Gel electrolytes have attracted increasing attention for solid-state supercapacitors. An ideal gel electrolyte usually requires a combination of advantages of high ion migration rate, reasonable mechanical strength and robust water retention ability at the solid state for ensuring excellent work durability. Here we report a zwitterionic gel electrolyte that successfully brings the synergic advantages of robust water retention ability and ion migration channels, manifesting in superior electrochemical performance. When applying the zwitterionic gel electrolyte, our graphene-based solid-state supercapacitor reaches a volume capacitance of 300.8 F cm−3 at 0.8 A cm−3 with a rate capacity of only 14.9% capacitance loss as the current density increases from 0.8 to 20 A cm−3, representing the best value among the previously reported graphene-based solid-state supercapacitors, to the best of our knowledge. We anticipate that zwitterionic gel electrolyte may be developed as a gel electrolyte in solid-state supercapacitors.

Aggregation and dissolution of poly(N-isopropylacrylamide) (PNIPAM) in water were investigated using an ultrasensitive differential scanning calorimetry (US-DSC) and a pressure perturbation calorimetry (PPC). US-DSC reveals that both the aggregation and dissolution of PNIPAM chains are greatly dependent on the scanning rate, indicating that the processes are kinetically controlled. The hysteresis in the dissolution process was found to have a nonequilibrium nature, which is thought to be related to the additional hydrogen bondings formed in the collapsed state of PNIPAM chains. A bimodal appearing in the cooling process at a slow scanning rate indicates the dissolution involves two different processes, i.e., the disruption of additional hydrogen bondings and the dissolution of the collapsed chains. PPC reveals that the solvent accessible surface area of PNIPAM chains in the cooling process is smaller than that in the heating process, which further indicates the dissolution of the PNIPAM aggregates involves such two processes.

Thermally sensitive poly(N-isopropylacrylamide) (PNIPAM) brushes grafted on SiO2-coated quartz crystal surface were prepared with a surface-immobilized initiator. Using quartz crystal microbalance (QCM), we investigated the collapse and swelling of the brushes in water in real time. Both frequency and dissipation of PNIPAM brushes were found to gradually change throughout a temperature range 20-38 degrees C, indicating that PNIPAM brushes undergo a continuous collapse transition in contrast with PNIPAM chains free in dilute solution exhibiting a sharp coil-to-globule transition. This result is in accordance with the previous theoretical prediction. The nonuniformity and stretching of PNIPAM brushes as well as the cooperativity between collapse and dehydration transitions are thought to be responsible for the continuity. On the other hand, a hysteresis was also observed in the cooling process. This is not only due to the intrachain and interchain interactions formed in the collapsed state but also to the nonuniform structure and stretching of the high-density brushes.