Bipotentiostatic control

Marken F, Compton RG (1996) Electrochemistry in the presence of ultrasound the need for bipotentiostatic control in sonovoltammetric experiments. Ultrason Sonochem 3 S131 —S134... 

The first STM experiments were performed under UHV conditions, and so the bias potential was simply applied as a difference across the tip and sample. However, introducing an electrolyte above the sample brought with it some particular problems. It is no longer sufficient simply to apply a bias voltage equal to the potential difference between tip and sample as this means that the potentials of the tip and sample are undefined with respect to any fixed reference, a wholly undesirable situation. Consequently, modern electrochemical STM systems operate under bipotentiostatic control with the tip and sample controlled and monitored independently with respect to the reference electrode. The bias potential is then still given by (Fs — FT), but VT and Fs are now potentials with respect to the reference electrode. 

A bipotentiostat controls the potential of two working electrodes independently, and measures the current that they pass. A typical circuit is shown in Fig. 7.8. Bipotentiostats are necessary in performing studies with double hydrodynamic electrodes (Sections 8.5-8.7). 

SECM instruments suitable for imaging require a PC equipped with an interface board to synchronize acquisition of the electrochemical data with the movement of the tip. Building an SECM for kinetic experiments at fixed tip position or approach curve measurements is relatively easy, but fairly sophisticated software and some electronic work is necessary to construct a computer-controlled apparatus for imaging applications. Details on the construction of SECM instruments can be found elsewhere [6, 13-18, 53, 55]. An SECM is now available commercially from CH Instruments, Inc. (Austin, TX, USA). The instrument employs piezoelectric actuators, a three-axis stage, and a bipotentiostat controlled by an external PC under a 32-bit Windows environment. Various standard electrochemical techniques are incorporated along with SECM imaging, approach curves, and the modes described in Sect. 3.3.I.I. 

Fig. 2 Schematic drawing of (A) a circuit with two working electrodes and one reference electrode and two independent potential sources and (B) a circuit with two working electrodes bipotentiostatically controlled.

Electrochemical experiments with two electrodes, each with independently controlled potential are conveniently performed with bipotentiostatic control (4-electrode ) or for low current systems with two independent current/voltage sources (3-electrode , see Fig. 2). The latter type of experiment offers the benefit of very low noise (no feedback) and therefore the... 

A diverse range of bipotentiostat controlled generator-collector-type electrochemical experiments have been developed over the last 60 years. ... 

Several designs for STM electrochemical cells have appeared in the literature [M]- hr addition to an airtight liquid cell and the tip insulation mentioned above, other desirable features include the incorporation of a reference electrode (e.g. Ag/AgCl in saturated KCl) and a bipotentiostat arrangement, which allows the independent control of the two working electrodes (i.e. tip and substrate) [ ] (figure BL19.11). 

The apparatus consists of a tip-position controller, an electrochemical cell with tip, substrate, counter and reference electrodes, a bipotentiostat and a data-acquisition system. The microelectrode tip is held on a piezoelectric pusher, which is mounted on an inchwomi-translator-driven x-y-z tliree-axis stage. This assembly enables the positioning of the tip electrode above the substrate by movement of the inchwomi translator or by application of a high voltage to the pusher via an amplifier. The substrate is attached to the bottom of the electrochemical cell, which is mounted on a vibration-free table [, and ]. A number... 

In the dual-electrode techniques, the potential of each electrode is controlled with a bipotentiostat so that a small constant potential difference is maintained across the polymer film as its potential is slowly scanned, relative to a reference electrode. Figure 10 shows the results of this type of experiment for poly(3-methylthiophene) in SO20).37... 

For in situ investigations of electrode surfaces, that is, for the study of electrodes in an electrochemical environment and under potential control, the metal tip inevitably also becomes immersed into the electrolyte, commonly an aqueous solution. As a consequence, electrochemical processes will occur at the tip/solution interface as well, giving rise to an electric current at the tip that is superimposed on the tunnel current and hence will cause the feedback circuit and therefore the imaging process to malfunction. The STM tip nolens volens becomes a fourth electrode in our system that needs to be potential controlled like our sample by a bipotentiostat. A schematic diagram of such an electric circuit, employed to combine electrochemical studies with electron tunneling between tip and sample, is provided in Figure 5.4. To reduce the electrochemical current at the tip/solution... 

In addition, a bipotentiostat is used to control the tip potential with respect to the surface and independent of control of the surface potential with respect to the reference electrode. The tip potential E, is given by E, = Eg + E , where Eg is the bias potential that generates the tnnneling current between tip and surface, and E (a vital variable not typical of other applications of STM and AFM) is the potential of the surface relative to the reference electrode. 

