Solar cell and battery combined

Carbon nitrides enable storage of sunlight and time delayed extraction of electrical energy

Filip Podjaski, Julia Kröger, Bettina V. Lotsch

The research group of Prof. Lotsch, professor at the Max Planck Institute for Solid State Research in Stuttgart and at the department of Chemistry, LMU Munich, developed graphitic carbon nitrides for solar battery applications. This material class has been mainly used and investigated for photocatalytic applications, mimicking natural photosynthesis; whereas now, they have been utilized to directly store solar energy, which can be extracted as electrical energy after significant time delay. These findings are reported in the scientific journal “Advanced Materials”.

Illustration of the dual solar cell – battery functionality enabled by the graphitic carbon nitride NCN-PHI.

Graphitic carbon nitrides are two dimensional, polymeric semiconductors. Owing to their suitable band gap, they can absorb visible light and transfer the harvested energy to co-catalysts, which can help to split water into hydrogen and oxygen. The photocatalytic synthesis of these “solar fuels” can make an important contribution to the storage of solar energy within highly energetic energy carriers. The Lotsch research group had shown previously that a carbon nitride called polyheptazine imide (NCN-PHI) can be used to decouple the absorption of light from the catalytic hydrogen evolution reaction on its surface. The carbon nitride was “light charged” with electrons that could be extracted later on for catalytic reactions on demand. By this means, it is possible to overcome the limitations of “artificial photosynthesis”, which are currently set by the alternation of day and night. The process has been researched in depth and the direct, delayed extraction of electrical energy is made possible. In that regard, this is the very first material being described which enables simultaneous coupling of two fundamental functionalities, light absorption and electrical energy storage, creating a solar cell with integrated storage capabilities. This way an important milestone for harvesting and storing renewable energy without using external storage technologies is being set, as this “solar battery” material allows for time delayed extraction of energy without using any secondary batteries or storage materials.

The coupling of these two abilities is a big advantage over existing concepts that are based on the combination of a solar cell and a battery or a capacitor. It significantly reduces the device complexity while having a positive influence on system costs. Furthermore, the capability of storing electrical energy in NCN-PHI is not limited to solar energy. The material can also be charged conventionally, which  is visualized by a color change from yellow to blue, thus enabling the storage of other renewable energy forms, such as electricity produced from wind turbines etc.

Another advantage of NCN-PHI is the increase in anode voltage, which is important for aqueous batteries since the battery potential is normally limited by water electrolysis. This enables a rise in the overall cell voltage together with the energy density of the battery.

The here described graphitic carbon nitride is a scalable, cheap and purely organic based polymer that can be used in aqueous environments. This is an important advantage over traditional toxic and environmentally harmful electrolytes and electrode materials that are used in batteries. This work opens up an interesting and innovative perspective with a new material for a sustainable energy supply for our society.

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