Carbonizing eggshell membranes in a way that preserves their unique porous structure could lead to improved electrodes for supercapacitors.
By Tyler Irving
Posted May 2012
A team from the University of Alberta has made a dramatic improvement in supercapacitor performance using an unlikely material: eggshell membranes.
Supercapacitors consist of two electrodes bathed in an electrolyte solution. During charge, positive ions accumulate on the surface of the negative electrode and vice versa, leading to an electrochemical potential that can later be released. Electrochemical supercapacitors hold promise as a way to store energy from intermittent sources, such as solar and wind. The electrodes are usually made of nanoporous carbon due to its high surface area, conductivity and ease of manufacture. Due to their high rate of discharge, supercapacitors are currently used in some niche applications, such as smoothing out electrical spikes in power grids. However, their low capacitance to date has prevented them from competing directly with rechargeable batteries.
David Mitlin and his group in the Department of Chemical and Materials Engineering at U of A have been working on generating nanoporous carbon from biological waste, such as eggshell membranes. These are readily available from industrial egg-cracking facilities, which separate egg shells and membranes from the whites and yolks used in everything from noodles to cakes. The carbon-rich membranes have a specialized three-dimensional structure of interwoven fibres that improve the transfer of charged particles through the material. This allows better performance using a lower surface area. As a bonus, carbons made from these membranes contain nitrogen and oxygen-based groups which get oxidized or reduced during charge, providing a second mechanism to store energy. The work is published in Advanced Energy Materials.
In the study, the team carbonized the membranes by carefully heating them to 800 C in an atmosphere of argon, preserving their structure. Their model capacitors reached 297 farads per gram, a capacitance significantly higher than unstructured nanoporous carbon, including that from biological sources. On top of that, the team detected only a three per cent fade in capacitance over 10,000 charge/discharge cycles. “Our egg-based capacitors show performance that is among the best in the world, while at the same time using a waste material,” says Mitlin. The group has received funding for three years to create commercial prototypes.
Photo credit: Zhi Li
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