DC current-voltage measurements

The electrical resistivity of fhe graphite bipolar plates can be measured either by passing DC current and measuring the voltage drop across the plate sample or by direct resistance measurement using a milliohmmeter. Using the measured electrical resistance R, the electrical resistivity is calculated as... 

Fig. 15-8 Synchronous current, voltage and potential recording with stray current interference from dc railways (a) Without protective measures, (b) direct stray current drainage to the rails, (c) rectified stray current drainage to the rails, (d) forced stray current drainage with uncontrolled protection rectifier, (e) forced stray current drainage with galvanostatically controlled protection rectifier (constant current), (f) forced stray current drainage with potentiostatically controlled protection rectifier (constant potential), (g) forced stray current drainage with potentiostatically controlled protection rectifier and superimposed constant current.

FIGURE 7.2 Schematic of a DNA sensor based on a capacitive EIS structure. For operation, a DC (direct current) polarization voltage (VG) is applied via the reference electrode (RE) to set the working point of the EIS sensor, and a small AC (alternating current) voltage (E ) is applied to the system in order to measure the capacitance of the sensor. ssDNA - single-stranded DNA, cDNA - complementary DNA, dsDNA - double-stranded DNA. 

The power consumption of the electrolyzer is generally calculated by multiplying the applied current by the stack voltage that are both continuously measured. However, this power consumption refers to DC current and therefore to the electrolysis stack. The AC power consumption is not usually measured, and most manufacturers assume a 5-10% energy loss for the AC/DC converter. 

For the usual dc measurement the constant dc current source should be capable of providing currents in the range 0.1-10 mA for a typical bar of 1 mm square cross-section, 1 cm length, and a resistivity at 100 K of 50 pOhm-cm the voltage measured for a 1 mA current source would be 1 / V. Since even for a typical low value of the critical current density, 100 A/cm2, the measurement current would be 1000 times less and thus have essentially no effect on the measurement. However, the measurement of 1 / V to a precision of 1% already requires care to assure that noise and thermal voltages are reduced well below this value. Currents of similar value are used for measurements in thin films. 

Another measurement that follows the line of the Porath et al. [14] experiment was performed by Yoo et al. [75]. In this experiment, long poly(dG)-poly(dC) and poly(dA)-poly(dT) molecules were electrostatically trapped between two planar metal electrodes that were 20 nm apart (see Fig. 11) on a Si02 surface, such that they formed a bundle that was -10 nm wide. A planar gate electrode added another dimension to this measurement. The current-voltage curves showed a clear current flow through the bundle and both temperature and gate dependencies. The resistivity for the poly(dG)-poly(dC) was calculated to be 0.025 flcm. 

A stable and sensitive conductivity cell has been described that measures the DC current that passes through a small volume of solution confined in a small capillary tube. The very simple apparatus is illustrated in Figure 6.19. When filled with 0.1 M KC1, the cell exhibits an Ohm s law behavior with an applied voltage from 5 to 100 V (approximately 5-100 mA of current). The cell also can be used for conductimetric titrations, with the titration carried out in vessel B. Because some liquid flows through the capillary during a titration, a correction factor is used to obtain the most accurate results (although corrections usually are <0.2%). 

To carry out this type of study, a small AC amplitude voltage perturbation, AV ico,/), is applied, superimposed onto a DC bias voltage component, and the resulting alternating current response and its phase, A/(co,t), is measured [123,132], Then, the electrochemical impedance of the system is thus defined as... 

Conventional two-electrode dc measurements on ceramics only yield conductivities that are averaged over contributions of bulk, grain boundaries and electrodes. Experimental techniques are therefore required to split the total sample resistance Rtot into its individual contributions. Four-point dc measurements using different electrodes for current supply and voltage measurement can, for example, be applied to avoid the influence of electrode resistances. In 1969 Bauerle [197] showed that impedance spectroscopy (i.e. frequency-dependent ac resistance measurements) facilitates a differentiation between bulk, grain boundary and electrode resistances in doped ZrC>2 samples. Since that time, this technique has become common in the field of solid state ionics and today it is probably the most important tool for investigating electrical transport in and electrochemical properties of ionic solids. Impedance spectroscopy is also widely used in liquid electrochemistry and reviews on this technique be found in Refs. [198 201], In this section, just some basic aspects of impedance spectroscopic studies in solid state ionics are discussed. 

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