Cathodes cell voltage drop

Instead of an asbestos membrane and an anion exchange membrane, Li et al. [46] suggested a Model 3 type cell in which a Nafion membrane was employed as the electrolyte to keep borohydride from crossing to the cathode. It was verified that cation (Na+) was the charge carrier in the Nafion membrane. Compared with anode polarization, cathode polarization was the main reason for the cell voltage drop. The power density of a Model 3 type cell can reach 190mWcm, as shown in Fig. 8.22. 

The existing control room will be used for both tankhouses, and will report the main production and quality parameters of individual cathode weighing, bundle weighing, temperature scanning, cell voltage drop, as well as other useful statistical measurements. 

Thus, Giddey et al. (2010) performed experiments without and with airflow to the cathode. When there was no air flowing to the cathode, H2 was produced there via conventional water electrolysis (Figure 15.24). However, with the introduction of air in the cathode, there was no H2 produced, and the cell voltage dropped in accordance with Eqn (15.36) as aresult of ORR occurring there, as described in Eqn (15.35). These... 

With respect to fuel cell catalysis, most research has been focused on cathode ORR catalysts development, because the ORR kinetics are much slower than flic anodic HOR kinetics in other words, the fuel cell voltage drop polarized by load is due mainly to the cathode ORR overpotential [7, 8]. However, in some cases the overpotential of the anodic HOR can also contribute a non-negligible portion of the overall fuel cell voltage drop [8]. Therefore, the catalytic HOR on the fuel cell anode catalyst is also worth examining. 

A traee amount of H2S in the fuel or air stream could degrade the cell performanee significantly, mainly through the poisoning effect of the Pt catalysts. A 0.1 ppm level of H2S in the fuel stream could cause a 250 mV cell voltage drop within 300 hours at 0.5A/cm load [59]. Unlike in the case of CO poisoning, the presence of Ru in the Pt catalyst caimot provide sufficient tolerance to H2S poisoning. Cathode exposure to 200 ppm H2S for 10.5 hours caused complete deterioration in cell performance [58], which was only partially recoverable when neat air was introduced to the cathode side for 70.5 hours. 

Hui et al. [29] conducted toluene contamination tests at different toluene concentrations. They also studied the effects of different operational conditions on toluene contamination, including the effects of fuel cell relative humidity (RH), of Pt loading in the cathode catalyst layer, of back pressure, and of air stoichiometry [30]. Figure 3.8 shows a set of representative results of contamination tests at 1.0 A cm with various levels of toluene concentration in the air. It can be seen that the cell voltage starts to decline immediately after the introduction of toluene, and then reaches a plateau (steady state). These plateau voltages indicate the saturated nature of the toluene contamination. For example, the cell voltage drops from 0.645 V to 0.522 V at 1.0 A cm-2 within 30 min of the cathode... 

Figure 6.5 shows the transient cell performance behaviors at different current densities and with different toluene inlet concentrations. On the one hand, the effect of toluene contamination becomes more severe with a higher toluene concentration at the same cell current density for example. Figure 6.5(d) indicates that at the same current density of 1.0 Acm, the cell voltage drops due to toluene concentrations of 250, 500, and 750 ppb are 37, 42, and 48 mV, respectively. On the other hand, the toluene contamination increases steadily with increasing cell current density for example, the voltage drops in response to 750 ppb toluene in the cathode flow channel are 9,16, 27, and 48 mV, corresponding to cell current densities of 0.5, 0.75, and 1.0 AcmV respectively, as shown in Figure 6.5. Furthermore, the time required for the cell voltage to reach steady state is also affected by both toluene concentration and current density, i.e., a larger toluene concentration and a lower current density result in a longer time before cell performance reaches steady state.

In the absence of oxygen, the cathode potential cannot be maintained and the cell voltage drops, and may even become slightly negative dne to the ohmic potential reqnired to transport the protons through the membrane. Under milder conditions of low air flow, the cell becomes oxygen limited and the cathode overpotential increases. 

Although we have seen that small fuel cells can be operated without additional or external humidification, in larger cells this is rarely done. Operating temperatures of over 60°C are desirable to reduce losses, especially the cathode activation voltage drops described in... 

Due to the cell voltage drop as a result of the decreasing fuel concentration towards the exhaust side of the stack, only a certain percentage of the available fuel can be electrochemically converted to electricity and heat. An overall utilisation of 85-90% is considered a practical maximum. At higher fuel utilisations, nickel may oxidise locally. A catalytic burner is used to burn the remaining fuel from the anode side with the surplus air from the cathode side. 

Fig. 5. Energy requirements of the HaH-Hfiroult cell (23—25). E, decomposition of alumina Eg, depolarization by carbon E, anode overvoltage E, counter electromotive force E, bath voltage drop E, bath bubble voltage F/, anode voltage drop Eg, cathode voltage drop E, external voltage drop ...

When measuring AU profiles, the voltage drop is measured along the interior of the casing [2]. The voltage drop is caused by cell currents as in Section 18.2.1 or by cathodic protection currents. The anodic and cathodic sections of the casing and also the effect of the cathodic protection can be determined from the AU profiles. 

If too large a current is drawn from a Ledanche cell, the ammonia forms a gaseous insulating layer around the carbon cathode. When this happens, the voltage drops sharply and then returns slowly to its normal value of 1.5 V. This problem can be avoided by using an alkaline dry cell, in which the paste between the electrodes contains KOH rather than NH4CL In this case the overall cell reaction is simply... 

The third limitation is concerned with the numerous contributions to the cell voltage Vceii, which, along with the difference in the electrode reversible potentials AEeq, comprises overpotentials at the cathode, tjc, and the anode, as well as the ohmic drop A ohmic ... 

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