Deep eutectic solvents (DESs) are known as low-cost and environmentally friendly electrolytes for supercapacitors. However, because DESs are particularly vulnerable to moisture adsorption in the air, the voltage window (<1.2 V) is significantly lower than expected. Herein, we report a new ferroelectric benzimidazole (BI) additive that, by restricting water electrochemical activity at the DES/carbon electrode interface, allows air-exposed DESs to reach a high voltage of 2.2 V. The optimized DES with 0.5 wt% BI addition not only increases the voltage but also the capacitance and energy density while maintaining excellent cycling stability. This study addresses the voltage drop of DESs in air, providing insights into the design of additives that inhibit interfacial water splitting.