Poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is one of the most widely studied conductive polymers, owing to its excellent electrical, optical, and mechanical properties, with various applications such as organic electrochemical transistors, electrochromics, and flexible/stretchable supercapacitors. The charging mechanism of PEDOT:PSS supercapacitors has been traditionally believed to be faradaic, which involves the transfer of charge across the electrode/electrolyte interface. In the present work, however, robust experimental evidence suggests that the PEDOT:PSS supercapacitors mainly store and deliver charge nonfaradaically. The various electrochemical properties of PEDOT:PSS electrical double layer capacitors (EDLCs) are clearly distinguishable from those of polyaniline (PANI) pseudocapacitors, which store charge faradaically. Owing to the nonfaradaic mechanism, the frequency response of PEDOT:PSS supercapacitors is comparable to that of state‐of‐the‐art ultrafast EDLCs with carbon‐based electrodes, making them suitable for high‐frequency applications such as 60 Hz AC line filtering. This result is of great importance for the fundamental understanding of the charging mechanism of mixed ionic‐electronic conducting polymers, such as PEDOT:PSS, and is expected to contribute to the development of various electrochemical devices based on this type of material.

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Summary Aqueous supercapacitors have received significant attention for their high safety, environmental friendliness, and high ionic conductivity. In particular, the supercapacitors with highly concentrated water‐in‐salt electrolytes (WiSEs) exhibit outstanding characteristics such as wide electrochemical stability windows (2‐3 V) and fast ion transport behaviors. However, previous studies on the WiSE supercapacitors have mostly focused on the effects of high concentrations, and few have examined the influence of the electrolytic structure upon the performance of supercapacitors with various concentrations. Hence, the present study examines how the concentration affects the performance of supercapacitors with various concentrations of NaClO4 (1‐17 mol kg−1) and LiFSI (1‐35 mol kg−1) electrolytes. The study reveals a strong correlation between the electrochemical performance and the concentration‐dependent properties of the electrolyte. Sudden increases in the cell voltages, along with the maximum response speeds of the supercapacitors (−Φ120 Hz = 80.5° and 77.9° for NaClO4 and LiFSI, respectively) are observed in the middle of the concentration range (i.e., at 5‐6 mol kg−1), and are strongly associated with a critical change in the ion and water aggregates in the unique biphasic aqueous electrolyte. The present work is of great importance in gaining a fundamental understanding of the aqueous electrolytes with chaotropic ion salts and is expected to contribute greatly to the development of ultrafast supercapacitors based on the highly conductive electrolytes.