Contribute of Electrochemical Impedance Spectroscopy (EIS) to monitoring long-term durability of hybrid solid-state supercapacitors

Recently, the development of high performance and cost-effective materials for electrodes and electrolytes of energy storage devices is highly desired. In this presentation, some carbon materials for electrodes and solid polymer electrolytes were studied for the fabrication of flexible solid-state supercapacitors. The supercapacitor cells were fabricated contacting face-to-face a SPEEK membrane and two 2 cm2 electrodes. The electrochemical features of these supercapacitors were investigated in a specific designed titanium cell provided of a reference electrode, which monitored the potentials of the positive and negative electrodes of the full cell working in the range from 0 to 1.6 Volt. The 1M Na2SO4 solution was used to exchange the SPEEK membrane, to impregnate a porous separator in a second type of supercapacitor; while a KI solution was added as an additive of the electrolyte on the positive electrode. The sulfonated poly(ether-ether-ketone) (SPEEK) membrane in the supercapacitors had the function of ionic conductor between electrodes and KI salt provided additional pseudocapacitance through I-/I2/I3- multiple redox reactions on the positive electrode. The designed solid-state hybrid supercapacitors were electrochemical investigated trough cyclic voltammetry (CV), DC galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS). As results of the study, it were found that the solid-state supercapacitor based on SPEEK membrane and with iodide species in the positive electrode exhibited higher specific capacitance (~200 F g-1), which was higher than supercapacitor (93 F g-1) with porous separator impregnated with 1M Na2SO4 electrolyte. The EIS and potentiodynamic impedance in-situ analysis highlighted very interesting features especially for solid-state hybrid supercapacitor based on polymer electrolyte (with KI additive). In particular, it showed high specific capacitance, low resistance, high capacity retention and long-term durability for tests up to 20000 cycles and more than 300 h in voltage holding condition at 1.6 Volt. Moreover, the EIS was also used to gain insight of electrochemical processes as the interaction between electrode and electrolyte, when the charge storage mechanism occurs also through faradaic reactions. Likewise, EIS was used to identify possible ageing mechanisms and to understand the causes of change in impedance that occurs by either the addition of new electrochemical processes or by enhancing of specific resistances of the supercapacitor. The remarkable electrochemical performances (e.g. ~ 200 F g-1, ~ 20 Wh kg-1) and exceptional long-term durability (~ 300 h at 1.6 V, under constant voltage test) demonstrated from this configuration of flexible solid-state supercapacitor are very promising for the development of next-generation low cost, high-performance, solid-state and flexible energy storage devices.