*2.6. Supercapacitor Characterization*

The electrochemical tests consisted in EIS, cyclic voltammetry (CV) and galvanostatic (GCPL) tests and were performed in a thermostatic oven at 30 ◦C using a BioLogic VSP multichannel potentiostat/galvanostat/FRA. EIS was performed with a 100 kHz–100 mHz frequency range and 5 mV AC perturbation, acquiring 10 points per decade. To evaluate the impedance of each of the EDLC electrodes, three electrode measurements, have been done. A silver disk has been used as pseudo reference. Here, the working electrode was the tested one and counter the other. To evaluate the

complete cell impedance, two electrode measurement have been done. For two electrode measurements, the silver disk was disconnected, the EDLC positive electrode was the working, and the EDLC negative electrode was connected to the counter and reference instrument plugs.

CV discharge curves were analyzed to get a first evaluation of the EDLC specific capacitance (CEDLC). Specifically, CEDLC was calculated from the slope of the voltammetric plots of the discharge capacity vs. cell voltage. The capacity was calculated by the integral of the CV current over time. The slope values were divided by the total composite mass of the two electrodes (mtot).

The GCPL curves were analyzed to quantify the equivalent series resistance (ESR) and the CEDLC, the specific energy and power of the devices at different discharge currents. ESR was calculated according to Equation (1), where ΔVohmic is the ohmic voltage drop at the beginning of discharge, and i is the current density (A cm<sup>−</sup>2):

$$\text{ESR} = \Delta \mathbf{V}\_{\text{ohmic}} / (2 \times \text{i}) \tag{1}$$

CEDLC was calculated from the reciprocal of the slope of the GCPL voltage profile during the discharge (dt/dV) by Equation (2):

$$\mathbf{C}\_{\rm EDLC} = \mathbf{i} \times \mathbf{d}t / \mathbf{d}V / \mathbf{m}\_{\rm tot} \tag{2}$$

The single electrode specific capacitance (Celectrode) was therefore calculated from the EDLC's one by Equation (3)

$$\mathbf{C}\_{\text{electrode}} = \mathbf{4} \times \mathbf{C}\_{\text{EDTA}} \tag{3}$$

The EDLCs specific energy (E) and power (P) were calculated from the GCPL discharge curves through Equations (4) and (5):

$$\mathbf{E} = \mathbf{i} \int \mathbf{V} \times \mathbf{d}t \langle \mathbf{3}600 \times \mathbf{m}\_{\text{tot}} \rangle \tag{4}$$

$$\mathbf{P} = \mathfrak{Z}\mathfrak{G}00 \times \mathbf{E} / \Delta \mathfrak{t} \tag{5}$$

where Δt is the discharge time in seconds.

## **3. Results**
