Limitations ohmic

As explained before, the open-circuit potential of the battery depends on concentration, temperature, and transport limitations. The real voltage delivered by a battery in a closed circuit is affected by ohmic limitations (ohmic potential), concentration limitations (concentration overpotential), and surface limitations (surface overpotential). The close circuit potential of the cell is given by the open-circuit potential of the cell minus the drop in potential due to ohmic potential, concentration overpotential, and surface overpotential. The ohmic potential is due to the ohmic potential drop in the solution. It is mostly affected by the applied charge/discharge current of the battery. The concentration overpotential is associated with the concentration variations in the solution near the electrodes. It is strongly affected by transport properties such as electrolyte conductivity, transference number, and diffusion coefficients. Finally, the surface overpotential is due to the limited rates of the electrode reactions. 

Ruan, R Ye, X Chen, P Doona, C.J. Ohmie heating. In P. Richardson (Ed.). Thermal technologies in food processing. Cambridge Woodhead Publishing Limited. Ohmic heating, p.165-241, 2001. 

Electrode materials play an important role in the performance (power output) and cost of bacterial fuel cells. This problem was the topic of two review papers. In a review by Rismani-Yazdi et al. (2008), some aspects of cathodic limitations (ohmic and mass transport losses, substrate crossover, etc.), are discussed. In a review by Zhou et al. (2011), recent progress in anode and cathode and filling materials as three-dimensional electrodes for microbial fuel cells (MFCs) has been reviewed systematically, resulting in comprehensive insights into the characteristics, options, modifications, and evaluations of the electrode materials and their effects on various actual wastewater treatments. Some existing problems of electrode materials in current MFCs are summarized, and the outlook for future development is also suggested. 

The exact solution of the instanton equation in the large ohmic friction limit has been found by Larkin and Ovchinnikov [1984] for the cubic parabola (3.18). At T = 0... 

Another issue that can be clarified with the aid of numerical simulations is that of the recombination profile. Mailiaras and Scott [145] have found that recombination takes place closer to the contact that injects the less mobile carrier, regardless of the injection characteristics. In Figure 13-12, the calculated recombination profiles arc shown for an OLED with an ohmic anode and an injection-limited cathode. When the two carriers have equal mobilities, despite the fact that the hole density is substantially larger than the electron density, electrons make it all the way to the anode and the recombination profile is uniform throughout the sample. 

However, under working conditions, with a current density j, the cell voltage E(j) decreases greatly as the result of three limiting factors the charge transfer overpotentials r]a,act and Pc,act at the two electrodes due to slow kinetics of the electrochemical processes (p, is defined as the difference between the working electrode potential ( j), and the equilibrium potential eq,i). the ohmic drop Rf. j, with the ohmic resistance of the electrolyte and interface, and the mass transfer limitations for reactants and products. The cell voltage can thus be expressed as... 

The third limitation is concerned with the numerous contributions to the cell voltage Vceii, which, along with the difference in the electrode reversible potentials AEeq, comprises overpotentials at the cathode, tjc, and the anode, as well as the ohmic drop A ohmic ... 

The above brief analysis underlines that the porous structure of the carbon substrate and the presence of an ionomer impose limitations on the application of porous and thin-layer RDEs to studies of the size effect. Unless measurements are carried out at very low currents, corrections for mass transport and ohmic limitations within the CL [Gloaguen et ah, 1998 Antoine et ah, 1998] must be performed, otherwise evaluation of kinetic parameters may be erroneous. This is relevant for the ORR, and even more so for the much faster HOR, especially if the measurements are performed at high overpotentials and with relatively thick CLs. Impurities, which are often present in technical carbons, must also be considered, given the high purity requirements in electrocatalytic measurements in aqueous electrolytes at room temperature and for samples with small surface area. 

In the ac circuit of the polarographic cell there is such an external ohmic resistance that via the alternating voltage (300 V) together with a superimposed dc the voltage over the cell alternates from 0 to -2V vs. an SCE within these limits oxidation of Hg and reduction of Na+ (electrolyte) to Na(Hg) remains sufficiently restricted. 

The dimensionless limiting current density N represents the ratio of ohmic potential drop to the concentration overpotential at the electrode. A large value of N implies that the ohmic resistance tends to be the controlling factor for the current distribution. For small values of N, the concentration overpotential is large and the mass transfer tends to be the rate-limiting step of the overall process. The dimensionless exchange current density J represents the ratio of the ohmic potential drop to the activation overpotential. When both N and J approach infinity, one obtains the geometrically dependent primary current distribution. 

It is convenient to distinguish three components of the overpotential, r. Two of these are associated respectively with mass-transfer restrictions in the electrolyte near the electrode (concentration overpotential, f/c), and with kinetic limitations of the reaction taking place at the electrode surface (surface overpotential, rjs) the third one is related to ohmic resistance. 

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