Ultra-high-vacuum electrochemical techniques

In this chapter we introduce and discuss a number of concepts that are commonly used in the electrochemical literature and in the remainder of this book. In particular we will illuminate the relation of electrochemical concepts to those used in related disciplines. Electrochemistry has much in common with surface science, which is the study of solid surfaces in contact with a gas phase or, more commonly, with ultra-high vacuum (uhv). A number of surface science techniques has been applied to electrochemical interfaces with great success. Conversely, surface scientists have become attracted to electrochemistry because the electrode charge (or equivalently the potential) is a useful variable which cannot be well controlled for surfaces in uhv. This has led to a laudable attempt to use similar terminologies for these two related sciences, and to introduce the concepts of the absolute scale of electrochemical potentials and the Fermi level of a redox reaction into electrochemistry. Unfortunately, there is some confusion of these terms in the literature, even though they are quite simple. 

The application of ultra-high vacuum surface spectroscopic methods coupled to electrochemical techniques t21-241 have provided valuable information on surface structure/reactivity correlations. These determinations, however, are performed ex-situ and thus raise important concerns as to their applicability to electrocatalytic systems, especially when very active intermediates are involved. 

We begin with the most routine characterization methods—electrochemical methods. We then discuss various instrumental methods of analysis. Such instrumental methods can be divided into two groups ex situ methods and in situ methods. In situ means that the film on the electrode surface can be analyzed while the film is emersed in an electrolyte solution and while electrochemical reactions are occurring on/in the film. Ex situ means that the film-coated electrode must be removed from the electrolyte solution before the analysis. This is because most ex situ methods are ultra-high-vacuum techniques. Examples include x-ray photoelectron spectroscopy [37], secondary-ion mass spectrometry [38,39], and scanning or transmission electron microscopies [40]. Because ex situ methods are now part of the classical electrochemical literature, we review only in situ methods here. 

Using ultra-high-vacuum (UHV) surface and electrochemical techniques, three interfacial systems have been studied, including KOH/Ni(lll), COj/K/AgClll) and THF/Li interfaces. All three studies were associated with electrochemical energy storage... 

The alternative route to the solution of the problem of surface structure and composition is the one to be described in this chapter as the ex-situ route. By transferring the electrode from the electrochemical cell into the ultra-high vacuum (UHV), a vast range of techniques developed in the past two decades by surface scientists becomes available, notably those which depend on electron spectroscopy or electron diffraction. The central difficulty then becomes that of verifying that any change in the electrode... 

The combination of ultra-high vacuum (UHV) surface science techniques with electrochemical methods of electrode surface characterization (voltammetry, chrono-coulometry) resulted in a spectacular progress in the investigation and molecular level understanding of some processes occurring at electrode/solution interfaces. Evidently the experimental approach strongly depends on the aims of the investigation and the systems to be studied. 

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