© 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.