Nafion, electron transport

The unique electronic stmcture of CNTs makes them ideal candidate materials for composite devices. SWNT/poly (3-octylthiophene) composite possesses properties suitable for use in photovoltaic cells.Composites with PMPV have been reported to be effective in organic light-emitting diodes as electron transport layers.Actuators based on SWNT/Nafion composites have potential applications as microswitches and artificial muscles. CNT/polyimide composites have been investigated for use as ultrafast optical switches. Such materials exhibited optical delay times less than 1 ps at a wavelength of 1.55 pm and showed great potential as all-optical switches. MWNTs dispersed in a photoresist epoxy can be applicable in the production of microelectromechanical systems such as electroplating molds, sensors, and aauators. ... 

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. 

Because the reaction in a CL requires three-phase boundaries (or interfaces) among Nafion (for proton transfer), platinum (for catalysis), and carbon (for electron transfer), as well as reacfanf, an optimized CL structure should balance electrochemical activity, gas transport capability, and effective wafer management. These goals are achieved through modeling simulations and experimental investigations, as well as the interplay between modeling and experimental validation. 

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. 

Fig. 24 A microtomed slice of a catalyzed Nafion 117 membrane used in a DMFC, imaged with an electron microscope. Two features gleaned from this image have been the sharpness of the catalyst layer/membrane interface (on the scale of the image shown) and a measured thickness of the catalyst layers about twice that calculated from perfect space filling by the composite catalyst layer material. The latter suggests a high fraction of free volume, facilitating gas transport [52].

The reasons for the success of this model are elaborated in the next section, which also addresses the physicochemical and transport properties of Nafion . It may be noted that the existence of cluster networks in the perfluorinated membranes was demonstrated by Gierke et al. [40] by transmission electron microscopy. 

In these two Ni-functionalized CNT materials, the Ni-molecular catalyst is located at the crossroads of the three interpenetrated networks allowing percolation of protons (the Nafion membrane), hydrogen (the pores in the gas diffusion layer), and electrons (the carbon fibers of the gas diffusion layer relayed by the conducting CNTs). In a way and even if it is not as well defined as in the protein, the catalyst environment in this membrane-electrode assembly reproduces that found in the active sites of hydrogenases buried into the polypeptidic framework but connected to the surfece of the protein via a gas diffusion channel, a network of hydrogen-bonded amino acids for proton transport and the array of electrontransferring iron-sulfur clusters. 

Oxidation of NO on classical conductive materials such as noble metals (platinum, gold, etc.) or carbon, which are used as electrodes, produces a relatively low current at neutral pH. This is due to a strong absorption of NO to the electrode surface and a slow rate of electron transfer between NO and the electrode. Typical differential pulse voltammograms (DPV) of NO on carbon liber covered with Nafion, and carbon fiber covered with porphyrinic film and Nafion are shown in Fig. 3. There is about a 190 mV difference between the oxidation potential of NO on carbon fiber and porphyrinic film. A concentration of 0.1-pM NO produces a very small current on the carbon fiber electrode operating in the DPV mode (Fig. 3a). However, this same carbon fiber covered with a layer of polymeric porphyrin produces a much larger current (Fig. 3b) for NO oxidation. The current generated on polymeric porphyrin is mass transport controlled and is linearly proportional to the concentration of NO. The linearity is observed over four orders of magnitude of NO concentration [45]. 

Ion exchange polymeric films, for instance, Nafion are electroactive by exchange of some of their charge-compensating counterions for electroactive ones [57, 58]. In such films, electrons are transported in part by physical diffusion of electroactive ions, which may also undergo self-exchange as part of the transport process. 

In the Z-scheme MR, one of the key components is the modified Nafion membrane that allows not only the transport of H2 ions, but also the exchange of the electron... 

Nafion, a polymer known to form hydrophilic and hydrophobic domains ( ), was shown by Buttry and Anson to transport electrons partially by "single-file diffusion of electroactive species as they competed for ion-exchange sites within the polymeric film ( ). The heterogeneity of Nafion domains is important in applications such as the electrocatalytic system described above, and represents one of the earliest moves toward architectural design of microstructures at the surfaces of electrodes (vide infra). 

From our study of the electroreduction of O2 in 0.5 M H2SO4 on Pt electrodes coated with a Nafion film [4, we recall the following conclusions i) the film does not alter the mechanism of the reduction reaction ii) the film concentrates O2 fi om the solution but, nevertheless, the current measured for the filmed electrodes only increases relatively to the uncoated electrodes near the onset of the reduction where electron transfer controls once diffiision becomes important, the current decreases because, whatever the thickness, the transport of O2 inside the film is slowed down iii) the values found of the O2 concentration, and the diffiision coeficient of O2, Df i, inside the film, showed that films of low thickness p) behave like Nafion membranes whereas thicker films behave like recast films. 

Besides the catalyst gradient in the catalyst layer, other components such as the hydrophobic agent (PTFE) and proton conductive polymer (Nafion) may also need to be adjusted in order to optimize gas/water transportation and electron/proton transfer. It can be expected that the catalyst layer adjacent to the gas diffusion layer side should be more hydrophobic to ensure much more of the reactants penetrates the inside of the electrode. While near the membrane side, more proton conductive polymer is needed to ensure a continuous network for proton conduction. Therefore, a non-uniform catalyst layer with a decreasing PTFE loading and an increasing Nafion content along the through-plane direction from GDL to membrane should be more efficient. 

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