons combine with protons from the water to form gaseous hydrogen. In this way, solar capture, storage, and hydrogen production can be carried out at different times, without continuous connection.
In laboratory tests, after one hour of light exposure, the system generated 13.5 micromoles of hydrogen in darkness. The maximum rate reached 3,220 micromoles per gram per hour, a record for a non-illuminated photocatalytic system. Under real sunlight, successful results were also obtained, with a rate of 954 micromoles per gram per hour without resorting to external electricity.
Confirmation of the mechanism
Advanced studies verified the process: luminescence tests showed that the electrons remain thanks to the storage; spectroscopy evidenced the reduction of tungsten atoms; and magnetic measurements detected these states only after exposure to light.
The authors summarized: “This system demonstrates remarkable efficiency in storing solar energy as electrons”.
Potential and challenges
The technology opens up the possibility of transporting captured solar energy from sunny regions to areas with less radiation, in liquid form and without the need for cables, batteries, or specialized hydrogen storage. If the stored electrons can remain stable for weeks and not just hours, solar energy could be distributed internationally and transformed into fuel when needed.
However, significant challenges remain: the system depends on methanol as an essential component and has not yet been tested for long periods outside the laboratory.
The transformation of solar energy into a truly portable and accessible resource is getting closer. This advancement demonstrates that it is possible to store solar energy in liquid form and release it as hydrogen without external electricity, bringing science one step closer to real applications that could revolutionize the global energy transition.
