Overall current efficiency

Current-potential curves obtained under steady state conditions are called polarization curves. If two or more faradaic processes occur at the electrode, the fraction of current (ir) driving the rth process is the instantaneous current efficiency. Over a period of operating time the fraction of the total number of coulombs used in the rth process ( r) is related to the overall current efficiency of that process (OCE), i.e. 

However, and as for all the processes, EDBM presents limits [23]. The performances of a water splitter are controlled by the permselectivities of the individual component membranes and by diffusive transport. Figure 21.7 illustrates the various processes which contribute to the overall current efficiency. Zone 1 concerns the specific process which generates acid and base from salt and water. Competing with this are the undesirable processes, which reduce the current efficiency. Zone 2 represents the loss of permselectivity of each homopolar layers of the BPM. In the three-compartment configuration (Figure 21.5), this factor directly acts on the purity of the acid and base produced. Zone 3 represents the loss of permselectivity of the associated homopolar membranes. Zone 4 concerns the diffusional losses due to the concentration gradients, which significantly occur for poorly ionized small molecules (HF, NH3, SO2, etc.). 

The reduction of benzene itself may then be achieved either in LiBr + HMPA solution (water as proton donor) [313] or in ethanol-HMPA (66.6 mol% ethanol) at an aluminum catghode [311] this gives a mixture of 22% cyclohexadiene, 10% cyclohexene, and 67% cyclohexane with an overall current efficiency of 95% [311]. The electrolysis must be carried out with a large excess of benzene. However, product distribution may depend on both the concentration of ethanol and the current density. As previously studied under chemical conditions (lithium metal added to ethanol in ammonia [314]), the product distribution would depend on the rates of reduction of benzene (/cb) and of cyclohexene (kc) by the reducing species (here the solvated electron), the nature of the solvent, and efficiency of the proton donor in the considered solvent. In ethylenediamine, the ratio /kc was found to be equal to 200, and this value explains why reduction stops when an isolated double bond has been formed. In contrast, in HMPA-ethanol mixtures, the ratio k[)/kc is only 1.4 this may explain the lack of selectivity and high yield of cyclohexane when a large excess of proton donor is used. 

When an electrical potential gradient is imposed on the stack, alternate compartments become enriched and depleted in sodium chloride. A typical module of an electrodialytic salt plant has 1500 pairs of membranes, each with an effective membrane area of 1 m2. The current density is 3.65 A/dm2 at 620 V with a membrane spacing of about 0.75 mm. A brine concentrate containing about 118 g/1 of chloride can be attained. The overall current efficiency is 73% for Na+ and 85% for Cl". Typically, the membranes are divinylbenzene cross-linked polystyrene with sul-phonic acid, or quaternary ammonium exchange groups the exchange capacity is 1.8 to 2.8 meq/g at 25 °C34). 

The overall current efficiency for the nth process is given by the ratio between the fraction of the number of coulombs (Q ) involved in the nth process and the total charge (Qj-) passed across the cell... 

A current efficiency of unity (or 100%) implies that only one process is occurring at an electrode. When one considers the result of an electrolysis over some period of time, the overall current efficiency represents the fraction of the total charge involved in the rth process ... 

If the solution in Problem 11.10 is electrolyzed at a constant current of 80 mA under the same conditions (a) What is the concentration of remaining in solution when the current efficiency drops below 100% (b) How long does it take to reach this point (c) How many coulombs have been passed to this point (d) How much longer will it take to decrease the concentration to 0.1% of its initial value What is the overall current efficiency for removal of 99.9% of by this constant-current electrolysis ... 

With two-compartment cells the cathodic efficiency also represents the total current efficiency of the process. In the three-compartment cells, the current efficiency for sulfuric acid production (anodic efficiency) must also be taken into consideration. The overall current efficiency of electrolysis is represented by the lower of the two efficiencies. If, for example, the anodic efficiency is lower than the cathodic, acidity builds up in the sodium sulfate compartment during operation so that the cathodic current efficiency progressively decreases when it reaches the same value as the anodic, stationary process conditions are maintained. Therefore, the process is provided with a self-regulating mechanism. Alternatively, if the accumulation of acidity in the sodium sulfate compartment cannot be allowed, then a portion of the caustic soda produced can be fed into the sodium sulfate loop. The quantity of caustic soda available for use outside the electrolysis plant again represents the overall current efficiency, which still remains a function of the anodic efficiency. The anodic and cathodic current efficiencies can be made independent when the control of acidity in the sodium sulfate compartment is performed by means of sodium carbonate addition. 

Early in the electrolysis, the C- 2-diacids are formed in good selectivity (> 80 5I>), but, as the electrolysis is continued, the pentenoic acids become the major products and the overall current efficiency drops markedly, probably because of electrode polarization. 

It can be seen that in a commercial cell, the major byproducts are sodium hypochlorite, sodium chlorate, oxygen and carbon dioxide, which lead to overall current efficiencies in the range 90-96%. The electrolysis current only converts about half the 25% w/w sodium chloride feed to caustic soda, which leads to a maximum of about 12% w/w caustic in the product sodium chloride/sodium hydroxide liquor. 

Pavlovid MG, Nikolic ND, Popov KI (2003) The current efficiency during the eathodie period of reversing current in copper powder deposition and the overall current efficiency. J Serb Chem Soc 68 649-656... 

FIf. 2.10 The tiaie-4 pfliito of (a) reactant eoncmitration, (b) logarithm of reactant oooeeittradofi, (c) overall current efficiency during batch processing using a constant current where initially i[Pg.108]

Depending on the solution being processed, the EW unit was operated with current densities from about 10 to 20 amp/ft at a temperature of 40 C. The concentration of strong electrolyte was approximately 26 g/L Cu and 125 g/L H2SO4, and the concentration of the lean electrolyte was approximately 20 g/L Cu and 160 g/L H2SO4. The second and third portions of this first field test were short in duration. Therefore, these EW data were combined to obtain an overall current efficiency for the portion of the test that dealt with these solutions. Typical copper cathode analyses are given in Table III. 

As an example of the technique, we may consider the reduction of penta-chlorophenol (PCP). The reactor consisted of two compartments, each 10 cm long, 2 cm wide and 0.5 cm deep, the cathode being a packed bed of carbon fibre, which was separated from the anode by a cation-exchange membrane (Nation 426). In the batch-recycle mode, constant current (10 A) electrolysis of 1 dm of 50 mg dm PCP in 0.1 mol dm " Na2S04 plus 0.1 mol dm NaOH removed PCP to a level < 0.5 mgdm within 30 min. The overall current efficiency for complete dechlorination was approximately 1 %, the energy consumption being 36 kWhm"l... 

When, however, Bi was added as an electron transfer mediator, the Mn02 films were oxidized by electrogenerated bismuthate [BiOs", Bi(V)J and the overall current efficiencies for... 

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