Effective catalyst layer proton conductivity

An effective catalyst layer must serve multiple functions simultaneously electron and proton conduction, oxygen or hydrogen supply, and water management. The composition and structure of a CL can affecf all fhese functions... 

The function of the electrolyte membrane is to facilitate transport of protons from anode to cathode and to serve as an effective barrier to reactant crossover. The electrodes host the electrochemical reactions within the catalyst layer and provide electronic conductivity, and pathways for reactant supply to the catalyst and removal of products from the catalyst [96], The GDL is a carbon paper of 0.2 0.5 mm thickness that provides rigidity and support to the membrane electrode assembly (MEA). It incorporates hydrophobic material that facilitates the product water drainage and prevents... 

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. 

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. 

An archetypal MEA consists of an electrolyte membrane sandwiched between two catalyst layers and two gas diffusion layers (GDLs) as shown in Fig. 1. The fuel and oxidant gases diffuse through the GDL to react in the catalyst layer between the electrode and electrolyte. The catalyst, typically Pt or Pt based alloy, are nanoparticles residing on carbon particles. In addition to its primary purpose as the center of reactivity, the catalyst must participate in the effective adsorption of the reactants, conduction of the electrons to/from the electrode and diffusion of protons to/from... 

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.

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