Electrode PTFE content

Figure 6.4 shows the polarization curves of fuel cells with different PTFE content in the gas diffusion layer of the gas diffusion electrodes. The optimal PTFE content in the gas diffusion layer is 30 wt%. The performance of the fuel cell containing a gas diffusion layer with PTFE content of 40 wt% is similar to one with 10 wt% PTFE content. 

Figure 6.4. The polarization curves of fuel cells with electrodes that contain various PTFE content in the gas diffusion layer ( ) 10 ( ) 20 (A) 30 (+) 40 wt% [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.)...

In Figure 6.5a it can be seen that the kinetic arc for the electrode with 30 wt% PTFE content in the gas diffusion layer has the smallest diameter. Indeed, the spectra for this electrode all have the minimum kinetic loop measured at all three cathode potentials, as seen in Figure 6.5b and c. This result is in agreement with that from the polarization curve measurements however, AC impedance spectra provide more information than polarization curves. This figure shows that the impedance arc due to mass transport in the low-frequency region grows with increasing electrode overpotential and is very sensitive to PTFE content in the gas diffusion layer. 

Song et al. [5] explained that for the electrode with 40 wt% PTFE content in the gas diffusion layer, the increase in the size of the kinetic arc was attributable to the substantial decrease in the active Pt area caused by low water content at the interface of the catalyst layer and the gas diffusion layer. This explanation has been verified by cyclic voltammetric results. A possible solution to improve the performance of this particular electrode is simply to raise the humidification temperature in order to increase the water content at the interface. The results at higher humidification temperatures are shown in Figures 6.6 and 6.7. 

Figure 6.6 proves that increasing the humidification temperature does improve fuel cell performance. Figure 6.7 also confirms that the size of the kinetic arc does decrease with increasing humidification temperature. From these results the authors concluded that it was the reduced water content at the interface that caused the increased charge-transfer resistance of the electrode with excessive PTFE content (40 wt%). 

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 . 

For polymer electrolyte membrane fuel cell (PEMFC) applications, platinum and platinum-based alloy materials have been the most extensively investigated as catalysts for the electrocatalytic reduction of oxygen. A number of factors can influence the performance of Pt-based cathodic electrocatalysts in fuel cell applications, including (i) the method of Pt/C electrocatalyst preparation, (ii) R particle size, (iii) activation process, (iv) wetting of electrode structure, (v) PTFE content in the electrode, and the (vi) surface properties of the carbon support, among others. ... 

For the investigation the oxygenated liquid fuels in half-cells, chemically prepared Pt-Sn (0.5 mg cm 8% Sn) and R-Ru (0.5 mg cm 50% Ru) electrodes supported on gas-diffusion type Vulcan XC-72/Toray paper were obtained from ETEK Inc. These electrodes generally have 15-20% PTFE content in the electrocatalytic layer. The electrochemically prepared electrodes consisted of electro-depositing R-Sn and R-Ru from an acid solution of hydrogen hexachloroplatinate(IV), tin(IV)chloride, and potassium pentachloro-aquoruthenate. 

FIGURE 3.9 Specific capacitance and energy density of carbon BP2000 electrode material as a function of PTFE contents. (Reprinted from Effects of electrode layer composition/ thickness and electrolyte concentration on both specific capacitance and energy density of supercapacitor, 60, Tsay, K. C., L. Zhang, and J. J. Zhang, Electrochimica Acta, 428-436, Copyright 2012, with permission from Elsevier.)... 

The electrode can also be made with multiple catalyst layers. Each layer has different composition and structure (e.g., catalyst type and loading, support type and loading, ionomer content, PTFE content, porosity, and thickness) in order to achieve higher performance and durability and lower total precious metal loading. Of course, the manufacturing process will be slower and more costly. 

A good catalyst for the bifunctional hydrogen electrode is platinum, and for the bifunctional oxygen electrode the most promising catalyst material is a mixture of platinum and iridium oxide. The use of thin catalyst layers in the electrode helps to minimize mass transport and ohmic limitations [57]. In addition to the catalyst composition, the ionomer content, the catalyst layer thickness, and the PTFE content are varied and the influence of these variations on performance has been described [58-63]. The highest efficiency can be achieved using a catalyst with a high amount of platinum and a low amount of iridium [58, 60]. 

Giorgi L, Antolini E, Pozio A, Passalacqua E. Infuence of the PTFE content in the diffusion layer of low-Pt loading electrodes for potymer electrotyte fuel cells. Electrochim Acta 1998 43 3675-80. 

Giorgi, L., Antolini, E., Pozio, A. and Passalacqua, E. (1998) Influence of PTFE content in the DiBiision Layer of Low-Pt Loading Electrodes for Polymer Electrolyte Fuel Cells, Electrochirmca Acta, 43, 3675-3680. 

Fig. 3.3. Pressure equalisation burette in a device for potentiometric titration. For full details of operation see original publication. A break-seal ampoule containing the titrating solution B which reacts with the contents of break-seal ampoules P in reactor R, C sintered filter, Z) 10 ml burette, E 2 mm capillary, F capillary tip, Pt wires from electrodes leading to potentiometer. T, T, 7 PTFE taps, 7J three-way glass vacuum tap.

Fig. 3.23. Conductivity cell with phial magazine Q containing phials and magnetic pusher. The electrode assembly E is that shown in Fig. 3.22. S is a stirrer shaft with a propeller at one end and a glass-enclosed magnet N at the other. The stirrer shaft is held in position by the PTFE bearings Tf Tf, and Tf and the glass tube spacer G Th ha. thermocouple pocket and B a magnetic breaker for the phial P. The propeller, driven by the rotating magnet M, pumps the cell contents around the loop L, so that when P is broken there is very fast mixing.

The test substance is mixed with a conducting substance and usually with a binder (polyethylene or PTFE) and a pore producing compound, pressed and, if necessary, sintered. Compact electrodes are obtained, many with a large content of the test material, which can be used without much modification in operating cells. The measure of the activity is the current density in mA/cm2. Despite the close simulation of operating conditions, this test method is unsuitable for the comparison of different substances. A relatively large quantity of catalyst is required, and the soft, hydrophobic binder can enclose the catalyst particles. 

A number of different methods exist for the production of catalyst layers [97-102]. They use variations in composition (contents of carbon, Pt, PFSI, PTFE), particle sizes and pds of highly porous carbon, material properties (e.g., the equivalent weight of the PFSI) as well as production techniques (sintering, hot pressing, application of the catalyst layer to the membrane or to the gas-diffusion layer, GDL) in order to improve the performance. The major goal of electrode development is the reduction of Pt and PFSI contents, which account for substantial contributions to the overall costs of a PEFC system. Remarkable progress in this direction has been achieved during the last decade [99, 100], At least on a laboratory scale, the reduction of the Pt content from 4.0 to 0.1 mg cm-2 has been successfully demonstrated. 

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