Com­bin­ing the best of ca­pac­i­tors and bat­ter­ies

The Witness - Wheels - - MOTORING - — WR.

SCI­EN­TISTS at UCLA’s Cal­i­for­nia Nano-Sys­tems In­sti­tute have de­vel­oped a new de­vice that com­bines the high en­ergy den­si­ties of bat­ter­ies and the quick charge and dis­charge rates of su­per­ca­pac­i­tors.

Their hy­brid su­per­ca­pac­i­tor was sub­mit­ted for peer re­view in Oc­to­ber 30, 2014 and the jour­nal Pro­ceed­ings of the Na­tional Academy of Sciences has now re­ported their char­ing de­vice is six times as en­ergy-dense as a com­mer­cially avail­able su­per­ca­pac­i­tor and packs nearly as much en­ergy per unit vol­ume as a lead-acid bat­tery.

Elec­tric Car ex­plains bat­ter­ies can store a lot of en­ergy in a small and light pack­age, but they can’t charge or dis­charge very quickly or last a long time the way su­per­ca­pac­i­tors can.

A sin­gle de­vice that com­bines all of th­ese pos­i­tive at­tributes could change the en­tire tech­no­log­i­cal land­scape of to­day, lead­ing to lighter, com­pact phones and elec­tric cars that charge in sec­onds in­stead of hours.

Pro­fes­sor Richard Kaner and Dr Ma­her El-Kady have made an im­por­tant step in this di­rec­tion by cre­at­ing a high­per­for­mance hy­brid su­per­ca­pac­i­tor. Like other su­per­ca­pac­i­tors, their de­vice charges and dis­charges very quickly and lasts more than 10 000 recharge cy­cles. But, ac­cord­ing the sci­en­tists, their in­ven­tion also stores six times more en­ergy than a con­ven­tional su­per­ca­pac­i­tor, hold­ing more than twice as much charge as a typ­i­cal thin-film lithium bat­tery in one fifth the thick­ness of a sheet of pa­per.

The amount of en­ergy that can be stored in such a de­vice de­pends in large part on the con­tact area be­tween the elec­trolyte and the two elec­trodes: the greater the con­tact area, the more en­ergy can be stored. Pre­vi­ous hy­brid su­per­ca­pac­i­tors used por­ous struc­tures in the elec­trode to max­i­mize this area, but the pores were sim­ply too big, and there­fore too few, bear­ing rel­a­tively lit­tle ef­fect on per­for­mance.

Kaner and El-Kady used man­ganese diox­ide (a ma­te­rial used for alkaline bat­ter­ies) for the elec­trodes, but also added a spe­cial three-di­men­sional laser­scribed graphene (LSG) struc­ture. Cru­cially, this graphene struc­ture was specif­i­cally de­signed for high con­duc­tiv­ity, poros­ity and sur­face area, al­low­ing the de­vice to pack much more en­ergy per unit vol­ume and mass. “Even though our elec­trodes are thin (around 15 mi­crons), they are ca­pa­ble of stor­ing more charge than the 100–200 mi­cron­meter thick com­mer­cial su­per­ca­pac­i­tor elec­trodes mainly be­cause our hy­brid LSG/ MnO2 elec­trodes are very en­ergy dense,” El-Kady told Giz­mag.

The re­searchers stated the su­per­ca­pac­i­tors can reach en­ergy den­si­ties of up to 42 Wh/l, com­pared with 7 Wh/l for state of the art com­mer­cial car­bon­based su­per­ca­pac­i­tors. Their de­vice also pro­vides power den­si­ties up to around 10 kW/l, which is 100 times more than lead acid bat­ter­ies.

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