Electrical and Electronic Devices, Circuits, and Materials. Группа авторов

Figure 3.1 (a) The development of supercapacitors in different countries [Reproduced with permission from Ref. [6], © AIP Publishing 2019]. (b) An illustration of key performance metrics, test methods, major affecting factors for the evaluation of SCs [Reproduced with permission from Ref. [7], © Wiley 2014].

Figure 3.2 Schematic diagram of (a) an electrostatic capacitor, (b) an electric double-layer capacitor, (c) a pseudocapacitor, and (d) a hybrid-capacitor [Reproduced with permission from Ref. [12], © Royal Society of Chemistry 2015].

       3.1.2 Key Characteristics of the Electrolyte

      In general, the electrolytes for the SC application need to follow some requirements: (1) broad potential window; (2) high ionic conductivity; (3) broad operating temperature range; (4) non-volatile and non-flammable nature; (5) better chemical and electrochemical stability; (6) chemically inert toward SC cell components such as electrodes, current collectors; and (7) cost-effective and environmentally friendly. The prepared polymer electrolyte needs to be examined on the basis of the characteristic parameter that influences the morphological, structural and electrical properties [14–16]. These important parameters are influenced by the host polymer, salt and nanofiller addition. This section discusses the important parameters.

       Morphology and Crystallinity

      Fast ion dynamics in polymer electrolytes is facilitated by high amorphous content and is examined before going ahead for electrical properties. The X-ray diffraction (XRD) and differential scanning calorimetry (DSC) are techniques to estimate the degree of crystallinity (ΧC).

Figure 3.3 Strategies for improving the energy density of supercapacitors [Reproduced with permission from Ref. [13], © Springer Nature 2016].

      It provides information about the crystalline and amorphous content in the polymer matrix. From XRD degree of crystallinity is evaluated from the area of crystalline (A_C) and amorphous peaks (A_A) using expression; . In DSC the melting enthalpy of crystalline host polymer (ΔH_m) and polymer matrix are used to evaluate the degree of crystallinity (Χ_C) via expression; .

       Ionic Conductivity

      The ion dynamics in the polymer matrix is examined by evaluating the ionic conductivity and is expressed by relation; (n_i is number of free charge carriers, z_i, is ion charge, and & μ_i is ion mobility). Ionic conductivity is linked with number of free charge carriers available in the polymer matrix and ion mobility. The ionic conductivity is examined via complex impedance spectroscopy (CIS) technique by applying ac signal (10-100 mV) across the cell assembly SS||PE||SS (SS refers to stainless steel electrode). From the obtained Nyquist plot (Z″ vs. Z′), bulk resistance (R_b) is extracted from the intercept on the real axis and ionic conductivity is obtained through this equation; ; where ‘t’ is the thickness of the polymer electrolyte (PE) film, A is the area of the SS electrodes and R_b is the bulk resistance.

The ionic conductivity also varies with temperature. The increase of temperature thermally activates the charge carriers and lowers the activation energy or potential barrier required for ion migration. The variation of ionic conductivity with temperature follows three behaviors depending upon temperature range: (i) Arrhenius behavior, (ii) Vogel-Tamman-Fulcher (VTF) behavior, and (iii) Williams-Landel-Ferry (WLF) behavior [17–22].

       Arrhenius behavior

      The increase of temperature in the polymer matrix thermally activates the charge carriers and increase in flexibility leads to fast ion migration via coordinating sites. This collectively favors the ion dynamics and Arrhenius’s behavior suggests the ion transport occurs via hopping mechanism. This behavior dominates when the temperature is lower than the glass transition temperature (T_g) [18]. To explore it further, activation energy is evaluated and the lower value of the activation energy is favorable for fast ion dynamics and hence promotes higher ionic conductivity. The activation energy (Ea) is slope of linear-least square fitting of the log σvs. 1/T plot by Arrhenius equation and is expressed as; [Here, σo is pre-exponential factor, k is Boltzmann constant].

       Vogel-Tamman-Fulcher (VTF) behavior

      The VTF σ vs. 1/T plot is the non-linear plot and ion transport occurs via the segmental motion of the polymer chain coupled with hopping. The ion diffusion within the polymer matrix occurs via the availability of free volume that is delivered by the polymer chains. The thermally activated charge carriers cross the potential barrier and contribute to conduction [19, 20]. The VTF equation is; . Here, σ is the ionic conductivity, A is the pre-exponential factor, B is a constant, and T_o is the temperature close to the T_g of material (where entropy is zero).

       Cation/Ion Transference Number

      As in polymer electrolytes, the main contribution is from the ion migration. So, the cation (t₊) and ion (t_ion) transference number is evaluated to check the exact contribution from ions and cation through cell configuration (SS|PE|SS), SS refers to stainless steel. The former is determined by a combination of ac impedance & d. c. polarization technique, while the latter is obtained from dc Wagner’s polarization technique [23, 24]. The cation transference number (t₊) is obtained via relation; ; [V is the applied voltage across cell configuration, I_i and I_s are the initial and steady-state currents, R_i, and R_s are the interfacial resistance before and after polarization]. The ion (t_ion) transference number is obtained via equation; ; [I_t and I_e are the total current and the residual current respectively and are related as i_t = i_ion + i_elec].

       Electrochemical Stability Window (ESW)

The voltage window of the electrolyte is a very significant parameter that needs to be examined before adopting polymer electrolytes for SC application. The energy density & capacity of the SC cell are directly linked with the voltage widow of electrolytes or devices. The voltage stability window is examined by the linear sweep voltammetry (LSV) technique which