Total Cell Voltage

Returning to Eq. 2.41, many interrelated parameters govern the required values of the open-cell voltage and ohmic resistances. We will mention the third component of the total cell voltage the overpotential or overvoltage—but polarization and electrode kinetics are the subject of Chapter 3. 

It is practical to linearize overpotential relationships over a limited current density change about a mean current density i  

THE PROBLEM A divided cell is to be used for an electrowinning application. The anticipated range of current densities is 500-3000 A/m. Show how variations in the composition of the anolyte will affect the total cell voltage if the specific conductance of the anolyte is given by  

Assume that the diaphragm contains catholyte then its effective conductivity is 60 X 0.2 = 12 mho/m. 

Rearranging and solving by iteration, we find that c = 6.61 kmol/m.  

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This is possible since the total cell voltage (AV+iR ) at t=40 min had reached approximately 2.5 V. As shown on the figure subse-... 

As the electrodes are typically made of highly conducting material, it seems justified to assume infinite conductivity of them, i.e. ke —> °o. Under this assumption, the potentials anode potential is identical to the reference... 

In a diaphragm cell operating at 2.3 kA/m2, for example, the percentages of the total cell voltage attributable to these three components are 67 percent thermodynamic, 23 percent IR, and 10 percent overvoltage. Typical voltage distribution in a membrane cell is given in Table 26.9. 

Asea Brown Boveri of Switzerland has commercialized the MEMBREL process for the electrolytic generation of ozone. The synthesis takes place in a cell whose anodic section is made of titanium and the cathodic section of stainless steel. A Nafion membrane (Du Pont) acts not only as the electrolyte for the system, but also as the separator. This membrane is sandwiched between the anode (lead dioxide) and the cathode (platinum). Electrolysis takes place at a total cell voltage of 3-5 V with a current density of 0.5 - 2.0 A/cml The corresponding cathodic reaction is hydrogen evolution. 

As explained earlier, an electrochemical cell requires at least two electrodes - an anode and a cathode - to enable a current to flow through it. The rate of electrolysis will depend on the kinetics of the two electrode reactions. It is usually essential to have an overpotential, rj, to increase the rate at which an electrode reaction occurs. The total cell voltage required to bring about chemical changes by electrolysis is given by ... 

Reducing the electrode spacing also reduces power requirements. According to Honey et al. (10), the electrolyte IR drop accounts for about 14.5% of the total cell voltage at 90 mm spacing. A 10 mm reduction would theoretically correspond to a 1.6% power saving, which is supported by Figure 6 in the same paper. 

Fig. 9.5 Deconvolution of the total cell voltage according to DoE targets. The blue line includes activation losses only, the red line includes ohmic losses as well, and the green line gives the overall performance in agreement with the DoE targets (A)...

MEA performance is also seriously limited by the sluggish electroreduction of oxygen on the state-of-the-art Pt/C cathode catalyst in both PEMFCs and DMFCs. As is shown in Figure 22.4, the overpotential for the oxygen reduction reaction (ORR) in a PEMFC contributes about 80% of the total cell voltage loss [30]. In... 

FIGURE 9.4 Calculated anode, cathode, and total cell voltage changes with backpressure as a function of current density. In the calculation, —... 

The description leads to the clear conclusion that we can divide the total cell voltage into two half-cell voltages (electrode potentials), provided we use a good ion conductor as an electrolyte. The two partial voltages A eft (composed of potential jump I -I- potential jump III) and A< ight (potential jump II) refer to the half cells Pt Ag AgCl and AgCllCb, Pt. 

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