Electrodes porosity

The consequences of shape change are densification and loss of electrode porosity, increased current density caused by loss of zinc surface area, and finally earlier passivation. Two different forms of pasi-vation can stop the discharge of a zinc electrode before the active material is exhausted. "Spontaneous" passivation occurs... 

Ohm.cm2 (of visible surface area) or even less. Some visible ways to lower the electrode resistivity are (a) fabricating thin electrodes, (b) increasing the conductivity of electrolyte and electrode material, (c) matching the electrode porosity with the size of species in the electrolyte in order to facilitate the ion transport along the pores, and (d) assembling bipolar devices. 

The reactant gas must diffuse through the electrode structure which contains air (02, N2) and any products of reaction (CO2, N02, NO, H2O vapor, etc.). Response characteristics are dependent on electrode material, Teflon content, electrode porosity, thickness and diffusion/reaction kinetics of the reactant gas on the catalytic surface. By optimizing catalytic activity for a given reaction and controlling the potentiostatic voltage on the sensing electrode, the concentration of reactant gas can be maintained at essentially zero at the electrode/electrolyte interface. All reactant species arriving at the electrode/electrolyte interface will be readily reacted. Under these conditions, the rate of diffusion is proportional to C, where... 

The permeability of an electrode can be estimated from the electrode porosity (e) and pore diameter (d) by the Kozeny-Carmen relation... 

The immittance analysis can be performed using different kinds of plots, including complex plane plots of X vs. R for impedance and B vs. G for admittance. These plots can also be denoted as Z" vs. Z and Y" vs. Y, or Im(Z) vs. Rc(Z), and Im( Y) vs. Re( Y). Another type of general analysis of immittance is based on network analysis utilizing logarithmic Bode plots of impedance or admittance modulus vs. frequency (e.g., log Y vs. logo)) and phase shift vs. frequency ( vs. log co). Other dependencies taking into account specific equivalent circuit behavior, for instance, due to diffusion of reactants in solution, film formation, or electrode porosity are considered in - electrochemical impedance spectroscopy. Refs. [i] Macdonald JR (1987) Impedance spectroscopy. Wiley, New York [ii] Jurczakowski R, Hitz C, Lasia A (2004) J Electroanal Chem 572 355... 

It must be emphasized that the mathematical simplicity of equations (13.1) and (13.2) is the consequence of a specific time-constant distribution. As shown in this chapter, time-constant distributions can result from nonuniform mass transfer, geometry-induced nonuniform current and potential distributions, electrode porosity, and distributed properties of oxides. At first glance, the associated impedance responses may appear to have a CPE behavior, but the frequency dependence of the phase angle shows that the time-constant distribution differs from that presented in equation (13.7). 

Fig, 5.22 Capacity retention of half-cells containing a surface-treated graphite-negative electrode top) and a positive LiCoO, respectively, with different fractions of TIMREX KS graphite and Super P Li as conductive additive (bottom) (electrode porosity ca. 35%, electrolyte 1-M LiPF in ethylene carbonate/ethyl methyl carbonate 1 3 (v v)) ... 

The influence of the electrode porosity is displayed in Fig. 9.35. At very low porosities, i.e., for very shallow pores (I = 0.005 cm), the electrode behaves practically as flat, and one semicircle is observed on the complex plane plots. With an increase in the pore length, two semicircles are observed (I = 0.05 cm), and with further increases, one semicircle is observed identical to the faradaic impedance and the influence of the double-layer capacitance disappears. 

Fig. 9.35 Influence of electrode porosity parameter on impedance plots, characterized by different pore lengths, indicated in figures in centimeters, r = 10 cm other parameters as in Fig. 9.34, at a constant potential — = 0 V continuous line, total impedance, dashed lines, faradaic...

Springer et al. [31] developed a semi-empirical model for use in a fuel cell with a partially hydrated membrane (as opposed to a fully hydrated membrane). Empirically determined relationships were developed correlating membrane conductivity and electrode porosity with water content in the Nafion membrane. Most of the models subsequently developed used these correlations to determine the conductivity of the Nafion membrane. 

If the surface path dominates, a large TPB length is clearly advantageous and can be achieved if porous cathodes are used. Composite cathodes consisting of electrolyte particles, electrode material and pores even lead to a three-dimensional network of TPBs and hence can further enhance the reaction rate. Simulations on the performance of composite electrodes and suggestions with respect to optimal parameters such as electrode porosity and thickness were given by Sunde [89, 90], Costamagna et al. [91] and Tanner et al. [92]. 

Internal resistance R, is defined as the opposition or resistance to the flow of an electric current within a cell or battery, i.e. the sum of the ionic and electronic resistances of the cell components. Electronic resistance includes the resistance of the materials of construction metal covers, carbon rods, conductive cathode components, and so on. Ionic resistance encompasses factors resulting from the movement of ions within the cell. These include electrolyte conductivity, ionic mobility, electrode porosity, electrode surface area, secondary reactions, etc. These fall into the category of factors that affect the ionic resistance. These factors are encompassed by the term polarization. Other considerations include battery size and construction as well as temperature, age and depth of discharge. 

Tortuosity is one of the most important parameters to characterize a porous medium, and reflects the reduction in transport within the electrode due to the complex porous structure comprised of active particles, binder, and conductive carbon [59-61]. Complex, tortuous nanostructures can lead to decreased effective electrolyte conductivity and diffusivity for porous electrodes by limiting transport in the electrolyte phase. The concept of electrode tortuosity (t) is used along with electrode porosity (e) as a measure for the decrease in effective electrolyte conductivity and diffusivity due to the structure of the electrode within the confines of the porous electrode description the tortuosity of a material should decrease as the porosity increases, and Bruggeman suggested a quantitative relationship where tortuosity is inversely proportional to the square root of porosity [62, 63] note that... 

Pan et al. [73] investigated the effect of electrode porosity on the performance of PA-doped PBI-membrane-based HT-PEM fuel cells. They optimized the porosity of the gas diffusion layers by applying different amounts of PTFE, and the porosity of the catalyst layer by adding ammonium oxalate into the catalyst layers, followed by heating to remove the additive and make the pores. They observed that an increase in the porosity of the electrode improved the mass transfer, leading to better cell performance (Fig. 10.16). 

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