Rotating Disk Electrode Device

Direct spectroscopic analysis of liquids is possible using a rotating graphite disk as the lower electrode. The disk rotates slowly with its lower edge dipping into the solution to be analyzed. The liquid is carried upward 

FIGURE 5-17. Photograph of a rotating disk electrode unit [Courtesy Jarrell-Ash Division, Fisher Scientific Co.] 

Several other devices to use with arc-spark stands are available but are not so frequently used as the three that have been described. Some of these include controlled-atmosphere chambers, briquette holders, rotating platform assemblies, and water-cooled electrode holders. These devices are available and are very useful for special applications. 

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To get relevant information about active materials, the working electrode is made as similar as possible to the electrode of an operational device. However, current collectors are usually made with corrosion resistant materials, with good electronic conductivity, and no concern is taken about its relative mass. Materials such as gold, platinum, and vitreous carbon are commonly used. The active mass is usually tested in small amounts, mixed with electronically conducting materials, such as acetylene black, and a binder, such as poly vinylidene fluoride PVDF or polytetrafluoroethylene. The working electrode may be flat, with a 1 cm2 surface, for example, a rotating disk electrode (RDE), or a microcavity electrode, or any geometrical convenient electrode. 

The characteristics of laminar flow can allow mathematical prediction of the solution velocity and this has led to a range of hydrodynamic devices which use forced convection as a transport component under laminar flow conditions, examples include, the -> rotating disk electrode [i],-> wall jet electrode [ii], and channel flow cell (see -> flow cell). 

To carry out theoretical studies of oxidation-reduction reactions, it is often of interest to know how in Equation 2S-6 is affected by the hydrodynamics of the system. A common method for obtaining a rigorous description of the hydrodynamic flow of stirred solution is based on measurements made with a rotating disk electrode (RDE), such as the one illustrated in Figure 25-21 a and b. When the disk electrode is rotated rapidly, the flow pattern shown by the arrows in the figure is set up. At the surface of the disk, the liquid moves out horizontally from the center of the device, which produces an upward axial flow to replenish the displaced liquid. A rigorous treatment of the hydrodynamics is possible in this case" and leads to the Levich equation ... 

Since a fuel cell is an electrochemical device, electrochemical mefliods are deemed to play important roles in characterizing and evaluating the cell and its components such as the electrode, the membrane, and the catalyst. The most popular eleetroehemical characterization methods include potential step, potential sweep, potential cycling, rotating disk electrode, rotating ring-disk eleetrode, and impedance spectroscopy. Some techniques derived from these methods are also used for fuel cell characterization. 

What sort of instrumentation would be needed for electrochemical experiments A potentiometry experiment requires little more than a pH meter. A potentiostat or galvanostat can be used for the controlling potential or current in an experiment. In a coulometric procedure, a device to integrate the current (i.e., a coulometer) would also be needed. A hydrodynamic voltammetry [e.g., a rotating disk electrode (RDE)] experiment would require an electrode rotor (to spin the electrode at a precisely known rotation speed), and the rotating ring-disk or RRDE refinement (see below) would necessitate the use of a bipotentiostat so that the disk and ring potentials can be independently controlled. An ac impedance measurement involves the use of a sine-wave oscillator and... 

With its axisymmetric transport and current distribution, the rotating hemispherical electrode complements the rotating disk as a tool for studying electrode processes. Der-Tau Chin provides a valuable overview and summary of the fundamental theory and applications of this interesting device. 

Some electrochemical experiments, such as those involving rotating ring-disk electrodes and the scanning electrochemical microscope, require simultaneous control of two working interfaces. A device that will meet this demand is called a bipotentiostat. 

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