Current densities at different

Electrochemical oxidation of hydroquinone was investigated on a rotating disk electrode in a solution containing 0.01 M quinone and hydroquinone in 0.5 M H2so4 at 298 K. The following values of current density at different electrode potential values and RDE rotation rates were obtained ... 

Figure 1. This figure shows the generation of ion current filaments. Here we see the jet head on. The four slices show the ion current density at different depths of the shock at a fixed time. The different depths are z= 60, 100, 120, 160 electron skin depths.

If we assume that the O2 solubility in Nafion is linearly proportional to its pressure, and the parameters for H2 are the same as those for O2, Table 3.15 lists the O2 and H2 crossover current densities at different pressures. 

TABLE 4.2 Apparent and Intrinsic Exchange Current Densities at Different Temperatures in the Low Current Density Range, Measured at AMBIENT Pressure and Zero RH ( o2/H20 Measured Apparent Exchange Current Density for the ORR, i° /H+ is the Simulated Apparent Exchange Current Density for the HOR, and fo2/H20 H2/h+ Intrinsic Exchange... 

FIGURE 10.3 Loss in voltage as a function of current density at different temperatures and different CO concentrations [20]. 

MEA 38 was made with the GEFC 112 membrane. This membrane is thinner than the Nation 117 used previously, and was specifically chosen to determine whether a thinner and more cost effective membrane could reduce cell resistance and improve output accordingly. The GEFC series of membranes properties differ from the Nafion counterparts, and further information can be found on the respective manufacturers websites [19,20]. The cell resistance was 146.91, 112.05, 77.14, and 69.72mS2 at temperatures of 23, 30, 50, and 70°C respectively. The current density at different temperatures is presented in Figure 8.9. 

Fig. 7 CeU G2 cathode catalyst-layer and GDL mass-transport overpotentials, oxygen reduction reaction (ORR) overpotential, and iR-corrected cell voltage as a function of current density at different durability testing times (1,054 total hours at constant 0.90 A cm Gore PRIMEA series 5620 MEA (50.0-cm active area, 0.40/0.60mg Pt cm, 35- am Gore SELECT membrane) SGL SIGRACET GDL 24BC (5wt%/23wt% PTFE substrate/MPL) at both sides. See the Appendix for durability testing conditions. RH Relative humidity, A Anode, C Cathode...

Fig. 88. Cell voltage of a fuel cell with Zfo.ss Cao.15 35 as solid electrolyte versus current density at different temperatures with hydrogen and oxygen as fuels...

The exchange current density at different pressures and temperatures (as per Equation 3-15) is defined as [7] ... 

Electrolytic plating rates ate controUed by the current density at the metal—solution interface. The current distribution on a complex part is never uniform, and this can lead to large differences in plating rate and deposit thickness over the part surface. Uniform plating of blind holes, re-entrant cavities, and long projections is especiaUy difficult. 

Figure 8.32. PEMFC potential as a function of current density for different CO contents in the hydrogen supply. Note the rapid drop in potential as soon as current is drawn, even for pure hydrogen. This is due to overpotentials in the system, while the monotonic decrease at higher current is attributed to the internal resistance of the PEM. [Adapted from H.F. Oetjen,...

If current passes through an electrolytic cell, then the potential of each of the electrodes attains a value different from the equilibrium value that the electrode should have in the same system in the absence of current flow. This phenomenon is termed electrode polarization. When a single electrode reaction occurs at a given current density at the electrode, then the degree of polarization can be defined in terms of the over potential. The overpotential r) is equal to the electrode potential E under the given conditions minus the equilibrium electrode potential corresponding to the considered electrode reaction Ec ... 

An experimental method is proposed for estimation of the transport hindrances in air gas-diffusion electrodes. As a measure of the transport hindrances in the air gas-diffusion electrodes is introduced the difference AE between the potentials of one and the same electrode when operating with air and with pure oxygen at one and the same current density. The difference AE can be theoretically described by the equation ... 

Typically the contributions of the two bands to the current are of rather unequal magnitude, and one of them dominates the current. Unless the electronic densities of states of the two bands differ greatly, the major part of the current will come from the band that is closer to the Fermi level of the redox system (see Fig. 7.6). The relative magnitudes of the current densities at vanishing overpotential can be estimated from the explicit expressions for the distribution functions Wled and Wox ... 

If one studies the growth rate as a function of anodization current density for different PS structures prepared in the same electrolyte, as shown in Fig. 6.5, some inherent laws can be observed. In the regime of stable macropore formation on n-type silicon the growth rate is found to be virtually independent of the applied current density. This is simply a consequence of JPS being present at any pore tip, as described by Eq. (9.5). For the growth rate rPS (in nm s 1) of micro PS in ethanoic... 

