Current transport analysis

G. Agostinelli, Photocurrent Analysis of CdTe Solar Cells, Ph.D. Thesis, Univer-siteit Gent, 2002. See also G. Agostinelli, et al., Light dependent current transport mechanisms in chalcogenide solar cells, in 3rd World Photovoltaic Energy Conference, Osaka, May 11-18, 2003. 

Important information about the material properties relevant for superconductivity is gained from the temperature and frequency dependence of thermodynamic and transport properties. Experiments indicate severe deviations from the universal behavior predicted by weak-coupling BCS theory. The current theoretical analysis of experimental data which relies on standard weak-coupling theory can yield only qualitative results. A quantitative treatment must account for strong-coupling effects. This, however, requires a microscopic picture of the normal state, i.e., the quasiparticles and their interactions. 

The processes that occur in the space between the cathode and anode of a vacuum arc are rather unevenful compared to those at the electrodes, but nonetheless they are very complicated and not easily subject to analysis. The interelectrode volume is tilled with a diffuse plasma whose main function is to provide a conductive medium for the arc current transport between the electrodes. In the absence of an anode spot, the vapor and most of this plasma is provided by the spray from the high-velocity jets that have their origin in the cathode spots. 

Improved sensitivities can be attained by the use of longer collection times, more efficient mass transport or pulsed wavefomis to eliminate charging currents from the small faradic currents. Major problems with these methods are the toxicity of mercury, which makes the analysis less attractive from an eiivironmental point of view, and surface fouling, which coimnonly occurs during the analysis of a complex solution matrix. Several methods have been reported for the improvement of the pre-concentration step [17,18]. The latter is, in fact. 

Fig. 5. Various current transients obtained by a time-of-fligbt method (a) nondispersive transport (b) dispersive transport and (c) analysis of disperse...

Mass Transport. Probably the most iavestigated physical phenomenon ia an electrode process is mass transfer ia the form of a limiting current. A limiting current density is that which is controlled by reactant supply to the electrode surface and not the appHed electrode potential (42). For a simple analysis usiag the limiting current characteristics of various correlations for flow conditions ia a parallel plate cell, see Reference 43. 

Charge Transport. Side reactions can occur if the current distribution (electrode potential) along an electrode is not uniform. The side reactions can take the form of unwanted by-product formation or localized corrosion of the electrode. The problem of current distribution is addressed by the analysis of charge transport ia cell design. The path of current flow ia a cell is dependent on cell geometry, activation overpotential, concentration overpotential, and conductivity of the electrolyte and electrodes. Three types of current distribution can be described (48) when these factors are analyzed, a nontrivial exercise even for simple geometries (11). 

The foregoing equations are coupled and are generally nonlinear no general solution exists. However, these equations serve as a starting point for most of the analysis that is relevant to electrophoretic transport in solutions and gels. Of course, the specific geometry and boundary conditions must be specified in order to solve a given problem. Boundary conditions for the electric field include specification of either (1) constant potential, (2) constant current, or (3) constant power. 

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. 

The electrolyte dropping electrode has found particular application in the study of ion transfer at the polarized ITIES, with an emphasis on analysis. A range of species have been detected amperometrically by measuring the transport-limited current ... 

---