Mitlin, a professor of chemical and materials engineering at the University of Alberta, Canada is working on a way to turn waste eggshell membranes and egg whites into materials for high-performance supercapacitors.
Supercapacitors are devices which offer many “need-of-the-hour” features like high power density, charging and discharging which are generally faster than regular rechargeable batteries. But unfortunately they store much less energy.
Mitlin thinks the membranes inside eggshells could help crack this problem. “If you could keep the very nice power of a supercapcitor but extend the energy density even to be that of a mediocre lithium-ion battery, you’d really enable the applications world in the automotive sector and in consumer products,” Mitlin said in a conference meeting.
Supercapacitors are essentially electrochemical devices that store a double layer of charge on activated carbon as electrodes.
Mitlin and colleague, Zhi Li, propose replacing the activated carbon with eggshell membranes from certain industrial chicken farms. These farms provide eggs yolks for the production of medicine and cosmetics, but they discard the shells.
Mitlin and Li start by heating the shells to 920 °C. They then activate the material by heating it to 300 °C in the air for two hours. “You take the membranes and transform them into this almost construction-paper-looking thing,” says Mitlin. The carbonized membranes retain their porous structure, providing plenty of surface area on which to hold an electrical charge. And their relatively high nitrogen content is about 8% after processing leads to capacitance that isn’t just chicken feed, though scientists are unsure why this happens.
Mitlin measured a capacitance of approximately 300 farads per gram in the eggshell supercapacitors, this is about 100 times as much as those that used activated carbon. The material retained 96% of its efficiency even after 10000 charge-discharge cycles, this is a highly satisfactory result because the performance of any capacitor reduces when charge-discharge cycle increases, this will indirectly increase the energy constrain in any electronic device.
Activated carbon is already very cheap, but Mitlin says the egg-based approach could compete on performance. The membrane material used here also meets one of the goals of Canada’s clean-energy initiative, which seeks to use bio-waste to produce energy.
2012 MRS Fall Meeting & Exhibit