Bi-potentiostat

A Gamry electrochemical measurements system and a Pine Bi-Potentiostat were used to study the experimental decomposition potential and current response to the applied voltage. The experimental variables were electrolyte flow rate and temperature. Linear sweep voltammetry (LSV) technique was the main method used to study the electrolytic processes. 

If the tunnel junction of Fig. 1 a is simply immersed in an electrolyte, the polarization between the tip and the sample will promote an electrolysis. A bi-potentiostat is necessary to ensure real tunneling between the sample and the tip. Such a device, classically used in electrochemistry, enables to split the tunnel junction into two sol-id/liquid interfaces, independently polarized against a reference of potential (Fig. 1 b). Using this configuration, also referred to as the four-electrode configuration and introduced very early by several groups, it is possible to avoid any electrochemical transfer between the sample and the tip [25,26]. The reference potential is an electrode whose potential is well defined and constant with respect to the vacuum level. The sample is biased against the reference electrode to monitor reactions at the surface, just as in a classical electrochemical cell. The tip potential is adjusted... 

The STM/AFM unit (Unversial SPC 400 SPM, Park Scifentific Instruments) consisted of an electronic unit which provided both the scan control for scanning the tip over the smface and an electronic feedback loop maintaining a constant working distance between tip sample. Additionally a four-electrode bi-potentiostat that was coimected to the control unit was capable of setting the potentials of both tip and substrate... 

Figure 9.35 SECM set-up described in reference (71) for the determination of spatial distribution of active sites on semiconductor electrodes. Substrate potential (EJ and the tip potential (Ep) were controlled independently by bi-potentiostat. Redox species generated on substrate were collected using UME tip, which is raster over surface by x-y piezo. The contrast is generated by the tip current ij measured at (x, y) coordinates. Reprinted with permission from reference (73). Cop5right 1998, American Chemical Society.

Cl was in form of NaCl (Aldrich, 99.99%). The electropolished C x hkt) surface was immersed at a potential of -0.2 V. A Pine Instruments bi-potentiostat (model AFRDE4) was used for the potentiodynamic measurements data were acquired digitally on an IBM PC using Lab View for Windows. A PAR EG G Model 173 potentiostat was used for the electropolishing experiments. 

ANanoScope I (Digital Instruments, CA) was modified to carry out the electrochemical tunneling spectroscopy (ETS) measurement. ETS measurement was carried out by monitoring the tunneling current under a constant tip-sample distance. Details of the present ETS system were desaibed elsewhere (7). The potential of the tip and the sample were controlled independently by a home-built bi-potentiostat (8) with respect to a common Pd-H reference electrode which was charged sufficiently at a current density of 10 fcA cm before each measurement Potentials were presented with respect an Ag/AgCl electrode in this article. The counter electrode was a Pt wire. The tip was a simply cut Pt wire (0.3 mm) insulated with apiezon wax. 

The two sets of ultramicrobands can also be addressed differently by using a bi-potentiostat to enable a generator-collector mode. One set of electrodes is set to a potential (or current) where a species is generated at the electrode, e.g. by reduction, and the other set of electrodes is set to a potential (or current) where the generated species is consumed, e.g. by oxidation. The generator-collector mode is illustrated in Fig. 12.17c. 

Both processes, mono-potentiostatic and bi-potentiostatic, are time dependent and the applied potential driving the oxidation was shown to be crucial in determining the overall rate (see Fig. 18A,B). This suggested that potential gradients into the trench (into the oil phase) and in particular at the triple phase boundary exist and that the oversimplified diffusion picture (see Fig. 18C) is not sufficient to explain reactivity patterns. This type of dual-plate oil-trench electrode system could in future be further developed with selectivity to specific ions or as an anti-oxidant level or oil-quality monitoring tool. " Particularly interesting will be applications of oil-trench electrode systems exploiting smaller nano-gap devices. 

The fact that the potentials of the two electrodes can be controlled independently makes the RRDE a very powerful device of detecting and exploring the behavior of electroactive intermediates or products of electrode reactions, and when properly operated with the aid of a bi-potentiostat, it can effectively be used even for the detection of short-lived intermediates. 

In the experiments reported in [7, 14-15,16] TINE AFE7R8 RRDE tips were used with a bi-potentiostatic setup of XPot devices manufactured by ZAHNER-Elektrik, Kronach, Germany both the ring and the disk were made of polycrystalline gold. The gap size between the ring and the disk was 178 pm and the collection efficiency of the RRDE was 22%. 

In order to operate an RRDE, one needs a four-electrode potentiostat, often referred to also as a "bi-potentiostat . Now, we began this book with simple two-electrode cells and proceeded to the three-electrode configuration. How does the fourth electrode fit into this scheme. The best way to xmderstand this is to view both the ring and the disc as working electrodes, with common counter and reference electrodes. The important point is that the four-electrode potentiostat controls the potentials of the disc and of the ring with respect to the reference electrode independently and measures the current going through each of them separately. 

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