Carbon of PTFE

The generic process for electrochemical synthesis of sp-carbon chains was electrochemical reductive carbonization (corrosion) of poly(tetrafluoro-ethylene) (PTFE) by alkali metal amalgams, pioneered by Jansta and dousek [6 9] (for review see Reference 3). The reaction occurs at the interface of a dry contact between PTFE and alkali metal amalgams, hence, it does not seem to recall an electrochemical synthesis in its classical sense. The purely electrochemical carbonization of PTFE on a Pt electrode in aprotic electrolyte solution is also possible [3], but the amalgam-driven process is superior, presenting a clean and well-defined alternative to classical (wet) electrochemistry. 

Table 4.1 summarizes some experimental values of rate constants Kq. The most precise data exist for the amalgam-driven carbonization of PTFE and various other fluoropolymers [3,10,11]. 

The carbonization of PTFE on a metal cathode in aprotic electrolyte solutions was pioneered by Brewis et al. [50], and further studied by this [12,13, 51-54] and other groups [55-63]. The charge consumed in reaction 4.17 slightly exceeds that for a total dehalogenation of PTFE [12,13,53]. This superstoichiometric reduction indicates n-doping of the formed carbon ... 

The electrochemical carbonization of PTFE is anisotropic, propagating rapidly along the oriented macromolecule chains [12,54]. Although the aim of early studies [12,13,50-54] was just surface modification of PTFE for improving of its adhesion, the authors have intuitively suggested the formation of polyyne (cf. Eq. 4.18) [52]. This prediction was later confirmed by IR spectroscopy [56,60,61]. The reactivity of ex-PTFE carbon was recently used for its subsequent functionalization with diazonium salts [62] and metallization [63]. 

In spite of the high effort focused on the carbon electrochemistry, very little is known about the electrochemical preparation of carbon itself. This challenging idea appeared in the early 1970s in connection with the cathodic reduction of poly(tetrafluoroethylene) (PTFE) and some other perfluorin-ated polymers. The standard potential of the hypothetical reduction of PTFE to elemental carbon ... 

In Moscow Power Engineering Institute (TU) portable air aluminum batteries with saline electrolyte were developed [7, 18, and 20], In our devices, the air electrodes consist of two layers. Diffusion layer contains PTFE, carbon black and metal screen active layer consists of activated carbon and PTFE. At 293 K and the range of current density 2-25 mA/ cm2 dependence of cathode potential E (in H-scale) upon current density J (Figure 2) may by written by the Tafel equation (12). 

Numerous determinations of the heat of formation of carbon difluoride, a transient intermediate in the production of PTFE, for example, have been made. The most recent one has combined kinetic and equilibrium approaches. The equilibrium C2F4 2CF2 was studied at 1150-1600 K at 0.07-46 bar in dilute argon mixtures using incident and reflected shock waves. The carbene concentration was monitored at 250 nm after a careful study of the extinction coefficient over a wide temperature range. Rate parameters were found for forward and back... 

The porous hydrophobic film of previous electrode designs has now been substituted with a new layer based on a mixture of particles of hydrophobic carbon and PTFE binder. This mixture is very similar in composition to the catalytic layer. This particular modification provides several advantages ... 

Ofher diffusion layer approaches can also be found in the literature. Chen-Yang et al. [81] made DLs for PEMFCs out of carbon black and unsintered PTFE comprising PTFE powder resin in a colloidal dispersion. The mixture of fhese materials was then heated and compressed at temperature between 75 and 85°C under a low pressure (70-80 kg/cm ). After this, the DLs were obtained by heating the mixture once more at 130°C for around 2-3 hours. Evenfually, fhe amount of resin had a direct influence on determining the properties of fhe DL. The fuel cell performance of this novel DL was shown to be around a half of that for a CFP standard DL. Flowever, because the manufacturing process of these carbon black/PTFE DLs is inexpensive, they can still be considered as potential candidates. 

Campbell et al. [84] developed DEs made out of glass fiber webs filled wifh carbon and PTFE particles. The same research group later designed special DEs made with different carbons claiming to improve the overall fluid diffusion toward the catalyst layer [85]. 

On the other hand, with the same amount of PTFE as the CFP, carbon cloth (E-TEK type A carbon cloth) performed better and was able to eject the gases within the DL more effectively, thus giving more access to the methanol. 

In DMFCs, Scott, Taama, and Argyropoulos [117] changed the PTFE content (from 0 to 40 wt%) of the anode DL (E-TEK type A CC) in order to observe how this affected the methanol and carbon dioxide transport through the DL. At very high levels of PTFE, the performance of the cell decreases due to an increase in resistance losses. On the other hand, when an untreated CC was used, the observed performance was the lowest of all the materials investigated. In this study it was concluded that the ideal amount of hydro-phobic agent for the anode DL is around 13-20 wt% (see Figure 4.17). 

