All-round supercapacitors: A rational design of sustainable hydrogel electrolyte utilizing calcium salts

Hydrogel electrolytes are widely employed in supercapacitors (SCs), but the water evaporation in electrolytes can lead to the device failure. Therefore, developing sustainable hydrogel electrolyte that can replenish water from air is highly desirable. Here, we propose a sustainable electrolyte utilizing the hygroscopic CaCl2. The introduction of CaCl2 can greatly enhance the water retentivity of the hydrogel, and ion conductivity and voltage window are also improved simultaneously. The water evaporation dynamics of the electrolyte are systematically studied and it reveals that the mass ratio between water and the polymer matrix played a key role in the performance of the hydrogel electrolyte. An all-round SC based on this sustainable electrolyte was prepared utilizing the all-in-one electrodes. The voltage window can reach 1.8 V, achieving an energy density of 164.5 μWh cm−2. Importantly, the all-round SC can operate sustainably in an open condition with fluctuated humidity (35 % RH ∼ 75 % RH) for more than 1 week. The performance of SC experiences a decline due to the water loss when the environment gets dried but recovers when the humidity rises by absorbing water from air. In contrast, the conventional SC, with a smaller energy storage capacity, quickly fails within few hours. The applications of the all-round SCs have been demonstrated, showing their potential in a wide range of wearable energy storage.