Electron Transfer Resistance

FIGURE 1.1 Typical discharge curves of a Cd/Ni cell. (From Linden, D. and Reddy, T.B. in Handbook of Batteries, 3rd edn., Me Graw Hill, New York, 2002. With permission.) 

(For a 12 V nominal battery, discharge is not allowed to proceed under 9 V.) Conversely, if the current I is kept below 0.1 x U/p° during the application, it means that the generator is oversized for the application. 

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From an analysis of data for polypyrrole, Albery and Mount concluded that the high-frequency semicircle was indeed due to the electron-transfer resistance.203 We have confirmed this using a polystyrene sulfonate-doped polypyrrole with known ion and electron-transport resistances.145 The charge-transfer resistance was found to decrease exponentially with increasing potential, in parallel with the decreasing electronic resistance. The slope of 60 mV/decade indicates a Nemstian response at low doping levels. 

Very often, the electrode-solution interface can be represented by an equivalent circuit, as shown in Fig. 5.10, where Rs denotes the ohmic resistance of the electrolyte solution, Cdl, the double layer capacitance, Rct the charge (or electron) transfer resistance that exists if a redox probe is present in the electrolyte solution, and Zw the Warburg impedance arising from the diffusion of redox probe ions from the bulk electrolyte to the electrode interface. Note that both Rs and Zw represent bulk properties and are not expected to be affected by an immunocomplex structure on an electrode surface. On the other hand, Cdl and Rct depend on the dielectric and insulating properties of the electrode-electrolyte solution interface. For example, for an electrode surface immobilized with an immunocomplex, the double layer capacitance would consist of a constant capacitance of the bare electrode (Cbare) and a variable capacitance arising from the immunocomplex structure (Cimmun), expressed as in Eq. (4). 

As the immunocomplex structure is generally electroinactive, its coverage on the electrode surface will decrease the double layer capacitance and retard the interfacial electron transfer kinetics of a redox probe present in the electrolyte solution. In this case, Ra can be expressed as the sum of the electron transfer resistance of the bare electrode CRbare) and that of the electrode immobilized with an immunocomplex (R immun) ... 

Electrochemical transduction of the hybridisation event can be classified into two categories label-based and label-free approaches. The label-based approach can be further subdivided into intercalator/groove binder, non-intercalating marker, and NP. The label-free approach is based on the intrinsic electroactivity of the DNA purine bases or the change in interfacial properties (e.g., capacitance and electron transfer resistance) upon hybridisation [49],... 

Novel and simple methods for the immobilization of NPs need to be developed to further reduce the electron transfer resistance and improve the stability, sensitivity, selectivity, and life-span of the electrodes. 

The irreversibility can be described as the energy losses between charge and discharge (for a given active mass) that strongly depend on exchange current j°. This leads to the definition of electron transfer resistance Rt, usually calculated for (q,./) —> 0 ... 

At low currents, near equilibrium, the power losses are close to R, x. P. At high currents, the electron transfer resistance becomes R x T/(nFinversely proportional to the current density. It is generally negligible in comparison with the ohmic resistance. 

Zhang et al. reported a nanocomposite membrane of shuttle shaped ceria nanocrystals (Guo et al., 2008), SWNTs, and ILsl-butyl-3-methyli-midazolium hexafluorophosphate (BMIMPFg), which was incorporated on the glassy carbon electrode for electrochemical sensing of the immobilization and hybridization of DNA (Zhang et al., 2009). The electron transfer resistance (Pgt) of the electrode surface increased after the immobilization of probe ssDNA on the Ce02-SWNTs-BMIMPF6 membrane and rose further after the hybridization of the probe ssDNA with its complementary sequence. 

The photoisomerization of a command interface resulting in different electrochemical kinetics of a soluble redox-probe also can be probed by faradic impedance spectroscopy. A small electron transfer resistance (R ) is found for the system when there is an attractive interaction between the charged redox-probe and the command interface, and a much larger one upon photoisomerization to the state when the repulsive interactions exist. This paradigm was demonstrated with a negatively charged redox-probe,... 

A label-free electrochemical impedance immunosensor for the rapid detection of E. coli 0157 H7 consists of immobilized anti- . coli antibodies on an indium-tin oxide IDA microelectrode [123]. The binding of E. coli cells to the IDA microelectrode surface increases the electron-transfer resistance, which is directly measured with electrochemical impedance spectroscopy in the presence of [Fe(CN)(6)] as a redox probe. The electron-transfer resistance correlates with the concentration of E. coli cells in a range from 4.36 X 10 to 4.36 X 10 cfu ml with a detection Umit of 10 cfu ml . ... 

Figure 3-5. (A) Assembly of reconstituted glucose oxidase on a PQQ-FAD monolayer linked to an Au-electrode. (H i Faradaic impedance spectra of the modified electrode at time intervals of reconstitution, (a) 0.1 h, (b) 0.25 h. (c) 0.5 h. (d) 1 h. (e) 2 h, (f) 4 h. Inset Interfacial electron transfer resistance of the modified electrode at time-intervals of reconstitution. (C) Cyclic voltammograms corresponding to the bioelectrocatalyzed oxidation of glucose, 80 mM, by the enzyme-functionalized electrode at time-intervals of reconstitution (a) 0 h, (b) 0.1 h, (c) 0.25 h, (d) 0.5 h, (e) 1 h, (f) 2 h, (g) 4 h. Inset Electrocatalytic currents transduced by the enzyme-modified electrode at time-intervals of reconstitution. Reproduced with permission from ref. 32. Copyright 2002 American Chemical Society.

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