## 2 Impedance Spectroscopy
### Scopes of Application
#### 2.1.2 Is Applied in Forest and Agriculture Management
Flexible electrode devices, sensors, biosensors, photoelectrochemical cells, supercapacitors, fuel cells, and protective coatings are emerging areas where Impedance Spectroscopy is applied in developing new functional materials [71; 72; 73; 74]. Metal oxides, metal nanoparticles, and carbon-based materials have been widely used to produce functional devices that meet emerging demands in these areas due to their viable electronic properties, sensitivity to variations in environmental parameters, high chemical and structural stabilities over a wide range of temperatures, nanoscale size particle distribution, high surface area/volume ratio, and interaction with other functional materials. IS in Flexible Structures and Sensing Applications Assessment  
IS has been used to study flexible materials applications, such as electrodes for miniaturized devices with reliable and reproducible responses (for usage in systems with high potential, on-demand solutions for emerging micro and nanoelectronics technologies). Conductive disks based on silver nanoparticles, with different formulations, were produced to develop inkjet-printed flexible electrodes [75]. The authors assessed by EleIS the electrical behavior of the electrodes, and demonstrated that it was directly influenced by the ink curing temperature (150-300 \({}^{\circ}\)C) and by the electrode design (measurements were performed using a four-points setup using an input AC voltage possessing an amplitude of 10 mV, in a frequency range from 0.1 Hz to 100 kHz). The resistivity of the inks was recorded in the range of 3 to 5 u\(\Omega\).cm, and the fabricated electrodes had an electrical impedance modulus of less than 200 \(\Omega\), confirming their viability for sensing applications employment. [PERSON] et al. [76] recently developed a new electrically conductive paint based on copper oxide (CuO) nanospheres and reduced graphene oxide (rGO) nanoplates for producing flexible electrodes. EleIS was used to model the electrical response of the paint as a function of the variation in the CuO/rGO ratio (impedance spectra were acquired while applying to the electrode specimens a constant current density of 50 mA-cm\({}^{-2}\), in a frequency range between 0.1 Hz and 10 kHz, using a four-point measurement arrangement). The results revealed that it was possible to develop electrodes that displayed energy-storing properties and, consequently, that could be used in batteries. [PERSON] and colleagues [77] used rGO nanoplates to develop three flexible electrode types for humidity sensors. Assuming that carbon-based nanomaterials actively respond to variations in moisture concentration, the authors produced sensors with reversible responses in short time intervals; using EleIS data analysis and interpretation tools, they related the results gathered for varying humidity levels directly with the alterations in the contact resistance of the samples, with the capacitive reactance at the interface between the rGO nanoplates, and with the observed electrical resistance of the nanoplates (EleIS data was obtained using a four-points configuration approach, with frequencies ranging from 20 Hz to 2 MHz). The electrical signature of humidity variations over a wide range of relative humidity (20-95%) was validated by repeated experiments, demonstrating the predominance of a proton-hopping conduction mechanism. [PERSON]'s group [78] studied a flexible electrode based on multi-layer carbon nanotubes (MWCNTs), which are eco-friendly and prepared without the presence of binding additives. Using EleIS, the Nyquist plots of the evaluated electrodes showed a straight line inclined at 45\({}^{\circ}\) in the low-frequency region, and a small semicircle at high frequencies (a four-points measuring setup was used while stimulating the electrodes in the range of 10 mHz to 100 kHz); accordingly, the authors concluded that the electrode possessed an electrical behavior with high similarity to that presented by a capacitor, displaying fast energy charge and discharge cycles. [PERSON] et al.