Haber-Luggin capillary

The potential dependence of the velocity of an electrochemical phase boundary reaction is represented by a current-potential curve I(U). It is convenient to relate such curves to the geometric electrode surface area S, i.e., to present them as current-density-potential curves J(U). The determination of such curves is represented schematically in Fig. 2-3. A current is conducted to the counterelectrode Ej in the electrolyte by means of an external circuit (voltage source Uq, ammeter, resistances R and R") and via the electrode E, to be measured, back to the external circuit. In the diagram, the current indicated (0) is positive. The potential of E, is measured with a high-resistance voltmeter as the voltage difference of electrodes El and E2. To accomplish this, the reference electrode, E2, must be equipped with a Haber-Luggin capillary whose probe end must be brought as close as possible to... 

Luggin Capillaries The location, where the potential of the electrolyte should be measured, is connected to the RE by the Luggin (Haber-Luggin) capillary. Its design can significantly influence the accuracy of the acquired potential values. Requirements are... 

The equivalent electrical circuit in the case of a three-electrode setup is given in Fig. 2.9. Working and counter electrode are identical as for a two-electrode setup, while the reference electrode, as a non-current conducting electrode, only has the role of potential reference and therefore does not contribute to the impedance. However, the position of the Haber-Luggin capillary determines the contribution of Re and Rcomp to Ra given by the following equation ... 

Fig. 45. (a) Cyclic voltammogram and (b) stationary potential domains during the reduction of S20g2 on a Ag ring electrode at fixed outer potential (t/ = —1.13V) [42], The end of a Haber-Luggin capillary was positioned on the axis of the ring and close to the WE. 

Conventional electrochemical methods require the use of a reference electrode to control the potential. Since the thickness of the membrane electrolyte is about 50-200 pm, the implementation of a Haber-Luggin capillary is complicated. To avoid as much disturbance of the fuel cell process as possible the diameter of the capillary has to be less than 1 mm [112]. Even though the disturbance of the water management and the conductivity of the membrane can be minimised a change in the distribution of the electrical potential has to be taken into account. 

Reference electrodes are generally used together with Haber-Luggin capillaries (for details, see Ref. [2]).The design and position of these capillaries pose current and potential distribution problems. In order to minimize ohmic drop they have to be placed as close as possible to the electrode surface. But if the distance is too small they act as a current shield and non-imiform current distribution arises. In practice the tip of the Luggin probe should be at a distance of about 2 d from the working electrode where d is the external diameter of the capillaiy. 

Moreover, the reference electrode tip, usually the orifice of the Haber-Luggin capillary, should be positioned close to the working electrode. Insulating objects in the electrolyte, however, mean distortions of the current distribution and, thus, local potential changes. Therefore, the capillary should not come too close to the electrode (screening effect, [10, 11]). A more distant position, however, means a larger electrolyte... 

Therefore the reference electrode is placed with its Haber-Luggin capillary as close to the electrode surface as possible, thus measuring the electrode potential without large deviations due to a possible ohmic drop within the electrolyte. However, it should not get too close in order to avoid the formation of an artificial crevice. As a general rule, the distance should be two to three times the diameter of the capilleuy. The remaining ohmic drop AUa may be compensated for by up to more than 90% by a feedback loop of the potentiostat. 

Figure 1-10. Electrochemical cell with a noble metal counter electrode (CE), a reference electrode (RE) with a Haber-Luggin capillary (HL), the working electrode (WE), a gas inlet for purging the electrolyte with nitrogen or argon, agitation of the electrolyte with magnetic stirring (MS), and a constant temperature by thermostated water (TW).

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