Instrumentation potentiostat

The instrument used to perform cyclic voltammetry and potential step experiments is a potentiostat, which controls the voltage and measures the current. The potentiostat maintains the potential of the working electrode at a desired value with respect to the potential of the reference electrode. The current flows between the working and the counter electrodes in response to the potential of the working electrode. 

Ref reference electrode C counter electrode W working electrode 

An electrometer measures the voltage difference between the reference and the working electrodes. An electrometer has extremely high input impedance so that the input current is nearly zero, which enables the reference electrode to keep a 

When the electronie circuit is slightly modified so that the current is eontrolled and the corresponding potential of the working electrode is measured, it beeomes a galvanostat. 

Pt is the most important catalyst material for PEMFCs, DMFCs, and PAFCs. Due to the relatively low operating temperatures of these fuel eells (from room temperature to about 200 °C), the reaction kinetics is slow, especially at the eathode for the ORR. Therefore, Pt and its alloys that have shown the highest eatalytic activity in these fuel cells have to be used as the catalysts. 

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The DBMS setup and experimental procedures used in this study were the same as described in more detail elsewhere [Jusys et al., 2001]. Briefly, the DBMS setup consisted of two differentially pumped chambers, a Balzers QMS 112 quadrupole mass spectrometer (MS), a Pine Instruments potentiostat, and a computerized data acquisition system. 

Most of the techniques employed can be traced back to polarography, which was already in use in 1925, to determine the concentrations of organic molecules [3]. Technical developments in instrumentation (potentiostats) [4], the use of nonaque-ous electrolytes [5], and the digital control of experiments [6] led to the spread of electroanalytical techniques. For example, cyclic voltammograms are frequently and routinely used today to define the redox... 

Note that this equation can also be used concerning the whole instrumentation - potentiostat - which also has a time constant, r = RC.)... 

It is possible to represent the entire electrochemical system including the instrumentation (potentiostat, etc.) as a single electrical circuit. The solution is usually spatially discretized into a network of resistance elements (see for example Coles et al., 1996). Double-layer charging can also be incorporated into these models by defining each element to contain a capacitor as well as a resistor. 

Developments in microprocessor-based instrumentation completely changed the situation. Nowadays, nearly all commercial instruments - potentiostats and galvanos-tats - are digitally based, which means that the programming of an almost infinite variety of step and pulse waveforms has become relatively easy to carry out. This opens up a multitude of exciting possibilities to the experimentalist, since he can adapt the applied waveforms to the kinetics and mechanism of the electrode reaction under study, so long as there... 

Electrochemical instruments (potentiostat) allow separation of the anodic and the cathodic reactions, the anodic reaction occur-... 

Time, Cost, and Equipment Controlled-potential coulometry is a relatively time-consuming analysis, with a typical analysis requiring 30-60 min. Coulometric titrations, on the other hand, require only a few minutes and are easily adapted for automated analysis. Commercial instrumentation for both controlled-potential and controlled-current coulometry is available and is relatively inexpensive. Low-cost potentiostats and constant-current sources are available for less than 1000. 

The potentiostatic technique discussed here involves the polarisation of a metal electrode at a series of predetermined constant potentials. Potentio-stats have been used in analytical chemistry for some time Hickling was the first to describe a mechanically controlled instrument and Roberts was the first to describe an electronically controlled instrument. Greene has discussed manual instruments and basic instrument requirements. 

Instruments very suitable for corrosion work are readily available, with several different models produced commercially. Although most, if not all, of the available potentiostats are properly designed, it should be kept in mind that corrosion studies require the instrument to have a low internal resistance and to react quickly to changes of potential of the working electrode. 

An ASTM recommended practice (A Standard Reference Method for Making Potentiostatic and Potentiodynamic Anodic Polarisation Measurements, G5 1972) has been issued. It provides a means of checking experimental technique and instrumentation using a specimen from a single heat of AISI Type 430 stainless steel, which is available from ASTM. ... 

The basic instrumentation required for controlled-potential experiments is relatively inexpensive and readily available commercially. The basic necessities include a cell (with a three-electrode system), a voltammetric analyzer (consisting of a potentiostatic circuitry and a voltage ramp generator), and an X-Y-t recorder (or plotter). Modem voltammetric analyzers are versatile enough to perform many modes of operation. Depending upon the specific experiment, other components may be required. For example, a faradaic cage is desired for work with ultramicroelectrodes. The system should be located in a room free from major electrical interferences, vibrations, and drastic fluctuations in temperature. 

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 16.4 Electrochemical screening instrumentation, consisting of a wave generator, a potentiostat, a current follower, and a PC with a data acquisition card [Guerin et al., 2004].

Electrochemical Measurements. The electrochemical instrumentation included (a) a PAR model 263A potentiostat, (b) a PAR PowerCV software for data acquisition and analysis, and (c) a Dell Pentium IV computer. 

The experimental setup included a three-electrode electrochemical cell with a liquid contact membrane electrode in which the internal Ag/AgCl electrode acted as a working electrode connected to a potentiostat/galvanostat. The instrument was capable of switching rapidly between potentiostatic and galvanostatic modes [51]. 

Electrochemical Equipment. Electrochemical experiments were performed using either a PAR Model 175 universal programmer and a PAR Model 363 potentiostat/galvanostat, or a Pine Instruments RDE-4 bipotentiostat, coupled with a Kipp and Zonen BD 91 X-y-y recorder. The current-time response for the chronoamperometry experiments was recorded with a Nicolet 4094 digital oscilloscope. All potentials were measured vs. a Ag/10"2 M Ag+ reference electrode. 

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. 

One must keep in mind that modern electrochemical instrumentation compensates for the potential drop i (Rn + Rnc) through the use of appropriate circuitry (positive feedback compensation). This adds a supplementary potential to the input potential of the potentiostat (equal to the ohmic drop of the potential), which is generated by taking a fraction of the faradaic current that passes through the electrochemical cell, such that in favourable cases there will be no error in the control of the potential. However, such circuitry can give rise to problems of reliability in the electrochemical response on occasions when an overcompensation is produced. 

In 1980 Bemhardsson et introduced an automated electrochemical method for CPT determination. The specimen is mounted as described in Section IV.2 (ii) using a stream of argon to avoid crevice corrosion and 0.02-5% sodium chloride as electrolyte. The CPT is determined by a potentiostatic test method using an instrument called the Santron CDT 400 for potential control, temperature control, and current measurements. 

Cyclic voltammetry was conducted using a Powerlab ADI Potentiostat interfaced to a computer. A typical three electrode system was used for the analysis Ag/AgCl electrode (2.0 mm) as reference electrode Pt disc (2.0 mm) as working electrode and Pt rod (2.0 mm) as auxiliary electrode. The supporting electrolyte used was a TBAHP/acetonitrile electrolyte-solvent system. The instrument was preset using a Metrohm 693 VA Processor. Potential sweep rate was 200 mV/s using a scan range of-1,800 to 1,800 mV. 

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