Technique, electrochemical experiment

DC (Direct current) techniques — Electrochemical experiments where the applied potential (in -> potentio-static techniques) or current (in -> galvanostatic tech-... 

The main technique employed for in situ electrochemical studies on the nanometer scale is the Scanning Tunneling Microscope (STM), invented in 1982 by Binnig and Rohrer [62] and combined a little later with a potentiostat to allow electrochemical experiments [63]. The principle of its operation is remarkably simple, a typical simplified circuit being shown in Figure 6.2-2. 

The most appropriate experimental procedure is to treat the metal in UHV, controlling the state of the surface with spectroscopic techniques (low-energy electron diffraction, LEED atomic emission spectroscopy, AES), followed by rapid and protected transfer into the electrochemical cell. This assemblage is definitely appropriate for comparing UHV and electrochemical experiments. However, the effect of the contact with the solution must always be checked, possibly with a backward transfer. These aspects are discussed in further detail for specific metals later on. 

Electrochemical processes are always heterogeneous and confined to the electrochemical interface between a solid electrode and a liquid electrolyte (in this chapter always aqueous). The knowledge of the actual composition of the electrode surface, of its electronic and geometric structure, is of particular importance when interpreting electrochemical experiments. This information cannot be obtained by classical electrochemical techniques. Monitoring the surface composition before, during and after electrochemical reactions will support the mechanism derived for the process. This is of course true for any surface sensitive spectroscopy. Each technique, however, has its own spectrum of information and only a combination of different surface spectroscopies and electrochemical experiments will come up with an almost complete picture of the electrochemical interface. XPS is just one of these techniques. 

While characterization of the electrode prior to use is a prerequisite for a reliable correlation between electrochemical behaviour and material properties, the understanding of electrochemical reaction mechanisms requires the analysis of the electrode surface during or after a controlled electrochemical experiment. Due to the ex situ character of photoelectron spectroscopy, this technique can only be applied to the emersed electrode, after the electrochemical experiment. The fact that ex situ measurements after emersion of the electrode are meaningful and still reflect the situation at the solid liquid interface has been discussed in Section 2.7. 

Although the instrumental techniques described here give detailed mechanistic information, they do not provide an insight into the structure of intermediates. If we, however, combine electrochemical and spectroscopic methods, this is advantageously accomplished (spectroelectrochemistry) [73]. Various spectroscopies have been coupled with electrochemical experiments, among them ESR [74], optical [75], and NMR spectroscopy [76, 77], as well as mass spectrometry [78, 79]. 

For many years, intramolecular reactions such as conformational changes, bond cleavage, bond formation, and valence isomerizations have been observed only when hydrocarbons were reduced with alkali metals in ethereal solvents. In most electrochemical experiments, these reactions were dominated by the electrophilic processes already described. However, progress in experimental techniques [8, 9, 27-29] has made these reactions accessible to electroanalytical investigations, providing new mechanistic insight. 

The techniques used in studying interfaces can be classified in two categories in situ techniques and ex situ techniques. In situ methods are those where a surface is probed by one or several techniques while immersed in solution and under potential control. In contrast, in ex situ methods, an electrochemical experiment is first carried out. Then the electrode is removed from solution and examined by one or several spectroscopic techniques, which generally require ultrahigh vacuum (UHV) conditions. Figures 6.10 and 6.11 show some of the most common ex situ and in situ techniques applicable to the study of the metal/solution interface. 

Many species dissolved in solution exhibit a tendency to adsorb on the electrode surface, a phenomenon that can markedly affect the results of electrochemical experiments. For example, the course of an electrode reaction can be altered, or the rate of electron exchange enhanced or virtually stopped. Adsorption is responsible for much unusual electrochemical behavior and is frequently blamed for unexplained results. Thus it is important for the chemist using electrochemical techniques to recognize phenomena that are attributable to adsorption and to realize which techniques are useful for studying adsorbed species. 

The objective of most electrochemical experiments is to allow the experimenter to investigate one or more of three types of parameters (1) the concentration and identity of one or more solution components, (2) the kinetics of chemical, charge transfer, or adsorption processes, and (3) the nature of the double-layer capacitance associated with the electrode-solution interface. Historically, most small-amplitude techniques have been developed in an attempt to allow an easier separation of the contributions of these basic parameters. 

