Discharge activity electrochemical discharges

Figure 2.2 Experimental apparatus used by Wehnelt for his studies on electrochemical discharges [119]. After a first series of experiments (left), he improved the set-up by enclosing the active electrode c in a glass tube d (right). 

Note that this classification is done for discharges where the current is carried by electrons. For electrochemical discharges this is the case when the active electrode is a cathode the electrons travel through the gas film from the electrode to the electrolyte, where they will somehow undergo electrochemical reactions with the ions of the electrolyte. It is harder to imagine that this would also be the case if the active electrode is an anode. Indeed, this would raise serious questions about how electrons would be emitted from the electrolyte and travel to the anode. As discussed in Section 2.5, the situation is very different for this case. For the time being, we restrict the discussion to the situation where the active electrode is a cathode. 

As shown in Fig. 2.9, the calculated amplification factor Mfor the gas film around the active electrode is far less than the critical value of 11 needed for self-sustained discharges. This is an indication that electrochemical discharges... 

Electrochemical discharges have all the characteristics of arc discharges. They occur in a very similar voltage range with similar currents and at atmospheric pressure. The question remains as to how these arcs can be initiated. We have proposed the hypothesis that the ignition is thermal [123]. The cathode temperature required is probably reached before the gas film is totally formed,3 when the active electrode bubble coverage fraction reaches its maximum value. 

This mechanism is consistent with the observation of H lines and OH bands in the emission spectra of electrochemical discharges at an active anode. 

In summary, today it is believed that the following mechanisms take place during electrochemical discharges at an active anode [98] ... 

Another significant difference is the nature of the discharges. As discussed previously in Section 2.4, in the case of an active cathode, the electrochemical discharges are most likely electrons emitted from the active electrode by thermionic emission, whereas in the case of an active anode, the discharges result from the accelerated ions across the gas film. 

By counting the current pulses experimentally it is possible to verify that electrochemical discharges follow a Poisson process. Therefore, one can count the number of current pulses N(t) during a fixed time interval t. The experiment shows that the distribution of N(t) follows the Poisson distribution (4.37) as plotted in Fig. 4.14. This figure presents the counts of current pulses obtained at 28 V with a cylindrical active cathode of 172 pm diameter [123]. The pulses were counted for a time interval of 40 ms. The experiment was repeated 500 times and the frequency distribution was evaluated. The superimposed curve is the fitted Poisson distribution. From this distribution, the parameter... 

This evolution equation allows the description of the current during discharge activity with time. In particular, the mean current and the fluctuations in the current can be calculated, as shown in next section. Both quantities are important for the application of electrochemical discharges to machining. 

Figure 9.1 Micrograph of nanoparticles formed by electrochemical discharges from H2PtCl6 and NaAuCl4 in HC104. The active cathode was rotated at 2000 rpm. Reprinted from [78] with permission from Elsevier.

We found just above that if we discharged the electrochemical cell (2.1.17) through an infinite load resistance, the discharge would be reversible. The potential difference is therefore always the equilibrium (open-circuit) value. Since the extent of reaction is supposed to be small enough that all activities remain constant, the potential also remains constant. Then, the energy dissipated in R is given by... 

The active cathodic material is an intercalation compound, namely a compound with an open structure which allows reversible insertion-deinsertion of lithium ions from-to the electrolyte medium. This makes intercalation compounds very suitable electrodes for rechargeable lithium batteries [10] and indeed they are currently used for this purpose in liquid electrolyte sytems [11]. In the case of the LPBs of interest here, the electrochemical discharge process may be described as the dissolution of lithium at the anode, the migration of the Li" ions across the PEO-LiX electrolytic membrane and its insertion within the structure of the hosting intercalation compound IC ... 

This product acts as a separator or mass transport barrier between the cathode and the anode to limit electrochemical self-discharge. If the integrity of this separator is breached, the battery can experience a thermal runaway condition, whereby the active electrochemical components are chemically consumed with accompanying generation of large amounts of excess heat. At the same time, if battery conditions are such that alloy formation exceeds usage, the excess alloy can cause periodic shorting, the alloy noise sometimes seen in cold-stored batteries. 

---