Three-electrode control systems are widely available in the market and there are also four-electrode systems for double working electrodes. The construction is either integral or modular. It is perfectly possible to construct the necessary electronics in-house and, in this case, modular construction is suggested as being more flexible. Operational amplifiers and other components of high quality should be used, particularly for kinetic applications. The elements of a bipotentiostat (independent control of two working electrodes) and a galvanostat are described in ref. 139. 

The scanning electrochemical microscope (SECM) consisted of a positioning system from Marzhauser (Wetzlar, Germany), a bipotentiostat CH701 (CH Instruments, Austin, TX, USA) and a homemade control software. 

Figure 1 shows a schematic diagram of the basic SECM instrument employing an amperometric microprobe. An UME tip is attached to a three-dimensional (3D) piezo positioner controlled by a computer, which is also used for data acquisition. A bipotentiostat (i.e., a four-electrode potentiostat) controls the potentials of the tip and/or the substrate versus the reference electrode and... 

Many SECM experiments require biasing the substrate. A bipotentiostat in Fig. 1 is used to control both the tip and substrate potentials. Unless transient measurements are made, the response of the bipotentiostat does not have to be fast. More importantly, it should be capable of measuring a broad range of current responses a picoamp scale (or even sub-pA) tip current and a much higher current at a macroscopic substrate. For this reason, it is convenient to have several choices of preamplifiers/current-to-voltage transducers. 

Fig. 7.8. Bipotentiostat circuit for control of the potential of two working electrodes. All resistances are equal, except RD and RK which are variable.

Bipotentiostat — An instrument that can control the potential of two independent -> working electrodes. A - reference electrode and an -> auxiliary electrode are also needed therefore the cell is of the four-electrode type. Bipotentiostats are most often employed in electrochemical work with rotating ring-disk electrodes and scanning electrochemical microscopes. They are also needed for monitoring the electrode-reaction products with probe electrodes that are independently polarized. All major producers of electrochemical equipment offer this type of potentiostat. The instruments that can control the potential of more than two working electrodes are called multipotentiostats. 

An often-adopted sonovoltammetric design is that shown in Fig. 35 built around a conventional three-electrode cell and which allows the ultrasound intensity and the distance between the horn and electrode to be continuously varied at a fixed ultrasound frequency of typically 20 kHz. This arrangement is much less sensitive to the shape and dimensions of the electrochemical cell than when a sonic bath is utilized. A further and important point of contrast is that the direct contact of the (metallic) horn with the electrochemical system may dictate the use of a bipotentiostat to control its electrical potential relative to that of the reference electrode (Marken and Compton, 1996). Alternatively, the horn may be electrically isolated (Huck, 1987 Klima et al., 1994). A significant merit of the design shown in Fig. 35 is that the mass transport characteristics may be empirically but reliably established. It is to this essential topic we next turn. 

Charge-transfer processes occurring across the liquid-liquid interface have also been studied by EPR. The Galvani potential difference between the two immiscible solvents, water and 1,2-dichloroethane (DCE), was controlled electrochemically by means of a bipotentiostat. The water phase contained potassium ferrocyanide, which, in the DCE phase, by electrochemical polarization of the interface, can reduce a compound such as tetracyanoquinodimethane to its radical anion or oxidize a compound such as tetrathiafulvalene to its cation radical. Both radicals were detected by EPR spectroscopy [79]. 

A bipotentiostat is simply a potentiostatic circuit designed to allow simultaneous potential control of two working electrodes in an electrochemical cell. A simplified schematic of a bipotentiostat circuit is illustrated in Figure 4. Note that connections in this schematic are always indicated by a dot at the intersection, and crossing lines without a dot are not connected. Part of this circuit is identical to the potentiostat circuit, with the addition of am-... 

As an alternative to a bipotentiostat, two separate potentiostats can be used to control two working electrodes. For conventional potentiostats, both working electrodes would be held at circuit common, which requires that one of the potentiostats be operated in a floating mode. With potentiostats that allow a floating working electrode, the circuit common can be shared at the reference electrode and no isolation is required. Whether to use a single bipotentiostat or dual potentiostats in a SECM experiment is up to... 

The model 900 SECM (CH Instruments) is the first commercial instrument specifically designed for SECM. Although STM instruments equipped with electrochemical accessories can be adapted for some SECM experiments, they cannot replace a general purpose SECM instrument. This section will describe the features of the model 900. The model 900 includes the cell and probe positioner illustrated in Figure 5 as well as a computer, motor controller, and bipotentiostat (35). 

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