The simulation of other electrochemical experiments will require different electrode boundary conditions. The simulation of potential-step Nernstian behavior will require that the ratio of reactant and product concentrations at the electrode surface be a fixed function of electrode potential. In the simulation of voltammetry, this ratio is no longer fixed it is a function of time. Chrono-potentiometry may be simulated by fixing the slope of the concentration profile in the vicinity of the electrode surface according to the magnitude of the constant current passed. These other techniques are discussed later a model for diffusion-limited semi-infinite linear diffusion is developed immediately. 

Nuclear magnetic resonance (NMR) is another spectroscopic technique that has many formal similarities to EPR. It is widely used for product analysis, as in studies leading to the development of electrosynthetic methods. The intrinsic sensitivity of NMR is several orders of magnitude lower than that of EPR, making simultaneous NMR-electrochemical experiments unattractive. 

The most convenient means of making time-resolved SH measurements on metallic surfaces is to use a cw laser as a continuous monitor of the surface during a transient event. Unfortunately, in the absence of optical enhancements, the signal levels are so low for most electrochemical systems that this route is unattractive. A more viable alternative is to use a cw mode-locked laser which offers the necessary high peak powers and the high repetition rate. The experimental time resolution is typically 12 nsec, which is the time between pulses. A Q-switched Nd YAG provides 30 to 100 msec resolution unless the repetition rate is externally controlled. The electrochemical experiments done to date have involved the application of a fast potential step with the surface response to this perturbation followed by SHG [54, 55,116, 117]. Since the optical technique is instantaneous in nature, one has the potential to obtain a clearer picture than that obtained by the current transient. The experiments have also been applied to multistep processes which are difficult to understand by simple current analysis [54, 117]. 

As the later chapters indicate, a given question concerning a chemical system usually can be answered by any one of several electrochemical techniques. However, experience has demonstrated that there is a most convenient or reliable method for a specific kind of data. For example, polarography with a static or dropping-mercury electrode remains the most reliable electrochemical method for the quantitative determination of trace-metal ion concentrations. This is true for two reasons (1) the reproducibility of the dropping-mercury electrode is unsurpassed and (2) the reference literature for analysis by polarography surpasses that for any other electrochemical method by at least an order of magnitude. 

Scanning electrochemical microscopy (SECM the same abbreviation is also used for the device, i.e., the microscope) is often compared (and sometimes confused) with scanning tunneling microscopy (STM), which was pioneered by Binning and Rohrer in the early 1980s [1]. While both techniques make use of a mobile conductive microprobe, their principles and capabilities are totally different. The most widely used SECM probes are micrometer-sized ampero-metric ultramicroelectrodes (UMEs), which were introduced by Wightman and co-workers 1980 [2]. They are suitable for quantitative electrochemical experiments, and the well-developed theory is available for data analysis. Several groups employed small and mobile electrochemical probes to make measurements within the diffusion layer [3], to examine and modify electrode surfaces [4, 5], However, the SECM technique, as we know it, only became possible after the introduction of the feedback concept [6, 7],... 

The second part of the book discusses ways in which information concerning electrode processes can be obtained experimentally, and the analysis of these results. Chapter 7 presents some of the important requirements in setting up electrochemical experiments. In Chapters 8—11, the theory and practice of different types of technique are presented hydrodynamic electrodes, using forced convection to increase mass transport and increase reproducibility linear sweep, step and pulse, and impedance methods respectively. Finally in Chapter 12, we give an idea of the vast range of surface analysis techniques that can be employed to aid in investigating electrode processes, some of which can be used in situ, together with photochemical effects on electrode reactions— photoelectrochemistry. 

At the present time, with the development of new electrochemical methods and new electrode materials, a large amount of research has been carried out in the electrochemistry of proteins, enzymes, and cellular components. Nevertheless, much remains to be done. Electrochemical experiments, in conjunction with other techniques, such as spectroscopy, may give a better answer to these questions. 

The electrochemical quartz crystal microbalance (EQCM) has emerged as a very powerful in situ technique to complement electrochemical experiments [3-5]. Nomura and Okuhara [15] first used the quartz crystal microbalance (QCM) to detect mass changes at a metal coated quartz resonator immersed in electrolyte during electrochemical experiments. 

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