Capacitance redox

The monotonic increase of immobilized material vith the number of deposition cycles in the LbL technique is vhat allo vs control over film thickness on the nanometric scale. Eilm growth in LbL has been very well characterized by several complementary experimental techniques such as UV-visible spectroscopy [66, 67], quartz crystal microbalance (QCM) [68-70], X-ray [63] and neutron reflectometry [3], Fourier transform infrared spectroscopy (ETIR) [71], ellipsometry [68-70], cyclic voltammetry (CV) [67, 72], electrochemical impedance spectroscopy (EIS) [73], -potential [74] and so on. The complement of these techniques can be appreciated, for example, in the integrated charge in cyclic voltammetry experiments or the redox capacitance in EIS for redox PEMs The charge or redox capacitance is not necessarily that expected for the complete oxidation/reduction of all the redox-active groups that can be estimated by other techniques because of the experimental timescale and charge-transport limitations. 

The terms itu CLF(/ E+i t) in the denominator of the potentiostatic EHD expression (6-33) explain why all the experimental diagrams are shifted towards lower p values with respect to the one obtained on a uniformly accessible Pt electrode, which matches the theoretical predictions and is independent of Q. The non-reducibility of the experimental diagrams is caused by the existence of a modulated current due to charging and discharging of the polymer redox capacitance. Since this... 

Recently, we have explored the use of anodic Ti02 nanotube layers as electrodes for electrochemical capacitors [94], Nickel oxide was incorporated into the Ti02 nanotubes, with nickel-to-titanium atom ratio being 9.6 at% and 36.4 at%, respectively, for redox capacitance application. The experimental results showed that the corresponding specific redox capacitance was 26 and 85 mF/cm2 with highly reversible charge-discharge stability,... 

Taniki, R., K. Matsumoto, T. Nohira, and R. Hagiwara. 2013. Evaluation of double-layer and redox capacitances of activated carbon electrodes in IV-ethyl-lV-methylpyrrolidinium fluorohydrogenate ionic Uqnid. Journal of the Electrochemical Society 160 A734-A738. 

Mousavi et al. have used PEDOT-CNT composite as ion-to-electron transducer in the fabrication of potassium ISEs [19]. In this work, PEDOT was electrochemically S5mthesized using negatively charged multi-walled CNTs (MWCNTs) as counterions. Results from cyclic voltammetric (CV) and electrochemical impedance spectroscopic (EIS) measurements shown in Fig. 11.1, reveal that the PEDOT-MWCNT film exhibits higher redox capacitance than a film based on PEDOT doped with chloride [Cr) ions, i.e., PEDOT-Cl. This sufficiently high redox capacitance is one of the conditions necessary for stable potential in all-solid-state ISEs having an ECP as the solid contact [23]. 

Figure 6.5 TVansmission line circuit where and R, are the electronic and ionic resistances, while Cp is the redox capacitance. Reproduced from Ref. 161 with permission of The Royal Society of Chemistiy.

Nyquist plots were also used to estimate the low-frequency redox capacitance (Q) of the composite fibers. It can be calculated from the equation... 

The theory of the impedance method for an electrode with diffusion restricted to a thin layer is well established [24,25,39,54,65,66,114-116,118,119,121,125,129, 130,133] however, the ideal response—separate Randles circuit behavior at high frequencies, a Warburg section at intermediate frequencies, and purely capacitive behavior due to the redox capacitance at low frequencies (see Fig. 3.7)—seldom appears in real systems. 

It is evident that the shape of the impedance spectra varies with the potential since the values of the charge transfer resistance (Ret), the low frequency (redox) capacitance (Cl) and the Warburg coefficient change with the potential more exactly, they depend on the redox state of the polymer. In many cases D is also potential-dependent. The double-layer capacitance (Cdi) usually shows only slight changes with potential. The ohmic resistance (Rq) is the sum of the solution resistance and the film resistance, and the latter may also be a function of potential due to the potential-dependent electron conductivity, the sorption of ions, and the swelling of the film. In Fig. 3.9 three spectra are displayed, which were constructed using the data obtained for a PTCNQ electrode at three different potentials near its equilibrium potential [23]. 

A characteristic of the redox capacitance response is the peak width at half-height S. This quantity is given by 8 = 26i, where — 1/2 when P - Pm - Under such conditions we note from Eqn. 225 that... 

This is a characteristic of the redox capacitance in the absence of interaction effects. We now examine the more realistic situation that takes interaction effects into account. 

We can show from detailed analysis of Eqn. 407 for Z that the low-frequency resistance and redox capacitance are given by... 

Cyclic voltammograms of reversible redox capacitance on RUO2 (top) and W oxide (bottom) in acidic electrolyte. Source Conway, B. E., V. Birss, and J. Wojtowicz. 1997. Journal of Power Sources, 66,1-14. With permission.)... 

---