Nafion® loading

To increase the surface area, the resin can be supported on porous carriers, or it can be directly incorporated into silica by a sol-gel preparation technique. Both methods have been used by Botella et al. (205), who compared several composite Nafion/silica samples with varying surface areas and Nafion loadings for isobutane/2-butene alkylation at 353 K. Furthermore, supported and unsupported Nafion samples were used. As expected, the unsupported resin with its low... 

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. 

Kamarajugadda and Mazumder [123] used numerical modeling and investigated the effects of ionomer (Nafion) loading, cafalyst (Pt)... 

Antolini, E., Giorgi, L., Pozio, A., and Passalacqua, E. Influence of Nafion loading in the catalyst layer of gas-diffusion electrodes for PEFC. Journal of Power Sources 1999 77 136-142. 

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. 

Li G, Pickup PG (2003) Ionic conductivity of PEMFC electrodes effect of Nafion loading. JElectrochem Soc (ll) C745-52... 

The function of a proton-conducting ionomer such as Nafion in the catalyst layer is to provide an ionic path for proton migration from the membrane to the reaction site at the catalyst surface. Therefore, the content of the proton-conducting ionomer in the catalyst layer will greatly influence the transport of protons to the catalyst sites. The impedance spectra of fuel cells with different Nafion loadings in the catalyst layers of both the cathode and the anode at OCV were compared by... 

Figure 6.9. Impedance spectra for the oxygen reduction reaction at an electrode potential of 0.6 V. The catalyst layer of the electrode contains various Nafion loadings ( ) 0.2 ( ) 0.8 (A) 2.0 mg/cm2 [5], (Reprinted from Journal of Power Sources, 94(1), Song JM, Cha SY, Lee WM. Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method, 78-84, 2001, with permission from Elsevier and the authors.)...

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... 

Figure 6.27. Nyquist plots (65000 to 0.82 Hz, 22 + 2°C, 1.00 V vs. H2) for nitrogen-bathed cathodes with various Nafion loadings. The inset shows an expansion of the high-frequency region of the plots [25], (Reproduced by permission of ECS—The Electrochemical Society, and the authors, from Li G, Pickup PG. Ionic conductivity of PEMFC electrodes.)...

The highest electronic resistances were observed at low Nafion loadings, indicating that the ionomer played a significant role as a binder [211], Meanwhile, kinetic losses pass throngh a minimnm correlated with the electrochemically active snrface area of the catalyst estimated from cyclic voltammograms [209] The higher the electrochemically active surface area, the lower the kinetic losses. This volcano type of cnrve reflects the optimnm in the metal utilization factor u. Below, we try to nnderstand how carbon properties may influence these characteristics. 

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. 

Coating of catalyst particles by an ionomer film is not likely to affect the availability of the catalyst surface for the electrochemical reaction. Only after the thickness of the coating exceeds about 23 nm, which is about 10 times the radius of the catalyst particle, will it affect the electrochemical reaction at a current density of 1.5 A cm 2 Pt. With a typical Nafion loading of 15-35% wt. in the catalyst layer, the coating of Nafion on the catalyst support is around several nanometers thick and thus it is not expected to have any negative impact on the availability of the catalyst particles underneath. The less than 100% catalyst utilization mainly arises from the lack of ionomer around some catalyst particles. 

Figure 12.22 Crossing wave patterns in the BZ reaction on the two surfaces of a Nafion membrane, (a) Nafion membrane loaded with ferroin to 16.7% capacity gives strong coupling, (b) Nafion loaded to 38.7% capacity gives weak coupling. (Reprinted with permission from Winston, D. Arora, M. ...

Fig. 8.15 SWV curve of [Cof complex 36 (1 x 10" M) and [Fc] complex 39 (2x10" M) at Nafion-loaded CPE (carbon paste electrode) in buffer containing 1% EtOH and 10% rabbit normal serum. (Data adapted from [55]).

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