Chemical diffusion coefficient electrodes

The primary question is the rate at which the mobile guest species can be added to, or deleted from, the host microstructure. In many situations the critical problem is the transport within a particular phase under the influence of gradients in chemical composition, rather than kinetic phenomena at the electrolyte/electrode interface. In this case, the governing parameter is the chemical diffusion coefficient of the mobile species, which relates to transport in a chemical concentration gradient. 

More importantly, the chemical diffusion coefficient >chcm, instead of Dself, is the parameter that is relevant to the behavior of electrode materials. They are related by... 

The experimental value for Agl is 1.97 FT cirT1 [16, 3], which indicates that the silver ions in Agl are mobile with nearly a thermal velocity. Considerably higher ionic transport rates are even possible in electrodes, by chemical diffusion under the influence of internal electric fields. For Ag2S at 200 °C, a chemical diffusion coefficient of 0.4cm2s, which is as high as in gases, has been measured... 

Kleinfeld, M. Wiemhofer, H.-D. 1988. Chemical diffusion coefficients and stabihty of CuInS2 and CuInSe2 from polarization measurements with point electrodes. Solid State Ionics. 28-30 1111-1115. 

In general, only the two fastest species have to be taken into consideration and all other ones may be neglected. The second fastest species are rate controlling since these limit the motion of the fastest ones. In general, electrons (or holes) and the fastest moving ionic species have to be considered in electrodes. For the combined motion, the proportionality constant between the flux and the concentration gradient is called the chemical diffusion coefficient D ... 

As discussed in Section 8.2 the relation between the chemical diffusion coefficient and diffusivity (sometimes also called the component diffusion coefficient) is given by the Wagner factor (which is also known in metallurgy in the special case of predominant electronic conductivity as the thermodynamic factor) W = d n ajd In where A represents the electroactive component. W may be readily derived from the slope of the coulometric titration curve since the activity of A is related to the cell voltage E (Nernst s law) and the concentration is proportional to the stoichiometry of the electrode material ... 

We will see that in the steady state of the blocking cells, we can extract partial conductivities, and from the transients chemical diffusion coefficients (and/or interfacial rate constants). Cell 7 combines electronic with ionic electrodes here a steady state does not occur but the cell can be used to titrate the sample, i.e., to precisely tune stoichiometry. Cell 1 is an equilibrium cell which allows the determination of total conductivity, dielectric constant or boundary parameters as a function of state parameters. In contrast to cell 1, cell 2 exhibits a chemical gradient, and can be used to e.g., derive partial conductivities. If these oxygen potentials are made of phase mixtures212 (e.g., AO, A or AB03, B203, A) and if MO is a solid electrolyte, thermodynamic formation data can be extracted for the electrode phases. 

A disc of reduced semiconducting rutile crystal 2 cm in diameter and 2 mm thick is heated in air for 10 s at 300 °C. After cooling, circular electrodes, lcm in diameter, are applied symmetrically to the two major surfaces. The chemical diffusion coefficient D for the oxidation reaction in reduced single-crystal TiC is given by... 

Figure 26 A comparison of the chemical diffusion coefficient of Li into graphite (calculated from PITT), the intensity of the major XRD peaks (e.g., 002, 004) during intercalation (vs. E), and a completed slow scan rate cycling voltammogram of a thin (10 pm) composite graphite electrode (KS-6 from Lonza) in EC-DMC/LiAsF6 solution. Note that as the electrode is thinner and the particles are more oriented (with their basal planes parallel to the current collector), the scan rate is slower and the CV peaks are sharper and better resolved. The various phases (intercalation stages) are indicated [87]. (With copyrights from Elsevier Science Ltd., 1998.)...

Technique applied to measure the chemical diffusion coefficient of the intercalating species within insertion-host electrode materials with the help of an electrochemical cell, followed by the current response on the staircase potential signal that is recorded as currenttime curve [i]. The theory of this technique is based on... 

As intensive studies on the ECPs have been carried out for almost 30 years, a vast knowledge of the methods of preparation and the physico-chemical properties of these materials has accumulated [5-17]. The electrochemistry ofthe ECPs has been systematically and repeatedly reviewed, covering many different and important topics such as electrosynthesis, the elucidation of mechanisms and kinetics of the doping processes in ECPs, the establishment and utilization of structure-property relationships, as well as a great variety of their applications as novel electrochemical systems, and so forth [18-23]. In this chapter, a classification is proposed for electroactive polymers and ion-insertion inorganic hosts, emphasizing the unique feature of ECPs as mixed electronic-ionic conductors. The analysis of thermodynamic and kinetic properties of ECP electrodes presented here is based on a combined consideration of the potential-dependent differential capacitance of the electrode, chemical diffusion coefficients, and the partial conductivities of related electronic and ionic charge carriers. 

FIGURE 5 Experimental setup for the measurement of the chemical-diffusion coefficient of wustite W, working electrode R, reference electrode C, counter electrode. 

In the second subsection we will show how data obtained from electrochemical three-electrode impedance spectroscopy can be used to obtain information about the chemical diffusion coefficient, D, as well as about energy barriers at the... 

Bisquert J (2004) Chemical diffusion coefficient of electrons in nanostructured semiconductra-electrodes and dye-sensitized solar cells. J Phys Chem B 108 2323-2332... 

The chemical diffusion coefficient describes the relaxation of compositional gradients to achieve a homogeneous composition. This quantity is important in the case of matty phenomena of practical interest, e.g., for corrosion processes or the performance of electrodes in batteries. The chemical diffusion is commonly determined by the simultaneous diffusion of (at least) two different charge carriers in order to maintain local electroneutrahty. In most cases it is the diffusion of one type of ions and electrons or holes. 

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