Green hydrogen is intended to make blast furnaces climate-neutral, convert steelworks and move heavy industry away from coal. So far, however, its production depends on whether there is enough sun or wind. This is also a core problem of the energy transition: Renewable energy is subject to strong fluctuationsindustrial processes need them to be reliable. A new study describes a material that could close this gap – by temporarily storing solar energy and only later converting it into hydrogen.

What’s special about it: The new system separates charging using light and the actual hydrogen production in time. So it’s not about storing hydrogen directly, but rather using a special polymer to absorb electrons from sunlight and hold them for days. Only when an acid and a catalyst are added does hydrogen form – even in the dark.

“Combination of solar cell and battery”

The material was developed by researchers from Ulm and Jena. The core is a water-soluble copolymer with so-called viologen units. These chemical building blocks can absorb electrons and store them stably. The polymer charges under visible light, supported by a ruthenium complex as a photosensitizer.

After around two hours of irradiation, the system reaches a charge level of 80 to 84 percent. This means: More than four fifths of the units capable of storage are occupied by electrons. “You can imagine it like a combination of solar cell and battery at the molecular level,” says Professor Sven Rau from Ulm University.

“If necessary, we retrieve the chemical energy in the form of hydrogen. The stored electrons are specifically used again,” adds his colleague Professor Ulrich S. Schubert from the Friedrich Schiller University Jena.

More than 80 percent efficiency

The stored electrons are not only retained for minutes or hours. In the experiment, the state of charge remained stable for 24 and even 72 hours as long as no oxygen entered the solution. In comparison, similar, non-polymer-bound molecules lose around 40 percent of their charge within 18 hours.

The amount of energy per mass of material is also significantly higher than with comparable systems. The polymer stores around 101 coulombs per gram. Metal-organic framework compounds that have been tested in other approaches come to around 15 coulombs per gram. The new material is more than six times higher.

Hydrogen is only produced when needed – even without light

The real advantage comes when unloading. Because hydrogen is not created immediately when solar energy is absorbed, but only later: If the solution is acidified to a pH value of 2 and a suitable catalyst is added, the previously stored electrons combine with protons – and hydrogen is formed.

The system worked best with tiny platinum particles. In this case, around 72 percent of the stored energy was actually converted into hydrogen. A rhodium catalyst tested achieved around 54 percent. Other variants remained significantly weaker. Without a suitable catalyst – or without the necessary acid – no measurable hydrogen was produced at all.

A simple switch restarts the system

After hydrogen production, the system is not used up. It can be returned to its original state. To do this, the acidic solution is simply neutralized. The polymer can then absorb solar energy again and recharge itself. This change between acidic and neutral acts like a switch that allows multiple charging and discharging processes.

The system does not work completely without losses. In the first run, the polymer stored around 71 percent of its possible energy, and in the second run it still stored 67 percent. Even after four charging cycles, the value was still in this range at 68 percent – so the material itself remains comparatively stable.

The decline in hydrogen production was more significant: with platinum as a catalyst, only around 32 percent of the stored energy was actually converted into hydrogen in the fourth round.

Solution for industry and energy transition?

Such a process would be interesting for industries such as steel, chemicals or refineries. Systems there run around the clock. You need reliably large amounts of hydrogen – not just when sun or wind is supplying a lot of electricity. Up to now, H₂ has either had to be produced immediately or stored in a technically complex manner. A polymer-based chemical buffer could decouple production processes and make them easier to plan.

However, the system is still purely laboratory research. It uses a ruthenium complex as a light collector and triethylamine as an electron donor. The solution must not contain any oxygen, and a strongly acidic environment is required for the release of hydrogen. For use on an industrial scale, more robust materials and simplified conditions are required.