Ohmic losses

In low temperature fuel ceUs, ie, AEG, PAEC, PEEC, protons or hydroxyl ions are the principal charge carriers in the electrolyte, whereas in the high temperature fuel ceUs, ie, MCEC, SOEC, carbonate and oxide ions ate the charge carriers in the molten carbonate and soHd oxide electrolytes, respectively. Euel ceUs that use zitconia-based soHd oxide electrolytes must operate at about 1000°C because the transport rate of oxygen ions in the soHd oxide is adequate for practical appHcations only at such high temperatures. Another option is to use extremely thin soHd oxide electrolytes to minimize the ohmic losses. 

Electrowinning from Aqueous Solutions. Electrowinriing is the recovery of a metal by electrochemical reduction of one of its compounds dissolved in a suitable electrolyte. Various types of solutions can be used, but sulfuric acid and sulfate solutions are preferred because these are less corrosive than others and the reagents are fairly cheap. From an electrochemical viewpoint, the high mobiUty of the hydrogen ion leads to high conductivity and low ohmic losses, and the sulfate ion is electrochemicaHy inert under normal conditions. 

A number of anode connections will be made to the d.c. power source using proprietary splice kits (approximately one for every 60-80 m of concrete to be protected). This will provide redundancy for anode failure and reduce ohmic losses along the anode cable. Care must also be taken not to expose the copper conductor during installation or anode failure could take place. Once fitted to the concrete surface a 15 mm thick cementitious overlay is applied above the anode mesh, as recommended by the anode manufacturer, although thickness of up to 35-40 mm have been applied in some instances. 

Micro reactors are seen to have smaller inhomogeneities of the electrical field and less temperature rise in the reaction medium due to the Joule heating effect between the electrodes [70]. Submillimeter interelectrode gaps are expected to reduce the ohmic loss. 

Interelectrode Gap The relative electrolyte volume available per unit surface area of the electrodes is determined by the distance (gap) between the electrodes. This distance is between fractions of a millimeter and some 10 cm. The ohmic losses in the electrolyte increase with the distance between the electrodes. On the other hand, when the electrolyte volume is too small, the reactant concentrations will change rapidly. Often, the electrolyte volume in a reactor is increased by providing space for the electrolyte not only between the electrodes but also above or below the block of electrodes. Sometimes the electrolyte is pumped around in an external circuit, including an additional electrolyte vessel. 

Consider a cell with one positive and two negative electrodes where the latter are at different distances, and I2, from the former (Fig. 18.3). We shall assume for the sake of simplicity that polarization of the electrodes is proportional to current density [i.e., AE = pi (p is the combined polarization resistance of the positive and negative electrodes)]. The voltages of the two halves of the cell, which are in parallel, are identical hence, the snm of ohmic losses and polarization in the two halves shonld also be identical. The ohmic losses in the electrolyte are given by IH<3. Thus,... 

In this example the current density distribution is nonuniform in the vertical, since at all heights x the sums of ohmic potential drops and polarization of the two electrodes must be identical. In the top parts of the electrodes, where the ohmic losses are minor, the current density will be highest, and it decreases toward the bottom. The current distribution will be more uniform the higher the polarization. 

The most general form of a differential equation for the distribution of potential and current density in a system with ohmic losses is obtained when Eq. (18.10) is differentiated. Let Xy (= 2/d) be the working surface area referred to unit volume (of the current collector). Considering that dE = owing to the constancy of the sums of ohmic losses and polarization (which includes the appropriate signs), we find that... 

One of the main reasons for a lower specific activity resides in the fact that electrodes with disperse catalysts have a porous structure. In the electrolyte filling the pores, ohmic potential gradients develop and because of slow difiusion, concentration gradients of the reachng species also develop. In the disperse catalysts, additional ohmic losses will occur at the points of contact between the individual crystallites and at their points of contact with the substrate. These effects produce a nonuniform current distribution over the inner surface area of the electrode and a lower overall reaction rate. 

Ohmic losses AEohmic originate from (i) membrane resistance, (ii) resistance of CLs and diffusion layers, and (iii) contact resistance between the flow field plates. Although it is common practice to split current-voltage characteristics of an MEA into three regions— kinetic (low currents), ohmic (intermediate currents), and mass transport (high currents) [Winter and Brodd, 2004]—implicit separation of vt Afiohmic is not always straightforward, and thus studies of size and... 

The voltage demand was the sum of the various overpotentials and ohmic loss, since there was little concentration difference in H2 across the membrane in these experiments. These polarizations were not individually identified. 

Ohmic losses, in fuel cell voltages, 12 207 Ohmic polarization, batteries, 3 425—426 Ohnesorge number, 23 183, 190 Oil absorption, by silica, 22 371 Oil additives... 

In the current-voltage curve in Fig. 14.15, three different regions can be discerned. At low current densities, the performance is kinetically limited. In the linear part, ohmic losses are significant. At high current densities, mass transport losses dominate. 

Fig. 3.3 The I—V curves, as recorded and compensated for ohmic losses (iRcor.), of Si electrodes in aqueous HF (1M HF, 0.5M NH4CI) are found to shift cathodically with increasing p-type doping density. In a V versus log(i) plot (inset) a p-type electrode (1 2 cm,...

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