Near-metal region, electrode-electrolyte

A major difference between electrochemistry performed at metal electrodes and that performed at semiconductor electrodes is that for a metal electrode, all the potential drop appears on the solution side of the metal-electrolyte interface, whereas for a semiconductor electrode, a portion of the potential drop occurs within the semiconductor material near the interface (within the so-called depletion or space-charge region, typically 10 nm to 1 pm thick). This additional built-in barrier to charge transfer at the interface means that the standard diagnostics for reversible electrochemical behavior are not applicable at a semiconductor electrode [ii]. 

A plane (m) is associated with an excess concentration of elections near the physical surface of the electrode, represented by a solid line. The inner Helmholtz plane (ihp) is associated with ions that are specifically adsorbed onto the metal surface. The outer Helmholtz plane (ohp) is the plane of closest approach for solvated ions that are free to move within the electrolyte. The ions within the electrolyte near the electrode surface contribute to a diffuse region of charge. The diffuse region of charge has a characteristic Debye length. 

The interaction between a metal in contact with a solution of its ions produces a local change in the concentration of the ions in solution near the metal surface. This causes charge neutrality not to be maintained in this region, which can result in causing the electrolyte surrounding the metal to be at a different electrical potential from the rest of the solution. Thus, a potential difference known as the halfcell potential is established between the metal and the bulk of the electrolyte. It is found that different characteristic potentials occur for different materials and different redox reactions of these materials. Some of these potentials are summarized in Table 4.2. These half-cell potentials can be important when using electrodes for low-frequency or dc measurements. 

Method numbers 2 and 3 are based on the assumption that the metal/liquid interphase and thus the polarization impedance is invariable. This is not always the case. Measuring on dry samples for instance implies poor control of the contact electrolyte. Also a sample may contain local regions of reduced conductivity near the electrode surface. The currents are then canalized with uneven current density at the metal surface (shielding effect). Electrode polarization impedance, in particular at low frequencies, is then dependent on the degree of shielding. An example of method 4 is Krizaj and Pecar (2012), who described such a method for removing the contribution from electrode polarization impedance on measured impedance data of a suspension of microcapsules. 

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