Nafion ionomer

Membrane Diffusion layer Nafion ionomer Catalyst Particle... 

Fig. 3. The effect of Nafion ionomer content on the MEA perfonnance. (a) E-TEK catalyst, (b) Pt-Ru/SRaw, (c) Pt-Ru/S700, (d) Pt-Ru/S900.

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. 

Ayato Y, Kunimatsu K, Osawa M, Okada T. 2006. Study of Pt electrode/Nafion ionomer interface in HCIO4 by in situ surface-enhanced ETIR spectroscopy. J Electrochem Soc 153 A203-A209. 

At the next level of abstraction are measurements performed at a thin film of fuel cell catalyst immobilized on the surface of an inert substrate, such as glassy carbon (GC) or gold (Fig. 15.2c). Essentially, three versions of this approach have been described in the fiterature. In the first case (a porous electrode ), an ink containing catalyst and Nafion ionomer is spread onto an inert nonporous substrate [Gloaguen et al., 1994 Gamez et al., 1996 Kabbabi et al., 1994]. In the second case (a thin-fihn electrode ), the ink does not contain Nafion , but the latter is... 

To overcome these disadvantages, a thin-film CL technique was invented, which remains the most commonly used method in PEM fuel cells. Thin-film catalyst layers were initially used in the early 1990s by Los Alamos National Laboratory [6], Ballard, and Johnson-Matthey [7,8]. A thin-film catalyst layer is prepared from catalyst ink, consisting of uniformly distributed ionomer and catalyst. In these thin-film catalyst layers, the binding material is not PTFE but rather hydrophilic Nafion ionomer, which also provides proton conductive paths for the electrochemical reactions. It has been found that the presence of hydrophobic PTFE in thin catalyst layers was not beneficial to fuel cell performance [9]. 

It is well known that Nafion ionomer contains both hydrophobic and hydrophilic domains. The former domain can facilitate gas transport through permeation, and the latter can facilitate proton transfer in the CL. In this new design, the catalyst loading can be further reduced to 0.04 mg/cm in an MEA [10,11]. However, an extra hydrophobic support layer is required. This thin, microporous GDL facilitates gas transport to the CL and prevents catalyst ink bleed into the GDL during applications. It contains both carbon and PTFE and functions as an electron conductor, a heat exchanger, a water removal wick, and a CL support. 

A well-distributed deposition of Pt/C nanocatalyst and Nafion ionomer on bofh hydrophilic and hydrophobic carbon-based electrodes has been successfully obfained using a Pt/C concentration of 1.0 g/L, an electrical field of 300 V/cm, and a deposition time of 5 minutes [118]. The deposition of Pt/C nanocatalysts and Nafion solution via the electrophoretic process gives rise to higher deposition efficiency and a uniform distribution of catalyst and Nafion ionomer on the PEMFC electrodes. 

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. 

How to balance Nafion ionomer contenf and Pf/C loading is a challenge for optimizing CL performance, due to fhe complexity induced by proton and electron conduction, reactant and product mass transport, as well as electrochemical reactions within the CL. The optimization of such a complex system is mainly implemented through multiple components and scale modeling, in combination with experimental validation. 

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. 

Lee, D., and Hwang, S. Effect of loading and distribution of Nafion ionomer in the catalyst layer for PEMECs. International Journal of Hydrogen Energy 2008 33 2790-2794. 

Mao et al. [174] recently presented research in which Nafion ionomer particles were used as hyperdispersant agents in the MPL of a cathode DL. It was shown that this ionomer helps to decrease the particle size of the PTFE in the MPL. Thus, increasing the Nafion particle content gradually decreased the PTFE size and decreased the hydrophobicity in the layer. In fuel cell testing, an MPL having 1 wt% ionomer showed the best performance it improved the gas permeability and electronic conductivity. 

For the DMFC, Zhang et al. [127] used the sessile drop method to study the wettabilities of liquid methanol solutions on the surface of the anode DLs and MPLs. They were able to observe that the contact angles of the materials were the smallest with low PTFE content. In addition, the effect of Nafion ionomer content on the MPL (to increase hydrophilicity see Section 4.3.2) was also shown through the contact angle measurements (i.e., smaller contact angles compared to MPLs with PTFE). 

Recently, 500 MHz i fluorine NMR was used to study adsorption of Nafion ionomer on PEFC catalysts and the supporting carbons in aqueous solution. It was observed that Nafion adsorbs strongly on carbon as well as on Pt and PtRu. The adsorption was classified into primary and secondary adsorption. At low concentration of Nafion ionomer, the adsorption was found to follow a Langmuir isotherm (primary adsorption). Although there was uncertainty in the types of adsorption isotherms at high concentration of Nafion ionomer, the secondary adsorption isotherms were fitted to a Langmuir isotherm as well. 

Nafion ionomers were developed and are produced by the E. I. DuPont Company. These materials are generated by copolymerization of a perfluorinated vinyl ether comonomer with tetrafluoroethylene (TEE), resulting in the chemical structure given below. 

Figure 10. Evolution of a smaller number of larger clusters with increased hydration of Nafion ionomers, according to Gierke, Munn, and Wilson. (Reprinted with permission ref 17. Copyright 1981 Wiley.)...

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. 

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],... 

Guo et al. [7], as shown by the Nyquist plots in Figure 6.10. In their impedance measurements, different amounts of Nafion ionomer in the catalyst layer, ranging from 0.33 to 1.13 mg/cm2 (dry weight) were examined. The active area of their fuel cells was 1.0 cm2. The fuel cells were operated in H2/air gas feeding mode with a flow rate of 220 cm3/min (at standard temperature and pressure) for both sides. The cell temperature as well as the humidification temperature for both electrodes were controlled at 70°C. The cell s AC impedance was measured using a Gamry PC4/750-DHC2 potentiostat. The perturbation amplitude was set at 5 mV in potentiostatic mode, and the frequency was scanned from 0.01 Hz to 100 kHz with 10 points per decade. 

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 5. Snapshots of the final configurations of the bulk hydrated Nafion ionomer with the ionomers made invisible at hydration levels (a) X = 4.4, and (b) X = 9.6. A more connected water network is found at the higher water content.

Figure 19 TEM micrograph of catalyst-Nafion interface showing metal particles supported on carbon agglomerate and Nafion ionomer micelles.

The methanol permeability of the nanocomposite membranes was shown to decrease on addition of the sulfonated titanate. Functionalized montmorillonite (MMT) was also employed to improve PFSA [58, 59] these composite membranes provide a low methanol crossover, without sacrifidng proton conductivity due to the introduction of sulfonic acid groups at the MMT surface, followed by blending with the Nafion ionomer. 

Until recently, perfluorinated ionomrs with high equivalent weights were believed to be insoluble. Covitch(50), however, has identified a number of solvents and dissolution procedures for the sulfonyl fluoride precursor and sulfonate and carboxyl ate Nafion ionomers with 1100 to 1200 equivalent weight. This development has great potential for the preparation of sulfonate and carboxyl ate ionomer blends, the... 

Komorosl tj and Mauritz(14) studied the hydration of Nafion ionomers by Na-NMR, and they concluded that the interactions of the Na counterions with the sulfonated resin can be treated in terms of an equilibrium between strongly associated ions and re-... 

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