Closed-circuit Behaviour

2 Closed-circuit Behaviour - When a voltage is applied to such a cell there is a tendency for oxygen to migrate from one electrode to the other. As a result such arrangements have been used as oxygen pumps.18 20 

When the potential difference across the electrode/electrolyte double-layer is not at its equilibrium value then the interface is said to be polarised. When the interface is polarised a net current will flow, the magnitude of the current being dependent upon the difference in the total anodic and total cathodic currents, 

Take as an example the oxygen charge-transfer reaction + 0o 0 + V0 + 2e  

Therefore, rearranging, the exchange current density can be expressed in terms of the rate constants and coverages, 

The actual potential difference across the electrode/electrolyte interface minus the equilibrium potential difference across the interface is known as the electrode overpotential, t, and is, in effect, the driving force for net charge-transfer, 

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In this section both the open-circuit and closed circuit behaviour of electrochemical cells will be briefly discussed. The mechanism of the charge-transfer process for oxygen-ion conducting systems will also be discussed. 

In this section the use of amperometric techniques for the in-situ study of catalysts using solid state electrochemical cells is discussed. This requires that the potential of the cell is disturbed from its equilibrium value and a current passed. However, there is evidence that for a number of solid electrolyte cell systems the change in electrode potential results in a change in the electrode-catalyst work function.5 This effect is known as the non-faradaic electrochemical modification of catalytic activity (NEMCA). In a similar way it appears that the electrode potential can be used as a monitor of the catalyst work function. Much of the work on the closed-circuit behaviour of solid electrolyte electrochemical cells has been concerned with modifying the behaviour of the catalyst (reference 5 is an excellent review of this area). However, it is not the intention of this review to cover catalyst modification, rather the intention is to address information derived from closed-circuit work relevant to an unmodified catalyst surface. 

Steady-state Current Overpotential Behaviour - For a simple single charge-transfer process equation (2.28) describes the closed-circuit behaviour. At low overpotentials, the current and overpotential are linearly related and the exchange current density can be evaluated from the gradient (see equation... 

It is assumed here that a large surface Ti02 photoelectrode mimics closely the behaviour of the Ti02 particles of not too small size, and, in particular, that the surface species, formed during irradiation of the Ti02 dispersion, respectively, the TiOz electrode at open circuit, are identical. 

In addition to these typical behaviours, for particular values of I, other phenomena may appear. For example, for I = 3f4 L, the initial defect starts in one compartment, passes through the opening into the second compartment and then is attracted to a small closed circuit at short distance from the opening (not shown). Another type of behaviour is also obtained for / = L/10. In this case, as the original defect describes a trajectory in the first compartment, a second defect is created in the next compartment. After a short period of coexistence, the original defect disappears and the new one is stabilized in a fixed position in the second compartment. 

The behaviour of the resistance Is as expected. At low salt concentration R decreases with increasing surface charge on the latex, but at high there Is no difference between the latlces because now the bulk conductivity dominates (R R ). There Is a tendency of R to level off at very low electrolyte concentration then the particles are effectively so close to each other that they are "short-circuited", leading to a constant polarization. The theory (capillary model) applies semlquantltatlvely it becomes more defective at low because polarization Is not accounted for. 

The radioelectric properties of two composites based on polyaniline impregnated glass textiles with different conductivities have been characterised. Changes of (fi. e") with frequency are shown in Figure 8.41. A dependence quite different from those already presented is observed. Indeed, for both conductivities, log e" decreases linearly with log/ with a slope close to unity, e.g., 0.7 and 0.8. This behaviour, independent of conductivity, is representative of a purely resistive material. The electrical equivalent circuit was found to be a resistance in parallel with a capacitance [84,86]. 

The next step, after all experimental parameters have been given their correct values, is usually a calibration. A dummy cell is used, consisting of electronic components that imitate the behaviour of the real cell as closely as possible. The simplest one, which also is in many cases a completely adequate one, is shown in Fig.5. It consists of a capacitance (double layer capacitance) in parallel with a resistance (charge transfer resistance), and then, in series with this circuit, another resistance (solution resistance). The admittance of the dummy cell is recorded in an ordinary experiment and the transfer function, T(u), of the instrument is set equal to the ratio of the calculated, 0( )5 to the measured, ym( )) admittance of the dummy cell i.e. 

Our own work d involved measurement of potentials developed by exposure of NASICON to H2 at different pressures in a gaseous concentration cell. The NASICON was prepared both by the sol-gel method and the hydrothermal route. Pressed sintered disks were then prepared which were approximately 1.27 cm ( ") in diameter and 1 mm thick. The disk separated two chambers, one of which contained Nj at 1 atm and the other Hj. The potential of this cell was measured on open circuit as a function of hydrogen pressure and the results at 25 °C are plotted in Fig. 15.4. The slope of the line is close to that expected for Nernst type behaviour. At elevated temperatures, the voltage increased considerably but large deviations from Nernstian behaviour were observed presumably due to conduction of the sodium ions. These results are comparable to those obtained with P-alumina . By applying a potential across the... 

Figure 15.6a shows the polarisation curves acquired after 24, 48 and 72 h of exposure to sodium chloride solution for the film grown in the presence of PPA. For this coating, the mechanism of protection is quite different. The film shows similar behaviour at 48 and 72 h with corrosion current and potential values very close introducing a rather stable and resistive coating. After an initial stable open circuit potential at... 

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