Nafion® content

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]. 

Fig. 2. Schematic representation of electrodes, (a) Content of Nafion too low not enough catalysts with ionic connection to membrane, (b) Optimal Nafion content electronic and ionic connections well balanced, (c) Content of Nafion too high catalyst particles electronically isolated from diflusion layer. Reproduced from [9].

Li and co-workers [139] synthesized Pt (5 30wt%) nanoparticles supported on CNFs using a modified ethylene glycol method. Pt-CNF based MEAs with 50 wt% Nafion exhibited higher cell performance than the carbon black based MEAs with an optimized 30 wt% Nafion content (Fig. 14.11). This was attributed to the larger length to diameter ratio of CNFs that allows the formation of conductive networks in the Nafion matrix [140]. 

Using this equation, the optimum Nafion loading is around 36 wt% for all Pt loadings. Sasikumar etal. [14] summarized the literature results and found that the optimum Nafion content is in the range of 30-36 wt% for the CL. 

Song et al. [122] modeled optimal performance as a function of Nafion content as well as Pt loading. 

The experimental optimization of Nafion ionomer loading within a catalyst layer has attracted widespread attention in the fuel cell community, mainly due to its critical role in dictating the reaction sites and mass transport of reactants and products [15,128-134]. Nafion ionomer is a key component in the CL, helping to increase the three-phase reaction sites and platinum utilization to retain moisture, as well as to prevent membrane dehydration, especially at low current densities. Optimal Nafion content in the electrode is necessary to achieve high performance. 

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. 

Song, Y, Xu, H., Wei, Y, Kunz, H. R., Bonville, L. J., and Fenton, J. M. Dependence of high-temperature PEM fuel cell performance on Nafion content. Journal of Power Sources 2006 154 138-144. 

An alkene mixture of industrial source (equal amounts of C9-C13 alkenes and alkanes) was used in the alkylation of benzene on three Nafion-silica catalysts with 5%, 13%, and 20% loadings.195 20% Nafion-silica showed high and stable activity and its performance exceeded that of a Y-zeolite-based material. The selectivity to 2-phenylalkanes (25%) was higher than in the Detal process using fluorinated silica-alumina but decreased somewhat with increasing Nafion content. 

Nafion-silica nanocomposites exhibit high selectivity in the synthesis of substituted 7-hydroxychromanones 158 in high yields691 [Eq. (5.247)]. Conversions increase with increasing Nafion loading and, consequently, Nafion SAC-80 (Nafion-silica with a Nafion content of 80 wt%) affords the highest yields. The catalysts could be recycled after treatment with nitric acid or hydrogen peroxide. 

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. 

Nafion content in the catalyst layer plays an important role in electrode performance. Incorporation of Nafion ionomer into carbon-supported catalyst particles to form the catalyst layer for the gas diffusion electrode can establish a three-dimensional reaction zone, which has been proven by cyclic voltammetric measurements. An optimal Nafion content in the catalyst layer of the electrode may minimize the performance loss that arises from ohmic resistance and mass transport limitations of the electrode [6],... 

In Song et al. s same work [5], the effect that Nafion content in the catalyst layer had upon electrode performance was also investigated, following their work on the optimization of PTFE content in the gas diffusion layer. The optimization of Nafion content was done by comparing the performance of electrodes with different Nafion content in the catalyst layer while keeping other parameters of the electrode at their optimal values. Figures 6.8 and 6.9 show the polarization curves and impedance spectra of fuel cells with electrodes made of catalyst layers containing various amounts ofNafion . 

As seen in Figure 6.8, the fuel cell with 0.8 mg/cm2 Nafion content in the catalyst layer exhibited the best performance. To the researchers surprise, the performance of the fuel cell with 2.0 mg/cm2 Nafion content dropped sharply when the current was over 300 mA/cm2. The linear zone of the V-I curve with 2.0 mg/cm2 Nafion content is much shorter than that for the other two cases. This certainly would contribute to mass transport limitations within the catalyst layer under high ionomer loading. 

Sasikumar G, Ihm JW, Ryu H (2004) Dependance of optimum Nafion content in catalyst layer on platinum loading. J Power Sources 132(1—2) 11—17... 

Optimal Nafion content in electrodes made with sulfonated sUane-treated Pt/C was around 10% wt., while the optimal Nafion content for electrodes made with untreated Pt/C was 30% wt. The performance of the former electrode with 10% Nafion was only slightly lower than that of the latter with 30% Nafion . When 10% Nafion was used with the untreated Pt/C to make the electrode, its performance was much lower than that obtained by silane-treated Pt/C (Fig. 9). Clearly, the presence of sulfonated silane contributed significantly to the proton conductivity in the electrodes. It was found that modification of the carbon support prior to the Pt deposition was more effective than modification of Pt-catalyzed carbon, presumably due to the blocking of the active Pt sites by the silane in the latter case. Also, estimates indicated that the sulfonate loading was similar at the optimal Nafion loadings for untreated and sUane-treated Pt/C electrodes. Fuel cells showed no performance loss in 12 hours of operation, indicating that the attached sulfonated silane groups were stable in the fuel cell environment. 

Although the sulfonated silane-treated Pt/C electrode reduced the optimal Nafion content to one third of the untreated Pt/C electrode, the best performance of the former was slightly... 

Higher diffusivity means higher CCL porosity, which is usually achieved at the cost of lower Nafion content and thus of lower proton conductivity. Equation (2.80) thus gives an optimal oxygen diffusion coefHcient in the CCL, and ) can be used as a reference point for optimal CCL design in terms of porosity and related Nafion content. 

It was found that the catalysts from different institutes behaved completely different when changing, e.g., the Nafion content or the catalyst loading in FC and RDE tests, as shown for FC measurements in Fig. 16.20. 

Figure 8.3. Side view of the MD simulation system used in [112] at the maximum Nafion content. The graphite layer is represented by the two parallel planes shown at bottom. (Reprinted from Elecfrochimica Acta, 51.26, Lamas EJ, Balbuena PB. Molecular dynamics studies of a model polymer-catalyst-carbon interface, 5904-11, 32006, with permission from Elsevier.)...

Figure 10.9. Electrochemically active surface (EAS) from cyclic voltammetry as a function of Nafion content in the catal5hic layer (20 wt% PTFE/C in diffusion layer, 0.2 mg Pt cm in catalyst layer) [70]. (Reproduced from Journal of Power Sources, 77(2), Antolini E, Giorgi L, Pozio A, Passalacqua E. Influence of Nafion loading in the catalyst layer of gas-diffusion electrodes for PEFC, 136-42,1999, with permission from Elsevier.)...

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