Nafion catalyst structure

Pt particles are in contact with Nafion. The other part is disconnected from the polymer electrolyte and therefore, is inactive. This effect is due to the relatively large size of Pt-black particles (Fig. 2a,b). Additional agglomeration of the Pt-black particles aggravates it even further (Fig. 2c). Flowever, in the case of carbon-supported catalyst the interfacial area, where polymer electrolyte, catalyst, and hydrogen as a reactant meet within the electrode approaches 90% (Table 1), which correlates with catalyst structure possessing evenly distributed Pt particles on the surface of spherical particles of carbon (Fig. 2d). 

When considering the morphology of prepared electro-catalysts are different to each other especially to the commercial one, one can think that the structure of electrode which was optimized to the commercial catalyst may not be optimum. So, the for the better electrode structures was conducted by investigating the effect of NFP. Fig. 2 is a schematic of electrode which depicts the effect of Nafion content[9]. For the conventional electrocatalysts, the range of 30 35 % NFP is reported as optimum value[10]. 

But when the contents of Nafion ionomer was increased from 30 to 45 % to find out the better electrode structures, the Pt-Ru/SRaw, which had showed the lowest single cell performance, became the best electro-catalyst. By this result one can conclude that as long as the structure of the electrode can be optimized for the each of new electro-catalysts, the active metal size is a more important design parameter rather than inter-metal distances. Furthermore, when the electro-catalysts are designed, the principal parameters should be determined in the consideration of the electrode structures which affect on the electron conduction, gas permeability, proton conductivity, and so on. 

Passalacqua, E., Lufrano, F., Squadrito, G., Patti, A., and Giorgi, L. Nafion content in the catalyst layer of polymer electrolyte fuel cells Effects on structure and performance. Electrochimica Acta 2001 46 799-805. 

Equilibrium structure of a catalyst blend composed of carbon (black), Nafion (dark gray), water (light gray), and implicit solvent. (Reproduced from K. Malek et al. Journal of Physical Chemistry C 111 (2007) 13627. Copyright 2007, with permission from ACS.)... 

Like many other fluoropolymers, Nafion is quite resistant to chemical attack, but the presence of its strong perfluorosulfonic acid groups imparts many of its desirable properties as a proton exchange membrane. Fine dispersions (sometimes incorrectly called solutions) can be generated with alcohol/water treatments. Such dispersions are often critical for the generation of the catalyst electrode structure and the MEAs. Films prepared by simply drying these dispersions are often called recast Nafion, and it is often not realized that its morphology and physical behavior are much different from those of the extruded, more crystalline form. 

The reaction was carried out using Nafion-H and polystyrene sul-phonic acid resin catalyst at 180°C and 140°C, respectively, which are their maximum temperatures of use 2-nitrotoluene solvent and 10% w/w catalyst. Conversions of 20% and 1.5% were obtained at the end of six hours indicating that higher temperatures will be necessary to achieve appreciable rates on these catalysts. Since these catalysts are not structurally stable above the respective temperatures, they appear to be unsuitable for this application. The reaction was also carried out using triflic acid as catalyst at various temperatures. The results are shown in Fig.3. Surprisingly, the reaction did not proceed beyond 40% conversion in spite of the high acidity of the catalyst. 

The MEA consists of a thin (10-200 pm) solid polymer electrolyte (a protonic membrane, such as Nation) on both sides of which are pasted the electrode structures (fuel anode and oxygen cathode) (Figure 9.5). The electrode structure comprises several layers the first layer made of carbon paper (or cloth) to strength the structure, on which are coated the GDL, and then the catalyst layer (CL), directly in contact with the protonic membrane (usually Nafion). 

As shown in Figure 1.6, the optimized cathode and anode structures in PEMFCs include carbon paper or carbon cloth coated with a carbon-PTFE (polytetrafluoroethylene) sub-layer (or diffusion layer) and a catalyst layer containing carbon-supported catalyst and Nafion ionomer. The two electrodes are hot pressed with the Nafion membrane in between to form a membrane electrode assembly (MEA), which is the core of the PEMFC. Other methods, such as catalyst coated membranes, have also been used in the preparation of MEAs. 

Separation of the individual contributors can provide useful information about performance optimization for fuel cells, helping to optimize MEA components, including catalyst layers (e.g., catalyst loading, Nafion content, and PTFE content), gas diffusion layers, and membranes. It assists in the down-selection of catalysts, composite structure, and MEA fabrication methods. It also helps in selecting the most appropriate operating conditions, including humidification, temperature, back-pressure, and reactant flow rates. 

Fuel cell performance is affected by MEA composition, including catalyst loading, PTFE content in the gas diffusion layer, and Nafion content in the catalyst layer and membrane, each of which affects the performance in different ways, yielding distinct characteristics in the electrochemical impedance spectra. Even different fabrication methods may influence a cell s performance and electrochemical impedance spectra. With the help of the model described above, impedance spectra can provide us with a useful tool to probe structure-performance relationships and thereby optimize MEA structure and fabrication methods. 

Antolini et al. [6] have provided empirical equations to calculate the optimal Nafion loading in the catalyst layer as a function of electrode structure. In the case of a catalyst layer containing Pt/C and Nafion ionomer, the optimal Nafion load (in mg/cm2) is expressed as... 

Pair correlation functions can also be used to show differences in structure between the bulk PEM and interfacial regions. Figure 13 shows the difference in the water network in the aqueous domain of bulk membrane of Nafion to those adsorbed on to a catalyst surface through the Oh o Oh o POF at all the water con-... 

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