Sunlight is one of the most abundant energy sources on Earth, but we can’t use it wherever and whenever we want. This is because storing solar energy and transporting it from sunny regions to places with limited sunlight is still costly and inefficient.
However, a team of researchers has now shown that sunlight can be stored inside a liquid using simple chemical materials and later converted into hydrogen gas in complete darkness. Plus, this method won’t require wires, batteries, or power grids to transport energy.
Moreover, until now, no system using simple, commercially available materials had managed to store solar energy and later release it as hydrogen without any external electricity. The new research suggests that this barrier has finally been crossed.
Turning sunlight into “stored electrons”
The research team built their system from two inexpensive, off-the-shelf materials. The first is graphitic carbon nitride, a yellow powder that can absorb visible light and act as a photocatalyst.
The second is ammonium metatungstate, a cluster of tungsten and oxygen atoms that can accept and hold multiple electrons, much like a tiny rechargeable battery. The process happens in water with a small amount of methanol added.
Methanol plays a crucial role by soaking up the “positive charges” created when light hits the carbon nitride. This prevents electrons from quickly recombining and disappearing, allowing them to be stored instead. As a result, the system does not split pure water and needs methanol as a sacrificial helper.
When the carbon nitride is illuminated with blue light, it generates pairs of electrons and holes. The electrons quickly jump into nearby tungsten clusters.
As more electrons accumulate, the solution visibly changes color from pale yellow to deep blue — a clear sign that tungsten atoms are being reduced from a +6 to a +5 charge state and that solar energy is now chemically stored.
This transfer works so well for two main reasons. First, under acidic conditions, the surface of carbon nitride becomes positively charged, while the tungsten clusters are negatively charged. Opposite charges attract, pulling the two materials tightly together and allowing electrons to move efficiently between them.
Second, their energy levels are well-matched, so electrons can flow naturally without needing an external push. Among several similar materials tested, this tungsten compound showed the best alignment and performance.
Testing energy storage without light
Once the light is turned off, the stored energy does not vanish. To release it, the researchers simply add a platinum-on-carbon catalyst to the darkened solution. Platinum provides spots where stored electrons can combine with protons from the water to form hydrogen gas.
In this way, sunlight capture, energy storage, and hydrogen production are separated into different steps and can even happen at different times. After one hour of light exposure, the system produced 13.5 micromoles of hydrogen in the dark.
The peak hydrogen production rate reached 3,220 micromoles per gram per hour, the highest ever reported for a dark photocatalytic system. Outdoor tests under real sunlight also worked, delivering 954 micromoles per gram per hour in darkness — all without any electrical input.
Advanced measurements confirmed how the system works. Light-emission studies showed that electrons survive longer because they have somewhere to go. Spectroscopy revealed tungsten atoms capturing electrons under light.
Magnetic measurements detected the reduced tungsten species only when illumination occurred. Together, these results confirmed that solar energy was truly being stored and later released on demand.
“This system demonstrates remarkable efficiency in storing solar energy as electrons,” the study authors said.
The technology must come out of the lab
This work shows that solar energy can be captured, stored, transported as a liquid, and later converted into hydrogen without high-pressure tanks, extreme cold, or electrical power.
If future experiments prove that the stored electrons remain stable for weeks instead of hours, this approach could enable solar energy harvested in sun-rich regions to be shipped to darker parts of the world and turned into fuel exactly when needed.
However, the current method also has some limitations. For instance, the system relies on methanol rather than pure water, and long-term storage beyond laboratory timescales has not yet been demonstrated.
Hopefully, future research will address these limitations and finally turn solar energy transport from a promising concept to a viable real-world technology.
The study is published in the journal Advanced Materials.