These points indicate that the continuum theory expression of the free energy of activation, which is based on the Born solvation equation, has no relevance to the process of activation of ions in solution. The activation of ions in solution should involve the interaction energy with the solvent molecules, which depends on the structure of the ions, the solvent, and their orientation, and not on the Born charging energy in solvents of high dielectric constant (e.g., water). Consequently, the continuum theory of activation, which depends on the Born equation,fails to correlate (see Fig. 1) with experimental results. Inverse correlations were also found between the experimental values of the rate constant for an ET reaction in solvents having different dielectric constants with those computed from the continuum theory expression. Continuum theory also fails to explain the well-known Tafel linearity of current density at a metal electrode. ... 

Another important point regarding the fabrication process of MPLs is the fact that, typically, when carbon fiber paper is used as the DL, the MPL is coated just on one surface of the CLP. However, when a carbon cloth is used, a homogeneous water suspension of carbon powder and PTFE is filtered under vacuum onto both faces of the carbon cloth material to form the MPLs [153,158,161,171], followed by drying and sintering as mentioned earlier. Antolini et al. [161] were able to demonstrate that carbon cloth with double MPLs, for both the anode and the cathode sides, showed better performance than when a CFP was used as the cathode DL with one MPL. At low current densities, the difference between the two DLs was not as obvious, but it became more evident at higher current densities because the limiting current densities for each case are quite different ( 1.6 A cm for CFP vs. 2.7 A cm for CC) (see Figure 4.20 for more details). 

Figure 25. A1 corrosion current density at 4.20 V (vs Li+/ Li) in 1.0 M PC solutions of various lithium salts showing a difference between Tf , Im , and other anions. (Reproduced with permission from ref 141 (Figure 1). Copyright 1997 Elsevier.)...

In some deposits, notably those of nickel, electrical resistance follows current density at low temperamres in the sense that films deposited at low current density (say, lOniA/cm ) show lower resistance than do those deposited at higher density. Although this is so in the low-temperature range 4 to 40 K, this difference in resistance disappears closer to room temperature. 

Now think of the n —>p hole current. When the holes from the n-type of medium reach the junction [see Fig. 7.21(c)], they do not see any barrier due to an electrical potential difference, so they simply tumble over the potential drop. Hence, the n —> p hole current density at equilibrium is controlled only by diffusion and is simply proportional19 to the number of holes, nhn, in the rz-type of material ... 

The amount of oxygen adsorbed in the three-phase region has been found to depend linearly on the exchange current density for different catalyst-electrodes under similar conditions.31,32 This indicates that the electrocatalytic reaction takes place at the three-phase boundary. Vayenas and co-workers pointed out that for less porous electrodes the charge-transfer reaction at the two-phase boundary might become important and that under some conditions oxygen on the electrolyte surface itself might play a role. 

Furthermore, Figs. 4.4.6 and 4.4.7 depict the i dependence on V for different values of R and 8 = 200,20, respectively. When the thickness of the unstirred layer is much larger than the typical distance between the conductive inhomogeneities of the membrane surface (8 1), the deviation in the limiting current density at a nonhomogeneous membrane from the appropriate value at a homogeneous surface is hardly noticeable, unless the proportion of the conductive membrane surface area is very small (R 1). 

Fig. 17. Cathodic overpotential versus current density for different annealing periods at 1000 °C for the cell LaCoO3/La0 9Bao,AIO2.95/YSZ in air at 800 °C. The variation of overpotential at the current density of 100mA/cm2 with the preannealing period is also shown. A 12 h, o 36 h, A 48 h, 60 h. From ref. [75]. Reprinted by permission of the American Ceramic Society.

Here / is the current density with the subscript representing a specific electrode reaction, capacitive current density at an electrode, or current density for the power source or the load. The surface overpotential (defined as the difference between the solid and electrolyte phase potentials) drives the electrochemical reactions and determines the capacitive current. Therefore, the three Eqs. (34), (35), and (3) can be solved for the three unknowns the electrolyte phase potential in the H2/air cell (e,Power), electrolyte phase potential in the air/air cell (e,Load), and cathode solid phase potential (s,cath), with anode solid phase potential (Sjan) being set to be zero as a reference. The carbon corrosion current is then determined using the calculated phase potential difference across the cathode/membrane interface in the air/air cell. The model couples carbon corrosion with the oxygen evolution reaction, other normal electrode reactions (HOR and ORR), and the capacitive current in the fuel cell during start-stop. 

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