© Gian Vaitl / Empa

The biodegradable battery

The number of data-transmitting microdevices, for example in packaging and transport logistics, will increase sharply in the future. All these devices need energy, but the amount of batteries required for them would have a huge impact on the environment. Empa researchers have developed a compostable mini-capacitor that can solve the problem. It consists only of carbon, cellulose, glycerine and common salt – and it works reliably.

The fabrication plant for the battery revolution looks quite harmless: It is a modified, commercially available 3D printer, located in a room in the Empa laboratory building. The real innovation is in the recipe for the gelatinous inks that this printer can squirt onto a surface. The mixture in question consists of cellulose nanofibres and cellulose nanocrystallites, plus carbon in the form of carbon black, graphite and activated carbon. To liquefy all this, the researchers use glycerol, water and two different kinds of alcohol. Add a pinch of table salt for ionic conductivity.

A sandwich of four layers
To build a functioning supercapacitor from these ingredients, four layers are needed, all of which flow out of the 3D printer one after the other: a flexible film, a current-conducting layer, then the electrode and finally the electrolyte. The whole thing is then folded up like a sandwich, with the electrolyte in the middle.
What emerges is an ecological miracle. The mini-capacitor from the Empa laboratory can store electricity for hours and can already power a small digital clock. It can withstand thousands of charge and discharge cycles and probably years of storage, even in freezing temperatures. Furthermore, the capacitor is resistant to pressure and shock.

Biodegradable power supply
But the best part is that when you don’t need it anymore, you can throw it in the compost or just leave it in nature. After two months the condenser has disintegrated into its components, only a few visible carbon particles remain of it. The researchers have already tried this out as well.
“It sounds quite simple, but it wasn’t at all,” says Xavier Aeby from Empa’s Cellulose & Wood Materials department. Long series of tests were necessary until all parameters were correct, until all components flowed reliably from the printer and the capacitor finally worked. Says Aeby, “As researchers, we don’t just want to experiment around, we want to understand what’s happening inside our materials.”
Together with his boss Gustav Nyström, Aeby developed and implemented the concept of the biodegradable electricity storage system. Aeby studied microsystems engineering at EPFL and moved to Empa for his doctorate. Nyström and his team have been researching functional gels based on nanocellulose for years. The material is not only an environmentally friendly, renewable raw material, but also extremely versatile due to its inner chemistry. “The project of a compostable electricity storage system has been close to my heart for a long time,” Nyström says. “We applied for internal Empa research funds with our ‘Printed Paper Batteries’ project and were then able to use these funds to launch our activities. Now we have achieved our first goal.

Application in the “Internet of Things
The supercapacitor could soon become a key component for the Internet of Things, Nyström and Aeby expect. “In the future, such capacitors could be briefly charged using an electromagnetic field, for example, and then they would provide power for a sensor or microtransmitter for hours.” This would make it possible, for example, to check the contents of individual packages during shipping. The power supply of sensors in environmental monitoring or in agriculture is also conceivable – one does not have to collect these batteries again, but could simply leave them in nature after work is done.
Point-of-care testing, which is currently booming, will also contribute to the growing number of electronic miniature devices. Small test devices for use at the bedside or self-testing devices for diabetics are among them. Gustav Nyström is convinced that the compostable cellulose capacitor could also be well suited for such applications.