Effective conductivity, ionomer

Figure 6.11. Nyquist plots for MEAs containing different proton-conducting ionomers at 0.85 V without external humidification catalyst loading = 0.4, 0.7 mg Pt/cm2 for anode and cathode, respectively TceU = 25°C Pressure = 1 atm and H2/02 flow = 400 cmVmin [8]. (Reprinted from Electrochimica Acta, 50(2-3), Ahn SY, Lee YC, Ha HY, Hong SA, Oh IH. Effect of the ionomers in the electrode on the performance of PEMFC under non-humidifying conditions, 673-6, 2004, with permission from Elsevier.)...

Transport properties of ionomer blends, characterized by a given type of spheroids and the aspect ratio, e/a, can now be analyzed by the effective medium theory discussed in the previous section. In this theory, the two phases are assumed randomly mixed and the probability of finding each phase is equal to its volume fraction f.. The effective conductivity, o, of the composite for either Na+ of OH ions is given by (15) ... 

Gas diffusion is a much more effective mechanism of reactant supply and water removal. Yet, CLs with sufficient gas porosity, usually in the range Yp - 30% -60%, have to be made much thicker, 10 pm - 20 pm. At such thicknesses, proton diffusion in liquid water is not sufficient for providing uniform reaction conditions. Porous gas diffusion electrodes are therefore impregnated with proton-conducting ionomer, usually Nafion [1-2, 4]. Resulting CLs are random composite media of carbon/Pt, ionomer, and a complex pore space. 

Curves on the left hand side of Figure 2.8, show the effect of ionomer load (/3aa) on (a) proton conduction efficiency and (b) electronic conduction efficiency of reconstructed CLs. with 30% porosity (O ). Each curve represents different pore size distribution. It can be observed that... 

Figure 2.8. Effect of ionomer load ( n) on proton conduction efficiency and the electronic conduction efficiency of reconstructed CLs. See main text for full description.

Curves on the right of Figure 2.8 show the effect of ionomer load (Pn) on the proton conduction (a) and the electronic conduction (b) efficiency of the reconstructed CLs. Each curve represents different porosities (O7). As the pore size distribution does not affect these results, conduction efficiency dependent on the pore size distribution, was averaged to be included in this analysis. As expected, one can also observe that the decay rate of the proton conduction efficiency increases, while the electronic conduction efficiency diminishes when p increases. 

In Figure 2.9, the results obtained for proton conduction (a) and electronic conduction (b) efficiency versus different total porosity in reconstructed CLs, now with 35% of ionomer loading, are shown. Each curve represents a different pore size distribution and it can be noticed that conduction efficiencies, both electronic and ionic, decrease almost linearly when the CLs porosity increases. This trend is caused by the smaller volumetric proportion of solid components (electronic and ionic) when porosity increases, which clearly affects the effective conductivity of the heterogeneous material. In addition, it should be noted that the slope for the electronic efficiency is different from that for the ionic efficiency. As we shall see later, the decay rate depends on Pj. Also from these figures, one can see that the pore size distribution does not affect the effective ionic or the effective electronic conduction for the same Pi value. 

Electrochemical polymeriza tion of heterocycles is useful in the preparation of conducting composite materials. One technique employed involves the electro-polymerization of pyrrole into a swollen polymer previously deposited on the electrode surface (148—153). This method allows variation of the physical properties of the material by control of the amount of conducting polymer incorporated into the matrix film. If the matrix polymer is an ionomer such as Nation (154—158) it contributes the dopant ion for the oxidized conducting polymer and acts as an effective medium for ion transport during electrochemical switching of the material. 

The catalyst layer is composed of multiple components, primarily Nafion ion-omer and carbon-supported catalyst particles. The composition governs the macro- and mesostructures of the CL, which in turn have a significant influence on the effective properties of the CL and consequently the overall fuel cell performance. There is a trade-off between ionomer and catalyst loadings for optimum performance. For example, increased Nafion ionomer confenf can improve proton conduction, but the porous channels for reactanf gas fransfer and water removal are reduced. On the other hand, increased Pt loading can enhance the electrochemical reaction rate, and also increase the catalyst layer thickness. 

Atkins, J. R., Sides, C. R., Greager, S. E., Harris, J. L., Pennington, W. T., Thomas, B. H. and DesMarteau, D. D. 2003. Effect of equivalent weight on water sorption, PTFE-like crystallinity, and ionic conductivity in bis[(perfluoroalkyl) sulfonyl] imide perfluorinated ionomers. Journal of New Materials for Electrochemical Systems 6 9-15. 

Microstructures of CLs vary depending on applicable solvenf, particle sizes of primary carbon powders, ionomer cluster size, temperafure, wetting properties of carbon materials, and composition of the CL ink. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules, which control the catalyst layer formation process. The choice of a dispersion medium determines whefher fhe ionomer is to be found in solubilized, colloidal, or precipitated forms. This influences fhe microsfrucfure and fhe pore size disfribution of the CL. i It is vital to understand the conditions under which the ionomer is able to penetrate into primary pores inside agglomerates. Another challenge is to characterize the structure of the ionomer phase in the secondary void spaces between agglomerates and obtain the effective proton conductivity of the layer. 

To examine the possibility that the changes in membrane properties were caused by permanent chemical changes in the Na- SPS, a portion of the annealed sample was redissolved in DMF, recast as a film and its conductance measured. As Table IV indicates, the recast film returned to its original conductance, which clearly proves that the annealing process only effected the physical properties of the Na-SPS ionomer film. This point is reinforced by our discovery that an annealed sample which was stored in water over a period of a week slowly changed back to its original conductance value. 

The anode layer of polymer electrolyte membrane fuel cells typically includes a catalyst and a binder, often a dispersion of poly(tetraflu-oroethylene) or other hydrophobic polymers, and may also include a filler, e.g., acetylene black carbon. Anode layers may also contain a mixture of a catalyst, ionomer and binder. The presence of a ionomer in the catalyst layer effectively increases the electrochemically active surface area of the catalyst, which requires a ionically conductive pathway to the cathode catalyst to generate electric current (16). 

Limoges et al. looked at the HOR catalytic activities of a series of heteropolyacids (HPAs) containing Mo and V. The CD is too low (a few mA/cm ) for them to be used as stand-alone anode catalysts, although it should be pointed out that the HPA loading of the anode used in this study was one to two orders of magnitude lower on a molar basis than that of a typical Pt anode. However, HPAs have been shown to be promising proton-conductive membrane/ionomer tillers and effective catalysts for H2O2 decomposition. On this basis, they may eventually become a part of fuel cell electrodes. 

---