Ohmic resistance, substrate

For a moderately or highly doped material the potential drop due to ohmic resistance in the substrate is very small. For example, for a substrate with a resistivity of 0.1 Qcm ( 1017/cm3) the potential drop in the substrate of 0.1 mm thick at a current density of 10... 

The recorded current is caused not only by the heterogeneous electron transfer to the substrate (the Faradaic current ), but also by the current used to charge the electrical double layer, which acts as a capacitor. The measured potentials include the potential drop caused by the ohmic resistance in the solution, the iR drop. Both the charging current ic and the iR drop grows with the sweep rate it is always desirable to compensate for ic and iR drop, but it becomes imperative at higher sweep rates. There exist different ways to compensate electrically for these phenomena, and this makes it possible to operate up to about 103 V sec-1. It is assumed below that the data are obtained with proper compensation. 

The solvent may influence the electrolytic reaction in different respects. The solubility of the substrate in the solvent is important for the attainment of a high initial current the solubility of a supporting electrolyte and the dielectric constant of the medium are reflected in the ohmic resistance. The adsorption of substrate on the electrode depends on the medium, and so does the availability of protons. Some solvents are, themselves, oxidized or reduced too easily to be useful. A review on the solvents of interest for electrolysis has recently been published 36 it includes information on the solubility of supporting electrolytes, useful reference electrodes, and attainable potential range. 

The parameters that control the macroscopic current distribution (in the absence of substrate resistance) can be represented in terms of the Wagner number, defined by the ratio of the activation resistance of the surface reaction, (Ra), to the electrolyte ohmic resistance, (Rn) ... 

The electrical resistance of coatings is composed of the active (ohmic) and reactive (polarization) resistances. The latter is a resistance to alternating current through the capacity and inductance of the coating-substrate system. Ohmic resistance makes up an insignificant portion of the total electrical resistance of pol3Tner coatings and characterizes electrolyte diffusion in the... 

Figure 16.19 Fitted values of the transmission line model. Rion denotes the ionic transport resistance, R the charge-transfer resistance, and Rq the ohmic resistance of the 8YSZ electrolyte substrate. riMi was set fixed to zero.

Currently, electrolyte-supported, cathode-supported, anode-supported, and metallic substrate-supported planar SOFCs are tmder development. In electrolyte-supported cells, the thickness of the electrolyte, typically YSZ, is 50-150 pm, making then-ohmic resistance high, and such cells are suitable only for operation at 1,000°C. In electrode-supported designs, the electrolyte thickness can be much lower, typically 5-20 pm, which decreases their ohmic resistance and makes them better suited for operation at lower temperatures. The anode (Ni/YSZ cermet) is selected as the supporting electrode, because it provides superior thermal and electrical conductivity, superior mechanical strength, and minimal chemical interaction with the electrolyte. Kim et al. [83] have reported power densities as high as 1.8 W/cm at 800°C for such anode-supported SOFCs. At Pacific Northwest National Laboratory [84, 85], similar anode-supported cells have been developed using 10 pm... 

For MFC, ferricyanides and manganese oxides have been used as alternatives for oxygen. The main application of MFC is to treat the wastewater. The ohmic resistance plays dominant role in the polarisation curves. The polarisation and power curves for MFC are shown in Fig. 2.12 for starch substrate. The polarisation curves for MFC are usually linear in nature and hence the internal resistance value can be easily obtained from their slope. The performance of MFC depends upon the electrochemical reactions occurring between the electron acceptor with high potential and organic substrate with low potential. In MFC, it becomes uncertain to have an idea of the cell voltage because the electron transfer takes place via the organic substrate and complex chain which varies from microbe to microbe. 

Physically, the sensitivity of reactions to surface curvature can be associated with the space change layer or the resistance of the substrate. For moderately or highly doped materials, this sensitivity is only associated with the space change layer because the ohmic potential drop in the semiconductor substrate is very small. However, for lowly doped material a significant amount of potential can drop in the semiconductor to cause the current flow inside semiconductor to be also sensitive to the curvature of the surface. 

Ohmic losses. The finite resistance of the electrolyte, the substrate and the membrane used in a fuel cell will induce a supplementary loss in efficiency. This reduction becomes severe at higher current densities since the power loss is proportional to the square of the current density. The solutions to this problem rely greatly on good engineering practice and on a fundamental understanding of the type of electrode used. 

The impregnation of porous nickel discs with CoPc was difficult because of the limited solubility of the chelate in the usual solvents. CoPc cathodes with carbon as substrate were therefore prepared for use in H2/O2 fuel cells. A mixture of 72 mg CoPc and 48 mg acetylene black, with PTFE as binder, was pressed into a nickel mesh of area 5 cm2. Electrodes of this type were tested in an H2/O2 fuel cell with 35% KOH electrolyte in an asbestos matrix at 80° C. Figure 5 compares the current/voltage characteristics of CoPc cathodes (14 mg/cm2) with those of other catalysts, including platinum (9 mg/cm2), silver (40 mg/cm2), and pure acetylene black (20 mg/cm2). An hydrogen electrode (9 mg Pt/cm2) was used as the anode in all tests. To facilitate comparison of the activity of different cathodes, the pure ohmic internal resistance of the cells (of the order of 0.02 ohm) was eliminated. 

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