Effective Proton Conductivity

The proton conductivity of a conventional CL is determined by the amount and network topology of the ionomer phase. It can be expressed as 

12 (Eikerling, 2006 Eikerling and Komyshev, 1998 Eikerling et al 2004, 2007a). It should be noted that this parameterization does not take into account the specific thin-film morphology of ionomer in CLs discussed in the section lonomer Structure in Catalyst Layers Redefined.  

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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. 

In order to maintain a A/1h+ across a membrane, and to ensure that it is used for the synthesis of ATP and not dissipated by leakage, the membrane must be closed and not leaky to protons. From the rate at which a pH gradient across the membrane decayed, it was shown that the effective proton conductance of the mitochondrial inner membrane [8], bacterial plasma membrane [9], and chloroplast thylakoid membrane [10] have a value of only some 0.5 jttS2/cm, or a million-fold less than the aqueous phases on either side. 

T, the effective protonic conductivity within the catalyst layer (S/cm). 

Fig. 20 Distributions at current density of 1 A cm 2, of electrode potential (top), reactant concentration (middle), and current generation (bottom) in a PEFC anode catalyst layer 5 pm thick, as result of limited transport rate of the hydrogen gas reactant and/or the limited transport rate of protons. Two cases of reactant concentration, 100% hydrogen and 10% hydrogen in the dry gas and two cases of effective protonic conductivity in the catalyst layer, 0.1 and 0.01 S cm-1, are considered in these calculations. A value of 2 x 10-4 cm2 sec-1 was used as estimate for effective Dh2 in the catalyst layer.

Intrinsic rate constant of evaporation Evaporation-penetration depth Liquid water viscosity Ratio of the distributed liquid vapor interfacial area to the apparent electrode surface area Active site fraction effective proton conductivity in CCL... 

The proton current in the CL is generated by the oxygen consumption, and the change in the proton current density, ih+, is satisfied with the balance expressed by Equation (1.4). The change in the cathode overpotential, rj, corresponds to the voltage decrease due to the proton transport it is expressed by Equation (1.5) using the proton current density, in+, and the effective proton conductivity,... 

In the catalyst layer, the effective proton conductivity of the polymer electrolyte and the effective diffusion coefficient in the pores of the catalyst layer depend on the catalyst layer structure. These properties, and were given using the Bruggeman correction factor as follows ... 

With high hydration in the electrolyte, a proton hopping, or Grotthuss mechanism [5] is observed, with concomitantly higher effective proton conductivity. In this mode of transport, protons hop from one H3O+ to another along a connected pathway in the ionomer structure. 

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