Polarization resistance exchange current density

The improvement in cell performance at higher pressure and high current density can be attributed to a lower diffusion polarization at the cathode and an increase in the reversible cell potential. In addition, pressurization decreases activation polarization at the cathode because of the increased oxygen and water partial pressures. If the partial pressure of water is allowed to increase, a lower acid concentration will result. This will increase ionic conductivity and bring about a higher exchange current density. The net outcome is a reduction in ohmic losses. It was reported (33) that an increase in cell pressure (100% H3PO4, 169°C (336°F)) from 1 to 4.4 atm (14.7 to 64.7 psia) produces a reduction in acid concentration to 97%, and a decrease of about 0.001 ohm in the resistance of a small six cell stack (350 cm electrode area). 

This review focuses on corroding systems. However, the concept of polarization resistance applies equally well to reduction-oxidation systems. Here, the exchange current density, iD, may be calculated from the polarization resistance, where R is the ideal gas constant, T is the temperature, and a, and ac are the anodic... 

EXAMPLE 12-2 According to Bockris and Huq the exchange current density for the oxygen electrode in acid solution is of the order of 10 A/cm on a platinum surface. The polarization resistance is RTIAFjo = 6 x 10 fl-cm. The double-layer capacity C j is of the order of 40 p/lcm, based on the true area, or perhaps 100 pjjcm based on the geometric or projected area, on which the current density was based. Thust = 6 X 10 X 100 X 10 = 6 x 10 s, and the time for overpotential decay from 10 to 1 mV is estimated to be 2.3t 4 h. Therefore, measurements accurate to... 

Although the potentiostat has an adjustable IR compensation circuit, this was not used. When it was used, the scan rate varied. Compensation did not seem to have much effect. Some researchers have developed their own IR compensation circuit for the high resistance solution ( ), some added a supporting electrolyte (12 11) And some simply ignored it (H). Since the behavior of electrolytes under supercritical conditions is not well known, no supporting electrolyte was added to eliminate the IR drop in this experiment. Furthermore, the IR circuit was not used for this study either, since the desired electrochemical data, such as exchange current density, open circuit potential, and transfer coefficients, can be obtained from the polarization curves without IR compensation. 

Figure 17. Dependencies of exchange current density jo at various electrodes on the ionic conductivity of various zirconia solid electrolytes (A) SDC (dashed, dash-dotted, and dotted lines) and Ru (0.5 mg/cm )-dispersed SDC anodes in humidified Hi. Reprinted from Ref. 45, Copyright (1997), with permission from The Electrochemical Society, (B) Pt cathodes in Oj, (C) LfCi jsSriu MnO, (LSM, dashed lines) and Pt (0.1 nig/cnr(-dispersed LSM cathodes in Oi. Each jo value was calculated from the polarization resistance (Rp, 2 tm ), since linear relationships were observed between 7 and j for rj < 0.1 V at all the electrodes and 7 ccii between 800 and 1000°C jo = (RT/nF)Rp. Reproduced by Ref. 46, Copyright (1999), by permission from The Electrochemical Society. 

Near the reversible potential, the current density is seen to be linearly dependent on overpotential with a slope that scales with exchange current density. This linearity of current to potential is similar to Ohm s law, and the proportionality is called polarization resistance, i = r]/Rp, where Rp = RT / ion F. 

A higher exchange current density impHes increased reaction rate, while lower exchange current density (higher polarization resistance) indicates sluggish corrosion kinetics. Equation (1.10) can be written in terms of charge transfer resistance as ... 

The exchange current densities for a hydrogen-evolution reaction on three different metals are 5 X A/cm, 3x10 A/cm, and 5xl0 A/cm, respectively. Calculate the current densities and the polarization resistances if the reactions proceed at t] = —0.0S V versus SFIE. The electrode surface area is 1 cm ... 

The rate of this process in aprotic electrolytes is rather high the exchange current density is fractions to several mA/cm. As pointed out already, the first contact of metallic lithium with electrolyte results in practically the instantaneous formation of a passive film on its surface conventionally denoted as solid electrolyte interphase (SEI). The SEI concept was formulated yet in 1979 and this film still forms the subject of intensive research. The SEI composition and structure depend on the composition of electrolyte, prehistory of the lithium electrode (presence of a passive film formed on it even before contact with electrode), time of contact between lithium and electrolyte. On the whole, SEI consists of the products of reduction of the components of electrolyte. In lithium thionyl chloride cells, the major part of SEI consists of lithium chloride. In cells with organic electrolyte, SEI represents a heterogeneous (mosaic) composition of polymer and salt components lithium carbonates and alkyl carbonates. It is essential that SEI features conductivity by lithium ions, that is, it is solid electrolyte. The SEI thickness is several to tens of nanometers and its composition is often nonuniform a relatively thin compact primary film consisting of mineral material is directly adjacent to the lithium surface and a thicker loose secondary film containing organic components is turned to electrolyte. It is the ohmic resistance of SEI that often determines polarization of the lithium electrode. 

As is evident from the spectra of Fig. 24 the polarization resistance, Rp, is also significantly decreased with increasing Ph2o-This eventually corresponds to the enhancement of the electrocata-lytic activity of the electrode/electrolyte electrochemical interface. This can be quantitatively rationalized through the exchange current density of the electrochemical interface, lo, as this is described by the Battler-Volmer eletrokinetic egitation " ... 

Based on an analytical model for PFFCs, Kulikovsky et al. presented a two-step procedure to evaluate the parameters Tafel slope, exchange current density, and cell resistance from two sets of polarization curves for an HT-PFFC [36]. The method was validated with experimental data. Shamardina et al. described an analytical model taht accounts for the crossover of gases through the membrane [37]. The model is pseudo-two dimensional and describes mainly the effects across the MFA. Temperature and pressure variations in the cell were not considered. From these analytical studies, it follows that the crossover effect has a considerable influence only at a low stoichiometry of oxygen. 

The situation with the activation polarization on the cathode side is more subtle. The assumption of constant activation resistivity is justified for cathodes with a high exchange current density For sufficiently... 

A polarization curve, i.e., an 1-V curve of a fuel cell stack under certain operating conditions, is frequently used to describe fuel cell performance and determine performance decay with time. Polarization curve analysis can provide the most important kinetic parameters, such as the Tafel slope, exchange current density, cell resistance, limiting current density, and specific activity of Pt. A single fuel cell potential can be described by ... 

Determination of the Exchange Current Density from Polarization Resistance... 

This equation permits determination of the exchange current density I o without knowing the total 77/i relationship (see also resistance polarization Rp, Sec. 1.5.4). 

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