Equivalent circuit model porous oxides

To elucidate methanol crossover at the DMFC cathode, the active electrode surface of the cathode was divided into two separate parts one for oxygen reduction and the other for oxidation of crossover methanol. In this model, the methanol oxidation and oxygen reduction occur in parallel at different sites or pores because of the porous structure of the catalyst layer. The equivalent circuit for this model is presented in Figure 6.69. 

When the polymer flhn is oxidized, its electronic conductivity can exceed the ionic conductivity due to mobile counterions. Then, the film behaves as a porous metal with pores of limited diameter and depth. This can be represented by an equivalent circuit via modified Randles circuits such as those shown in Figure 8.4. One Warburg element, representative of linear finite restricted diffusion of dopants across the film, is also included. The model circuit includes a charge transfer resistance, associated with the electrode/fllm interface, and a constant phase element representing the charge accumulation that forms the interfacial double... 

An equivalent circuit can be derived for the surface-bound membrane formed in this work similar in a manner to the approach taken for porous anodic films and porous electrodes (41-46). An equivalent circuit network, proposed in Figure 8a, corresponds to the model in Figure 7. This network has three RC subnetworks that represent the oxide layer, the surface-bound membrane layer, and the double layer. Cox and Rox are the capacitance and resistance of oxide. and Rdl are the double-layer capacitance and the polarization resistance, known as the charge transfer resistance at the membrane-water interface. For the subnetwork of the surface-bound membrane layer, one branch represents a tightly packed alkylsilane and lipid bilayer in series, and the other branch represents the pores and defects through the bilayer. Calk, Clip and Ralk, Rhp are the capacitances and resistances of... 

Capacitive humidity sensors commonly contain layers of hydrophilic inorganic oxides which act as a dielectric. Absorption of polar water molecules has a strong effect on the dielectric constant of the material. The magnitude of this effect increases with a large inner surface which can accept large amounts of water. An example of this type of dielectric is porous j8-alumina. Colloidal ferric oxide, certain semiconductors, perowskites and certain polymers have also been used. /1-alumina is characterized by ionic conductance. Materials of this type can be characterized by a complex resistance composed of real (ohmic) as well as capacitive terms. The behaviour of such solids can be symbolized by a model and an associated equivalent circuit as given in Fig. 5.8. 

The impedance of a porous electrode can be simulated with the transmission line model, and the penetration depth can be evaluated [24]. For the non-porous Pt-modified as-deposited surface, the methanol oxidation reaction can be simulated as a simple Randles equivalent circuit comprising a parallel combination of a double layer capacitance and a semi-infinite Warburg impedance in series with a charge transfer resistance. 

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