Humidity-modulated properties of hydrogel polymer electrolytes for flexible supercapacitors

Hydrogel polymer electrolytes (HPEs) are essential components in building flexible solid-state supercapacitors, which are promising power sources for future flexible and wearable electronics. HPEs reported so far have shown a wide range of properties with various compositions. In this paper, we demonstrate that relative humidity (RH) significantly influences the electrochemical and mechanical properties of HPEs, which in turn, determines the power output of supercapacitors assembled with such HPEs. Using polyvinyl alcohol and phosphoric acid hydrogel (PVA-H3PO4) as model HPE, we showed that water content in PVA-H3PO4 increases with RH and can reach equilibrium at each RH level. By using atomic force microscope, electrochemical impedance spectroscopy, and cyclic voltammetry, we found that the mechanical stiffness, ionic conductivity, and interfacial capacitance of PVA-H3PO4 prepared at precisely controlled RH levels can vary orders of magnitude. As a result, the flexibility and the energy and power performance of PVA-H3PO4 based supercapacitors can all be modulated by RH. We believe that the result provides a new approach to optimizing HPEs for flexible supercapacitor applications.