# Portable and Highly Versatile Impedance Meter for Very Low Frequency Measurements
## 3 Results and Discussion
### System Testing: An Example of Application  
Following the preliminary tests, performed on well-known DUTs and intended to verify the soundness of the approach we propose, we performed the characterization of a prototype supercapacitor using symmetric electrodes based on polypyrrole (PPY). In common with other conjugated polymers, PPY has alternating single and double carbon-carbon bonds and the ability to undergo electrically driven reversible transition between its oxidized and reduced form, which has been exploited in actuators emulating artificial muscles [43], in electrochromic windows [44], and in a variety of applications going from electrochemical sensing [45; 46] to energy storage and conversion [47; 48; 49; 50; 51; 52]. Conducting electrodes to be used in electrochemical energy storage devices can be developed starting from commercially available insoluble PPY powder, as we attempt to do here, or by using PPY polymerized from its monomer and doped, or by combining PPY with a variety of other materials including nanostructured carbon and metal oxides, to yield nanocomposites with tailored electrical, and electrochemical properties [53; 54; 55]. Because of their electrochemical activity, electrodes based on PPY and its nanocomposites, when applied in a planar supercapacitor configuration, namely when impregnated with an electrolyte and separated from another electrode through an ion-permeable medium, confer to the device a faradaic character. Meaning that the energy storage mechanism in supercapacitors having PPY based electrodes is not only the electrically driven absorption-desorption of anions and cations at the electrodes, as is the case in electric double layer capacitors but is also the occurrence of fast redox transition at the surface of the PPY electrode, as it happens in the so-called pseudo-capacitor. The performance of each supercapacitor, which strictly depends on the composition, and on the structural, electrical, and electrochemical properties of its electrodes, can be evaluated using impedance spectroscopy, and low frequency measurements provide important information on the device characteristics. The DUT examined here is a flexible symmetric pseudo-capacitor developed using a substrate transparency sheet having a gold film evaporated in a vacuum on the top. The electrode material is obtained as a clear aqueous suspension by diluting in deionized water the doped polypyrrole (PPY) powder (purchased from Aldrich, Merck KGaA, Darmstadt, Germany) dispersed in Dodecylbenzenesulfonic acid (DBSA, 70% in isopropanol, Aldrich), at a ratio of 1:4 by weight. The PPy-DBSA dispersion is used as ink to fill rechargeable pens and is applied on the top of the gold-coated transparency. After few minutes of storage at about 60 \({}^{\circ}\)C to remove the residual solvent, a layer deposited from a mixture of 300 mg of lithium hexafluorophosphate (LiPF6) and 3 mL of Nafion (D2029, purchased from Ion Power GmbH, Terminalstrasse Mitte 18, Munchen, Germany) in 10 mL of ethanol, is applied on the top of the PYR-DBSA electrode. After this latter layer is left to dry slowly, an ion-permeable Nafion membrane impregnated with the lithium salt, forms on the PYR-DBSA electrode. At this stage, the transparency sheet is cut into a couple of pieces that are faced to each other and stored at about 60 \({}^{\circ}\)C for a few tens of seconds, in order to achieve adherence of the two pieces. The developed device is a symmetric system the schematic view of which is shown in Figure 11.  
Figure 11: Schematic view of the supercapacitor structure subjected to low frequency impedance measurements. The substrate (SUB) is cut out from a copier grade transparency sheet on top of which a gold layer (GOLD) was thermally evaporated. Electrodes are obtained from polypyrrole powder dispersed in Dodecylbenzenesulfonic acid (PPY-DBSA) and are separated by a mixture of Nafion and lithium hexafluorophosphate (Nafion-LiPF6). Full characterization of the device in Figure 11 is under development.