Fuel Cell Performance 29

A perfect fuel cell would be able to deliver as much power as fuel delivered, but in practice the maximum power deliverable by a fuel cell is limited by several factors: (1) the electrolyte conductivity, (2) catalytic activity, (3) gas transport processes, and (4) inherent “leakages” across the electrolyte membrane. The performance of a fuel cell is generally characterzied by measuring the electric potential, while drawing increasing amounts of current, which is then used to generate a performance “polarization” curve, such as that depicted by

the solid–line in Figure 1.9. Deviations from the theoretical open cell voltage, E0, result

from these aforementioned limitations. These “losses,” indicated in Figure 1.9, are termed Ohmic polarization, activation polarization, and concentration polarization. A significant limitation to the performance of a fuel cell is a result Ohmic losses, a direct consequence of slow ionic conduction in the electrolyte. A second significant source of losses, are the result of activation polarization, which is a consequence of sluggish reaction kinetics at the anode and cathode, and is directly affected by the activity of the fuel cell catalyst. Losses can also occur at high current densities, at which point concentration polarization losses result from the generation of fuel cell reaction products at a rate faster than they can be transported out of the cell—thereby diluting the fuel and lowering the overall chemical potential of the cell. The sum of these losses limits the overall performance of a fuel cell such that the measured power (I× E) output of a fuel cell, as a function of current, results in a characteristic peak power, indicated by the dotted-line in Figure 1.9. Last of all, inherent leakages across an electrolyte membrane can be the result of electronic conduction or fuel/oxidant cross-over through the electrolyte membrane, both of which lower the open cell voltage.

Figure 1.9: Diagram of a typical fuel cell performance characterization curves. Polarization curve, electric potential as a function of current—solid-line, and power as function of current—dotted-line.

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Chapter 2

Experimental Methods

2.1

Synthesis

One advantage to carrying out research on solid acids is the relatively easy synthesis of most compounds from aqueous solutions. In general, starting from the appropriate ions in aqueous solution in stoichiometric quantities yields the desired MHnXO4-type solid acid,

M+_{(aq) + H}

nXO4−(aq)→ MHnXO4(s) + H2O

where M = Li, Na, K, Rb, Tl, NH4, and Cs; X = S, Se, P, and As; and n= 1, 2. Various

methods are employed in precipitating solid acids compounds from solution: (1) heating to evaporate excess water, (2) freeze-drying to submilate excess water, and (3) solvent introduction to exceed the solubility limit of the desired solid acid compound. The last method is preferred, as it is an inexpensive and expeditious synthesis method. For each solid acid some methods are more effective than others, and in the latter case some solvents are more productive than others. Other factors which have an effect on the synthesis of solid acid compounds are pH, synthesis temperature, and ion concentration. In the Appendix are recipes for the preparation of various solid acid compounds. In this work, single crystals were grown by slow evaporation of water at ambient temperature from aqueous solutions, where the solutions were prepared from re-dissolved powders of the desired solid acid compounds.