Image: Empa

A drop of water is enough: A paper bat­tery with water switch

A team of Empa researchers has devel­oped a dis­pos­able paper bat­tery that can be acti­vat­ed by water. This could be used to pow­er a wide range of small, dis­pos­able elec­tron­ic devices with low pow­er con­sump­tion, such as smart tags for track­ing objects, envi­ron­men­tal sen­sors or med­ical diag­nos­tic devices — and min­i­mize their envi­ron­men­tal impact to boot. The proof-of-con­cept study has just been pub­lished in Sci­en­tif­ic Reports.

The bat­tery devel­oped by Empa researcher Gus­tav Nys­tröm and his team con­sists of at least one elec­tro­chem­i­cal cell mea­sur­ing around one square cen­time­ter. Three dif­fer­ent inks are print­ed on a rec­tan­gu­lar paper strip. Salt, in this case sim­ply sodi­um chlo­ride or table salt, is dis­trib­uted through­out the paper strip, and one of the two short­er ends of the strip has been dipped in wax. On one side of the paper, an ink is print­ed that con­tains graphite flakes and acts as the pos­i­tive pole of the bat­tery — the cath­ode; on the reverse side, a sec­ond ink is print­ed that con­tains zinc pow­der and acts as the neg­a­tive pole of the bat­tery — the anode. A third ink, con­tain­ing graphite flakes and car­bon black, is print­ed on both sides of the paper over the oth­er two inks. This forms the cur­rent col­lec­tors that con­nect the two poles of the bat­tery to two wires locat­ed at the end of the paper strip dipped in wax.

If a small amount of water is added, the salt con­tained in the paper dis­solves, charged ions are released, and the elec­trolyte becomes ion­i­cal­ly con­duc­tive. This step acti­vates the bat­tery: the ions dis­trib­ute in the paper, caus­ing the zinc on the anode to oxi­dize and release elec­trons. By clos­ing the (exter­nal) cir­cuit, these elec­trons can then flow from the zinc-con­tain­ing anode — via the graphite- and car­bon black-con­tain­ing ink and wires — to the graphite cath­ode, where they are trans­ferred to the oxy­gen from the ambi­ent air, there­by reduc­ing it. These two “redox reac­tions” (a reduc­tion and an oxi­da­tion) pro­duce an elec­tric cur­rent that can be used to pow­er an elec­tri­cal device.

“Proof of con­cept”: a sus­tain­able ener­gy source for low-pow­er electronics

To demon­strate the via­bil­i­ty of their bat­tery for pow­er­ing low-pow­er elec­tron­ics, Nys­tröm’s team com­bined two iden­ti­cal cells — increas­ing the bat­tery’s oper­at­ing volt­age — and used them to pow­er an alarm clock with a liq­uid crys­tal dis­play. When the researchers ana­lyzed the per­for­mance of a (sin­gle-cell) bat­tery, they found that after adding two drops of water, the bat­tery was acti­vat­ed with­in 20 sec­onds and reached a sta­ble volt­age of 1.2 volts. For com­par­i­son, the volt­age of a nor­mal AA alka­line bat­tery is 1.5 volts.

After one hour, the per­for­mance of the sin­gle-cell bat­tery decreased sig­nif­i­cant­ly as the paper dried out. How­ev­er, when the researchers added two more drops of water, the bat­tery main­tained a sta­ble oper­at­ing volt­age of 0.5 volts for more than anoth­er hour.

What’s spe­cial about the new bat­tery is that because both paper and zinc and the oth­er com­po­nents are biodegrad­able, it could sig­nif­i­cant­ly min­i­mize the envi­ron­men­tal impact of dis­pos­able, low-pow­er elec­tron­ics. “And unlike many met­al-air bat­ter­ies, which use a met­al foil that is grad­u­al­ly used up as the bat­tery is used, in our design we only put just the amount of zinc into the ink that is actu­al­ly need­ed for the appli­ca­tion,” Nys­tröm adds. In oth­er words, the more zinc the ink con­tains, the longer the bat­tery will last. Met­al foils, on the oth­er hand, are much more dif­fi­cult to “dose”, i.e. they are not always com­plete­ly used up, which leads to a waste of material.

One minor weak­ness of the new water-acti­vat­ed bat­tery con­cept is the amount of time the bat­tery remains wet — and thus func­tion­al — as Nys­tröm admits. “But I’m sure we can solve this prob­lem by build­ing it dif­fer­ent­ly.” And for envi­ron­men­tal sens­ing appli­ca­tions above a cer­tain humid­i­ty or in wet and humid envi­ron­ments, dry­ing out would not be an issue anyway.

Two com­ple­men­tary mini pow­er stor­age units

Recent­ly, Nys­tröm’s team had already devel­oped a degrad­able paper-based super­ca­pac­i­tor that could be charged and dis­charged thou­sands of times with­out los­ing effi­cien­cy. Com­pared to bat­ter­ies of the same weight, super­ca­pac­i­tors have an ener­gy den­si­ty that is about ten times low­er — but a pow­er den­si­ty that is ten to one hun­dred times high­er. Super­ca­pac­i­tors can there­fore be charged and dis­charged much faster. They can also with­stand many more charge and dis­charge cycles. “So the two tech­nolo­gies com­ple­ment each oth­er very well,” Nys­tröm says. The idea behind the new water-acti­vat­ed bat­tery, he said, was to make small pow­er stor­age devices that are ful­ly charged and pro­vide that ener­gy only after a stim­u­lus is trig­gered, in this case sim­ply a drop of water.