PAFC electrode

As in the case with PAFC s, voltage obtained from an AFC is affected by ohmic, activation, and concentration losses. Figure 4-7 presents data obtained in the 1960 s (18) which summarizes these effects, excluding ohmic losses, for a catalyzed reaction (0.5-2.0 mg noble metal/cm ) with carbon-based porous electrodes for H2 oxidation and O2 reduction in 9 N KOH at 55-60 C. The electrode technology was similar to that employed in the fabrication of PAFC electrodes. Performance of AFC s with carbon-based electrodes has not changed dramatically since these early results were obtained. 

First it will be explained what an electrode ink is, what requirements are set on it and how it defines the properties of the electrode. This is followed by typical ways to manufacture gas diffusion electrodes for fuel cells. Afterwards the dependence between the electrode and MEA fabrication will be discussed. Due to the distinct affinity between HT-PEM, LT-PEM, and PAFC electrodes, the most here invented techniques are usable for fabrication of GDEs for aU three fuel cell types. 

PAFC electrodes are made out of planar, porous substrate on which the catalyst is coated along with a binder. The reaction takes place at the catalyst surface, and the electrons are transferred from the catalyst particles to the electrode substrate. Further, the electrons are passed to the conducting separator gas plate and transmitted to the external circuit. 

The first generation PAFC electrodes were made of Tantalum 100 mesh, woven screens with gold... 

The fifth generation PAFC electrode substrates are basically refinements of the Kureha s carbon substrate as mentioned before. These substrates manufactured by Toray have a mean pore size of about 20-30 p, about 0.3-0.4 mm thick and are very successfully used in UTCs 200 kw PC-25 power plants, being one of... 

Fig. 6 Electrochemical surface area determination of Pt based PAFC electrode by cyclic voltammetry.

Combination of the above methods is useful to analyze change of catalyst structure and activity of PAFC electrode in the course of operation. Such an aging study helps to determine the various modes of catalyst decay. Maoka et al. (1996), reported various physical and electrochemical methods to determine PAFC cathode catalyst deactivation over time. 

As many of the parameters like inlet humidity, cell temperature, gas flow rates and catalyst state of health are interrelated, there is a need to develop comprehensive online fault diagnosis system. There is a big gap in the knowledge base of online techniques for understanding problems of PAFC electrodes. For example, for loss of power in an operating PAFC stack, there is a need to understand the state of health of... 

Westinghouse Electric Corp. initiated a program to develop air-cooled PAFC stacks, containing cooling plates at six-ceU intervals. Full size 100-kW stacks (468 cells, 0.12-m electrode area) were built, and a module containing four of these stacks was tested. An air-cooled stack operated at 0.480 MPa yielded a cell voltage of 0.7 V at 267 m A /cm (187 mW/cm ). Demonstration of this technology is plarmed for a site in Norway. 

In applications where Nafion is not suitable, at temperatures above 200 °C with feed gas heavily contaminated with CO and sulfur species, a phosphoric acid fuel cell (PAFC)-based concentrator has been effective [15]. Treating the gas shown in Table 1, a H2 product containing 0.2% CO, 0.5%CO2 and only 6 ppm H2S was produced. The anode electrode was formed from a catalyst consisting basically of Pt-alloy mixed with 50% PTFE on a support of Vulcan XC-72 carbon. The cathode was... 

In MCFCs, which operate at relatively high temperature, no materials are known that wet-proof a porous structure against ingress by molten carbonates. Consequently, the technology used to obtain a stable three-phase interface in MCFC porous electrodes is different from that used in PAFCs. In the MCFC, the stable interface is achieved in the electrodes by carefully tailoring the pore structures of the electrodes and the electrolyte matrix (LiA102) so that the capillary forces establish a dynamic equilibrium in the different porous structures. Pigeaud et al. (4) provide a discussion of porous electrodes for MCFCs. 

The porous electrodes used in PAFCs are described extensively in the patent literature (6) see also the review by Kordesch (5). These electrodes contain a mixture of the electrocatalyst supported on carbon black and a polymeric binder, usually PTFE (about 30 to 50 wt%). The PTFE binds the carbon black particles together to form an integral (but porous) structure, which is supported on a porous carbon paper substrate. The carbon paper serves as a structural support for the electrocatalyst layer, as well as the current collector. A typical carbon paper used in PAFCs has an... 

Oxidant The oxidant composition and utilization are parameters that affect the cathode performance, as evident in Figure 2-3. Air, which contains -21% Oi, is the oxidant of choice for PAFCs. The use of air with -21% Oi instead of pure Oi results in a decrease in the current density of about a factor of three at constant electrode potential. The polarization at the cathode increases with an increase in Oi utilization. Experimental measurements (38) of the change in overpotential (Aric) at a PTFE-bonded porous electrode in 100% H3PO4 (191°C, atmospheric pressure) as a function of O2 utilization is plotted in Figure 5-4 in accordance with Equation (5-7) ... 

M. Watanabe, C. Shirmura, N. Hara, K. Tsurumi, "An Advanced Gas-Diffusion Electrode for Long-Life and High Performance PAFC," in The International Fuel Cell Conference Proceedings, NEDO/MITI, Tokyo, Japan, 1992. 

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