A new copolymer-based battery developed by researchers at Ulm and Jena universities in Germany stores energy from sunlight for days and can release it when required as green hydrogen. The battery is rechargeable, and the charge and discharge process can be activated by flipping a pH switch, a press release said.
With the focus on switching away from fossil fuels, countries are adopting large-scale solar and wind power plants. However, for applications requiring higher energy density, hydrogen is a more viable alternative. It can be burnt, much like a fossil fuel, but produces only water as a byproduct, offering a carbon-free solution for energy-intensive applications.
However, hydrogen production itself can be a carbon-emitting process. Large-scale hydrogen plants use methane reforming to produce hydrogen, since it is cost-effective. For hydrogen to be an ideal replacement for fossil fuels, it must be produced using solar or wind energy, also known as green hydrogen.
Copolymer to the rescue
Green hydrogen can be produced using sunlight through a photocatalytic process. Once the gas is produced, it needs to be stored separately in tanks and processed when required. However, a research team led by Ulrich Schubert at Jena University and Sven Rau at Ulm University decided to use copolymer molecules instead.
Copolymers are macromolecules that consist of different organic building blocks. They have a stable framework and can be equipped with specific functional units. For this solar battery, the researchers used a water-soluble copolymer with reinforced redox activity as its chief functional unit.
When exposed to sunlight, the system achieves 80% charging efficiency. Once charged, the system can maintain its charged state for several days. To retrieve the energy, the researchers added an acid and a hydrogen-evolution catalyst to cause the electrons stored in the system to combine with protons, thereby releasing hydrogen. Here, the system’s efficiency is high again, reaching 72 percent.
Uses pH as a switch
The copolymer-based system features redox reactions that are completely reversible. So, when the battery is discharged, it can be left in the Sun to recharge, facilitating multiple catalytic and storage cycles.
To reset the system, the researchers simply change its pH value. But pH is not just a switch; it is also an indicator of the polymer’s state of charge. When discharged, the presence of the acid changes the colour from violet to yellow.
When placed in sunlight to charge, the system changes colour from yellow to violet again, showing that the battery has a charge it can release as hydrogen when necessary. The hydrogen released could be used for a wide variety of applications, from running electric cars to manufacturing steel or generating clean electricity on demand.
“The project is also of scientific significance because it combines very different concepts from the field of chemistry that otherwise have few points of contact: namely, macromolecular polymer chemistry and photocatalysis,” added Rau in the press release.
“The results open up new perspectives for cost-effective, scalable solar storage technologies – and provide an important building block on the way to a sustainable, chemical-based energy economy,” concluded Schubert.
The research findings were published in the journal Nature Communications.
