*3.1. Materials*

Notably, 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF4, ≥98%), 1-butyl-3 methylimidazolium tetrafluoroborate ([Bmim]BF4, ≥99%), 1-hexyl-3-methylimidazolium tetrafluoroborate

([Hmim]BF4, ≥98%), 1-octyl-3-methylimidazolium tetrafluoroborate ([Omim]BF4, ≥98%), 1-decyl-3 methylimidazolium tetrafluoroborate ([Dmim]BF4, ≥98%), and tetrabutylammonium tetrafluoroborate ([Bu4N]BF4, ≥99%) were all supplied by Chengjie Chem. Co., Ltd. (Shanghai, China). Propylene carbonate (PC, ≥99.9%), iodine (I2, ≥99.5%), and tetrabutylammonium iodide (TBAI, ≥99.9%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Ferrocene (Fc, ≥99.9%) was purchased from Tianjin Deen Chemical Reagent Co., Ltd. (Tianjin, China). Nafion N-117 membrane (0.180 mm thick, ≥0.90 meg/g exchange capacity) was purchased from Alfa Aesar China Co., Ltd. (Tianjin, China). Ag electrodes (10mm × 10mm, 1mm in thickness, >99.99%) and graphite rod (5 mm diameter, length 15 cm, ≥99.99%) were acquired from Tianjin Aida Hengsheng Technology Development Co., Ltd. (Tianjin, China).

#### *3.2. The Construction of Pt*/*(I*−/*I 3–) Reference Electrode*

In this work, Pt/(I–/I3 –) was used as the reference electrode in the ILs/PC non-aqueous system. The Pt/(I–/I3 –) reference electrode was prepared by inserting Pt wire into a mixture solution of I2 (0.05 M) and TBAI (0.1 M). In addition, in order to avoid the junction potential and the pollution of the cathodic electrolyte solution from the electrolyte in Pt/(I–/I3 –) reference electrode, a salt bridge filled with the same cathodic electrolyte solution is also used. Both bottoms of the Pt/(I–/I3 –) reference electrode and the salt bridge were sealed with porous polytetrafluoroethylene.
