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September, 2017

Supercapacitors promise recharging of phones and other devices in seconds and minutes vs hours for batteries. Current technologies, however, are not usually flexible, have insufficient capacities, and their performance quickly degrades with charging cycles. Researchers in the Smoukov and Kumar labs have found improved all three properties in one stroke. The prototyped polymer electrode achieves storage close to the theoretical limit, but also shows flexibility and resilience to charge/discharge cycling. The technique is general and could be applicable to many types of materials for supercapacitors, specifically a class exhibiting pseudocapacitance.

Pseudocapacitance is a property of polymer and composite supercapacitors that allows ions to enter inside the material and thus pack much more charge than carbon ones that mostly store the charge as concentrated ions (in the so-called double layer) near the surface. The problem with polymer supercapacitors, however, is that the ions necessary for these chemical reactions can only access the top few nanometers below the material surface, leaving the rest of the electrode as dead weight. Growing polymers as nano-structures is one way to increase the amount of accessible material near the surface, but this can be expensive, hard to scale up, and often results in poor mechanical stability. Using reactions and phase separation, however, the researchers have interwoven active nanostructures within a bulk material, just like the intertwined red and white regions in a candy cane. The thin active material (the conductive polymer) is created in thin strands and always in contact with a second polymer which contains ions, allowing full utilization of the material and quick charging. This interpenetration also enables the material to bend, swell and shrink without cracking, leading to greater longevity. The Smoukov group had previously pioneered a combinatorial route to multifunctionality using interpenetrating polymer networks (IPN) and this time they applied the method to energy storage. In an accepted review in Sustainable Energy & Fuels, they overview techniques used to improve multiple performance dimensions in supercapacitors. It is notable that the first author, Kara Fong, was a Master’s student in the AIM lab, as well as a Churchill Scholar now continues her interest in energy storage by pursuing a PhD at Berkeley. 

K. D. Fong, T. Wang, H.-K. Kim, R. V. Kumar, and S. K. Smoukov, "Semi-Interpenetrating Polymer Networks for Enhanced Supercapacitor Electrodes", ACS Energy Lett. 2, 2014–2020 (2017)