Performance of AFC

Till now, we have been familiar with the concept that the activation overvoltage on the cathode is reduced upon increase in pressure and temperature. Upon increasing the pressure, the change in OCV is given by the second term of Eq. (2.74), i.e.. 

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As in the case of PAFC discussed in the previous section, an increase in cell operating pressure enhances performance of AFC s. Figure 4-3 presents a plot of the increase in the reversible e.m.f of alkaline cells with pressure over a wide range of temperatures (14). The increase in cell open circuit voltage will be somewhat less because of the greater gas solubility at increasing pressure which produces higher lost currents. 

As in the case with PAFC s, voltage obtained from an AFC is affected by ohmic, activation, and concentration losses. Figure 4-7 presents data obtained in the 1960 s (18) which summarizes these effects, excluding ohmic losses, for a catalyzed reaction (0.5-2.0 mg noble metal/cm ) with carbon-based porous electrodes for H2 oxidation and O2 reduction in 9 N KOH at 55-60 C. The electrode technology was similar to that employed in the fabrication of PAFC electrodes. Performance of AFC s with carbon-based electrodes has not changed dramatically since these early results were obtained. 

The major cause of the degrading performance of AFCs is the consequent precipitation of metal carbonate crystals (most commonly Na2C03 or K2CO3, depending on the alkaline electrolyte used) in the electrolyte-filled pores of the electrodes, blocking pores and mechanically disrupting and destroying active layers. 

The ideal performance of a fuel cell depends on the electrochemical reactions that occur with different fuels and oxygen as summarized in Table 2-1. Low-temperature fuel cells (PEFC, AFC, and PAFC) require noble metal electrocatalysts to achieve practical reaction rates at the anode and cathode, and H2 is the only acceptable fuel. With high-temperature fuel cells (MCFC, ITSOFC, and SOFC), the requirements for catalysis are relaxed, and the number of potential fuels expands. Carbon monoxide "poisons" a noble metal anode catalyst such as platinum (Pt) in low-temperature... 

Fig. 13.17. Performance of advanced lightweight pressurized alkaline fuel cells. The dashed lines show initial advanced AFC cell results. A, 149 °C, 17 bar B, 140 °C, 17 bar C, 127 °C, 17 bar D, 110 °C, 4 bar E, 82°C, 4 bar F, 82 °C, 1 bar G, 0.2 MgPt-C and the same conditions as F (IR-free) H, 10 mg/cm2 Au/Pt, 127 °C, 1 bar (IR-free). , nominal performance of space shuttle cell (1000 h) , United Technologies target goal (1000 hr). Solid lines show solid polymer electrolyte cells for comparison under different pressure and temperature conditions. (Reprinted from Assessment of Research Needs for Advanced Fuel Cells, S. S. Penner, ed., Pergamon Press, 1986, pp. 14,87.)...

Previous research on the perception threshold has shown that the elderly were able to perform 2-AFC or 3-AFC type tasks (Mojet et al., 2001) for a review see Methven et al. (2012). As a consequence, it may be relevant to conduct discriminative tasks with elderly people so as to, for example, check if any improvement brought to the product has actually been perceived by this population or not. Yet, as attentional capacities decline with age, we recommend choosing duo-trio tests, or paired comparison tests, rather than a triangle test or tetrad. Indeed, the latter require comparing simultaneously more samples than the former, and are therefore more costly from a cognitive point of view. 

Alloying bismuth (Bi) with Pd/C has resulted in a slight decrease in performance but an increase in stability during a constant DP AFC operation at 0.3 V (Fig. 4.7) [71]. Unfortunately after 2 h of continued operation, the performance of PdBi/C... 

AFCs that use liquid electrolytes have been well developed from l%0s to 1980s, and were successfully applied in space programs. They are the best performing of all known fuel cell types operable below 150 °C owing to their facile kinetics at the cathode and anode [1-3]. Compared to the harsher acidic enviromnent, AFCs not only offer advantages in cathode and anode kinetics but also improved material stability. Many less expensive non-Pt electrocatalysts, such as Pd, Ag, and Ni, have been successfully applied in AFCs [4—8]. However, the fundamental difficulty with the AFCs is that the aqueous KOH electrolyte reacts with CO2 from the air to form carbonate species that lower the AFC performance and reduce the lifetime of the cell through the formation of carbonate precipitates oti the electrodes. 

A decreased electrochemical performance, induced by electrochemical stress, was observed at DLR for GDEs with nickel catalysts in the anodes of AFCs. For aU electrodes, a time constant of 250h is observed at a current density of lOOmAcm". This time constant depends on the current density, a dependence that is stronger for the type 3 electrodes than for the type 2 electrodes. The second time constant for describing the degradation of the electrochemical performance of the type 2 electrodes depends strongly on the current density. 

Silver has been widely used as a cathode material in AFCs, in part due to its relatively low cost ( 1 % that of Pt). The performance of Ag-based electrodes is typically reported to be slightly lower than that of Pt, but under high pH conditions, the ORR activity of Ag compares favorably with that of Pt [8]. The application of Ag as a catalyst in low temperature fuel cells is favorable only in alkaline media, as Ag is unstable in acidic electrolytes in the potential range of interest for the ORR. 

AUcaline fuel cells (AFCs, hydrogen-fuelled cells with an alkaline liquid electrolyte such as KOH(aq)) are the best performing of all known conventional hydrogen-oxygen fuel cells operable at temperatures below 200 C. This is due to the facile kinetics at the cathode and at the anode cheaper non-noble metal catalysts can be used (such as nickel and silver [3,4]), reducing cost. McLean et al. gave comprehensive review of alkaline fuel cell technology [5]. The associated fuel cell reactions both for a traditional AFC and also for an AMFC are ... 

Stoica et al. [140,141] developed and characterized another anion-conducting membrane based on a poly(epichlorohydrin) copolymer using allyl glycidyl ether as cross-linking agent. In order to introduce anionic properties, two cyclic diamines were incorporated DABCO and l-azabicyclo-[2.2.2]-octane (quinuclidine) (Fig. 5.13). To stabilize the membrane, the film was thermally or photochemically cross-linked. High conductivities were obtained without any KOH addition. At 60 °C and 98% relative humidity, the membrane exhibited a hydroxyl conductivity of 1.3 X 10 S/cm and an ion exchange capacity of 1.3 X 10 equiv./g. The performance of this MEA in H2/O2 AFC operation was 100 X 10 W/cm for 270 X 10 A/cm [142]. 

For alkaline electrolytes, the oxidizer reduction reaction (ORR) kinetics are more efficient than acid-based electrolytes (e.g., PEFC, PAFC). Many space appUcations utiUze pure oxygen and hydrogen for chemical propulsion, so the AFC was well suited as an APU. However, the alkaline electrolyte suffers an intolerance to even small fractions of carbon dioxide (CO2) found in air which react to form potassium carbonate (K2CO3) in the electrolyte, gravely reducing performance over time. For terrestrial applications, CO2 poisoning has limited lifetime of AFC systems to well below that required for commercial application, and filtration of CO2 has proven too expensive for practical use. Due to this limitation, relatively little commercial development of the AFC beyond space applications has been realized. Some recent development of alkaline-based solid polymer electrolytes is underway, however. The AFC is discussed in greater detail in Chapter 7. 

In an AFC, carbon dioxide impurity can degrade the performance of the electrol3de over time. How can the effect of CO2 impurity on the electrol3de be precisely quantified and modeled ... 

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