This image shows the candy cane-like structure of the new polymer supercapacitor. Image: Stoyan Smoukov.Supercapacitors hold out the promise of recharging phones and other devices in seconds and minutes rather than hours. But current supercapacitor technologies have several limitations: they are not usually flexible, have insufficient energy storage capacity and their performance quickly degrades with charging cycles.
Researchers at Queen Mary University of London (QMUL) and the University of Cambridge, both in the UK, have now found a way to overcome all three limitations, by developing a prototyped polymer electrode that resembles a candy cane usually hung on a Christmas tree. This polymer electrode achieves energy storage close to the theoretical limit, but also demonstrates flexibility and resilience to charge/discharge cycling.
Their technique could be applied to many types of materials for supercapacitors, leading to fast charging of mobile phones, smart clothes and implantable devices. The research was published in a paper in ACS Energy Letters.
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 supercapacitors that mostly store 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 first few nanometers below the polymer 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. By developing a way to interweave nanostructures within a bulk material, the researchers have been able to achieve the benefits of conventional nanostructuring without using complex synthesis methods or sacrificing material toughness.
"Our supercapacitors can store a lot of charge very quickly, because the thin active material (the conductive polymer) is always in contact with a second polymer which contains ions, just like the red thin regions of a candy cane are always in close proximity to the white parts. But this is on a much smaller scale," explains project leader Stoyan Smoukov from QMUL.
"This interpenetrating structure enables the material to bend more easily, as well as swell and shrink without cracking, leading to greater longevity. This one method is like killing not just two but three birds with one stone."
The Smoukov group had previously pioneered a combinatorial route to multifunctionality using interpenetrating polymer networks (IPN), in which each component would have its own function, rather than using trial-and-error chemistry to try to fit all functions in one molecule. This time they applied the method to energy storage, specifically supercapacitors, because of the known problem of poor material utilization deep beneath the electrode surface.
Their interpenetration technique drastically increases the material's surface area, or more accurately the interfacial area between the different polymer components. Interpenetration also happens to solve two other major problems in supercapacitors. It brings flexibility and toughness, because the interfaces stop the growth of any cracks that may form in the material. It also allows the thin regions to swell and shrink repeatedly without developing large stresses, ensuring they are electrochemically resistant and maintain their performance over many charging cycles.
The researchers are currently rationally designing and evaluating a range of materials that can be adapted into the interpenetrating polymer system for even better supercapacitors.
In an upcoming review, accepted for publication in Sustainable Energy and Fuels, they provide an overview of the different techniques scientists have used to improve the multiple parameters required for novel supercapacitors. Such devices could be made as soft and flexible freestanding films for powering electronics embedded in smart clothing, wearable and implantable devices, and soft robotics.
This story is adapted from material from Queen Mary University of London, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.