A novel bilayer heterogeneous poly(ionic liquid) electrolyte for high-performance flexible supercapacitors with ultraslow self-discharge

Flexible supercapacitors with high power density and long cyclic stability represent a promising candidate to be used as power supplies for portable electronics, but often suffer from the disadvantages of a limited working voltage and rapid self-discharge (spontaneous drop of open-circuit voltage). Here, we design a bilayer heterogeneous poly(ionic liquid) electrolyte (BHPE) consisting of a polycation complex and a polyanion complex with different zeta potentials to suppress the self-discharge of flexible symmetric supercapacitors. The resultant BHPE-based supercapacitors using active carbon/carbon nanotube composite electrodes exhibit a high working potential of 3.0 V and an energy density of 33 W h kg^-1, which are comparable with those of devices obtained by using a homogeneous poly(ionic liquid) electrolyte (HPE). More significantly, the developed BHPE-based supercapacitor charged under forward bias exhibits a self-discharge time of 23.2 h, which is at least twice that of the device charged under reverse bias and is also much superior to those of HPE-based supercapacitors. The BHPE-based supercapacitors also possess excellent mechanical flexibility and stability, due to the stabilized interface contact between two layers of poly(ionic liquid)s.

Phase-Transitional Ionogel-Based Supercapacitors for a Selective Operation

As the demand for energy storage devices increases, the importance of electrolytes for supercapacitors (SCs) is further emphasized. However, since ions in electrolytes are always in an active state, it is difficult to store energy for a long time due to ion diffusion. Here, we have synthesized a phase-transitional ionogel and fabricated an SC based on the ionogel. The 1-ethyl-3-methylimidazolium nitrate ([EMIM]+[NO3]-) ionogel changes its phase from crystal to amorphous when the temperature was elevated above its phase transition temperature (∼44 °C). When the temperature is elevated from 25 to 45 °C, the resistivity of the gel is decreased from 2318.4 kΩ·cm to 43.2 Ω·cm. At the same time, the capacitance is boosted from 0.02 to 37.35 F g-1, and this change was repeatable. Furthermore, the SC exhibits an energy density of 7.77 Wh kg-1 with a power density of 4000 W kg-1 at 45 °C and shows a stable capacitance retention of 87.5% after 3000 cycles of test. The phase transition can switch the SCs from "operating mode" to "storage mode" when the temperature drops. A degree of self-discharge is greatly suppressed in the storage mode, storing 89.51% of charges after 24 h in self-discharge tests.

A Redox-Mediator-Integrated Flexible Micro-Supercapacitor with Improved Energy Storage Capability and Suppressed Self-Discharge Rate

To effectively improve the energy density and reduce the self-discharging rate of micro-supercapacitors, an advanced strategy is required. In this study, we developed a hydroquinone (HQ)-based polymer-gel electrolyte (HQ-gel) for micro-supercapacitors. The introduced HQ redox mediators (HQ-RMs) in the gel electrolyte composites underwent additional Faradaic redox reactions and synergistically increased the overall energy density of the micro-supercapacitors. Moreover, the HQ-RMs in the gel electrolyte weakened the self-discharging behavior by providing a strong binding attachment of charged ions on the porous graphitized carbon electrodes after the redox reactions. The micro-supercapacitors with HQ gel (HQ-MSCs) showed excellent energy storage performance, including a high energy volumetric capacitance of 255 mF cm-3 at a current of 1 µA, which is 2.7 times higher than the micro-supercapacitors based on bare-gel electrolyte composites without HQ-RMs (b-MSCs). The HQ-MSCs showed comparatively low self-discharging behavior with an open circuit potential drop of 37% compared to the b-MSCs with an open circuit potential drop of 60% after 2000 s. The assembled HQ-MSCs exhibited high mechanical flexibility over the applied external tensile and compressive strains. Additionally, the HQ-MSCs show the adequate circuit compatibility within series and parallel connections and the good cycling performance of capacitance retention of 95% after 3000 cycles.

Electrochemical performance of the homologous molybdenum(vi) redox-active gel polymer electrolyte system

PVA–H₃PO₄–Na₂MoO₄ and PVA–H₃PO₄–PMo₁₂ are assembled into a single solid-state supercapacitor to improve the specific capacitance. Homologous molybdenum (vi) of PMo₁₂ and Na₂MoO₄ provides synergistic effect to improve faradaic capacitance performance.

A Regenerable Hydrogel Electrolyte for Flexible Supercapacitors

Easy regenerability of core components such as electrode and electrolyte is highly required in advanced electrochemical devices. This work reports a reliable, regenerable, and stretchable hydrogel electrolyte based on ionic bonds between polyacrylic acid (PAA) and polyallylamine (PAH). PAA-PAH electrolyte (1M LiCl addition) exhibits high ionic conductivity (0.050 S·cm-1) and excellent mechanical property (fracture strain of 1,688%). Notably, the electrolyte can be regenerated to any desired shape under mild conditions and remains 96% and 90% of the initial ionic conductivity after the first and second regeneration, respectively. PAA-PAH/LiCl-based supercapacitor exhibits nearly 100% capacitance retention upon rolling, stretching, and 5,000 charge-discharge cycles, whereas the regenerated device holds 97.6% capacitance of the initial device and 90.9% after 5,000 cycles. This low-cost, high-efficiency, and regenerable hydrogel electrolyte reveals very promising use in solid-state/flexible supercapacitors and possibly becomes a standard commercial hydrogel electrolyte for sustainable electrochemical energy devices.