Charge-transfer resistance coatings

AC Impedance measurements enable the determination of charge transfer resistance and double layer capacitance and other parameters related to coated systems. 

Decrease In charge transfer resistance and Increase In double layer capacitance Is observed with Increasing time of Immersion or with Increasing test temperature and gives Information on the degree of protection efficiency of a coating. 

In another study also, electrochemical impedance technique has been shown to be a useful method for a DNA biosensor using a multinuclear nickel(II) salicylaldimine metallodendrimer platform [164], Both the preparation of the dendrimer-modified GCE surface and the immobilization of DNA have been effectively done by simple drop-coating procedures. The metallodendrimer is electroactive exhibiting two redox couples in phosphate buffer solution. The impedance study demonstrated that the DNA biosensor responded well to 5 nM of target DNA by displaying a decrease in charge transfer resistance in phosphate buffer solution and increase in charge transfer resistance in the presence of the [Fe(CN)6]3/4" redox probe. 

The effective area of the OTS-coated PtO electrode can be derived if the charge transfer resistance (K ) is known. Rct can be obtained from impedance data measured at a potential near the reversal potential (37, 33) Rct = RT/(nFAI0), where R is the universal gas constant, T is absolute temperature, n is the number of electrons transferred per molecule of TONE, F is Faraday s constant, I0 is the exchange current density, and A is the effective surface area. Because the impedance spectra of the PtO and PtO-OTS electrodes were measured under the same conditions, the value of Rct may be assumed to be affected only by the effective surface area. In Figure 3, the impedance data are replotted as 2 versus 1 /a)1 2, where a) is the angular frequency (2 tt/). Rct is estimated from the intercept on the Z axis by extrapolation. The Rct values are 95 and 980 fl for PtO and PtO-OTS, respectively. An OTS coverage factor, 0, can then be estimated from (1 — 0) = ct(Pto)/ ct(Pto-OTS> In is case 0 = 0.9. 

Film application EiS in 3.5% NaCl EB-PANi is added to the hardener. The coating resistance and the charge transfer resistance are the highest for 2.5 wt.% PANi-EB, and the lowest for epoxy coating without EB-PANi. [84]... 

Impedance spectra of partially degraded coatings cannot only be used to estimate the porosity of the film but also to estimate the area of the metal electrode wetted by electrolyte. Since the interface metal-electrolyte is represented by the parallel combination of the charge-transfer resistance and the double-layer capacitance both of these can be used to estimate the fraction of the electrode surface W wetted by electrolyte... 

Electrochemical Impedance Spectroscopy. Electrochemical Impedance Spectroscopy (EIS), a non-destructive investigative technique enables an insight into the corrosion process not obtained by DC techniques. EIS provides information on reaction parameters, corrosion rates, oxide characteristics and coating integrity, data on electrode interfacial capacitance and charge transfer resistance. It provides... 

FIG. 12—Electrical equivalent circuit to simulate a coated steel panel with a defect [146,147,149. Rg is the coating defect resistance, R is a charge transfer resistance (similar to R) at the metal Interface where water has penetrated. Is the coating capacitance. 

AC impedance relies on the ability to monitor the behavior of a coating that absorbs water by modeling it as a capacitor. By measuring capacitance increase as a function of immersion time, the diffusion coefficient of water and the amount of saturated water within the coating film can be calculated. In addition, any severe under-film corrosion can be monitored by a drastic decrease in the coating resistance and charge transfer resistance. 

An ES electrode is traditionally composed of a conductive metallic (generally aluminum) current collector coated with an active electric double-layer carbon or psuedocapacitive component. The primary technical challenge in using a metallic foil current collector lies at the interface of the collector and the active material. This interface induces a charge transfer resistance that will inevitably increase the internal resistance of the entire system and reduce overall ES performance. 

In addition to the equivalent circuit method, the impedance results can also be analyzed using mathematical models based on physicochemical theories. Guo and White developed a steady-state impedance model for the ORR at the PEM fuel cell cathode [15]. They assumed that the electrode consists of flooded ionomer-coated spherical agglomerates surrounded by gas pores. Stefan-Maxwell equations were used to describe the multiphase transport occurring in both the GDL and the catalyst layer. The model predicted a high-frequency loop due to the charge transfer process and a low-frequency loop due to the combined effect of the gas-phase transport resistance and the charge transfer resistance when the cathode is at high current densities. 

The coated electrodes were also analysed in a system without the permeability marker. In this case the charge transfer resistance was infinity, and the equivalent eireuit in Fig. 6b was used for fitting the experimental data. Fig. 5 shows the frequency response for two electrodes and the theoretical response from the derived equivalent circuit. 

---