Sodium iron dioxide (NaFeO2) with high specific capacity, has been widely used as a kind of electrode materials for Na-ion batteries. When NaFeO2 is used as the battery-type electrode for aqueous Na-ion hybrid capacitors, there are a lot of technical challenges to meet, especially the narrow voltage window limited by the thermodynamic decomposition of water. We propose a novel decoupling electrolyte strategy to suppress the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) on the NaFeO2 anode and the activated carbon (AC) cathode respectively, in order to expand the voltage window of the aqueous AC//NaFeO2 Na-ion hybrid capacitor. The alkaline anolyte (2 M NaOH) and the neutral catholyte (1 M Na2SO4) can be decoupled by the cation exchange membrane, which selectively allows the Na+ ions through the membrane to carry the electric currents in the charge/discharge process. The NaFeO2 sample with the stable crystal structure and oxygen vacancy defects, can provide a gravimetric specific capacitance of 101.1 F g−1 at 1 A g−1. Electrochemical kinetic analysis indicates that the capacitor-type behaviour is predominant in the electrochemical energy storage process of the AC//NaFeO2 Na-ion capacitor at the scan rates higher than 40 mV s−1. The AC//NaFeO2 Na-ion capacitor with the stable voltage window of 2 V, can provide an energy density of 12.6 Wh kg−1 at 1 kW kg−1. Due to its inhibiting effects on HER and OER, this decoupling electrolyte strategy holds great potential for constructing high-voltage aqueous energy storage devices.