State-of-Health Monitoring of Hybrid Solid-State Supercapacitors through Electrochemical Impedance Spectroscopy

In this study, we report a diagnostic technique to identify and monitor the long-term performance developments and state-of-health (SOH) of hybrid solid-state supercapacitors through electrochemical impedance spectroscopy. This technique applies a small AC signal (voltage) to the supercapacitor over a specific frequency range, which is distinctive to a supercapacitor features. The supercapacitor impedance response to the AC perturbation give information about the electrical properties of materials (current collector, electrodes, electrolyte and electrode/electrolyte interfaces) as well as faradaic and double layers capacitances of the whole cell. The different processes occurring in cell can be distinguished using impedance measurements because they can have different time constant and taken over a broad range of frequencies (i.e.: 100 kHz to 0.001 Hz). We present here the EIS study on various solid-state supercapacitors realized by contacting face-to-face a SPEEK electrolyte membrane and the two 2 cm2 carbon xerogel electrodes. In addition, a KI solution was used as additive to electrolyte and impregnate in the positive electrode, while the Na2SO4 was employed to make a cation exchange SPEEK membrane and impregnate the negative electrode. This added KI salt had the function providing additional pseudocapacitance (through I-/I3-redox reactions) overlying to the EDLC and may allow a widen voltage window, even without water electrolysis at 1.6 Volt. The studied supercapacitors were electrochemical investigated by usual cyclic voltammetry (CV), DC galvanostatic charge/discharge and as well as AC electrochemical impedance spectroscopy (EIS) and Potentiodynamic impedance (PDEIS). This latter technique was applied on the full voltage range from 0 to 1.6 V and at voltage scan of 0.2V, which can give additional information on the reversibility of different processes and that contribute to the frequency response of device. The EIS analysis highlighted that the solid-state supercapacitor had very low resistance and full capacitance retention during long-term durability test of 20 k cycles (at 2 Ag-1) and additional 300 h at maximum voltage of 1.6 Volt. Moreover, EIS and PDEIS analysis carried out a configuration of SCs with SPEEK membrane and KI redox-species shows several benefits such us exceptional long-term durability (20 K cycles and additional 300 h at constant voltage of 1.6 V), high specific capacitance (? 200 F g-1), high energy density (?20 Wh kg-1) and slow self-discharge rate. These benefits are due to lowering of the positive potential (during the stability test) that was achieved by adding redox-active (I-/I3-) species. Thus maintaining this positive electrode under the limits of oxygen evolution reaction (OER), which is about 0.8-0.9Vvs SHE, the supercapacitor can work reversibly and for long time without appreciable decay. The impedance analysis shows that during the long stability occurs a slight increase of ionic resistance, and a higher increasing of charge transfer (Rct), which can likely lead to a slight decay of EDLC contribution and an increase in pseudo capacitance so that the total capacitance remain unchanged for the supercapacitor.