Charge-current efficiency

Charge-current efficiency is the ratio of the current that is actually used for electrochemical conversion of the active material from lead sulfate to lead and lead dioxide to the total current supplied to the cell on recharge. The current which is not used for charging is consumed in parasitic reactions within the cell such as corrosion and gas production. 

Improved sensitivities can be attained by the use of longer collection times, more efficient mass transport or pulsed wavefomis to eliminate charging currents from the small faradic currents. Major problems with these methods are the toxicity of mercury, which makes the analysis less attractive from an eiivironmental point of view, and surface fouling, which coimnonly occurs during the analysis of a complex solution matrix. Several methods have been reported for the improvement of the pre-concentration step [17,18]. The latter is, in fact. 

In controlled-potential coulometry, accuracy is determined by current efficiency and the determination of charge. Provided that no interferents are present that are easier to oxidize or reduce than the analyte, current efficiencies of greater than 99.9% are easily obtained. When interferents are present, however, they can often be eliminated by applying a potential such that the exhaustive electrolysis of the interferents is possible without the simultaneous electrolysis of the analyte. Once the interferents have been removed the potential can be switched to a level at... 

The ion transport number is defined as the fraction of current carried through the membrane by counterions. If the concentration of fixed charges in the membrane is high compared to the concentration of the ambient solution, then the mobile ions in the IX membrane are mosdy counterions, co-ions are effectively excluded, and the ion transport number then approaches 1. Commercial membranes have ion transport numbers in dilute solutions of ca 0.85—0.95. The relationship between ion transport number and current efficiency is shown in Figure 3 where is the fraction of current carried by the counterions (anions) through the AX membrane and is the fraction of current carried by the counterions (cations) through the CX membrane. The remainder of the current (1 — in the case of the AX membranes and (1 — in the case of the CX membranes is carried by co-ions and... 

Table 3). However, their cycle life depends on the discharge and charge currents. This problem results from the low cycling efficiency of lithium anodes. Another big problem is the safety of lithium-metal cells. One of the reasons for their poor thermal stability is the high reactivity and low melting point (180 °C) of lithium. 

OS 86] [R 29] [P 66] The Faradaic current efficiency, the electrical charge equivalent for conversion as a fraction of the total electrical charge, was measured for a... 

Figure 4.95 Faradaic current efficiency as electrical charge equivalent for conversion of D-gluconic acid given for two values of average current density. The symbols represent measured conversion and the solid and dashed lines calculated results [65. ...

The current efficiency of an electrolytic process is a measure of the current or the charge actually used in carrying out the desired electrochemical reaction as compared to the theoretical requirement. It is, therefore, defined as the ratio of the theoretical current requirement to the actual current requirement for the desired reaction alternatively, it may also be expressed as the ratio of amount of material actually deposited at the electrode to that which should have deposited on the basis of Faraday s law, by the passage of the same charge, assuming that no side reactions take place at the electrode. The current efficiency, can be expressed as... 

As mentioned earlier, the current efficiency also depends on the presence of additives and/or of impurities which may co-deposit or may influence the electrochemical reaction or may affect the overvoltages of the desirable and the undesirable reactions. The impurities which are more noble would be deposited this would not only contaminate the metal but would also consume charge for undesirable reactions. Additives may be deliberately added when depositing alloys, so that the deposition potentials of the different metals involved could be brought closer however, in most other cases these are considered as harmful impurities. The electrolyte, therefore, needs to be purified with respect to such impurities in order to improve the current efficiency. 

The two-step charge transfer [cf. Eqs. (7) and (8)] with formation of a significant amount of monovalent aluminum ion is indicated by experimental evidence. As early as 1857, Wholer and Buff discovered that aluminum dissolves with a current efficiency larger than 100% if calculated on the basis of three electrons per atom.22 The anomalous overall valency (between 1 and 3) is likely to result from some monovalent ions going away from the M/O interface, before they are further oxidized electrochemically, and reacting chemically with water further away in the oxide or at the O/S interface.23,24 If such a mechanism was operative with activation-controlled kinetics,25 the current-potential relationship should be given by the Butler-Volmer equation... 

Satisfactory agreement of experiments with kinetic laws, described by Eqs. (44) and (45), are observed only for tantalum and niobium, when the current efficiency approaches 100%. Even for these metals, certain deviations occur which could be attributed to space charge effects,82 electronic leakage currents,83 or other factors. In the case of aluminum, these deviations are relatively large, as, even in barrier-forming electrolytes, some oxide dissolution takes place from the very beginning of voltage supply to an anodized sample.32... 

Umax = (Qd/Qch) -100% - is the maximum value of current efficiency Qch- charge electrode capacity) ... 

On the basis of the charge passed during growth (and assuming 100% current efficiency), the authors calculated that the him thickness was 420 A. From Figures 3.80(a) and (b), it can be seen that the best fit to the data was obtained with a him thickness of 669 A, suggesting that the as-grown him is 63% pyrrole, 37% solvent and electrolyte. 

Schiff bases with intramolecular charge transfer complexes such as 2,3-bis[(4-diethylamino-2-hydroxybenzylidene)amino]but-2-enedinitrile zinc (II) (BDPMB-Zn, 187) emit red fluorescence with fluorescent quantum yields up to 67%. OLEDs with a structure of ITO/TPD/ TPD BDPMB-Zn/Alq3 BDPMB-Zn/Alq3/Mg-Ag showed very bright saturated red emission with CIE (0.67, 0.32) with a luminance of 2260 cd/m2 at 20 V and a current efficiency of 0.46 cd/A (at 20 mA/cm2). In addition, the EL spectra do not change with the doping concentration in the range of 0.5—3% [229]. 

Quite differently, Pleux et al. tested a series of three different organic dyads comprising a perylene monoimide (PMI) dye linked to a naphthalene diimide (NDI) or C60 for application in NiO-based DSSCs (Fig. 18.7) [117]. They corroborated a cascade electron flow from the valance band of NiO to PMI and, finally, to C60. Transient absorption measurements in the nanosecond time regime revealed that the presence of C60 extends the charge-separated state lifetime compared to just PMI. This fact enhanced the device efficiencies up to values of 0.04 and 0.06% when CoII/m and P/Ij electrolytes were utilized, respectively. More striking than the efficiencies is the remarkable incident photon-to-current efficiency spectrum, which features values of around 57% associated to photocurrent densities of 1.88 mA/cm2. 

In addition, the current efficiency ( current yield ) is typical for an electrolysis process, the fraction of the electrical cell current - or (integrated over the time) the fraction of the transferred charge - which is used to form the product. The theoretical charge transfer for one mol product is given by the Faraday constant F, the charge of one mol electrons, F = 96 485 As/mol = 26, 8 Ah/mol, multiplied by the number of transferred electrons. 

Charge (current) balance and calculation of the current efficiency (see Sect. 2.3.1), that is, the electrochemical products on both electrodes, including product gases, have to be equivalent to the consumed electrical charge. 

Microelectrodes were mentioned previously in Chapter 5, where we saw how their small size increased the faradaic efficiency since the interfacial capacitance Cdi is decreased, itself minimizing the charging currents. Microelectrodes can be purchased relatively cheaply, and in a variety of types, e.g. hemispherical and flat circular rings or bands, with a wide range of diameters. Such electrodes were discussed previously in Section 5.3. 

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