3.2.1. High Binder Low Mass Loading Electrodes (HBLME)

In this section, the results of the electrochemical characterization of the HBLME-EDLCs are reported. Figure 4a shows the Nyquist plots of the HBLME-EDLC single electrodes and the full cell. The three Nyquist plots share all the same shape. They can be divided into three components: (i) a high frequencies semicircle, (ii) a middle frequencies line with a slope of ca. 45◦, and (iii) a low frequency line that approaches a slope of 90◦. The intercepts at the highest frequencies of the semicircles represents the ohmic resistances (electronic and ionic) of the electrodes and electrolyte-separator system. Values of 1.4, 1.5 and 3.3 Ohm cm2 have been measured, respectively for the negative, the positive electrode and the full cell. The small semicircle has been attributed to (i) the ion transport at the electrolyte-carbon interface and (ii) the contact between the electrode and the current collector [34]. For the full cell the semicircle diameter is 0.3 Ohm cm2. The middle frequency line with 45◦ slope is representative of diffusion limited phenomenon. Specifically, it refers to diffusion of ions required to charge inner pores of the carbon electrodes. The low frequency line represents the capacitive behavior of the electrodes and the EDLC. For an ideal EDLC, a vertical line is expected. In Figure 4a the lines deviate from this ideal behavior because of the presence of different class of pores [35]. The real axis intercept of the linear fit of the cell low frequency line gives the ESR that was quantified in 6.4 ohm cm2.

Figure 4b reports the CVs of the full HBLME-EDLC cell at different scan rate, between 0 and 3.2 V. The voltammogram are symmetric and box shaped, which indicates the absence of faradic secondary process and an electrical double layer driven process. The maximum current of 3 A g−<sup>1</sup> (25 mA cm<sup>−</sup>2) is reached with a scan rate of 200 mV s<sup>−</sup>1, this value is comparable with the ILs based EDLC already reported in literature [8]. Figure 4c reports the trend of CEDLC versus the scan rate. The highest specific

capacitance of HBLME-EDLC is 18 F g−<sup>1</sup> at 5 mV s−<sup>1</sup> and decreases to 14 F g−<sup>1</sup> at 200 mV s−1. This trend has been widely discussed in literature and is attributed to the ionic diffusion limitation upon the double layer formation in the smallest pores at fast scan rates [36]. Indeed, micropores with an internal area less exposed to the electrolytes need more time for the creation of the electrical double layer than bigger pores. At low scan rate, the polarization is slow and ions have enough time to access the internal area of micro-pores. Increasing the scan rate, only the external surface of the pores becomes easily accessible. This process also explains the 45◦ Warburg line of the Nyquist plot of Figure 4a.

**Figure 4.** Electrochemical characterization of HBLME-EDLC (**a**) Nyquist plots of the (black) full cell, (red) positive and (blue) negative electrodes (500 kHz and 100 mHz), (**b**) 2-electrode CVs at different scan rate from 5 mV s–1 to 200 mV s–1, between 0 V and 3.2 V, (**c**) Capacitance of the EDLC evaluated by CV reported as function of the scan rate; and (**d**) selected galvanostatic charge/discharge cycles between 0 V and 3.2 V at different current densities from 0.5 A g–1 to 4 A g–1.

Figure 4d reports selected voltage profiles of the HBLME-EDLC under galvanostatic charge/discharge cycles at different current density, between 0 and 3.2 V. The voltage profile of the cell has a symmetric, triangular shape which is characteristic of electrical double layer driven process. Increasing the current from 0.5 to4Ag−<sup>1</sup> leads, as expected, to the decrease of the charge/discharge time. Coulombic efficiency (ηc), i.e., the ratio between the charge released during discharge and the charge stored during charge, is reported as inset in Figure 3d. This quantity is always greater than 98% and reaches the highest value of 100% at 4 A g<sup>−</sup>1. The GCPL ohmic drops were analyzed to quantify ESR of the device and resulted in 5.9 Ohm cm2, that well compares with the value obtained by EIS. EDLC. Specific capacitance CEDLC has been calculated from the slope of the GCPL discharge profile and for HBLM-EDLC resulted in 15.9, 15.4, 14.6 and 13.7 F g−<sup>1</sup> at 0.5, 1, 2 and4Ag<sup>−</sup>1, respectively. The corresponding single electrode specific capacitances (Celectrode) are 63, 61, 58 and 54.8 F g−1. These values well compare with those of electrodes featuring the same electrolyte and carbon but employing

a fluorinated binder [8]. Table 3 reports the ESR and CEDLC at 0.5 A g−<sup>1</sup> of the HBLME-EDLC along with the EDLC areal capacitance.


**Table 3.** Gravimetric and surface quantities of HBLME-EDLC and LBHME-EDLC.

\* Capacitance has been calculated from the CV at 50 mV s−1, \*\* Specific energy has been calculated from GCPL at minimum current (0.5 A g<sup>−</sup>1), \*\*\* Specific power has been calculated at maximum current (4 and5Ag−1).
