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3.2. ELECTROCHEMICALMEASUREMENTS 45 venting precise measurements of the electrode potential. This potential gradient is known as the IR drop. The IR drop in the electrolyte will cause the measured electrode potential to differ from that associated with the surface reaction alone. Therefore, the IR drop has to be corrected for. In the present thesis, the current in- terrupt method was used to correct for the electrolyte IR-drop. The method utilizes the fact that the voltage component associated with the electrical resistance of the electrolyte will disappear immediately once the electrical current is switched off. However, the electrode potential will attain a new value more slowly, as the poten- tial decay of the electrode is associated with the conversion of surface species. This means that if the electrode potential is followed after the current is interrupted, the transient potential decay of the electrode can be recorded and used to determine the electrode potential just after the current has switched off. This electrode potential is the IR-corrected electrode potential. If the perfect equipment for analysis of this potential decay were available, a com- plete transient might be measured, and the value just after the current is turned off would be measured precisely. However, in practice, the time resolution of the instrument is often a limiting factor. For potential decay on oxide electrodes such as DSA, the potential decay occurs over several tens of microseconds, and it is in general not possible to obtain accurate measurements of this decay during the first few microseconds. This is mainly associated with the delayed response of the reference electrode used. To reduce this time-lag, a dual-reference electrode setup was used in the present work. The additional reference electrode was a Pt wire immersed in the electrolyte, and connected in series with a capacitor. In turn, the Pt wire and capacitor were connected in parallel with the reference electrode. The response of the Pt wire then dominates during the first few microseconds[198]. In the present work, it was found that this allows a stable signal to be read after ca 4 μs to 5 μs rather than after 10 μs without the dual reference electrode setup. Further- more, to reduce the noise in the measurements, the mean value of several transients (between 24 and 72), measured upon interruption at the same current density, was used. Between each interruption, the electrode was polarized for several ms, to make sure that each transient would be measured under the same conditions. Nev- ertheless, the electrode potential just after the interruption of the current is still not measured, so some type of fitting must be used to obtain an expression that can yield the potential at t = 0 s. In the present work, the expression due to Morley and Wetmore [199] was used: E (t ) = E (t = 0) − b × ln 1 + t . (3.1) τ In this expression, E (t = 0) is the IR-corrected potential, b is the Tafel slope of the reaction occurring during the potential decay and τ is a constant expressed by τ = bCj , (3.2)PDF Image | Studies of Electrode Processes in Industrial Electrosynthesis
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