Ion reservoir and binder-like effects of gel polymer electrolytes (GPEs) are suggested for working mechanisms to enhance rate capability and cycling stability of activated carbon (AC) supercapacitors (SCs) even at 3.4 V. Analysis on kinetics from cyclic voltammetry, electrochemical reactions through in situ Fourier-transform infrared spectroscopy, and differential information of galvanostatic curves reveals that the increased rate-capability is derived dominantly by an improved non-faradaic process by the ion reservoir effect of GPEs in the AC. Although the designed GPEs induce slightly higher bulk and diffusion resistance at the incipient stage, the GPEs play a binder-like function to suppress detachment of AC particles and aggravation of impedance parameters during cycling at 3.4 V.