Metal-electrolyte interface diagnostics

While many of the standard electroanalytical techniques utilized with metal electrodes can be employed to characterize the semiconductor-electrolyte interface, one must be careful not to interpret the semiconductor response in terms of the standard diagnostics employed with metal electrodes. Fundamental to our understanding of the metal-electrolyte interface is the assumption that all potential applied to the back side of a metal electrode will appear at the metal electrode surface. That is, in the case of a metal electrode, a potential drop only appears on the solution side of the interface (i.e., via the electrode double layer and the bulk electrolyte resistance). This is not the case when a semiconductor is employed. If the semiconductor responds in an ideal manner, the potential applied to the back side of the electrode will be dropped across the internal electrode-electrolyte interface. This has two implications (1) the potential applied to a semiconducting electrode does not control the electrochemistry, and (2) in most cases there exists a built-in barrier to charge transfer at the semiconductor-electrolyte interface, so that, electrochemical reversible behavior can never exist. In order to understand the radically different response of a semiconductor to an applied external potential, one must explore the solid-state band structure of the semiconductor. This topic is treated at an introductory level in References 1 and 2. A more complete discussion can be found in References 3, 4, 5, and 6, along with a detailed review of the photoelectrochemical response of a wide variety of inorganic semiconducting materials. 

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]. 

Zhuiykoy S. (2005) Sensors measuring oxygen activity in melts Development of impedance method for in-situ" diagnostics and control electrolyte/liquid-metal electrode interface. Ionics, 11, 352-61. 

IMPEDANCE METHOD FOR THE ANALYSIS OF IN-SITU DIAGNOSTICS AND THE CONTROL OF AN ELECTROLYTE/LIQUID-METAL ELECTRODE INTERFACE... 

The results of the present work may be applicable for diagnostics of oxygen sensors at more complicated applications, such as measurement of oxygen activity in liquid sodium, lithium, or lead-bismuth heat carriers for atomic power plants. Corrosion and mass transfer in nonisothermal lead-bismuth circuits with temperatures of a heat carrier of 300-500°C do usually occur at a concentration of dissolved O2 of 10 - 10 mass %. The proposed impedance method is developed for determining the level and the character of polarization at the electrolyte-electrode interface, which ensures a continuous oxide protection of materials against corrosion by means of zirconia sensors in all tanperature regimes of exploitation of liquid-metal circuits. 

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