Previously, self-mending polymeric materials based on physical crosslinking or external stimuli – such as heat and light – have been successfully developed and employed for the mechanical recovery and electrical restoration of devices.
Besides an indispensable layer of electrolyte sandwiched between two electrodes, an extra layer of self-healing polymer is wrapped onto electrodes or used as a substrate for all self-healable supercapacitors reported thus far.
"However, these devices have suffered from low healing efficiency and cyclability," Chunyi Zhi, an assistant professor in the Department of Physics & Materials Science at the City University of Hong Kong, tells Nanowerk. "After merely a few breaking/healing cycles (no more than 5), the capacitances decrease dramatically (14.3–28.2%). Another desirable feature missing in these devices is volume/mass economy owing to the use of an additional component, lowering the volumetric/mass capacitance."
Given that all electrocapacitive materials are not intrinsically stretchable, various modified structures (for example, non-coplanar buckled, coplanar serpentine and wavy, percolating nanostructured) and electron/ion-inactive stretchable substrates (such as elastomers and stretchable textiles) have been utilized to introduce stretchability into conventionally rigid supercapacitors.
However, most achieved strains did not exceed 100% and the performance usually deteriorated at super-high strains.
"The low healing efficiency of self-healable supercapacitors and the small strain of stretchable supercapacitors are fundamentally limited by the widely used polyvinyl alcohol (PVA)-based acidic electrolytes which are intrinsically neither self-healable nor very stretchable," says Zhi. "This gives rise to disadvantages of unsatisfactory performance, additional components, and complex multi-step design."
"Therefore, it is primarily important to develop a multifunctional polyelectrolyte to realize intrinsic self-healability and high stretchability," he points out.
A team led by Zhi has now developed a multifunctional polyelectrolyte, achieving an electrochemically complete self-healability and 600% stretchability of supercapacitors. This work can be applied to other energy conversion and storage devices such as batteries, fuel cells, etc.
Schematics of fabrication strategies for specific functional supercapacitors. (a) Schematic of the supercapacitor comprising the VSNPs-PAA polyelectrolyte and PPy@CNT paper electrodes. (b) Fabrication of the patch-assisted non-autonomic self-healable supercapacitor. (c) Schematic of the fabrication of a super-stretchable supercapacitor. (Reprinted with permission by Nature Publishing Group)
They fabricated a multifunctional polyelectrolyte by polyacrylic acid dual crosslinked by hydrogen bonding and vinyl hybrid silica nanoparticles. As the team demonstrates, this polyelectrolyte with tunable ionic conductivity can be easily stretched to greater than 3,700% strain and, once cut, can be simply self-healed by combing the broken interfaces in ambient conditions.
"Movable protons in the polyelectrolyte provide an equivalent electrode capacitance compared with the commonly used PVA/H3PO4 electrolyte," explains Zhi. "It can be easily self-repaired at room temperature and the repaired samples show ionic properties similar to pristine samples after repeated instances of breaking and healing. In addition, it can be stretched more than 36-fold without any crack."
In addition to the intrinsically self-healable polyelectrolite, the researchers used small patches of carbon nanotube (CNT) paper to overcome the well-known misalignment problem of self-healing supercapacitors.
"Our study of the superior multifunctionality at the device level with facile fabrication and the fewest components creates considerable potential for the wide-scale application of multifunctional devices in many fields such as energy storage and biomimetic sensing," concludes Zhi.