The biodegradable battery

The fabrication plant for the battery revolution looks quite innocuous: It's a modified, off-the-shelf 3D printer sitting in a room in Empa's lab building. The real innovation lies in the recipe for the gelatinous inks that this printer can squirt onto a surface. The mixture involved is cellulose nanofibers and cellulose nanocrystallites, plus carbon in the form of carbon black, graphite and activated carbon. To liquefy all this, the researchers use glycerin, water and two different types of alcohol. Plus 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 lab can store electricity for hours and can already power a small digital clock. It can withstand thousands of charge and discharge cycles and is expected to last for years in storage, even at freezing temperatures. In addition, the capacitor is resistant to pressure and shock.

Biodegradable power supply

But the best thing about it is that when you no longer need it, you can throw it in the compost or simply leave it in nature. After two months, the capacitor has decomposed into its components, leaving only a few visible carbon particles. The researchers have already tried this out, too.

"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 the parameters were right, until all the components flowed reliably from the printer and the capacitor finally worked. Aeby: "As researchers, we don't just want to try things out, we also 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 its internal chemistry makes it extremely versatile.

"The project of a compostable electricity storage system had been close to my heart for a long time," says Nyström. "We applied for Empa-internal research funds with our 'Printed Paper Batteries' project and were then able to start our activities with these funds. Now we have achieved a 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 could be used, for example, to check the contents of individual packages during shipping. Powering sensors in environmental monitoring or agriculture is also conceivable - you don't have to collect these batteries again, but could simply leave them in nature after the work is done.

Point-of-care testing, which is currently booming, will also contribute to the growing number of electronic miniature devices. These include small test devices for use at the bedside or self-testing devices for diabetics. Gustav Nyström is convinced that the compostable cellulose capacitor could also be well suited for such applications.