Arrhenius plots, electrochemical

As described in the previous sections, a stable Pt skin of a few nanometers is formed on the Pt-Fe, Pt-Co, and Pt-Ni alloy surfaces after electrochemical stabilization. Figure 10.12 shows Arrhenius plots of kapp on the alloy electrodes at —0.525 V vs. E° in comparison with that of a pure Pt electrode. In the low temperature region (20-50 °C for Pt54Fe45, 20-60 °C for Pt6gCo32 and Ptg3Ni37), linear relationships between log kapp and 1 / Tare observed at all the electrodes, corresponding to the following Arrhenius equation ... 

The effect of the addition of water and molecular solvents such as propylene carbonate, N-methylformamide, and 1-methylimidazole on the conductivity of [C4Cilm][Br] and [C2Cilm][BF4] was measured at 298 K [211]. The mixture of the IL and the molecular solvent or water showed a maximum on the conductivity/mole fraction IL curves. The maximum for nonaqueous solvents was at the level of approximately 18-30 mScm at low mole fraction of the IL and the maximum for water was at level approximately 92-98 mScm [211]. The conductivity of a mixture of these two ILs depends monotonically on the composition. The temperature dependence of the conductivity obeys the Arrhenius law. Activation energies, determined from the Arrhenius plot, are usually in the range of 10-40 kj mol / The mixtures of two ILs or of an IL with molecular solvents may find practical applications in electrochemical capacitors [212]. 

Fig. 2F Arrhenius plots, showing the variation of the apparent electrochemical enthalpy of activation (calculated from the slope of the straight lines) as a function of potential. All lines extrapolate to the same value at infinite temperature, showing that the entropy of activation is independent of potential.

In Case 1, treated by the conventional method (1), linear electrochemical Arrhenius plots result with slopes that decrease in the expected way with increasing 17. Case 2 gives curvature in such plots as does case 4 (but with the curvature in the opposite direction). Case 3, with a linear in T, as often found, gives log vs. 1/ T plots that are parallel for various 17 values so that the AH° values that are recovered are apparently independent of 17. This is an interesting and significant case. The behavior of Case 3 follows obviously from Eq. (48) when a = yT since then the exponent in Eq. (48) only involves the term AH° / as a T-dependent quantity... 

Care must therefore be taken in interpreting apparent activation energies as /(17) from electrochemical Arrhenius plots when a = yT, i.e., when b is independent of T... 

When a is /(T), complications arise in electrochemical Arrhenius plots for evaluation of the apparent AH then... 

Big, 59. Arrhenius plots of propagation constants, k , of polybutadiene ion-pairs in THFforLi+, Na+, andK+ salts. The full circles represent the results of Funt derived from electrochemical studies... 

Figure 16J4. Arrhenius plot of reciprocal decay of conductivity in dry conditions for polythiophene. Adapted from Bull. Electrochem. 10(11/12), 508 (1994), with permission of C.E.C.R.I. (India).

Figure 6. Arrhenius plots of the linear (a) and parabolic (b) rate constants. (Reproduced with permission from Ref. 18. Copyright 1978 The Electrochemical Society.)...

Figure 5.7. Plots of the gas permeation coefficients in 1100 equivalent weight Nafion . (a) Hydrogen and oxygen permeation coefficients at 30°C as a function of water content. The dotted lines signify the transition-region values, (b) Arrhenius plot of the oxygen permeation coefficient as a function of temperature for a liquid-equilibrated membrane, a vapor-equilibrated membrane, and a dry membrane. Also plotted are the oxygen permeation coefficients in water [79] and Teflon [80, 81]. (Figure b is reproduced from Ref. [39] with permission of The Electrochemical Society, Inc.)...

Figure 6.19 Arrhenius plot of the conductivities of a simple (PE0)8LiC104 and of a composite (PEO)8LiClO4.10w/oy-LiAlO2 membrane. From ref [39] by permission of the Electrochemical Society, Inc.

Many examples of Arrhenius plots are found in the literature, but for an electrochemical flavor the one shown in Fig. 3.2, taken from a paper by Kreysa and Medin, refers to the chemical step in the indirect electrochemical oxidation of p-methoxytoluene to p-methoxybenzaldehyde using a Ce /Ce redox couple. This reaction has an activation energy obtained from the slope of Fig. 3.2 ... 

Fig. 12.6 Arrhenius plot of conductivity of BaCeo.9Mo.1O3 a(M = Y, Tm, Yb, Lu, In, or Sc) in moist H2 closed symbols) and O2 (open symbols) in the temperature range of 400°-900°C P(H20)=1.9x 10 Pa. Dashed lines indicate isoconductivity lines in S cm (Reprinted from [5] with permission from Electrochemical Society)...

Fig. 9.5 (a) Structures of the ionic liquids, (b) Photograph of gel polymer electrolyte with a diameter of 13 mm and a thickness of 200 pm. (c) Conductivity Arrhenius plots of four gel polymer membranes. The data are obtained from impedance spectroscopy at 10-50 °C. (d) Composition, conductivity data, and ECW of gel polymer electrolytes with varied ionic liquids, mp melting point, EON electrochemical window electrodes, stainless steel/stainless steel first cycle v = 25 mV/s T=50 °C potential range, E vs Stainless Steel= 4 V [57]... 

Figure 9. Arrhenius plots of molar conductivity of EMIBF and BPBF. From reference 50, reproduced by permission of The Electrochemical Society, Inc.

Figure 12. Arrhenius plot of the ionic conductivity of UQ04 (IM), EC-DEC liquid eiectrolyte in comparison with that of the P(EO)ioLiCI04-t-lOwt% nano-partide SO2 composite membrane swelled (300%) in an EC(25 % molar)-DEC solvent mixture. From reference 31 reproduced by permission of The Electrochemical Society, Inc.

If In k is plotted versus, the activation energy a corresponds to the slope of the resulting straight line, whereas the pre-exponential factor A can be determined from the intersection with the T axis. By Eyring s theory of the activated complex, rearrangement processes of the ionic atmosphere are important in electrochemical reactions. Hence it is useful to introduce the activation entropy A5. Since the pre-exponential factor A of Arrhenius equation can be expressed as a function of entropy, we get (with activation enthalpy, AH and Boltzmann constant b)-... 

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