Electrolyte Ohmic losses

Anode ohmic loss or overpotential Cathode ohmic loss or overpotential Electrolyte ohmic loss or overpotential... 

Self-supporting Electrolyte supported Relatively strong structural support from dense electrolyte Less susceptible to failure due to anode re-oxidation Higher resistance due to low electrolyte conductivity Higher operating temperatures required to minimise electrolyte ohmic losses... 

After the part where the dominant role is played by the activation losses, the current-voltage curve becomes almost linear in its middle range. This part of the curve is described by the losses associated with the resistance of the electrolyte (ohmic losses) by the following relationship ... 

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. 

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,... 

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 Polarization Ohmic losses occur because of resistance to the flow of ions in the electrolyte and resistance to flow of electrons through the electrode materials. The dominant ohmic losses, through the electrolyte, are reduced by decreasing the electrode separation and enhancing the ionic conductivity of the electrolyte. Because both the electrolyte and fuel cell electrodes obey Ohm s law, the ohmic losses can be expressed by the equation... 

The tape casting and electrophoretic deposition processes are amenable to scaleup, and thin electrolyte structures (0.25-0.5 mm) can be produced. The ohmic resistance of an electrolyte structure and the resulting ohmic polarization have a large influence on the operating voltage of MCFCs (14). FCE has stated that the electrolyte matrix encompasses 70% of the ohmic loss (15). At a current density of 160 mA/cm, the voltage drop (AVohm) of an 0.18 cm thick electrolyte structure, with a specific conductivity of -0.3 ohm cm at 650°C, was found to obey the relationship (13). 

Electrolyte Structure Ohmic losses contribute about 65 mV loss at the beginning of life and may increase to as much as 145 mV by 40,000 hours (15). The majority of the voltage loss is in the electrolyte and the cathode components. The electrolyte offers the highest potential for reduction because 70% of the total cell ohmic loss occurs there. FCE investigated increasing the porosity of the electrolyte 5% to reduce the matrix resistance by 15%, and change the melt to Li/Na from Li/K to reduce the matrix resistivity by 40%. Work is continuing on the interaction of the electrolyte with the cathode components. At the present time, an electrolyte loss of 25% of the initial inventory can be projected with a low surface area cathode current collector and with the proper selection of material. 

The solid oxide electrolyte must be free of porosity that permits gas to permeate from one side of the electrolyte layer to the other, and it should be thin to minimize ohmic loss. In addition, the electrolyte must have a transport number for O as close to unity as possible, and a transport and a transport number for electronic conduction as close to zero as possible. Zirconia-based electrolytes are suitable for SOFCs because they exhibit pure anionic conductivity over a wide... 

The voltage losses in SOFCs are governed by ohmic losses in the cell components. The contribution to ohmic polarization (iR) in a tubular cell" is 45% from cathode, 18% from the anode, 12% from the electrolyte, and 25% from the interconnect, when these components have thickness (mm) of 2.2, 0.1, 0.04 and 0.085, respectively, and specific resistivities (ohm cm) at 1000°C of 0.013, 3 X 10, 10, and 1, respectively. The cathode iR dominates the total ohmic loss despite the higher specific resistivities of the electrolyte and cell interconnection because of the short conduction path through these components and the long current path in the plane of the cathode. 

Figure 48. Kenjo s ID macrohomogeneous model for polarization and ohmic losses in a composite electrode, (a) Sketch of the composite microstructure, (b) Description of ionic conduction in the ionic subphase and reaction at the TPB s in terms of interpenetrating thin films following the approach of ref 302. (c) Predicted overpotential profile in the electrode near the electrode/electrolyte interface, (d) Predicted admittance as a function of the electrode thickness as used to fit the data in Figure 47. (Reprinted with permission from refs 300 and 301. Copyright 1991 and 1992 Electrochemical Society, Inc. and Elsevier, reepectively.)...

The cell potential is simply the work that can be accomplished by the electrons produced in the SOFC, and this potential decreases from the equilibrium value due to losses in the electrodes and the electrolyte. For YSZ electrolytes, the losses are purely ohmic and are equal to the product of the current and the electrolyte resistance. Within the electrodes, the losses are more complex. While there can be an ohmic component, most of the losses are associated with diffusion (both of gas-phase molecules to the TPB and of ions within the electrode) and slow surface kinetics. For example, concentration gradients for either O2 (in the cathode) or H2 (in the anode) can change the concentrations at the electrolyte interface,which in turn establish the cell potential. Similarly, slow surface kinetics could result in the surface at the electrolyte interface not being in equilibrium with the gas phase. 

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. 

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