Influence of PTFE content in the anode DL of a DMFC. Operating conditions 90°C cell temperature anode at ambient pressure cathode at 2 bar pressure methanol concentration of 2 mol dm methanol flow rate of 0.84 cm min. The air flow rate was not specified there was a parallel flow field for both sides. The anode catalyst layer had 13 wt% PTFE, Pt 20 wt%, Ru 10 wt% on Vulcan XC-73R carbon TGP-H-090 with 10 wt% PTFE as cathode DL. The cathode catalyst layer had 13 wt% PTFE, Pt 10 wt% on carbon catalyst with a loading 1 mg cm Pt black with 10 wt% Nafion. The membrane was a Nafion 117. (Reprinted from K. Scott et al. Journal of Applied Electrochemistry 28 (1998) 1389-1397. With permission from Springer.)... 

Yu et al. [139] developed a dry-deposition technique for coating the MPL onto a diffusion layer. This method consisted of forcing a mixture of carbon and PTFE powder through a mesh with the help of a vacuum pump located underneath the DL material. Once the mixture passed through the mesh, it was deposited on the surface of fhe substrate (still with the help of the vacuum pump). After this, the DL, with the MPL, was sintered at 350°C in order to melt the PTFL particles and bind all the particles together. Once the thermal stage was completed, the MPL was subjected to a rolling step in order to adjust the total thickness of the layer (MPL and DL). 

In addition to the way in which the MPL is manufactured, other MPL parameters directly affect fuel cell performance. These include fhickness of fhe MPL, carbon loading, PTFE content, type of carbon parficles, efc. The following subsection will briefly discuss them. 

One final example of multiple layer MPL was presented by Karman, Cindrella, and Munukutla [172]. A four-layer MPL was fabricated by using nanofibrous carbon, nanochain Pureblack carbon, PIPE, and a hydrophilic inorganic oxide (fumed silica). The first three layers were made out of mixtures of the nanofibrous carbon, Pureblack, carbon, and PTFE. Each of these three layers had different quantities from the three particles used. The fourth layer consisted of Pureblack carbon, PTPE, and fumed silica to retain moisture content to keep the membrane humidified. Therefore, by using these four layers, a porosity gradient was created that significantly improved the gas diffusion through the MEA. In addition, a fuel cell with this novel MPL showed little performance differences when operated at various humidity conditions. 

Peled ef al. [177] also designed a novel MEA in order to improve the water back diffusion from fhe cathode to the anode side. They used a liquid-water barrier layer (LWBL), which consisted of a paste, made out of PTFE and carbon black particles, fhat was inserted in the pores of fhe CFP to form a layer inside fhe paper. Up to seven layers were necessary in order to achieve a uniform layer of 20-50 pm in thickness. Testing showed that the LWBL on the cathode DL creates a hydraulic pressure that forces (or pushes) the water back from fhe cafhode toward the anode, thus improving the cell s water management at different operating conditions. 

D. Bevers, R. Rogers, and M. von Bradke. Examination of the influence of PTFE coating on the properties of carbon paper in polymer electrolyte fuel cells. Journal of Power Sources 63 (1996) 193-201. 

A twin electrode thin layer Kissinger cell was designed in which the channel volume could be varied through the use of PTFE spacers [174]. The working and counter electrodes were carbon paste (3.14 m ) and the reference electrode was Ag/AgCl. The performance of the cell was tested on 50 pL portions of chlorpromazine solutions in 0.01 M HCl, and the three cited methods were compared. Linear sweep voltammetry was found to be the simplest to apply and showed moderate sensitivity. 

Teflon is the brand name for the chemical compound called polytetrafluoroethylene (PTFE). One molecule of PTFE contains two carbon atoms bonded to four fluorine atoms. Many molecules of PTEE can form polymer chains known as fluoropolymers. 

Tetrafluoroethylene (TFE), also known as perfluoroethylene, is a colorless, flammable, toxic gas. It is the monomer used for polytetrafluoroethylene (PTFE), which is sold under the DuPont tradename of Teflon. TFE is co-polymerized with other compounds to produce a variety of Teflons. TFE is produced by heating chlorodifluoromethane (CHC1F2, Freon-22) or trifluoromethane (CldF3, Freon-23). TFE is used almost exclusively as a monomer in the production of PTFE. PTFE is a vinyl polymer, which means it is made from a monomer with carbon-carbon double bonds. PTFE is made from TFE by free radical polymerization. 

The exceptional thermal and chemical stability of PTFE originate in the strength of its primary chemical bonds. The carbon-fluorine bond energy is the highest currently known among organic compounds [3, 4]. In addition, the fluorine atoms... 

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