A review of neutral pH polymer electrolytes for electrochemical capacitors: Transitioning from liquid to solid devices

The development of neutral pH polymer electrolytes has enabled high-performance solid-state, thin, and flexible electrochemical capacitors (ECs) to provide power for future consumer electronics and Internet-of-Thing devices. Notwithstanding their promising prospect, there is still some lack of understandings or disconnections from fundamental science to practical applications of these electrolytes. In this review, we provide an overview of stateof-the-art studies on ECs with neutral pH electrolytes in both liquid and solid configurations. Starting from the fundamental studies on the voltage window and ion conduction of salt species in liquid solution to polymer electrolytes, key considerations in developing neutral pH polymer electrolytes are discussed. The performance of the polymer electrolytes along with their enabled solid symmetric and asymmetric EC devices, as well as some enhanced functionalities are presented. The future directions for research on neutral pH polymer electrolytes are proposed, expected to provide reference for further enriching the fundamental knowledge and improving the device performances.