Dopant-Free and Self-Charged Gel-Type Polyelectrolytes for Supercapacitors

With the increasing demand for wearable and flexible energy storage devices, there has been significant interest in developing safe and mechanically stable prototypes. For this, liquid electrolytes for batteries and supercapacitors (SC) need to be replaced by self-charged gel-type polyelectrolytes (SCGPE), which exhibit good ionic mobility and offer the advantage of being incorporated into solid-state electronic devices. Here, the SCGPE studied has been synthesized by a polyhydroxyalkylation reaction in a superacid medium of 4-acetylpyridine and the nonactivated aromatic compounds para-terphenyl and biphenyl. The reaction was carried out in a single step at room temperature, without the use of metal catalysts, and yielded water as the only byproduct. Chemical modification reactions were then carried out by quaternizing 4-acetylpyridine using a bromohexyltrimethylammonium salt, which incorporated positively charged elements onto the polymer backbone. The functionalization degree and viscosity of the gel polyelectrolyte were the two main factors affecting SC performance. These SCGPEs exhibit ionic conductivity without the need for doping with a conducting salt, ionic liquid, or acid, thereby ensuring that the PE maintains its mechanical stability and safety. Molecular dynamics simulations have confirmed the key role of the solvent in influencing the polymer conformation and ion transport. SCGPEs enable the sparing of dopants, such as ionic liquids, conducting salts, or acids, as the SCGPE exhibits good ionic conductivity (on the order of 10^-4 S/cm) and high specific capacitance (up to 123 mF cm^-2) when used in textile carbon-based SCs. It also showed only slight differences with a well-known gel polyelectrolyte (GPE), poly-(vinyl alcohol)-potassium hydroxide (PVA-KOH). GPEs used with textile carbon electrodes pave the way for developing all-solid-state wearable SCs without leaking or spilling of liquid electrolytes.