*2.4. Electrical Characterization*

To describe the properties of the capacitors, the electrical behavior of these devices has been analyzed by integrating them on a real electronic circuit. In order to do this, two stages have been differentiated according to the nature of the current supplied to the device. First, the properties have been studied in a dynamic regime, that is, by supplying an alternating current, from 40 Hz to 100 MHz on frequency range. In this work, a precision impedance analyzer model Agilent 4294A (Keysight Technologies, Santa Rosa, CA, USA) has been used. This study allows knowing the characteristic magnitudes of the capacitor, such as the impedance module and phase as a function of the input AC frequency. Secondly, the capacitors were tested in static regime, that is, with continuous input current. This section is essential when checking the energy storage properties of the device because the procedure consists of charging and discharging the device, analyzing the intensity of the current flowing through the capacitor at all times. To allow this, the assembly of a charge–discharge RC circuit is required, as shown in Figure 4.

*V* represents the DC voltage supplied by the power supply, *R*<sup>L</sup> is the load resistance, *R*<sup>D</sup> is the discharge resistance, and the values are set as *V* = 2 V and *R*<sup>L</sup> = *R*<sup>D</sup> =2MΩ. In addition, the circuit is fitted with an ammeter (*A*) to measure the current intensity (*I*) flowing through the capacitor branch as well as a switch (*S*) to select the working mode of the capacitor (*C*), either charging or discharging. A Keithley 2410 1100 V Source-Meter (Tektronix, Inc., Beaverton, OR, USA) has been used as the power source and a Keithley 2700 1100 V Multimeter/Data Acquisition System (Tektronix, Inc., Beaverton, OR, USA) as the ammeter. Through the data of current intensity transient collected by this device, it is possible to know the energy storage capacity of the capacitor.

**Figure 4.** Schematic view of the RC charge–discharge circuit. In the charge configuration, (**a**) the capacitor (*C*) is powered by the power supply (*V*) through the load resistance (*R*<sup>L</sup> =2MΩ). In the discharge configuration, (**b**) the discharge resistance (*R*D) is powered by the capacitor. Note that the current (*I*) flows in different directions through the ammeter (*A*) depending on whether the configuration is charge or discharge.

By modifying the circuit (see Figure 5), it is possible to find the maximum operating voltage of the device or breakdown voltage, in which the dielectric medium loses its insulating properties and the current circulates through it as if it were a conductive medium, thus losing the properties for energy storage.

**Figure 5.** Circuit diagram for breakdown voltage measurements. The power supply (*V*) is directly connected to the capacitor (*C*), so the current flowing through the device can be simply monitored by the ammeter (*A*). A linear increase in the current read by the ammeter indicates the loss of the insulating properties of the capacitor.
