3.2.2. Lower Binder High Mass Loading Electrodes (LBHME)

In this section, the results of the electrochemical characterization of the LBHME-EDLCs are reported. Figure 5a shows the Nyquist plots of the LBHME-EDLC single electrodes and full cell.

**Figure 5.** Electrochemical characterization of LBHME-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 5 A g–1.

Like for HBLME, the three Nyquist plots share all the same shape. For the physical interpretation of the Nyquist plots, the considerations that have been drawn in the previous section are still valid. The high frequency intercepts with the real axis of the semicircles are 0.8, 0.9 and 1.7 Ohm cm<sup>2</sup> for the positive and negative electrodes and full cell, respectively. Noticeably these values are halved with respect to those of the HBLME electrodes and EDLC (cf. Figure 4a). In LBHME formulation, the quantity of binder and conductive carbon are halved compared to the HBLME one. Therefore, the decrease of the high frequency impedance achieved by LBHME can be explained with the decrease of the insulating component of the electrode, i.e., the binder. Comparing the high frequency semicircles in Figures 4a and 5a, it is possible to notice that the LBHME's is wider than HBLME's. Indeed, the LBHME-EDLC semicircle diameter is 1.3 Ohm cm<sup>2</sup> while the HBLME's is 0.3 Ohm cm2. This difference is due to the high mass loading of LBHME with respect to HBLME (1.5-fold), that brings about a worse ionic and electronic connection between the carbon particles [10]. The LBHME-EDLC middle frequency line (45◦ slope) span across the same range of resistance with respect to the HBLME-EDLC. The ESR of the LBHME-EDLC was evaluated from the real axis intercept of the low frequency line and resulted in 7.6 Ohm cm2.

Figure 5b reports the CVs of the full LBHME-EDLC cell at different scan rate, between 0 and 3.2 V. From these measurements, voltammetric specific capacitance values have been calculated and are reported as function of the scan rate in Figure 5c. The highest specific capacitance is 14 F g <sup>−</sup><sup>1</sup> at 5 mV s−<sup>1</sup> and decreases to 7 F g−<sup>1</sup> at 200 mV s<sup>−</sup>1. Therefore, from 5 mV s−<sup>1</sup> to 200 mV s−<sup>1</sup> there is a 50% specific capacitance reduction, that is higher than what observed for HBLME-EDLC (22%). This can be related to a not optimized electronic and ionic connection of the electrodes carbon particles that has been highlighted by the Nyquist plot analysis reported above (Figure 5a).

The LBHME-EDLC galvanostatic charge/discharge profiles at different current are reported in Figure 5d. The coulombic efficiency was 98.3%, 99.6%, 100%, 100% at 0.5, 1, 2, 5 A g−1, respectively. These values are slightly higher than those that have been observed for the HBLME. The ESR was 7.9 Ohm cm2 in agreement with the EIS value. The CEDCL was 6.2, 5.8, 5.3 and 4.2 F g−<sup>1</sup> at 0.5, 1, 2 and 5 A g<sup>−</sup>1. These values are lower than those featured by HBLME-EDLC and this can be explained with the not optimized ionic and electronic connection highlighted by Table 3 that reports the ESR and CEDLC at 0.5 A g−<sup>1</sup> of LBHME-EDLC. The EDLC areal capacitance is also reported in the Table 3.
