Polarization ohmic

Ohmic polarization takes place on account of resistance to the flow of ions and electrons in the battery. More precisely, ohmic polarization results from the resistance that arises as a result of the presence of such components in the battery as the electrolyte, electrodes, current collectors, and terminals. The overpotential generated is expressed by the term IE, in which R is the specific area resistance [6,8,66] and I is the flowing current. This type of polarization emerges and vanishes instantly, when the current flows and ceases, respectively. This is given by the Ohm s law relationship, t n = IE, between the current, I, and the overpotential, i)Q, due to the ohmic resistance in the cell. 

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In contrast to signal spread, according to Eq. (3-48) for a coating with few defects, in this case a locally almost constant conductivity is assumed. For the extreme case of an uncoated pipe and neglecting the ohmic polarization resistances, there is a distance x = a where both voltage drops of Eqs. (3-52) and (3-53) are equal... 

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... 

Useful work (electrical energy) is obtained from a fuel cell only when a reasonable current is drawn, but the actual cell potential is decreased from its equilibrium potential because of irreversible losses as shown in Figure 2-2". Several sources contribute to irreversible losses in a practical fuel cell. The losses, which are often called polarization, overpotential, or overvoltage (ri), originate primarily from three sources (1) activation polarization (r act), (2) ohmic polarization (rjohm), and (3) concentration polarization (ricoiic)- These losses result in a cell voltage (V) for a fuel cell that is less than its ideal potential, E (V = E - Losses). 

The activation polarization loss is dominant at low current density. At this point, electronic barriers have to be overcome prior to current and ion flow. Activation losses show some increase as current increases. Ohmic polarization (loss) varies directly with current, increasing over the whole range of current because cell resistance remains essentially constant. Gas transport losses occur over the entire range of current density, but these losses become prominent at high limiting currents where it becomes difficult to provide enough reactant flow to the cell reaction sites. 

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... 

Summing of Cell Voltage The cell voltage includes the contribution of the anode and cathode potentials and ohmic polarization ... 

To determine actual cell performance, three losses must be deducted from the Nernst potential activation polarization, ohmic polarization, and concentration polarization. Definition of the ohmic polarization is simply the product of cell current and cell resistance. Both activation polarization and concentration polarization required additional description for basic understanding. 

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). 

The electrolyte composition affects the performance and endurance of MCFCs in several ways. Higher ionic conductivities, and hence lower ohmic polarization, are achieved with Li-rich electrolytes because of the relative high ionic conductivity of Li2C03 compared to that of Na2C03 and K2CO3. However, gas solubility and diffusivity are lower, and corrosion is more rapid in Li2C03. 

It has been observed that solid oxide fuel cell voltage losses are dominated by ohmic polarization and that the most significant contribution to the ohmic polarization is the interfacial resistance between the anode and the electrolyte (23). This interfacial resistance is dependent on nickel distribution in the anode. A process has been developed, PMSS (pyrolysis of metallic soap slurry), where NiO particles are surrounded by thin films or fine precipitates of yttria stabilized zirconia (YSZ) to improve nickel dispersion to strengthen adhesion of the anode to the YSZ electrolyte. This may help relieve the mismatch in thermal expansion between the anode and the electrolyte. 

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. 

Ohmic polarization arises from the resistance of the electrolyte, the conductive diluent, and materials of construction of the electrodes, current collectors, terminals, and contact between particles of the active mass and conductive diluent or from a resistive film on the surface of the electrode. Ohmic polarization appears and disappears instantaneously (<10 s) when current flows and ceases. Under the effect of ohmic resistance, R, there is a linear Ohm s Law relationship between /and rj. 

Other contributions to the polarization are activation polarizations (nact) caused by inhibition of the passage of ions through the phase boundary which may arise in the discharge mechanism. Films on the electrode may also contribute (e.g. oxide, metal already deposited, impurity) by offering a resistance to current flow differing from the bath resistance (ohmic polarization, Tlohm)- Hence the observed overvoltage is given by... 

At lower temperature, ionic conductivity is lowered, ohmic polarization is increased, and the battery capacity decrease is observed mainly due to diffusion limitations. 

Fig. 13.7. Graphical representation of the influence of the internal resistance of an electrochemical energy converter on the cell potential when mass-transfer polarization is negligible. The early nonlinear part of the curve represents the effect of the activation overpotential on the cell potential before ohmic polarization has become important.

Ohmic polarization which is a phenomenon linked to the resistance of particular cell components and to the resistance as a result of contact problems among the cell components. 

Figure 3.3.7 Theoretical (dashed dotted) and real (solid) cell voltage (V) - current density (I) performance characteristics of a fuel cell. Overpotentials are responsible for the difference between theoretical and real performance and cause efficiency losses. They split into (i) activation polarization overpotentials at anode and cathode due to slow chemical kinetics, (ii) ohmic polarization overpotential due to ohmic voltage losses along the circuit, and (iii) concentration polarization overpotentials due to mass-transport limitations. The activation overpotentials of the cathode are typically the largest contribution to the total overvoltage.

The majority of fuel cells including SOFC are limited largely by ion transport rate through membranes, i.e., ohmic polarization. Studies on the SOFC with the SDC (Ceo.8Smo.2OL9) electrolyte at 500 °C showed that decreasing the thickness of the electrolyte from 0.35 mm to 0.15 mm led to the increase in the current density from 0.2 W/cm to 0.4 Therefore, it is highly desirable to use a... 

The corrosion resistance of stainless steels and nickel-based alloys in aqueous solutions can often be increased by addition of chromium or aluminum. " Chromium protects the base metal from corrosion by forming an oxide layer at the surface. Chromium is also considered to be an important alloying metal for steels in MCFC applications. Chromium containing stainless steel, however, leads to the induced loss of electrolyte. Previous studies done to characterize the corrosion behavior of chromium in MCFC conditions have shown the formation of several lithium chromium oxides by reaction with the electrolyte. This corrosion process also results in increased ohmic loss because of the formation of scales on the steel. Aluminum additions similarly have a positive effect on corrosion resistance. " However, corrosion scales formed in aluminum containing alloys show low conductivity leading to a significant ohmic polarization loss. 

Active query methods measure cell impedance, which is then correlated to SoC. The technique often superimposes an active signal (a low amplitude, characteristic high-frequency square or sinusoidal current pulse) onto the battery and then uses a transfer function on the response waveform to determine the ohmic polarization or a direct correlation to SoC. One permutation of this technique uses the voltage response to indigenous current spikes to map impedance in a similar way. This method provides reasonable results, but if hardware is involved it is often complex and expensive even sensors will require a relatively high-speed data acquisition bus to minimize the slew between voltage and current. 

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