Working electrode potentiostatic circuit

Apparatus. The source of current is a potentiostat which is used in conjunction with a reference electrode (commonly a saturated calomel electrode) to control the potential of the working electrode. The circuit will be essentially that shown in Fig. 12.2(a) but with the addition of the integrator or of a coulometer. 

The most convenient and reliable electrical biasing method for use with a hydrated SPE cell has been shown to be a three electrode potentiostatic circuit which maintains the sensing electrode at a predetermined potential vs. a stable reference (1 >3.>j0e The most reversible reference is a Pt/Hp, H+, static or dynamic. In practical instruments, however, good accuracy and convenience are achieved using a large surface area platinoid metal black/air (Op). All work reported in this study utilized the air reference which has a potential of approximately +1.05 V vs. a standard hydrogen electrode (SHE). For convenience, all potentials reported are vs. the SHE ... 

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... 

Figure 1. Schematic diagram illustrating Langmuir-Blodgett transfer under potentiostatic conditions. A gold-coated glass slide is acting as a working electrode in a three-electrode potentiostatic circuit. Below, an inset shows the pattern of the vapor deposited gold film. The central rectangular area (A = 0.20 cm ) is coated with an LB monolayer as the substrate is withdrawn from the subphase. The two lines mark the initial and the final position of the water meniscus in the LB experiments.

Figure 6.2-1 Simplified circuit of a potentiostat with working electrode (WE) on ground.

Fig. 19.36 Basic circuit for a poiemiostat. (a) Basic circuit for a potentiostat and electrochemical cell, (b) Equivalent circuit, (c) Circuit of a basic potentiostat. A.E. is the auxiliary electrode, R.E. the reference electrode and W.E. the working electrode (6 and c are from Polen-tiostat and its Applications by J. A. von Fraunhofer and C. H. Banks, Butlerworths (1972))...

The electrode will of course be incorporated in a circuit similar to that previously described in which a potentiostat controls the potential of the working electrode and may also provide a counter-current facility to nullify the background current an electronic integrator will also be included. 

The potentiostat has a three-electrode system a reference electrode, generally a saturated calomel electrode (SCE) a platinum counter, or amdliary, electrode through which current flows to complete the circuit and a working electrode that is a sample of interest (Fig. 25-10). The potentiostat is an instrument that allows control of the potential, either holding constant at a given potential, stepping from potential to potential, or changing the potential anodically or cathodically at some linear rate. 

FIGURE 2.45. Equivalent circuit for the cell and instrument. WE, RE, and CE, working, reference, and counter electrodes, respectively iph, photocurrent ij/, double-layer charging current Q, double-layer differential capacitance Rc, Ru, cell compensated (by the potentiostat) and uncompensated resistances, respectively Rs, sampling resistance RP, potentiostat resistance E, potential difference imposed by the potentiostat between the reference and working electrodes Vpu, photo-potential as measured across the sampling resistor. Adapted from Figure 1 of reference 51, with permission from Elsevier. 

Figure 6.2-1 Simplified circuit of a potentiostat with working electrode (WE) on ground. Reference electrode (RE) and potentiostatic setpoint are fed to the inverting and noninverting input of an operational amplifier. The counter-electrode (CE) is connected to the output of the operational amplifier. I(EC) electrochemical current.

It is reasonable to ask at this point Are there other approaches to reach stability with grace in true potentiostatic circuits The answer is indeed affirmative, but unfortunately with qualifications. One technique is discontinuous control of cell potential. It is not a new approach it was, in fact, the method used in the very first electronic potentiostat by Hickling in 1942. The principle is quite simple Current pulses are applied to the counterelectrode so that the desired potential is maintained between reference and working electrode. Since the potential can be measured between pulses, there is no iR drop. Cell potential is not steady it depends on the sensitivity of the comparator circuit and the rate at which current demand can be met. 

An equivalent circuit of the three-electrode cell discussed in Chapters 6 and 7 is illustrated in Figure 9.1. In this simple model, Rr is the resistance of the reference electrode (including the resistance of a reference electrode probe, i.e., salt bridge), Rc is the resistance between the reference probe tip and the auxiliary electrode (which is compensated for by the potentiostat), Ru is the uncompensated resistance between the reference probe and the working-electrode interphase (Rt is the total cell resistance between the auxiliary and working electrodes and is equal to the sum of Rc and Ru), Cdl is the double-layer capacitance of the working-electrode interface, and Zf is the faradaic impedance of the electrode reaction. 

Ohmic drop in three-electrode circuits. In modem coulometry and voltammetry the use of a potentiostat and a three-electrode configuration is the routine practice. The three electrodes are usually called the working, reference, and counter (or auxiliary) electrodes (see Figure 5.2). The cell current passes between the working electrode immersed in the test solution and the counter electrode, which may be in the test solution but is usually isolated from it by a single- or double-junction glass frit. 

The external leads from the potentiostat to the electrodes may also contribute significant resistance and capacitance that must be taken into account if the cell currents are large and if fast response is desired. Most metallic working electrodes will have very low resistance, but a typical diopping-mercury electrode (DME) may have a resistance as large as 100 Q because the mercury-filled lumen of the capillary is so small (— 0.005-cm diameter). This resistance makes a contribution to the total cell resistance and to the uncompensated resistance in a three-electrode circuit. 

Usually, that is except under open circuit conditions, the working electrode is embedded in an electric circuit, which imposes a constraint on WE(f) and thus defines its value with respect to some potential scale (in our case Eq. (14b)). Moreover, WE will in general evolve in time and thus the external constraint directly influences the dynamics of the system. The two most important operation modes of electrochemical systems are the potentiostatic and the galvanostatic operation. 

Fig. 7.7. Potentiostat circuit for control of working electrode potential. All resistances are equal, except / D which is variable.

Potential or current step transients seem to be more appropriate for kinetic studies since the initial and boundary conditions of the experiment are better defined unlike linear scan or cyclic voltammetry where time and potential are convoluted. The time resolution of the EQCM is limited in this case by the measurement of the resonant frequency. There are different methods to measure the crystal resonance frequency. In the simplest approach, the Miller oscillator or similar circuit tuned to one of the crystal resonance frequencies may be used and the frequency can be measured directly with a frequency meter [18]. This simple experimental device can be easily built, but has a poor resolution which is inversely proportional to the measurement time for instance for an accuracy of 1 Hz, a gate time of 1 second is needed, and for 0.1 Hz the measurement lasts as long as 10 seconds minimum to achieve the same accuracy. An advantage of the Miller oscillator is that the crystal electrode is grounded and can be used as the working electrode with a hard ground potentiostat with no conflict between the high ac circuit and the dc electrochemical circuit. 

Potentiostat — A potentiostat is an electronic amplifier which controls the potential drop between an electrode (the -> working electrode, (WE)) and the - electrolyte. The WE is normally connected to ground potential the potential of the electrolyte is measured by a special probe, the -> reference electrode (RE). Effects of the -> counter electrode (CE), (e.g., potential drop at the CE electrolyte interface) and the electrolyte (esp. the solution resistance) can be suppressed by this technique. Potentiostats are based on -> operational amplifiers (OPA) the simplest circuit is given in Fig (a). The difference between the desired potential Ureference electrode potential Ure is amplified, resulting in currents via counter and working electrode until this difference becomes (almost) zero. 

Potentiostatic circuit — The potentiostatic circuit consists of the electrochemical cell with three electrodes (-> working electrode WE, reference electrode RE, and -> counter electrode CE) and a special electronic amplifier, the potentiostat. 

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