Reverse-current mechanism

Fig. 1 Cross-sectional electron microprobe images of four locations of a membrane electrode assembly (MEA) from a polymer-electrolyte fuel cell (PEFC) stack that was subjected to 1,994 uncontrolled start/stop cycles. The stack utilized two fuel passes, as shown. As expected by the reverse-current mechanism, the amount of damage depends on the distance from the fuel inlet. Note the changes in the cathode catalyst layer and the presence of platinum in the membrane, especially in the second pass...

In the case when the preceding chemical reaction occurs at a rate of the same order as the intervention time scale of cyclic voltammetry, the repercussions of the chemical complication on the potential of the electrode process are virtually negligible, whereas there is a significant effect on the current. In particular, it is characteristic of this mechanism that the forward current decreases with the scan rate much more than the reverse current. This implies that the current ratio ipr/ipf is always greater than 1, increasing as scan rates are increased. 

The current for a reversible EE mechanism can achieve a stationary feature when microelectrodes are used since in these conditions the function fG(t, qa) that appears in Eq. (3.150) transforms into fG,micro given in Table 2.3 of Sect. 2.6. For microelectrode geometries for which fo.micro is constant, the current-potential responses have a stationary character, which for microdiscs and microspheres can be written as [16] ... 

Fig. 4.26 Variation of the dimensionless peak current of a catalytic mechanism (solid lines) and of a reversible E mechanism (dashed line) for spherical and disc electrodes with the parameter s/Dti/tq, with ra being the sphere or disc radius. The values of A (= r lki + ki)/D) are indicated on the curves. A = 50mV, ti = 1 s, t-2 0.050 s, K = 1 //feq = 1, T = 298.15 K,...

The influence of the chemical kinetics is analyzed in Fig. 4.31 where ADDPV curves are plotted for different values of the dimensionless rate constant %2(= (k + ki)zi). For comparison, the curve corresponding to a simple, reversible charge transfer process (Er) of species C + B for the CE mechanism and of species A for the EC one has also been plotted (dashed line in Fig. 4.31a, b). As can be observed, the behavior of ADDPV curves with is very different depending on the reaction scheme. For the CE mechanism with K = (1 /Kepeak current increases and the peak potential shifts toward more negative values as the kinetics is faster, that is, as xi increases. For very fast chemical reactions, the ADDPV signal is equivalent to that of a reversible E mechanism (Er) with... 

The relative magnitude of the peaks of the ADDPV curve is also very informative about the electrode process [55, 81, 82]. It can be observed that for the CE mechanism it is fulfilled that/ lane < / ll ll ine whereas for the EC one / i"L > / I I "L. This behavior contrasts with the case of the first-order catalytic mechanism and with the reversible E mechanism for which the value of the peak currents is equal (/ lane = / lane ). Therefore, this simple criterion allows us to discriminate between these mechanisms. 

Fig. 7.34 Theoretical SWV curves for a reversible EE mechanism coupled to a very fast chemical reaction. AT) = OmV, Sw = 50mV, AEs = 5mV. The values of the chemical equilibrium constant Keq are 0.01 (a), 1 (b), and 100 (c). The dimensionless net current (A) and their forward (b) components are shown. Reproduced from [51 ] with permission...

Although the reverse current of an ideal Schottky barrier is J, in practice there are other current soitfces. Imperfect contacts have a leakage current which generally increases exponentially with bias. Even with an ideal contact, there is a thermal generation current caused by the excitation of electrons and holes from bulk gap states to the band edges. This mechanism determines the Fermi energy position under deep depletion conditions. The current density is the product of the density of states and the excitation rate and is approximately. 

Reiser, C.A. et al., A reverse-current decay mechanism for fuel cells, Electrochem. Solid-State Lett., 8, A273, 2005. 

It is possible that the steep, steady-state current-voltage (/-F) curves frequently observed with PbNe and other azides prior to detonation are reverse currents at Schottky barriers, dominated by interface states. They do not appear to be single-injection currents because the same electrode that causes the steep, steady-state j-V dependences fails to produce the characteristic transient /-F-r dependences expected from the single-injection mechanism [21,22]. The reverse-bias current mechanism is also consistent with temperature-dependence measurements reported on AgNa [17] and with electrode material-dependence measurements reported on TIN3 and CuN [18]. It is essential to settle this point before one can use such measurements to establish initiation mechanisms. 

The other approach is based on the use of current systematic (although limited) knowledge of the reversed-phase mechanism in combination with the principles of the analyte behavior in the mobile phase. This offers some flexibility in the controlling the selectivity of your method and certain predictability of components retention. Although some black magic may still be needed there also. 

C. A. Reiser, L. Bregoli, T. W. Patterson, J. S. Yi, J. D. Yang, M. L. Perry, and T. D. Jarvi, A Reverse-Current Decay Mechanism for Fuel Cells, Electrochem. Solid-State Lett., 8, A273 (2005). 

So far, the most popular solution for this problem has been to sample the current in SCV not at the end of the pulse but at an appropriate time (sampling time, ts) so that the SCV voltammogram is equivalent to the CV one. The optimal value for the sampling time depends on the experimental system (electrode kinetics, reaction mechanism, step potential, etc.) and it has been established for some frequent situations. For example, for a reversible E mechanism at a macroelectrode there is an acceptable agreement between SCV and CV for Ea [Pg.81]

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